COMPARISON OF PSBO AND PSBH DELETION MUTANTS OF SYNECHOCYSTIS PCC-6803 INDICATES THAT DEGRADATION OF D1 PROTEIN IS REGULATED BY THE Q(B) SITE AND DEPENDENT ON PROTEIN-SYNTHESIS

Mutants of the cyanobacterium Synechocystis PCC 6803 lacking the psbO or psbH gene are more vulnerable to photoinhibition than the wild type (WT), In the case of the psbO-less mutant, the increased sensitivity to photodamage is also accompanied by accelerated turnover of the D1 protein and a rapid rate of recovery on transfer to non-photoinhibitory conditions. In contrast, in low light the psbH-less mutant has a poor ability to recover after photoinhibition and has a reduced rate of D1 turnover as compared with WT, Since the psbO gene encodes the 33 kDa manganese-stabilizing protein associated with the water-splitting reaction, the increased sensitivity to photoinduced damage is attributed to perturbation of electron transfer processes on the donor side of photosystem II (PSII). In contrast, the absence of H protein, encoded by the psbH gene, affects the acceptor side of PSII with preferential photoinhibitory damage occurring at the Q(B) site. The apparent consequence of this is that the psbH-less mutant, unlike the psbO-less mutant, is not able to regulate the rate of turnover of the D1 protein. In all cases it was shown that chloramphenicol, which blocks protein synthesis, enhances the rate of photoinhibition as judged by a decrease in oxygen evolution but slows down the rate of degradation of D1 protein compared to that observed during normal turnover, We conclude either that a factor or enzyme that is rapidly turned over is required to allow the D1 degradation to occur at in vivo rates or that the degradation and removal of the D1 protein from damaged reaction centers is synchronized with the availability of newly synthesized D1 protein. We favor the latter on the basis of the relationship between turnover rates and message level. Our findings also support the concept that D1 turnover is in some way regulated by the state of the Q(B)-binding pocket.