The Oligonucleotide/oligosaccharide binding (OB)-fold domain of ATP-dependent DNA ligase LigD 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. Bacterial DNA ligases are divided into two broad classes: NAD-dependent and ATP-dependent. All bacterial species have a NAD-dependent DNA ligase (LigA). Some bacterial genomes contain multiple genes for DNA ligases that are predicted to use ATP as their cofactor, including Mycobacterium tuberculosis LigB, LigC, and LigD. This group is composed of Mycobacterium tuberculosis LigD and similar bacterial proteins. LigD, or DNA ligase D, catalyzes the end-healing and end-sealing steps during nonhomologous end joining. 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.