ELECTRON-TRANSFER BETWEEN PRIMARY AND SECONDARY DONORS IN RHODOSPIRILLUM-RUBRUM - EVIDENCE FOR A DIMERIC ASSOCIATION OF REACTION CENTERS

Light-induced oxidation of the primary electron donor P and of the secondary donor cytochrome c2 was studied in whole cells of Rhodospirillum rubrum in the presence of myxothiazole to slow down their reduction. 1. The primary and secondary electron donors are close to thermodynamic equilibrium during continuous illumination when the rate of the electron transfer is light-limited. This implies a long-range thermodynamic equilibration involving the diffusible cytochrome c2. A different behavior is observed with Rhodobacter sphaeroides R26 whole cells, in which the cytochrome c2 remains trapped within a supercomplex including reaction centers and the cytochrome b/c complex [Joliot, P., et al. (1989) Biochim. Biophys. Acta 975, 336–345]. 2. Under weak flash excitation, the reduction kinetics of the photooxidized primary donor are nearly exponential with a half-time in the hundred microseconds time range. 3. Under strong flash excitation, the reduction of the photooxidized primary donor follows a second-order kinetics. About half of the photooxidized primary donor is reduced in a few milliseconds while the remainder stays oxidized for hundreds of milliseconds despite an excess of secondary donors in their reduced form. The flash intensity dependence of the amplitude of the slow phase of P+ reduction is proportional to the square of the fraction of reaction centers that have undergone a charge separation. These results are correctly described with a model in which (1) reaction centers are associated in dimers, (2) the affinity of cytochrome c2 for the reaction center is low when the cytochrome c2 is reduced but high when oxidized, and (3) electrostatic or steric interactions prevent the binding of two cytochrome c2 molecules on the same dimer P-P. © 1990, American Chemical Society. All rights reserved.