D1 Protein Degradation and psbA Transcript Levels in Synechocystis PCC 6803 during Photoinhibition in vivo

Abstract

The relation of photoinhibition of Photosystem II (PS II) to the rate of degradation of the reaction center protein D1 was studied in vivo in the cyanobacterium Synechocystis 6803. Exposure of cells to a PPFD of 1500 μmol 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 D1 protein as determined by quantitative immunoblotting analysis. However, in the presence of protein synthesis inhibitors degradation of the D1 protein occurred rapidly and photoinhibition of PS II was accelerated. Pulse and chase experiments under strong illumination revealed that the D1 protein turned over rapidly with a half-life of about 30 min. The presence or absence of psbA mRNA carrying polyribosomes on the thylakoid membranes did not significantly affect the rate of D1 protein degradation. Our results indicate that no direct synchronization exists between degradation and synthesis of the D1 protein and it is probable that reassembly and activation of PS II after insertion of a new copy of the 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 inhibitors demonstrated that protein factors are needed to regulate the expression of the psbA genes in Synechocystis 6803.