The cyanobacterium Synechococcus elongatus exhibits circadian rhythmicity, and the abundance of the KaiB and KaiC proteins display a circadian cycle. Iwasaki et al. and Williams et al. both provide evidence for a phosphorylation-dependent mechanism controlling the circadian rhythm of this simple organism. In the absence of kaiA, kaiB, or kaiC, rhythmic expression of a kaiBC reporter construct is abolished. However, Iwasaki et al. found that the stimulation of kaiBC expression associated with overexpression of KaiA did not occur in KaiC-deficient cells, suggesting that KaiC is required for the stimulatory effects of KaiA. Iwasaki et al. and Williams et al. both showed that in vitro KaiA stimulated the autophosphorylation activity of KaiC; although only the COOH-domain of the protein had this activity. In vivo, the rhythmic phosphorylation of KaiC was decreased in kaiA- cells. Iwasaki et al. further demonstrated that the kaiA2 mutant strain that exhibits an increased period of clock-controlled gene expression also showed decreased KaiC circadian phosphorylation, which was restored in the kaiC15 mutant that suppressed the kaiA2 phenotype. Williams et al. analyzed the structure of the NH2- and COOH-domains of KaiA by circular dichroism and 15N heteronuclear single-quantum coherence spectrum. The NH2-domain resembled a receiver domain, when compared with receiver domains from two-component systems. However, certain essential residues were not conserved, which suggests that the KaiA NH2-domain may be a pseudoreceiver domain. The importance of this domain in regulating the circadian clock is apparent from the abundance of mutations in KaiA that alter periodicity and that occur in this region. Williams et al. suggest that the pseudoreceiver domain receives input from a light sensor (possibly the phytochrome encoded by cikA) and then may regulate the COOH-domain to control the KaiC autophosphorylation.
H. Iwasaki, T. Nishiwaki, Y. Kitayama, M. Nakajima, T. Kondo, KaiA-stimulated KaiC phosphorylation in circadian timing loops in cyanobacteria. Proc. Natl. Acad. Sci. U.S.A. 99, 15788-15793 (2002). [Abstract] [Full Text]
S. B. Williams, I. Vakonakis, S. S. Golden, A. C. LiWang, Structure and function from the circadian clock protein KaiA of Synechococcus elongatus: A potential clock input mechanism. Proc. Natl. Acad. Sci. U.S.A. 99, 15357-15362 (2002). [Abstract] [Full Text]