The C termini of Arabidopsis cryptochromes mediate a constitutive light response

Cryptochrome blue light photoreceptors share sequence similarity to photoly ases, flavoproteins that mediate light-dependent DNA repair. However, crypt ochromes lack photolyase activity and are characterized by distinguishing C -terminal domains. Here we show that the signaling mechanism of Arabidopsis cryptochrome is mediated through the C terminus. On fusion with beta -gluc uronidase (GUS), both the Arabidopsis CRY1 C-terminal domain (CCT1) and the CRY2 C-terminal domain (CCT2) mediate a constitutive light response. This constitutive photomorphogenic (COP) phenotype was not observed for mutants of cct1 corresponding to previously described cry1 alleles. We propose that the C-terminal domain of Arabidopsis cryptochrome is maintained in an inac tive state in the dark. Irradiation with blue light relieves this repressio n, presumably through an intra- or intermolecular redox reaction mediated t hrough the flavin bound to the N-terminal photolyase-like domain.