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White Collar-2 (wc-2) is a gene in Neurospora crassa that codes for the protein WC-2 (54 kDa). WC-2 is a GATA transcription factor necessary for regulating circadian rhythm s in Frequency (FRQ) expression in fungi. FRQ is a light-induced clock protein involved in an autoregulatory transcription-translation negative feedback loop that controls rhythmic behaviors including conidiation. WC-2 binds to its non-redundant counterpart White Collar-1 (WC-1) through PAS domain s to form the White Collar Complex (WCC), which is transcriptionally active. In the early subjective night, the WCC binds to the frequency (frq) promoter via zinc finger (ZnF) DNA-binding domains and positively regulates FRQ expression. While FRQ and WC-1 are rhythmically expressed, WC-2 is constitutively expressed.

Protein Structure
WC-2 is a soluble nuclear protein composed of 530 amino acids (aa).

WC-2 has a putative GATA-family ZnF DNA-binding motif that allows the WCC to bind to promoter elements of light-induced genes such as frq. WC-2 also possesses a putative PAS domain that allows protein-protein interactions. WC-2 and WC-1 heterodimerize using PAS domains to form the WCC in vivo.

Role of WC-2
WC-2 has been shown to be necessary for both clock-driven and light-driven expression of frq. While the ZnF and DBD regions of WC-1 are not required for induction of light-responsive genes, the ZnF region of WC-2—which is very similar to that of WC-1 (58% identity match)—is necessary for mediating the association of the frq promoter of WCC in light conditions. WC-2 mutants with altered ZnF and 499-504 aa sequences failed to bind to the frq promoter via the Clock box (C box) and FRQ expression and DNA binding activity were both significantly decreased/impaired; therefore, the WC-2 ZnF and 499-504 aa sequences are essential for FRQ expression in light conditions and for WCC circadian function. A WC-2 null allele (ER33), after hit with a light pulse, showed partial frq transcription induction. This shows that light-driven frq induction is dependent on WC-2; however, it is partially independent of the WC-2 ZnF region, suggesting an alternative mechanism for light-driven frq induction as compared to that of other Neurospora light-induced genes and that of frq regulation in dark conditions.

Interaction between WC-1 and WC-2
WC-1 is a photopigment that requires flavin adenine dinucleotide (FAD) and blue light to begin its conformational change. After exposure to light, WC-1’s Cys71 residue changes its hydrogen bonding partner. This causes a change in the alpha helix which leads to an open pocket to which WC-2 can bind. In light conditions, WCC takes on the L-WCC conformation, a heterotrimer consisting of two WC-1s and one WC-2. In dark conditions, the WCC adopts a heterodimer conformation, (D-WCC) consisting of WC-1 and WC-2. WC-1 and WC-2 are thought to be the only non-redundant and non-essential genes that are involved in the positive regulation of light-induced carotenogenesis in Neurospora, and are analogous to Clock and Bmal proteins in the Mouse and Drosophila circadian systems in their positive functions in the circadian loops.