PRD-2 directly regulates and counteracts nonsense-mediated decay in the Neurospora circadian clock.

Circadian clocks in fungi and animals are driven by a functionally conserved transcription-translation feedback loop. In , negative feedback is executed by a complex of Frequency (FRQ), FRQ-interacting RNA helicase (FRH), and casein kinase I (CKI), which inhibits the activity of the clock's positive arm, the White Collar Complex (WCC). Here, we show that the () gene, whose mutation is characterized by recessive inheritance of a long 26 hr period phenotype, encodes an RNA-binding protein that stabilizes the transcript, resulting in CKI protein levels sufficient for normal rhythmicity. Moreover, by examining the molecular basis for the short circadian period of mutants, we uncovered a strong influence of the Nonsense Mediated Decay pathway on CKI levels. The finding that circadian period defects in two classically derived Neurospora clock mutants each arise from disruption of regulation is consistent with circadian period being exquisitely sensitive to levels of .