An enzyme found in bacteria and certain lower eukaryotes (e.g. yeasts) that repairs damage to DNA induced by exposure to ultraviolet (UV) light. In the dark it binds to thymine dimers formed by the UV light, and on subsequent exposure to blue light it absorbs the light energy and uses it to split the dimer and restore normal structure. This process is called photoreactivation; the DNA photolyase of E. coli is encoded by the phr gene (for ‘photoreactivation’). Compare cryptochrome.