Inhibits the supercoiling activity of DNA gyrase. Acts by inhibiting DNA gyrase at an early step, prior to (or at the step of) binding of DNA by the gyrase. It protects cells against toxins that target DNA gyrase, by inhibiting activity of these toxins and reducing the formation of lethal double-strand breaks in the cell. Protects cells against the natural plasmid-encoded toxins microcin B17 (MccB17) and CcdB, and synthetic quinolones. Can also protect cells against alkylating agents that act independently of DNA gyrase, suggesting a more general role in protecting cells against DNA damage. Interacts with DNA gyrase. Interacts preferentially with the holoenzyme composed of GyrA and GyrB, but can also weakly interact with the individual GyrA and GyrB subunits. By DNA-damaging agents and by the entry of cells into the stationary growth phase. Belongs to the DNA gyrase inhibitor family.