The role of c-type cytochromes in the photosynthetic electron transport pathway of Rhodobacter capsulatus

(1) Short flash excitation of membrane vesicles of a cytochrome-c2-deficient mutant of Rhodobacter capsulatus (strain MT-G4/S4) led to rapid oxidation of a c-type cytochrome. In redox titrations, the photooxidation of c-type cytochrome was attenuated with a midpoint of approx. +360 mV. Vesicles from a control strain, MT1131, gave similar results. These findings are consistent with those of Prince et al. (Prince, R.C., Davidson, E., Haith, L.E. and Daldal, F. (1986) Biochemistry 25, 5208-5214). (2) In anaerobic intact cells the extent of rapid re-reduction of c-type cytochrome oxidised after a flash was less in MT-G/S4 than in MT1131. Cytochrome c reduction in both strains was inhibited by myxothiazol. The myxothiazol-sensitive component of the electrochromic absorbance change in cells indicated that rapid charge separation through the cytochrome bc1 complex was less extensive after a flash in MT-G4/S4 than in MT 1131. (3) In anaerobic intact cells and in chromatophores of Rb. capsulatus strain MT-GS18, a mutant deficient in both cytochrome c1 and cytochrome c2, flash excitation led to the oxidation of c-type cytochrome. Redox titrations and spectra of chromatophores suggested that this is the same cytochrome as was photooxidized in vesicles of MT-G4/S4 and MT1131. This result is in contrast with earlier findings (Prince, R.C. and Daldal, F. (1987) Biochim. Biophys, Acta 894, 370-378) in which it was reported that no photooxidation of c-type cytochrome occurred in the absence of c1 and c2, and argues against the possibility that cytochrome c1 can rapidly and directly donate electrons to the reaction centre. (4) It is proposed that a previously uncharacterized, membrane-bound c-type cytochrome (Em7 approximately +360 mV) is present in Rb-capsulatus MT1131, in the c2-deficient mutant MT-G4/34 and in the c1/c2-deficient mutant MTGS18. This cytochrome and cytochrome c2 are alternative electron donors to the reaction centre in strain MT1131.