DNA gyrase action involves the introduction of transient double-strand breaks into DNA

DNA gyrase from Escherichia coli, in the presence of ATP, can both separate catenated DNA circles and unknot knotted DNA. Both these reactions require passage of a DNA segment through a transient double-strand break in DNA. Evidence that transient double-strand breaks are also involved in the supercoiling and relaxing activities of DNA gyrase is derived from experiments showing that the linking number of circular DNA is changed in steps of two. A mechanism is proposed for the action of the enzyme.