Plasmid DNA Supercoiling by DNA Gyrase

DNA gyrase catalyzes DNA supercoiling in a reaction coupled to ATP hydrolysis (1 ). The enzyme has been found in many eubacterial species and is unique among the topoisomerases in promoting negative supercoiling of DNA (2 ,3 ). A variety of studies have shown that gyrase is essential for bacterial growth with roles in DNA replication, transcription, and recombination. Moreover, in combination with the relaxing activity of DNA topoisomerase I, gyrase is responsible for the homeostatic regulation of DNA supercoiling in bacteria (reviewed in refs. 3 ,4 ). The enzyme from Escherichia coli has been the most extensively characterized, and is a tetramer made up of two GyrA and two GyrB subunits encoded by the gyrA and gyrB genes, respectively. DNA supercoiling takes place by the directional crossing of a DNA duplex through a transient enzyme-bridged double-strand break in a 120–150 bp segment of DNA wrapped on the enzyme (5 –7 ). This process changes the linking number of DNA in steps of two, and together with an ability to form and resolve DNA knots and catenanes, establishes gyrase as a type II topoisomerase.