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Abstract
Light signals perceived by the phytochrome family of sensory photoreceptors are transduced to photoresponsive genes by an unknown mechanism. Here, we show that the basic helix-loop-helix transcription factor PIF3 binds specifically to a G-box DNA-sequence motif present in various light-regulated gene promoters, and that phytochrome B binds reversibly to G-box–bound PIF3 specifically upon light-triggered conversion of the photoreceptor to its biologically active conformer. We suggest that the phytochromes may function as integral light-switchable components of transcriptional regulator complexes, permitting continuous and immediate sensing of changes in this environmental signal directly at target gene promoters.
↵* To whom correspondence should be addressed. E-mail: quail{at}nature.berkeley.edu
