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

Light-induced electron transfer has been studied in whole cells of Rhodopseudomonas viridis. No photooxidation of the low-potential hemes of the RC bound cytochrome is observed under the more reducing physiological conditions used in this study, i.e. anaerobiosis. On the other hand, photooxidation of the high-potential heme c556 of the RC-bound cytochrome is observed 10 mus after flash excitation for cells placed both in aerobic or anaerobic conditions. The photooxidized c556 is re-reduced by the soluble cytochrome c2. This electron transfer occurs with an half-time of 110 mus and 40 mus under aerobic and anaerobic conditions, respectively. The half-time of re-reduction of the soluble cytochrome c2 is also dependent on the redox state of the cells (25 ms and 8 ms under aerobiosis and anaerobiosis, respectively). This re-reduction process is inhibited by addition of 1 mM myxothiazol or stigmatellin, showing that the bc1 complex is involved in this reaction. Participation of the bc1 complex in the cyclic electron transfer is also confirmed by the observation of photooxidation of a cytochrome b under anaerobic conditions. Varying the flash intensity or the viscosity of the medium 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 strongly affected under such conditions. We conclude from this series of experiments that the soluble cytochrome c2 forms a stable complex with the RC tetraheme cytochrome. Once oxidized, the cytochrome c2 diffuses and is re-reduced via a cyclic electron transfer involving the bc1 complex.