The Role Of The N-terminal Domain-mediated Homodimerization In The Light Activation Of Arabidopsis CRYPTOCHROME 1 Photoreceptor

Arabidopsis cryptochromes (CRY) are blue light receptor that share sequencesimilarity with photolyases, flavoproteins that catalyze the repair of UV light–damagedDNA. CRY1 is the major photoreceptor mediating blue light inhibition of hypocotylselongation. CRY1 has an N-terminal photolyase-like domain and a distinguishingC-terminal domain that is absent in photolyase. Transgenic Arabidopsis thalianaseedlings expressing the C-terminal domains of the Arabidopsis CRY fused toβ-glucuronidase (GUS) display a constitutive photomorphogenic (COP) phenotype,indicating that the signaling mechanism of Arabidopsis CRY is mediated through theC-terminal domain. The role of the Arabidopsis CRY N-terminal photolyase-likedomain in CRY action remains poorly understood. Here, we report the essential roleof the Arabidopsis CRY1 N-terminal domain (CNT1) in the light activation of CRY1photoreceptor activity. Yeast two-hybrid assay studies indicate that CRY1homodimerizes in a light-independent manner. Studies of transgenic plantsexpressing point mutation and deletions within CNT1 demonstrate thatCNT1-mediated dimerization is required for light activation of the C-terminal domain ofCRY1 (CCT1). Transgenic data suggest that multimerization of GUS is bothresponsible and required for mediating a COP phenotype on fusion to CCT1. Deletionfragments of GUS did not interact with GUS and transgenic seedlings expressingthese fragments fused to CCT1 did not mediate a COP phenotype. These resultsindicate that the properties of the GUS multimer are analogous to those of thelight-modified CNT1 dimer. Irradiation with blue light modifies the properties of theCNT1 dimer, resulting in a change in CCT1, activating CCT1, and eventuallytriggering the CRY1 signaling pathway.