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Reiko Murakami, Ayumi Miyake, Ryo Iwase, Fumio Hayashi, Tatsuya Uzumaki and Masahiro Ishiura^*

Center for Gene Research, Nagoya University, Furocho, Chikusa, Nagoya 464-8602, Japan

KaiA, KaiB and KaiC constitute the circadian clock machinery in cyanobacteria. KaiC is a homohexamer; its subunit contains duplicated halves, each with a set of ATPase motifs. Here, using highly purified KaiC preparations of the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 produced in Escherichia coli, we found that the N- and C-terminal domains of KaiC had extremely weak ATPase activity. ATPase activity showed temperature compensation in wild-type KaiC, but not in KaiC_S431A/T432A, a mutant that lacks two phosphorylation sites. We concluded that KaiC phosphorylation is involved in the ATPase temperature-compensation mechanism—which is probably critical to the stability of the circadian clock in cyanobacteria—and we hypothesized the following temperature-compensation mechanism: (i) The C-terminal phosphorylation sites of a KaiC hexamer subunit are phosphorylated by the C-terminal domain of an adjacent KaiC subunit; (ii) the phosphorylation suppresses the ATPase activity of the C-terminal domain; and (iii) the phosphorylated KaiC spontaneously dephosphorylates, resulting in the recover of ATPase activity.

These authors have equally contributed to this study.

Communicated by: Shunsuke Ishii

^* Correspondence: Email: ishiura{at}gene.nagoya-u.ac.jp

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