Steps on the Way to Building Blocks, Topologies, Crystals and X-ray Structural Analysis of Photosystems I and II of Water-oxidizing Photosynthesis.

Basic structural elements of the two photosystems and their component electron donors, acceptors, and carriers were revealed by newly developed spectroscopic methods in the 1960s and subsequent years. The spatial organization of these constituents within the functional membrane was elucidated by electrochromic band shift analysis, whereby the membrane-spanning chlorophyll-quinone couple of Photosystem (PS) II emerged as reaction center and as a model relevant also to other photosystems. A further step ahead for improved structural information was realized with the use of thermophilic cyanobacteria instead of plants which led to isolation of supramolecular complexes of the photosystems and their identification as PS I trimers and PS II dimers. The preparation of crystals of the PS I trimer, started in the late 1980s. Genes encoding the 11 subunits of PS I from Synechococcus elongatus were isolated and the predicted sequences of amino acid residues formed a basis for the interpretation of X-ray structure analysis of the PS I crystals. The crystallization of PS I was optimized by introduction of the 'reverse of salting in' crystallization with water as precipitating agent. On this basis the PS I structure was successively established from 6 A resolution in the early 1990s up to a model at 2.5 A resolution in 2001. The first crystals of the PS II dimer, capable of water oxidation, were prepared in the late 1990s; a PS II model at 3.8-3.6 A resolution was presented in 2001. Implications of the PS II structure for the mechanism of transmembrane charge separation are discussed. With the availability of PS I and PS II crystals, new directional structural results became possible also by application of different magnetic resonance techniques through measurements on single crystals in different orientations.