Photo-induced cyclic electron transfer operates in frozen cells of Rhodobacter sphaeroides

The three phases of the flash-induced membrane potential rise were measured in Rhodobacter sphaeroides, at temperatures between 16 degrees C and -30 degrees C, in the liquid or the frozen state. We show that phase II, which is myxothiazol-insensitive, includes phase IIa and phase IIb, completed at -8 degrees C in about 2 ms and 10 ms, respectively. Phase IIa is very likely associated with the protonation of the doubly-reduced quinone acceptor Q(B)(-) (Drachev, L.A., Mamedov, A., Mulkidjanian, A.Y., Semenov, A.Y., Shinkarev, V.P., and Verkhovsky, M.J. (1988) FEES Lett. 233, 315-318); phase IIb is associated with the oxidation of cytochrome c(2), (Jackson, J.B. and Dutton, P.L., 1973, Biochim. Biophys. Acta 325, 102-113). Freezing the sample does not modify the kinetics of phase IIa but slows down phase IIb by a factor of 2. The amplitude of phase III, which is exclusively related to electron and proton transfer within the cytochrome b/c(1) complex, is temperature-independent between room temperature and -16 degrees C. In the frozen state, the rate of phase III is mainly limited by the electron transfer from cytochrome c(2) to the reaction centers and not by the movement of cytochrome c(2) between the two membrane complexes. These results are interpreted assuming that one cytochrome c(2) is trapped in a supercomplex formed by the association of two reaction centers and one cytochrome b/c(1) complex.