UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

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2004 - 20102

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    Genomics and ecology of Integrative conjugative elements (ICE) in Vibrio cholerae
    (2010) Taviani, Elisa; Colwell, Rita; Huq, Anwar; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Lateral gene transfer (LGT) is recognized as a major contributor in bacterial evolution. In this study, the importance of LGT was assessed by analyzing a class of mobile elements, Integrative Conjugative Elements (ICE), and two genomic islands, Vibrio Seventh Pandemic islands (VSP-I and VSP-II) in the bacterial species, Vibrio cholerae. The objective was to understand how acquisition of heterologous genetic material by V. cholerae has influenced its differentiation and adaptation to different niches in the aquatic environment, as well as its role as a human pathogen. ICEs are a class of self-transmissible mobile elements that mediate LGT via conjugation. Members of this class of mobile elements have been isolated from several species of Gram-negative and Gram-positive bacteria, and have been observed to serve as vectors of drug resistance, virulence factors and genetic tools modulating ecological adaptation in V. cholerae. In this study, two new ICEs discovered in seventh pandemic V. cholerae O1 El Tor strains associated with cholera epidemics in Asia and Africa, were analyzed. Overall, comparative analysis of the ICEs revealed an epidemiological relationship between new and old pandemic clones in cholera endemic areas. Furthermore, analysis of a novel ICE found in a clinical isolate of V. cholerae O37 revealed an unusual genetic organization and also association with a major pathogenicity island in V. cholerae, offering evidence of ICEs as progenitors and/or potential dissemination tools for PAIs, likely associated with the pathogenic potential of non-O1/O139 V. cholerae. It is concluded from an extensive survey of the geographical distribution of ICEs that they are present in environmental V. cholerae populations worldwide. In summary, through analysis of ICEs and VSP-I and II it was possible to demonstrate the extent to which environmental V. cholerae, autochthonous to the aquatic environment worldwide, comprise a reservoir of mobile genetic elements. It is clear that mobile genetic elements have undergone significant molecular rearrangement, with a significant range of profiles, confirming their genetic plasticity. Finally, their variable content most likely contributes significantly to genomic evolution responsible for adaptation of this bacterial host to new ecological niches.
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    Pseudomonas syringae pathogenesis: Regulation of type III secretion and identification of a secreted effector
    (2004-11-01) Bretz, James Robert; Hutcheson, Steven W; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Pseudomonas syringae hrp pathogenicity island encodes a type III secretion system (TTSS) which is used to translocate effector proteins into host cells to facilitate pathogenesis. Expression of the hrp TTSS is controlled by the alternative sigma factor, HrpL, whose expression in turn is positively controlled by two truncated enhancer binding proteins, HrpR and HrpS. Although a number of environmental conditions are known to modulate hrp TTSS expression, such as stringent conditions and pathogenesis, the mechanism by which the activities of these transcriptional factors are modulated had not been established. Both HrpR and HrpS were shown to be required for full expression of hrpL. hrpRS were shown to be expressed as an operon and a promoter was identified 5' to hrpR. The hrpRS promoter and coding sequence were found to be conserved among P. syringae strains. The hrpRS operon was constitutively expressed under conditions in which the hrpL promoter was inactive, indicating the involvement of a negative regulatory factor. Transposome (Tnp) mutagenesis was used to identify Lon protease as a negative regulator of hrpL expression, suggesting an effect on HrpR and/or HrpS. HrpR was observed to be unstable in wild-type P. syringae strains grown in non-inductive media. The apparent half-life of HrpR increased more than 10-fold in the P. syringae lon::Tnp mutants or upon transfer to inductive (stringent) conditions. As a result, an interaction between factors involved in the stringent response, Lon protease, and hrp regulation was also investigated. The regulatory system described above was used to develop a HrpL-dependent promoter trap to identify effectors secreted by the Hrp TTSS. One of these potential effectors, HopPtoD2, was shown to encode a protein tyrosine phosphatase that was translocated into Arabidopsis thaliana cells via the hrp-encoded TTSS. A hopPtoD2 mutant exhibited strongly reduced virulence in Arabidopsis thaliana. Expression of hopPtoD2 delayed the development of several defense-associated responses in infected plants. These results indicate that HopPtoD2 is a translocated effector with protein tyrosine phosphatase activity that modulates plant defense responses.