Heterogeneous Pools of Cytochrome c₂ in Photo-Denitrifying Cells of Rhodobacter sphaeroides forma sp. denitrificans

Abstract

When cells of the denitrifying phototrophic bacterium Rhodobacter sphaeroides forma sp. denitrificans were grown anaerobically under illumination in the presence of nitrate, the content of photosynthetic reaction centers per cellular protein was less than that in cells grown photosynthetically without nitrate under the same light intensity. The contents of cytochromes c₁ and c₂, which work in both photosynthetic and denitrifying electron transport systems, were almost constant, being independent of the presence of nitrate during growth. Consequently, the ratio of cytochromes c₁ and c₂ to the reaction center was more than three in the photo-denitrifying cells, whereas it was close to one in the photosynthetic cells under light-limiting conditions. In spite of the excess of cytochromes c₁+c₂, over the reaction center in the photo-denitrifying cells, all cytochromes c₁+ c₂, were oxidized by illumination within hundreds of milliseconds in the presence of antimycin. When glycerol was added to increase the viscosity in the periplasm, biphasic oxidation of cytochromes c₁+c₂ was apparent in the photo-denitifying cells with repetitive flashes. The fast phase oxidation, which took place instantaneously (<1 ms) after the first and second flashes, showed a similar pattern to the oxidation in the light-limiting photosynthetic cells. The rate of the slow phase oxidation was sensitive to viscosity and was thought toreflect a diffusion-controlled second-order reaction between cytochrome c₂ and the reaction center. The biphasic oxidation of cytochromes c₁+ c₂suggests that these cytochromes exist in the photo-denitrifying cells as two different pools in relation to the reaction center.