The long-term goal is to understand the molecular mechanism of quinone reduction by photosynthetic reaction centers. The focus of the work in this area is to define the unique role played by bound bicarbonate anions in plastoquinone reduction and protonation by photosystem II of plants and cyanobacteria. Use of site- directed and site-selected mutants of cyanobacteria has established the involvement of specific amino acids of two separate photosystem II reaction center proteins, D1 and D2, in the binding and functioning of bicarbonate. Using the powerful combination of molecular biology and advanced biophysical techniques, the experimental work proposed here has two specific goals. Additional site-directed mutants of D1 and D2 in Synechocystis sp. PCC 6803 and D1 mutants in Chlamydomonas reinhardtii will be constructed. These will be used to identify the amino acid ligands and structural requirements for bicarbonate binding and resolve the unique role of bicarbonate in the protonation of plastoquinone (i.e., QB- and/or QB2-) and define the identity and role of amino acids involved in this function.
