Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0046872 metal ion binding	IEA GO_REF:0000043	REMOVE	Summary: Retired SwissProt-keyword (SPKW) annotation derived from the "Metal-binding" / "Iron" UniProt keywords. PsaC genuinely binds metal (iron) atoms within its two [4Fe-4S] clusters, so the term is not factually wrong, but it is a broad, imprecise generalization. The precise function is already captured by the still-current GO:0051539 "4 iron, 4 sulfur cluster binding". Reason: GOA's removal of this SPKW annotation is JUSTIFIED. PsaC does bind iron, but only as part of two [4Fe-4S] cluster cofactors, not as free metal ions. In the GO hierarchy GO:0051539 "4 iron, 4 sulfur cluster binding" is NOT a child of GO:0046872 "metal ion binding": GO:0051539 sits on the metal cluster binding branch (small molecule binding to metal cluster binding to iron-sulfur cluster binding to 4Fe-4S cluster binding), whereas metal ion binding sits on the ion binding / cation binding branch. They are non-redundant sibling branches that share only the broad ancestor "small molecule binding". "Metal ion binding" is therefore an imprecise descriptor for a cluster-binding protein; it conveys no information beyond what the precise, retained GO:0051539 annotation already states. Removing it loses no correct biological information and improves annotation precision. This gene exemplifies an SPKW annotation that is broadly true but redundant against a better, current term. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ... spectroscopically characterized electron acceptor FA and cluster 1 is ... most probably FB. PMID:8993322 The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit. file:CHLRE/psaC/psaC-deep-research-falcon.md binds the terminal/secondary [4Fe-4S] iron-sulfur clusters FA and FB that relay light-generated electrons from PSI to soluble acceptors ... PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters and sits adjacent to the FX site of the PsaA-PsaB core
GO:0016491 oxidoreductase activity	IEA GO_REF:0000043	REMOVE	Summary: Retired SwissProt-keyword (SPKW) annotation derived from the "Oxidoreductase" UniProt keyword (UniProt assigns EC 1.97.1.12 to the whole PSI complex via HAMAP rule MF_01303). PsaC is an electron-carrier subunit, not a standalone catalytic oxidoreductase. The precise function of PsaC is captured by the still-current GO:0009055 "electron transfer activity". Reason: GOA's removal of this SPKW annotation is JUSTIFIED. The EC 1.97.1.12 oxidoreductase designation applies to the intact PSI holocomplex (plastocyanin/cytochrome c6-ferredoxin oxidoreductase), not to PsaC as an isolated catalyst. PsaC itself simply carries electrons through its FA and FB [4Fe-4S] clusters from FX to soluble ferredoxin; it does not catalyze a redox reaction in the enzymatic sense. In current GO, GO:0009055 "electron transfer activity" is a direct child of molecular_function and is deliberately NOT placed under GO:0016491 "oxidoreductase activity"; electron carriers were separated from catalytic oxidoreductases. So "oxidoreductase activity" is an imprecise, borderline-incorrect descriptor for this subunit, and the precise current term GO:0009055 is retained. No correct information is lost by removal. This gene exemplifies an SPKW annotation that is broad/imprecise and subsumed in intent by a better, current term. Proposed replacements: electron transfer activity Supporting Evidence: PMID:10438510 The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC. Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC. file:CHLRE/psaC/psaC-uniprot.txt Apoprotein for the two 4Fe-4S centers FA and FB of ... essential for photochemical activity. FB is the ... terminal electron acceptor of PSI, donating electrons to ferredoxin. file:CHLRE/psaC/psaC-deep-research-falcon.md The UniProt record associates PsaC with EC 1.97.1.12, reflecting PSI's overall oxidoreductase activity of transferring electrons from reduced plastocyanin/cytochrome c6 to ferredoxin through the PSI cofactor chain, with PsaC specifically functioning as the protein scaffold that binds FA/FB and enables terminal electron transfer to soluble acceptors.
GO:0009055 electron transfer activity	IEA GO_REF:0000120	ACCEPT	Summary: Electron transfer activity is the core, precise molecular function of PsaC. The protein carries electrons through its two [4Fe-4S] clusters FA and FB, relaying them from cluster FX of the PSI core to soluble ferredoxin. This is strongly supported by experimental work in Chlamydomonas. Reason: This is the correct, appropriately specific molecular function term for PsaC. PsaC binds the two terminal [4Fe-4S] electron acceptors of PSI and relays electrons from FX through FA to FB to ferredoxin. Site-directed mutagenesis in C. reinhardtii directly demonstrated electron transfer through PsaC and identified FB as the cluster donating to ferredoxin. The IEA evidence (UniRule, from the conserved PSI iron-sulfur center family) is fully consistent with experimental data. Supporting Evidence: PMID:10438510 The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC ... These data indicate that F(B) is the cluster interacting with Fd and therefore the outermost iron-sulfur cluster of PSI. PMID:9463363 the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas the K35R change has no effect on Fd reduction. file:CHLRE/psaC/psaC-deep-research-falcon.md PsaC is not merely a passive Fe-S carrier but actively shapes encounter complex formation, orientation, and ET efficiency for stromal acceptors ... PSI electron flow is described as plastocyanin -> P700 -> internal cofactors -> PsaC FA/FB -> ferredoxin.
GO:0009522 photosystem I	IEA GO_REF:0000002	ACCEPT	Summary: PsaC is a bona fide subunit of the Photosystem I complex. It is the small stromal subunit that binds the FA/FB clusters and is required for stable assembly of the complex. Cryo-EM structures of the Chlamydomonas PSI-LHCI supercomplex (e.g. PDB 6IJJ/6JO5; Su et al. 2019) resolve PsaC carrying the terminal [4Fe-4S] clusters FA and FB on the stromal face of the PSI core. Reason: Correct cellular component (complex) annotation. PsaC is an extrinsic stromal subunit of PSI; it is resolved as chain C in numerous Chlamydomonas PSI cryo-EM structures, and its inactivation destabilizes the whole PSI complex. The InterPro2GO mapping (IPR017491, PSI PsaC) is accurate. Supporting Evidence: PMID:1712288 neither PSI reaction center subunits nor the seven small subunits belonging to PSI accumulate stably in the thylakoid membranes of the transformants. PMID:9463363 PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB. PMID:30850819 multiple units of light-harvesting complex I (LHCI) bind to the PSI core and function as peripheral antennae, forming a PSI-LHCI supercomplex ... Here, we report structures of CrPSI-LHCI, solved by cryo-electron microscopy
GO:0009535 chloroplast thylakoid membrane	IEA GO_REF:0000120	ACCEPT	Summary: PsaC is localized to the chloroplast thylakoid membrane, where the PSI complex resides. It is a peripheral membrane protein on the stromal side. Reason: Correct and appropriately specific subcellular localization. PsaC is part of the PSI complex embedded in the chloroplast thylakoid membrane; UniProt records it as a peripheral membrane protein on the stromal side. This is more specific than the generic "thylakoid membrane" annotation and is preferred. Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Plastid, chloroplast thylakoid membrane ... Peripheral membrane protein ... Stromal side
GO:0009773 photosynthetic electron transport in photosystem I	IEA GO_REF:0000002	ACCEPT	Summary: PsaC mediates the terminal step of photosynthetic electron transport within PSI, carrying electrons from FX through FA and FB to ferredoxin. This is the precise biological process for the gene. Reason: This is the correct and appropriately specific biological process term. PsaC carries the FA/FB part of the PSI electron transfer chain; experimental mutagenesis in Chlamydomonas directly demonstrated its role in photosynthetic electron transport to ferredoxin. The InterPro2GO mapping is accurate and more informative than the parent term "photosynthesis". Supporting Evidence: PMID:8993322 The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit. PMID:10438510 Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC.
GO:0015979 photosynthesis	IEA GO_REF:0000120	MARK AS OVER ANNOTATED	Summary: Photosynthesis is correct but is a broad parent of the more specific and better term "photosynthetic electron transport in photosystem I" (GO:0009773), which is also annotated to this gene. Reason: PsaC genuinely functions in photosynthesis, so the term is not wrong, but it is unnecessarily broad. The more specific child term GO:0009773 "photosynthetic electron transport in photosystem I" is already annotated and precisely captures PsaC's role. Retaining the generic "photosynthesis" term adds no information beyond the specific annotation; it is best treated as an over-annotation rather than a core function term. Supporting Evidence: PMID:1712288 Transformants ... are unable to grow on minimal medium lacking acetate and are deficient in PSI activity
GO:0042651 thylakoid membrane	IEA GO_REF:0000002	MARK AS OVER ANNOTATED	Summary: Thylakoid membrane is correct but is the generic parent of the more specific "chloroplast thylakoid membrane" (GO:0009535), which is also annotated to this gene. Reason: PsaC is in the thylakoid membrane, so the term is accurate, but it is less specific than GO:0009535 "chloroplast thylakoid membrane", which is already annotated and is appropriate for this chloroplast-encoded alga protein. The generic term adds no information beyond the specific one; mark as over-annotated in favor of the chloroplast-specific term. Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Plastid, chloroplast thylakoid membrane ... Peripheral membrane protein ... Stromal side
GO:0051539 4 iron, 4 sulfur cluster binding	IEA GO_REF:0000002	ACCEPT	Summary: PsaC binds two [4Fe-4S] clusters (FA and FB) via eight cysteine ligands arranged in two 4Fe-4S ferredoxin-type domains. This is a core, precise molecular function of the protein. Reason: This is the correct and appropriately specific molecular function term for cofactor binding by PsaC. UniProt documents binding of two [4Fe-4S] clusters with eight cysteine BINDING-site residues, and experimental work in Chlamydomonas characterized clusters FA and FB coordinated by PsaC. This precise term supersedes the retired, broader SPKW term "metal ion binding". Supporting Evidence: file:CHLRE/psaC/psaC-uniprot.txt Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ... spectroscopically characterized electron acceptor FA and cluster 1 is ... most probably FB. PMID:9463363 This subunit resembles 2[4Fe-4S] bacterial ferredoxins but contains two additional sequences: an internal loop and a C-terminal extension.

Core Functions

Binds the two terminal [4Fe-4S] iron-sulfur clusters FA and FB of Photosystem I as the apoprotein PsaC, providing the redox cofactors for the terminal segment of PSI electron transfer.

Molecular Function:

4 iron, 4 sulfur cluster binding

Directly Involved In:

photosynthetic electron transport in photosystem I

Cellular Locations:

chloroplast thylakoid membrane

Supporting Evidence:

file:CHLRE/psaC/psaC-uniprot.txt
Apoprotein for the two 4Fe-4S centers FA and FB of ... essential for photochemical activity.
PMID:9463363
PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB.
file:CHLRE/psaC/psaC-deep-research-falcon.md
PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters and sits adjacent to the FX site of the PsaA-PsaB core.

Acts as an electron carrier within Photosystem I, relaying electrons from cluster FX of the PSI core through FA and then to FB, the most distal cluster, which donates electrons to soluble ferredoxin on the stromal side.

