Figure 6. Counterbalancing regulation of casein kinase I (CKI) provides a unifying genetic model for the action of PRD-2 and UPF1^PRD-6 in the circadian oscillator.

The NMD complex (UPF1^PRD-6, UPF2, and UPF3) targets the frq and ck-1a transcripts for degradation (upstream uORFs in frq; long 3’-UTR in ck-1a). PRD-2 binds to and stabilizes ck-1a transcripts (dashed lines), which could also promote local translation and complex formation for the negative arm of the clock. In the absence of PRD-2, the long period phenotype is due to low CKI levels, and in the absence of NMD, the short period phenotype is due to high CKI levels.