Blue Light Receptors and Signal Transduction

Abstract

Functions of Blue Light Receptors in Phototropism, Photomorphogenesis, and Photoperiodic Flowering. Solid arrows depict light, and dashed arrows depict signal transduction of photoreceptors. Cryptochromes share sequence similarity to the DNA repair enzyme photolyase but have no DNA repair activity. Cryptochromes and DNA photolyases share similarities not only in amino acid sequences but also in chromophore composition and in the light-dependent nature of their respective biochemical activities. Cryptochromes appear to be evolutionarily derived from gene duplication events of ancestral photolyase genes, because many organisms, including Arabidopsis and Drosophila, are known to have both cryptochromes and photolyases, functioning as photoreceptors and DNA repair enzymes, respectively (Cashmore et al., 1999). Cryptochromes, which were historically defined by their action spectra, are photolyase-like blue light receptors (Gressel, 1979; Briggs and Huala, 1999; Cashmore et al., 1999; Lin, 2000b). Most organisms examined to date have more than one cryptochrome, and different cryptochromes of the same organism often mediate related light responses. No biochemical activity has been demonstrated for a cryptochrome, but the expression of a cryptochrome gene may be regulated by light via different mechanisms from transcription to degradation. DNA photolyases are ∼55 to 65 kD flavoproteins widely found in microbes including bacteria, Archaea, and yeast (Sancar, 1994). Photolyases catalyze blue/UV-A light–dependent repair of DNA damage resulting from exposure to high-energy short-wavelength (<350 nm) UV light (UV-B and UV-C). There are two types of structurally related DNA photolyases, one (called photolyase) that repairs cyclobutane pyrimidine dimers and another (called 6-4 photolyase) that repairs pyrimidine-pyrimidine 6-4 photoproducts (Sancar, 1994, 2000). A phylogenetic analysis indicated that plant cryptochromes are more closely related to the photolyases than to the 6-4 photolyases, whereas animal cryptochromes are more closely related to 6-4 photolyases than to the photolyase (Cashmore et al., 1999). Therefore, it appears that plant cryptochromes and animal cryptochromes arose from independent gene duplications of their respective ancestral photolyase genes. Most plant cryptochromes are 70- to 80-kD proteins with two recognizable domains, an amino terminal PHR (for photolyase-related) domain that shares sequence homology with photolyases and a carboxyl terminal domain that has no strong sequence similarity to known protein domains (Cashmore et al., 1999; Mockler and Lin, unpublished data). Photolyase contains two chromophores, a light-harvesting chromophore, which is either a folate (methenyltetrahydrofolate) or a deazaflavin, and a catalytic chromophore that is flavin adenine dinucleotide (FAD). Almost all residues known to be important for chromophore binding in photolyase are conserved in cryptochromes, whereas residues of photolyases that are critical for the binding of DNA lesions and the catalysis of DNA repair are not equally conserved in cryptochromes (Ahmad and Cashmore, 1993; Todo et al., 1996; Imaizumi et al., 2000). The chromophore composition of cryptochromes has been investigated using recombinant cryptochrome purified from heterologous expression systems. The full-length Arabidopsis CRY1 expressed and purified from insect cells non-covalently binds a stoichiometric amount of FAD (Lin et al., 1995b). The PHR domain of Arabidopsis CRY1 expressed as a fusion protein to maltose binding protein (CRY1N-MBP) and purified from Escherichia coli also contained non-covalently bound FAD, indicating that the PHR domain is indeed the chromophore binding domain of cryptochrome. In addition, the CRY1N-MBP recombinant protein purified from E. coli contains the light-harvesting chromophore found in many photolyases (Sancar, et al., with the that cryptochromes are no photolyase activity in different in and expression of cryptochromes not the E. coli (Lin et al., et al., et al., 1996; Imaizumi et al., et al., 2000). Cryptochromes have been found in and Cashmore, 1993; 1993; et al., et al., and Cashmore, 1996; Imaizumi et al., et al., and Imaizumi et al., Imaizumi et al., and et al., Most plant Arabidopsis has two cryptochrome genes, CRY1 and (Ahmad and Cashmore, 1993; et al., 1996; et al., and and have cryptochrome genes, and et al., and have and two cryptochrome genes, respectively and Imaizumi et al., The amino acid sequences of CRY1 or and are more to their Arabidopsis than to that the gene duplication resulting in CRY1 and the of and et al., et al., 2000). Most plant cryptochromes a in to the PHR and the domain has been found to be critical to the of Arabidopsis and (Ahmad et al., et al., and Lin, unpublished data). the domains of different cryptochromes in from amino in amino and amino in Arabidopsis CRY1 and to no in the of and of the The PHR gene to a DNA photolyase but it found to have no DNA repair activity