Abstract
Photosystem II (PSII) complexes, isolated from spinach and the thermophilic cyanobacterium Synechococcus elongatus, were characterized by electron microscopy and single-particle image-averaging analyses. Oxygen-evolving core complexes from spinach and Synechococcus having molecular masses of about 450 kDa and dimensions of approximately 17.2 x 9.7 nm showed twofold symmetry indicative of a dimeric organization. Confirmation of this came from image analysis of oxygen-evolving monomeric cores of PSII isolated from spinach and Synechococcus having a mass of approximately 240 kDa. Washing with Tris at pH 8.0 and analysis of side-view projections indicated the possible position of the 33-kDa extrinsic manganese-stabilizing protein. A larger complex was isolated that contained the light-harvesting complex II (LHC-II) and other chlorophyll a/b-binding proteins, CP29, CP26, and CP24. This LHC-II-PSII complex 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 latter is located in the center of the larger particle, with additional peripheral regions accommodating the chlorophyll a/b-binding proteins. It 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 PSII may exist as a dimer in vivo.