DNA ligase III is recruited to DNA strand breaks by a zinc finger motif homologous to that of poly(ADP-ribose) polymerase - Identification of two functionally distinct DNA binding regions within DNA ligase III

Mammalian DNA ligases are composed of a conserved catalytic domain flanked by unrelated sequences. At the C-terminal end of the catalytic domain, ther e is a 16-amino acid sequence, known as the conserved peptide, whose role i n the ligation reaction is unknown. Here we show that conserved positively charged residues at the C-terminal end of this motif are required for enzym e-AMP formation. These residues probably interact with the triphosphate tai l of ATP, positioning it for nucleophilic attack by the active site lysine. Amino acid residues within the sequence RFPR, which is invariant in the co nserved peptide of mammalian DNA ligases, play critical roles in the subseq uent nucleotidyl transfer reaction that produces the DNA-adenylate intermed iate. DNA binding by the N-terminal zinc finger of DNA ligase III, which is homologous with the two zinc fingers of poly(ADP-ribose) polymerase, is no t required for DNA ligase activity in vitro or in vivo. However, this zinc finger enables DNA ligase III to interact with and ligate nicked DNA at phy siological salt concentrations. We suggest that in vivo the DNA ligase III zinc finger may displace poly(ADP-ribose) polymerase from DNA strand breaks , allowing repair to occur.