Tim Yahr Laboratory

  • Projects: Regulation of the Pseudomonas aeruginosa type III secretion system

    The labs current research projects focus on the Pseudomonas aeruginosa type III secretion system (TTSS). P. aeruginosa is an opportunistic pathogen responsible for both acute and chronic infections in humans. Acute infections are most often seen in patients with severe burns, corneal ulcerations, or those requiring mechanical ventilation. Chronic infections are most often seen in the airways of individuals with cystic fibrosis.

    One of the many virulence determinants of P. aeruginosa is a TTSS. TTSSs are unique to Gram-negative bacteria and function like hypodermic needles to inject toxins into eukaryotic host cells. P. aeruginosa uses a TTSS to inject at least four toxins into host cells. Injection of these toxins leads to disruption of Ras and Rho mediated signal transduction pathways, collapse of the actin cytoskeleton, and cell death. Although the TTSS is crucial for the pathogenesis of acute P. aeruginosa infections, its role in the establishment and/or progression of chronic infections is unclear. Nearly all isolates from patients with acute P. aeruginosa infections express the TTSS. In contrast, most isolates from the airways of cystic fibrosis patients do not express the TTSS. The laboratory is focused on defining the regulatory pathways required for expression of the TTSS in acute infections and repression in the cystic fibrosis lung. 

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    The P. aeruginosa TTSS consists of ~45 genes. Normally, expression of the TTSS is repressed. Expression is induced when P. aeruginosa contacts eukaryotic cells or is grown in the presence of serum or low Ca2+ concentrations. We are interested in how these environmental cues are sensed by P. aeruginosa and how this leads to expression of the TTSS.

    At least two genes play a role in controlling expression of the TTSS. ExsA is a positive regulatory factor that activates transcription of the TTSS regulon. ExsD is a negative regulator of the TTSS. In the absence of environmental cues, we postulate that ExsD binds ExsA to prevent expression of the TTSS. In the presence of environmental cues, ExsD is released from ExsA allowing for expression of the TTSS. Future research projects will address the following questions: (i) Does ExsD interact with ExsA?, (ii) Do environmental cues influence the interaction of ExsA and ExsD, and if so how?, (iii) Do additional proteins interact with either ExsA or ExsD?