DNA ligases are critical enzymes of DNA metabolism. The reaction they catalyse (the joining
of nicked DNA) is required in DNA replication and in DNA repair pathways that require the re-
synthesis of DNA. Most organisms express DNA ligases powered by ATP, but eubacteria
appear to be unique in having ligases driven by NAD+. Interestingly, despite protein
sequence and biochemical differences between the two classes of ligase, the structure of
the adenylation domain is remarkably similar. Higher organisms express a variety of …