ActI-ORF1 (SCO5087) is the ketosynthase subunit (KSalpha) of the actinorhodin "minimal" type II polyketide synthase in Streptomyces coelicolor. Together with ActI-ORF2, the chain-length factor (KSbeta/CLF), it forms the heterodimeric KS-CLF that catalyzes iterative decarboxylative (Claisen) condensation of malonyl units to polymerize the poly-beta-keto chain of the aromatic polyketide antibiotic actinorhodin. The KS subunit carries the catalytic activity and must catalyze both chain initiation and elongation, while CLF, which lacks an active site, governs the final chain length; chain elongation and the first cyclization occur within an amphipathic tunnel at the heterodimer interface (PMID:15286722; PDB 1TQY). Although the KS-CLF is evolutionarily related to fatty-acid beta-ketoacyl synthases, it is a polyketide synthase, not part of fatty-acid metabolism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of cytosolic localization. The actinorhodin type II PKS is a soluble cytosolic enzyme system, so this is appropriate. Reason: Correct cellular component for a soluble type II PKS subunit. |
| GO:0004315 3-oxoacyl-[acyl-carrier-protein] synthase activity | IEA GO_REF:0000120 | MODIFY | Summary: InterPro/sequence-based (IEA) assignment from the beta-ketoacyl synthase (KAS) signature. The chemistry (decarboxylative condensation) is correct, but the specific term GO:0004315 denotes the fatty-acid-synthase (FAS) ketosynthase. ActI-ORF1 is the ketosynthase of a type II POLYKETIDE synthase, so the polyketide-specific term is more accurate. Reason: Wrong-specific: the enzyme is a polyketide ketosynthase, not a fatty-acid KAS. Replace with GO:0016218 (polyketide synthase activity). Proposed replacements: polyketide synthase activity Supporting Evidence: PMID:15286722 Although CLF regulates chain length, it does not have an active site; KS must catalyze both chain initiation and elongation. |
| GO:0006633 fatty acid biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: InterPro-based (IEA) over-propagation from the KAS/FAS signature. ActI-ORF1 is part of actinorhodin (aromatic polyketide antibiotic) biosynthesis, not fatty acid biosynthesis. Reason: Wrong biological process. Replace with the specific, accurate term GO:1901112 (actinorhodin biosynthetic process). Proposed replacements: actinorhodin biosynthetic process Supporting Evidence: PMID:15286722 The synthesis of aromatic polyketides, such as actinorhodin, tetracycline and doxorubicin, begins with the formation of a polyketide chain. |
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based (IEA) general acyltransferase activity. This broad parent is a true (if non-specific) description of the ketosynthase condensation reaction. Reason: Correct general molecular function (true parent of polyketide synthase activity); retained as accurate but non-specific. Supporting Evidence: PMID:15286722 KS must catalyze both chain initiation and elongation |
| GO:0030497 fatty acid elongation | IEA GO_REF:0000117 | REMOVE | Summary: Orthology-based (IEA) fatty acid elongation. This is a fatty-acid-specific process term; ActI-ORF1 elongates a POLYKETIDE chain, not a fatty acid, and the polyketide chain-extension activity is already captured by the actinorhodin biosynthetic process and polyketide synthase activity assignments. Reason: Wrong-specific fatty-acid term that mischaracterizes polyketide chain extension; redundant once the actinorhodin/polyketide terms are applied. Supporting Evidence: PMID:15286722 In type II polyketide synthases (PKSs), chains are polymerized by the heterodimeric ketosynthase-chain length factor (KS-CLF). |
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Download this section (compressed HTML)Q: The minimal PKS additionally requires the holo-ACP (ActI-ORF3) for chain assembly; should the type II PKS complex annotation be extended to include the ACP subunit?
Experiment: Active-site cysteine mutagenesis of ActI-ORF1 with in vitro reconstitution of the KS-CLF + ACP to confirm that catalysis (initiation and elongation) resides solely in the KS subunit.
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