The circadian clock protein Period opposes Cryptochrome in controlling phosphorylation of CLOCK

CLOCK phosphorylation levels exhibit a circadian oscillation.

• CLOCK phosphorylation may be essential for autonomous circadian oscillation in clock gene expression.
• Cryptochrome (CRY) impairs CLOCK phosphorylation, while Period (PER) protects it from CRY's effects.
• PER3 does not protect CLOCK phosphorylation, unlike PER1 and PER2, correlating with differences in mice lacking Per genes.
• Further research is needed to clarify the regulatory mechanism of CLOCK phosphorylation and its role in transcriptional repression.

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