title: Genomic comparisons among Escherichia coli strains B, K-12, and O157:H7 using IS elements as molecular markers.
abstract: BACKGROUND
  Insertion Sequence (IS) elements are mobile genetic elements widely distributed among bacteria. Their activities cause mutations, promoting genetic diversity and sometimes adaptation. Previous studies have examined their copy number and distribution in Escherichia coli K-12 and natural isolates. Here, we map most of the IS elements in E. coli B and compare their locations with the published genomes of K-12 and O157:H7.
  
  
  RESULTS
  The genomic locations of IS elements reveal numerous differences between B, K-12, and O157:H7. IS elements occur in hok-sok loci (homologous to plasmid stabilization systems) in both B and K-12, whereas these same loci lack IS elements in O157:H7. IS elements in B and K-12 are often found in locations corresponding to O157:H7-specific sequences, which suggests IS involvement in chromosomal rearrangements including the incorporation of foreign DNA. Some sequences specific to B are identified, as reported previously for O157:H7. The extent of nucleotide sequence divergence between B and K-12 is < 2% for most sequences adjacent to IS elements. By contrast, B and K-12 share only a few IS locations besides those in hok-sok loci. Several phenotypic features of B are explained by IS elements, including differential porin expression from K-12.
  
  
  CONCLUSIONS
  These data reveal a high level of IS activity since E. coli B, K-12, and O157:H7 diverged from a common ancestor, including IS association with deletions and incorporation of horizontally acquired genes as well as transpositions. These findings indicate the important role of IS elements in genome plasticity and divergence.
authors.affiliation: Laboratoire Plasticité et Expression des Génomes Microbiens, CNRS FRE2383, Université Joseph Fourier, Grenoble, Fance. dominique.schneider@ujf-grenoble.fr
authors.forename: Dominique
authors.initials: D
authors.lastname: Schneider
authors.forename: Esther
authors.initials: E
authors.lastname: Duperchy
authors.forename: Joelle
authors.initials: J
authors.lastname: Depeyrot
authors.forename: Evelyne
authors.initials: E
authors.lastname: Coursange
authors.forename: Richard
authors.initials: R
authors.lastname: Lenski
authors.forename: Michel
authors.initials: M
authors.lastname: Blot
mesh_terms: D002874:Chromosome Mapping / Q000379:methods; D002876:Chromosomes, Bacterial / Q000235:genetics; D004251:DNA Transposable Elements / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics*; D019453:Escherichia coli O157 / Q000235:genetics*; D005819:Genetic Markers / Q000235:genetics*; D016680:Genome, Bacterial; D005810:Multigene Family / Q000235:genetics; D010641:Phenotype; D012703:Serotyping; D013045:Species Specificity
chemical_list: D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D005819:Genetic Markers