DctA- and Dcu-independent transport of succinate in Escherichia coli: Contribution of diffusion and of alternative carriers

Quintuple mutants of Escherichia coli deficient in the C₄-dicarboxylate carriers of aerobic and anaerobic metabolism (DctA, DcuA, DcuB, DcuC, and the DcuC homolog DcuD, or the citrate/succinate antiporter CitT) showed only poor growth on succinate (or other C₄-dicarboxylates) under oxic conditions. At acidic pH (pH 6) the mutants regained aerobic growth on succinate, but not on fumarate. Succinate uptake by the mutants could not be saturated at physiological succinate concentrations (≤5 mM), in contrast to the wild-type, which had a K_m for succinate of 50 μM and a V_max of 35 U/g dry weight at pH 6. At high substrate concentrations, the mutants showed transport activities (32 U/g dry weight) comparable to that of the wild-type. In the wild-type using DctA as the carrier, succinate uptake had a pH optimum of 6, whereas succinate uptake in the mutants was maximal at pH 5. In the mutants succinate uptake was inhibited competitively by monocarboxylic acids. Diffusion of succinate or fumarate across phospholipid membranes (liposomes) was orders of magnitude slower than the transport in the wild-type or the mutants. The data suggest that mutants deficient in DctA, DcuA, DcuB, DcuC, DcuD (or CitT) contain a carrier, possibly a monocarboxylate carrier, which is able to transport succinate, but not fumarate, at acidic pH, when succinate is present as a monoanion. Succinate uptake by this carrier was inhibited by addition of an uncoupler. Growth by fumarate respiration (requiring fumarate/succinate antiport) was also lost in the quintuple mutants, and growth was not restored at pH 6. In contrast, the efflux of succinate produced during glucose fermentation was not affected in the mutants, demonstrating that, for succinate efflux, a carrier different from, or in addition to, the known Dcu and CitT carriers is used.