<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 I topoisomerases are ATP-independent enzymes (except for reverse gyrase), and can be subdivided according to their structure and reaction mechanisms: type IA (bacterial and archaeal topoisomerase I, topoisomerase III and reverse gyrase) and type IB (eukaryotic topoisomerase I and topoisomerase V). These enzymes are primarily responsible for relaxing positively and/or negatively supercoiled DNA, except for reverse gyrase, which can introduce positive supercoils into DNA. </p><p>Type IA topoisomerases are comprised of four domains that together form a toroidal structure with a central hole large enough to accommodate single- and double-stranded DNA: an N-terminal alpha/beta Toprim domain, domain 2 and the C-terminal domain 4 are winged-helix domains, and domain 3 is a beta-barrel. Domains 1 (Toprim) and 3 form the active site of the enzyme, while the winged helix domains 2 and 4 form a single-strand DNA-binding groove [<cite idref="PUB00020796"/>, <cite idref="PUB00020799"/>]. This entry represents the central portion of the enzyme, which covers domains 2 and 3 in topoisomerase type IA enzymes.</p><p>More information about this protein can be found at Protein of the Month: DNA Topoisomerase [<cite idref="PUB00035961"/>].</p>