Abstract
Cyanobacteria are the simplest organisms known to possess a circadian oscillator. The central oscillator of the cyanobacterium Synechococcus elongatus PCC. 7942 comprises three clock proteins termed KaiA, KaiB, and KaiC. In vitro reconstruction of the KaiC phosphorylation cycle achieved simply by incubating KaiA, KaiB, and KaiC in the presence of ATP, has provided a means of studying the detailed mechanisms of the Kai-protein oscillator.
We used time-resolved fluorescence spectro scopy, small-angle X-ray scattering, and mutational analyses to track and visualize the ticking of KaiC in solution. During the phosphorylation cycle, the C-terminal domains of KaiC were repositioned in a stepwise manner to effect global expansion and contraction motions of the C-terminal ring. We found that Arg393 of KaiC played a critical role in expanding the C-terminal ring and its replacement with Cys affected the temperature compensation of the period, a fundamental property of circadian clocks. We will discuss mechanisms of KaiC oscillator based on structural information.
