PROJECTION STRUCTURES OF 3 PHOTOSYNTHETIC COMPLEXES FROM RHODOBACTER-SPHAEROIDES - LH2 AT 6 ANGSTROM LH1 AND RC-LH1 AT 25 ANGSTROM

Three photosynthetic complexes, light-harvesting complex 2 (LH2), ligh t-harvesting complex 1 (LH1), and the reaction centre-light-harvesting complex 1 photounit (RC-LH1), were purified from a single species of a purple bacterium, Rhodobacter sphaeroides, and reconstituted into tw o-dimensional (2-D) crystals. Vesicular 2-D crystals of LH1 and RC-LH1 were imaged in negative stain and projection maps at 25 Angstrom. res olution were produced. The rings formed by LH1 have approximately the same mean diameter as the LH1 rings from Rhodospirillum rubrum (simila r to 90 Angstrom) and therefore are likely to be composed of 15 to 17 alpha beta subunits. In the projection map calculated from the RC-LH1 2-D crystals, the reaction centre is represented by an additional dens ity in the centre of the ring formed by the LH1 subunits. The marked i mprovement of shape and fine structure after a rotational pre-alignmen t of the RC-LH1 unit cells before averaging strongly suggests that the RC is not in a unique orientation within the LH1 rings. Tubular cryst als of LH2 showed a high degree of order and allowed calculation of a projection map at 6 Angstrom resolution from glucose-embedded specimen s. The projection structure shows a ring of nine alpha beta subunits. Variation of the alpha-helical projection densities suggests that the 9-fold symmetry axis is tilted with respect to the membrane normal. (C ) 1998 Academic Press.