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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.
DNA ligase reconnects DNA strands when they are broken. It uses a cofactor molecule (shown in red) for power and a special lysine amino acid (shown in magenta) to perform the reaction. Our DNA ligases and the DNA ligase from the bacteriophage T7 (shown at the top from PDB entry 1a0i ) use ATP as the cofactor. Many bacteria, on the other hand, use NAD in the reaction, as with the DNA ligase shown at the bottom (from PDB entry 1dgs ). In both cases, a lysine in the DNA ligase forms a bond to the phosphate in the cofactor, holding onto the AMP portion and discarding the rest. Later in the reaction, this AMP is transferred to the broken DNA strand, and then is released when the strand is rejoined.