Evidence is presented that lactose uptake into whole cells of Escherichia coli occurs by symport with a single proton over the range of external pH 6.5–7.7. The proton/lactose stoicheiometry has been measured directly over this pH range by comparison of the initial rates of proton and lactose uptake into anaerobic resting cell suspensions of E. coli ML308. Further, the relationship between the protonmotive force and lactose accumulation has been studied in E. coli ML308-225 over the range of external pH 5.9–8.7. At no point was the accumulation of the beta-galactoside in thermodynamic equilibrium with the protonmotive force. It is concluded that the concentration of lactose within the cell is governed by kinetic factors rather than pH-dependent changes in the proton/substrate stoicheiometry. The relevance of these findings to the model of pH-dependent proton/substrate stoicheiometries derived from studies with E. coli membrane vesicles is discussed.

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