crtI (P21685, Pantoea ananatis / Erwinia uredovora) — curation notes

Research journal for the de-novo review. Every assertion carries inline provenance.

1. Identity

UniProt P21685 is CRTI_PANAN, "Phytoene desaturase (lycopene-forming)", EC 1.3.99.31,
gene crtI, organism Pantoea ananas (Erwinia uredovora), NCBI taxon 553, 492 aa
[file:PANAN/crtI/crtI-uniprot.txt "RecName: Full=Phytoene desaturase (lycopene-forming);"].

The identity chain across the three primary papers is unambiguous:

• The gene was cloned from Erwinia uredovora 20D3 (ATCC 19321)
  PMID:2254247.
  UniProt's reference [1] is this paper and cites the same strain
  [file:PANAN/crtI/crtI-uniprot.txt "STRAIN=ATCC 19321 / DSM 30080 / NCPPB 800 / NRRL B-14773 / 20D3;"].
• The 1992 biochemistry paper expressed "the complete crtI gene encoding phytoene desaturase
  from Erwinia uredovora" PMID:1400305.
• The 2012 structure paper states it purified this same protein under the modern genus name
  PMID:22745782.
  Its structure is PDB 4DGK, which is the structure cross-referenced from P21685
  [file:PANAN/crtI/crtI-uniprot.txt "DR PDB; 4DGK; X-ray; 2.35 A; A=1-492."].

So all three papers describe P21685 itself, not a paralog or a same-symbol gene from another
organism. No name-confusion caveat is needed anywhere in this review.

2. Cached publication status

PMID:22745782 and PMID:39322757 were not in the cache and were fetched with
uv run ai-gene-review fetch-pmid. The two GOA-cited papers were already cached.

Note on PMID:2254247: although the cached file has a "## Full Text" heading, the content under
it is the abstract text again (full_text_extraction_method: html_abstract_only), so all quotes
drawn from it are abstract quotes.

3. What the enzyme does

Overall reaction. One polypeptide performs the whole phytoene-to-lycopene conversion. This
was the central conclusion of the original pathway reconstruction
PMID:2254247,
and it is what UniProt records as the catalytic activity
[file:PANAN/crtI/crtI-uniprot.txt "Reaction=15-cis-phytoene + 4 A = all-trans-lycopene + 4 AH2;"],
mapped to Rhea:RHEA:15585 and EC 1.3.99.31
[file:PANAN/crtI/crtI-uniprot.txt "Xref=Rhea:RHEA:15585, ChEBI:CHEBI:13193, ChEBI:CHEBI:15948,"].

Purified-enzyme demonstration (the IDA source). Fraser et al. purified the recombinant
enzyme to homogeneity, restored activity after urea removal, and showed the product directly
PMID:1400305.
The same abstract is the source of the FAD cofactor claim and of the NAD/NADP inhibition that
UniProt records under ACTIVITY REGULATION
PMID:1400305.

Confirmation in a defined membrane system. Schaub et al. reconstituted the reaction with
phytoene embedded in liposomes and saw clean conversion with no intermediate build-up
PMID:22745782.
The transformation comprises four desaturations plus one isomerization
PMID:22745782.

In vivo, heterologous. The 2024 study expressed P. ananatis CrtI (PaCrtI) in engineered
yeast and quantified its substrate and product intracellularly
PMID:39322757,
corroborating that phytoene in, lycopene out is the activity of this specific protein in a cell
and not only in a purified assay.

4. Cofactor and electron acceptor

FAD is the only redox cofactor
PMID:22745782,
consistent with UniProt's COFACTOR line
[file:PANAN/crtI/crtI-uniprot.txt "Name=FAD; Xref=ChEBI:CHEBI:57692;"].
FAD is not only catalytic but structural: it is required at the moment the enzyme associates
with the membrane, and cannot be supplied afterwards
PMID:22745782;
holoenzyme formation and membrane binding are one event
PMID:22745782.
This is a strong, direct demonstration of FAD binding beyond the 1992 abstract's
"FAD was involved" phrasing, so GO:0071949 FAD binding is solidly grounded.

Terminal acceptor: oxygen aerobically
PMID:22745782,
with quinones able to substitute anaerobically
PMID:22745782.
Which acceptor operates in native P. ananatis is not established by any retrieved study —
recorded below as an open question rather than asserted.

5. Localization

UniProt: "SUBCELLULAR LOCATION: Cell membrane", evidenced from the 1992 paper
[file:PANAN/crtI/crtI-uniprot.txt "SUBCELLULAR LOCATION: Cell membrane {ECO:0000269|PubMed:1400305}."].
That 1992 paper described the enzyme as membrane-integrated
PMID:1400305.

The 2012 structure revises the topology but not the compartment: the protein is peripheral,
not integral
PMID:22745782,
possibly monotopic
PMID:22745782.
A peripheral or monotopic protein that does its chemistry on a bilayer-embedded substrate is
still correctly located_in the membrane it works on, so GO:0005886 plasma membrane stands.
I considered GO:0009898 cytoplasmic side of plasma membrane as a more precise replacement and
rejected it: no retrieved source establishes which leaflet, and the liposome work says nothing
about the native bacterial membrane's sidedness. Left as a question instead.

