Bacteria respond to changing environments by altering gene expression. Two-component regulatory systems (TCRS) are important mediators of signal transduction that enable bacteria to detect physical and/or chemical changes and then relay this signal through the cytoplasm to the bacterial nucleoid, where modulation of gene expression occurs.
In its most basic form, a TCRS consists of a membrane-bound sensor kinase and a DNA-binding response regulator. In response to specific stimuli, the sensor kinase is phosphorylated at a conserved histidine residue and then the phosphoryl group is transferred to the conserved aspartate on the response regulator. Phosphorylation of the response regulator triggers a conformational change, driving dimerization and high affinity DNA binding. For example, EnvZ is a sensor kinase that spans the inner cytoplasmic membrane and has two domains- a sensory domain and a transmitter domain. In response to osmotic changes, EnvZ is phosphorylated at a conserved histidine residue and then the phosphoryl group is transferred to the conserved aspartate on the response regulator, OmpR (see Figure 1). Phosphorylation of OmpR triggers a conformational change, driving dimerization and high affinity DNA binding. Bacteria differ from eukaryotes in that their signal transduction systems employ histidine phosphorylation rather than tyrosine phosphorylation .
While many two-component systems control basic cellular processes such as motility, nutrient uptake and cell division, bacterial pathogens use two-component systems to control virulence. A prominent example is the gram-negative pathogen Salmonella Typhimurium, which is a frequent cause of bacterial gastroenteritis. After ingestion of contaminated food, Salmonella can travel to the lower intestinal tract where it crosses the intestinal epithelium and is engulfed by resident macrophages. The low acidity and low osmolality of the macrophage vacuolar environment serves as an activating signal for various two-component systems, including: EnvZ/OmpR, PhoP/PhoQ, SsrA/SsrB, which enables the pathogen to survive inside host cells. The PhoP/PhoQ TCRS responds to changes in the level of cations like Ca2+ and Mg2+, whereas EnvZ-OmpR, the master regulator, responds to changes in the acidic environment of the macrophage . Both PhoQ/PhoP  and EnvZ/OmpR ,  regulate SsrA/SsrB which eventually regulates SPI-2 expression and secretion of effectors (Figure 2).
In particular, the SsrA sensor kinase is activated by low pH and phosphorylates the response regulator SsrB. Phosphorylated SsrB de-represses H-NS silencing and activates a set of virulence genes located at the Salmonella Pathogenicity island-2 (SPI-2) region , . These genes encode various components of a molecular syringe (Type-III secretion system) which allows the delivery of ~30 specialized effector proteins that manipulate the host cellular machinery to ensure the survival and proliferation of intracellular Salmonella.
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