This proposal is based on a fundamentally new approach to the isolation of bacterial virulence genes involved in pathogenesis, termed IVET (in vivo expression technology), that allows for the positive selection of bacterial genes that are specifically induced in host tissues. This methodology will be used to characterize the genetic and environmental factors that regulate the expression of in vivo induced genes (ivi) in the human pathogen, Salmonella typhimurium. Such studies will provide insights into the molecular mechanisms governing the regulation of pathogenicity, which is fundamental to the understanding of infectious disease. Genetic analysis of genes that answer the IVET selection is facilitated because the genes are transcriptionally fused to the reporter gene, lacZ, allowing the recovery of regulatory mutants that constitutively express ivi genes in vitro (normally repressing conditions). Our efforts will be focused on those regulatory factors that coordinately regulate ivi genes, thus defining the regulatory components that ensure a multifaceted bacterial response to a multifaceted host challenge. Trans-acting regulatory mutations will be used to characterize the positive and negative ivi regulatory elements that activate ivi transcription in vivo and repress ivi transcription in vitro. These mutants will be screened for those that confer virulence defects in an animal model, suggesting that they, or some of the products they regulate, play an important role in pathogenesis. Cis-acting mutations will be used to define the ivi promoter region with an emphasis on establishing a consensus site for coordinate ivi gene expression. Defining the nature of coordinate regulation provides a means to under stand the sophisticated signal transduction mechanisms that enable bacteria to grow under such different environmental conditions (e.g., laboratory media vs. host tissues). Additionally, the IVET selection will be extended to screening for ivi fusions whose survival is limited to a particular host tissue and thus may contain tissue-specific ivi genes, whose restricted expression will be examined by genetic analysis. These studies have practical applications for both vaccine and antimicrobial development.
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