fabB (PA1609) review notes

UniProt: Q9I3B2 (unreviewed, 405 aa), "3-oxoacyl-[acyl-carrier-protein]
synthase 1" (beta-ketoacyl-ACP synthase I, KAS I; EC 2.3.1.41). Locus PA1609,
cotranscribed with fabA.

What is established

fabA and fabB were cloned from P. aeruginosa and encode
beta-hydroxyacyl-ACP dehydratase and beta-ketoacyl-ACP synthase I
PMID:9286984. They form an operon
PMID:9286984.
Chromosomal fabB mutants are auxotrophic for unsaturated fatty acids
PMID:9286984. The same point is restated in the
structural paper
PMID:35136566.
The reaction FabB performs in unsaturated fatty acid synthesis is elongation
of the cis-unsaturated intermediate produced by FabA, so the auxotrophy is a
direct consequence of the enzyme's own catalysis rather than a downstream
effect
PMID:35136566. FabB is not responsible for the final vaccenic
acid step, which is FabF's
PMID:35136566.
The P. aeruginosa enzyme has been crystallised as the active-site C161A
variant, confirming the condensing-enzyme architecture and the catalytic
cysteine
PMID:35136566.

So FabB has both a general role (chain elongation in fatty acid biosynthesis,
the KAS I condensation step) and a specific one it cannot be replaced in
(elongation of the unsaturated intermediate). Both GO:0006633 and GO:0006636 are
therefore kept on this gene: the general term is not merely entailed by the
specific one, because FabB also elongates saturated acyl-ACPs.

Literature search

Europe PMC, 2026-11-05: aeruginosa AND FabB AND "fatty acid". Cached and used:
PMID:9286984, PMID:35136566. Also returned: PMID:41652533 / PMID:38335573 on
fabF1/fabF2 and on FabB in P. putida, which corroborate the division of labour
between FabB and FabF but were not needed for the annotation calls.

PseudoCAP IBA-seed assessment

PTN node	node term	n_seeds	leaf IBAs	seed annotation (term, evidence, PMID)	verdict	rationale	other seeds	IBD stands without this seed?
PTN000918584	GO:0006633 fatty acid biosynthetic process	8	436 (128 taxa)	GO:0006636 unsaturated fatty acid biosynthetic process, IMP, PMID:9286984 (PseudoCAP)	SOUND	The seed is a mutant phenotype, but for this gene the phenotype and the activity coincide: FabB is the condensing enzyme that elongates the cis-unsaturated intermediate, so an unsaturated-fatty-acid auxotrophy is the expected consequence of losing its own catalytic step, not an indirect downstream effect. The seeded node term is the parent GO:0006633, which is the right level for a node of KAS enzymes spanning bacteria, plants and the human mitochondrial OXSM; the seed's more specific GO:0006636 is a descendant, so it supports the node term without pushing a lineage-specific claim up the tree.	E. coli FabB (P0A953) and FabF (P0AAI5), M. tuberculosis KasA/KasB (P9WQD7, P9WQD9), human OXSM (Q9NWU1), Arabidopsis AT1G74960 (KASI), PAO1 Q9HU15	Yes. E. coli FabB and FabF alone carry direct experimental support for fatty acid biosynthesis. No `GO_CENTRAL` defect.

The GO:0004315 IBD at the same node (7 seeds, this gene not among them) is
likewise sound: KAS I/II activity is the family-defining activity and PAO1 FabB
has been crystallised with the catalytic cysteine identified.

PseudoCAP defects from this gene: none. The seed cites the right paper for the
right locus, and the evidence it rests on is appropriate for the term.