Molecular Function:

electron transfer activity

Directly Involved In:

photosynthetic electron transport in photosystem I

Cellular Locations:

chloroplast thylakoid membrane

Supporting Evidence:

PMID:10438510
These data indicate that F(B) is the cluster interacting with Fd and therefore the outermost iron-sulfur cluster of PSI.
PMID:9463363
the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy.
file:CHLRE/psaC/psaC-deep-research-falcon.md
PSI electron flow is described as plastocyanin -> P700 -> internal cofactors -> PsaC FA/FB -> ferredoxin ... under iron deprivation, flavodoxin can substitute for ferredoxin and accept electrons from a PsaC-coordinated [4Fe-4S] cluster.

Required for stable assembly of the Photosystem I complex and, together with PsaD and PsaE, forms the stromal ferredoxin-docking site; loss of PsaC destabilizes the whole PSI complex and abolishes photoautotrophic growth.

Directly Involved In:

photosynthetic electron transport in photosystem I

Cellular Locations:

chloroplast thylakoid membrane

Supporting Evidence:

PMID:1712288
the iron sulfur binding protein encoded by the psaC gene is an essential component, both for photochemical activity and for stable assembly of PSI.
file:CHLRE/psaC/psaC-uniprot.txt
The C-terminus interacts with PsaA/B/D and helps assemble the protein ... into the PSI complex. Required for binding of PsaD and PsaE to PSI.
file:CHLRE/psaC/psaC-deep-research-falcon.md
removing PsaC eliminates FA/FB and is associated with incomplete FX formation and missing stromal subunits PsaD and PsaE, supporting a model in which PsaC binding promotes conformational states needed for proper FX maturation and subsequent stromal-subunit incorporation.

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

SwissProt keyword-derived (SPKW) annotations present in the Sept 2025 goa_uniprot_gcrp snapshot but removed from the current GOA release after GOA retired the keyword2GO pipeline for cellular organisms.

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:1712288

Directed chloroplast transformation in Chlamydomonas reinhardtii: insertional inactivation of the psaC gene encoding the iron sulfur protein destabilizes photosystem I.

Insertional inactivation of the chloroplast psaC gene abolishes photoautotrophic growth and PSI activity; PsaC is essential both for PSI photochemical activity and for stable assembly of the PSI complex.

"Transformants ... are unable to grow on minimal medium lacking acetate and are deficient in PSI activity ... We conclude that the iron sulfur binding protein encoded by the psaC gene is an essential component, both for photochemical activity and for stable assembly of PSI."
The PsaC amino acid sequence is highly conserved relative to higher plants and cyanobacteria.

"The deduced amino acid sequence is highly related to that of higher plants and cyanobacteria."

PMID:8993322

Targeted mutations in the psaC gene of Chlamydomonas reinhardtii: preferential reduction of FB at low temperature is not accompanied by altered electron flow from photosystem I to ferredoxin.

PsaC coordinates the two [4Fe-4S] centers FA and FB, which form the terminal part of the electron transfer pathway within the PSI complex.

"The terminal part of the electron pathway within the photosystem I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated by the PsaC subunit."

PMID:9463363

The PsaC subunit of photosystem I provides an essential lysine residue for fast electron transfer to ferredoxin.

PsaC is the stromal subunit of PSI that binds the two terminal electron acceptors FA and FB; it resembles 2[4Fe-4S] bacterial ferredoxins with an extra internal loop and C-terminal extension.

"PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB. This subunit resembles 2[4Fe-4S] bacterial ferredoxins but contains two additional sequences: an internal loop and a C-terminal extension."
Lys35 of PsaC is a main electrostatic interaction site with ferredoxin and is required for fast electron transfer from PSI to ferredoxin.

"K35 is a main interaction site between PsaC and ferredoxin (Fd) and that it plays a key role in the electrostatic interaction between Fd and PSI ... the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd ... whereas the K35R change has no effect on Fd reduction."

PMID:10438510

Site-directed mutagenesis of the PsaC subunit of photosystem I. F(b) is the cluster interacting with soluble ferredoxin.

The two [4Fe-4S] clusters FA and FB are the terminal electron acceptors of PSI and are bound by the stromal subunit PsaC; FB is the outermost cluster that donates electrons to soluble ferredoxin.

"The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC. Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced by the outermost iron-sulfur cluster of PsaC ... These data indicate that F(B) is the cluster interacting with Fd and therefore the outermost iron-sulfur cluster of PSI."

PMID:30850819

Antenna arrangement and energy transfer pathways of a green algal photosystem-I-LHCI supercomplex.

PMID:31182847

Structure of the green algal photosystem I supercomplex with a decameric light-harvesting complex I.

file:CHLRE/psaC/psaC-uniprot.txt

UniProt entry Q00914 (PSAC_CHLRE), Photosystem I iron-sulfur center

PsaC is the apoprotein for the two 4Fe-4S centers FA and FB of PSI; FB is the terminal electron acceptor donating electrons to ferredoxin; PsaC is required for binding of PsaD and PsaE to PSI.

"Apoprotein for the two 4Fe-4S centers FA and FB of ... essential for photochemical activity. FB is the ... terminal electron acceptor of PSI, donating electrons to ferredoxin ... Required for binding of PsaD and PsaE to PSI."
PsaC is localized to the chloroplast thylakoid membrane as a peripheral membrane protein on the stromal side.

"Plastid, chloroplast thylakoid membrane ... Peripheral membrane protein ... Stromal side"
PsaC binds two [4Fe-4S] clusters via eight cysteine ligands organized in two 4Fe-4S ferredoxin-type domains.

"Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ... spectroscopically characterized electron acceptor FA and cluster 1 is ... most probably FB."

file:CHLRE/psaC/psaC-deep-research-falcon.md

Deep research report (falcon/Edison): Chlamydomonas reinhardtii psaC (Q00914), Photosystem I iron-sulfur center subunit PSI-C

PsaC is the stromal, membrane-extrinsic PSI subunit that binds the terminal [4Fe-4S] clusters FA and FB, relaying light-generated electrons from cluster FX of the PSI core to soluble ferredoxin (and, under iron deprivation, flavodoxin). This is the precise electron-carrier molecular function, distinct from the catalytic oxidoreductase activity of the intact PSI holocomplex.

"PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters and sits adjacent to the FX site of the PsaA-PsaB core, accepting electrons from upstream cofactors and passing them onward to ferredoxin ... PSI electron flow is described as plastocyanin -> P700 -> internal cofactors -> PsaC FA/FB -> ferredoxin."
The EC 1.97.1.12 oxidoreductase designation applies to the intact PSI holocomplex acting as a plastocyanin/cytochrome c6-ferredoxin oxidoreductase, not to PsaC as a standalone catalyst; PsaC functions as the protein scaffold that binds FA/FB and enables terminal electron transfer to soluble acceptors.

"The UniProt record associates PsaC with EC 1.97.1.12, reflecting PSI's overall oxidoreductase activity ... with PsaC specifically functioning as the protein scaffold that binds FA/FB and enables terminal electron transfer to soluble acceptors."
PsaC is indispensable for PSI in Chlamydomonas - chloroplast deletion of psaC abolishes detectable functional PSI and photoautotrophic growth and strongly impairs CO2 fixation and H2 photoproduction (CO2 uptake ~1500 -> ~210 and H2 evolution ~1200 -> ~3 nmol/min/mg Chl).

"Chloroplast deletion of psaC causes loss of photoautotrophic growth and PSI detectability by immunological and spectroscopic assays, with CO2 uptake reduced from 1500 to 210 ... and H2 evolution reduced from 1200 to 3"
Site-directed mutagenesis of PsaC residues (Lys35; the K52/R53 region) in Chlamydomonas shows PsaC actively shapes ferredoxin/flavodoxin docking and electron-transfer kinetics; Lys35 is a key electrostatic contact whose charge inversion severely slows electron transfer to soluble acceptors.

"These data support a mechanistic model where PsaC is not merely a passive Fe-S carrier but actively shapes encounter complex formation, orientation, and ET efficiency for stromal acceptors."
Cryo-EM of a PsaC-less PSI assembly intermediate indicates PsaC binding promotes conformational states needed for proper FX maturation and subsequent incorporation of the stromal subunits PsaD and PsaE, consistent with PsaC being required for stable PSI assembly.

"removing PsaC eliminates FA/FB and is associated with incomplete FX formation and missing stromal subunits PsaD and PsaE, supporting a model in which PsaC binding promotes conformational states needed for proper FX maturation and subsequent stromal-subunit incorporation."

Suggested Experiments

Experiment: Use ultrafast flash-absorption and EPR spectroscopy on Chlamydomonas PSI to quantify electron transfer rates along the FX, FA, FB, ferredoxin chain and confirm the FA-proximal / FB-distal orientation of PsaC under physiological conditions.

Experiment: Generate cysteine-ligand substitution mutants of PsaC in C. reinhardtii and assay cluster occupancy, redox potential, and PSI assembly to dissect the individual contributions of FA versus FB to electron delivery to ferredoxin.

Experiment: Solve high-resolution cryo-EM structures of Chlamydomonas PSI-ferredoxin complexes to map the PsaC/PsaD/PsaE ferredoxin-docking interface and the role of PsaC Lys35.

Deep Research

Falcon

(psaC-deep-research-falcon.md)

Research Report: *Chlamydomonas reinhardtii* **psaC** (UniProt **Q00914**) — Photosystem I iron–sulfur center subunit (PSI-C) Falcon Edison Scientific Literature 47 citations 2 artifacts 2026-05-21T14:18:08.672039

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000040 Figure 2B on page 5 provides a schematic representation of the Photosystem I (PSI) electron transfer chain. The 'Right side view' e (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research Report: Chlamydomonas reinhardtii psaC (UniProt Q00914) — Photosystem I iron–sulfur center subunit (PSI-C)

Executive summary

The Chlamydomonas reinhardtii chloroplast gene psaC (UniProt Q00914) encodes the stromal, membrane-extrinsic Photosystem I (PSI) subunit PsaC/PSI‑C, a small ferredoxin-like protein that binds the terminal/secondary [4Fe–4S] iron–sulfur clusters FA and FB that relay light-generated electrons from PSI to soluble acceptors such as ferredoxin (Fd) (and in some conditions flavodoxin) to support NADPH production and broader reductive metabolism. This identity is directly supported by a Chlamydomonas proteomics study that explicitly labels PsaC (Q00914) as a PSI iron–sulfur center protein. (ford2019inhibitionoftor pages 16-18, ford2019inhibitionoftor pages 14-16)

Functionally, PsaC is essential for PSI integrity and oxygenic photosynthesis in Chlamydomonas: deletion of psaC eliminates detectable functional PSI, photoautotrophic growth, and strongly impairs CO2 fixation and H2 photoproduction, with residual electron transport signatures reflecting non-PSI processes. (redding1999photosystemiis pages 7-8, redding1999photosystemiis pages 5-7, redding1999photosystemiis pages 2-3)

1) Key concepts and definitions (current understanding)

1.1 Photosystem I as a plastocyanin–ferredoxin oxidoreductase

PSI is widely described as a light-driven plastocyanin–ferredoxin oxidoreductase, transferring electrons from lumenal donors (plastocyanin; in algae often also cytochrome c6) to stromal acceptors (ferredoxin) to generate reducing power for metabolism. (rolo2024structurefunctionand pages 3-4, rochaix2000chloroplastsitedirectedmutagenesis pages 2-3)

