title: Sequential strand exchange by XerC and XerD during site-specific recombination at dif.
abstract: Successful segregation of circular chromosomes in Escherichia coli requires that dimeric replicons, produced by homologous recombination, are converted to monomers prior to cell division. The Xer site-specific recombination system uses two related tyrosine recombinases, XerC and XerD, to catalyze resolution of circular dimers at the chromosomal site, dif. A 33-base pair DNA fragment containing the 28-base pair minimal dif site is sufficient for the recombinases to mediate both inter- and intramolecular site-specific recombination in vivo. We show that Xer-mediated intermolecular recombination in vitro between nicked linear dif "suicide" substrates and supercoiled plasmid DNA containing dif is initiated by XerC. Furthermore, on the appropriate substrate, the nicked Holliday junction intermediate formed by XerC is converted to a linear product by a subsequent single XerD-mediated strand exchange. We also demonstrate that a XerC homologue from Pseudomonas aeruginosa stimulates strand cleavage by XerD on a nicked linear substrate and promotes initiation of strand exchange by XerD in an intermolecular reaction between linear and supercoiled DNA, thereby reversing the normal order of strand exchanges.
authors.affiliation: Division of Molecular Genetics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom.
authors.forename: G W
authors.initials: GW
authors.lastname: Blakely
authors.forename: A O
authors.initials: AO
authors.lastname: Davidson
authors.forename: D J
authors.initials: DJ
authors.lastname: Sherratt
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002876:Chromosomes, Bacterial / Q000235:genetics*; D004254:DNA Nucleotidyltransferases / Q000737:chemistry / Q000378:metabolism*; D004278:DNA, Superhelical / Q000235:genetics; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D029968:Escherichia coli Proteins; D019426:Integrases; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides; D010957:Plasmids / Q000235:genetics; D045522:Recombinases; D011995:Recombination, Genetic; D012093:Replicon; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D004278:DNA, Superhelical; D029968:Escherichia coli Proteins; D007527:Isoenzymes; D009838:Oligodeoxyribonucleotides; D045522:Recombinases; C473360:XerC protein, E coli; D004254:DNA Nucleotidyltransferases; D019426:Integrases; C473361:XerD protein, E coli; C115006:integron integrase IntI1