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. Author manuscript; available in PMC: 2013 Jan 1.
Published in final edited form as: FEMS Microbiol Rev. 2011 Aug 1;36(1):95–110. doi: 10.1111/j.1574-6976.2011.00288.x

Figure 1. Oscillator model for the circadian clock.

Figure 1

(A) Circadian systems comprise three essential elements: an endogenous self-sustaining oscillator, an ability to sense environmental time cues (Input), and physiological output tied to the oscillator at distinct phases. Environmental variables such as light and temperature can entrain or couple the core clock via input pathways. The central oscillator is based on the interplay between positive acting factors driving the expression of negative factors which feedback to inhibit the positive complex. A critical component to this feedback is a mechanism for time delay of the negative state variable that reflects the ~24 hour nature of the physiological rhythms. (B) The organization of molecular components of the feedback loop in Neurospora crassa. The entraining variables of light and temperature impact different parts of the oscillator, light acting through the photoreceptor function of WC-1 to induce transcription of frq and temperature acting to modulate amounts of FRQ. WC-1 and WC-2 form the WCC complex. FRQ and FRH form the FFC complex. Rhythmic output is primarily generated through rhythmic expression of clock-controlled genes (ccgs) but can also be due to changes in mRNA stability and possibly phosphorylation. Kinases contribute to the time-delay by influencing the stability of FRQ.