Patterns of electrochemical proton gradient formation by membrane vesicles from an obligately acidophilic bacterium

Isolated membrane vesicles from the obligately acidophilic bacterium Bacillus acidocaldarius generated an electrochemical gradient of protons (delta mu- H+) upon energization with ascorbate-phenazine methosulfate at pH 6.0 or 3.0. At pH 6.0, there was little or no transmembrane pH gradient (delta pH), but a transmembrane electrical potential (delta psi) of ca. -77 mV, positive out, was observed. At pH 3.0, a delta pH equivalent to - 100 mV, acid out, and a delta psi of -73 mV, positive out, were observed upon energization. The total magnitude of the delta mu- H+ was higher than that of whole cells at acid pH, but the very large delta pHs and the reversed delta psi s, i.e., inside positive, that are typical of acidophile cells were not observed in the vesicles. The vesicles exhibited energy-dependent accumulation of alpha-aminoisobutyric acid that was inhibited by both nigericin and valinomycin (plus K+) at pH 3.0 but was inhibited little by nigericin at pH 6.0.