Direct targeting of light signals to a promoter element-bound transcription factor

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 specif ically to a G-box DNA-sequence motif present in various Light-regulated gen e promoters, and that phytochrome B binds reversibly to G-box-bound PIF3 sp ecifically upon light-triggered conversion of the photoreceptor to its biol ogically active conformer. We suggest that the phytochromes may function as integral Light-switchable components of transcriptional regulator complexe s, permitting continuous and immediate sensing of changes in this environme ntal signal directly at target gene promoters.