Evidence for regulation in vivo of the ATP synthase in intact cells of the photosynthetic bacterium Rhodopseudomonas capsulata

(1) The cytoplasmic membrane potential (Δψ) of intact cells of Rhodopseudomonas capsulata, measured either from the uptake of butyltriphenylphosphonium cation or from the electrochromic carotenoid band shift, increased upon illumination (negative on the cytoplasmic side) and then, within the next 20 s, partly declined while the light was still on. In the presence of the F₀ inhibitor venturicidin the light-induced Δψ was increased by 30% and the partial decline was abolished. (2) From the ionic current/Δψ curves for the bacterial membranes it was concluded that the slow, partial decline of Δψ after the onset of illumination was the result of an increase in membrane conductance. The conductance increase seen in the ionic current/Δψ curves was blocked by venturicidin suggesting that it was caused by increased proton flux through the ATP synthase. (3) Analysis of the light-induced changes in adenine nucleotide levels in intact bacterial cells showed that the apparent increase in ATP synthase activity was not the result of a decrease in phosphorylation potential. The data were consistent with either an increase in the catalytic activity of the ATP synthase or with an increase in H⁺ flux through the enzyme without a proportionate increase in the rate of phosphorylation (increased ‘slip’). (4) This slow change in the properties of the ATP synthase, as judged by the venturicidin-sensitive partial decline of Δψ, required a minimum initial value of Δψ. When Δψ was reduced, either by decreasing the actinic light intensity or by adding carbonylcyanide trifluoromethoxyphenylhydrazone the partial decline in Δψ was abolished. (5) The slow change in ATP synthase properties reversed upon darkening the bacterial cell suspension. A second illumination period shortly after the first elicited a smaller initial Δψ and a smaller Δψ decline. The relaxation of the ATP synthase in the dark was measured from the dependence of the initial increase in Δψ after the second illumination period upon the dark-time between the two illumination periods.