PRD-2 directly regulates <i>casein kinase I</i> and counteracts nonsense-mediated decay in the Neurospora circadian clock.

Abstract

Circadian clocks in fungi and animals are driven by a functionally conserved transcription-translation feedback loop. In <i>Neurospora crassa</i>, 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 <i>prd-2</i> (<i>period-2</i>) gene, whose mutation is characterized by recessive inheritance of a long 26 hr period phenotype, encodes an RNA-binding protein that stabilizes the <i>ck-1a</i> transcript, resulting in CKI protein levels sufficient for normal rhythmicity. Moreover, by examining the molecular basis for the short circadian period of <i>upf-1^prd-6</i> 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 <i>ck-1a</i> regulation is consistent with circadian period being exquisitely sensitive to levels of <i>casein kinase I</i>.