Molecular mechanism of the cyanobacterial clockwork

The S. elongatus luciferase reporter system was used to screen for clock gene mutants, of which many were isolated. These mutants were used to identify a gene cluster composed of three genes, named kaiA, kaiB and kaiC; (Ishiura et al., 1998; "kai" means "rotation" or "cycle number" in Japanese). These genes encode the proteins KaiA, KaiB, and KaiC, which are essential for clock function in S. elongatus and constitute a core circadian oscillator. No significant similarity was found among the kai genes and any other previously reported genes in eukaryotes, but there are potential homologs in the genomic sequences of other bacteria (both eubacteria and archaea).

At first, the cyanobacterial clockwork appeared to be a transcription and translation feedback loop in which clock proteins autoregulate the activity of their own promoters by a process that was similar in concept to the circadian clock loops of eukaryotes. Subsequently, however, several lines of evidence indicated that transcription and translation was not necessary for circadian rhythms of Kai proteins,the most spectacular being that the three purified Kai proteins can reconstitute a temperature-compensated circadian oscillation in a test tube.The rhythm that is measurable in vitro is the phosphorylation status of the clock protein KaiC. This is the first (and so far, only) example of the reconstitution of a circadian clock in vitro.