"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 specificity 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 revealed a long (≈130 bp) and degenerate binding motif that can explain the existence of SGSs. The gyrase motif reflects wrapping of DNA around the enzyme complex and DNA flexibility. It contains two periodic regions in which GC-rich islands are alternated with AT-rich patches by a period close to the period of DNA double helix (≈10.5 bp). The two regions correspond to DNA binding by C-terminal domains of GyrA subunits and resemble eukaryotic nucleosome binding motif. Inhibition by antibiotics Gyrase is present in prokaryotes and some eukaryotes, but the enzymes are not entirely similar in structure or sequence, and have different affinities for different molecules. This makes gyrase a good target for antibiotics. Two classes of antibiotics that inhibit gyrase are: The aminocoumarins (including novobiocin and Coumermycin A1), which work by competitive inhibition of energy transduction of DNA gyrase by binding to the ATPase active site on the GyrB subunit. The quinolones (including nalidixic acid and ciprofloxacin) are