The mechanism by which the phytochrome (phy) photoreceptor family transduces informational light signals to photoresponsive genes is unknown. Using a yeast two-hybrid screen, we have identified a phytochrome-interacting factor, PIF3, a basic helix-loop-helix protein containing a PAS domain. PIF3 binds to wild-type C-terminal domains of both phyA and phyB, but less strongly to signaling-defective, missense mutant-containing domains. Expression of sense or antisense PIF3 sequences in transgenic Arabidopsis perturbs photoresponsiveness in a manner indicating 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 includes a direct pathway involving physical interaction between the photoreceptor and a transcriptional regulator.
Bibliographical noteFunding Information:
We thank Drs. J. Kim and A. Theologis for the pACT two-hybrid library; Dr. K. Franke for the binary vector pKF111; Dr. K. Halliday for advice and help with fluence rate response experiments; Dr. N. Wei for the CAB , CHS , and 18S rRNA probes; Arabidopsis Biological Resources Center for the CD4-14 λZAP cDNA and CD4-21 CIC YAC libraries; D. Hantz for greenhouse plant care; Drs. J. Colasanti, C. Fairchild, U. Hoecker, M. Hudson, and E. Huq for comments on the manuscript; lab members for discussion and support; O. Barajas and J. Ryu for technical assistance; and R. Wells for help with the manuscript. Research was supported by grants from National Institutes of Health (GM47475); Department of Energy, Basic Energy Sciences (DE-FG03-87ER13742); and U.S. Department of Agriculture, Current Research Information Service (5335-21000-006-00D).