DNA breaking-rejoining enzyme, catalytic core <p>Phage integrases are enzymes that mediate unidirectional site-specific recombination between two DNA recognition sequences, the phage attachment site, attP, and the bacterial attachment site, attB [<cite idref="PUB00014061"/>]. Integrases may be grouped into two major families, the tyrosine recombinases and the serine recombinases, based on their mode of catalysis. Tyrosine family integrases, such as <taxon tax_id="10710">Bacteriophage lambda</taxon> integrase, utilise a catalytic tyrosine to mediate strand cleavage, tend to recognise longer attP sequences, and require other proteins encoded by the phage or the host bacteria.</p><p>The 356 amino acid lambda integrase consists of two domains: an N-terminal domain that includes residues 1-64 and is responsible for binding the arm-type sites of attP, and a C-terminal domain (CTD) that binds the lower affinity core-type sites and contains the catalytic site. The CTD can be further divided into the core-type binding domain (residues 65-169) and the catalytic core domain (170-356), the later representing this entry. The catalytic core adopts an alpha3-beta3-alpha4 fold, where one side of the beta sheet is exposed.</p><p>The recombinases Cre from phage P1, XerD from <taxon tax_id="562">Escherichia coli</taxon> and Flp from yeast are members of the tyrosine recombinase family, and have a two-domain motif resembling that of lambda integrase, as well as sharing a conserved binding mechanism [<cite idref="PUB00014062"/>]. The structural fold of their catalytic core domains resemble that of Lambda integrase.</p><p>The catalytic core of the eukaryotic DNA topoisomerase I shares significant structural similarity with the bacteriophage family of DNA integrases [<cite idref="PUB00005230"/>]. Topoisomerases I promote the relaxation of DNA superhelical tension by introducing a transient single-stranded break in duplex DNA and are vital for the processes of replication, transcription, and recombination.</p>