DNA gyrase, or simply gyrase, is an enzyme within the class of topoisomerase and is a subclass of Type II topoisomerases that reduces topological strain in an ATP dependent manner while double-stranded DNA is being unwound by elongating RNA-polymerase or by helicase in front of the progressing replication fork. The enzyme causes negative supercoiling of the DNA or relaxes positive supercoils. It does so by looping the template so as to form a crossing, then cutting one of the double helices and passing the other through it before releasing the break, changing the linking number by two in each enzymatic step. This process occurs in bacteria, whose single circular DNA is cut by DNA gyrase and the two ends are then twisted around each other to form supercoils. Gyrase is also found in eukaryotic plastids: it has been found in the apicoplast of the malarial parasite Plasmodium falciparum and in chloroplasts of several plants. Bacterial DNA gyrase is the target of many antibiotics, including nalidixic acid, novobiocin, albicidin, and ciprofloxacin. The unique ability of gyrase to introduce negative supercoils into DNA at the expense of ATP hydrolysis is what allows bacterial DNA to have free negative supercoils. The ability of gyrase to relax positive supercoils comes into play during DNA replication and prokaryotic transcription. The helical nature of the DNA causes positive supercoils to accumulate ahead of a translocating enzyme, in the case of DNA replication, a DNA polymerase. The ability of gyrase (and topoisomerase IV) to relax positive supercoils allows superhelical tension ahead of the polymerase to be released so that replication can continue. Gyrase structure DNA gyrase is a tetrameric enzyme that consists of 2 GyrA ("A") and 2 GyrB ("B") subunits. Structurally the complex is formed by 3 pairs of "gates", sequential opening and closing of which results into the direct transfer of DNA segment and introduction of 2 negative supercoils. N-gates are formed by ATPase domains of GyrB subunits. Binding of 2 ATP molecules leads to dimerization and, therefore, closing of the gates. Hydrolysis, on the contrary, opens them. DNA cleavage and reunion is performed by a catalytic center located in DNA-gates build by all gyrase subunits. C-gates are formed by GyrA subunits. Mechanochemical model of gyrase activity A single molecule study has characterized gyrase activity as a function of DNA tension (applied force) and ATP, and proposed a mechanochemical model. Upon binding to DNA (the