Kinetics of cobalamin repression of the cob operon in Salmonella typhimurium.

The cob operon in Salmonella typhimurium encodes 25 proteins involved in the biosynthesis of cobalamin. Expression of the cob operon is negatively feedback regulated by cobalamin via a translational control mechanism. The concentration of cobalamin required to repress cob expression to half-maximal was determined in vivo and in vitro to 0.4 microM and 0.6 microM, respectively. These results suggest that cob expression in wild-type cells is partially repressed by de novo synthesized cobalamin.