The recently identified plant photoreceptor UVR8 triggers regulatory changes in gene expression in response to ultraviolet-B (UV-B) light via an unknown mechanism. Here, crystallographic and solution structures of the UVR8 homodimer, together with mutagenesis and far-UV circular dichroism spectroscopy, reveal its mechanisms for UV-B perception and signal transduction. β-propeller subunits form a remarkable, tryptophan-dominated, dimer interface stitched together by a complex salt-bridge network. Salt-bridging arginines flank the excitonically coupled cross-dimer tryptophan “pyramid” responsible for UV-B sensing. Photoreception reversibly disrupts salt bridges, triggering dimer dissociation and signal initiation. Mutation of a single tryptophan to phenylalanine re-tunes the photoreceptor to detect UV-C wavelengths. Our analyses establish how UVR8 functions as a photoreceptor without a prosthetic chromophore to promote plant development and survival in sunlight.

Cryptochromes regulate the circadian clock in animals and plants. Humans and mice have two cryptochrome (Cry) genes. A previous study showed that mice lacking the Cry2 gene had reduced sensitivity to acute light induction of the circadian gene mPer1 in the suprachiasmatic nucleus (SCN) and had an intrinsic period 1 hr longer than normal. In this study, Cry1 ؊͞؊ and Cry1 ؊͞؊ Cry2 ؊͞؊ mice were generated and their circadian clocks were analyzed at behavioral and molecular levels. Behaviorally, the Cry1 ؊͞؊ mice had a circadian period 1 hr shorter than wild type and the Cry1 ؊͞؊ Cry2 ؊͞؊ mice were arrhythmic in constant darkness (DD). Biochemically, acute light induction of mPer1 mRNA in the SCN was blunted in Cry1 ؊͞؊ and abolished in Cry1 ؊͞؊ Cry2 ؊͞؊ mice. In contrast, the acute light induction of mPer2 in the SCN was intact in Cry1 ؊͞؊ and Cry1 ؊͞؊ Cry2 ؊͞؊ animals. Importantly, in double mutants, mPer1 expression was constitutively elevated and no rhythmicity was detected in either 12-hr light͞12-hr dark or DD, whereas mPer2 expression appeared rhythmic in 12-hr light͞12-hr dark, but nonrhythmic in DD with intermediate levels. These results demonstrate that Cry1 and Cry2 are required for the normal expression of circadian behavioral rhythms, as well as circadian rhythms of mPer1 and mPer2 in the SCN. The differential regulation of mPer1 and mPer2 by light in Cry double mutants reveals a surprising complexity in the role of cryptochromes in mammals.gene targeting ͉ photoreceptor ͉ suprachiasmatic nucleus C ircadian rhythms are oscillations with daily periodicities in physiological and behavioral functions of organisms (1-3). The rhythms are generated by a cell-autonomous circadian oscillator (4) that is synchronized with the environment by light. Recently, it was proposed that, in mammals, the nonopsin pigments, cryptochrome blue-light photoreceptors (5, 6), may be the photoactive pigments that synchronized the molecular oscillator and, ultimately, the organismic circadian rhythm with the daily light-dark cycle (7,8). In humans and mice there are two genes encoding the apoproteins of the cryptochromes: CRY1 and CRY2 in humans and Cry1 and Cry2 in mice (6,8,9). Both genes are expressed throughout the body (8-11). Of particular interest, both Cry1 and Cry2 are expressed at high levels in the ganglion cells and the inner nuclear layer of the retina, which are known to be important for circadian photoreception, and Cry1 is expressed with a robust circadian rhythm in the suprachiasmatic nucleus (SCN) (8).In a previous study (12), we found that mice lacking a functional Cry2 gene (i) had reduced sensitivity to acute light induction of the clock gene mPer1 (mouse period gene 1) in the SCN, (ii) had an intrinsic circadian period about 1 hr longer than normal, and (iii) exhibited high amplitude phase shifts in response to light pulses administered at circadian time (CT) 17. These data, and related findings in cryptochrome mutants of Drosophila melanogaster (13) and Arabidopsis thaliana (14), supported the notion that cr...