Catalyzes the two successive C-2 and C-7 methylation reactions involved in the conversion of uroporphyrinogen III to precorrin-2 via the intermediate formation of precorrin-1. It is a step in the biosynthesis of both cobalamin (vitamin B12) and siroheme. Neither uroporphyrin III nor the chlorin (factor I) is a substrate of SUMT. 2 S-adenosyl-L-methionine + uroporphyrinogen III = H(+) + precorrin-2 + 2 S-adenosyl-L-homocysteine S-adenosyl-L-methionine + uroporphyrinogen III = H(+) + precorrin-1 + S-adenosyl-L-homocysteine precorrin-1 + S-adenosyl-L-methionine = precorrin-2 + S-adenosyl-L-homocysteine S-adenosylhomocysteine is an extremely powerful competitive inhibitor of the uroporphyrinogen III methylation. SUMT exhibits a substrate inhibition phenomenon at uroporphyrinogen III concentrations above 2 uM; this property might play a regulatory role in cobalamin biosynthesis. The enzyme activity is completely insensitive to feedback inhibition by cobalamin and corrinoid intermediates. kcat is 38 h(-1). Optimum pH is 7.7. Cofactor biosynthesis; adenosylcobalamin biosynthesis; precorrin-2 from uroporphyrinogen III: step 1/1. Porphyrin-containing compound metabolism; siroheme biosynthesis; precorrin-2 from uroporphyrinogen III: step 1/1. Homodimer. Cells lacking this gene are deficient in cobalamin biosynthesis. Uroporphyrinogen III is a key intermediate because of its position at the branch point of two divergent pathways. One pathway leads to protoheme, and the other one is the cobalamin and siroheme pathway. 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.