Catalyzes the methylation of 27-O-demethylrifamycin SV (DMRSV) to rifamycin SV. 27-O-demethylrifamycin SV + S-adenosyl-L-methionine = H(+) + rifamycin SV + S-adenosyl-L-homocysteine Slightly inhibited by Ca(2+) and Mg(2+). Strongly inhibited by Zn(2+), Ni(2+) and Co(2+). kcat is 87 sec(-1). Optimum pH is 7.5-8.0. Antibiotic biosynthesis; rifamycin B biosynthesis. Exists probably as a trimer. Mutant loses its ability to produce rifamycin B, but accumulates 27-O-demethylrifamycin SV (DMRSV) as the major new metabolite, together with a small quantity of 27-O-demethyl-25-O-desacetylrifamycin SV (DMDARSV). Belongs to the class I-like SAM-binding methyltransferase superfamily.