Studies of propionate toxicity in Salmonella enterica identify 2-methylcitrate as a potent inhibitor of cell growth

Salmonella enterica serovar Typhimurium LT2 showed increased sensitivity to propionate when the 2-methylcitric acid cycle was blocked. A derivative of a prpC mutant (which lacked 2-methylcitrate synthase activity) resistant t o propionate was isolated, and the mutation responsible for the newly acqui red resistance to propionate was mapped to the citrate synthase (gltA) gene . These results suggested that citrate synthase activity was the source of the increased sensitivity to propionate observed in the absence of the 2-me thylcitric acid cycle. DNA sequencing of the wild-type and mutant gltA alle les revealed that the ATG start codon of the wild-type gene was converted t o the rare GTG start codon in the revertant strain. This result suggested t hat lower levels of this enzyme were present in the mutant. Consistent with this change, cell-free extracts of the propionate-resistant strain contain ed 12-fold less citrate synthase activity. This was interpreted to mean tha t, in the wild-type strain, high levels of citrate synthase activity were t he source of a toxic metabolite. In vitro experiments performed with homoge neous citrate synthase enzyme indicated that this enzyme was capable of syn thesizing 2-methylcitrate from propionyl-CoA and oxaloacetate, This result lent further support to the in vivo data, which suggested that citrate synt hase was the source of a toxic metabolite.