TRANSPORT OF C-4-DICARBOXYLATES BY ANAEROBICALLY GROWN ESCHERICHIA-COLI - ENERGETICS AND MECHANISM OF EXCHANGE, UPTAKE AND EFFLUX

Transport activities for uptake, efflux and exchange of C-4-dicarboxyl ates were observed in anaerobically grown Escherichia coli. All three transport modes were only present in strains containing the transcript ional activator FNR of anaerobic respiration, and were repressed by ni trate and O-2. The kinetic and energetic parameters of C-4-dicarboxyla te transport and the mechanism of the uptake, efflux and exchange reac tions were analyzed in whole cells and in membrane vesicles. Fumarate/ succinate exchange could be characterized as homologous or heterologou s 1:1 counter-exchange, The external substrate was determined as dival ent fumarate(2-) (or succinate(2-)) at pH 6-9, whereas monovalent H-fu marate dominated as the substrate at pH 3-4. The exchange was not inhi bited by dissipation of Delta p or constituents of it (Delta psi or De lta pH). We conclude that this transport mode functions as an electron eutral exchange of C-4-dicarboxylates. The uptake of C-4-dicarboxylate s did not depend on internal counter-substrate and resulted in an accu mulation of the substrate. Similar to antiport, fumarate was accepted in the divalent form at pH values greater than or equal to 6 and in th e monovalent form at pH 3.5-6. The uptake was inhibited by dissipation of Delta p or Delta psi. Artificially imposed Delta pH, Delta psi or fumarate gradients were able to drive fumarate uptake. An involvement of Na+ could not be detected. Thus the uptake is likely to operate as an electrophoretic H+/fumarate symport. Independent of the presence of an external counter-substrate, the substrates were secreted from cell s or membrane vesicles loaded with succinate or fumarate. The efflux w as electrogenic. Energizing the cells or membrane vesicles inhibited e fflux, maximal efflux rates were obtained only after dissipation of De lta p or Delta psi. An imposed K+-diffusion potential (outside positiv e) inhibited succinate excretion. The efflux of succinate from de-ener gized membrane vesicles generated a Delta psi of -70 mV. It is thus su ggested that succinate efflux functions as a H+/succinate symport.