Department of Electrical Engineering and Bioscience, Faculty of Science and Engineering, Waseda University, Japan.
PLoS Genet. 2020 Nov 30;16(11):e1009230. doi: 10.1371/journal.pgen.1009230. eCollection 2020 Nov.
Most organisms harbor circadian clocks as endogenous timing systems in order to adapt to daily environmental changes, such as exposure to ultraviolet (UV) light. It has been hypothesized that the circadian clock evolved to prevent UV-sensitive activities, such as DNA replication and cell division, during the daytime. Indeed, circadian control of UV resistance has been reported in several eukaryotic organisms, from algae to higher organisms, although the underlying mechanisms remain unknown. Here, we demonstrate that the unicellular cyanobacterium Synechococcus elongatus PCC 7942 exhibits a circadian rhythm in resistance to UV-C and UV-B light, which is higher during subjective dawn and lower during subjective dusk. Nullification of the clock gene cluster kaiABC or the DNA-photolyase phr abolished rhythmicity with constitutively lower resistance to UV-C light, and amino acid substitutions of KaiC altered the period lengths of the UV-C resistance rhythm. In order to elucidate the molecular mechanism underlying the circadian regulation of UV-C resistance, transposon insertion mutants that alter UV-C resistance were isolated. Mutations to the master circadian output mediator genes sasA and rpaA and the glycogen degradation enzyme gene glgP abolished circadian rhythms of UV-C resistance with constitutively high UV-C resistance. Combining these results with further experiments using ATP synthesis inhibitor and strains with modified metabolic pathways, we showed that UV-C resistance is weakened by directing more metabolic flux from the glycogen degradation to catabolic pathway such as oxidative pentose phosphate pathway and glycolysis. We suggest glycogen-related metabolism in the dark affects circadian control in UV sensitivity, while the light masks this effect through the photolyase function.
大多数生物体都拥有生物钟作为内源性时间系统，以适应日常环境变化，如暴露在紫外（UV）光下。人们假设，生物钟的进化是为了防止在白天进行 UV 敏感的活动，如 DNA 复制和细胞分裂。事实上，已经在从藻类到高等生物的几种真核生物中报告了生物钟对 UV 抗性的控制，尽管其潜在机制尚不清楚。在这里，我们证明单细胞蓝藻 Synechococcus elongatus PCC 7942 对 UV-C 和 UV-B 光的抗性表现出昼夜节律，在主观黎明时更高，在主观黄昏时更低。时钟基因簇 kaiABC 或 DNA 光解酶 phr 的缺失消除了节律性，导致对 UV-C 光的抗性始终较低，并且 KaiC 的氨基酸取代改变了 UV-C 抗性节律的周期长度。为了阐明生物钟对 UV-C 抗性调控的分子机制，我们分离了改变 UV-C 抗性的转座子插入突变体。改变主生物钟输出调节剂基因 sasA 和 rpaA 以及糖原降解酶基因 glgP 的突变使 UV-C 抗性的昼夜节律消失，同时对 UV-C 抗性始终较高。将这些结果与使用 ATP 合成抑制剂和具有修饰代谢途径的菌株进行的进一步实验相结合，我们表明，通过将更多的代谢通量从糖原降解引导到分解代谢途径（如氧化戊糖磷酸途径和糖酵解），UV-C 抗性会减弱。我们认为，黑暗中与糖原相关的代谢会影响 UV 敏感性的生物钟控制，而光通过光解酶功能掩盖了这种影响。
