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. 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. The gibepyrone A derivatives gibepyrones B and D are produced by cluster-independent P450 monooxygenases, probably to protect the fungus from the toxic product. In contrast, the formation of gibepyrones E and F from gibepyrone A is a spontaneous process and independent of enzymatic activity