Supramolecular organization of the photosynthetic apparatus of Rhodobactersphaeroides

Native tubular membranes were purified from the purple non-sulfur bacterium Rhodobacter sphaeroides. These tubular structures contain all the membrane components of the photosynthetic apparatus, in the relative ratio of one c ytochrome bc(1) complex to two reaction centers, and similar to 24 bacterio chlorophyll molecules per reaction center. Electron micrographs of negative -stained membranes diffract up to 25 Angstrom and allow the calculation of a projection map at 20 Angstrom. The unit cell (a = 198 Angstrom, b = 120 A ngstrom and gamma = 103 degrees) contains an elongated S-shaped supercomple x presenting a pseudo-2-fold symmetry. Comparison with density maps of isol ated reaction center and light-harvesting complexes allowed interpretation of the projection map. Each supercomplex is composed of light-harvesting 1 complexes that take the form of two C-shaped structures of similar to 112 A ngstrom in external diameter, facing each other on the open side and enclos ing the two reaction centers. The remaining positive density is tentatively attributed to one cytochrome bc(1) complex. These features shed new light on the association of the reaction center and the light-harvesting complexe s. In particular, the organization of the light-harvesting complexes in C-s haped structures ensures an efficient exchange of ubihydroquinone/ubiquinon e between the reaction center and the cytochrome bc(1) complex.