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Enzyme

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. It is the only known enzyme to actively contribute negative supercoiling to DNA, while it also is capable of relaxing positive supercoils. It does so by looping the template 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 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.

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 "Gyrase DNA" state), there is a competition between DNA wrapping and dissociation, where increasing DNA tension increases the probability of dissociation. According to the catalytic cycle proposed, binding of 2 ATP molecules causes dimerization of ATPase domains of GyrB subunits and capturing of a T segment of DNA (T from transferring) in a cavity between GyrB subunits. On a next step the enzyme cleaves a G segment of DNA (G from gate) making a double strand break. Then the T segment is transferred through the break, which is accompanied by the hydrolysis of the first ATP molecule. DNA gyrase ligates the break in a G segment back and T segment finally leaves the enzyme complex. Hydrolysis of the second ATP returns the system to the initial step of a cycle. As the result of a catalytic cycle two ATP molecules are hydrolyzed and two negative supercoils are introduced into the DNA template. The number of superhelical turns introduced into an initially relaxed circular DNA has been calculated to be approximately equal to the number of ATP molecules hydrolyzed by gyrase. Therefore, it can be suggested that two ATP molecules are hydrolyzed per cycle of reaction by gyrase, leading to the introduction of a linking difference of 2.

Gyrase has a pronounced specificity to DNA substrates. Strong gyrase binding sites (SGS) were found in some phages (bacteriophage Mu group) and plasmids (pSC101, pBR322). Recently, high throughput mapping of DNA gyrase sites in the Escherichia coli genome using Topo Seq approach
The quinolones (including nalidixic acid and ciprofloxacin) are known as topoisomerase poisons. By binding to the enzyme they trap it in a transient step of the catalytic cycle, preventing the reunion of a G segment. This results in an accumulation of double strand breaks, stalling of replication forks and cell death. Quinolone resistant bacteria frequently harbor mutated topoisomerases that resist quinolone binding. The subunit A is selectively inactivated by antibiotics such as oxolinic and nalidixic acids. The subunit B is selectively inactivated by antibiotics such as coumermycin A1 and novobiocin. Inhibition of either subunit blocks supertwisting activity.

Phage T4 genes 39, 52 and 60 encode proteins that form a DNA gyrase that is employed in phage DNA replication during infection of the E. coli bacterial host. The phage gene 52 protein shares homology with the bacterial gyrase gyrA subunit and the phage gene 39 protein shares homology with the gyrB subunit. Since the host E. coli DNA gyrase can partially compensate for the loss of the phage gene products, mutants defective in either genes 39, 52 or 60 do not completely abolish phage DNA replication, but rather delay its initiation. A mutant defective in gene 39 also shows increased sensitivity to inactivation by ultraviolet irradiation during the stage of phage infection after initiation of DNA replication when multiple copies of the phage chromosome are present.