Supercoiling is a process employed by bacteria to allow them to fit a large chromosome into their small cell. Winding the DNA tightly allows it to be condensed.
This winding it done by topoisomerases. Topoisomerase I, III, and IV all induce positive supercoils. However, topoisomerase II (gyrase) is able to introduce negative supercoils – bacterial DNA is negatively supercoiled.
Supercoiling is able to regulate gene expression in bacteria, allowing them to tightly wind regions of their chromosome that they want to suppress expression of. Regions of DNA that are less tightly wound will be more transcriptionally active.
Topoisomerase II is formed of 2 subunits: gyrA and gyrB. GyrA binds to the DNA, forming a double-strand break. GyrB then passes one end of the DNA through the ds break to introduce a negative supercoil. This is repeated many times, condensing the DNA.
Gyrase (topoisomerase II) can be inhibited by quinolone antibiotics. As supercoiling is exclusively found in bacteria, this allows for the selective toxicity necessary with an antibiotic drug.