T4DNA ligase is an Mg 2+ -dependent andATP-dependent enzyme that seals DNA nicks in three steps: it covalently binds AMP,transadenylates the nick phosphate,and cata-lyses formation of the phosphodiester bond releasing AMP. In this kinetic study,we further detail the reaction mechan-ism,showing that the overall ligation reaction is a super-impositionof twoparallel processes: a processive ligation,in which the enzyme transadenylates and seals the nickwithout dissociating fromdsDNA,and a nonprocessive ligation,in which the enzyme takes part in the abortive adenylation cycle (covalent binding of AMP,transadenylation of the nick,and dissociation)
