Bacterial photosynthetic phosphorylation under conditions of limited electron flow.

In the absence of antimycin interpretation of the absorbance changes is further complicated by the relaxation kinetics associated with electron flow through the site sensitive to antimycin. Inspection of the time-resolved spectra and the uncompensated traces (not shown) reveals an obvious cytochrome-c change and a less obvious, but discernible, cytochrome-b change. In the compensated spectra [Fig. 2(d); subtraction as for Fig. 2(c)] the contributions from the cytochrome b are more readily seen, and separate spectra for cytochrome c (developing between 0.12 and 0.6ms, relaxing between 0.6 and 2.2ms) can be resolved over appropriate time domains. Individual traces at 550-540 and 560-570nm (Fig. 26) show the difference kinetics at wavelength pairs commonly used tomeasure the c and b cytochromes. There is some contribution to changes measured at the latter wavelength pair, of fast components from reaction-centre and cytochrome-c changes, but the dominant change is that due to cytochrome b. The timeresolved spectra and traces above support our previous interpretation of the kinetics observed at these wavelength pairs (Evans & Crofts, 1974; Crofts et al., 1975). They are difficult to reconcile with any ‘Q-cycle’ type of mechanism (Mitchell, 1975; Crofts et al., 1975; Prince & Dutton, 1976), in which electrons from a semiquinone reduce cytochrome b following reduction of cytochrome c by the dihydroquinone. They are compatible with a linear electron-transport chain in which electrons from cytochrome 6 reduce cytochrome c through intermediate steps which include the site of inhibition by antimycin.