Site-directed mutations at D1-His198 and D1-Thr179 of photosystem II in Synechocystis sp. PCC 6803: deciphering the spectral properties of the PSII reaction centre.

Site-directed mutations were constructed in photosystem II of Synechocystis sp. PCC6803 in which the axial ligand, D1-His198, of special pair chlorophyll PD1 was replaced with Gln and where D1-Thr179, which overlies monomeric chlorophyll ChlD1, was replaced with His. The D1-His198Gln mutation produces a 3nm displacement to the blue of the bleaching minimum in the Soret and in the Qy region of the (P+QA--PQA) absorbance difference spectrum. To a first approximation, the bleaching can be assigned to the low-energy exciton transition of the special pair chlorophylls PD1/PD2. The D1-Thr179His mutation produces a 2nm displacement to the red of the bleaching minimum in the Qy region of the (3P-1P) absorbance difference spectrum. Analysis of the flash-induced (P+QA--PQA) and (3P-1P) absorbance difference spectra of both mutants compared with wild-type at 80K indicate that the cation of the oxidized donor P+ is predominantly localized on the chlorophyll PD1 of the special pair and that the reaction centre triplet state, produced upon charge recombination from 3[P+Pheo-], when the primary quinone electron acceptor QA is doubly reduced, is primarily localized on ChlD1.