Fast oxidation of the primary electron acceptor under anaerobic conditions requires the organization of the photosynthetic chain of Rhodobacter sphaeroides in supercomplexes

The kinetics of reoxidation of the primary acceptor Qa has been followed by measuring the changes in the fluorescence yield induced by a series of saturating flashes in intact cells of Rhodobacter sphaeroides in anaerobic conditions. At 0 °C, about half of Qa− is reoxidized in about 200 ms while reoxidation of the remaining fraction is completed in several seconds to minutes. The fast phase is associated with the transfer of ubiquinone formed at site Qo of the cytochrome bc1 complex while the slowest phase is associated with the diffusion of ubiquinone present in the membrane prior to the flash excitation. The biphasic kinetics of Qa− oxidation is interpreted assuming that the electron chain is organized in supercomplexes that associate two RCs and one cyt bc1 complex, which allows a fast transfer of quinone formed at the level of cyt bc1 complex to the RCs. In agreement with this model, the fast phase of Qa− reoxidation is inhibited by myxothiazol, a specific inhibitor of cyt bc1. The PufX-deleted mutant displays only the slowest phase of Qa− oxidation; it is interpreted by the lack of supramolecular organization of the photosynthetic chain that leads to a larger average distance between cyt bc1 and RCs.