Date of Degree
PhD (Doctor of Philosophy)
Timothy L. Yahr
Pseudomonas aeruginosa is a Gram-negative bacterium that causes acute nosocomial infections as well as chronic infections in cystic fibrosis (CF) patients. P. aeruginosa utilizes a type III secretion system (T3SS) during acute infections to promote host cell cytotoxicity and inhibit phagocytosis. Regulation of T3SS expression can be classified into two distinct categories: intrinsic and extrinsic. T3SS intrinsic regulation involves the well-characterized ExsECDA cascade that controls T3SS gene transcription. Extrinsic regulation involves global regulatory systems that affect T3SS expression. Despite general knowledge of global regulation of T3SS expression, few specific mechanisms have been elucidated in detail. The overall goal of my thesis work was to provide clarity to global regulatory mechanisms controlling T3SS expression.
One well-documented observation is that P. aeruginosa isolates from CF patients commonly have reduced T3SS expression. In chapter II, I describe how the MucA/AlgU/AlgZR system, commonly activated in CF isolates through mutation of the mucA gene, inhibits T3SS gene expression. My experiments demonstrate that the AlgZR two-component system inhibits ExsA expression through two separate global regulatory systems. First, as previously described, AlgZR inhibits ExsA expression by reducing activity of the cAMP/Vfr signaling pathway. Vfr, a homolog of Escherichia coli Crp, regulates T3SS gene expression through an unknown mechanism. Second, AlgZR alters the activity of the RsmAYZ system to specifically reduce ExsA expression. The RNA-binding protein RsmA, a homolog of E. coli CsrA, activates ExsA expression at a post-transcriptional level.
Previous studies in our laboratory identified several transposon insertion mutants that appeared to be novel extrinsic regulators of T3SS gene expression. One of those candidates, named DeaD, is a putative ATP-dependent RNA helicase. My experiments in chapter III reveal that DeaD regulates T3SS expression by directly stimulating exsA translation. Mutants lacking deaD have reduced exsA translational reporter activity and ExsA expression in trans fails to complement a deaD exsA double mutant for T3SS gene expression. I demonstrate that purified DeaD stimulates ExsA expression in a coupled in vitro transcription/translation assay, confirming our in vivo findings.
In chapter II, I observed that RsmA activates the transcription of RsmY and RsmZ, two small non-coding RNAs that act to sequester RsmA from target mRNAs. My experiments in chapter IV begin to dissect the RsmA-activation mechanism of RsmY/Z expression. I show that RsmA activation requires the previously described Gac/Lad/Ret system that controls RsmY/Z expression. RsmA, however, does not alter Gac/Lad/Ret gene transcription or translation. Interestingly, an RsmA variant deficient in RNA-binding, RsmA R44A, was able to complement an rsmA mutant for RsmY/Z expression. I hypothesized that RsmA interacts with an unknown protein to activate RsmY/Z expression and identified several potential interaction partners using co-purification assays. Together, my combined experiments elucidate novel global regulatory pathways controlling T3SS gene expression during acute and chronic P. aeruginosa infections, and provide a foundation towards the goal of developing future treatment options.
Pseudomonas aeruginosa is a bacterial species that commonly causes infections in hospital settings and in most individuals with cystic fibrosis. P. aeruginosa utilizes a needle-like structure called a type III secretion system (T3SS) to inject toxins into host cells and promote infections. Expression of the T3SS is controlled by the master transcriptional activator ExsA. Regulation of ExsA expression is complex and involves multiple factors, many of which remain unidentified. The focus of my thesis was to elucidate novel regulatory mechanisms of T3SS expression.
My initial experiments uncovered an intersection of regulatory pathways that occurs during CF infections, resulting in reduced ExsA expression. P. aeruginosa acquires many mutations during CF infections, many of which reduce T3SS expression. My data indicate that a common mutation in the mucA gene results in reduced ExsA expression at a post-transcriptional level. Additionally, my work identifies a new T3SS regulator, DeaD, that functions by stimulating exsA translation. I show that deaD mutants are deficient in ExsA expression and that DeaD promotes ExsA expression directly. Finally, I reveal a novel autoregulatory mechanism that has implications affecting our current understanding of P. aeruginosa gene expression, including the T3SS.
Novel treatment options must be developed as antibiotic treatments become less effective. One such option involves targeting virulence factors, such as T3SS. Together, my data provides new understanding of T3SS regulation in P. aeruginosa. Additionally, my efforts provide a foundation for development of treatment options targeting T3SS.
publicabstract, DeaD, ExsA, MucA, Pseudomonas aeruginosa, RsmA, type III secretion
Copyright 2015 Peter J Intile