THE COBC GENE OF SALMONELLA-TYPHIMURIUM CODES FOR A NOVEL PHOSPHATASEINVOLVED IN THE ASSEMBLY OF THE NUCLEOTIDE LOOP OF COBALAMIN

We report the identification of a new locus, designated cobC, involved in the assembly of the nucleotide loop of cobalamin in Salmonella typ himurium. The cobC gene has been mapped, cloned, and sequenced. DNA se quence analysis suggested that cobC is divergently transcribed from th e adjacent cobD gene and suggests that the regulatory region of these genes overlap. The cobC gene codes for a predicted polypeptide of 26 k Da with striking homology to phosphoglycerate mutase, fructose-2,6-bis phosphatase, and acid phosphatase enzymes. In vitro experiments demons trated that CobC dephosphorylated the cobalamin biosynthetic intermedi ate N-1-(5 phospho-alpha-D-ribosyl)-5,6-dimethylbenzimidazole to gener ate N-1-alpha-D-ribosyl-5,6-dimethylbenzimidazole. In vivo data showed that the lack of cobC function blocks the synthesis of cobalamin from its precursors cobinamide and 5,6-dimethylbenzimidazole, i.e. it prev ents the assembly of the nucleotide loop of cobalamin. Additionally, e xogenous N-1-alpha-D-ribosyl-5,6-dimethylbenzimidazole rescues the def ect of a cobC mutant. We propose that cobC codes for a novel phosphata se whose primary role is in cobalamin biosynthesis. A model for the se quence of biosynthetic steps that as semble the nucleotide loop of cob alamin in S. typhimurium is presented.