Photo-induced cyclic electron transfer involving cytochrome bc(1) complex and reaction center in the obligate aerobic phototroph Roseobacter denitrificans

Flash-induced redox changes of b-type and c-type cytochromes have been stud ied in chromatophores from the aerobic photosynthetic bacterium Roseobacter denitrificans under redox-controlled conditions. The flash-oxidized primar y donor P+ of the reaction center (RC) is rapidly re-reduced by heme H-1 (E -m,E-7 = 290 mV), heme H-2 (E-m,E-7 = 240 mV) or low-potential hemes L-1/L- 2 (E-m,E-7 = 90 mV) of the RC-bound tetraheme, depending on their redox sta te before photoexcitation. By titrating the extent of flash-induced low-pot ential heme oxidation, a midpoint potential equal to -50 mV has been determ ined for the primary quinone acceptor Q(A). Only the photooxidized heme H-2 is re-reduced in tens of milliseconds, in a reaction sensitive to inhibito rs of the bc(1) complex, leading to the concomitant oxidation of a cytochro me c spectrally distinct from the PC-bound hemes. This reaction involves cy tochrome c(551) in a diffusional process. Participation of the bc(1) comple x in a cyclic electron transfer chain has been demonstrated by detection of flash-induced reduction of cytochrome b(561), stimulated by antimycin and inhibited by myxothiazol. Cytochrome b(561), reduced upon flash excitation, is re-oxidized slowly even in the absence of antimycin. The rate of reduct ion of cytochrome b561 in the presence of antimycin increases upon lowering the ambient redox potential, most likely reflecting the progressive prered uction of the ubiquinone pool. Chromatophores contain approximately 20 ubiq uinone-10 molecules per RC. At the optimal redox poise, approximately 0.3 c ytochrome b molecules per RC are reduced following flash excitation. Cytoch rome b reduction titrates out at E-h < 100 mV, when low-potential heme(s) r apidly re-reduce P+ preventing cyclic electron transfer. Results can be rat ionalized in the framework of a Q-cycle-type model.