title: Xer Site Specific Recombination: Double and Single Recombinase Systems.
abstract: The separation and segregation of newly replicated bacterial chromosomes can be constrained by the formation of circular chromosome dimers caused by crossing over during homologous recombination events. InEscherichia coliand most bacteria, dimers are resolved to monomers by site-specific recombination, a process performed by two Chromosomally Encoded tyrosine Recombinases (XerC and XerD). XerCD recombinases act at a 28 bp recombination sitedif, which is located at the replication terminus region of the chromosome. The septal protein FtsK controls the initiation of the dimer resolution reaction, so that recombination occurs at the right time (immediately prior to cell division) and at the right place (cell division septum). XerCD and FtsK have been detected in nearly all sequenced eubacterial genomes including Proteobacteria, Archaea, and Firmicutes. However, in StreptococciandLactococci, an alternative system has been found, composed of a single recombinase (XerS) genetically linked to an atypical 31 bp recombination site (difSL). A similar recombination system has also been found in 𝜀-proteobacteria such asCampylobacterandHelicobacter, where a single recombinase (XerH) acts at a resolution site calleddifH. Most Archaea contain a recombinase called XerA that acts on a highly conserved 28 bp sequencedif, which appears to act independently of FtsK. Additionally, several mobile elements have been found to exploit thedif/Xer system to integrate their genomes into the host chromosome inVibrio cholerae, Neisseria gonorrhoeae, andEnterobacter cloacae. This review highlights the versatility ofdif/Xer recombinase systems in prokaryotes and summarizes our current understanding of homologs ofdif/Xer machineries.
authors.affiliation: Département de Microbiologie, Infectiologie et Immunologie, Université de Montréal, Montréal QC, Canada.
authors.forename: Fabio
authors.initials: F
authors.lastname: Castillo
authors.affiliation: Département de Microbiologie, Infectiologie et Immunologie, Université de Montréal, Montréal QC, Canada.
authors.forename: Amal
authors.initials: A
authors.lastname: Benmohamed
authors.affiliation: Département de Microbiologie, Infectiologie et Immunologie, Université de Montréal, Montréal QC, Canada.
authors.forename: George
authors.initials: G
authors.lastname: Szatmari
keywords: IMEX; XerA; XerH; XerS; single recombinases; site-specific recombination; tyrosine recombinases