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Site-directed mutations were created in the cyanobacterium Synechocystis 6803 to alter specific histidine residues of the photosystem II (PS II) D2 protein. In one mutant (tyr-197). the his-197 residue was replaced by tyrosine, in another mutant (asn-214), his-214 was changed into asparagine. The tyr-197 mutant did not show any low-temperature fluorescence attributable to PS II. but contained a PS II chlorophyll-protein, CP-47, in significant quantities. Another PS II chlorophyll-protein, CP-43, was absent, as was PS II-related herbicide binding. The asn-214 mutant showed a blue-shifted low-temperature fluorescence maximum around 682 nm. but did not have a significant amount of membrane-incorporated CP-43 or CP-47. Herbicide binding was also absent in this mutant. These data indicate a very important role of the his-197 and his-214 residues in the D 2 protein, and are interpreted to support the hypothesis that the D2 protein and the M subunit from the photosynthetic reaction center of purple bacteria have analogous functions. According to this hypothesis, his-197 is involved in binding of P680. and his-214 forms ligands with Q ᴀ  and Fe 2+ . In absence of a functional D2 protein, the PS II core complex appears to be destabilized as evidenced by loss of chlorophyll-proteins in the mutants.

Site-Directed Mutations of Two Histidine Residues in the D2 Protein Inactivate and Destabilize Photosystem II in the Cyanobacterium Synechocystis 6803