Electron transfer between Q(A) and Q(B) in photosystem II is thermodynamically perturbed in phototolerant mutants of Synechocystis sp. PCC 6803

Several random mutations have been generated in the psbA2 gene of Synechocy stis sp. PCC 6803 [Narusaka, Y,, Murakami, A., Saeki, J., Kobayashi, H., an d Satoh, K, (1996) Plant Sci. 115, 261-266]. The phototolerant mutant (I6) carrying all the amino acid substitutions in the lumenal side of D1 protein (S322I, I326F, and F328S) and a site-directed mutant of the same phenotype (NDFS) substituted in the stromal side of the protein (N234D and F260S) we re characterized by thermoluminescence measurements. We observed (1) no sig nificant differences in their growth rates at either low or high light irra diance, (2) a downshifted B-band in the NDFS mutant, (3) an upshifted Q-ban d in the 16 mutant, and (4) a damped period four oscillation of thermolumin escence in the B-band of both mutants. By examining the possible implicatio ns of these results on the redox properties of the PS II components in the mutants, we concluded that equilibrium constants for sharing an electron be tween the primary (Q(A)) and secondary acceptor plastoquinones (Q(B)) are d ecreased in both mutants.