D1 PROTEIN-DEGRADATION AND PSBA TRANSCRIPT LEVELS IN SYNECHOCYSTIS PCC-6803 DURING PHOTOINHIBITION IN-VIVO

The relation of photoinhibition of Photosystem II (PS II) to the rate of degradation of the reaction center protein D1 was studied in vivo i n the cyanobacterium Synechocystis 6803. Exposure of cells to a PPFD o f 1500 mumol m-2 s-1 induced 75 % photoinhibition of oxygen evolution of PS II within 2 h. In spite of severe photoinhibition, only a slight net decrease was observed at the steady-state level of the Dl protein as determined by quantitative immunoblotting analysis. However, in th e presence of protein synthesis inhibitors degradation of the D1 prote in occurred rapidly and photoinhibition of PS II was accelerated. Puls e and chase experiments under strong illumination revealed that the D1 protein turned over rapidly with a half-life of about 30 min. The pre sence or absence of psbA mRNA carrying polyribosomes on the thylakoid membranes did not significantly affect the rate of D1 protein degradat ion. Our results indicate that no direct synchronization exists betwee n degradation and synthesis of the Dl protein and it is probable that reassembly and activation of PS II after insertion of a new copy of th e D1 protein to the PS II complex are the rate-limiting factors of the repair of photodamaged PS II in cyanobacteria. Northern blot analysis of psbA mRNA in the presence and absence of protein synthesis inhibit ors demonstrated that protein factors are needed to regulate the expre ssion of the psbA genes in Synechocystis 6803.