Highly reducing polyketide synthase; part of the cluster that mediates the biosynthesis of acurin A, a highly reduced polyketide coupled to a serine via a peptide bond (PubMed:32234543). The activities of the highly reducing polyketide synthase acrA and the nonribosomal peptide synthetase acrB are collectively responsible for the synthesis of the acurin A core structure with a heptaketide backbone produced by acrA covalently fused to a L-serine by acrB (PubMed:32234543). After the formation of the PK-NRP hybrid product, it is detached from acrB by reductive release to set up the formation of the lactam ring by aldol condensation (Probable). The hydrolyase acrC then catalyzes water loss to generate a double bond in the ring (Probable). This double bond is probably reduced, which is followed by three oxidations at C-22 to generate the carboxylic acid moiety, involving probably the FAD-binding monooxygenase acrE and the cytochrome P450 monooxygenases acrD and acrF (Probable). Finally, a last methylation step performed by the O-methyltransferase acrG leads to the production of acurin A (Probable). Secondary metabolite biosynthesis. Expression is positively regulated by the acurin A cluster-specific transcription regulator acrR. 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 (MT) 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. Abolishes the production of acurin A.