Light-induced oxygen reduction as a probe of electron transport between respiratory and photosynthetic components in membranes of Rhodopseudomonas capsulata

Abstract

Membrane vesicles prepared from photosynthetically grown cells of Rhodopseudomonas capsulata strain M7 are able to perform light-induced oxygen uptake. In contrast, chromatophores from mutant strain M6, lacking the cytochrome b 260-containing pathway of oxygen reduction, are completely devoid of oxygen uptake induced by light. Therefore, cytochrome b 260 (cyt b 260), previously demonstrated to be the alternative oxidase in photosynthetic and aerobic membranes of R. capsulata ( La Monica, R. F., and Marrs, B. L. (1976) Biochim. Biophys. Acta 449, 431–439; Zannoni, D., Melandri, B. A., and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta 449, 386–400), also appears to be involved in light-induced oxygen uptake. Light-driven oxygen reduction activity is inhibited by high concentrations of cyanide (5 × 10 −3 m) and by carbon monoxide in accord with the sensitivity of cyt b 260 to these inhibitors reported in aerobic membranes ( Zannoni, D., Melandri, B. A., and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta 423, 413–430). Further evidence for a direct involvement of the cyt b 260-containing pathway in light-induced oxygen uptake has been obtained by comparing membranes from semiaerobically grown cells to those from photosynthetic cells of R. capsulata M7. The substrate-dependent dark oxidase activity associated with the cyt b 260-containing pathway is 4.6 times higher in semiaerobic than in photosynthetic membranes, and a parallel enhancement of light-induced oxygen uptake is observed. The data presented are in agreement with and extend previous results on the composition and function of the respiratory and photosynthetic apparatus, supporting an association of cytochrome b 260 with both the aerobic and photosynthetic systems present in membranes of R. capsulata. These findings clearly demonstrate that respiratory electron carriers have access to electrons flowing from the photosynthetic reaction center, i.e., the two systems are “electrically connected” in membrane fragments from this organism.