Abstract
Author SummaryMany organisms from bacteria to humans have evolved circadian mechanisms for regulating biological processes on a daily time scale. In cyanobacteria, a minimal system for such cyclical regulation can be reconstituted in vitro from three proteins, called KaiA, KaiB, and KaiC. This three-protein oscillator is believed to regulate the cyclical activities in vivo through a post-translational mechanism that involves rhythmic phosphorylation of KaiC. Although this post-translational oscillator (PTO) is sufficient for generating rhythms in vitro, the cyanobacterial circadian system in vivo also includes a transcriptional/translational feedback loop (TTFL). The precise roles of the PTO and the TTFL and their interdependence in forming the complete clock system in vivo are unclear. By manipulating wild-type and mutant clock protein expression in vivo, we here show that the cyanobacterial circadian system is dependent upon the biochemical oscillator provided by the PTO and that suppression of the PTO leads to a residual damped (slave) oscillation that results from the TTFL. Mathematical modeling shows that the experimental data are compatible with a mechanism in which the PTO acts as a pacemaker to drive the activity of the TTFL. Moreover, our analyses suggest a mechanism by which the TTFL can feed back into the PTO such that new synthesis of the Kai proteins entrains the core PTO pacemaker. Therefore, the PTO and TTFL appear to have a definite hierarchical interdependency: the PTO is a self-sustained core pacemaker that can oscillate independently of the TTFL, but the TTFL is a slave oscillator that damps when the phosphorylation status of KaiC in the PTO is clamped. The core circadian pacemaker in eukaryotes is thought to be a TTFL, but our results with cyanobacteria have important implications for eukaryotic clock systems in that they can explain how a TTFL could appear to be the core clock when in fact the true pacemaker is an embedded biochemical oscillator.
Highlights
The mechanism of circadian clocks in eukaryotes is generally thought to be based upon autoregulatory transcriptional/translational feedback loops (TTFLs) [1,2]
When the essential components of the circadian clock in the prokaryotic cyanobacterium Synechococcus elongatus were identified [3] as the proteins KaiA, KaiB, and KaiC, the initial interpretation that the core of this prokaryotic clockwork might be a TTFL was based on the same kind of evidence that supports TTFL oscillators in eukaryotes, namely: (i) rhythms of abundance for mRNAs and proteins encoded by ‘‘clock genes,’’ (ii) feedback of clock proteins on their gene’s transcription, and (iii) phase setting by experimental expression of clock proteins [3,4,5]
By manipulating wild-type and mutant clock protein expression in vivo, we here show that the cyanobacterial circadian system is dependent upon the biochemical oscillator provided by the post-translational oscillator (PTO) and that suppression of the PTO leads to a residual damped oscillation that results from the TTFL
Summary
The mechanism of circadian (daily) clocks in eukaryotes is generally thought to be based upon autoregulatory transcriptional/translational feedback loops (TTFLs) [1,2]. In 2005 came the astonishing discovery that the phosphorylation status of KaiC in vitro continued to cycle when the three Kai proteins were combined in a test tube with ATP [10]. This in vitro rhythm persists with a circa-24 h period for at least 10 d and is temperature compensated [10,11], which indicates that a circadian temperature compensation mechanism is encoded in the characteristics of the three Kai proteins and the nature of their interactions. A TTFL is not necessary for the circadian rhythm of KaiC phosphorylation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
