The regulatory role of photosystem II photoinactivation and de novo protein synthesis in the degradation and exchange of two forms of the D1 protein in the cyanobacterium Synechococcus PCC 7942

Abstract

After exposure of cells of the cyanobacterium Synechococcus PCC 7942 to increased irradiance, there is a good correlation between the rate of exchange of the ‘low-light’ form D1:1 for the ‘high-light’ form D1:2, and the rate of photosystem II photoinactivation (PSIIPI) assessed in chloramphenicol (CAP)-treated cells. This result indicates that PSIIPI triggers the exchange of D1:1 for D1:2. On the other hand, the reverse exchange of D1:2 for D1:1 after return of the cells to low irradiance is apparently independent of PSIIPI. The D1:2 degradation is partially inhibited by the protease inhibitor Pefabloc, suggesting the role of a serine protease in this process. The degradation of both D1:1 under high irradiance and D1:2 under low irradiance is slowed down in the presence of CAP, confirming previous results obtained with other cyanobacterial strains. A comparison of the turnover rates of the D1 forms under high irradiance shows that the radiolabelled D1:2 turns over significantly faster than D1:1. The presence of D1:2 in the membrane therefore improves the protection of PSII against photodamage and this can be the primary reason for the preferential synthesis of this form under high irradiance.