Circadian clocks are a mechanism that evolved in diverse groups of species. Although their details differ, their purpose is always the same. Through a central oscillator with a period of ~24 hours, daily changes in the environment due to the diurnal cycle can be anticipated. This 24-hour rhythm is robust, even in varying temperature conditions, and can be reset by external stimuli. In the cyanobacterial clock model Synechococcus elongatus PCC 7942 the central oscillator consists of the proteins KaiA, KaiB and KaiC. Through rhythmic phosphorylation and dephosphorylation of KaiC, this oscillator regulates a multitude of cellular processes. Synechocystis sp. PCC 6803 possesses orthologs of this core oscillator but additionally exhibits two paralogs of KaiB and KaiC. Our recent findings have uncovered a novel chimeric KaiA-like protein (KaiA3) which completes the KaiB3-KaiC3 oscillator¹. In a previous study, our group could show that backscatter measurements can detect oscillations in the growth signal of Synechocystis that follow the circadian rhythm². To investigate the newly discovered KaiA3-KaiB3-KaiC3 oscillator and its influence on the circadian rhythm of Synechocystis, we used online backscatter measurements of several knockout mutants of the kai genes. Our results show that knockouts of kaiC3 and kaiA3 lead to reduced amplitude and shifted phase whereas knockout of kaiB3 almost completely abolishes oscillation. These results suggest a connection between the two oscillators of Synechocystis which indicates that bacterial circadian clocks may be more complex than previously thought. Since kai homologs were detected in several prokaryotes including archaea, the impact of these results may reach beyond cyanobacteria.

Köbler, C. et al. Two circadian oscillators in one cyanobacterium. bioRxiv preprint (2023) doi:10.1101/2021.07.20.453058.Berwanger, L. C. et al. Self-sustained rhythmic behavior of Synechocystis PCC 6803 under continuous light conditions in the absence of light-dark entrainment. bioRxiv preprint (2023) doi:10.1101/2023.09.26.559469.