Both regulated expression of the clock genes kaiA, kaiB, and kaiC and interactions among the Kai proteins are proposed to be important for circadian function in the cyanobacterium Synechococcus sp. strain PCC 7942. We have identified the histidine kinase SasA as a KaiC-interacting protein. SasA contains a KaiB-like sensory domain, which appears sufficient for interaction with KaiC. Disruption of the sasA gene lowered kaiBC expression and dramatically reduced amplitude of the kai expression rhythms while shortening the period. Accordingly, sasA disruption attenuated circadian expression patterns of all tested genes, some of which became arrhythmic. Continuous sasA overexpression eliminated circadian rhythms, whereas temporal overexpression changed the phase of kaiBC expression rhythm. Thus, SasA is a close associate of the cyanobacterial clock that is necessary to sustain robust circadian rhythms.
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