NEUROSPORA WC-1 AND WC-2 - TRANSCRIPTION, PHOTORESPONSES, AND THE ORIGINS OF CIRCADIAN RHYTHMICITY

Circadian rhythmicity is universally associated with the ability to pe rceive light, and the oscillators (''clocks'') giving rise to these rh ythms, which are feedback loops based on transcription and translation , are reset by light. Although such loops must contain elements of pos itive and negative regulation, the clock genes analyzed to date-frq in Neurospora and per and tim in Drosophila-are associated only with neg ative feedback and their biochemical functions are largely inferred. T he white collar-1 and white collar-2 genes, both global regulators of photoresponses in Neurospora, encode DNA binding proteins that contain PAS domains and are believed to act as transcriptional activators. Da ta shown here suggest that wc-1 is a clock-associated gene and wc-2 is a clock component; both play essential roles in the assembly or opera tion of the Neurospora circadian oscillator. Thus DNA binding and tran scriptional activation can now be associated with a clock gene that ma y provide a positive element in the feedback loop. In addition, simila rities between the PAS-domain regions of molecules involved in light p erception and circadian rhythmicity in several organisms suggest an ev olutionary link between ancient photoreceptor proteins and more modern proteins required for circadian oscillation.