1.2 What is PsaC?

PsaC is the stromal-side PSI subunit that carries the terminal iron–sulfur electron acceptors FA and FB (both [Fe4–S4] clusters) and participates in forming the stromal “ridge” (with PsaD and PsaE) that supports ferredoxin docking for productive electron transfer. (rolo2024structurefunctionand pages 4-5, ozawa2018configurationoften pages 1-2)

A 2024 review summarizes that PsaC together with the PsaA–PsaB reaction-center heterodimer carries the cofactors for plastocyanin→ferredoxin oxidoreduction, explicitly including the three [Fe4–S4] clusters FX, FA, and FB. (rolo2024structurefunctionand pages 3-4)

1.3 Biochemical reaction context / EC number interpretation

The UniProt record associates PsaC with EC 1.97.1.12, reflecting PSI’s overall oxidoreductase activity of transferring electrons from reduced plastocyanin/cytochrome c6 to ferredoxin through the PSI cofactor chain, with PsaC specifically functioning as the protein scaffold that binds FA/FB and enables terminal electron transfer to soluble acceptors. Consistent with this, PSI electron flow is described as plastocyanin → P700 → internal cofactors → PsaC FA/FB → ferredoxin. (rolo2024structurefunctionand pages 4-5)

2) Molecular function: electron-transfer role of PsaC and its partners

2.1 Cofactors bound by PsaC

PsaC contains the terminal/secondary FA and FB [4Fe–4S] clusters and sits adjacent to the FX site of the PsaA–PsaB core, accepting electrons from upstream cofactors and passing them onward to ferredoxin. (rolo2024structurefunctionand pages 4-5)

Figure evidence: a Plant Cell 2024 schematic shows the PSI electron-transfer chain with PsaC housing FA/FB on the stromal side. (rolo2024structurefunctionand media e293fb55)

2.2 Electron donor(s) to PSI vs acceptor(s) from PSI

Structural evidence indicates that the binding site of the reduced electron carriers (plastocyanin and cytochrome c6) is on the luminal face of PSI. (caspy2020thestructureof pages 3-4)

On the stromal side, ferredoxin binds near PsaC. In a PSI–ferredoxin complex structure, multiple close contacts between PsaC and ferredoxin are reported, including specific residue–residue proximity constraints (Å-level distances) and an experimental record that mutagenesis of several PsaC residues impacts ferredoxin binding. (caspy2020thestructureof pages 3-4)

2.3 Quantitative kinetics: PsaC residues tune ferredoxin/flavodoxin electron transfer

Targeted psaC mutations in Chlamydomonas provide strong, quantitative evidence that PsaC residues tune electron-transfer kinetics and docking affinity for soluble acceptors:

K52/R53 region (near FA): A psaC double mutant K52S/R53A accumulates only ~20–30% PSI and shows altered low-temperature FA/FB behavior (preferential FB photoreduction at 15 K) but maintains near-normal PSI→ferredoxin electron transfer at room temperature. Quantitatively, ferredoxin binding/dynamics were similar (Kd 6.1 µM WT vs 7.3 µM mutant) and second-order rate constants were 3.5 × 10^8 M⁻¹ s⁻¹ (WT) vs 5.6 × 10^8 M⁻¹ s⁻¹ (mutant), with a fast reduction phase measurable in the microsecond regime. (fischer1997targetedmutationsin pages 7-8, fischer1997targetedmutationsin pages 8-9, fischer1997targetedmutationsin pages 1-2)
Lys35 (key electrostatic contact): Mutations at PsaC Lys35 strongly impact photoreduction of external acceptors. For flavodoxin reduction, half-times and second-order constants show that charge inversion (Lys→Glu/Asp) severely slows electron transfer, while Lys→Arg largely preserves WT kinetics. Example quantitative outcomes include oxidized-flavodoxin t1/2 2.4 ms (WT) vs <9.9 ms (Lys35Glu), semiquinone t1/2 0.35 ms (WT) vs <9.9 ms (Lys35Glu), and second-order constants dropping from 0.96 × 10^8 M⁻¹ s⁻¹ (WT) to 0.05 × 10^8 M⁻¹ s⁻¹ (Lys35Glu). Cross-linking experiments also indicate Lys35 is necessary for forming a flavodoxin–PsaC adduct. (meimberg1999lys35ofpsac pages 3-4, meimberg1999lys35ofpsac pages 4-6, meimberg1999lys35ofpsac pages 6-7)

These data support a mechanistic model where PsaC is not merely a passive Fe–S carrier but actively shapes encounter complex formation, orientation, and ET efficiency for stromal acceptors.

2.4 Stress/redox regulation signals involving PsaC

In a TOR-inhibition proteomics study in Chlamydomonas, PsaC (Q00914) is explicitly described as a PSI subunit containing an iron–sulfur center and showed a substantial increase in reversible cysteine oxidation (reported fold-changes include 2.46 for a significant oxidation identifier and 3.36 in discussion). These redox changes were discussed in the context of altered electron-flow partitioning (e.g., shifts toward cyclic electron flow or downstream bottlenecks). (ford2019inhibitionoftor pages 16-18, ford2019inhibitionoftor pages 14-16)

3) Cellular and subcellular localization

3.1 Subcellular location

PsaC functions in the chloroplast thylakoid membrane system as part of PSI. It is membrane-extrinsic on the stromal side (i.e., chloroplast stroma-facing surface of PSI), where it presents FA/FB and forms part of the ferredoxin-docking ridge. (caspy2020thestructureof pages 3-4, rolo2024structurefunctionand pages 4-5, ozawa2018configurationoften pages 1-2)

3.2 Topological context (two-sided electron-transfer)

PSI is topologically polarized: electron donation occurs on the luminal face (plastocyanin/cytochrome c6), while electron acceptance occurs on the stromal face (ferredoxin binding near PsaC). (caspy2020thestructureof pages 3-4, rolo2024structurefunctionand pages 3-4)

4) Pathways and biological processes involving psaC/PsaC

4.1 Linear electron flow (LEF) and NADPH generation

Reduced ferredoxin produced by PSI is primarily reoxidized by ferredoxin–NADP+ reductase (FNR) to generate NADPH in LEF. (rolo2024structurefunctionand pages 3-4)

4.2 Alternative acceptors and iron-stress substitution

A 2024 review notes that under iron deprivation, flavodoxin can substitute for ferredoxin and accept electrons from a PsaC-coordinated [4Fe–4S] cluster, emphasizing functional flexibility at the PSI acceptor side. (tian2024photosystemiaa pages 2-4)

4.3 Cyclic electron flow (CEF) and photoprotection/ATP balancing

Ferredoxin can also route electrons into cyclic electron flow from the PSI acceptor side back to cytochrome b6f, increasing proton motive force without NADPH production and thereby increasing ATP/NADPH ratio and protecting PSI against over-reduction. (rolo2024structurefunctionand pages 3-4)

5) Mutant and phenotype evidence in Chlamydomonas reinhardtii

5.1 psaC is essential for PSI function and photoautotrophy

Chloroplast deletion of psaC causes loss of photoautotrophic growth and PSI detectability by immunological and spectroscopic assays, with CO2 uptake reduced from ~1500 to ~210 nmol·min⁻¹·mg Chl⁻¹ and H2 evolution reduced from ~1200 to ~3 nmol·min⁻¹·mg Chl⁻¹ (WT vs psaC-deletion strain values as reported). (redding1999photosystemiis pages 5-7, redding1999photosystemiis pages 2-3)

These data support that PsaC is indispensable for establishing the PSI acceptor side capable of reducing ferredoxin sufficiently to drive CO2 fixation and hydrogenase-linked H2 photoevolution. (redding1999photosystemiis pages 7-8, redding1999photosystemiis pages 2-3)

5.2 psaC mutations impact PSI stability, light sensitivity, and acceptor-side ET

Site-directed mutant analysis shows multiple mechanistic phenotypes:
* K52/R53 mutations destabilize PSI to varying degrees, alter low-temperature FA/FB behavior, and can produce aerobic photosensitivity while preserving near-normal PSI→ferredoxin ET at room temperature in some alleles. (fischer1997targetedmutationsin pages 1-2)
* Lys35 mutations strongly reduce rates of electron transfer to flavodoxin and (by parallel mechanistic rationale) ferredoxin. (meimberg1999lys35ofpsac pages 3-4, meimberg1999lys35ofpsac pages 4-6)
* A Chlamydomonas PSI mutagenesis review describes that psaC mutations can decrease acceptor-side ET, increase ferredoxin affinity, and lead to high light sensitivity; it also notes an estimated FA–FB redox potential difference on the order of ~50 mV and frames PsaC/FA placement as protective against electron escape to oxygen. (rochaix2000chloroplastsitedirectedmutagenesis pages 8-9)

6) Recent developments (prioritizing 2023–2024) and latest research directions

6.1 2024 structural and assembly perspectives: PsaC’s role in biogenesis

A major 2024 Plant Cell review synthesizes current knowledge of PSI assembly, including the idea that intermediates exposing or lacking Fe–S clusters (FX/FA/FB) can be oxidation sensitive and that correct incorporation/stabilization of PsaC and its clusters is integrated with formation of the stromal ridge and ferredoxin docking features. (rolo2024structurefunctionand pages 13-14)

A 2024 high-resolution cryo-EM study of a cyanobacterial PsaC-less assembly intermediate reports that removing PsaC eliminates FA/FB and is associated with incomplete FX formation and missing stromal subunits PsaD and PsaE, supporting a model in which PsaC binding promotes conformational states needed for proper FX maturation and subsequent stromal-subunit incorporation (note: cyanobacterial system, but informative mechanistically). (naschberger20241.8åresolution pages 1-5, naschberger20241.8åresolution pages 5-8)

6.2 2024 expert analysis: conservation vs flexibility

A 2024 expert review emphasizes that PSI has a remarkably conserved core but flexible peripheral architecture, and describes PsaC as a conserved Fe–S protein (FA/FB) that helps stabilize extremely low-potential electrons and protect PSI from radical damage; it also advances an interpretation that PSI maintains two electron-transfer branches, potentially balancing efficiency with protective “slippage.” (nelson2024investigatingthebalance pages 3-5, nelson2024investigatingthebalance pages 1-3)

6.3 2024 algal PSI supercomplex diversity (context for psaC functional environment)

A 2024 IJMS review on PSI adaptations highlights that Chlamydomonas PSI can exist in multiple supramolecular forms (e.g., PSI–LHCI–LHCII supercomplex in state transitions; PSI–LHCI dimers under low light/anoxia) with hundreds of cofactors, underscoring that while PsaC’s Fe–S function is conserved, the antenna and supercomplex context that modulates excitation/electron flow is highly plastic. (tian2024photosystemiaa pages 7-9)

7) Current applications and real-world implementations

7.1 Photosynthetic hydrogen production and bioenergy engineering

Because PsaC-mediated terminal electron transfer is required to reduce ferredoxin, and reduced ferredoxin is a key electron donor for algal hydrogenases, psaC integrity is a prerequisite for photobiological H2 production. This is supported by direct experiments showing H2 photoevolution is essentially absent in psaC deletion mutants (vs robust WT H2 evolution). (redding1999photosystemiis pages 7-8, redding1999photosystemiis pages 2-3)

