Evidence that a B12-adenosyl transferase is encoded within the ethanolamine operon of Salmonella enterica

Adenosylcobalamin (Ado-B-12) is both the cofactor and inducer of ethanolamine ammonia lyase (EA-lyase), a catabolic enzyme for ethanolamine. De novo synthesis of Ado-B-12 by Salmonella enterica occurs only under anaerobic conditions. Therefore, aerobic growth on ethanolamine requires import of Ado-B-12 or a precursor (CN-B-12 or OH-B-12) that can be adenosylated internally. Several known enzymes adenosylate corrinoids. The CobA enzyme transfers adenosine from ATP to a biosynthetic intermediate in de novo B,, synthesis and to imported CN-B-12, OH-B-12, or Chi (a B-12 precursor). The PduO adenosyl transferase is encoded in an operon (pdu) for cobalamin-dependent propanediol degradation and is induced by propanediol. Evidence is presented here that a third transferase (EutT) is encoded within the operon for ethanolamine utilization (eut). Surprisingly, these three transferases share no apparent sequence similarity. CobA produces sufficient Ado-B-12 to initiate eat operon induction and to serve as a cofactor for EA-lyase when B-12 levels are high. Once the eut operon is induced, the EutT transferase supplies more Ado-B-12 during the period of high demand. Another protein encoded in the operon (EutA) protects EA-lyase from inhibition by CN-B-12 but does so without adenosylation of this corrinoid.