Role of a white collar‐1–white collar‐2 complex in blue‐light signal transduction

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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 .

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