A requirement for ATP for beta-galactoside transport by Bacillus alcalophilus

Lactose-grown cells of Bacillus alcalophilus actively transported methylthio-beta, D-galactoside (TMG) in a range of pH values from 7.5 to 10.5 with a pH optimum at 8.5. The TMG was accumulated in a chemically unmodified form, and cell extracts failed to catalyze either ATP or P-enolpyruvate-dependent phosphorylation of TMG. At pH 8.5, the lactose-grown cells exhibited a transmembrane proton gradient (deltapH) of 1.38 units, interior acid, and a transmembrane electrical potential (delta psi) of -132 mV. Accordingly, the total protonmotive force at this pH was very low, -51mV. Several lines of evidence indicate that the protonmotive force or delta psi did not directly energize TMG transport but, rather, that ATP was directly required: (a) in cells treated with arsenate so that the delta psi was unaffected and cellular ATP levels were markedly lowered, TMG transport was inhibited in proportion to the reduction of cellular ATP, while electrogenic alpha-aminoisobutyric acid transport was not; (b) when a valinomycin-induced potassium diffusion potential was established in starved cells, alpha-aminoisobutyric acid transport, but not TMG transport, was stimulated; and (c) in a series of experiments in which the delta psi was rapidly abolished by treatment with gramicidin, ATP levels declined slowly and the rate of TMG transport correlated directly with ATP levels rather than with the delta psi. Consumption of cellular ATP concomitant with TMG transport could be demonstrated.