White Collar-1, a circadian blue light photoreceptor, binding to the frequency promoter.

In the fungus Neurospora crassa, the blue light photoreceptor(s) and signaling pathway(s) have not been identified. We examined light signaling by exploiting the light sensitivity of the Neurospora biological clock, specifically the rapid induction by light of the clock component frequency (frq). Light induction of frq is transcriptionally controlled and requires two cis-acting elements (LREs) in the frq promoter. Both LREs are bound by a White Collar-1 (WC-1)/White Collar-2 (WC-2)-containing complex (WCC), and light causes decreased mobility of the WCC bound to the LREs. The use of in vitro-translated WC-1 and WC-2 confirmed that WC-1, with flavin adenine dinucleotide as a cofactor, is the blue light photoreceptor that mediates light input to the circadian system through direct binding (with WC-2) to the frq promoter.