Being in time: the importance of post-translational processes in circadian clocks

The well-known daily variations in behaviour, physiology and metabolism are generated by endogenous timekeepers known as circadian clocks. The core clockwork is an oscillator that keeps an approximate 24-hour rhythm and is synchronised to the outside day mainly by environmental light. This central oscillator operates at the level of a single cell, generating circadian rhythms of clock gene expression through negative transcriptional feedback. Accumulating evidence points to the importance of post-translational processes for the exact timing of clock protein oscillations. Output pathways then translate the protein cycles into physiological and molecular rhythms, directing these processes to an appropriate time of the day.