7.2 Engineering/biotechnology: tuning stromal acceptor interactions

Quantitative mutagenesis at PsaC residues (e.g., Lys35; K52/R53 region) demonstrates that electron-transfer rates and binding affinities to soluble acceptors are tunable by targeted changes. This principle is broadly relevant to synthetic biology strategies aiming to rewire electron flow to desired metabolic sinks (e.g., hydrogenases, reductive biosynthesis, or redox enzymes), even if direct industrial deployment remains developmental. (fischer1997targetedmutationsin pages 8-9, meimberg1999lys35ofpsac pages 3-4)

7.3 Structural biology and modeling platforms

High-resolution structural studies of PSI with bound partners (ferredoxin, plastocyanin) provide atomistic interaction maps—PsaC contributes multiple close-contact residues to ferredoxin binding—supporting rational design of PSI–acceptor interfaces. (caspy2020thestructureof pages 3-4)

8) Key statistics and data points (from the cited studies)

PSI-LHCI stoichiometry in Chlamydomonas: ~10 LHCI subunits per PSI-LHCI, including LHCA1 at 1.81 ± 0.07 copies and ~1 copy each of LHCA2–LHCA9; PSAD ~1.17 ± 0.19 and PSAF ~1.00 ± 0.19; apparent complex size ~700 kDa. (ozawa2018configurationoften pages 2-3, ozawa2018configurationoften pages 8-9)
psaC deletion phenotype (gas exchange/H2): WT CO2 uptake 1500 vs psaC deletion 210 nmol·min⁻¹·mg Chl⁻¹; WT H2 evolution 1200 vs psaC deletion ~3 nmol·min⁻¹·mg Chl⁻¹; phototrophic growth absent. (redding1999photosystemiis pages 2-3)
PsaC K52S/R53A mutant: PSI accumulation ~20–30%; Fd Kd 6.1 µM (WT) vs 7.3 µM; second-order constants 3.5 × 10^8 (WT) vs 5.6 × 10^8 M⁻¹ s⁻¹; ~25% complexes with damaged redox cofactors. (fischer1997targetedmutationsin pages 7-8, fischer1997targetedmutationsin pages 8-9)
PsaC Lys35 mutants (flavodoxin ET): oxidized flavodoxin t1/2 2.4 ms (WT) vs <9.9 ms (Lys35Glu); semiquinone t1/2 0.35 ms (WT) vs <9.9 ms (Lys35Glu); second-order constant 0.96 × 10^8 (WT) vs 0.05 × 10^8 M⁻¹ s⁻¹ (Lys35Glu). (meimberg1999lys35ofpsac pages 3-4, meimberg1999lys35ofpsac pages 4-6)
Redox proteomics under TOR inhibition: PsaC (Q00914) oxidation fold-changes reported as 2.46 (significant oxidation identifier) and 3.36 (discussion). (ford2019inhibitionoftor pages 16-18, ford2019inhibitionoftor pages 14-16)

Evidence map (tabular)

The following table consolidates the most directly relevant experimental findings and quantitative data for functional annotation of Chlamydomonas PsaC (Q00914).

Study (author year)	System/approach	Key PsaC-related finding	Quantitative data (rates, Kd, stoichiometry, fold changes)	URL/DOI
Ford et al. 2019	Chlamydomonas reinhardtii; redox proteomics after TOR inhibition	PsaC was explicitly identified as the PSI iron-sulfur center protein corresponding to UniProt Q00914; cysteine oxidation changes were linked to altered photosynthetic electron flow/cyclic electron transport hypotheses. (ford2019inhibitionoftor pages 16-18, ford2019inhibitionoftor pages 14-16)	PsaC oxidation fold change 3.36 in discussion; significant oxidation identifier fold change 2.46 in results. (ford2019inhibitionoftor pages 16-18, ford2019inhibitionoftor pages 14-16)	https://doi.org/10.3390/cells8101171
Fischer et al. 1997	Chloroplast site-directed mutagenesis of psaC in C. reinhardtii; low-temperature EPR, flash absorption, ferredoxin reduction assays	PsaC residues K52/R53 affect PSI stability and low-temperature FA/FB behavior; K52S/R53A caused preferential FB photoreduction at low temperature but did not measurably impair PSI→ferredoxin electron transfer at room temperature. (fischer1997targetedmutationsin pages 7-8, fischer1997targetedmutationsin pages 3-4, fischer1997targetedmutationsin pages 8-9, fischer1997targetedmutationsin pages 1-2)	K52S/R53A accumulated ~20–30% PSI; WT ferredoxin Kd 6.1 µM vs mutant 7.3 µM; second-order rate constants 3.5 × 10^8 M^-1 s^-1 (WT) vs 5.6 × 10^8 M^-1 s^-1 (mutant); WT P700 kinetics half-times 10 µs, 22 ms, 1.2 s; mutant 20 µs, 1.6 ms, 27 ms, 1.2 s; ~25% mutant complexes had damaged redox cofactors. (fischer1997targetedmutationsin pages 7-8, fischer1997targetedmutationsin pages 3-4, fischer1997targetedmutationsin pages 8-9, fischer1997targetedmutationsin pages 1-2)	https://doi.org/10.1021/bi962244v
Meimberg et al. 1999	Site-directed mutagenesis of PsaC Lys35; flavodoxin photoreduction kinetics, flash absorption, EDC cross-linking	Lys35 is an important electrostatic contact for docking/reduction of soluble acceptors; charge inversion at Lys35 strongly impairs PSI-mediated flavodoxin reduction, while Lys35Arg largely preserves WT behavior. (meimberg1999lys35ofpsac pages 7-7, meimberg1999lys35ofpsac pages 3-4, meimberg1999lys35ofpsac pages 1-2, meimberg1999lys35ofpsac pages 4-6, meimberg1999lys35ofpsac pages 6-7)	Oxidized flavodoxin t1/2: WT 2.4 ms; Lys35Arg <2.2 ms; Lys35Asp <7.5 ms; Lys35Glu <9.9 ms. Semiquinone t1/2: WT 0.35 ms; Lys35Arg ~0.32 ms; Lys35Asp <5.8 ms; Lys35Glu <9.9 ms. Second-order constants: WT 0.96 × 10^8 M^-1 s^-1; Lys35Arg 1.1 × 10^8; Lys35Asp 0.115 × 10^8; Lys35Glu 0.05 × 10^8; semiquinone reduction for Lys35Asp/Lys35Glu reported as 1 × 10^7 and 5 × 10^6 M^-1 s^-1, respectively; steady-state rates ~49.7–63.4 mmol flavodoxin·mg chl^-1·h^-1; ~0.6–0.9 flavodoxin molecules reduced per PSI center. (meimberg1999lys35ofpsac pages 3-4, meimberg1999lys35ofpsac pages 1-2, meimberg1999lys35ofpsac pages 4-6, meimberg1999lys35ofpsac pages 6-7)	https://doi.org/10.1046/j.1432-1327.1999.00474.x
Redding et al. 1999	Chloroplast deletion of psaC or psaA; immunoblotting, spectroscopy, mass spectrometry, growth and gas-exchange assays	PsaC is essential for functional PSI in C. reinhardtii; deleting psaC abolished photoautotrophic growth, CO2 fixation, detectable PSI, and essentially all H2 photoproduction. (redding1999photosystemiis pages 7-8, redding1999photosystemiis pages 1-2, redding1999photosystemiis pages 5-7, redding1999photosystemiis pages 2-3, redding1999photosystemiis pages 3-5)	In psaC-deletion strain: residual PsaA ~5–10% of WT, no detectable PsaD, no photooxidizable P700, CO2 uptake ~210 nmol·min^-1·mg Chl^-1, H2 evolution ~3 nmol·min^-1·mg Chl^-1, phototrophic growth absent; WT values listed as CO2 uptake 1500, O2 evolution 1530, H2 evolution 1200 nmol·min^-1·mg Chl^-1. Reverted psaA strain restored P700 to ~40% of WT. (redding1999photosystemiis pages 5-7, redding1999photosystemiis pages 2-3, redding1999photosystemiis pages 3-5)	https://doi.org/10.1074/jbc.274.15.10466
Ozawa et al. 2018	Quantitative [14C]-labeling, SDS-PAGE, cross-linking analysis of C. reinhardtii PSI-LHCI	PsaC was described as a peripheral PSI subunit on the stromal side that binds FA and FB and, with PsaD/PsaE, forms the stromal ridge for ferredoxin docking. (ozawa2018configurationoften pages 1-2)	PSI-LHCI contained 10 LHCI subunits total: LHCA1 at 1.81 ± 0.07 copies and ~1 copy each of LHCA2–LHCA9; apparent complex size ~700 kD; PSAD 1.17 ± 0.19 and PSAF 1.00 ± 0.19 copies. (ozawa2018configurationoften pages 1-2, ozawa2018configurationoften pages 2-3, ozawa2018configurationoften pages 8-9)	https://doi.org/10.1104/pp.18.00749
Caspy et al. 2020	Structural analysis of PSI with ferredoxin and plastocyanin	PsaC lies on the stromal side of PSI, houses the terminal Fe-S centers, and makes multiple close contacts with ferredoxin; plastocyanin/cytochrome c6 bind on the luminal face. (caspy2020thestructureof pages 3-4, caspy2020thestructureof pages 4-5)	PsaC residues Ile12, Gly13, Cys14, Thr15, Gln16, Ala36, Pro59 were within ≤6 Å of ferredoxin; specific contacts included Arg19(PsaC)-Glu92(Fd) 5.0 Å and Lys35(PsaC)-Asp34(Fd) 3.5 Å; Cu+ center of plastocyanin located 14.7 Å from P700. In C. reinhardtii, PsaB substitutions D612H/E613H reduced plastocyanin Kd by >5-fold. (caspy2020thestructureof pages 3-4, caspy2020thestructureof pages 4-5)	https://doi.org/10.1038/s41477-020-00779-9
Rolo et al. 2024	Expert review of PSI structure/function/assembly	PsaC is the stromal PSI subunit that contains FA and FB [Fe4-S4] clusters; PSAD/PSAE stabilize PsaC attachment and support ferredoxin docking. Loss of PsaC abolishes PSI accumulation and photoautotrophy. (rolo2024structurefunctionand pages 3-4, rolo2024structurefunctionand pages 4-5)	PSI core mass 422 kDa; PsaA-PsaB heterodimer 257 kDa; PSI-LHCI ~584 kDa; LHCI belt ~162 kDa. Electron flow proceeds plastocyanin → P700 → FX → FA/FB → ferredoxin. (rolo2024structurefunctionand pages 4-5, rolo2024structurefunctionand pages 3-4)	https://doi.org/10.1093/plcell/koae169

Table: This table summarizes core experimental and review evidence for Chlamydomonas reinhardtii PsaC/PSI-C (UniProt Q00914), focusing on function, localization, docking interactions, mutant phenotypes, and quantitative kinetic or stoichiometric data. It is useful as a compact evidence map for functional annotation.

