A Cyanobacterial Circadian Clock Based on the Kai Oscillator

Apr 19, 2008Cold Spring Harbor symposia on quantitative biology

A cyanobacteria internal clock based on the Kai protein cycle

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Abstract

The circadian oscillation of KaiC phosphorylation persisted for at least three cycles in vitro.

KaiA, KaiB, and KaiC are essential components of the circadian clock in the cyanobacterium Synechococcus elongatus PCC 7942.
The oscillation of KaiC phosphorylation is identified as the primary pacemaker of the cyanobacterial circadian clock.
In vitro observations showed that period lengths of KaiC mutants were consistent with those measured in living organisms.
Four aspects of the KaiC phosphorylation cycle were analyzed, including protein interactions and energetics influencing the circadian period.
A circadian program consisting of KaiA, KaiB, and KaiC has been proposed to regulate biological time in cells.

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In the cyanobacterium Synechococcus elongatus PCC 7942, the products of three genes (kaiA, kaiB, and kaiC) have been identified as essential components of the circadian clock. Recently, we reconstituted the self-sustainable circadian oscillation of the KaiC phosphorylation state by incubating purified KaiC with KaiA, KaiB, and ATP. This in vitro oscillation persisted for at least three cycles and the period was compensated against temperature changes. Period lengths observed in vivo in various kaiC mutants were consistent with those measured using in vitro mixtures containing the respective mutant KaiC proteins. These results demonstrate that the oscillation of KaiC phosphorylation is the primary pacemaker of the cyanobacterial circadian clock and reveal a novel function of proteins as timing devices that govern cellular metabolism. We further analyzed four aspects of the KaiC phosphorylation cycle in vitro: the interactions among KaiA, KaiB, and KaiC; the functions of the two phosphorylation sites, the energetics that determine the circadian period, and the mechanisms that synchronize the components of the Kai oscillator. From these analyses, we have proposed a circadian program consisting of the three proteins that keeps biological time in a living cell.

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A Cyanobacterial Circadian Clock Based on the Kai Oscillator

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