THE PHOTOINDUCED CYCLIC ELECTRON-TRANSFER IN WHOLE CELLS OF RHODOPSEUDOMONAS-VIRIDIS

Light-induced electron transfer has been studied in whole cells of Rho dopseudomonas viridis. No photooxidation of the low-potential hemes of the RC bound cytochrome is observed under the more reducing physiolog ical conditions used in this study, i.e. anaerobiosis. On the other ha nd, photooxidation of the high-potential heme c556 of the RC-bound cyt ochrome is observed 10 mus after flash excitation for cells placed bot h in aerobic or anaerobic conditions. The photooxidized c556 is re-red uced by the soluble cytochrome c2. This electron transfer occurs with an half-time of 110 mus and 40 mus under aerobic and anaerobic conditi ons, respectively. The half-time of re-reduction of the soluble cytoch rome c2 is also dependent on the redox state of the cells (25 ms and 8 ms under aerobiosis and anaerobiosis, respectively). This re-reductio n process is inhibited by addition of 1 mM myxothiazol or stigmatellin , showing that the bc1 complex is involved in this reaction. Participa tion of the bc1 complex in the cyclic electron transfer is also confir med by the observation of photooxidation of a cytochrome b under anaer obic conditions. Varying the flash intensity or the viscosity of the m edium did not affect the half-time of the oxidation of cytochrome c2. On the other hand, the rate of re-reduction of cytochrome c2 is strong ly affected under such conditions. We conclude from this series of exp eriments that the soluble cytochrome c2 forms a stable complex with th e RC tetraheme cytochrome. Once oxidized, the cytochrome c2 diffuses a nd is re-reduced via a cyclic electron transfer involving the bc1 comp lex.