REPRESSION OF THE COB OPERON OF SALMONELLA-TYPHIMURIUM BY ADENOSYLCOBALAMIN IS INFLUENCED BY MUTATIONS IN THE PDU OPERON

The cob operon encodes functions needed for the biosynthesis of adenos ylcobalamin (Ado-B-12). Propanediol induces transcription of the cob o peron and the neighboring pdu operon, which encodes proteins for the B -12-dependent degradation of propanediol. Expression of the cob (but n ot the pdu operon is repressed by exogenous cyanocobalamin. Evidence i s provided that cob operon repression is signaled by internally genera ted Ado-B-12, which can be formed either by the CobA adenosyltransfera se or by an alternative adenosyltransferase (AdoT) that we infer is en coded within the pdu operon. Repression is also affected by mutations (AdoB) in the pdu operon that map upstream of the inferred pdu adenosy ltransferase gene, Such mutations allow cobalamin to mediate repressio n at concentrations 100-fold lower than those needed in the wild type. It is proposed that these mutations eliminate a component of the prop anediol dehydratase enzyme complex (PduCDE) and that this complex comp etes with the cab regulatory mechanism for a limited supply of Ado-B-1 2.