Studies of regulation of expression of the propionate (prpBCDE) operon provide insights into how Salmonella typhimurium LT2 integrates its 1,2-propanediol and propionate catabolic pathways

Expression of the prpBCDE operon of Salmonella typhimurium LT2 required (i) the synthesis of propionyl-coenzyme A (CoA) by the PrpE protein or the ace tyl-CoA-synthesizing systems of the cell and (ii) the synthesis of 2-methyl citrate from propionyl-CoA and oxaloacetate by the PrpC protein. We propose that either 2-methylcitrate or a derivative of it signals the presence of propionate in the environment. This as yet unidentified signal is thought t o serve as a coregulator of the activity of PrpR, the member of the sigma-5 4 family of transcriptional activators needed for activation of prpBCDE tra nscription. The CobB protein was also required for expression of the prpBCD E operon, but its role is less well understood. Expression of the prpBCDE o peron in cobB mutants was restored to wild-type levels upon induction of th e propanediol utilization (pdu) operon by 1,2-propanediol. This effect did not require catabolism of I,2-propanediol, suggesting that a Pdu protein, n ot a catabolite of 1,2-propanediol, was responsible for the observed effect . We explain the existence of these redundant functions in terms of metabol ic pathway integration. In an environment with 1,2-propanediol as the sole carbon and energy source, expression of the prpBCDE operon is ensured by th e Pdu protein that has CobB-like activity. Since synthesis of this Pdu prot ein depends on the availability of 1,2-propanediol, the cell solves the pro blem faced in an environment devoid of 1,2-propanediol where propionate is the sole carbon and energy source by having cobB located outside of the pdu operon and its expression independent of 1,2-propanediol. At present, it i s unclear how the CobB and Pdu proteins affect prpBCDE expression.