5 ' to 3 ' Single strand DNA exonuclease activity in a preparation of human Ku protein

We describe a novel 5' to 3' single-strand exonuclease activity exhibited b y a Ku preparation purified from a human cell line, The enzyme removes 5' s ingle-strand extensions from duplex DNA molecules. The exonuclease and heli case activities respond reciprocally to changes in ATP concentrations: Nucl ease activity is inhibited at the ATP concentrations that are optimal for t he helicase. The exonuclease activity does not require divalent cations. Th e potential implications of the exonuclease activity findings for repair of double-strand breaks and recombination processes are discussed.