DNA ligase that seals nicks in double-stranded DNA during DNA replication, DNA recombination and DNA repair. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Not essential for growth, no effect on NHEJ. In quadruple ligB-ligC1-ligC2-ligD deletions NHEJ on blunt and 5'-overhangs is 0.22 and 0.12% of wild-type respectively; only 4-fold decrease in 3'-overhang NHEJ. Belongs to the ATP-dependent DNA ligase family. Extended N-terminus.