The Oligonucleotide/oligosaccharide binding (OB)-fold domain of ATP-dependent DNA ligase I is a DNA-binding module that is part of the catalytic core unit.
ATP-dependent polynucleotide ligases catalyze phosphodiester bond formation using nicked nucleic acid substrates with the high energy nucleotide of ATP as a cofactor in a three step reaction mechanism. DNA ligases play a vital role in the diverse processes of DNA replication, recombination and repair. ATP-dependent ligases are present in many organisms such as viruses, bacteriohages, eukarya, archaea and bacteria. There are three classes of ATP-dependent DNA ligases in eukaryotic cells (I, III and IV). This group is composed of eukaryotic DNA ligase I, Sulfolobus solfataricus DNA ligase and similar proteins. DNA ligase I is required for the ligation of Okazaki fragments during lagging-strand DNA synthesis and for base excision repair (BER). ATP dependent DNA ligases have a highly modular architecture consisting of a unique arrangement of two or more discrete domains including a DNA-binding domain, an adenylation (nucleotidyltransferase (NTase)) domain, and an oligonucleotide/oligosaccharide binding (OB)-fold domain. The adenylation and C-terminal OB-fold domains comprise a catalytic core unit that is common to most members of the ATP-dependent DNA ligase family. The catalytic core unit contains six conserved sequence motifs (I, III, IIIa, IV, V and VI) that define this family of related nucleotidyltransferases. The OB-fold domain contacts the nicked DNA substrate and is required for the ATP-dependent DNA ligase nucleotidylation step. The RxDK motif (motif VI), which is essential for ATP hydrolysis, is located in the OB-fold domain.