PROJECT SUMMARY: Nontyphoidal Salmonella infections are frequently associated with diarrhea in healthy people. Some serovars such as Typhimurium are also common causes of bacteremia in HIV-infected people, and life- threatening disseminated complications in immunocompromised individuals with defects in neutrophils, macrophages or CD4 T cells. Sensor kinases and their cognate response regulators in two-component systems orchestrate many virulence programs in Salmonella and many other pathogenic bacteria. In the canonical activation of two-component systems, the sensor kinase is phosphorylated in response to cues encountered during colonization and infection of the mammalian host. The transfer of the phosphoryl group from the sensor kinase to the receiver domain of its cognate response regulator turns on virulence programs essential for bacterial pathogenesis. We have made the unexpected discovery that two-component response regulators are controlled by previously unknown allosteric interactions with thioredoxin. Our research has shown that thioredoxin post-translationally controls several response regulators such as OmpR, PhoP and SsrB, all of which govern key aspects of Salmonella pathogenesis. Strikingly, the post-translational control exerted by thioredoxin on two-component signaling does not rely on the universally conserved thiol-disulfide oxidoreductase enzymatic activity of this ancestral protein, but is contingent upon a hitherto uncharacterized hydrophobic interfacial surface that has been preserved throughout the evolution of thioredoxin in bacteria, archaea and eukaryotes. Our investigations indicate that most contributions of thioredoxin to Salmonella pathogenesis are independent of its oxidoreductase activity but are carried out by this newly discovered interfacial surface. The proposed research will test the hypothesis that thioredoxin leverages the binding attributes of a conserved hydrophobic patch to establish protein-protein interactions with multiple response regulators, thereby exerting broad post-translational control of two-component signaling. Specifically, we will identify the interfacial residues that mediate oxidoreductase-independent functions of thioredoxin, and will quantify the extent that the novel thioredoxin-binding face enables response regulators to activate Salmonella virulence programs. Our research will elucidate previously unappreciated elements in the regulation of two- component signaling, and will ascertain unprecedented, oxidoreductase-independent functions of thioredoxin. The knowledge gained on the novel function of thioredoxin will not only shed light on key aspects of Salmonella pathogenesis, but may ultimately broaden our understanding of a primordial function of universally-expressed thioredoxin proteins. Our work will also provide far reaching insight into the regulation of two-component systems, which represent a dominant signaling pathway in bacteria. Drugs that specifically inhibit interactions...