Cryptochrome Overview

Abstract

Cryptochromes were first discovered in Arabidopsis where a mutation conferring a deficiency in blue light signaling was shown to reside in a gene encoding a protein with similarities to photolyases ([Ahmad and Cashmore 1993]).The latter are flavoproteins that mediate the repair of pyrimidine dimers, generated as a result of exposure of DNA to UV-B light ([Sancar 2003]). This DNA repair activity of photolyases is dependent on irradiation with blue or UV-A light and results from transfer of an electron from the photolyase-bound flavin to the damaged pyrimidine dimer, which then undergoes isomerization to yield the monomer; the electron is returned to the photolyase. In these respects photolyases are photoreceptors mediating blue light-dependent redox reactions, and in view of the similarities between the Arabidopsis cry1 gene and photolyases it was proposed that CRY1 was also a blue light photoreceptor. Cryptochromes lack the DNA repair activity of photolyases and, at least in plants, cryptochromes are characterized by a distinguishing C-terminal extension ([Cashmore 2003]). Cryptochromes have now been characterized for several additional plant species including tomato ([Ninu et al 1999],[Weller et al 2001]) and rice ([Matsumoto et al 2003]). In both cases, as in Arabidopsis, these cryptochromes apparently play a role in blue light-mediated de-etiolation and photomorphogenesis.