WHITE COLLAR-1, a multifunctional Neurospora protein involved in the circadian feedback loops, light sensing, and transcription repression of Wc-2

WHITE COLLAR-1 (WC-1) and WC-2, the two PAS domain-containing transcription factors, are the positive elements of the circadian feedback loops in Neurospora. In addition, both proteins are essential components for the light input of various blue light responses, including the light entrainment of the circadian clock. Recently, we identified WC-1 as the blue light photoreceptor responsible for these light responses. In this study, we show that the formation of the FRQ-WC complex in vivo, a step critical in closing the circadian negative feedback loop, requires WC-1. In addition, we show that WC-1 negatively regulates the expression of wc-2 at the level of the transcription, forming another interacting loop. In a wc-1 mutant, we demonstrate that there is alternative protein initiation of WC-1, and the requirements of WC-1 for the light induction of frq and other genes differ significantly, suggesting the existence of different WC complexes in the cell. Consistent with this interpretation, our results show that there are at least two different types of WC-1/WC-2 complexes in vivo, and that the larger WC-1/WC-2 complex contains more than one WC-1 molecule. Using a series of wc-1 mutants, we show that the WC-1 PASC domain and its C-terminal region are essential for the formation of the WC-1/WC-2 complex. Functional analyses reveal that the DNA-binding domain of WC-1 is required only for the activation of frq in the dark and not for the light function of the protein, confirming that WC-1 is a multifunctional protein with separable protein domains.