Effect of pH on the cyanobacterial circadian oscillator in vitro

The cyanobacterial clock protein KaiC exhibits robust 24-h oscillation of phosphorylation when incubated with KaiA, KaiB, and ATP in vitro. This study shows that the period of the in vitro phosphorylation rhythm of KaiC was correlated with solution pH, varying from 15 h at pH 6.5-36 h at pH 8.5, without affecting the period's temperature compensation. The solution pH altered the autophosphorylation and autodephosphorylation of KaiC and the effect of KaiB on KaiC but had little effect on its ATPase activity. It also modified the surface charge of the interface between two ATPase domains in KaiC, thereby affecting the autophosphorylation and autodephosphorylation activity of this protein via interdomain communication. These findings not only reveal key biochemical properties of the Kai oscillator but also provide insight into its evolutionary adaptation to environmental changes in cyanobacteria.