ATP synthesis coupled to light-dependent non-cyclic electron flow in chromatophores of Rhodopseudomonas capsulata

Abstract

Light-dependent non-cyclic electron transport in membranes from aerobic dark-grown cells of Rhodopseudomonas capsulata, i.e., light-induced oxygen uptake, is clearly linked to energy transduction. It is proposed that the open chain performing the electron transfer from exogenous donors such as 2,6-dichlorophenolindophenol (DCIP) or N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) to oxygen includes cytochrome c 2, reaction centre bacteriochlorophyll, ubiquinone molecules and the alternate oxidase (cytochrome b-260). In contrast, cytochrome b-60 previously suggested to act at the branching point of the dual electron transport system of Rps. capsulata (Zannoni, D., Melandri, B.A. and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta 449, 386–400) does not appear to be essential for aerobic photophosphorylation. This conclusion is derived mainly from experiments with Rps. capsulata R126 chromatophores deficient in cytochrome b-60 photoreduction and electron transfer through the cytochrome b- c segment of the chain. Indeed, these mutant membranes present normal rates of light-induced oxygen consumption. Moreover, this activity is coupled to Δμ H + formation which in turn drives ATP synthesis.