AMP-dependent DNA relaxation catalyzed by DNA ligase occurs by a nicking–closing mechanism

In the presence of AMP and Mg2+, a covalently closed duplex DNA containing negative superhelical turns was treated with DNA ligase isolated from bacteriophage T4-infected E. coli. This resulted in the gradual and not sudden loss of superhelical turns as for example in the case of type I DNA topoisomerase. All DNA products remain covalently closed. Since T4 enzyme-mediated DNA relaxation is inhibited by both pyrophosphate and by ATP this suggests that DNA relaxing and DNA joining activities probably coincide. EDTA addition in the presence of a large excess of enzyme, induces the formation of nicked DNA products while protein denaturing treatments are not very effective. Our observations might suggest an involvement of the relaxing activity of DNA ligase during the ligation process.