O-methyltransferase; 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. Abolishes the production of acurin A. Belongs to the methyltransferase superfamily. Type-7 methyltransferase family.