An alkaline-tolerant bacterium, which grew on various carbohydrates between pH 6 and 9-5, was isolated from soil and identified as . Lactose-grown organisms exhibited a transmembrane pH gradient (ΔpH) of −47 mV at pH 6-6, but no ΔpH at pH 9-0. The transmembrane electrical potential (Δψ) was −66 mV at pH 6.6 and −115mV at pH 90. Thus the total protonmotive forces at the two pH values were essentially the same. Lactose-grown organisms transported thiomethyl β-D-galactopyranoside (TMG) at pH 6.6 and at pH 9.0, but transport at the alkaline pH was dependent upon addition of ascorbate/ -tetramethyl--phenylenediamine (TMPD) or preincubation with lactose. In the presence of ascorbate/TMPD, the TMG transport system exhibited similar kinetics and substrate specificities at pH 6.6 and pH 9.0, and resulted in accumulation of chemically unmodified TMG to a concentration approximately 180 times greater than the external concentration. Experiments in which a diffusion potential was generated in starved organisms or in which organisms were treated with nigericin indicated a lack of correlation between the rate of TMG uptake and the magnitude of Δψ. By contrast, the rate of TMG uptake correlated with cellular ATP levels in organisms incubated at different pH values and in organisms treated with -dicyclohexylcarbodiimide, arsenate or nigericin.


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