Practice question

DNA ligation efficiency depends on enzyme type and end structure. T4 DNA ligase, encoded by bacteriophage T4, utilizes ATP to adenylate its active-site lysine and subsequently seal nicks bearing 3'-hydroxyl and 5'-phosphate groups. Unlike ATP-independent bacterial ligases such as Escherichia coli ligase that require cohesive overhangs for stable association, T4 ligase effectively joins both sticky and blunt termini. For blunt-end joining, higher enzyme concentration and inclusion of polyethylene glycol or other crowding agents enhance molecular collisions, making T4 ligase the universal enzyme for fragment assembly, vector circularization, and adaptor cloning.