PIF3, A PHYTOCHROME-INTERACTING FACTOR NECESSARY FOR NORMAL PHOTOINDUCED SIGNAL-TRANSDUCTION, IS A NOVEL BASIC HELIX-LOOP-HELIX PROTEIN

The mechanism by which the phytochrome (phy) photoreceptor family tran sduces informational light signals to photoresponsive genes is unknown . Using a yeast two-hybrid screen, we have identified a phytochrome-in teracting factor, PIF3, a basic helix-loop-helix protein containing a PAS domain. PIF3 binds to wild-type C-terminal domains of both phyA an d phyB, but less strongly to signaling-defective, missense mutant-cont aining domains. Expression of sense or antisense PIF3 sequences in tra nsgenic Arabidopsis perturbs photoresponsiveness in a manner indicatin g that PIF3 functions in both phyA and phyB signaling pathways in vivo . PIF3 localized to the nucleus in transient transfection experiments, indicating a potential role in controlling gene expression. Together, the data suggest that phytochrome signaling to photoregulated genes i ncludes a direct pathway involving physical interaction between the ph otoreceptor and a transcriptional regulator.