Reconstitution of photosynthetic electron transport and photophosphorylation in cytochrome- c2-deficient membrane preparation of Rhodopseudomonas capsulata

Abstract

Cytochrome c 2 was removed by washing from heavy chromatophores prepared from Rhodopseudomonas capsulata cells. The easy removal of the cytochrome could indicate that it was attached on the outside of the membrane. Therefore, the membrane was probably oriented inside out in relation to the membrane of regular chromatophores, from which cytochrome c 2 could not be removed. Washing of the heavy chromatophores caused loss of photphosphorylation activity. The activity was restored to the resolved heavy chromatophores by the supernatant obtained during the washing or by the native cytochrome c 2, which was found to be the active component in this supernatant. The activity could not be restored by other c-type cytochromes. Ascorbate, which enhanced photophosphorylation activity in the heavy chromatophores at the optimal concentration of 8 m m, restored this activity to the washed heavy chromatophores, but at an optimum concentration of 50 m m. Cytochrome c 2 and dichlorophenol indophenol reduced the optimum of the ascorbate concentration to 7 m m. This might indicate that the effect of ascorbate is mediated through cytochrome c 2. Washing the heavy chromatophores caused 70% loss of the light-induced electron transport from ascorbate and from ascorbate-reduced dichlorophenol indophenol to O 2. However, this effect was only observed with the lower concentrations of ascorbate and the dye. The activity was restored either by the supernatant obtained from the washing or by various c-type cytochromes, reduced by ascorbate. Washing the heavy chromatophores did not affect succinate oxidation in the dark. It is suggested that cytochrome c 2 is one of the cytochromes catalyzing the photosynthetic cyclic electron transport, as has been seen from its high specificity in the reconstitution experiments. Light can induce oxidation of various c-type cytochromes and other redox reagents. However, reduction was specific for cytochrome c 2 from Rps. capuslata, since it was the only one which could be both reduced and oxidized as required from a component which is part of a cyclic electron transport chain. It is also suggested that cytochrome c 2 was not part of the succinate oxidase system.