Abstract

Mutations in either white collar-1 (wc-1) or white collar-2 (wc-2) lead to a loss of most blue-light-induced phenomena in Neurospora crassa. Sequence analysis and in vitro experiments show that wc-1 and wc-2 are transcription factors regulating the expression of light-induced genes. The WC proteins form homo- and heterodimers in vitro; this interaction could represent a fundamental step in the control of their activity. We demonstrate in vivo that the WC proteins are assembled in a white collar complex (WCC) and that wc-1 undergoes a change in mobility due to light-induced phosphorylation events. The phosphorylation level increases progressively upon light exposure, producing a hyperphosphorylated form that is degraded and apparently replaced in the complex by a newly synthesized wc-1. wc-2 is unmodified and also does not change quantitatively in the time frame examined. Light-dependent phosphorylation of wc-1 also occurs in a wc-2 mutant, suggesting that a functional wc-2 is dispensable for this light-specific event. These results suggest that light-induced phosphorylation and degradation of wc-1 could play a role in the transient expression of blue-light-regulated genes. Our findings suggest a mechanism by which wc-1 and wc-2 mediate light responses in Neurospora.