ABSTRACT
Exposure of DNA to ultraviolet light results in the formation of cyclobutane dimers between adjacent pyrimidine residues. Since direct excitation of pyrimidine dimers cannot occur with visible light, it was long suspected that the photolyase reaction might be a photosensitized process involving one or more enzyme-bound chromophores that absorbed visible light. Highly purified photolyase has been isolated from Escherichia coli, Saccharomyces cerevisiae, Streptomyces griseus, the green alga Scenedesmus acutus, and the cyanobacteria Anacystis nidulans. Plasmids which overproduce photolyase have been constructed for the enzymes from E. coli and yeast. Several factors indicate that the binding of E. coli photolyase to dimers in DNA involves interaction of the enzyme with a limited region immediately surrounding the dimer. The identification of 8-hydroxy-5-deazaflavin in Scenedesmus acutus photolyase is the direct evidence for the presence of the chromophore in eukaryotes. The chromophores in S. acutus photolyase are released as a nearly equimolar mixture of oxidized flavin adenine dinucleotide plus an 8-hydroxy-5-deazaflavin derivative.
