Involved in the biosynthesis of the anthracyclines carminomycin and daunorubicin (daunomycin) which are aromatic polyketide antibiotics that exhibit high cytotoxicity and are widely applied in the chemotherapy of a variety of cancers. In vivo, catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the 4-O-position of carminomycin to form daunorubicin. In vitro, it also methylates the anthracyclines rhodomycin D (10-carbomethoxy-13-deoxycarminomycin), 10-carboxy-13-deoxycarminomycin, 13-deoxy-carminomycin and 13-dihydrocarminomycin at the 4-hydroxyl position. carminomycin + S-adenosyl-L-methionine = daunorubicin + H(+) + S-adenosyl-L-homocysteine Strongly inhibited by S-adenosyl-L-homocysteine and weakly by adenine and methionine. Optimum pH is 8 and the values at pH 7.5 and 7.0 result in 60% and 80% decreases in activity, respectively. No pH values above pH 8.0. Optimum temperature is 37 degrees Celsius with about 10% decrease in activity at 30 and 45 degrees Celsius. Antibiotic biosynthesis; daunorubicin biosynthesis. Antibiotic biosynthesis; carminomycin biosynthesis. Homodimer and homotetramer in equilibrium. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.