STEPWISE PHOTOINHIBITION OF PHOTOSYSTEM-II - STUDIES WITH SYNECHOCYSTIS SPECIES PCC-6803 MUTANTS WITH A MODIFIED D-E LOOP OF THE REACTION-CENTER POLYPEPTIDE D1

Several mutant strains of Synechocystis sp. PCC 6803 with large deleti ons in the D-E loop of the photosystem II (PSII) reaction center polyp eptide D1 were subjected to high light to investigate the role of this hydrophilic loop in the photoinhibition cascade of PSII. The toleranc e of PSII to photoinhibition in the autotrophic mutant Delta R225-F239 (PD), when oxygen evolution was monitored with 2,6-dichloro-p-benzoqu inone and the equal susceptibility compared with control when monitore d with bicarbonate, suggested an inactivation of the Q(B)-binding nich e as the first event in the photoinhibition cascade in vivo. This step in PD was largely reversible at low light without the need for protei n synthesis. Only the next event, inactivation of Q(A) reduction, was irreversible and gave a signal for D1 polypeptide degradation. The het erotrophic deletion mutants Delta G240-V249 and Delta R225-V249 had se verely modified Q(B) pockets, yet exhibited high rates of 2,6-dichloro -p-benzoquinone-mediated oxygen evolution and less tolerance to photoi nhibition than PD. Moreover, the protein-synthesis-dependent recovery of PSII from photoinhibition was impaired in the Delta G240-V249 and D elta R225-V249 mutants because of the effects of the mutations on the expression of the psbA-2 gene. No specific sequences in the D-E loop w ere found to be essential for high rates of D1 polypeptide degradation .