Characterization of QSEA and QSED in the Quorum Sensing Cascade of Enterohemorrhagic Escherichia Coli

Enterohemorrhagic E. coli O157:H7 (EHEC) is an enteric pathogen that has been implicated in many outbreaks of bloody diarrhea worldwide. EHEC senses its environment through quorum sensing, a mechanism by which bacteria use chemical signals, termed autoinducers, to regulate key genes. In the gastrointestinal tract, EHEC responds to AI-3 produced by the endogenous gastrointestinal microbial flora and epinephrine/norepinephrine produced by the host to regulate expression of virulence genes. In particular, EHEC utilizes quorum sensing to regulate virulence processes, including motility and chemotaxis and the production of attaching and effacing lesions. Motility and chemotaxis processes are controlled under the complex flagella regulon in EHEC. The expression of genes within the locus of enterocyte effacement (LEE) results in the production of the characteristic attaching and effacing lesions created as a result of production of a type III secretion apparatus. The LEE1 operon encodes for a transcriptional activator, Ler, which is responsible for the activation of other genes within the pathogenicity island. The virulence mechanisms that enable EHEC to circumvent the host defenses and compete for essential nutrients for survival are controlled by several transcriptional regulators, many of which are controlled in response to quorum sensing in EHEC. Quorum sensing E. coli regulator A, QseA, recently was described as a transcription factor that is activated via quorum sensing in EHEC. QseA, which belongs to the family of LysR transcription factors, activates the transcription of LEE1/ler directly; therefore, QseA indirectly activates the expression of other genes within the LEE pathogenicity island. The work in the first specific aim of this thesis examines the specific regulation of the LEE1/ler promoter by QseA through the use of genetic and biochemical methods. Quorum sensing E. coli regulator D, QseD, is a previously uncharacterized transcription factor that is repressed through quorum sensing in EHEC. QseD appears to play a significant role in the overall quorum sensing cascade, as it is involved in the modulation of both motility and type III secretion in EHEC. The second aim of this thesis is to study the role of QseD modulation in quorum sensing signaling in EHEC.