Figure 1. Proposed monarch butterfly circadian clock mechanism. The main gear of the clock mechanism is an autoregulatory transcription feedback loop in which CLK and CYC heterodimers drive the transcription of the per, tim, and cry2 genes through E box enhancer elements; in addition to per, there are CACGTG E box elements within the 1.5 kb 5’ flanking regions of the butterfly tim and cry2 genes (data not shown). TIM (T), PER (P), and CRY2 (C2) form complexes in the cytoplasm and CRY2 is shuttled into the nucleus where it shuts down CLK:CYC-mediated transcription. PER is progressively phosphorylated and likely helps translocate CRY2 into nucleus. CRY1 (C1) is a circadian photoreceptor which, upon light exposure (lightning bolt) causes TIM degradation to gain access to the central clock mechanism. The thick gray arrows represent output functions for CRY1 and for CRY2.
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