Action of a type II topoisomerase

DNA gyrase of E. coli; the example shown is a tetrameric protein with two A and two B subunits.

The enzyme is shown introducing two negative turns and changing the linking number from +1 to -1.

The enzyme catalyzes a double-strand break, and the two DNA ends are bound by A subunits, which move the DNA ends apart so that the unbroken duplex can pass through the gap.

Resealing converts the positive supertwist to a negative one, giving the overall molecule a DL of -2.

Type II topoisomerases can relax underwound duplexes by the reverse of the above pathway.