The action of Escherichia coli endonuclease III on multiply damaged sites in DNA.

Energy deposition by ionizing radiation can lead to the formation of clustered DNA damage, i.e. more than one lesion situated within a helical turn of DNA. Among the postulated lesions are those characterized by damaged bases and abasic sites on opposite strands. Enzymatic removal of such lesions may inadvertently lead to the formation of double-strand breaks. To test this hypothesis, we have constructed model substrates containing damaged bases (5,6-dihydrothymine) or abasic sites set one, three, five and seven bases apart on opposite strands, and examined the reactivity of Escherichia coli endonuclease III towards these substrates. Endonuclease III demonstrates two activities; as a glycosylase that removes saturated pyrimidine bases, such as dihydrothymine, and as an AP lyase that cleaves DNA strands at abasic sites. Analysis of endonuclease III-treated dihydrothymidine containing plasmid DNA by agarose gel electrophoresis indicated that the enzyme generated only single-strand breaks when the base damage was set one and three base-pairs apart, and only slowly introduced double-strand breaks in the other substrates. Endonuclease III treatment of the abasic site-containing DNA, however, readily yielded double-strand breaks. Taken together, these results indicate that the glycosylase activity of the enzyme, but not the AP lyase activity, is inhibited by the presence of a closely positioned break in the opposite strand.