We showed previously that a weak 4-fold overexpression of UVR8W285A in transgenic Arabidopsis plants (in contrast with 40-fold overexpression of wild-type UVR8) results in a phenotype resembling a constitutive UV-B response (Heijde et al., 2013). to mimic UV-B signaling. We further show, in contrast with COP1, that this WD40 repeat proteins REPRESSOR OF UV-B PHOTOMORPHOGENESIS1 (RUP1) and RUP2 interact only with the UVR8 C27 domain name. This coincides with their facilitation of UVR8 reversion to the ground state by redimerization and their potential to interact with UVR8 in a UV-B-independent manner. Collectively, our results provide insight into a key mechanism of photoreceptor-mediated signaling and its unfavorable feedback regulation. INTRODUCTION The unavoidable exposure of plants to UV-B radiation (280 to 315 nm) is usually mitigated by effective toleration mechanisms. UV RESISTANCE LOCUS8 (UVR8) is usually a unique UV-B photoreceptor that, following the absorption of UV-B photons, initiates changes in gene expression (Heijde and Ulm, 2012; Li et al., 2013; Tilbrook et al., 2013; Jenkins, 2014). Targets include genes involved in phenylpropanoid biosynthesis, resulting in the accumulation of phenolic sunscreen metabolites (e.g., flavonols and sinapates) and antioxidants (anthocyanins), as well as genes encoding photolyases, which are involved in DNA repair (Kliebenstein et al., 2002; Brown et al., 2005; Favory et al., 2009; Stracke et al., 2010). The induction of genes associated with UV-B protection and repair highlights the importance of UVR8 for UV-B acclimation (Favory et al., 2009), which is usually distinct from the UV-B stress pathway involving mitogen-activated protein kinase signaling (Gonzlez Besteiro et al., 2011). In contrast with a number of UV-B light-induced genes, auxin-responsive genes are widely and rapidly repressed in response to UV-B light, and this response is also dependent on EAI045 UVR8 (Favory et al., 2009; Hayes et al., 2014; Vandenbussche et al., 2014). This may be the basis of photomorphogenic responses to UV-B such as hypocotyl growth inhibition (Ballare et al., 1995; Kim et al., 1998; Favory et al., 2009; Hayes et al., 2014; Huang et al., 2014; Vandenbussche et al., 2014). In addition to UV-B stress acclimation and hypocotyl growth inhibition, UVR8 also has been implicated in UV-B entrainment of the circadian clock, stomatal closure, phototropic bending, inhibition of shade avoidance, leaf development, and defense responses (Wargent et al., 2009; Fehr et al., 2011; Demkura and Ballar, 2012; Hayes et al., 2014; Tossi et al., 2014; EAI045 Vandenbussche et al., 2014). The UVR8 signaling pathway includes CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) and ELONGATED HYPOCOTYL5 (HY5) (Ulm et al., 2004; Brown et al., 2005; Oravecz et al., 2006; Stracke et al., 2010; Huang et al., 2012; Binkert et al., 2014) and the unfavorable feedback regulators REPRESSOR OF UV-B PHOTOMORPHOGENESIS1 (RUP1) and RUP2 (Gruber et al., 2010; Heijde and Ulm, 2013). UVR8 is usually a -propeller protein EAI045 in which intrinsic Trp residues are the basis of UV-B photoreception (Rizzini et al., 2011; Wu et al., 2011, 2012; Christie et al., 2012; Liu et al., 2014). UVR8 exists as a homodimer that readily monomerizes in response to UV-B (Rizzini et al., 2011; Christie et al., 2012; Wu et al., 2012). UV-B-activated UVR8 interacts with COP1 (Favory et al., 2009), which is a major factor in the UVR8-mediated signal transduction pathway (Oravecz et al., 2006). The EAI045 C-terminal C27 domain name (UVR8397-423) was found to be necessary and sufficient for UVR8 conversation with COP1, and thus C27 represents the COP1-conversation domain name (Cloix et al., 2012). In support of this, UVR8C27 is usually UV-B-responsive (monomerization, nuclear accumulation) but is usually impaired in UV-B-dependent COP1 conversation (Cloix et al., 2012). Furthermore, C27 was found to interact constitutively with COP1 in a yeast two-hybrid assay (Cloix et al., 2012). However, it was not known whether the C27 domain name is sufficient to activate UV-B-related responses in vivo. To better understand UVR8-mediated early UV-B signaling, we focused on the -propeller and the C-terminal regions of UVR8, including the C27 Rabbit Polyclonal to LFNG domain, in yeast and plants. We show here that this -propeller domain name of UVR8 interacts with COP1 in a UV-B-dependent manner in the absence of the C-terminal 44 amino acids and, thus, the C27 domain name. However, the -propeller domain name alone is not sufficient to activate early UV-B signaling. We further demonstrate that this C-terminal 44 amino acids alone interact constitutively with COP1 and that this depends on a Val-Pro (VP) pair in the C27 domain name. Chemically induced expression of the C-terminal 44 amino acids is sufficient to mimic early UVR8 responses. Thus, UVR8 conversation with COP1 is usually 2-fold:.