Gene Ontology annotation through association of InterPro records with GO terms

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.

Combined Automated Annotation using Multiple IEA Methods

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."