Formate transport,growth inhibition and the membrane protonmotive force in two methylotrophs (T15 and L3)

Summary Formate transport and the effect of formate on growth and the membrane protonmotive force were studied in two ribulose-monophosphate-type obligate methylotrophs (bacterial strains T15 and L3). Formate was accumulated very fast by the membrane pH according to the general transport mechanism of short-chain organic acids. Formate accumulation was reduced or abolished by a number of factors (protonophores, high extracellular pH, cell-starvation conditions) that reduced or abolished the pH. Formate transport was accompanied by removal of protons from the medium by the cells. Since protons are released by the cells upon substrate oxidation, the stoichiometry of proton uptake upon formate transport could not be directly determined, although data suggest that formate is cotransported with one proton. The net rate of proton removal from the medium by the cells due to formate transport and oxidation increased with increasing formate concentrations or decreasing medium-pH values. The membrane protonmotive force of strain T15 was also studied as a function of the pH. High formate concentrations (of 100 to 400 mM) reduced the membrane pH by ca. 20 to 60% and the growth rate by ca. 20 to 100% for both strains but to a different extent. For example, 400 mM formate inhibited growth by ca. 60% in strain T15 and by 100% in strain L3. The nature of growth inhibition by formate is discussed in some detail.