The Synechococcus Circadian Oscillator

At subjective dawn or sunrise, CikA protein, either through its own chromophore or the chromophores of other postulated interactive proteins, recognizes light input information, and relays that information to the Kai clock for synchronization to local time. The daily light (and temperature) information is integrated into the Kai mechanism by the amino-terminal, pseudo-receiver domain of KaiA. The information relay is probably via a protein-protein interaction, with an as-yet unidentified protein(s), given the structural similarity of amino-terminal KaiA to bacterial receivers that propagate conformational changes based on information from protein interactions. At dawn and during the subjective day, transcription of the kaiA and kaiBC operons and subsequent translation of KaiA, KaiB, and KaiC monomers occurs. KaiB protein is sequestered to the cell membrane by an as-yet unidentified, membrane-associated or membrane-bound KaiB-interactive protein. Meanwhile, KaiC protein monomers bind ATP, inducing the hexamerization of KaiC, and the carboxy-terminal domain of KaiA aids in KaiC autophosphorylation.

By subjective dusk, KaiC is fully phosphorylated and associated with KaiA, and kaiBC transcription has reached its peak level. KaiB protein is still membrane localized. Interactions between the amino terminus of SasA (which has high sequence similarity to full-length KaiB) with KaiC may be occurring at this point, allowing for the autophosphorylation of SasA to occur while KaiB is still localized to the cell membrane. Phosphorylated SasA could then transfer its phosphoryl group to activate its cognate, as-yet unidentified, response regulator (SasR), which transmits timing information to downstream genes. Additionally, the putative SasR protein could react, either directly or indirectly, with the kaiBC promoter, to negatively inhibit transcription. At CT20, KaiB is released from the membrane, and association of KaiB with the KaiC/KaiA/SasA complex is initiated, reducing the autophosphorylation of KaiC, and subsequently the phosphorylation of SasA. Introduction of KaiB to the clock complex itself, or the action of inhibiting KaiC phosphorylation, may induce dissociation of the of the Kai clock complex by the subsequent subjective dawn.

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Ditty JL, Williams SB, Golden SS. A Cyanobacterial circadian timing mechanism. (2003) Annu. Rev. Genet. 37:513–43.

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