Prokaryotic Circadian Systems: Cyanobacteria and Beyond

Circadian clocks are biological timekeeping mechanisms that synchronize physiology with the 24-h day–night cycle and provide temporal order to cellular events that recur daily as circadian rhythms. The cyanobacterium Synechococcus elongatus displays robust circadian rhythms and for more than 30 years has served as a model organism for uncovering the principles of prokaryotic timekeeping. The fundamental driving force behind these rhythms is a three-protein oscillator composed of KaiA, KaiB, and KaiC. In this review, we summarize current knowledge of the molecular mechanism of the Kai oscillator and focus on the dynamic conformational changes of these proteins over the period of a day. We also discuss how timing information is relayed from the oscillator to regulate downstream gene expression, thereby influencing cellular physiology. Furthermore, we explore circadian or circadian-like timing systems identified in other prokaryotes. We hope this review can inspire the discovery of new clock mechanisms in the microbial world and beyond.