In molecular biology, DNA ligase is a specific type of enzyme, a ligase, (EC 6.5.1.1) that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. DNA ligase III: complexes with DNA repair protein XRCC1 to aid in sealing DNA during the process of nucleotide excision repair and recombinant fragments. 2  Of the all known mammalian DNA ligases, only Lig III has been found to be present in mitochondria. 3  DNA ligase IV: complexes with XRCC4.