ARE THE LIGHT-HARVESTING-I COMPLEXES FROM RHODOSPIRILLUM-RUBRUM ARRANGED AROUND THE REACTION-CENTER IN A SQUARE GEOMETRY

The basic photosynthetic unit containing the reaction centre and the l ight-harvesting I complex (RC-LHI) of the purple non-sulphur bacterium Rhodospirillum rubrum was purified and reconstituted into two-dimensi onal (2D) membrane crystals. Transmission electron microscopy using co nventional techniques and cryoelectron microscopy of the purified sing le particles and of 2D crystals yielded a projection of the RC-LHI com plex at a resolution of at least 1.6 nm. In this projection the LHI ri ng appears to have a square symmetry and packs in a square crystal lat tice. The square geometry of the LHI ring was observed also in images of single isolated particles of the RC-LHI complex. However, although the LHI units are packed identically within the crystal lattice, a new rotational analysis developed here showed that the reaction centres t ake up one of four possible orientations within the ring. This fourfol d disorder supports our interpretation of a square ring symmetry and s uggests that a hitherto undetected component may be present within the photosynthetic unit. (C) 1998 Academic Press.