Q_A Binding to D2 Contributes to the Functional and Structural Integrity of Photosystem II

Two D2 mutants were created with a site-directed mutation near the presumable binding site of QA. In one of the mutants, in which Trp-253, the aromatic residue potentially involved in facilitating electron transport from pheophytin to QAand/or in binding of QA, had been replaced by Leu, PS II was undetectable in thylakoids. This mutant is an obligate photoheterotroph. In another mutant the Gly-215 residue, located next to the His residue that is proposed to bind QA and Fe2+, was mutated to Trp. This mutation leads to a rapid inactivation of oxygen evolution capacity in the light, and to a virtual elimination of the potential to grow photoautotrophically, but does not greatly affect the number of photosystem II reaction centers on a chlorophyll basis. We propose that proper binding of QA to the photosystem II reaction center complex is a prerequisite for stability of the photosystem II complex. Impairment of QA binding leads to rapid inactivation of photosystem II, which may be followed by a structural disintegration of the complex.