DNA gyrase, subunit B <p>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 [<cite idref="PUB00005437"/>]. 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 [<cite idref="PUB00020794"/>, <cite idref="PUB00016842"/>]. DNA topoisomerases are divided into two classes: type I enzymes (<db_xref db="EC" dbkey="5.99.1.2"/>; topoisomerases I, III and V) break single-strand DNA, and type II enzymes (<db_xref db="EC" dbkey="5.99.1.3"/>; topoisomerases II, IV and VI) break double-strand DNA [<cite idref="PUB00020793"/>].</p><p>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 [<cite idref="PUB00020795"/>].</p><p> 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.</p><p>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 [<cite idref="PUB00020803"/>].</p><p>More information about this protein can be found at Protein of the Month: DNA Topoisomerase [<cite idref="PUB00035961"/>].</p>