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. 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 [PMID:12042765, PMID:11395412]. DNA topoisomerases are divided into two classes: type I enzymes (EC: 5.99.1.2; topoisomerases I, III and V) break single-strand DNA, and type II enzymes (EC: 5.99.1.3; topoisomerases II, IV and VI) break double-strand DNA [PMID:12596227].

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.

This entry describes type IA topoisomerases, which are highly conserved enzymes that are structurally distinct from type IB enzymes. The structures of both topoisomerases I and III have been elucidated, and consist of four domains that together form a toroidal molecule with a central hole that is large enough to accommodate single- and double-stranded DNA [PMID:11239459]. It is believed that the domains transiently separate from one another to allow the entrance and exit of DNA strands.

More information about this protein can be found at Protein of the Month: DNA Topoisomerases.