O-methyltransferase; part of the gene cluster that mediates the biosynthesis of the linear tetracyclic TAN-1612 neuropeptide Y receptor antagonist (PubMed:21866960). The decaketide backbone of TAN-1612 is synthesized by the non-reducing polyketide synthase adaA via condensation of one acetyl-CoA starter unit with 9 malonyl-CoA units. The FAD-dependent monooxygenase adaC then performs hydroxylation at C2 while the polaketide chain is still attached to the NRPKS adaA (PubMed:21866960). The alpha-hydroxylation step at C2 appears to be crucial for the following C18-C1 Claisen cyclization and release of the C9-hydroxyl version of TAN-1612 from the NRPKS adaA, two steps performed by the lactamase-like protein adaB (PubMed:21866960). Finally, the O-methyltransferase adaD performs the C9 O-methylation to complete the biosynthesis of TAN-1612 (PubMed:21866960). 2-acetyl-3,4a,8,10,11,12a-hexahydroxy-1,4,4a,5,12,12a-hexahydrotetracene-1,12-dione + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + TAN-1612 Secondary metabolite biosynthesis. Belongs to the methyltransferase superfamily.