Site-directed mutations at D1-Thr179 of photosystem II in Synechocystis sp PCC 6803 modify the spectroscopic properties of the accessory chlorophyll in the D1-branch of the reaction center

D1-Thr179, which overlies the reaction center chlorophyll ChlD(1) of Photosystem II was replaced with His and Glu through site-directed mutation in Synechocystis sp. PCC 6803. Spectroscopic characterization of the mutants indicates that, compared to wild type, the main bleaching in the triplet-minus-singlet absorbance difference spectrum and the electrochromic band shift in the (P680(+)Q(A)(-)-P680Q(A)) absorbance difference spectrum are displaced to the red by similar to 2 nm in the D1-Thrl79His mutant and to the blue by similar to 1 nm in the D1-Thr179Glu mutant. These difference spectra are compared with the absorbance difference spectra, measured on the same states in the D1-His198Gln mutant in which the axial ligand D1-His198 of the special pair chlorophyll, P-D1, was replaced by glutamine. Together, these results give direct evidence that (a) the reaction center triplet state, produced upon charge recombination from 3[P(+)Pheo(-)], is primarily localized on Chl(D1); (b) the cation of the oxidized donor PI is predominantly localized on chlorophyll P-D1, of the special pair; and (c) the Q(Y) band of the accessory chlorophyll Chl(D1) is electrochromically shifted in response to charges on P+ and Q(A)(-). Light-induced absorbance difference spectra (between 650 and 7 10 nm), associated with the oxidation of secondary donors and the reduction of QA, exhibit a bleaching attributed to the oxidation of a Chl(Z) and strong electrochromic band shifts. On the basis of mutation-induced spectroscopic changes and of structure-based calculations, we conclude that the experimental spectra are best explained by a blue-shift of the Q(Y) band of the accessory chlorophyll Chl(D1), arising from charges on Car(D2) + and Chl(ZD2)(+) and on reduced Q(A).