Inamdar, K. V., Yu, Y. and Povirk, L. F. Resistance of 3′-Phosphoglycolate DNA Ends to Digestion by Mammalian DNase III. Radiat. Res. 157, 306 – 311 (2002).
An essential step in the repair of free radical-mediated DNA strand breaks is the removal of sugar fragments such as phosphoglycolate from the 3′ termini. While the abasic endonuclease Ape1 can remove phosphoglycolate from single-strand breaks in double-stranded DNA, an enzyme capable of removing it from 3′ overhangs of double-strand breaks has yet to be identified. We therefore tested DNase III, the predominant 3′ → 5′ exonuclease in mammalian cell extracts, for possible 3′-phosphoglycolate-removing activity. However, all 3′-phosphoglycolate substrates, as well as a 3′-phosphate substrate, were resistant to DNase III under conditions in which the analogous 3′-hydroxyl substrates were extensively degraded. The DNA end-binding protein Ku (an equimolar mixture of Ku70, now known as G22P1, and Ku86, now known as XRCC5) did not alter the resistance of the 3′-phosphoglycolate substrates, but the protein modulated the susceptibility of 3′-hydroxyl substrates, allowing DNase III to remove a 3′ overhang but inhibiting digestion of the double-stranded portion of the substrate.