bchF (P26165) — Rhodobacter capsulatus SB1003 — curation notes

Deep research was unavailable for this review. No deep-research API keys exist in this
container, so just deep-research was not run and no -deep-research-<provider>.md file
exists. All literature below was found and read directly (cached publications/PMID_*.md,
NCBI E-utilities, Europe PMC, Crossref, QuickGO, InterPro, and web search), and every claim
carries inline provenance.

1. Identity and locus

2. The reaction BchF catalyses

BchF hydrates the C-3 vinyl group of (bacterio)chlorophyllide a to a 3-(1-hydroxyethyl)
group; BchC then oxidises that to the 3-acetyl group of bacteriochlorophyllide a:

"followed by hydration of the 3V group to the 3-hydroxyethyl (3HE) group of 3HE-Bchlide a
by 3-vinyl (bacterio)chlorophyllide a [3V-(B)Chlide a] hydratase (BchF)"
PMID:36971575

"oxidation of the 3HE group by 3HE-Bchlide a dehydrogenase (BchC) to form the acetyl group
of Bchlide a, which is a direct precursor of Bchl a" PMID:36971575

Direct enzymology exists for the family, on the Chlorobaculum tepidum orthologue:
BchF and its paralogue BchV, expressed in E. coli, hydrate the C3-vinyl group in vitro and
run the reaction reversibly
PMID:36822492,
PMID:36822492,
PMID:36822492.
A wider in vitro survey of 3-vinyl hydratases from Chlorobaculum tepidum,
Chlorobaculum limnaeum, Chloroflexus aurantiacus and Chloracidobacterium thermophilum
found that "All the hydratases showed hydration activities." PMID:28643169, and notes that
"This 3-vinyl hydration also occurs in BChl a biosynthesis, followed by oxidation that
introduces an acetyl group at the 3-position." PMID:28643169.

There is no in vitro assay of the R. capsulatus BchF protein that I could find. The
R. capsulatus evidence is genetic.

3. R. capsulatus-specific genetic evidence

Burke, Alberti & Hearst 1993 (PMID:8385667) is the paper that defined this locus. It is
reference [1] of the UniProt entry and it both sequences and assigns bchF:

"The pigments accumulated in strains with physically mapped transposon insertion mutations
are analyzed by absorbance and fluorescence spectroscopy, allowing us to assign the genes
as bchF, bchN, bchB, and bchH, in that order." PMID:8385667

"bchF encodes a bacteriochlorophyll alpha-specific enzyme that adds water across the
2-vinyl group." PMID:8385667

("2-vinyl" is the pre-1990s numbering for the same carbon as today's C-3.) The same abstract
establishes that R. capsulatus BChl synthesis runs in the dark:
PMID:8385667.
This paper is not cited by any GOA row for P26165 — it is the strongest R. capsulatus
evidence for the gene and is uncited.

Bollivar, Suzuki, Beatty, Dobrowolski & Bauer 1994 (PMID:8158642) — the paper the CACAO
row cites — is a cluster-wide mutational analysis of the same 46 kb photosynthesis gene
cluster (cached abstract only; full_text_available: false):

"Previous studies have established that most if not all of the genes required for synthesis
of the Rhodobacter capsulatus essential photosystem are clustered on a 46 kb region of the
chromosome known as the photosynthesis gene cluster." PMID:8158642

"we have undertaken a systematic directed mutational analysis of 12 open reading frames in
the photosynthesis gene cluster to evaluate whether individual open reading frames have a
role in photopigment biosynthesis" PMID:8158642

"mutations constructed in seven open reading frames resulted in a loss of
bacteriochlorophyll biosynthesis, concomitant with the accumulation of specific
intermediates in the Mg-tetrapyrrole biosynthetic pathway" PMID:8158642

The abstract names no individual ORF, so whether bchF is one of the twelve cannot be read
off it. It can be established indirectly: a 2023 peer-reviewed paper cites this exact DOI
as a source for the R. capsulatus bchF deletion phenotype —

"Deletion of bchF in Rhodobacter sphaeroides and Rhodobacter capsulatus results in the
accumulation of Chlide a and 3V-Bchlide a" PMID:36971575

— whose references 2–4 for that sentence include 10.1006/jmbi.1994.1260, i.e. PMID:8158642
(verified against the Crossref reference list for 10.1128/spectrum.03878-22; reference 3 of
that list is 10.1006/jmbi.1994.1260). So an independent group treats Bollivar 1994 as
reporting a bchF mutant of R. capsulatus that accumulates the unhydrated intermediate —
exactly the phenotype expected of a lost 3-vinyl hydratase, and exactly the claim the CACAO
row makes.

