| Research Abstract |
The structure, function and biodynamics of the photosystem II (PSII) reaction center were analyzed in this study from two different viewpoints.
In the first approach, targeted in vitro random mutagenesis was applied to a wide region of the psbAII gene coding for the Ser148-Ala357 segment of the D1 subunit in Synechocystis sp.PCC 6803, in order to discover unexpected structure-function relationships in the PSII reaction center. A method based on the susceptibility of photosynthetic organisms to nitrofurantoin under illumination was established to screen mutants deficient in the function of photosynthetic electron transport. By this method we have succeeded in isolating more than 150 random mutants impaired in the function of PSII caused by 1-9 amino acid substitution (s) in the targeted amino acid region on D1 protein. By the analysis of these random mutants, several amino acid residues crucial for the function of water oxidation and charge separation in PSII were newly identified and the mechanism of functional modification of PSII by the amino acid substitution on D1 protein was investigated for each mutant.
In the second approach, the phenomenon of carboxyl-terminal (C-terminal) processing of precursor D1 protein of PSII reaction center was analyzed from two aspects, i.e., enzyme and substrate. We have succeeded in over-expressing the enzyme, CtpA, responsible for the process in E.coli and purifying it in homogeneity. The enzymatic properties and the mode of action in vivo were analyzed by using the over-expressed enzyme and the substrate in different molecular environments. This analysis suggested the presence of specific interaction of CtpA with factor (s) in the PSII complex that modulates the proteolysis in response to physiological conditions.
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