CRYPTOCHROMES confer robustness, not rhythmicity, to circadian timekeeping.

Circadian rhythms are a pervasive property of mammalian cells, tissues and behaviour, ensuring physiological adaptation to solar time. Models of cellular timekeeping revolve around transcriptional feedback repression, whereby CLOCK and BMAL1 activate the expression of PERIOD (PER) and CRYPTOCHROME (CRY), which in turn repress CLOCK/BMAL1 activity. CRY proteins are therefore considered essential components of the cellular clock mechanism, supported by behavioural arrhythmicity of CRY‐deficient (CKO) mice under constant conditions. Challenging this interpretation, we find locomotor rhythms in adult CKO mice under specific environmental conditions and circadian rhythms in cellular PER2 levels when CRY is absent. CRY‐less oscillations are variable in their expression and have shorter periods than wild‐type controls. Importantly, we find classic circadian hallmarks such as temperature compensation and period determination by CK1δ/ε activity to be maintained. In the absence of CRY‐mediated feedback repression and rhythmic Per2 transcription, PER2 protein rhythms are sustained for several cycles, accompanied by circadian variation in protein stability. We suggest that, whereas circadian transcriptional feedback imparts robustness and functionality onto biological clocks, the core timekeeping mechanism is post‐translational. SYNOPSIS: CRYPTOCHROME (CRY) proteins are central regulators of the circadian clock transcription/translation feedback loop. The finding that circadian timekeeping persists, albeit with reduced robustness, in CRY‐deficient cells and mice suggests that clock gene activity is determined by evolutionarily‐conserved post‐translational timing mechanisms. CRY‐mediated transcriptional feedback is dispensable for circadian timekeeping in mammalian cells, but functions to make rhythms more robust.CRY knockout mice exhibit behavioural rhythmicity under specific environmental conditions.Circadian variation in the abundance of core clock component PER2 is amplified by, but does not require, rhythmic Per2 transcription.CK1 and GSK3 kinases regulate PER2 stability in the absence of CRY. [ABSTRACT FROM AUTHOR]