A novel DNA N-glycosylase activity of E. coli T4 endonuclease V that excises 4,6-diamino-5-formamidopyrimidine from DNA, a UV-radiation- and hydroxyl radical-induced product of adenine.

Abstract

We report on a novel activity of T4 endonuclease V. This enzyme is well known to be specific for the excision of pyrimidine dimers from UV-irradiated DNA. In this work, we show that T4 endonuclease V excises 4,6-diamino-5-formamidopyrimidine from DNA. 4,6-Diamino-5-formamidopyrimidine is formed as a product of adenine in DNA upon action of hydroxyl radicals and upon UV-irradiation. DNA substrates were prepared by UV-or gamma-irradiation of DNA in aqueous solution. DNA substrates were incubated either with active T4 endonuclease V or with heat-inactivated T4 endonuclease V or without the enzyme. After incubation, DNA was precipitated and supernatant fractions were separated. Supernatant fractions after derivatization, and pellets after hydrolysis and derivatization were analyzed by gas chromatography/isotope-dilution mass spectrometry. The results provide evidence for the excision of 4,6-diamino-5-formamidopyrimidine by T4 endonuclease V from both gamma-and UV-irradiated DNA. Kinetics of excision were also determined. Fifteen other pyrimidine- and purine-derived base lesions that were identified in DNA samples were not substrates for this enzyme. It was concluded that, in addition to its well known activity for pyrimidine photodimers, T4 endonuclease V possesses an N-glycosylase activity for a major UV-radiation- and hydroxyl radical-induced monomeric product in DNA.