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The chemical nature of electron donor(s) in photosystem II in photosynthetic membranes was analyzed by site-directed mutagenesis of the gene encoding the protein D2 of the photosystem II reaction center. Mutation of the Tyr-160 residue of the D2 protein into phenylalanine results in the disappearance of the electron paramagnetic resonance signal IIS originating from D+ , the oxidized form of the slow photosystem II electron donor D. Signal IIS is still present if a neighboring residue in D2, Met-159, is mutated into arginine. Both mutants have normal rereduction kinetics of the oxidized primary electron donor, P680+ , in octyl glucoside-extracted thylakoids, indicating that D is not directly involved in P680+ reduction. However, overall photosystem II activity appears to be impaired in the Tyr-160-Phe mutant: photosystem II-dependent growth of this mutant is slowed down by a factor of 3-4, whereas photoheterotrophic growth rates in wild type and mutant are essentially identical. Binding studies of diuron, a photosystem II herbicide, show that there is no appreciable decrease in the number of photosystem II centers in the Tyr-160-Phe mutant. The decrease in photosystem II activity in this mutant may be interpreted to indicate a role of D in photoactivation, rather than one as an important redox intermediate in the photosynthetic electron-transport chain.

Site-directed mutagenesis in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: Donor D is a tyrosine residue in the D2 protein