The cryptochrome blue light photoreceptors were first described in the plant Arabidopsis. These photoreceptors are widespread, if not ubiquitous, throughout the plant kingdom, apparently present in all higher plants as well as ferns and algae. Cryptochrome-mediated blue light signaling affects a variety of plant processes including stem elongation, pigment production, gene expression, flowering time, and the entrainment of circadian rhythms. Cryptochromes have also been described for animals, including humans, where they play an essential role in circadian behavioral rhythms. Dark-grown transgenic Arabidopsis plants expressing the C-terminal domain of cryptochrome exhibit features that are normally associated with light-grown plants, indicating that the signaling properties of cryptochrome reside within this C-terminal domain. Apparently this signaling potential is repressed in the native cryptochrome molecule in the dark, this repression being overridden by the action of light. Here we propose to define the factors that are necessary to reconstruct in vitro the light-dependent signaling activity of cryptochrome. In addition, we will define the mechanism and the domain of the cryptochrome protein necessary for repressing cryptochrome signaling activity in the dark. We will use microarrays to identify genes whose expression is dependent on cryptochrome and we will perform chromatin immunoprecipitation studies to characterize the association of cryptochrome and other signaling partners with chromatin. Finally, we will further characterize new mutants of Arabidopsis that, like cryptochrome mutants, are selectively deficient in blue light signaling and therefore define new genes that regulate the light signaling process.