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Functional consequences of genome variation in E. coli O157:H7: Lineage-specific genomic variation associated with the PERC-homologues contributes to LEE island gene expression

Zhijie Yang, University of Nebraska - Lincoln

Abstract

E. coli O157:H7 is a member of the Enterohemorrhagic Escherichia coli (EHEC) and is responsible for serious gastrointestinal illness (hemorrhagic colitis) that is commonly foodborne. The O157:H7 populations share recent common ancestry and are genetically highly related. Nevertheless, detailed molecular evolution studies have shown that these populations have recently diverged through two primary evolutionary pathways (lineages) and the two primary lineages appear to have unique virulence characteristics as lineage II strains are statistically underrepresented among human clinical samples. ^ Because the genotyping methods necessary to discriminate the two lineages are tedious and subjective, and are not suited for studying large sets of strains, we have developed a lineage-specific polymorphism assay (LSPA) that can readily distinguish between the lineage I and lineage II subpopulations. Analysis of 1,429 O157:H7 strains with the LSPA-6 demonstrated that five genotypes comprise over 91% of the strains, suggesting that these subpopulations may be widespread. ^ Comparative genome analysis was used to identify genome alterations that are specific to either of the lineages and led us to lineage-specific sets of genes (called pchABCDE) encoding putative regulators of LEE island gene expression. PchABCDE gene content shows a tremendous diversity in O157:H7 populations. In-frame deletions were introduced into pchC and pchD in the NE018 strain background, which represents one of the common pchABCDE genotypes in lineage I. Phenotypic effects of the mutations on LEE island gene expression were monitored from LEE transcription fusions. Both mutations nearly abolished transcription from the tir promoter, but enhanced transcription from LEE1. Introduction of the LEE1 and tir transcription fusions into 40 different wildtype strains representing the genetic diversity in lineage I and II strains showed that strains with similar pchABCDE genotypes have more uniform distribution of expression patterns. ^ Collectively, our data support the hypothesis that allele combinations of pchABCDE present in O157:H7 strains have a significant impact on the virulence characteristics. Given the variation observed in the pchABCDE, we believe selection is acting on these genes, leading to evolution of populations with unique attachment characteristics.^

Subject Area

Biology, Molecular|Agriculture, Food Science and Technology|Biology, Microbiology|Health Sciences, Pathology

Recommended Citation

Yang, Zhijie, "Functional consequences of genome variation in E. coli O157:H7: Lineage-specific genomic variation associated with the PERC-homologues contributes to LEE island gene expression" (2006). ETD collection for University of Nebraska - Lincoln. AAI3237485.
http://digitalcommons.unl.edu/dissertations/AAI3237485

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