The repair of photosystem II is the site of regulation by environmental stresses

Abstract

The photosynthetic machinery, in particular the photosystem II complex (PSII), is rapidly damaged by light, but it is also rapidly repaired by light in vivo. Therefore, the extent of photodamage in living organisms is a result of balance between the photo-induced damage to PSII and the repair of the photodamaged PSII. We developed a method to separately measure the damage and repair processes in the cyanobacterium Synechocystis sp. PCC 6803. The initial rate of strong light-induced inactivation of PSII was measured in the presence of lincomycin which blocks the repair process by inhibiting the protein synthesis, whereas the initial rates of weak light-induced recovery of PSII activity was measured after switching the light intensity from strong to weak. We observed that the initial rate of photodamage was proportional to light intensity and this proportionality was not affected by the presence of DCMU or reactive oxygen species (ROS) such as superoxide radicals and H2O2 and by stress conditions due to high salt and cold. In contrast, the initial rate of repair was fast at low intensities of light and was saturated at 300 µE m-2 s-1, and this rate of repair at the saturation level was depressed by the environmental factors. Based on these observations we conclude that the photodamage to PSII depends solely on light intensity without regulation and that the repair of the PSII is the site of regulation by environments. The results of Northern blotting and Western blotting analyses suggest that the major site of inhibitory action by environmental factors is the translation of psbA genes.