Photoinduced cyclic electron transfer in Rhodocyclus tenuis cells: Participation of HiPIP or cyt c(8) depending on the ambient redox potential

We demonstrate the participation of a cytochrome cs and a high-potential iron-sulfur protein (HiPIP) in the photoinduced electron transfer in whole cells of Rhodocyclus tenuis depending on the redox state or background continuous illumination; At high redox potentials (above 350 mV) or under a strong background illumination (5 W m(-2)), the cytochrome c(8) acts as the physiological electron donor to the photo-oxidized high-potential hemes of the tetraheme cytochrome bound to the reaction center. For redox potentials ranging from 200 to 310 mV or under weak background illumination (1.25 W m(-2)), the electron carrier is the HiPIP. The electron transfer between cyt c(8) and HiPIP and the tetraheme cytochrome has half-times of 300 and 480 mu s, respectively. A slow electrogenic phase of the membrane potential is linked to their rereduction. This phase is sensitive to a specific inhibitor of the cyt bc(1) complex, indicating involvement of cyt c(8) and HiPIP in the photoinduced cyclic electron transfer at these two redox conditions.