A sigma factor and anti-sigma factor that control swarming motility and biofilm formation in Pseudomonas aeruginosa
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CitationMcGuffie, Bryan A. 2015. A sigma factor and anti-sigma factor that control swarming motility and biofilm formation in Pseudomonas aeruginosa. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractPseudomonas aeruginosa is an environmental bacterium and opportunistic human pathogen of major clinical significance. It is the principal cause of morbidity and mortality in patients with cystic fibrosis (CF) and a leading cause of nosocomial infections. Although the organism is unicellular, P. aeruginosa exhibits two forms of multicellular behaviors when associated with a surface under the right conditions: swarming motility and biofilm formation. Swarming motility is a multicellular cooperative form of flagella-dependent surface motility, while biofilm formation produces a sessile community of bacteria enclosed by a self-produced extracellular polymeric matrix. P. aeruginosa is thought to grow as a biofilm in the lungs of CF patients and the growth of P. aeruginosa biofilms on indwelling medical devices, such as endotracheal tubes and catheters, is a significant source of nosocomial infection. By growing as a biofilm, P. aeruginosa resists clearance by the immune system and increases its resistance to antimicrobial therapy. In this thesis, I describe the characterization of a sigma factor and anti-sigma factor implicated in P. aeruginosa virulence and cell envelope stress that control the expression of a novel regulator of swarming motility and biofilm formation. In addition, I describe work done to investigate the role of a post-translational regulator of flagellar motility that has been described in other bacteria, but has not been studied extensively P. aeruginosa.
In bacteria, RNA polymerase (RNAP) requires sigma factors for promoter-specific transcription initiation. sigma factors guide RNAP to promoters by recognizing conserved DNA sequences within the promoter called the -10 and -35 elements. Most bacteria encode a primary sigma factor and several alternative sigma factors, each of which recognizes different promoter -10 and -35 sequences. By modulating the activity of alternative sigma factors, bacteria can rapidly alter their transcriptional program in response to changes in growth, morphological development, and environmental conditions. The extracytoplasmic function (ECF) sigma factors are the largest and most diverse group of bacterial sigma factors. The gene encoding an ECF sigma factor is often cotranscribed with its own negative regulator, called an anti-sigma factor, which directly binds to and inhibits its partner sigma factor until the appropriate extracytoplasmic signal stimulates sigma factor release and expression of the sigma factor’s regulon.
In this thesis, I describe the characterization of the P. aeruginosa ECF sigma factor PA2896 and its cognate anti-sigma factor PA2895. Using immunoprecipitation, we show that the ECF sigma factor PA2896 and RNAP co-purify in vivo. Utilizing DNA microarrays, we identify the genes that constitute the PA2895 and PA2896 regulon and infer the putative -10 and -35 consensus sequences recognized by PA2896. Genetic analysis revealed a subset of genes within the PA2896 regulon that share the putative promoter consensus sequence are positively regulated by the ECF sigma factor PA2896 and negatively regulated by the anti-sigma factor PA2895. Using a bacterial two-hybrid assay, we show that PA2895 directly interacts with PA2896. We present evidence that increased expression of the PA2896 regulon in ∆PA2895 mutants cells leads to the inhibition of swarming motility and enhanced biofilm formation. We further show that one gene in the PA2896 regulon, PA1494, is necessary and sufficient for the inhibition of swarming motility and promotion of biofilm formation. Thus we report the discovery of a system that may respond to a stress signal by activating PA2896-dependent expression of PA1494 to inhibit swarming motility and promote the formation of a protective biofilm.
In many bacteria, swarming motility and biofilm formation are controlled by the second messenger c-di-GMP. In P. aeruginosa, elevated intracellular c-di-GMP generally results in the inhibition of flagellar-dependent swarming motility and enhanced biofilm formation. In some species of bacteria, c-di-GMP-mediated repression of flagellar motility is achieved by repressing the expression of the flagellar genes. However, flagellar gene expression in P. aeruginosa does not appear to be influenced by elevated c-di-GMP, suggesting c-di-GMP controls flagellar motility post transcriptionally in this bacterium. Mechanisms by which c-di-GMP controls flagellar function post translationally have been described in both Gram-negative and Gram-positive bacteria, however the mechanism in P. aeruginosa remains unclear. Gram-negative bacteria appear to utilize a “flagellar brake” to control flagellar function in response to c-di-GMP. P. aeruginosa encodes a homolog of this brake and in this thesis I present evidence that this homolog is involved in controlling flagellar function in P. aeruginosa.
Together, the characterization of PA2895 and PA2896, the identification of PA1494 as a novel regulator of swarming motility and biofilm formation, and evidence of a functional flagellar brake in P. aeruginosa advance our understanding of how this bacterium controls the transition from motile cell to sessile biofilm.
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