The CobII and CobIII regions of the cobalamin (vitamin B12) biosynthetic operon of Salmonella typhimurium.

A detailed deletion map of the CobII and CobIII regions of the cobalamin biosynthetic (cob) operon of Salmonella typhimurium LT2 has been constructed. The CobII region encodes functions needed for the synthesis of lower ligand 5,6-dimethylbenzimidazole (DMB); CobIII encodes functions needed for the synthesis of the nucleotide loop that joins DMB to the corrin macrocycle. The genetic analysis of 117 deletion, insertion, and point mutations indicates that (i) the CobII and CobIII mutations are contiguous--that is, they are grouped according to function; (ii) the CobII region is composed of four complementation groups (cobJKLM); (iii) cobM mutations do not complement mutations in any of the other three CobII groups; and (iv) CobIII mutations include three complementation groups that correspond to the cobU, cobS, and cobT genes.