DNA ligase that seals nicks in double-stranded DNA during DNA replication, DNA recombination and DNA repair. Can also use NAD, but less efficiently than ATP. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. NAD(+) + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + beta-nicotinamide D-nucleotide. Optimum pH is 8.0. Still active at 100 degrees Celsius. Thermostable. Monomer. Belongs to the ATP-dependent DNA ligase family.