The effect of transfer from low to high light intensity on electron transport in Rhodospirillum rubrum membranes

Abstract

The effects of transfer from low to high light intensity on membrane bound electrontransport reactions of Rhodospirillum rubrum were investigated. The experiments were performed with cultures which did not form bacteriochlorophyll (Bchl) for about two cell mass doublings during the initial phase of adaptation to high light intensity. Lack of Bchl synthesis causes a decrease of Bchl contents of cells and membranes. Also, the cellular amounts of photosynthetically active intracytoplasmic membranes decrease. In crude membrane fractions containing both cytoplasmic and intracytoplasmic membranes the initial activities of NADH oxidizing reactions increase only slightly (about 1.2 times) per protein, but the initial activities of succinate oxidizing reactions decrease (multiplied by a factor of 0.7). On a Bchl basis activities of NADH oxidizing reactions increase 3.4 times while activities of succinate dependent reactions increase 1.9 times. With isolated intracytoplasmic membranes activities of NADH as well as succinate dependent reactions increase to a comparable extent on a Bchl basis (about 1.8 times) and stay nearly constant on a protein basis. Cyochrome c oxidase responds like succinate dependent reactions. The data indicate that in cells growing under the conditions applied NADH oxidizing electrontransport systems are incorporated into both, cytoplasmic and intracytoplasmic membranes, while incorporation of succinate oxidizing systems is confined to intracytoplasmic membranes only. Activities of photophosphorylation and succinate dependent NAD+ reduction in the light increase per Bchl about 1.8 times. On a Bchl basis increases of the fast light induced "on" reactions at 422 nm and increases of soluble cytochrome c2 levels are comparable to increases of photophosphorylations and succinate dependent activities. But increases of slow light "off" reactions at 428 nm and of beta-type cytochrome levels become three times greater then increases of cytochrome c2 reactions and levels. These results infer that although electrontransport reactions of intracytoplasmic membranes change correlated to each other, Bchl, cytochrome c2 and beta-type cytochromes cellular levels are independent of each other. Furthermore, the data indicate that cytochrome c2 rather than beta-type cytochrome is involved with steps rate limiting for photophosphorylation.