The induction kinetics of bacterial photophosphorylation. Threshold effects by the phosphate potential and correlation with the amplitude of the carotenoid absorption band shift.

1. ATP synthesis (monitored by luciferin-luciferase) can be elicited by a single turnover flash of saturating intensity in chromatophores from Rhodopseudomonas capsulata, Kb1. The ATP yield from the first to the fourth turnover is strongly influenced by the phosphate potential: at high phosphate potential (-11.5 kcal/mol) no ATP is formed in the first three turnovers while at lower phosphate potential (-8.2 kcal/mol) and the yield in the first flash is already one half of the maximum, which is reached after 2-3 turnovers. 2. The response to ionophores indicates that the driving force for ATP synthesis in the first 20 turnovers is mainly given by a membrane potential. The amplitude of the carotenoid band shift shows that during a train of flashes an increasing delta psi is built up, which reaches a stationary level after a few turnovers; at high phosphate potential, therefore, more turnovers of the same photosynthetic unit are required to overcome an energetic threshold. 3. After several (six to seven) flashes the ATP yield becomes constant, independently from the phosphate potential; the yield varies, however, as a function of dark time (td) between flashes, with an optimum for td = 160-320 ms. 4. The decay kinetics of the high energy state generated by a long (125 ms) flash have been studied directly measuring the ATP yield produced in post-illumination by one single turnover flash, under conditions of phosphate potential (-10 kcal/mol), which will not allow ATP formation by one single turnover. The high energy state decays within 20 s after the illumination. The decay rate is strongly accelerated by 10(-8) M valinomycin. 5. Under all the experimental conditions described, the amplitude of the carotenoid signal correlates univocally with the ATP yield per flash, demonstrating that this signal monitores accurately an energetic state of the membrane directly involved in ATP synthesis. 6. Although values of the carotenoid signal much larger than the minimal threshold are present, relax slowly, and contribute to the energy input for phosphorylation, no ATP is formed unless electron flow is induced by a single turnover flash. 7. The conclusions drawn are independent from the assumption that a delta psi between bulk phases is evaluable from the carotenoid signal.