The interaction of quinolone antibacterials with DNA gyrase.

Quinolone drags are a clinically significant family of antibacterial compounds that are known to affect the activity of DNA gyrase, an essential bacterial enzyme involved in controlling the topological state of DNA, Gyrase holoenzyme, a complex of two A and two B subunits, can introduce negative supercoils into DNA using energy derived from the hydrolysis of ATP. Although addition of quinolones rapidly inhibits the supercoiling activity of gyrase, it was found that quinolone-dependent DNA cleavage was a slow process, leading to the suggestion that there may be two levels of interaction between quinolones and the gyrase-DNA complex. Rapid gel-filtration experiments have shown that stable quinolone binding requires the presence of both gyrase and DNA; no significant binding was found to either gyrase or DNA alone. Enzyme containing gyrase A protein with the mutation Ser83 to Trp (which is known to confer quinolone resistance) showed greatly reduced drug binding. It is concluded that efficiency of binding is primarily determined by the gyrase A subunits. Investigation of transcription by T7 and Escherichia coli RNA polymerases has found that quinolone-mediated stabilisation of a gyrase-DNA complex prevents passage of polymerase along the template. Inhibition of transcription required the presence of gyrase and quinolone together; RNA polymerase was unaffected by either quinolone or gyrase alone, implying that polymerase can normally pass or displace gyrase. In the presence of ciprofloxacin, gyrase was found to shield a region of about 26 bp of DNA from transcription by T7 RNA polymerase, with especially strong protection of a 20 bp core. Preliminary experiments performed using an in vitro DNA replication system suggest that DNA polymerases may be similarly interrupted by a gyrase-quinolone-DNA complex.