Two component regulation of the ysa type three secretion system in Yersinia enterocolitica

Type three secretion systems play an important role during infection by helping bacteria evade host immune responses by translocation of virulence factors into host cells. Regulation of type three secretion systems (TTSS) often includes various environmental stimuli that trigger a response through two-component signaling systems. Yersinia enterocolitica has two TTSS that vary in the secreted effectors and mode of regulation. One TTSS, Ysc-Yop, is encoded on the virulence plasmid while the other (Ysa-Ysp) is encoded on the chromosome. The focus of this thesis is to further our understanding of the latter, the chromosomal ysa TTSS, by studying the interplay between two, two-component signaling systems (TCS) and their effects on the expression of secreted proteins.; YsrRS is a putative two component system encoded in the ysa locus. In the presence of NaCl, YsrRS has been shown to regulate expression of ysa genes, though it is still unknown how this system functions as a TCS to control ysa gene expression. Based on homology with other systems, the YsrRS TCS is classified as a hybrid TCS. The YsrRS TCS likely engages in a phosphorelay cascade that includes a histidine-phosphotransfer (Hpt) intermediate. However the Hpt intermediate for the YsrRS TCS previously has been undefined and was proposed to be the RcsD protein that is a component of the RcsBCD TCS based on the analogous E. coli Rcs system. Work in this thesis demonstrates that RcsD is not required for the YsrRS TCS and that a novel Hpt (YsrT) intermediate is necessary for ysa gene expression. We further show that RcsC is also not necessary for the activation of ysa genes though RcsB is necessary for ysa gene expression. Finally, we demonstrate that expression of ysa genesis regulated upon exposure to solid surface in the presence of NaCl in a ysrRS and rcsB dependent manner, which only had been shown to be important for rcs regulated genes in E. coli.