The Action Pattern of Mammalian Deoxyribonuclease IV

The mode of action of an exonuclese that probably is involved in DNA repair, mammalian DNase IV, has been studied. By kinetic measurements with both uniformly and terminally labeled polynucleotides it is shown that enzyme breaks severa adjacent phosphodiester bonds each time it binds to its substrate, but tht the number of hydrolytic events is small compared to the degree of polymerization of the substrate. At 37° the enzyme releases 50‐200 nucelotides in an exonucleolytic fashion at each encounter with native DNA. The significance of these observations in relation to DNA repair in vivo is discussed.