Conclusion (functional annotation statement)

psaC (UniProt Q00914) encodes the PSI stromal iron–sulfur center subunit PsaC (PSI‑C) in Chlamydomonas reinhardtii. PsaC binds the terminal [4Fe–4S] clusters FA and FB, forms part of the stromal ferredoxin docking ridge with PsaD/PsaE, and is indispensable for productive PSI electron transfer to soluble acceptors (ferredoxin/flavodoxin), enabling CO2 fixation and hydrogen photoproduction in vivo. This function is supported by structural interaction mapping, quantitative acceptor-side kinetics from site-directed mutants, and strong loss-of-function phenotypes in psaC deletion strains. (caspy2020thestructureof pages 3-4, rolo2024structurefunctionand pages 4-5, fischer1997targetedmutationsin pages 8-9, meimberg1999lys35ofpsac pages 3-4, redding1999photosystemiis pages 2-3)

References

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(tian2024photosystemiaa pages 7-9): Li-Rong Tian and Jing-Hua Chen. Photosystem i: a paradigm for understanding biological environmental adaptation mechanisms in cyanobacteria and algae. International Journal of Molecular Sciences, 25:8767, Aug 2024. URL: https://doi.org/10.3390/ijms25168767, doi:10.3390/ijms25168767. This article has 7 citations.
(ozawa2018configurationoften pages 2-3): Shin-Ichiro Ozawa, Till Bald, Takahito Onishi, Huidan Xue, Takunori Matsumura, Ryota Kubo, Hiroko Takahashi, Michael Hippler, and Yuichiro Takahashi. Configuration of ten light-harvesting chlorophyll a/b complex i subunits in chlamydomonas reinhardtii photosystem i. Plant Physiology, 178:583-595, Aug 2018. URL: https://doi.org/10.1104/pp.18.00749, doi:10.1104/pp.18.00749. This article has 89 citations and is from a highest quality peer-reviewed journal.
(ozawa2018configurationoften pages 8-9): Shin-Ichiro Ozawa, Till Bald, Takahito Onishi, Huidan Xue, Takunori Matsumura, Ryota Kubo, Hiroko Takahashi, Michael Hippler, and Yuichiro Takahashi. Configuration of ten light-harvesting chlorophyll a/b complex i subunits in chlamydomonas reinhardtii photosystem i. Plant Physiology, 178:583-595, Aug 2018. URL: https://doi.org/10.1104/pp.18.00749, doi:10.1104/pp.18.00749. This article has 89 citations and is from a highest quality peer-reviewed journal.
(fischer1997targetedmutationsin pages 3-4): Nicolas Fischer, Pierre Sétif, and Jean-David Rochaix. Targeted mutations in the psac gene of chlamydomonas reinhardtii: preferential reduction of fb at low temperature is not accompanied by altered electron flow from photosystem i to ferredoxin. Biochemistry, 36 1:93-102, Jan 1997. URL: https://doi.org/10.1021/bi962244v, doi:10.1021/bi962244v. This article has 108 citations and is from a peer-reviewed journal.
(meimberg1999lys35ofpsac pages 7-7): Karen Meimberg, Nicolas Fischer, Jean‐David Rochaix, and Ulrich Mühlenhoff. Lys35 of psac is required for the efficient photoreduction of flavodoxin by photosystem i from chlamydomonas reinhardtii. European journal of biochemistry, 263 1:137-44, Jul 1999. URL: https://doi.org/10.1046/j.1432-1327.1999.00474.x, doi:10.1046/j.1432-1327.1999.00474.x. This article has 21 citations.
(meimberg1999lys35ofpsac pages 1-2): Karen Meimberg, Nicolas Fischer, Jean‐David Rochaix, and Ulrich Mühlenhoff. Lys35 of psac is required for the efficient photoreduction of flavodoxin by photosystem i from chlamydomonas reinhardtii. European journal of biochemistry, 263 1:137-44, Jul 1999. URL: https://doi.org/10.1046/j.1432-1327.1999.00474.x, doi:10.1046/j.1432-1327.1999.00474.x. This article has 21 citations.
(redding1999photosystemiis pages 1-2): Kevin Redding, Laurent Cournac, Ilya R. Vassiliev, John H. Golbeck, Gilles Peltier, and Jean-David Rochaix. Photosystem i is indispensable for photoautotrophic growth, co2 fixation, and h2 photoproduction inchlamydomonas reinhardtii *. The Journal of Biological Chemistry, 274:10466-10473, Apr 1999. URL: https://doi.org/10.1074/jbc.274.15.10466, doi:10.1074/jbc.274.15.10466. This article has 92 citations.
(redding1999photosystemiis pages 3-5): Kevin Redding, Laurent Cournac, Ilya R. Vassiliev, John H. Golbeck, Gilles Peltier, and Jean-David Rochaix. Photosystem i is indispensable for photoautotrophic growth, co2 fixation, and h2 photoproduction inchlamydomonas reinhardtii *. The Journal of Biological Chemistry, 274:10466-10473, Apr 1999. URL: https://doi.org/10.1074/jbc.274.15.10466, doi:10.1074/jbc.274.15.10466. This article has 92 citations.
(caspy2020thestructureof pages 4-5): Ido Caspy, Anna Borovikova-Sheinker, Daniel Klaiman, Yoel Shkolnisky, and Nathan Nelson. The structure of a triple complex of plant photosystem i with ferredoxin and plastocyanin. Nature Plants, 6:1300-1305, Oct 2020. URL: https://doi.org/10.1038/s41477-020-00779-9, doi:10.1038/s41477-020-00779-9. This article has 79 citations and is from a highest quality peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Citations

rolo2024structurefunctionand pages 3-4
rolo2024structurefunctionand pages 4-5
caspy2020thestructureof pages 3-4
tian2024photosystemiaa pages 2-4
fischer1997targetedmutationsin pages 1-2
rochaix2000chloroplastsitedirectedmutagenesis pages 8-9
rolo2024structurefunctionand pages 13-14
tian2024photosystemiaa pages 7-9
redding1999photosystemiis pages 2-3
ozawa2018configurationoften pages 1-2
ford2019inhibitionoftor pages 16-18
ford2019inhibitionoftor pages 14-16
redding1999photosystemiis pages 7-8
redding1999photosystemiis pages 5-7
rochaix2000chloroplastsitedirectedmutagenesis pages 2-3
fischer1997targetedmutationsin pages 7-8
fischer1997targetedmutationsin pages 8-9
nelson2024investigatingthebalance pages 3-5
nelson2024investigatingthebalance pages 1-3
ozawa2018configurationoften pages 2-3
ozawa2018configurationoften pages 8-9
fischer1997targetedmutationsin pages 3-4
redding1999photosystemiis pages 1-2
redding1999photosystemiis pages 3-5
caspy2020thestructureof pages 4-5
4Fe–4S
Fe4–S4
14C
Fe4-S4
https://doi.org/10.3390/cells8101171
https://doi.org/10.1021/bi962244v
https://doi.org/10.1046/j.1432-1327.1999.00474.x
https://doi.org/10.1074/jbc.274.15.10466
https://doi.org/10.1104/pp.18.00749
https://doi.org/10.1038/s41477-020-00779-9
https://doi.org/10.1093/plcell/koae169
https://doi.org/10.3390/cells8101171,
https://doi.org/10.1074/jbc.274.15.10466,
https://doi.org/10.1093/plcell/koae169,
https://doi.org/10.1016/s0300-9084(00
https://doi.org/10.1104/pp.18.00749,
https://doi.org/10.1038/s41477-020-00779-9,
https://doi.org/10.1021/bi962244v,
https://doi.org/10.1046/j.1432-1327.1999.00474.x,
https://doi.org/10.3390/ijms25168767,
https://doi.org/10.21203/rs.3.rs-4640615/v1,
https://doi.org/10.3390/ijms25105073,

📚 Additional Documentation

Notes

(psaC-notes.md)

psaC (Chlamydomonas reinhardtii) — research notes

UniProt: Q00914 (PSAC_CHLRE). Chloroplast (plastid)-encoded gene psaC.
NCBI taxon: 3055. Protein: 81 aa, ~9 kDa (mature ~8.86 kDa).

Summary of gene function

PsaC is the small stromal (extrinsic) subunit of Photosystem I (PSI). It is the
apoprotein that binds the two terminal [4Fe-4S] iron-sulfur clusters of PSI,
named FA and FB. Together with PsaA/PsaB (which carry P700, A0, A1 and the [4Fe-4S]
cluster FX) PsaC completes the linear electron transfer chain of PSI. FA receives
electrons from FX (in the PsaA/PsaB core); FB is the most distal cluster and is the
immediate electron donor to soluble ferredoxin on the stromal side. PsaC, together
with PsaD and PsaE, forms the docking/ferredoxin-reduction site on the stromal face
of PSI.

Structural facts

PsaC structurally resembles 2[4Fe-4S] bacterial dicluster ferredoxins, but has
two extra elements: an internal loop and a C-terminal extension that mediate
binding into the PSI complex PMID:9463363.
The 81-aa protein has two 4Fe-4S ferredoxin-type domains (residues ~2-31 and
~39-68). Eight cysteine residues coordinate the two clusters: cluster 1 (FB)
ligated by Cys11, Cys14, Cys17, Cys58; cluster 2 (FA) ligated by Cys21, Cys48,
Cys51, Cys54 (UniProt Q00914 BINDING features; UniProt COFACTOR note: "Binds 2
[4Fe-4S] clusters. Cluster 2 is most probably ... FA and cluster 1 is most
probably FB.").
Numerous Chlamydomonas PSI cryo-EM structures resolve PsaC as chain C (e.g. PDB
7R3K at 2.52 A, 7ZQC at 2.31 A, 6IJJ, 6JO5; UniProt cross-references).

PsaC is essential for PSI: insertional inactivation

The chloroplast psaC gene of C. reinhardtii was cloned; its deduced sequence is
highly conserved relative to higher plants and cyanobacteria PMID:1712288.
Directed chloroplast transformation disrupting psaC with an aadA cassette gave
transformants that "are unable to grow on minimal medium lacking acetate and are
deficient in PSI activity" PMID:1712288.
In psaC knockout cells, neither the PSI reaction center subunits nor the seven
small PSI subunits accumulate stably; the reaction center subunits are made but
turn over rapidly PMID:1712288.
Conclusion: "the iron sulfur binding protein encoded by the psaC gene is an
essential component, both for photochemical activity and for stable assembly of
PSI" PMID:1712288.

