Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of the phytotoxin stemphyloxin II (PubMed:31553484). The first step of the pathway is the synthesis of dehydroprobetaenone I by the polyketide synthase sthA and the enoyl reductase sthE via condensation of one acetyl-CoA starter unit with 7 malonyl-CoA units and 5 methylations (PubMed:31553484). The C-terminal reductase (R) domain of sthA catalyzes the reductive release of the polyketide chain (PubMed:31553484). Because sthA lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase sthE (PubMed:31553484). The short-chain dehydrogenase/reductase sthC then catalyzes reduction of dehydroprobetaenone I to probetaenone I (PubMed:31553484). The cytochrome P450 monooxygenase sthF catalyzes successive epoxidation, oxidation (resulting from epoxide opening) and hydroxylation to install a tertiary alcohol in the decaline ring to yield betaenone C from dehydroprobetaenone I and betaenone B from probetaenone I (PubMed:31553484). The FAD-linked oxidoreductase sthB is responsible for the conversion of betaenone C to betaenone A via an intramolecular aldol reaction between C-1 and C-17 to form the bridged tricyclic system in betaenone A (PubMed:31553484). Finally, the cytochrome P450 monooxygenase sthD catalyzes the hydroxylation of C-15 to afford the final metabolite stemphyloxin II (PubMed:31553484). acetyl-CoA + 10 AH2 + 2 H(+) + 7 malonyl-CoA + 5 S-adenosyl-L-methionine = 10 A + 7 CO2 + 8 CoA + dehydroprobetaenone I + 6 H2O + 5 S-adenosyl-L-homocysteine Binds 1 phosphopantetheine covalently. Mycotoxin biosynthesis. Expression is highly up-regulated during plant infection. 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; an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm; and a C-terminal reductase (R) domain that catalyzes the reductive release of the polyketide chain.