cao-2 (CAO2_NEUCR, A7UXI1 / NCU11424) — review notes

Gene identity and function

cao-2 encodes CAO-2, the second carotenoid cleavage oxygenase (CCO) of Neurospora crassa and the
paralog of cao-1. Unlike cao-1 (a stilbenoid cleaver), CAO-2 is a genuine carotenoid-cleaving
enzyme: it is a torulene dioxygenase (EC 1.13.11.59) that catalyzes the committed cleavage step of
the neurosporaxanthin (apocarotenoid) biosynthetic pathway.

Reaction: torulene (a C40 carotene) + O2 → 4'-apo-β-carotenal (β-apo-4'-carotenal, C35) +
3-methyl-2-butenal. The C35 apocarotenal is then oxidized to the xanthophyll neurosporaxanthin by
the aldehyde dehydrogenase YLO-1.
Pathway: Carotenoid biosynthesis (neurosporaxanthin branch); KEGG ncr00906; KEGG KO K17842
(torulene dioxygenase).
Localization: cytosol (UniProt Cytoplasm, cytosol).

Key experimental evidence — PMID:17610084 (Saelices et al. 2007)

CAO-2 was identified and characterized by genetics + biochemistry:
- Torulene-accumulating, neurosporaxanthin-lacking mutants map to cao-2; targeted disruption gives
PMID:17610084.
- Direct in vitro assay with purified enzyme:
PMID:17610084.
- Substrate specificity: PMID:17610084.
- Regulation: PMID:17610084 —
the opposite of cao-1 (light-independent), as expected for a genuine carotenoid-pathway gene.
- Conclusion: PMID:17610084.

Two IBA lessons in one paralog pair (cross-link to IBA_REVIEW.md)

CAO-2 is the positive-control paralog of cao-1. Both carry the identical family IBA terms
(GO:0010436 carotenoid dioxygenase activity, GO:0016121 carotene catabolic process; both from
GO_REF:0000033), because they share PANTHER family PTHR10543. For cao-1 those terms are wrong
(refuted; it cleaves stilbenes); for cao-2 they are correct (it cleaves the carotene torulene). The
same phylogenetic inference is right for one paralog and wrong for the other — only target-specific
experimental evidence separates them.

But CAO-2 also illustrates the inverse IBA problem — under-curation / IBA incompleteness.
Despite a full experimental characterization (disruption phenotype + purified-enzyme assay,
PMID:17610084), GOA carries only IBA/IEA annotations for CAO-2 — no IDA. The experimental
torulene-dioxygenase activity and the neurosporaxanthin/apocarotenoid-biosynthesis role are not
captured as experimental annotations. So the same gene is simultaneously a case where IBA is correct
and a case where experimental biology that should be annotated is missing.

Annotation review decisions (4 GOA annotations)

GO term	Evidence	Decision	Rationale
GO:0010436 carotenoid dioxygenase activity	IBA	ACCEPT (core)	Correct; CAO-2 is a torulene dioxygenase (EC 1.13.11.59). Experimentally supported (PMID:17610084) though currently only IBA. No specific torulene-dioxygenase GO term exists.
GO:0016121 carotene catabolic process	IBA	ACCEPT	Torulene (a carotene) is cleaved. Accurate but partial: the informative process is apocarotenoid/neurosporaxanthin biosynthesis (GO:0043289), captured in core_functions.
GO:0005829 cytosol	IEA	ACCEPT	Consistent with UniProt cytoplasm/cytosol.
GO:0016702 oxidoreductase (2 O atoms; dioxygenase)	IEA	ACCEPT	Correct general dioxygenase MF, corroborates GO:0010436.

Core function: torulene dioxygenase (GO:0010436; EC 1.13.11.59) in apocarotenoid biosynthetic process
(GO:0043289) / neurosporaxanthin biosynthesis; cytosol. Under-annotated experimentally — an
apocarotenoid-biosynthesis BP and IDA upgrades from PMID:17610084 are warranted.