Purification and characterization of an endonuclease from calf thymus acting on irradiated DNA

An endonuclease acting on DNA exposed to ultraviolet light or gamma-rays has been extensively purified from calf thymus. The enzyme has a pH optimum at pH 7.0-7.5, acts with equal efficiency in the presence of EDTA or divalent cations (Mg-2+ or Ca-2+), is inhibited by NaCl and tRNA and is inactivated by incubation at 50 degrees C. Its molecular weight, determined by Sephadex chromatography or sodium dodecylsulfate gel electrophoresis, is approx. 30 000. The enzyme catalyzes the formation of breaks with 5'-phosphate termini in double-stranded DNA irradiated with ultraviolet or gamma-rays. It does not act on unirradiated DNA or denatured DNA. Since in all these properties the enzymatic activity on ultraviolet- and gamma-irradiated DNA behaved similarly and since the two activities cochromatographed in all systems used during purification, we conclude that they are associated with the same protein. The site of action of the enzyme in ultraviolet-irradiated DNA is a photoproduct other than pyrimidine dimers. Such a photoproduct can also be induced by irradiation of the DNA in vivo, i.e. within the cells.