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RESEARCH PAPER

Molecular mechanism of light responses in Neurospora: from light-induced transcription to photoadaptation

Department of Physiology, The University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA

Blue light regulates many molecular and physiological activities in a large number of organisms. In Neurospora crassa, a eukaryotic model system for studying blue-light responses, the transcription factor and blue-light photoreceptor WHITE COLLAR-1 (WC-1) and its partner WC-2 are central to blue-light sensing. Neurospora's light responses are transient, that is, following an initial acute phase of induction, light-regulated processes are down-regulated under continuous illumination, a phenomenon called photoadaptation. The molecular mechanism(s) of photoadaptation are not well understood. Here we show that a common mechanism controls the light-induced transcription of immediate early genes (such as frq, al-3, and vvd) in Neurospora, in which light induces the binding of identical large WC-1/WC-2 complexes (L-WCC) to the light response elements (LREs) in their promoters. Using recombinant proteins, we show that the WC complexes are functional without the requirement of additional factors. In vivo, WCC has a long period photocycle, indicating that it cannot be efficiently used for repeated light activation. Contrary to previous expectations, we demonstrate that the light-induced hyperphosphorylation of WC proteins inhibits bindings of the L-WCC to the LREs. We show that, in vivo, due to its rapid hyperphosphorylation, L-WCC can only bind transiently to LREs, indicating that WCC hyperphosphorylation is a critical process for photoadaptation. Finally, phosphorylation was also shown to inhibit the LRE-binding activity of D-WCC (dark WC complex), suggesting that it plays an important role in the circadian negative feedback loop.

[Keywords: white collar-1; light; LOV domain; photoadaptation; phosphorylation; circadian]

Received August 29, 2005; revised version accepted September 26, 2005.

¹ Corresponding author.

E-MAIL Yi.Liu{at}UTSouthwestern.edu; FAX (214) 645-6049.

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