Ancient cyanobacterial proto-circadian clock adapted to short day-night cycles ~ 0.95 billion years ago

In the early history, the Earth rotation was much faster; however, how ancient organisms adapted to the day-night cycles of that time remains unclear. We resurrected the ancestor KaiABC (anKaiABC) genes circa 0.95 billion years (Ga) ago when the daily-night cycling period was ~ 18 h. Compared with its contemporary counterpart KaiC, the anKaiC protein shows subtle structural differences, and the activities of kinase, phosphatase activities, and adenosine triphosphatase (ATPase) of anKaiC are lower. The anKaiAB proteins are less effective in regulating KaiC/anKaiC phosphorylation status. The anKaiABC system does not sustain an endogenous oscillation but it can be entrained by an 18-h light/dark cycle. The strain expressing anKaiABC shows better adaptation under 9-h light/9-h dark cycles (LD9:9) which mimic the 18-h day-night cycles. These findings suggest that the ancient cyanobacterial proto-circadian system may not be endogenous, but it conferred the capability to adapt to daily cycles ~ 0.95 Ga ago. ### Competing Interest Statement The authors have declared no competing interest.