Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of alternapyrone derivatives (PubMed:16356847). Alternapyrone is a decaketide with octa-methylation from methionine on every C2 unit except the third unit (PubMed:16356847). All the domains in the polyketide synthase alt5 are apparently involved in alternapyrone synthesis, that is, the 8 CMeT, 7 KR, 7 DH, and 4 ER reactions in the 9 KS-mediated condensation steps required for alternapyrone synthesis (PubMed:16356847). the alternapyrone produced by alt5 might be intensively modified by cytochrome P450 monooxygenases alt1, alt2 and alt3 and FAD-dependent oxidoreductase alt4 present in the alt gene cluster (PubMed:16356847). Binds 1 phosphopantetheine covalently. Secondary metabolite biosynthesis. 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.