Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.
Ordered Phosphorylation Governs Oscillation of a Three-Protein Circadian Clock
Michael J. Rust,1*Joseph S. Markson,1,2*William S. Lane,3Daniel S. Fisher,4Erin K. O'Shea1
The simple circadian oscillator found in cyanobacteria can bereconstituted in vitro using three proteins—KaiA, KaiB,and KaiC. The total phosphorylation level of KaiC oscillateswith a circadian period, but the mechanism underlying its sustainedoscillation remains unclear. We have shown that four forms ofKaiC differing in their phosphorylation state appear in an orderedpattern arising from the intrinsic autokinase and autophosphataserates of KaiC and their modulation by KaiA. Kinetic and biochemicaldata indicate that one of these phosphoforms inhibits the activityof KaiA through interaction with KaiB, providing the crucialfeedback that sustains oscillation. A mathematical model constrainedby experimental data quantitatively reproduces the circadianperiod and the distinctive dynamics of the four phosphoforms.
1 Howard Hughes Medical Institute, Faculty of Arts and Sciences Center for Systems Biology, Departments of Molecular and Cellular Biology and of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA. 2 Graduate Program in Biophysics, Harvard University, Cambridge, MA 02138, USA. 3 Microchemistry and Proteomics Analysis Facility, Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, MA 02138, USA. 4 Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
* These authors contributed equally to this work.
To whom correspondence should be addressed. E-mail: erin_oshea{at}harvard.edu
To whom correspondence should be addressed. E-mail: 
