DNA gyrase

DNA gyrase, or simply gyrase, is an enzyme within the class of topoisomerase and is a subclass of Type II topoisomerases[1] that reduces topological strain in an ATP dependent manner while double-stranded DNA is being unwound by elongating RNA-polymerase[2] or by helicase in front of the progressing replication fork.

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.

[7] Bacterial DNA gyrase is the target of many antibiotics, including nalidixic acid, novobiocin, albicidin, and ciprofloxacin.

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.

[8] 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.

DNA cleavage and reunion is performed by a catalytic center located in DNA-gates build by all gyrase subunits.

[9] A single molecule study[10] has characterized gyrase activity as a function of DNA tension (applied force) and ATP, and proposed a mechanochemical model.

Recently, high throughput mapping of DNA gyrase sites in the Escherichia coli genome using Topo-Seq approach [2] revealed a long (≈130 bp) and degenerate binding motif that can explain the existence of SGSs.