Electron transfer: FB is the terminal cluster donating to ferredoxin

PsaC binds "the two terminal electron acceptors FA and FB" of PSI PMID:9463363.
FB is the cluster that interacts with soluble ferredoxin. Site-directed mutants in
C. reinhardtii placing positive charges next to FB (I12V/T15K/Q16R, "FB1" mutant)
gave a 60-fold increase in ferredoxin affinity and faster, near-monophasic Fd
reduction; mutations next to FA (V49I/K52T/R53Q, "FA2") only modestly decreased Fd
affinity. "These data indicate that F(B) is the cluster interacting with Fd and
therefore the outermost iron-sulfur cluster of PSI." PMID:10438510.
Lysine 35 in the PsaC internal loop is a key electrostatic interaction site with
ferredoxin: K35T/K35D/K35E mutations "drastically affect electron transfer from
PSI to Fd", and abolish chemical cross-linking of Fd to PsaC; the conservative
K35R change has no effect PMID:9463363.
Cross-linking shows ferredoxin contacts not only PsaD and PsaE but also PsaC
PMID:9463363.
Mutations of K52/R53 near FA (K52S/R53A) can make FB preferentially photoreduced
at low temperature (as in green sulfur bacterium Chlorobium), but room-temperature
measurements show "electron transfer from PSI to ferredoxin proceeds at normal
rates in the mutant" — i.e. the FA-region change did not block the FX->FA->FB->Fd
flow PMID:8993322.

PSI overall reaction (UniProt / HAMAP MF_01303)

PSI is a plastocyanin/cytochrome c6-ferredoxin oxidoreductase (EC 1.97.1.12):
light-driven charge separation transfers an electron from P700 through A0, A1, FX,
FA and FB in turn, ending in reduction of soluble ferredoxin
(reduced plastocyanin + hnu + oxidized ferredoxin -> oxidized plastocyanin +
reduced ferredoxin; Rhea:RHEA:30407). PsaC carries the FA/FB part of this chain.
The EC number and the whole-PSI reaction describe the holocomplex; PsaC's own
contribution is electron carriage via its two clusters, not catalysis of the
overall photochemical reaction.

Localization

PsaC is a peripheral membrane protein on the stromal side of the chloroplast
thylakoid membrane (UniProt SUBCELLULAR LOCATION; HAMAP-Rule MF_01303). It is an
extrinsic subunit, not an integral membrane protein, but is part of the
membrane-embedded PSI complex.

GO term closure observations (OLS, checked 2026-05-21)

Relevant to the retired SPKW annotations:

GO:0051539 "4 iron, 4 sulfur cluster binding" ancestry:
binding -> small molecule binding -> metal cluster binding (GO:0051540) ->
iron-sulfur cluster binding (GO:0051536) -> 4Fe-4S cluster binding.
It does NOT pass through GO:0046872 "metal ion binding". Metal ion binding sits on
the ion binding / cation binding branch (binding -> small molecule binding ->
ion binding -> cation binding -> metal ion binding). So "4Fe-4S cluster binding"
and "metal ion binding" are NOT in a parent/child relationship in GO; they share
only the broad ancestor "small molecule binding". A [4Fe-4S] cluster is bound as a
cluster cofactor, not as free metal ions; "metal ion binding" is a related but
imprecise generalization for this protein. The precise, biologically meaningful
term is the still-current GO:0051539.
GO:0009055 "electron transfer activity" ancestry: it is a direct child of
molecular_function (GO:0003674). It is NOT under GO:0016491 "oxidoreductase
activity". In current GO, electron transfer activity (electron carrier) was
deliberately separated from oxidoreductase activity (catalysis of a redox
reaction). PsaC carries electrons through FA/FB; it is not itself a catalytic
oxidoreductase enzyme. So "oxidoreductase activity" is an imprecise/borderline
description, and "electron transfer activity" is the precise current term.
(The EC 1.97.1.12 oxidoreductase designation belongs to the whole PSI complex,
not to PsaC as a standalone catalyst.)

Bottom line for the SPKW retrospective

Both retired SPKW annotations (metal ion binding, oxidoreductase activity) were
broad/imprecise descriptions of well-characterized PsaC functions that are better
captured by still-current, more specific annotations: GO:0051539 (4Fe-4S cluster
binding) and GO:0009055 (electron transfer activity). No correct information is lost
by their removal; GOA's retirement of these SPKW terms is justified.

