Uroporphyrinogen-III C-methyltransferase (), uroporphyrinogen methyltransferase, uroporphyrinogen-III methyltransferase, adenosylmethionine-uroporphyrinogen III methyltransferase, S-adenosyl-L-methionine-dependent uroporphyrinogen III methylase, uroporphyrinogen-III methylase, SirA, CysG, CobA, uroporphyrin-III C-methyltransferase, S-adenosyl-L-methionine:uroporphyrin-III C-methyltransferase) is an enzyme with systematic name S-adenosyl-L-methionine:uroporphyrinogen-III C-methyltransferase. This enzyme catalyses the following overall chemical reaction
The enzyme catalyses two methylation reactions. The first reaction converts uroporphyrinogen III into precorrin-1 and the second forms dihydrosirohydrochlorin (precorrin-2). In both cases the methyl group comes from the cofactor, S-adenosyl methionine (SAM), which loses its methyl group and becomes S-adenosyl-L-homocysteine (SAH). These reactions are part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in both anaerobic and aerobic bacteria.