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The role of cbiK, a gene found encoded within the Salmonella typhimurium 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 (encoding 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
the uroporphyrinogen III methyltransferase from Pseudomonas denitrificans) and
cbiK. Third, complementation of the cysteine auxotrophy of the E. coli cysG
deletion strain 302delta a could be attained by the combined presence of cobA(P.
denitrificans) and the S. typhimurium cbiK gene. Collectively these results
suggest that CbiK can function in fashion analogous to that of the N-terminal
domain of CysG (CysG(B)), which catalyzes the final two steps in siroheme
synthesis, i.e., NAD-dependent dehydrogenation 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 comparison 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 role is that of a cobalt chelatase in corrin biosynthesis.
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