PqsB (PA0997) is the non-catalytic subunit of the heterodimeric condensing enzyme PqsBC, which catalyzes the second step of 2-alkyl-4(1H)-quinolone (AQ/HAQ) biosynthesis in the Pseudomonas aeruginosa pqs quorum-sensing pathway. PqsBC couples 2-aminobenzoylacetate (2-ABA, made by PqsD) with an octanoyl group carried on the catalytic subunit PqsC to form 2-heptyl-4(1H)-quinolone (HHQ), the direct precursor of the Pseudomonas quinolone signal PQS. PqsB shares the beta-ketoacyl-ACP synthase III (FabH/KAS III) fold with PqsC but lacks the catalytic residues (the active-site Cys-129/His-269 are contributed by PqsC); it is nonetheless tightly associated with PqsC and required for the condensation reaction (PMID:24239007, PMID:26811339). PqsBC is thus an obligate heterodimer that is only functional when assembled. A pqsB mutant is defective in extracellular quinolone signal production (PMID:12426334).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Subcellular-location IEA, consistent with the experimental cytoplasmic localization (EXP, PMID:24239007) below and with PqsBC being a soluble cytoplasmic condensing enzyme. Reason: Correct cellular component; corroborated by experimental evidence. |
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro-based (IEA) acyltransferase activity from the condensing-enzyme (thiolase-like) signature. PqsB has this fold but is catalytically inactive: the active-site Cys-129/His-269 and the octanoyl substrate are carried by PqsC. The acyltransferase activity is a property of the assembled PqsBC heterodimer, catalyzed by PqsC, not enabled by PqsB on its own. Reason: Attributing the catalytic MF to the non-catalytic subunit over-annotates PqsB. PqsB contributes to (is required for) the activity of the PqsBC complex but does not itself catalyze acyl transfer; a contributes_to qualifier, or annotation of the activity to the PqsBC complex, would be more accurate. Supporting Evidence: PMID:26811339 does not form a catalytic triad with His-269 and Cys-129, and in its place a valine (Val-299) is present PMID:24239007 the decarboxylating coupling of 2-ABA to an octanoate group linked to PqsC produces HHQ |
| GO:0005737 cytoplasm | EXP PMID:24239007 The end of an old hypothesis: the pseudomonas signaling mole... | ACCEPT | Summary: Experimental cytoplasmic localization, consistent with PqsB acting as part of the soluble cytoplasmic PqsBC condensing enzyme. Reason: Experimentally supported cellular component. |
| GO:0044550 secondary metabolite biosynthetic process | IMP PMID:12426334 Functions required for extracellular quinolone signaling by ... | ACCEPT | Summary: Experimental (IMP) evidence that pqs genes, including pqsB, are required for extracellular quinolone signal synthesis. PqsB is required for the PqsBC condensation step and thus for HHQ/HAQ production. Reason: Core biological process with direct experimental (mutant phenotype) support. This is the defining process for the gene. Supporting Evidence: PMID:24239007 PqsB is tightly associated with PqsC and required for the second step |
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Download this section (compressed HTML)Q: Per GO guidelines the EC 2.3.1.230 activity is annotated to the catalytic subunit PqsC (enables); PqsB, being required but non-catalytic, is best represented with a contributes_to (not as a co-equal enabler) plus the protein-containing complex CC. Is the contributes_to warranted here, or should PqsB carry only the complex and process annotations?
Experiment: Co-expression/co-purification and activity assays of PqsC alone versus the PqsBC heterodimer to quantify the requirement of PqsB for HHQ synthase activity and complex stability.
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