CRYSTALLIZATION OF THE REACTION CENTER OF PHOTOSYSTEM II. Noam Adir, Faculty of Chemistry, The Technion, Technion City, Haifa 32000 Israel

The reaction center of Photosystem II (RCII), a ~250 kDa membrane bound protein/pigment complex has been crystallized. RCII catalyzes the photochemically driven transfer of electrons from water, resulting in the formation of reduced and protonated quinones and the evolution of molecular oxygen. It is the source of linear electron flow utilized by all oxygenic photosynthetic organisms for both the reduction of NADP and the formation of the proton gradient needed for ATP synthesis. It is highly conserved in all species at all levels of structure and function. Isolation procedures have been developed for RCII from a variety of photosynthetic organisms with an emphasis not only on purity, but homogeneity and stability as well. Isolated RCII has been crystallized by the vapor diffusion method. Two crystal forms of isolated spinach RCII have been obtained: hexagonal rods and rectangular rods (with dimensions of 0.3 x 0.3 x 1.0 mm). These crystals diffract to a maximum resolution of 7 Å, using synchrotron radiation (SSRL, Stanford). The crystals were however quite mosaic, apparently due to their sensitivity to physical stress. Both crystal forms were obtained in the presence of mixtures of two non-ionic detergents and heptane-triol. In addition to these conditions, more than 15 additional conditions have been found to promote the growth of crystals with various needle morphologies from spinach, pea, squash, maize and Chlamydomonas reinhardtii RCII. Preliminary work on the crystallization of RCII was performed in the laboratory of Profs. George Feher and Mel Okamura, of the Dept. of Physics, University of California, San Diego

(1). 1) Adir et al. (1992) in Research in Photosynthesis, Vol II (N. Murata ed.) pp. 195-8. Kluwer Academic Publishers, Dordrect Supported by the Womenís Division of the American Technion Society