InterPro domain: IPR011557

DNA topoisomerases regulate the number of topological links between two DNA strands (i.e. change the number of superhelical turns) by catalysing transient single- or double-strand breaks, crossing the strands through one another, then resealing the breaks [ 1 ]. These enzymes have several functions: to remove DNA supercoils during transcription and DNA replication; for strand breakage during recombination; for chromosome condensation; and to disentangle intertwined DNA during mitosis [ 2 , 3 ]. DNA topoisomerases are divided into two classes: type I enzymes ( 5.99.1.2 ; topoisomerases I, III and V) break single-strand DNA, and type II enzymes ( 5.99.1.3 ; topoisomerases II, IV and VI) break double-strand DNA [ 4 ].

Type II topoisomerases are ATP-dependent enzymes, and can be subdivided according to their structure and reaction mechanisms: type IIA (topoisomerase II or gyrase, and topoisomerase IV) and type IIB (topoisomerase VI). These enzymes are responsible for relaxing supercoiled DNA as well as for introducing both negative and positive supercoils [ 5 ].

Topoisomerase II (called gyrase in bacteria) primarily introduces negative supercoils into DNA. In bacteria, topoisomerase II consists of two polypeptide subunits, gyrA and gyrB, which form a heterotetramer: (BA)2. In most eukaryotes, topoisomerase II consists of a single polypeptide, where the N- and C-terminal regions correspond to gyrB and gyrA, respectively.

This entry represents the B subunit (gyrB) as found predominantly in bacteria, but does not include the topoisomerase II enzymes composed of a single polypeptide, as are found in most eukaryotes. GyrB has two functional domains: an N-terminal ATPase and a C-terminal responsible for subunit interactions, the latter differing between subunit B and single polypeptide topoisomerase II [ 6 ].

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2. Cellular roles of DNA topoisomerases: a molecular perspective. Nat. Rev. Mol. Cell Biol. 3, 430-40
3. DNA topoisomerases: structure, function, and mechanism. Annu. Rev. Biochem. 70, 369-413
4. Phylogenomics of type II DNA topoisomerases. Bioessays 25, 232-42
5. Structure and function of type II DNA topoisomerases. Biochem. J. 303 ( Pt 3), 681-95
6. Bacterial diversity based on type II DNA topoisomerase genes. Annu. Rev. Genet. 30, 79-107