Proton pumps in bacterial photosynthesis.

We have previously suggested that the cyclic electron-transport chain in Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata is arranged as two proton pumps operating in series, one pump being associated with the photochemical reaction centre and its immediate secondary donor (cytochrome c2) and acceptor (ubiquinone), and the other with the 'dark' electron transport through which the cycle is completed, from ubiquinone back to cytochrome c2 (Jackson & Crofts, 1971 ; Jackson & Dutton, 1973; Crofts et af. , 1971). Evidence for these two distinct sites has come from several lines of investigation, including kinetic studies of flash-induced redox changes, pH changes, and changes in membrane electrical gradient as indicated by the carotenoid change, and especially the way in which these parameters change as the ambient redox potential is changed (Jackson & Dutton, 1973; Cogdell et a f . , 1972; Dutton & Jackson, 1972; Evans &Crofts, 1974; Crofts etal. , 1975; Crofts, 1974; Prince &Dutton, 1975). In the present paper we discuss extensions of this work in which aspects of both proton pumps are examined further.