erythromycin 3''-o-methyltransferase activity
erythromycin 3''-o-methyltransferase activity
Catalysis of the reaction: erythromycin C + S-adenosyl-L-methionine = erythromycin A + H+ + S-adenosyl-L-homocysteine. Also converts erythromycin D into erythromycin B.