ORGANIZATION OF ELECTRON-TRANSFER COMPONENTS IN RHODOBACTER-SPHAEROIDES FORMA SP DENITRIFICANS WHOLE CELLS

Two pools of cytochrome c(t) can be observed in whole cells of Rhodobacter sphaeroides forma sp. denitrificans upon excitation by continuous light or saturating flashes. The first pool is connected only to the photosynthetic chain. The second one is preferentially coupled to the respiratory and the denitrifying chains. This second pool is in large excess compared to the first when cells are grown under denitrifying and/or aerobic conditions. These two pools equilibrate in less than 50 ms at pH lower than 7.5, but not at higher pH or in the presence of glycerol or divalent cations. For the first pool, the rate of electron transfer between cytochrome c(1) and cytochrome c(2) is not affected by the medium viscosity. Measurements of cytochrome c(t) re-reduction in the presence of subsaturating concentrations of myxothiazol show that a given cytochrome c(2) can only react with a single bc(1) complex. This is interpreted in terms of a supramolecular organization of the photosynthetic electron transfer components. Under conditions where the synthesis of the photosynthetic chain is repressed, i.e., addition of nitrate or dark semi-aerobic conditions, the LHII/LHI ratio decreases. This induces the formation of tubular membranes. Freeze-etching pictures of these tubes show a well-ordered dimeric organization of the membrane proteins.