A circadian rhythm in the accumulation of the core clock protein KaiC has been proposed to be important for proper circadian timing in the cyanobacterium Synechococcus elongatus PCC 7942 under continuous light conditions. Cycling in the abundance of the KaiC protein is delayed to the rhythm of its mRNA by ∼8 h, consistent with the proposed function of KaiC as a negative feedback regulator of kaiBC transcription. Here, we present temporal profiles of the synthesis and degradation of KaiC protein that determine the rhythm of its accumulation. The rate of KaiC synthesis shows a robust circadian oscillation, which is delayed to the mRNA rhythm slightly and advances the rhythm of KaiC accumulation by ∼6 h. The stability of KaiC protein also shows circadian fluctuations, such that KaiC degradation is suppressed during the mid-subjective night. These results suggest that transcriptional, translational, and posttranslational processes are important for the proper circadian changes in KaiC accumulation. Moreover, the turnovers of the phosphorylated and non-phosphorylated forms of KaiC show robust circadian rhythms with an anti-phase relationship to each other. Interestingly, when translation was inhibited, KaiC degradation and phosphorylation proceeded within at least 4 h in a circadian phase-dependent manner. Thus, the circadian timing seems flexible even when any perturbation in protein synthesis occurs.
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