Different mechanisms of the binding of soluble electron donors to the photosynthetic reaction center of Rubrivivax gelatinosus and Blastochloris viridis

The tetraheme cytochrome subunits of the photosynthetic reaction centers (R Cs) in two species of purple bacteria, Rubrivivax gelatinosus and Blastochl oris (Rhodopseudomonas) viridis, were compared in terms of their capabiliti es to bind different electron-donor proteins. The wild-type RCs from both s pecies and mutated forms of R gelatinosus Res (with amino acid substitution s introduced to the binding domain for electron-donor proteins) were tested for their reactivity with soluble cytochromes and high potential iron-sulf ur protein. Cytochromes from both species mere good electron donors to the B. viridis RC and the R. gelatinosus RC, The reactivity in the R. gelatinos us RC showed a clear dependence on the polarity of the charges introduced t o the binding domain, indicating the importance of the electrostatic intera ctions. In contrast, high potential iron-sulfur protein, presumed to operat e according to the hydrophobic mechanism of binding, reacted significantly only with the R gelatinosus RC. Evolutionary substitution of amino acids in a region of the binding domain on the cytochrome subunit surface probably caused the change in the principal mode of protein-protein interactions in the electron-transfer chains.