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PNAS 99 (24): 15357-15362

Copyright © 2002 by the National Academy of Sciences.

Nonlinear partial differential equations and applications


Structure and function from the circadian clock protein KaiA of Synechococcus elongatus: A potential clock input mechanism

Stanly B. Williams*,{dagger}, Ioannis Vakonakis{dagger},{ddagger}, Susan S. Golden*,§, and Andy C. LiWang{ddagger}

Departments of *Biology and {ddagger}Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843

Received for publication August 26, 2002.

Abstract: In the cyanobacterium Synechococcus elongatus (PCC 7942) the proteins KaiA, KaiB, and KaiC are required for circadian clock function. We deduced a circadian clock function for KaiA from a combination of biochemical and structural data. Both KaiA and its isolated carboxyl-terminal domain (KaiA180C) stimulated KaiC autophosphorylation and facilitated attenuation of KaiC autophosphorylation by KaiB. An amino-terminal domain (KaiA135N) had no function in the autophosphorylation assay. NMR structure determination showed that KaiA135N is a pseudo-receiver domain. We propose that this pseudo-receiver is a timing input-device that regulates KaiA stimulation of KaiC autophosphorylation, which in turn is essential for circadian timekeeping.

{dagger} S.B.W. and I.V. contributed equally to this work.

§ To whom correspondence regarding circadian aspects should be addressed. E-mail: sgolden{at}

To whom correspondence regarding structural aspects should be addressed. E-mail: andy-liwang{at}

Edited by Robert Haselkorn, University of Chicago, Chicago, IL, and approved September 17, 2002

This paper was submitted directly (Track II) to the PNAS office.

Data deposition: The crystal structures and coordinates reported in this paper have been deposited in the Protein Data Bank, (PBD ID codes 1M2E and 1M2F).

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