Phytochrome-interacting factors

The phytochrome family of sensory photoreceptors transduces environmental l ight signals to responsive nuclear genes by poorly defined pathways. The re cent application of yeast two-hybrid library screens to the identification of components that physically interact with members of the phytochrome fami ly has dramatically altered previous views of the likely intracellular sign aling pathways. The evidence indicates that one pathway involves light-trig gered translocation of the photoreceptor molecule from cytoplasm to nucleus where it Binds specifically in its biologically active form to a promoter- bound basic helix-loop-helix protein. The phytochrome molecules are propose d to function as integral, light-switchable components of transcriptional r egulator complexes targeting environmental light signals directly and insta ntly to specific gene promoters.