4. Testing the predicted failure shape: cross-isolate transfer — ABSENT

The screen ("references spanning sibling taxa", projects/CACAO/batch3-screens.yaml) flagged
PMID:8158642 because its four CACAO rows span two taxon ids:

accession entry gene taxon id taxon name
P26165 BCHF_RHOCB bchF 272942 R. capsulatus strain ATCC BAA-309 / NBRC 16581 / SB1003
P26168 BCHE_RHOCB bchE 272942 same
P26169 BCHJ_RHOCB bchJ 272942 same
P26236 BCHM_RHOCA bchM 1061 Rhodobacter capsulatus (species level, no strain)

Findings:

  1. No isolate boundary is crossed. The paper works in R. capsulatus and all four
    entries are R. capsulatus. Taxon 1061 is the species node and 272942 is a strain node
    beneath it; they are not sibling isolates.
  2. The split is a property of the UniProt entries, not of the annotation. P26236 is the
    legacy _RHOCA entry (created before strain-level proteomes, no RC STRAIN= line, not
    mapped to the SB1003 reference proteome), while the other three were re-filed as _RHOCB
    on the SB1003 genome. A CACAO student annotating four genes from one paper picked the
    UniProt entry that exists for each gene; the taxon id came with the entry.
  3. One paper, three/four bch genes is legitimate here. The alternative hypothesis in the
    launch prompt — neighbour mis-attribution within the cluster, since bchF (RCAP_rcc00666),
    bchE (rcc00669) and bchJ (rcc00670) are adjacent — is also not supported: Bollivar 1994
    is explicitly a systematic analysis of twelve ORFs of that very cluster, so citing it for
    several of them is correct use of a cluster-wide paper, not over-harvesting.

Verdict: shape absent. The screen fired on UniProt taxon-granularity drift between sibling
entries of one species, which is a property of the protein records and not an annotation
defect. Nothing here is reportable against CACAO.

5. What is wrong: a GO term homonym in InterPro2GO

GO:0019685 "photosynthesis, dark reaction" is the Calvin cycle, not "the
light-independent reactions of bacteriochlorophyll synthesis". Its GO definition:

"A complex cycle of enzyme-mediated reactions which catalyzes the reduction of carbon
dioxide to sugar. As well as carbon dioxide the cycle requires reducing power in the form of
reduced nicotinamide adenine dinucleotide phosphate (NADP) and chemical energy in the form of
adenosine triphosphate (ATP). The reduced NADP (NADPH) and ATP are produced by the 'light'
reactions." (QuickGO, 2026-11-01)

BchF has no role in carbon fixation. The row arrives from InterPro2GO:

InterPro:IPR009905 2-vinyl bacteriochlorophyllide hydratase > GO:photosynthesis, dark reaction ; GO:0019685

and the InterPro entry's own description explains how: it says IPR009905 "is involved in the
light-independent bacteriochlorophyll biosynthesis pathway by adding water across the 2-vinyl
group". "Light-independent" was mapped to the term named "dark reaction".

This is systematic, not a one-off. All four InterPro2GO rules that use GO:0019685 are
light-independent (bacterio)chlorophyll-synthesis families
(current.geneontology.org/ontology/external2go/interpro2go, retrieved 2026-11-01):

InterPro:IPR005969 Light-independent protochlorophyllide reductase, B subunit   > GO:0019685
InterPro:IPR005970 Light-independent protochlorophyllide reductase, N subunit   > GO:0019685
InterPro:IPR005971 Light-independent protochlorophyllide reductase, iron-sulphur ATP-binding protein > GO:0019685
InterPro:IPR009905 2-vinyl bacteriochlorophyllide hydratase                     > GO:0019685

QuickGO (2026-11-01) returns 3,731 annotations to GO:0019685, and 3,731 of them cite
GO_REF:0000002 (InterPro2GO) — i.e. every annotation to this Calvin-cycle term in GOA comes
from these four chlorophyll-pathway rules. The correct term for all four is GO:0036070 light-independent bacteriochlorophyll biosynthetic process (for the bch families) or the
chlorophyll counterpart. Recorded as a source_defects entry (WRONG_TERM, resource OTHER =
InterPro / InterPro2GO) on the GO:0019685 row.

6. Ontology gap: no GO term for the BchF reaction

GO has GO:0036354 bacteriochlorophyllide-a dehydrogenase activity for the next step
(BchC) but no term for the hydratase step itself; searching QuickGO for
"bacteriochlorophyllide" returns only GO:0036354, and GO:0016836 hydro-lyase activity has no
suitable child (checked 2026-11-01). So the IEA MF row sits at the generic hydro-lyase parent
with nowhere more specific to go. Filed as a proposed_new_terms entry,
"3-vinyl bacteriochlorophyllide a hydratase activity" (EC 4.2.1.165, RHEA/MetaCyc
MONOMER-13252, BRENDA 4.2.1.165), and used as the proposed_replacement_terms NTR for that
row and as the core function's proposed_molecular_function.

7. GONUTS — now permanently retired

projects/CACAO.md records that gowiki.tamu.edu was "temporarily offline while we perform
upgrades" through batches 1–2. Retried 2026-11-01: the host now returns HTTP 410 Gone with
the page title "GONUTS has been retired" and the body "The GO wiki that was at gowiki.tamu.edu
is no longer online. For Gene Ontology terms and annotations, see the Gene Ontology
Consortium. The lab's other wikis are still here: EcoliWiki and OMP." The Wayback availability
API has no snapshot for UniProtKB:P26165, PMID:8158642 or
Rhodobacter_capsulatus:bchF on that host. The GONUTS route is gone for good, so the
project's standing plan to "re-check the abstract-only rows when GONUTS returns" cannot be
carried out; abstract-only CACAO rows must be judged from the literature, as here.

8. Open points