Citrate transport in Salmonella typhimurium: studies with 2-fluoro-L-erythro-citrate as a substrate

The citrate analogue, 2-fluoro-L-erythro-[3,4,5,6-(14)C]citrate was synthesized as a probe for the citrate transport system of Salmonella typhimurium. This analogue was actively transported by an inducible energy-dependent transport system with high affinity for fluorocitrate (Km = 3.3 microM), and this transport system was inhibited competitively by citrate and isocitrate. Fluorocitrate was shown to be a competitive inhibitor of the citrate-binding protein (C protein) of this organism (Ki = 4-5 microM). Analogue resistant mutants were simultaneously defective in fluorocitrate transport as well as the C protein and the affected allele, tctC, was located at 59 units on the S. typhimurium chromosome map. These tctC mutants were shown to be specifically defective in K+-dependent fluorocitrate transport but still retained another system capable of transporting fluorocitrate in the presence of both Na+ and K+.