Methylates precorrin-2 at the C-20 position to produce precorrin-3A. precorrin-2 + S-adenosyl-L-methionine = H(+) + precorrin-3A + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 1/10. Homodimer. Belongs to the precorrin methyltransferase family. Was originally thought to originate from Pseudomonas denitrificans, but similarity searches show that the sequence is much closer to Sinorhizobium. The entry's taxonomy has been changed.