An Endonuclease from Escherichia coli That Acts Preferentially on UV-Irradiated DNA and Is Absent from the uvrA and uvrB Mutants

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Abstract
At least two endonucleolytic activities that preferentially incise ultraviolet (UV)-irradiated DNA exist in extracts of E. coli . These two activities can be separated by phosphocellulose chromatographic fractionation. The subject of this paper is one of these activities, which elutes from phosphocellulose with 0.25 M potassium phosphate, pH 7.5. This endonucleolytic activity specific for UV-irradiated DNA is absent from partially purified extracts of uvrA and uvrB mutants, which are defective in excision of pyrimidine dimers, but is present in normal amounts in the uvrC excision-defective mutant. The enzyme binds very tightly and specifically to UV-irradiated DNA. Binding can be prevented by prior treatment of the irradiated DNA with photoreactivating enzyme. This binding activity is absent in uvrA and uvrB mutants, but present in uvrC and uvrD mutants.