Abstract
In the
Neurospora circadian system, the transcription factors White Collar-1 (WC-1) and White Collar-2 (WC-2) activate expression of
frq, whose gene product inhibits its own expression. The WC proteins are thought to form an obligate complex; however, chromatin immunoprecipitation (ChIP) indicates that WC-2 binds to the
frq promoter in a rhythmic fashion, whereas WC-1 is bound continuously. Small oscillations in histone acetylation are detected over the circadian cycle with a marked reduction upon light-induced activation. Nuclease accessibility experiments indicate chromatin rearrangement at the
frq promoter; therefore, 19 genes with homology to ATP-dependent chromatin-remodeling enzymes were deleted and the strains examined for clock phenotypes. One gene, designated
clockswitch (
csw-1), is required for clock function; its product localizes to the
frq promoter, is required for proper
frq expression, and has an impact on chromatin structure. The data suggest that CSW-1 regulates accessibility of promoter DNA, thus generating the sharp transition from the transcriptionally active to the repressed state.