 Gyrase alters DNA topology by effecting a transient double-strand break in the DNA backbone, passing the double helix through the gate and resealing the gaps.The CcdB poison acts by trapping DNA gyrase in a cleaved complex with the gyrase A subunit covalently closed to the cleaved DNA, causing DNA breakage and cell death in a way closely related to quinolones antibiotics.In absence of the antitoxin, the CcdB poison traps DNA-gyrase cleavable complexes, inducing breaks into DNA and cell death.