The phytochrome family of sensory photoreceptors transduces environmental light signals to responsive nuclear genes by poorly defined pathways. The recent application of yeast two-hybrid library screens to the identification of components that physically interact with members of the phytochrome family has dramatically altered previous views of the likely intracellular signaling pathways. The evidence indicates that one pathway involves light-triggered 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 proposed to function as integral, light-switchable components of transcriptional regulator complexes targeting environmental light signals directly and instantly to specific gene promoters.