Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of gibepyrone A, a 2H-pyran-2-one metabolite exhibiting a moderate antimicrobial activity against Gram-positive bacteria and yeasts (PubMed:27856636). The highly reducing polyketide synthase GPY1 is sufficient to produce gibepyrone A. GPY1 uses an acetyl-CoA starter unit, three malonyl-CoA extender units, and two SAM-dependent methylations to establish the gibepyrone A carbon backbone, followed by product release upon intramolecular cyclization (PubMed:27856636). The gibepyrone A derivatives gibepyrones B and D are produced by cluster-independent P450 monooxygenases, probably to protect the fungus from the toxic product (PubMed:27856636). In contrast, the formation of gibepyrones E and F from gibepyrone A is a spontaneous process and independent of enzymatic activity (PubMed:27856636). Expression is up-regulated by gibepyrone A which is able to induce its own production by an elevated GPY1 gene expression in a positive feedback loop (PubMed:27856636). Expression is down-regulated by the cluster-specific ABC transporter GPY2 (PubMed:27856636). Members of the velvet complex, VEL1, VEL2, and LAE1, negatively affect GPY1 gene expression and gibepyrone A product formation, whereas SGE1 represents a positive regulator of gibepyrone biosynthesis (PubMed:27856636). Multidomain protein; including a ketosynthase (KS) that catalyzes repeated decarboxylative condensation to elongate the polyketide backbone; a malonyl-CoA:ACP transacylase (MAT) that selects and transfers the extender unit malonyl-CoA; a dehydratase (DH) domain that reduces hydroxyl groups to enoyl groups; a methyltransferase (CMeT) domain responsible for the incorporation of methyl groups; an enoylreductase (ER) domain that reduces enoyl groups to alkyl group; a ketoreductase (KR) domain that catalyzes beta-ketoreduction steps; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm. Results in total loss of gibepyrone A biosynthesis.