Coupled transport of citrate and magnesium in Bacillus subtilis.

Citrate transport in Bacillus subtilis is dependent on the presence of Mgz+. Other divalent cations like MnZ+, Co2+, or Ni2+ can replace magnesium ions. 2*Mg2+ or j4Mn2+ are taken up by the cells simultaneously with the uptake of citrate. Cells which are not induced for citrate transport do not show the citrate-dependent uptake of either 64Mn2+ or 2sMg2+. Citrate transport and the citrate-dependent transport of a divalent cation are inhibited by the uncoupling agent m-chlorophenyl carbonylcyanidehydrazone. Therefore, using this uncoupler, it is possible to distinguish between the citrate-independent binding of divalent cations to the cell surface and the citrate-dependent transport of these ions across the cytoplasmic membrane. The kinetic constants of citrate uptake and of citrate-dependent Mg2f transport are very similar. Citrate transport may be stoichiometrically reduced by limiting the concentrations of Mg2+. Our results suggest that a complex of citrate and Mg2+ is the transported substrate. The coupled transport of citrate and Mg2f does function normally in a Cozf-resistant mutant of B. subtilis which is defective in another transport system of high affinity for Mg2+. The eRlux of phosphate from B. subtilis cells during uptake of citrate is too slow to account for electroneutrality of the system. No cotransport of Kf and citrate can be demonstrated. Valinomycin inhibits the citrate-Mg2+ transport activity, which can be restored by the addition of external K+. In a potassium-retention mutant of B. subtilis, citrate transport activity decreases in parallel with the loss of K+ from these cells. Thus a high concentration of K+ inside the cells is necessary for the proper function of the citrateMg*+ transport system.