<p>KaiA is a component of the kaiABC clock protein complex, which constitutes the main circadian regulator in cyanobacteria. The kaiABC complex may act as a promoter-nonspecific transcription regulator that represses transcription, possibly by acting on the state of chromosome compaction. In the complex, KaiA enhances the phosphorylation status of kaiC. In contrast, the presence of kaiB in the complex decreases the phosphorylation status of kaiC, suggesting that kaiB acts by antagonising the interaction between kaiA and kaiC. The activity of KaiA activates kaiBC expression, while KaiC represses it. The overall fold of the KaiA monomer is that of a four-helix bundle, which forms a dimer in the known structure [<cite idref="PUB00015216"/>]. KaiA functions as a homodimer. Each monomer is composed of three functional domains: the N-terminal amplitude-amplifier domain, the central period-adjuster domain and the C-termianl clock-oscillator domain. The N-terminal domain of KaiA, from cyanobacteria, acts as a psuedo-receiver domain, but lacks the conserved aspartyl residue required for phosphotransfer in response regulators [<cite idref="PUB00014089"/>]. The C-terminal domain is responsible for dimer formation, binding to KaiC, enhancing KaiC phosphorylation and generating the circadian oscillations [<cite idref="PUB00031979"/>]. The KaiA protein from <taxon tax_id="103690">Anabaena sp. (strain PCC 7120)</taxon> lacks the N-terminal CheY-like domain.</p> Circadian clock protein KaiA, N-terminal