Photolyases are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. This enzyme requires visible light, preferentially from the end of the spectrum. Photolyase is an old enzyme which is present and functional in many species, from the bacteria to the fungi to plants. 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. Photolyases bind complementary DNA strands and break certain types of pyrimidine dimers that arise when a pair of thymine or cytosine bases on the strand of DNA become covalently linked. These dimers result in a bulge of the DNA structure, referred to as a lesion, the more common covalent linkage involves the formation of a cyclobutane bridge. Photolyases have an affinity for these lesions and reversibly bind. Photolyases are flavoproteins and contain two light-harvesting cofactors, 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, on the basis of sequence similarities DNA photolyases can be grouped into two classes. The first class contains enzymes from Gram-negative and Gram-positive bacteria, the halophilic archaea Halobacterium halobium, fungi, the branch named Cryptochrome-Drosophila, Arabidopsis, Synechocystis, Human were previously assumed to have no DNA repair activity because of negligible activity on double-stranded DNA. Their study showed that VcCry1 from Vibrio cholerae, X1Cry from Xenopus laevis, some sunscreens include photolyase in their ingredients, claiming a reparative action on UV-damaged skin
A deazaflavin photolyase from Anacystis nidulans, illustrating the two light-harvesting cofactors: FADH− (yellow) and 8-HDF (cyan).
A UV radiation induced thymine-thymine cyclobutane dimer (right) is the type of DNA damage which is repaired by DNA photolyase. Note: The above diagram is incorrectly labelled as thymine as the structures lack 5-methyl groups.