abstract
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Bacteria demonstrate an extraordinary capacity to survive and adapt to changing environments. In part, this ability to adapt can be attributed to horizontal gene transfer, a phenomenon which introduces novel genetic information that can be appropriated for use in particular niches. Nowhere is this more relevant than in pathogenic bacteria, whose acquisition of virulence genes have provided an arsenal that permits them to thrive within their selected host. Regulatory evolution is necessary for timely regulation of these acquired virulence genes in the host environment. Salmonella enterica serovar Typhimurium is an intracellular pathogen which possesses numerous horizontally-acquired genomic islands encoding pathogenic determinants that facilitate its host lifestyle. One island, Salmonella Pathogenicity Island (SPI)-2, encodes a type-III secretion system (T3SS) which is regulated by the two-component regulatory system SsrA-SsrB. This system coordinates expression of the SPI-2 T3SS as well as an array of virulence effectors encoded in horizontally-acquired regions throughout the Salmonella genome. The studies presented here investigated the mechanisms in which the transcription factor SsrB functions to integrate virulence processes through regulatory adaptation. This work identified the regulatory logic controlling SsrB and defined the associated SsrB regulon. Furthermore, SsrB was found to induce a regulatory cascade responsible for the expression of bacteriophage genes encoded within SPI-12, an island that also contributes to Salmonella virulence. These findings demonstrate the important contribution of regulatory evolution in pathogen adaptation to the host, and show that horizontally-acquired genes, once integrated into appropriate regulatory networks, can contribute to pathogen fitness in specific niche environments.