Time-of-day dependent responses of cyanobacterial cellular viability against oxidative stress

As an adaptation to periodic fluctuations of environmental light, photosynthetic organisms have evolved a circadian clock. The gene expression of ROS scavenging enzymes is regulated by circadian clock genes, which has been considered to help deal with the diurnal photogeneration of reactive oxygen species (ROS). Although a series of recent discoveries have suggested the correlation between circadian clock control and ROS scavenging mechanisms, circadian rhythms of ROS stress tolerance have not been experimentally demonstrated to date. In the present work, we constructed a novel assay using methyl viologen (MV) which generates ROS under light irradiation and experimentally verified the circadian rhythms of ROS stress tolerance in photosynthetic cells of cyanobacterium Synechococcus elongatus PCC7942, a standard model species for the investigation of circadian clock. Here, we report that ROS generated by MV treatment causes damage to stroma components and not to the photosynthetic electron transportation chain, leading to reduced cell viability. The degree of decrease in cell viability was dependent on the subjective time at which ROS stress was applied. Thus, ROS stress tolerance was shown to exhibit circadian rhythms. Notably, rhythms of ROS stress tolerance disappeared in mutant cells lacking the essential clock genes. Further, ROS stress tolerance showed irregular behaviors under irregular light/dark cycles that mismatched the subjective time. These results clearly show that the antioxidant ability in the stroma varies periodically under the control of clock genes. This is the first demonstration of ROS stress tolerance in cyanobacterial cells at a phenotypic level showing circadian oscillation.