Energy Coupling of the β-Methylgalactoside Transport System of Escherichia coli

Energy coupling for the concentrative uptake of galactose via the β-methylgalactoside transport system occurs via an intermediate produced during oxidative phosphorylation but before ATP formation. The membrane-bound ATPase, necessary for ATP formation during oxidative phosphorylation, is not obligatory for energy coupling of transport but can, under anaerobic conditions, accomplish energy coupling by ATP hydrolysis. These conclusions are supported by the following findings: (a) galactose uptake in intact cells can be stimulated by substrates of the respiratory chain and inhibited by inhibitors of the respiratory chain and by uncouplers of oxidative phosphorylation; (b) cells made anaerobic for the transport assay are fully transport active; (c) a mutant defective in the membrane-bound ATPase has lost transport activity unless supplied with exogenous d-lactate, and the additional presence of arsenate completely blocks the stimulation by d-lactate; (d) a mutant defective in the respiratory chain due to a defect in ubiquinone biosynthesis still exhibits transport activity, which cannot be stimulated by d-lactate.