FIG 7.

Model for the operation of the 2MCC in Pa. (A) During growth in the absence of propionate or propionyl-CoA generating substrates, the 2MCC operon (prp) expression is repressed through the binding of PrpR to its upstream promoter region. Incomplete repression of the operon (from basal cellular propionyl-CoA or competing transcriptional activators) results in a basal, low level of prpC transcription. (B) As the cellular propionyl-CoA levels rise, this metabolite is condensed with oxaloacetate by PrpC, resulting in the formation of 2-MC; 2-MC likely then binds to PrpR, inducing conformational changes that lead to the dissociation of PrpR from the DNA. This derepresses the prp operon, allowing expression of the 2MCC enzymes. However, as the concentration of propionyl-CoA falls (due to depletion of propionate or BCAAs due to 2MCC activity) so too does the concentration of 2-MC, which, in turn, leads to rebinding of PrpR to the prp promoter region and a resumption in prp operon repression. (C) In the absence of citrate synthase (GltA), Pa can survive because of the low-level basal expression of PrpC, a promiscuous enzyme that also has citrate synthase activity. However, this low total citrate synthase activity is unable to meet cellular demand, resulting in a severe growth defect and a strong selection pressure to acquire mutations that increase prpC expression. Based on our work, it seems that mutations in prpR that abolish its repressor activity are the most commonly selected mechanism for achieving this. These mutations lead to constitutive expression of the prp genes and, thus, an increase total cellular citrate synthase activity (compensating for the loss of GltA activity).