A ROLE FOR SALMONELLA-TYPHIMURIUM CBIK IN COBALAMIN (VITAMIN-B-12) AND SIROHEME BIOSYNTHESIS

The role of cbiK, a gene found encoded within the Salmonella typhimuri um cob operon, has been investigated by studying its in vivo function in Escherichia coli. First, it was found that cbiK is not required for cobalamin biosynthesis in the presence of a genomic cysG gene (encodi ng siroheme synthase) background. Second, in the absence of a genomic cysG gene, cobalamin biosynthesis in E. coli was found to be dependent upon the presence of cobA(P. denitrificans) (encoding tile uroporphyr inogen III methyltransferase from Pseudomonas denitrificans) and cbiK. Third, complementation of the cysteine auxotrophy of the E, coli cysG deletion strain 302 Delta a could be attained by the combined presenc e of cobA(P. denitrificans) and the S. typhimurium cbiK gene. Collecti vely these results suggest that CbiK can function in fashion analogous to that of the N-terminal do!nain of CysG (CysG(B)), which catalyzes the final two steps in siroheme synthesis, i.e., NAD-dependent dehydro genation of precorrin-2 to sirohydrochlorin and ferrochelation, Thus, phenotypically CysG(B) and CbiK have very similar properties in vivo, although the two proteins do not have any sequence similarity. In comp arison to CysG, CbiK appears to have a greater affinity for Co2+ than for Fe2+, and it is likely that cbiK encodes an enzyme whose primary r ole is that of a cobalt chelatase in corrin biosynthesis.