The minimal photounit of the photosynthetic membranes of the purple non-sulfur bacterium Rhodospirillum rubrum, comprising the reaction center and the light-harvesting complex has been purified and crystd. in two dimensions in the presence of added phospholipids, and subsequently visualized by electron microscopy after neg.-staining. The position of the reaction centers within the light-harvesting ring has been detd. at low resoln. by the application of a new anal. for rotationally disordered identical units (here the reaction centers) within a two-dimensional cryst. lattice comprised of perfectly aligned unit cells (here the light-harvesting complexes). The reaction center was found to preferentially occupy one of four orientations within the light-harvesting complex. The light-harvesting complex appears to be distored to C4 symmetry,thus assuming a squarish shape when visualized by neg. staining. A tentative structural model of the reaction center-light-harvesting complex photounit which fits the exptl. data is proposed. [on SciFinder (R)]