8.5 Å PROJECTION MAP OF THE LIGHT HARVESTING COMPLEX I FROM RHODO SPIRILLUM RUBRUM REVEALS A RING COMPOSED OF 16 SUBUNITS. Simone Karrasch, Per A. Bullough, Robin Ghosh, MRC Laboratory of Molecular Biology Hills Road Cambridge CB2 2QH, U.K

Two-dimensional crystals from light-harvesting complex I (LHC I) of the purple non-sulphur bacterium Rhodospirillum rubrum have been reconstituted from detergent-solubilized protein complexes. Frozen-hydrated samples have been analyzed by electron microscopy. The crystals diffract beyond 8 Å and a projection map was calculated to 8.5 Å. The projection map shows 16 subunits in a 116 Å diameter ring with a 68 Å hole in the centre. These dimensions are aufficient to incorporate a reaction centre in vivo. Within each subunit, density for the [[alpha]]- and the [[beta]]-polypeptide chains is clearly resolved and the density for the bacteriochlorophylls can be assigned. The experimentally determined structure contradicts models of the LHC I presented so far.