Two modes of interaction between photosynthetic and respiratory electron chains in whole cells of Rhodopseudomonas capsulata

Abstract (1) The inhibition of respiration by continuous or short flashing light has been studied in whole cells of Rhodopseudomonas capsulata , wild type and mutants M 6 and M 7 , which lack the alternative and the cytochrome c 2 oxidases, respectively. Continuous illumination inhibits oxygen uptake for the three strains studied. This inhibition is prevented in the presence of carbonyl cyanide m -chlorophenyl hydrazone (CCCP) (10 μM). (2) Upon excitation by flashes at low frequency (1 Hz), respiration is inhibited after each flash, but stimulated after even flashes for wild-type and M 6 strains. This oscillatory pattern is not influenced by the presence of CCCP. On the other hand, the strain M 7 exhibits no change in respiratory activity under flash excitation unless the flash frequency is higher than 10 Hz. (3) Addition of both antimycin A and low concentration (0.4–1 μM) of CCCP prevents the light inhibition of respiration, whereas addition of these chemicals separately does not impair the light inhibition in agreement with previous results (Cotton, N.P.J., Clark, A.J. and Jackson, J.B. (1983) Eur. J. Biochem. 130, 581–587). This synergistic effect is, however, observed only in wild-type and M 7 strains. For M 6 , addition of both antimycin A and CCCP in a wide range of concentration (1–10 μM) does not impair the inhibition of respiration by light. (4) These results are taken as evidence that two types of light inhibition occur in whole photosynthetic bacteria. The first type, predominant under flashing light, is due to the diversion of electrons from the respiratory chain, at the level of cytochrome c 2 , to the photooxidized reaction center. The second type, prevalent under continuous illumination, is a control of respiration by the photoinduced proton electrochemical gradient at the dehydrogenase level. Finally, in presence of antimycin A, CCCP, and light, the respiratory activity is sustained by the alternative oxidase.