Simultaneous reinforcement of the antioxidant defense and translational systems enhances the thermotolerance of photosystem II in Synechocystis sp. PCC 6803.

In the cyanobacterium Synechocystis sp. PCC 6803, the capacity for the repair of photosystem II (PSII) is closely associated with the tolerance of PSII to strong light. However, it remains unclear whether the capacity for PSII repair also contributes to the tolerance of PSII to high-temperature stress. In this study, we examined the tolerance of PSII to high temperatures in transformants of Synechocystis that had been engineered to enhance the capacity for PSII repair, specifically via the overexpression of reactive oxygen species- (ROS-) scavenging enzymes (superoxide dismutase and catalase) and exploiting a mutated derivative of EF-Tu, a translation factor that is sensitive to oxidation by ROS. When cells were exposed to the high temperature of 46 °C under standard growth illumination, the activity of PSII in transformants declined more slowly than that in wild-type cells. This mitigation of the heat-induced inactivation of PSII was attributed to enhancements of both the capacity for PSII repair and the thermostability of the PSII complex itself. The synthesis de novo of the D1 protein at 46 °C in the transformants was maintained at higher levels than that in wild-type cells. The heat-induced accumulation of intracellular ROS was significantly suppressed in the transformants. Furthermore, net photosynthetic activity and cell viability at high temperatures were enhanced in the transformants. Thus, reinforcement of both antioxidant defenses and translational systems might improve not only the repair of PSII but also the thermostability of PSII, leading to the enhanced thermotolerance of net photosynthesis and overall cell viability at elevated temperatures.