Essential protein required for siroheme biosynthesis (PubMed:20592802). 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 (PubMed:9006913). It is a step in the biosynthesis of siroheme (PubMed:9006913). Promotes nitrogen and sulfur assimilation as well as photosynthesis efficiency by triggering chlorophyll, nitrite reductase (NiR) and sulfite reductase (SiR) biosynthesis (PubMed:29472934). 2 S-adenosyl-L-methionine + uroporphyrinogen III = H(+) + precorrin-2 + 2 S-adenosyl-L-homocysteine Porphyrin-containing compound metabolism; siroheme biosynthesis; precorrin-2 from uroporphyrinogen III: step 1/1. Mostly expressed in leaves, and, to a lower extent, in stems, flowers and siliques. By light (PubMed:15326282, PubMed:27729721). Accumulates within two hours in etiolated seedlings exposed to light (PubMed:27729721). Embryo lethal. Belongs to the precorrin methyltransferase family.