Molecular Organization of Photsystem II

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I.Structural organization of photosystem IIThe photosystem II complexes of different integrities were isolated from thermophilic cyanobacteria. The structure and molecular interactions of the primary donor in the photosystem II reaction center have been investigated by detecting light-induced FT-IR difference spectra upon the formation of its triplet state. Considering the orientation of P-680 analyzed by EPR and the structure of bacteria reaction center determined by X-ray crystallography, together with the sequence homology between the D1 and D2 subunits of photosystem II and the L and M subunits of purple bacteria, a model of the structure of P-680 and its interactions with apoproteins has been proposed. Site-directed modification using Chlamydomonas reinhardtti of the amino acid side chains on D1 protein presumably involved in the hydrogen-bonding interaction (Ser-191 and Thr-192) has been succeeded and the analysis of these mutants is now in progress in order to prove this hypothesis. Chemical cross-linking analysis has also been conducted for the isolated photosystem II complexes to analyze the gross structure.II.Dynamic aspects of the organization of photosystem IIThe enzyme involved in the processing of D1 precursor protein has been identified and the partial amino acid sequences have been determined for spinach. Based on these data, a gene coding for the enzyme has been identified and sequenced. The recognition signal on substrate was analyzed for the C-terminal processing protease. Photo-tolerant mutants of an unicellular cyanobacterium Synechocystis PCC 6803 were obtained by in vitro random mutagenesis of psbAII (gene for D1 protein) by PCR under a condition for reduced fidelity of amplification, in order to analyze the damage-repair cycle of D1 protein ni photoinhibition of photosystem II reaction center.