Supramolecular organization of the photosynthetic chain in chromatophores and cells of Rhodobacter sphaeroides

Flash-induced kinetics of the membrane potential increase related to electron transfer within the cytochrome (cyt) b/c(1) complex (Phase III) and that of c(1) + c(2) reduction have been measured as a function of myxothiazol concentration in isolated chromatophores and whole cells of Rhodobacter sphaeroides. Upon addition of nonsaturating concentrations of myxothiazol, kinetics of Phase III display two phases, Phase IIIa and Phase IIIb. The amplitude of Phase IIIa, completed in about 10 ms, is proportional to the fraction of non-inhibited cyt b/c(1) complexes, while its half-time is independent of the myxothiazol concentration. A fast cyt c(1) + c(2) reduction phase is correlated to Phase IIIa. These experiments demonstrate that, in a range of time of several ms, diffusion of cyt c(2) is restricted to domains formed by a supercomplex including two reaction centers (RCs) and a single cyt b/c(1) complex, as proposed by Joliot et al. (Biochim Biophys Acta 975: 336-345, 1989). Phase IIIb, completed in about 100 ms, shows that positive charges or inhibitor molecules are exchanged between supercomplexes in this range of time. These exchanges occur within domains including 2 to 3 supercomplexes, i.e. in membrane domains smaller than a single chromatophore. These conclusions apply to both isolated chromatophores and whole cells.