The DNA gyrase of Escherichia coli plays an essential role in the life of this microorganism.It is unique among all topoisomerases because of its ability to introduce negative supercoils into DNA.This study investigated the single molecular interaction of E.coli gyrase with DNA using magnetic tweezers.The results showed that,in the absence of ATP,gyrase weakly binds the G and T segments.The stretched force of 0.7 pN can gradually destroy the binding,whereas that of 5.9 pN directly destroys it.Addition of high concentrations of norfloxacin enhances gyrase binding to both segments,making them adapt to 5.9 pN.DNA gyrase reduces the plectonemic dimension,which was determined by the bacterial enzyme and not by the pull force.Moreover,it has different affinities for positive supercoils,which it prefers,and negative supercoils.The time distribution of the dissociation of gyrase from DNA has a double-exponential form.We herein propose a model to explain this distribution and compare the results with those of other models.