📄 View Raw YAML

id: Q00914
gene_symbol: psaC
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:3055
  label: Chlamydomonas reinhardtii
description: >-
  psaC is the chloroplast-encoded small (~9 kDa, 81 aa) stromal subunit PsaC of
  Photosystem I (PSI) in the green alga Chlamydomonas reinhardtii. PsaC is the
  apoprotein that binds the two terminal [4Fe-4S] iron-sulfur clusters of PSI,
  FA and FB, via two 4Fe-4S ferredoxin-type domains and eight cysteine ligands.
  Structurally it resembles a bacterial 2[4Fe-4S] dicluster ferredoxin but
  carries an extra internal loop and C-terminal extension that lock it onto the
  stromal face of the PSI core. PsaC completes the linear PSI electron transfer
  chain: cluster FA accepts electrons from FX (in the PsaA/PsaB core) and the
  most distal cluster FB is the immediate electron donor to soluble ferredoxin.
  Together with PsaD and PsaE, PsaC forms the ferredoxin docking site; Lys35 in
  the internal loop is an essential electrostatic contact for fast electron
  transfer to ferredoxin. PsaC is essential: chloroplast insertional
  inactivation of psaC abolishes photoautotrophic growth and destabilizes the
  entire PSI complex.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000043
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
  findings:
  - statement: SwissProt keyword-derived (SPKW) annotations present in the Sept 2025
      goa_uniprot_gcrp snapshot but removed from the current GOA release after GOA
      retired the keyword2GO pipeline for cellular organisms.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:1712288
  title: 'Directed chloroplast transformation in Chlamydomonas reinhardtii: insertional
    inactivation of the psaC gene encoding the iron sulfur protein destabilizes photosystem I.'
  findings:
  - statement: Insertional inactivation of the chloroplast psaC gene abolishes
      photoautotrophic growth and PSI activity; PsaC is essential both for PSI
      photochemical activity and for stable assembly of the PSI complex.
    reference_section_type: ABSTRACT
    supporting_text: 'Transformants ... are unable to grow on minimal medium lacking
      acetate and are deficient in PSI activity ... We conclude that the iron sulfur
      binding protein encoded by the psaC gene is an essential component, both for
      photochemical activity and for stable assembly of PSI.'
  - statement: The PsaC amino acid sequence is highly conserved relative to higher
      plants and cyanobacteria.
    reference_section_type: ABSTRACT
    supporting_text: The deduced amino acid sequence is highly related to that of
      higher plants and cyanobacteria.
- id: PMID:8993322
  title: 'Targeted mutations in the psaC gene of Chlamydomonas reinhardtii: preferential
    reduction of FB at low temperature is not accompanied by altered electron flow
    from photosystem I to ferredoxin.'
  findings:
  - statement: PsaC coordinates the two [4Fe-4S] centers FA and FB, which form the
      terminal part of the electron transfer pathway within the PSI complex.
    reference_section_type: ABSTRACT
    supporting_text: The terminal part of the electron pathway within the photosystem
      I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated
      by the PsaC subunit.
- id: PMID:9463363
  title: The PsaC subunit of photosystem I provides an essential lysine residue for
    fast electron transfer to ferredoxin.
  findings:
  - statement: PsaC is the stromal subunit of PSI that binds the two terminal electron
      acceptors FA and FB; it resembles 2[4Fe-4S] bacterial ferredoxins with an extra
      internal loop and C-terminal extension.
    reference_section_type: ABSTRACT
    supporting_text: 'PsaC is the stromal subunit of photosystem I (PSI) which binds
      the two terminal electron acceptors FA and FB. This subunit resembles 2[4Fe-4S]
      bacterial ferredoxins but contains two additional sequences: an internal loop
      and a C-terminal extension.'
  - statement: Lys35 of PsaC is a main electrostatic interaction site with ferredoxin
      and is required for fast electron transfer from PSI to ferredoxin.
    reference_section_type: ABSTRACT
    supporting_text: K35 is a main interaction site between PsaC and ferredoxin (Fd)
      and that it plays a key role in the electrostatic interaction between Fd and
      PSI ... the mutations K35T, K35D and K35E drastically affect electron transfer
      from PSI to Fd ... whereas the K35R change has no effect on Fd reduction.
- id: PMID:10438510
  title: Site-directed mutagenesis of the PsaC subunit of photosystem I. F(b) is the
    cluster interacting with soluble ferredoxin.
  findings:
  - statement: The two [4Fe-4S] clusters FA and FB are the terminal electron acceptors
      of PSI and are bound by the stromal subunit PsaC; FB is the outermost cluster
      that donates electrons to soluble ferredoxin.
    reference_section_type: ABSTRACT
    supporting_text: The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron
      acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC.
      Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is reduced
      by the outermost iron-sulfur cluster of PsaC ... These data indicate that F(B)
      is the cluster interacting with Fd and therefore the outermost iron-sulfur cluster
      of PSI.
- id: PMID:30850819
  title: Antenna arrangement and energy transfer pathways of a green algal photosystem-I-LHCI
    supercomplex.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Cryo-EM structures of the C. reinhardtii PSI-LHCI supercomplex (PDB
      6IJJ/6JO5) resolve PsaC as a stromal subunit of the Photosystem I core, supporting
      its membership in PSI and its role carrying the terminal [4Fe-4S] clusters FA/FB.
- id: PMID:31182847
  title: Structure of the green algal photosystem I supercomplex with a decameric light-harvesting
    complex I.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Independent cryo-EM structure of the C. reinhardtii PSI-LHCI supercomplex
      (PDB 6JO5/6JO6) confirming the Photosystem I architecture in which PsaC sits on
      the stromal face of the PSI core mediating terminal electron transfer.
- id: file:CHLRE/psaC/psaC-uniprot.txt
  title: UniProt entry Q00914 (PSAC_CHLRE), Photosystem I iron-sulfur center
  findings:
  - statement: PsaC is the apoprotein for the two 4Fe-4S centers FA and FB of PSI;
      FB is the terminal electron acceptor donating electrons to ferredoxin; PsaC is
      required for binding of PsaD and PsaE to PSI.
    reference_section_type: DATABASE_ENTRY
    supporting_text: 'Apoprotein for the two 4Fe-4S centers FA and FB of ... essential
      for photochemical activity. FB is the ... terminal electron acceptor of PSI,
      donating electrons to ferredoxin ... Required for binding of PsaD and PsaE to
      PSI.'
  - statement: PsaC is localized to the chloroplast thylakoid membrane as a peripheral
      membrane protein on the stromal side.
    reference_section_type: DATABASE_ENTRY
    supporting_text: 'Plastid, chloroplast thylakoid membrane ... Peripheral membrane
      protein ... Stromal side'
  - statement: PsaC binds two [4Fe-4S] clusters via eight cysteine ligands organized
      in two 4Fe-4S ferredoxin-type domains.
    reference_section_type: DATABASE_ENTRY
    supporting_text: 'Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the ...
      spectroscopically characterized electron acceptor FA and cluster 1 is ... most
      probably FB.'
- id: file:CHLRE/psaC/psaC-deep-research-falcon.md
  title: 'Deep research report (falcon/Edison): Chlamydomonas reinhardtii psaC (Q00914),
    Photosystem I iron-sulfur center subunit PSI-C'
  findings:
  - statement: PsaC is the stromal, membrane-extrinsic PSI subunit that binds the
      terminal [4Fe-4S] clusters FA and FB, relaying light-generated electrons from
      cluster FX of the PSI core to soluble ferredoxin (and, under iron deprivation,
      flavodoxin). This is the precise electron-carrier molecular function, distinct
      from the catalytic oxidoreductase activity of the intact PSI holocomplex.
    supporting_text: PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters
      and sits adjacent to the FX site of the PsaA-PsaB core, accepting electrons from
      upstream cofactors and passing them onward to ferredoxin ... PSI electron flow
      is described as plastocyanin -> P700 -> internal cofactors -> PsaC FA/FB ->
      ferredoxin.
  - statement: The EC 1.97.1.12 oxidoreductase designation applies to the intact PSI
      holocomplex acting as a plastocyanin/cytochrome c6-ferredoxin oxidoreductase,
      not to PsaC as a standalone catalyst; PsaC functions as the protein scaffold
      that binds FA/FB and enables terminal electron transfer to soluble acceptors.
    supporting_text: The UniProt record associates PsaC with EC 1.97.1.12, reflecting
      PSI's overall oxidoreductase activity ... with PsaC specifically functioning
      as the protein scaffold that binds FA/FB and enables terminal electron transfer
      to soluble acceptors.
  - statement: PsaC is indispensable for PSI in Chlamydomonas - chloroplast deletion
      of psaC abolishes detectable functional PSI and photoautotrophic growth and
      strongly impairs CO2 fixation and H2 photoproduction (CO2 uptake ~1500 -> ~210
      and H2 evolution ~1200 -> ~3 nmol/min/mg Chl).
    supporting_text: Chloroplast deletion of psaC causes loss of photoautotrophic
      growth and PSI detectability by immunological and spectroscopic assays, with
      CO2 uptake reduced from 1500 to 210 ... and H2 evolution reduced from 1200 to
      3
  - statement: Site-directed mutagenesis of PsaC residues (Lys35; the K52/R53 region)
      in Chlamydomonas shows PsaC actively shapes ferredoxin/flavodoxin docking and
      electron-transfer kinetics; Lys35 is a key electrostatic contact whose charge
      inversion severely slows electron transfer to soluble acceptors.
    supporting_text: These data support a mechanistic model where PsaC is not merely
      a passive Fe-S carrier but actively shapes encounter complex formation,
      orientation, and ET efficiency for stromal acceptors.
  - statement: Cryo-EM of a PsaC-less PSI assembly intermediate indicates PsaC binding
      promotes conformational states needed for proper FX maturation and subsequent
      incorporation of the stromal subunits PsaD and PsaE, consistent with PsaC being
      required for stable PSI assembly.
    supporting_text: removing PsaC eliminates FA/FB and is associated with incomplete
      FX formation and missing stromal subunits PsaD and PsaE, supporting a model in
      which PsaC binding promotes conformational states needed for proper FX maturation
      and subsequent stromal-subunit incorporation.
existing_annotations:
- term:
    id: GO:0046872
    label: metal ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  retired: true
  review:
    summary: >-
      Retired SwissProt-keyword (SPKW) annotation derived from the
      "Metal-binding" / "Iron" UniProt keywords. PsaC genuinely binds metal
      (iron) atoms within its two [4Fe-4S] clusters, so the term is not factually
      wrong, but it is a broad, imprecise generalization. The precise function is
      already captured by the still-current GO:0051539 "4 iron, 4 sulfur cluster
      binding".
    action: REMOVE
    reason: >-
      GOA's removal of this SPKW annotation is JUSTIFIED. PsaC does bind iron,
      but only as part of two [4Fe-4S] cluster cofactors, not as free metal ions.
      In the GO hierarchy GO:0051539 "4 iron, 4 sulfur cluster binding" is NOT a
      child of GO:0046872 "metal ion binding": GO:0051539 sits on the metal
      cluster binding branch (small molecule binding to metal cluster binding to
      iron-sulfur cluster binding to 4Fe-4S cluster binding), whereas metal ion
      binding sits on the ion binding / cation binding branch. They are
      non-redundant sibling branches that share only the broad ancestor "small
      molecule binding". "Metal ion binding" is therefore an imprecise descriptor
      for a cluster-binding protein; it conveys no information beyond what the
      precise, retained GO:0051539 annotation already states. Removing it loses
      no correct biological information and improves annotation precision. This
      gene exemplifies an SPKW annotation that is broadly true but redundant
      against a better, current term.
    proposed_replacement_terms:
    - id: GO:0051539
      label: 4 iron, 4 sulfur cluster binding
    supported_by:
    - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
      supporting_text: 'Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the
        ... spectroscopically characterized electron acceptor FA and cluster 1 is
        ... most probably FB.'
    - reference_id: PMID:8993322
      supporting_text: The terminal part of the electron pathway within the photosystem
        I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated
        by the PsaC subunit.
    - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
      supporting_text: binds the terminal/secondary [4Fe-4S] iron-sulfur clusters
        FA and FB that relay light-generated electrons from PSI to soluble acceptors
        ... PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters and sits
        adjacent to the FX site of the PsaA-PsaB core
- term:
    id: GO:0016491
    label: oxidoreductase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  retired: true
  review:
    summary: >-
      Retired SwissProt-keyword (SPKW) annotation derived from the
      "Oxidoreductase" UniProt keyword (UniProt assigns EC 1.97.1.12 to the
      whole PSI complex via HAMAP rule MF_01303). PsaC is an electron-carrier
      subunit, not a standalone catalytic oxidoreductase. The precise function
      of PsaC is captured by the still-current GO:0009055 "electron transfer
      activity".
    action: REMOVE
    reason: >-
      GOA's removal of this SPKW annotation is JUSTIFIED. The EC 1.97.1.12
      oxidoreductase designation applies to the intact PSI holocomplex
      (plastocyanin/cytochrome c6-ferredoxin oxidoreductase), not to PsaC as an
      isolated catalyst. PsaC itself simply carries electrons through its FA and
      FB [4Fe-4S] clusters from FX to soluble ferredoxin; it does not catalyze a
      redox reaction in the enzymatic sense. In current GO, GO:0009055 "electron
      transfer activity" is a direct child of molecular_function and is
      deliberately NOT placed under GO:0016491 "oxidoreductase activity";
      electron carriers were separated from catalytic oxidoreductases. So
      "oxidoreductase activity" is an imprecise, borderline-incorrect descriptor
      for this subunit, and the precise current term GO:0009055 is retained. No
      correct information is lost by removal. This gene exemplifies an SPKW
      annotation that is broad/imprecise and subsumed in intent by a better,
      current term.
    proposed_replacement_terms:
    - id: GO:0009055
      label: electron transfer activity
    supported_by:
    - reference_id: PMID:10438510
      supporting_text: The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron
        acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC.
        Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions and is
        reduced by the outermost iron-sulfur cluster of PsaC.
    - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
      supporting_text: 'Apoprotein for the two 4Fe-4S centers FA and FB of ... essential
        for photochemical activity. FB is the ... terminal electron acceptor of PSI,
        donating electrons to ferredoxin.'
    - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
      supporting_text: The UniProt record associates PsaC with EC 1.97.1.12, reflecting
        PSI's overall oxidoreductase activity of transferring electrons from reduced
        plastocyanin/cytochrome c6 to ferredoxin through the PSI cofactor chain, with
        PsaC specifically functioning as the protein scaffold that binds FA/FB and
        enables terminal electron transfer to soluble acceptors.
- term:
    id: GO:0009055
    label: electron transfer activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      Electron transfer activity is the core, precise molecular function of
      PsaC. The protein carries electrons through its two [4Fe-4S] clusters FA
      and FB, relaying them from cluster FX of the PSI core to soluble
      ferredoxin. This is strongly supported by experimental work in
      Chlamydomonas.
    action: ACCEPT
    reason: >-
      This is the correct, appropriately specific molecular function term for
      PsaC. PsaC binds the two terminal [4Fe-4S] electron acceptors of PSI and
      relays electrons from FX through FA to FB to ferredoxin. Site-directed
      mutagenesis in C. reinhardtii directly demonstrated electron transfer
      through PsaC and identified FB as the cluster donating to ferredoxin. The
      IEA evidence (UniRule, from the conserved PSI iron-sulfur center family) is
      fully consistent with experimental data.
    supported_by:
    - reference_id: PMID:10438510
      supporting_text: The two [4Fe-4S] clusters F(A) and F(B) are the terminal electron
        acceptors of photosystem I (PSI) that are bound by the stromal subunit PsaC
        ... These data indicate that F(B) is the cluster interacting with Fd and therefore
        the outermost iron-sulfur cluster of PSI.
    - reference_id: PMID:9463363
      supporting_text: the mutations K35T, K35D and K35E drastically affect electron
        transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas
        the K35R change has no effect on Fd reduction.
    - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
      supporting_text: PsaC is not merely a passive Fe-S carrier but actively shapes
        encounter complex formation, orientation, and ET efficiency for stromal
        acceptors ... PSI electron flow is described as plastocyanin -> P700 ->
        internal cofactors -> PsaC FA/FB -> ferredoxin.
- term:
    id: GO:0009522
    label: photosystem I
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      PsaC is a bona fide subunit of the Photosystem I complex. It is the small
      stromal subunit that binds the FA/FB clusters and is required for stable
      assembly of the complex. Cryo-EM structures of the Chlamydomonas PSI-LHCI
      supercomplex (e.g. PDB 6IJJ/6JO5; Su et al. 2019) resolve PsaC carrying the
      terminal [4Fe-4S] clusters FA and FB on the stromal face of the PSI core.
    action: ACCEPT
    reason: >-
      Correct cellular component (complex) annotation. PsaC is an extrinsic
      stromal subunit of PSI; it is resolved as chain C in numerous Chlamydomonas
      PSI cryo-EM structures, and its inactivation destabilizes the whole PSI
      complex. The InterPro2GO mapping (IPR017491, PSI PsaC) is accurate.
    supported_by:
    - reference_id: PMID:1712288
      supporting_text: neither PSI reaction center subunits nor the seven small subunits
        belonging to PSI accumulate stably in the thylakoid membranes of the transformants.
    - reference_id: PMID:9463363
      supporting_text: PsaC is the stromal subunit of photosystem I (PSI) which binds
        the two terminal electron acceptors FA and FB.
    - reference_id: PMID:30850819
      supporting_text: multiple units of light-harvesting complex I (LHCI) bind to
        the PSI core and function as peripheral antennae, forming a PSI-LHCI supercomplex
        ... Here, we report structures of CrPSI-LHCI, solved by cryo-electron microscopy
- term:
    id: GO:0009535
    label: chloroplast thylakoid membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      PsaC is localized to the chloroplast thylakoid membrane, where the PSI
      complex resides. It is a peripheral membrane protein on the stromal side.
    action: ACCEPT
    reason: >-
      Correct and appropriately specific subcellular localization. PsaC is part
      of the PSI complex embedded in the chloroplast thylakoid membrane; UniProt
      records it as a peripheral membrane protein on the stromal side. This is
      more specific than the generic "thylakoid membrane" annotation and is
      preferred.
    supported_by:
    - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
      supporting_text: 'Plastid, chloroplast thylakoid membrane ... Peripheral membrane
        protein ... Stromal side'
- term:
    id: GO:0009773
    label: photosynthetic electron transport in photosystem I
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      PsaC mediates the terminal step of photosynthetic electron transport
      within PSI, carrying electrons from FX through FA and FB to ferredoxin.
      This is the precise biological process for the gene.
    action: ACCEPT
    reason: >-
      This is the correct and appropriately specific biological process term.
      PsaC carries the FA/FB part of the PSI electron transfer chain;
      experimental mutagenesis in Chlamydomonas directly demonstrated its role in
      photosynthetic electron transport to ferredoxin. The InterPro2GO mapping is
      accurate and more informative than the parent term "photosynthesis".
    supported_by:
    - reference_id: PMID:8993322
      supporting_text: The terminal part of the electron pathway within the photosystem
        I (PSI) complex includes two [4Fe-4S] centers, FA and FB, which are coordinated
        by the PsaC subunit.
    - reference_id: PMID:10438510
      supporting_text: Soluble ferredoxin (Fd) binds to PSI via electrostatic interactions
        and is reduced by the outermost iron-sulfur cluster of PsaC.
- term:
    id: GO:0015979
    label: photosynthesis
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      Photosynthesis is correct but is a broad parent of the more specific and
      better term "photosynthetic electron transport in photosystem I"
      (GO:0009773), which is also annotated to this gene.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      PsaC genuinely functions in photosynthesis, so the term is not wrong, but
      it is unnecessarily broad. The more specific child term GO:0009773
      "photosynthetic electron transport in photosystem I" is already annotated
      and precisely captures PsaC's role. Retaining the generic "photosynthesis"
      term adds no information beyond the specific annotation; it is best treated
      as an over-annotation rather than a core function term.
    supported_by:
    - reference_id: PMID:1712288
      supporting_text: Transformants ... are unable to grow on minimal medium lacking
        acetate and are deficient in PSI activity
- term:
    id: GO:0042651
    label: thylakoid membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      Thylakoid membrane is correct but is the generic parent of the more
      specific "chloroplast thylakoid membrane" (GO:0009535), which is also
      annotated to this gene.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      PsaC is in the thylakoid membrane, so the term is accurate, but it is less
      specific than GO:0009535 "chloroplast thylakoid membrane", which is already
      annotated and is appropriate for this chloroplast-encoded alga protein. The
      generic term adds no information beyond the specific one; mark as
      over-annotated in favor of the chloroplast-specific term.
    supported_by:
    - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
      supporting_text: 'Plastid, chloroplast thylakoid membrane ... Peripheral membrane
        protein ... Stromal side'
- term:
    id: GO:0051539
    label: 4 iron, 4 sulfur cluster binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      PsaC binds two [4Fe-4S] clusters (FA and FB) via eight cysteine ligands
      arranged in two 4Fe-4S ferredoxin-type domains. This is a core, precise
      molecular function of the protein.
    action: ACCEPT
    reason: >-
      This is the correct and appropriately specific molecular function term for
      cofactor binding by PsaC. UniProt documents binding of two [4Fe-4S]
      clusters with eight cysteine BINDING-site residues, and experimental work
      in Chlamydomonas characterized clusters FA and FB coordinated by PsaC. This
      precise term supersedes the retired, broader SPKW term "metal ion binding".
    supported_by:
    - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
      supporting_text: 'Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the
        ... spectroscopically characterized electron acceptor FA and cluster 1 is
        ... most probably FB.'
    - reference_id: PMID:9463363
      supporting_text: 'This subunit resembles 2[4Fe-4S] bacterial ferredoxins but
        contains two additional sequences: an internal loop and a C-terminal extension.'
core_functions:
- description: >-
    Binds the two terminal [4Fe-4S] iron-sulfur clusters FA and FB of Photosystem
    I as the apoprotein PsaC, providing the redox cofactors for the terminal
    segment of PSI electron transfer.
  supported_by:
  - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
    supporting_text: 'Apoprotein for the two 4Fe-4S centers FA and FB of ... essential
      for photochemical activity.'
  - reference_id: PMID:9463363
    supporting_text: PsaC is the stromal subunit of photosystem I (PSI) which binds
      the two terminal electron acceptors FA and FB.
  - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
    supporting_text: PsaC contains the terminal/secondary FA and FB [4Fe-4S] clusters
      and sits adjacent to the FX site of the PsaA-PsaB core.
  molecular_function:
    id: GO:0051539
    label: 4 iron, 4 sulfur cluster binding
  directly_involved_in:
  - id: GO:0009773
    label: photosynthetic electron transport in photosystem I
  locations:
  - id: GO:0009535
    label: chloroplast thylakoid membrane
- description: >-
    Acts as an electron carrier within Photosystem I, relaying electrons from
    cluster FX of the PSI core through FA and then to FB, the most distal cluster,
    which donates electrons to soluble ferredoxin on the stromal side.
  supported_by:
  - reference_id: PMID:10438510
    supporting_text: These data indicate that F(B) is the cluster interacting with
      Fd and therefore the outermost iron-sulfur cluster of PSI.
  - reference_id: PMID:9463363
    supporting_text: the mutations K35T, K35D and K35E drastically affect electron
      transfer from PSI to Fd, as measured by flash-absorption spectroscopy.
  - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
    supporting_text: PSI electron flow is described as plastocyanin -> P700 ->
      internal cofactors -> PsaC FA/FB -> ferredoxin ... under iron deprivation,
      flavodoxin can substitute for ferredoxin and accept electrons from a
      PsaC-coordinated [4Fe-4S] cluster.
  molecular_function:
    id: GO:0009055
    label: electron transfer activity
  directly_involved_in:
  - id: GO:0009773
    label: photosynthetic electron transport in photosystem I
  locations:
  - id: GO:0009535
    label: chloroplast thylakoid membrane
- description: >-
    Required for stable assembly of the Photosystem I complex and, together with
    PsaD and PsaE, forms the stromal ferredoxin-docking site; loss of PsaC
    destabilizes the whole PSI complex and abolishes photoautotrophic growth.
  supported_by:
  - reference_id: PMID:1712288
    supporting_text: the iron sulfur binding protein encoded by the psaC gene is an
      essential component, both for photochemical activity and for stable assembly
      of PSI.
  - reference_id: file:CHLRE/psaC/psaC-uniprot.txt
    supporting_text: The C-terminus interacts with PsaA/B/D and helps assemble the
      protein ... into the PSI complex. Required for binding of PsaD and PsaE to PSI.
  - reference_id: file:CHLRE/psaC/psaC-deep-research-falcon.md
    supporting_text: removing PsaC eliminates FA/FB and is associated with incomplete
      FX formation and missing stromal subunits PsaD and PsaE, supporting a model in
      which PsaC binding promotes conformational states needed for proper FX
      maturation and subsequent stromal-subunit incorporation.
  directly_involved_in:
  - id: GO:0009773
    label: photosynthetic electron transport in photosystem I
  locations:
  - id: GO:0009535
    label: chloroplast thylakoid membrane
proposed_new_terms: []
suggested_questions:
- question: >-
    What chloroplast assembly factors are required to insert the FA and FB
    [4Fe-4S] clusters into PsaC and to dock holo-PsaC onto the PSI core in
    Chlamydomonas?
- question: >-
    How do the internal loop and C-terminal extension of PsaC (absent in
    bacterial 2[4Fe-4S] ferredoxins) fine-tune the redox potentials of FA and FB
    and the directionality of electron flow?
suggested_experiments:
- description: >-
    Use ultrafast flash-absorption and EPR spectroscopy on Chlamydomonas PSI to
    quantify electron transfer rates along the FX, FA, FB, ferredoxin chain and
    confirm the FA-proximal / FB-distal orientation of PsaC under physiological
    conditions.
- description: >-
    Generate cysteine-ligand substitution mutants of PsaC in C. reinhardtii and
    assay cluster occupancy, redox potential, and PSI assembly to dissect the
    individual contributions of FA versus FB to electron delivery to ferredoxin.
- description: >-
    Solve high-resolution cryo-EM structures of Chlamydomonas PSI-ferredoxin
    complexes to map the PsaC/PsaD/PsaE ferredoxin-docking interface and the role
    of PsaC Lys35.

