ABSTRACT
Photolyases are enzymes that catalyze repair of damaged nucleic acid bases in a light-driven cyclic electron transfer reaction (1-4), a process known as photoreactivation. The primary substrates of these enzymes are cyclobutane-type pyrimidine dimmers (5-7) and pyrimidine-pyrimidone (4-6) photoproducts (4,8), each of which forms efficiently when adjacent pyrimidine bases in nucleic acids absorb far UV radiation (250-300 nm). Photolyases bind to these lesions, absorb a photon of near UV or visible light (350-450 nm), and utilize this energy to repair the damaged bases with a quantum yield of 0.1-1.0 (see Ref. 9 for a recent review). Given the large UV flux that prevailed during the early evolution of life on earth, it is likely that photolyases appeared quite early. Indeed, both their relatively simple composition (see in what follows) and the fact that photolyases have been identified in all phylogenetic lineages including viruses, Archae, Eubacteria, and Eukaryotes, suggest an early origin. However, among extant organisms, the occurrence of these enzymes is by no means universal [the most notable absence being placental mammals (10,11)], nor, surprisingly, does it correspond precisely to potential exposure to sunlight (12).
