Studies on the interaction of 4-quinolones with DNA by DNA unwinding experiments

It has been recently found that, contrary to prior belief, norfloxacin, a member of the 4-quinolone family of antibacterial drugs that specifically inhibit DNA gyrase, does not bind to the enzyme but instead to DNA. We have performed DNA unwinding experiments in order to decide whether binding of norfloxacin to DNA introduces changes in its supercoiled conformation. We have found that: (i) norfloxacin and nalidixic acid are capable of unwinding the double helix, thus confirming the binding of these antibiotics to DNA; (ii) DNA unwinding can be observed only in the presence of Mg2+ and decreases with increasing KCl concentration; (iii) the extent of unwinding varies in different DNA molecules, suggesting a sequence preference of norfloxacin binding to DNA.