SUPRAMOLECULAR STRUCTURE OF THE PHOTOSYSTEM-II COMPLEX FROM GREEN PLANTS AND CYANOBACTERIA

Photosystem II (PSII) complexes, isolated from spinach and the thermop hilic cyanobacterium Synechococcus elongatus, were characterized by el ectron microscopy and single-particle image-averaging analyses. Oxygen -evolving core complexes from spinach and Synechococcus having molecul ar masses of about 450 kDa and dimensions of approximate to 17.2 x 9.7 nm showed twofold symmetry indicative of a dimeric organization. Conf irmation of this came from image analysis of oxygen evolving monomeric cores of PSII isolated from spinach and Synechococcus having a mass o f approximate to 240 kDa. Washing with Tris at pH 8.0 and analysis of side-view projections indicated the possible position of the 33-kDa ex trinsic manganese-stabilizing protein. A larger complex was isolated t hat contained the light-harvesting complex II (LHC-II) and other chlor ophyll a/b-binding proteins, CP29, CP26, and CP24. This LHC-II-PSII co mplex had a mass of about 700 kDa, and electron microscopy revealed it also to be a dimer having dimensions of about 26.8 and 12.3 nm. From comparison with the dimeric core complex, it was deduced that the latt er is located in the center of the larger particle, with additional pe ripheral regions accommodating the chlorophyll a/b-binding proteins. I t is suggested that two LHC-II trimers are present in each dimeric LHC -II-PSII complex and that each trimer is linked to the reaction center core complex by CP24, CP26, and CP29. The results also suggest that P SII may exist as a dimer in vivo.