Diverse organisms time their cellular activities to occur at distinct phases of Earth's solar day, not through the direct regulation of these processes by light and darkness but rather through the use of an internal biological (circadian) clock that is synchronized with the external cycle. Input pathways serve as mechanisms to transduce external cues to a circadian oscillator to maintain synchrony between this internal oscillation and the environment. The circadian input pathway in the cyanobacterium Synechococcus elongatus PCC 7942 requires the kinase CikA. A cikA null mutant exhibits a short circadian period, the inability to reset its clock in response to pulses of darkness, and a defect in cell division. Although CikA is copurified with the Kai proteins that constitute the circadian central oscillator, no direct interaction between CikA and either KaiA, KaiB, or KaiC has been demonstrated. Here, we identify four proteins that may help connect CikA with the oscillator. Phenotypic analyses of null and overexpression alleles demonstrate that these proteins are involved in at least one of the functions-circadian period regulation, phase resetting, and cell division-attributed to CikA. Predictions based on sequence similarity suggest that these proteins function through protein phosphorylation, iron-sulfur cluster biosynthesis, and redox regulation. Collectively, these results suggest a model for circadian input that incorporates proteins that link the circadian clock, metabolism, and cell division.
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