Three-dimensional structure of Chlamydomonas reinhardtii and Synechococcuselongatus photosystem II complexes allows for comparison of their oxygen-evolving complex organization

Electron microscopy and single-particle analyses have been carried out on n egatively stained photosystem Il (PSII) complexes isolated from the green a lga Chlamydomonas reinhardtii and the thermophilic cyanobacterium Synechoco ccus elongatus, The analyses have yielded three-dimensional structures at 3 0-Angstrom resolution. Biochemical analysis of the C. reinhardtii particle suggested it to be very similar to the light-harvesting complex II (LHCII)P SII supercomplex of spinach, a conclusion borne out by its three-dimensiona l structure. Not only was the C, reinhardtii LHCII PSII supercomplex dimeri c and of comparable size and shape to that of spinach, but the structural f eatures for the extrinsic OEC subunits bound to the lumenal surface were al so similar thus allowing identification of the PsbO, PsbP, and Psba OEC pro teins. The particle isolated from S. elongatus was also dimeric and retaine d its OEC proteins, PsbO, PsbU, and PsbV (cytochrome c(550)), which were ag ain visualized as protrusions on the lumenal surface of the complex. The ov erall size and shape of the cyanobacterial particle was similar to that of a PSII dimeric core complex isolated from spinach for which higher resoluti on structural data are known from electron crystallography, By building the higher resolution structural model into the projection maps it has been po ssible to relate the positioning of the OEC proteins of C. reinhardtii and S. elongatus with the underlying transmembrane helices of other major intri nsic subunits of the core complex, D1, D2, CP47, and CP43 proteins. It is c oncluded that the PsbO protein is located over the CP47 and D2 side of the reaction center core complex, whereas the PsbP/PsbQ and PsbV/PsbU are posit ioned over the lumenal surface of the N-terminal region of the D1 protein. However, the mass attributed to PsbV/PsbU seems to bridge across to the Psb O, whereas the PsbP/PsbQ proteins protrude out more fi om the lumenal surfa ce. Nevertheless, within the resolution and quality of the data, the relati ve positions of the center of masses for OEC proteins of C. reinhardtii and S. elongatus are similar and consistent with those determined previously f or the OEC proteins of spinach.