Uroporphyrin-III C-methyltransferase (EC 2.1.1.107) (SUMT) [1,2] catalyzes the transfer of two methyl groups from S-adenosyl-L-methionine to the C-2 and C-7 atoms of uroporphyrinogen III to yield precorrin-2 via the intermediate formation of precorrin-1. SUMT is the first enzyme specific to the cobalamin pathway and precorrin-2 is a common intermediate in the biosynthesis of corrinoids such as vitamin B12, siroheme and coenzyme F430.

The sequences of SUMT from a variety of eubacterial and archaebacterial species are currently available. In species such as Bacillus megaterium (gene cobA), Pseudomonas denitrificans (cobA) or Methanobacterium ivanovii (gene corA) SUMT is a protein of about 25 to 30 Kd. In Escherichia coli and related bacteria, the cysG protein, which is involved in the biosynthesis of siroheme, is a multifunctional protein composed of a N-terminal domain, probably involved in transforming precorrin-2 into siroheme, and a C-terminal domain which has SUMT activity.

The sequence of SUMT is related to that of a number of P. denitrificans and Salmonella typhimurium enzymes involved in the biosynthesis of cobalamin which also seem to be SAM-dependent methyltransferases [3,4]. The similarity is especially strong with two of these enzymes: cobI/cbiL which encodes S-adenosyl-L-methionine--precorrin-2 methyltransferase and cobM/cbiF whose exact function is not known.

We developed two signature patterns for these enzymes. The first corresponds to a well conserved region in the N-terminal extremity (called region 1 in [1,3]) and the second to a less conserved region located in the central part of these proteins (this pattern spans what are called regions 2 and 3 in [1,3]).