PHOTOINACTIVATION OF PHOTOSYSTEM-II AS STUDIED WITH SITE-DIRECTED D2 MUTANTS OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

To study the primary sites of photoinactivation in Photosystem II, electron transport stability in cells and thylakoids of wild type and four site-directed D2 mutants of the cyanobacterium Synechocystis sp. PCC 6803 with greatly increased rates of photoinactivation has been determined. In two mutants, E69Q and P161L, both impaired at or near the water-splitting complex, photoinactivation is very rapid. The cause of the high rate of photoinactivation probably is the lack of sufficient electron donation to highly oxidizing species, including Z+ and P680+. Addition of an artificial electron donor to Z+ causes a decrease in the rate of photoinactivation. Photoinactivation also is rapid in H197Q, a mutant in which one of the presumed ligands to P680 has been changed, leading to a decrease in the operating redox midpoint potential of the P680/P680+ couple, and thus to a higher steady-state radical concentration at the donor side (Z+ + P680+) in the light. In wild type and all mutants studied the rate of photoinactivation continued to increase upon increasing the light intensity beyond that required for maximal steady-state electron transport rates. We hypothesize that the Z+, P680+, or other radicals associated with the Photosystem II donor side are the main triggers for the photoinactivation process. To test the hypothesis of double-reduction of Q(A) leading to loss of the quinone and to photoinactivation, the rapidly photoinactivating mutant G215W (which is mutated near the Q(A) site) was illuminated in the presence and absence of DCMU. The rate of photoinactivation of PS II in G215W in the presence of DCMU was similar to that of wild type and of the other D2 mutants, even though in the presence of DCMU Q(A) will tend to be more reduced than in its absence. Therefore, we do not find evidence for direct Q(A) involvement in photoinactivation. In all of the mutants in vivo photodamage to PS II cannot be fully repaired without de novo protein synthesis.