Interaction between the Respiratory and Photosynthetic Electron Transport Chains of Intact Cells of Rhodopseudomonas capsulata Mediated by Membrane Potential

1 The inhibition of respiration by light and the relief from this inhibition by uncoupling agents have been studied in intact cells of Rhodopseudomonas capsulata . 2 The same concentration range of carbonylcyanide p ‐trifluoromethoxyphenylhydrazone [CF 3 OPhC(CN) 2 ] prevented light‐inhibition of respiration and depressed the membrane potential (ΔΨ) in illuminated, aerobic bacterial suspensions. 3 For a wide range of CF 3 OPhzC(CN) 2 concentration the dependence of the respiration rate on ΔΨ in darkened cells coincided with that in illuminated cells. 4 When present separately, antimycin A and low concentrations of CF 3 OPhzC(CN) 2 had little effect on light‐inhibition of respiration or on ΔΨ in illuminated, aerobic cells, but added together they gave relief from light inhibition and they lowered ΔΨ. 5 In the absence of CF 3 OPhzC(CN) 2 even very low light intensities were sufficient to inhibit respiration and increase ΔΨ. In the presence of CF 3 OPhzC(CN) 2 much higher light intensities were necessary. 6 The results are interpreted as evidence that light‐inhibition of respiration is mediated by ΔΨ. The modification of the response by uncouplers, inhibitors and with light intensity is critically determined by the ionic conductance properties of the membrane in accordance with the findings in [Clark, A. J., Cotton, N. P. J., and Jackson, J. B. (1983) Eur. J. Biochem. 130 , 575–580].