Supramolecular organization of the photosynthetic apparatus of Rhodobacter sphaeroides

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 cytochrome bc(1) complex to two reaction centers, and similar to 24 bacteriochlorophyll 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 Angstrom and gamma = 103 degrees) contains an elongated S-shaped supercomplex presenting a pseudo-2-fold symmetry. Comparison with density maps of isolated 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 Angstrom in external diameter, facing each other on the open side and enclosing 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 complexes. In particular, the organization of the light-harvesting complexes in C-shaped structures ensures an efficient exchange of ubihydroquinone/ubiquinone between the reaction center and the cytochrome bc(1) complex.