Cryptochromes are blue-light receptors that have presumably evolved from the DNA photolyase protein family, and the genomes of many organisms contain genes for both types of molecules. Both protein structures resemble each other, which suggests that light control and light protection share a common ancient origin. In the genome of the filamentous fungus Aspergillus nidulans, however, only one cryptochrome/photolyase-encoding gene, termed cryA, was identified. Deletion of the cryA gene triggers sexual differentiation under inappropriate culture conditions and results in upregulation of transcripts encoding regulators of fruiting body formation. CryA is a protein whose N-and C-terminal synthetic green fluorescent protein fusions localize to the nucleus. CryA represses sexual development under UVA 350-370 nm light both on plates and in submerged culture. Strikingly, CryA exhibits photorepair activity as demonstrated by heterologous complementation of a DNA repair-deficient Escherichia coli strain as well as overexpression in an A. nidulans uvsB⌬ genetic background. This is in contrast to the single deletion cryA⌬ strain, which does not show increased sensitivity toward UV-induced damage. In A. nidulans, cryA encodes a novel type of cryptochrome/photolyase that exhibits a regulatory function during light-dependent development and DNA repair activity. This represents a paradigm for the evolutionary transition between photolyases and cryptochromes.
INTRODUCTIONCryptochromes (CRYs) are blue-light receptors that regulate growth, development, and the circadian clock in higher eukaryotes and that are believed to have evolved from the DNA photolyase protein family (Daiyasu et al., 2004;Lin and Todo, 2005). Photolyases, in contrast, repair UV-induced DNA damage by using a mechanism referred to as photorepair. They absorb light in the blue spectrum and transfer an excited electron from the cofactor FAD to an enzyme-bound cyclobutane pyrimidine dimer (CPD), which is thereby cleaved (Sancar, 1990(Sancar, , 1994Sancar, 1994). Cryptochromes are characterized by an N-terminal photolyase-related (PHR) region without significant photorepair activity and by a C-terminal domain of varying length, which is absent in members of the CRY-DASH subfamily (Daiyasu et al., 2004). The exact function of the CRY-DASH proteins was elusive; however, recent data imply that they are actually photolyases that specifically repair CPDs in single-stranded DNA (Selby and Sancar, 2006). The PHR domain is the most conserved region of the CRY proteins and contains the cofactor FAD required for electron transfer reactions. In Xenopus and Drosophila, the PHR domain is physiologically active even in the absence of the C-terminal domain. The C-terminal domain is important for either localization or protein stability, and its expression in Arabidopsis results in constitutive growth (Lin and Todo, 2005). Plant CRYs are phosphorylated under illumination, and gene expression control ranges from protein interactions to direct chromatin interactions (Shalitin et al....