The complex of DNA gyrase and quinolone drugs with DNA forms a barrier to transcription by RNA polymerase.

The effects of DNA gyrase and quinolone drugs on in vitro transcription of a template containing a preferred gyrase cleavage site have been investigated. We have found that gyrase-quinolone complexes with DNA lead to blocking of transcription by Escherichia coli and bacteriophage T7 RNA polymerases. Either gyrase or quinolone alone has no effect on transcription. With DNA gyrase containing a point mutation in the gyrase A protein, known to confer quinolone resistance, blocking was found to occur only at much higher concentrations of the drug. Other agents that inhibit gyrase-catalysed supercoiling (novobiocin and 5'-adenylyl-beta,gamma-imidodiphosphate) do not arrest transcription in the presence of gyrase. Mapping of the transcription termination sites in the presence of gyrase and quinolones shows that blocking occurs about 10 to 20 base-pairs upstream of the gyrase cleavage site. Analysis of transcription in the absence of drug suggests that RNA polymerase does not displace gyrase from the template. These results are discussed in the light of models for the bactericidal effects of quinolone drugs.