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Both the soluble cytochrome c2 and the membrane-bound cytochrome cy act as secondary electron carriers in photoinduced cyclic electron transfer chain of Rhodobacter capsulatus [Jenney and Daldal (1993) EMBO J 12: 1283–1292]. In this work, we have studied the kinetics of electron transfer between these secondary electron donors and the reaction center in intact cells of two mutants, MT-G4/S4 and MT-GS18 deleted in cytochrome c2 and in cytochrome c2 plus cytochrome bc1 complex, respectively. In the MT-G4/S4 mutant, only about one third of the primary electron donor is reduced by cytochrome cy in less than five ms. The remaining fraction is reduced in several seconds, although about 90% of the photoxidized cytochrome cy is reduced in less than 10 ms by the cytochrome bc1 complex. This implies that cytochrome cy is not in thermodynamic equilibrium with the large fraction of primary donors which are slowly reduced. As shown by energy transfer measurements, the reaction centers connected to cytochrome cy and the disconnected reaction centers are localized in the same membrane region. We propose that the movement of cyt cy is restricted to a small membrane domain which includes a single cytochrome bc1 complex. The kinetics of cytochrome cy photooxidation in the MT-G4/S4 mutant in the presence of myxothiazol presents a fast phase (t1/2 ∼ 3 µs) followed by a slower phase (t1/2 ∼ 20 µs). In the case of the double mutant MT-GS18, the kinetics of electron transfer between cytochrome cy and the reaction center is highly multiphasic and much slower than those observed for the MT-G4/S4 mutant. In particular, the amplitude of the fast phase is decreased by more than a factor 2 and the 20-µs phase is not observed. This implies an important structural role of the cytochrome bc1 complex in the interaction between reaction center and cytochrome cy, and their formation in supercomplex. The more problable stoichiometry of electron carriers in this supercomplex is 2 reaction centers, 2 cytochrome cy and 1 cytochrome bc1 complex.