May catalyze sequential reactions to synthesize uroporphyrinogen III from hydroxymethylbilane (HMB) and then precorrin-2, which are intermediate compounds in both vitamin B12 and siroheme biosyntheses. 2 S-adenosyl-L-methionine + uroporphyrinogen III = H(+) + precorrin-2 + 2 S-adenosyl-L-homocysteine hydroxymethylbilane = H2O + uroporphyrinogen III 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. In the N-terminal section; belongs to the precorrin methyltransferase family. In the C-terminal section; belongs to the uroporphyrinogen-III synthase family.