psaC

Gene Description

Existing Annotations Review

Core Functions

Binds the two terminal [4Fe-4S] iron-sulfur clusters FA and FB of Photosystem I as the apoprotein PsaC, providing the redox cofactors for the terminal segment of PSI electron transfer.

Acts as an electron carrier within Photosystem I, relaying electrons from cluster FX of the PSI core through FA and then to FB, the most distal cluster, which donates electrons to soluble ferredoxin on the stromal side.

Required for stable assembly of the Photosystem I complex and, together with PsaD and PsaE, forms the stromal ferredoxin-docking site; loss of PsaC destabilizes the whole PSI complex and abolishes photoautotrophic growth.

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Research Report: Chlamydomonas reinhardtii psaC (UniProt Q00914) — Photosystem I iron–sulfur center subunit (PSI-C)

Executive summary

1) Key concepts and definitions (current understanding)

1.1 Photosystem I as a plastocyanin–ferredoxin oxidoreductase

1.2 What is PsaC?

1.3 Biochemical reaction context / EC number interpretation

2) Molecular function: electron-transfer role of PsaC and its partners

2.1 Cofactors bound by PsaC

2.2 Electron donor(s) to PSI vs acceptor(s) from PSI

2.3 Quantitative kinetics: PsaC residues tune ferredoxin/flavodoxin electron transfer

2.4 Stress/redox regulation signals involving PsaC

3) Cellular and subcellular localization

3.1 Subcellular location

3.2 Topological context (two-sided electron-transfer)

4) Pathways and biological processes involving psaC/PsaC

4.1 Linear electron flow (LEF) and NADPH generation

4.2 Alternative acceptors and iron-stress substitution

4.3 Cyclic electron flow (CEF) and photoprotection/ATP balancing

5) Mutant and phenotype evidence in Chlamydomonas reinhardtii

5.1 psaC is essential for PSI function and photoautotrophy

5.2 psaC mutations impact PSI stability, light sensitivity, and acceptor-side ET

6) Recent developments (prioritizing 2023–2024) and latest research directions

6.1 2024 structural and assembly perspectives: PsaC’s role in biogenesis

6.2 2024 expert analysis: conservation vs flexibility

6.3 2024 algal PSI supercomplex diversity (context for psaC functional environment)

7) Current applications and real-world implementations

7.1 Photosynthetic hydrogen production and bioenergy engineering

7.2 Engineering/biotechnology: tuning stromal acceptor interactions

7.3 Structural biology and modeling platforms

8) Key statistics and data points (from the cited studies)

Evidence map (tabular)

Conclusion (functional annotation statement)

Artifacts

Citations

📚 Additional Documentation

Notes

psaC (Chlamydomonas reinhardtii) — research notes

Summary of gene function

Structural facts

PsaC is essential for PSI: insertional inactivation

Electron transfer: FB is the terminal cluster donating to ferredoxin

PSI overall reaction (UniProt / HAMAP MF_01303)

Localization

GO term closure observations (OLS, checked 2026-05-21)

Bottom line for the SPKW retrospective

📄 View Raw YAML