Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of betaenones, phytotoxic polyketides involved in leaf spot disease in sugar beets (PubMed:25530455). The first step of the pathway is the synthesis of dehydroprobetaenone I by the polyketide synthase bet1 and the enoyl reductase bet3 via condensation of one acetyl-CoA starter unit with 7 malonyl-CoA units and 5 methylations (PubMed:25530455). The C-terminal reductase (R) domain of bet1 catalyzes the reductive release of the polyketide chain (PubMed:25530455). Because bet1 lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase bet3 (PubMed:25530455). The short-chain dehydrogenase/reductase bet4 then catalyzes reduction of dehydroprobetaenone I to probetaenone I (PubMed:25530455). The cytochrome P450 monooxygenase bet2 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:25530455). The FAD-linked oxidoreductase (orf1) is probably 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 (By similarity). 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. 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.