Highly reducing polyketide synthase (HR-PKS); part of the gene cluster that mediates the biosynthesis of the polyenes aspernidgulenes (PubMed:30302871). The carbon backbone of aspernidgulenes is synthesized by the HR-PKS sdgA, which accepts acetyl-CoA as the starter unit and performs malonyl-CoA extensions as well as regioselective methylation and reduction (PubMed:30302871). The resulting nonaketide offloads the HR-PKS by intramolecular lactonization to yield the 5,6-dihydro-alpha-pyrone-containing hexaenoic acids preaspernidgulene A1 and A2 (PubMed:30302871). The FAD-dependent monooxygenase sdgC then installs the first epoxide on the penultimate double bond (PubMed:30302871). Subsequently, the FAD-dependent monooxygenase sdgF presumably generates a ketone intermediate through Meinwald rearrangement involving a hydride shift (PubMed:30302871). Next, sdgC introduces another epoxide on the last olefin of the ketone intermediate after E/Z isomerization (PubMed:30302871). The epoxide hydrolase sdgD then catalyzes stereospecific cyclization of the 5,6-dihydro-alpha-pyrone and opening of the epoxide ring to form an oxygenated trimethylcyclopentanone and an oxabicyclo[2.2.1]heptane unit (PubMed:30302871). Finally, the bicyclic unit undergoes hydrolytic cleavage, either spontaneously or catalyzed by sdgD, to assemble the dimethyl-gamma-lactone moiety in aspernidgulene A1 (PubMed:30302871). 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; 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.