ligA (Escherichia coli K12): DNA ligase, NAD(+)-dependent; DNA ligase that catalyzes the formation of phosphodiester linkages between 5'-phosphoryl and 3'-hydroxyl groups in double- stranded DNA using NAD as a coenzyme and as the energy source for the reaction. It is essential for DNA replication and repair of damaged DNA; Belongs to the NAD-dependent DNA ligase family. LigA subfamily.