Photolyases () are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. These enzymes require visible light (from the violet/blue end of the spectrum) both for their own activation and for the actual DNA repair. The DNA repair mechanism involving photolyases is called photoreactivation. They mainly convert pyrimidine dimers into a normal pair of pyrimidine bases. Function Photolyases bind complementary DNA strands and break certain types of pyrimidine dimers that arise when a pair of thymine or cytosine bases on the same strand of DNA become covalently linked. The bond length of this dimerization is shorter than the bond length of normal B-DNA structure which produces an incorrect template for replication and transcription. The more common covalent linkage involves the formation of a cyclobutane bridge. Photolyases have a high affinity for these lesions and reversibly bind and convert them back to the original bases. Evolution Photolyase is a phylogenetically old enzyme which is present and functional in many species, from the bacteria to the fungi to plants and to the animals. Photolyase is particularly important in repairing UV induced damage in plants. The photolyase mechanism is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism, although they do retain many cryptochromes. Photolyases are flavoproteins and contain two light-harvesting cofactors. Many photolyases have an N-terminal domain that binds a second cofactor. All photolyases contain the two-electron-reduced FADH−; they are divided into two main classes based on the second cofactor, which may be either the pterin methenyltetrahydrofolate (MTHF) in folate photolyases or the deazaflavin 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) in deazaflavin photolyases. Although only FAD is required for catalytic activity, the second cofactor significantly accelerates reaction rate in low-light conditions. The enzyme acts by electron transfer in which the reduced flavin FADH− is activated by light energy and acts as an electron donor to break the pyrimidine dimer. On the basis of sequence similarities DNA photolyases can be grouped into a few classes: Class 1 CPD photolyases are enzymes that process cyclobutane pyrimidine dimer (CPD) lesions from Gram-negative and Gram-positive bacteria, the halophilic archaea Halobacterium halobium. Class 2 CPD photolyases also process CPD lesions. They are found in plants like the thale cress Arabidopsis thaliana and the rice. The plant and fungi cryptochromes are similar to Class 1 CPDs. They are blue light photoreceptors that mediate blue light-induced gene expression and