Abstract
Circadian rhythms exist in most if not all organisms on the Earth and manifest in various aspects of physiology and behavior. These rhythmic processes are believed to be driven by endogenous molecular clocks that regulate rhythmic expression of clock-controlled genes (CCGs). CCGs consist of a significant portion of the genome and are involved in diverse biological pathways. The transcription of CCGs is tuned by rhythmic actions of transcription factors and circadian alterations in chromatin. Here, we review the circadian control of CCG transcription in five model organisms that are widely used, including cyanobacterium, fungus, plant, fruit fly, and mouse. Comparing the similarity and differences in the five organisms could help us better understand the function of the circadian clock, as well as its output mechanisms adapted to meet the demands of diverse environmental conditions.
Highlights
Circadian rhythms, controlled by endogenous circadian clocks, are rhythmic oscillations in our behavior and physiological processes with a period close to 24 h
Circadian clocks in diverse organisms are composed of molecular feedback loops, but these are coordinated in somewhat different ways with different factors that are presented in detail here, based on our knowledge from five widely used model systems: the freshwater cyanobacterium Synechococcus elongatus, the filamentous fungus Neurospora crassa, the thale cress Arabidopsis thaliana, the fruit fly Drosophila melanogaster, and the house mouse Mus musculus
clockcontrolled genes (CCGs) consist of a substantial portion of the genome
Summary
Circadian rhythms, controlled by endogenous circadian clocks, are rhythmic oscillations in our behavior and physiological processes with a period close to 24 h. The circadian clocks are organized around three major physiological components: an input pathway that receives environmental cues and entrain the oscillator, a central oscillator that keeps circadian time and generates rhythms, and an output pathway that generates manifested rhythmic processes throughout the body [7]. Fruit flies, and mammals, the positive elements of the circadian negative feedback loops are heterodimeric complexes of two PER-ARNT-SIM domaincontaining transcription factors that activate the transcription of negative elements. The cyclic activation, repression, and reactivation of circadian negative elements generate circadian rhythmicity, which regulates the circadian output pathway by driving downstream clock-controlled gene (CCG) expression [8,9,10]
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