Circadian rhythms, endogenous biological oscillations with a period of about a day, have been observed in innumerable physiological processes in most of organisms from cyanobacteria to higher plants and mammals. These rhythms are thought to be adaptive to daily environmental changes on the earth. These rhythms are daily fluctuations in cellular, physiological, and behavioral parameters, which are observed ubiquitously in many organisms. The circadian system is composed schematically of a central oscillator and inputs to and outputs from the clock. The cyanobacterium is the simplest organism known to harbor circadian clocks, and now becomes one of most successful model organisms for comprehensive understanding the oscillatory system at the molecular level. Molecular genetic studies on circadian clocks in the cyanobacterium Synechococcus identified two histidine kinases, SasA, and CikA. SasA interacts with a well-established clock protein KaiC and is necessary to sustain robust circadian oscillation, whereas CikA functions in a photic input pathway to the clock. Thus, multiple His-to-Asp signaling pathways are likely to play important roles in the Synechococcus circadian system.
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