The role of the proton-motive force and electron flow in light-driven solute transport in Rhodopseudomonas sphaeroides.

The initial rate of uptake of alanine was studied in whole cells of Rhodopseudomonas sphaeroides under conditions such that the proton-motive force generated by light-induced cyclic electron transfer, was composed of the membrane potential only. The proton-motive force was varied by varying the light intensity or by pretreatment of the cells with the uncoupler carbonylcyanide p-trifluoromethoxyphenylhydrazone. At constant light intensity a threshold delta psi is required before alanine uptake occurs. Above this threshold value the rate of alanine uptake increases with the delta psi. The threshold value of the delta psi is higher the lower the light intensity. Under conditions of constant delta psi the rate of alanine uptake increases linearly with the light intensity. At low delta psi values a threshold light intensity is required which is higher the lower the delta psi. These results demonstrate that both a proton-motive force and electron transfer are required for alanine uptake.