The D2 protein is a central component of the Photosystem II complex in photosynthetic membranes, but its precise function is unknown. The role of specific amino acid residues and polypeptide regions of D2 in function, structure and assembly of the Photosystem II complex is being studied by directed mutagenesis techniques. Specific mutations are induced D2. The psbD-I gene (encoding D2) carrying site-directed mutations is introduced into a mutant of the transformable photoheterotrophic cyanobacterium Synechocystis 6803, from which the two psbD gene copies (psbD-I and psbD-II) have been deleted. Target regions to create D2 mutations are identified in part by comparison with the partially homologous M subunit of the reaction center from purple bacteria, for which the detailed structure is known. The properties of the mutant Photosystem II complex are analyzed in a variety of ways, including mRNA studies, and analysis of Photosystem II protein composition and synthesis, electron transport, chlorophyll fluorescence, specific EPR signals, herbicide binding, and flash-induced absorbance changes. The results of this project will lead to a detailed understanding of the role of specific residues and regions in an important but poorly understood protein in photosynthetic membranes. Insight will be gained on how the oxidizing complex is organized. Potentially significant is that the protein complex can be altered to result in more efficient operation.***//