6. Reasoning on each contentious annotation

Principle applied across the nine rows. The electronic rows (InterPro2GO and ARBA) are
family- or domain-level rules; a correct but broad term is the appropriate output of such a
rule, and CLAUDE.md/the annotation-reviewer skill explicitly permit accepting IEAs that are
broader than the literature supports. The two IDA rows are different: they rest on an
experiment that pins the exact substrate and exact product, so the term should be as specific
as that experiment. I therefore ACCEPT all electronic rows and MODIFY only the two IDA rows
whose evidence pins the chemistry (the FAD-binding IDA is already precise, so it is ACCEPTed).

• GO:0016491 oxidoreductase activity IEA / InterPro:IPR002937 — IPR002937 is the generic
  amine-oxidase (Pfam PF01593) domain shared with monoamine oxidase and protoporphyrinogen IX
  oxidoreductase, a kinship the structure confirms
  PMID:22745782.
  The root oxidoreductase term is all that domain can legitimately assert. ACCEPT, not MODIFY:
  the mapping is doing the right thing for the evidence it has.
• GO:0016627 ... CH-CH group of donors IDA/PMID:1400305 — true but one level above the
  demonstrated activity. GO:0016166 phytoene dehydrogenase activity is a verified
  (non-obsolete) child of GO:0016627 — checked via QuickGO ancestors, which returns
  ['GO:0016627', 'GO:0003824', 'GO:0016491', 'GO:0016166', 'GO:0003674'] — and matches the
  assayed reaction exactly. MODIFY.
• GO:0016120 carotene biosynthetic process IDA/PMID:1400305 — the experiment made lycopene
  specifically, and GO:1901177 lycopene biosynthetic process is a verified child of
  GO:0016120 (QuickGO ancestors for GO:1901177 include GO:0016120). MODIFY.
  I deliberately did not touch the GO:0016117 / GO:0016120 branch relationship: in the
  current ontology GO:0016120 is not a descendant of GO:0016117, and that is not this review's
  business to assert otherwise or "fix".
• GO:0016117 carotenoid biosynthetic process IEA / InterPro:IPR014105 — IPR014105 is the
  carotenoid/retinoid oxidoreductase family, and TIGR02734 crtI_fam matches this protein
  [file:PANAN/crtI/crtI-uniprot.txt "DR NCBIfam; TIGR02734; crtI_fam; 1."]. Lycopene is a
  carotenoid and the module-level process is right. ACCEPT.
• Duplicated terms (GO:0016120 x2, GO:0016627 x2, GO:0071949 x2) are fine; duplicates with
  different evidence codes need no reconciliation, per the reviewer guidance.

No REMOVE anywhere. There is no InterPro2GO row here assigning a paralog activity the
protein lacks (nothing like squalene synthase or GGPP synthase activity appears in this GOA
file), no GO:0005515 protein binding row, and no negated row. Every one of the nine rows is
biologically true of this protein at some level of granularity.

No UNDECIDED. The two abstract-only papers are the IDA sources, but their abstracts state
the assayed result explicitly (product, cofactor, inhibitors), and the full-text 2012 paper
independently confirms every one of those claims. Nothing here required deferring.

7. The isomerase question (why I did not propose GO:0046608)

CrtI's overall reaction includes a cis-to-trans isomerization at the central double bond
PMID:22745782,
and the enzyme can act as an isomerase under anaerobic, reduced-FAD conditions
PMID:22745782.

It is tempting to add GO:0046608 carotenoid isomerase activity as a NEW annotation. I checked
the term definition against what the experiment actually produced, and it fails:

• GO:0046608 is defined (QuickGO) as "Catalysis of the isomerization of poly-cis-carotenoids to
  all-trans-carotenoids."
• What CrtI actually did to prolycopene was partial:
  PMID:22745782.
  Tri-cis and di-cis products are not all-trans carotenoids, so the demonstrated activity does
  not satisfy the term's definition. This is exactly the CRTISO-versus-CrtI distinction the
  paper is drawing, not evidence that CrtI is a CRTISO.
• The physiological central-bond isomerization is real, but it is an internal step of the
  overall 15-cis-phytoene -> all-trans-lycopene conversion that GO:0016166 plus Rhea:15585
  already represent; annotating it separately would double-count one reaction.

So: no NEW annotation. The gap is recorded as a suggested question and as a proposed new term
(below) instead.

8. Granularity gap -> proposed new term

GO:0016166 phytoene dehydrogenase activity is defined as "Catalysis of the dehydrogenation of
phytoene to produce a carotenoid intermediate such as phytofluene" (QuickGO), and it has no
children (QuickGO returns an empty child list). It is therefore used for both the plant/
cyanobacterial plastoquinone-dependent PDS reaction (EC 1.3.5.5) and the bacterial four-step
lycopene-forming CrtI reaction (EC 1.3.99.31). The repository's own pathway module records the
same problem
[file:../../../modules/carotene_backbone_biosynthesis.yaml "GO has no child of GO:0016166 specific to the"].
A child term for the lycopene-forming EC 1.3.99.31 / RHEA:15585 activity would let GO
distinguish the two mutually exclusive desaturation routes by function term rather than only by
Rhea and taxon. Entered under proposed_new_terms.

9. Open questions carried into the review

1. Native terminal electron acceptor in P. ananatis (oxygen vs. membrane quinone) — both work
   in vitro; neither is established in vivo.
2. Membrane sidedness/topology — peripheral and possibly monotopic is established, the leaflet
   is not, so no GO:0009898 annotation.
3. Whether the anaerobic prolycopene isomerization has any physiological role, and whether a
   partial-isomerization GO term is warranted.