Role of transcriptional regulatory networks in influencing Yersinia pseudotuberculosis pathogenesis

The aim of this dissertation is to better understand how facultative pathogens sense their environment, and coordinate the expression of genes important for establishing disease. This dissertation focuses on a facultative enteropathogen, Yersinia pseudotuberculosis, which is a Gram-negative bacteria that can be found in the environment or can cause disease in the mammalian host. In order for Y. pseudotuberculosis to be successful in causing disease, Y. pseudotuberculosis must modulate its transcriptome to reflect its environment. For example, in the mammalian host Y.pseudotuberculosis upregulates the type III secretion system (T3SS) an important virulence factor which is crucial for surviving and causing disease in the host. The first part of this dissertation focused on determining the regulon of the transcription factor, IscR, in Y. pseudotuberculosis. This transcription factor was previously shown to positively regulate the T3SS and a heme acquisition pathway in Yersinia, but not much else was known regarding the other genes IscR regulates. I employed ChIP-Seq and RNA-Seq to determine the IscR regulon and showed that IscR modulates many cellular processes including predicted virulence factors. The second part of my dissertation reports on the molecular mechanism of how IscR positively regulates the T3SS in Yersinia. Genes that encode the Yersinia T3SS are activated by the transcriptional activator LcrF. Previous data in our lab showed that IscR promotes LcrF transcription, thus activating expression of T3SS genes. Interestingly, IscR does not directly activate RNA-polymerase at the LcrF promoter, but instead antagonizes a repressor of LcrF. In this chapter I demonstrate that the nucleoid proteins H-NS and YmoA repress lcrF, and that IscR antagonizes H-NS-YmoA mediated repression of LcrF by binding to the lcrF promoter. The third part of my dissertation introduces the two-component regulatory system CpxRA. Previous data has shown the CpxRA system represses the T3SS in many pathogens including the T3SS in Yersinia. The previous model suggested that CpxR directly represses LcrF, however in this chapter I demonstrate that CpxR does not bind to the lcrF promoter and evokes indirect regulation of LcrF. I later show that CpxR activates expression of the known repressor of LcrF, YmoA and suggests that CpxR represses the T3SS through an indirect mechanism. A great deal of my work focuses on how transcriptional regulatory networks govern expression of virulence factors, however not much is known how the expression of virulence factors affects regulatory networks. The last part of my dissertation provides data to suggest that the disruption to assembly of the Yersinia T3SS induces CpxRA activity. Interestingly, specific T3SS mutants induce the CpxRA pathway suggesting that expression of the T3SS can modulate the CpxRA regulon. Together these findings enhance our understanding of how facultative pathogens sense environmental signals and how environmental signals modulate gene regulatory networks