The O-methyltransferase SrsB catalyzes the decarboxylative methylation of alkylresorcylic acid during phenolic lipid biosynthesis by Streptomyces griseus.

Streptomyces griseus contains the srs operon, which is required for phenolic lipid biosynthesis. The operon consists of srsA, srsB, and srsC, which encode a type III polyketide synthase, an O-methyltransferase, and a flavoprotein hydroxylase, respectively. We previously reported that the recombinant SrsA protein synthesized 3-(13'-methyltetradecyl)-4-methylresorcinol, using iso-C(16) fatty acyl-coenzyme A (CoA) as a starter substrate and malonyl-CoA and methylmalonyl-CoA as extender substrates. An in vitro SrsA reaction using [(13)C(3)]malonyl-CoA confirmed that the order of extender substrate condensation was methylmalonyl-CoA, followed by two extensions with malonyl-CoA. Furthermore, SrsA was revealed to produce an alkylresorcylic acid as its direct product rather than an alkylresorcinol. The functional SrsB protein was produced in the membrane fraction in Streptomyces lividans and used for the in vitro SrsB reaction. When the SrsA reaction was coupled, SrsB produced alkylresorcinol methyl ether in the presence of S-adenosyl-l-methionine (SAM). SrsB was incapable of catalyzing the O-methylation of alkylresorcinol, indicating that alkylresorcylic acid was the substrate of SrsB and that SrsB catalyzed the conversion of alkylresorcylic acid to alkylresorcinol methyl ether, namely, by both the O-methylation of the hydroxyl group (C-6) and the decarboxylation of the neighboring carboxyl group (C-1). O-methylated alkylresorcylic acid was not detected in the in vitro SrsAB reaction, although it was presumably stable, indicating that O-methylation did not precede decarboxylation. We therefore postulated that O-methylation was coupled with decarboxylation and proposed that SrsB catalyzed the feasible SAM-dependent decarboxylative methylation of alkylresorcylic acid. To the best of our knowledge, this is the first report of a methyltransferase that catalyzes decarboxylative methylation.