CLOCK is an acronym for Circadian Locomotor Output Cycles Kaput, identifying a core transcriptional factor that is indispensable for the molecular machinery of the circadian clock in mammalian cells. This protein is a helix-loop-helix transcription factor, functioning as a positive regulator within the intricate genetic feedback loop that generates the body’s 24-hour rhythm. Its activity is essential for the rhythmic expression of thousands of downstream genes that control metabolism and behavior.
Origin
The gene was initially identified and named in mice in the 1990s following a forward genetic screen where a mutation in this locus caused a significant disruption (kaput) in the normal circadian rhythm of locomotor activity. This discovery was a landmark event in chronobiology, revealing the genetic basis of the internal biological clock. The naming convention is highly specific to the observed phenotype in the original model.
Mechanism
CLOCK functions by forming a heterodimer complex with another transcription factor, BMAL1. This CLOCK:BMAL1 complex binds to specific DNA sequences known as E-box elements in the promoter regions of target genes, including the negative regulators Period (Per) and Cryptochrome (Cry). The transcriptional activation of these negative regulators initiates a feedback loop that ultimately represses the CLOCK:BMAL1 complex, completing the cycle and driving the approximately 24-hour oscillation of gene expression.
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