Structures of ATP-dependent DNA ligases. (a,b) Three-dimensional structures of (a) bacteriophage T7 DNA ligase complexed with ATP and (b) the Chlorella virus of Paramecium bursaria (PBCV-1) DNA ligase enzyme-adenylate complex, determined by X-ray crystallography. For the PBCV-1 enzyme, domains 1 and 2 (an OB fold) are indicated. (c,d) Structures of the BRCT domains from (c) the human DNA-repair factor Xrcc1 and (d) DNA ligaseIIIα, determined by X-ray crystallography and NMR (nuclear magnetic resonance), respectively. In each case, four short β strands form the core of the BRCT structure. The Xrcc1 core is flanked by three α helices (α1, α2 and α3) whereas that of DNA ligase IIIα is flanked by two only (α1 and α2). Theinteraction between the Xrcc1 and DNA ligase IIIα proteins in vivo is mediated by these BRCT domains. (e) Structure of a homodimer of Xrcc4 bound to a short peptide corresponding to amino acids 748-784 of human DNA ligase IV (shown in green). (f) Close-up view of the DNA ligase IV peptide bound to the helical tails of the Xrcc4 dimer. The peptide comprises a β hairpin followed by an α helix and lies asymmetrically across both Xrcc4 monomers. See text for details and references.