Circadian RNA expression is essential to regulate a plethora of downstream rhythmic processes. Both transcriptional and posttranscriptional mechanisms are important to drive rhythmic RNA expression; however, the extent to which each process contributes to this rhythmicity remains controversial. By monitoring RNA dynamics during a circadian cycle in mouse fibroblasts, we found that rhythmic RNA synthesis is the primary contributor of 24-h RNA rhythms, while rhythmic degradation is more important for 12-h RNA rhythms. Interestingly, core clock RNAs are regulated by multiple rhythmic processes and have the highest amplitude of synthesis and degradation, presumably critical to drive robust cell-autonomous rhythmicity. Overall, our study yields invaluable insights into the temporal dynamics of both 24- and 12-h RNA rhythms.
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