THE ACTION OF DNA-LIGASE AT ABASIC SITES IN DNA

Apurinic/apyrimidinic (AP) sites occur frequently in DNA as a result o f spontaneous base loss or following removal of a damaged base by a DN A glycosylase. The action of many AP endonuclease enzymes at abasic si tes in DNA leaves a 5'-deoxyribose phosphate (dRP) residue that must b e removed during the base excision repair process. This 5'-dRP group m ay be removed by AP lyase enzymes that employ a beta-elimination mecha nism. This beta-elimination reaction typically involves a transient Sc hiff base intermediate that can react with sodium borohydride to trap the DNA-enzyme complex. With the use of this assay as well as direct 5 '-dRP group release assays, we show that T4 DNA ligase, a representati ve ATP-dependent DNA ligase, contains AP lyase activity. the AP lyase activity of T4 DNA ligase is inhibited in the presence of ATP, suggest ing that the adenylated lysine residue is part of the active site for both the ligase and lyase activities. A model is proposed whereby the AP lyase activity of DNA ligase may contribute to the repair of abasic sites in DNA.