AUTHOR=Castillo Fabio , Benmohamed Amal , Szatmari George 

TITLE=Xer Site Specific Recombination: Double and Single Recombinase Systems

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.00453

DOI=10.3389/fmicb.2017.00453

ISSN=1664-302X

ABSTRACT=<p>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. In <italic>Escherichia coli</italic> and most bacteria, dimers are resolved to monomers by site-specific recombination, a process performed by two <underline>C</underline>hromosomally <underline>E</underline>ncoded tyrosine <underline>R</underline>ecombinases (XerC and XerD). XerCD recombinases act at a 28 bp recombination site <italic>dif</italic>, 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 S<italic>treptococci</italic> and <italic>Lactococci</italic>, an alternative system has been found, composed of a single recombinase (XerS) genetically linked to an atypical 31 bp recombination site (<italic>difSL</italic>). A similar recombination system has also been found in 𝜀-proteobacteria such as <italic>Campylobacter</italic> and <italic>Helicobacter</italic>, where a single recombinase (XerH) acts at a resolution site called <italic>difH</italic>. Most Archaea contain a recombinase called XerA that acts on a highly conserved 28 bp sequence <italic>dif</italic>, which appears to act independently of FtsK. Additionally, several mobile elements have been found to exploit the <italic>dif</italic>/Xer system to integrate their genomes into the host chromosome in <italic>Vibrio cholerae, Neisseria gonorrhoeae</italic>, and <italic>Enterobacter cloacae</italic>. This review highlights the versatility of <italic>dif</italic>/Xer recombinase systems in prokaryotes and summarizes our current understanding of homologs of <italic>dif</italic>/Xer machineries.</p>