A Mammalian Endonuclease Specific for Apurinic Sites in Double-stranded Deoxyribonucleic Acid

Double-stranded, covalently closed circular DNA from phage PM2 was either treated with methyl methanesulfonate, irradiated with ultraviolet light, or irradiated with γ-rays to introduce a small number of lesions per DNA molecule. The damaged DNA preparations were then exposed to an endonuclease from calf thymus that catalyzes the formation of single strand interruptions at apurinic sites in double-stranded DNA. DNA containing 2 to 3 alkylated residues per molecule was not attacked by the enzyme. DNA exposed to ultraviolet light was sensitized to the ultraviolet endonuclease from Micrococcus luteus, which attacks DNA containing pyrimidine dimers, but was resistant to the thymus endonuclease. On incubation in solution, alkali-labile sites appeared in the damaged DNA molecules as secondary lesions, and simultaneously the DNA became sensitive to the enzyme. Similar results were obtained with γ-irradiated DNA. The thymus endonuclease apparently is specific for apurinic sites and analogous lesions involving loss of a base residue from DNA.