Two-Component Regulators Control hilA Expression by Controlling fimZ and hilE Expression within Salmonella enterica Serovar Typhimurium

Abstract

c Salmonellae initiate disease through the invasion of host cells within the intestine. This ability to invade requires the coordinated action of numerous genes, many of which are found within Salmonella pathogenicity island 1 (SPI-1). The key to this process is the ability of the bacteria to respond to the environment, thereby upregulating the necessary genes under optimal conditions. Central to the control of SPI-1 is the transcriptional activator hilA. Work has identified at least 10 different activators and 8 different repressors responsible for the control of hilA. We have previously shown that hilE is a Salmonella-specific negative regulator that is able to repress hilA expression and invasion. Additionally, fimZ, a transcriptional activator responsible for the expression of type I fimbriae as well as flagellar genes, has also been implicated in this process. fimZ is homologous to response regulators from other two-component regulatory systems, although a sensor for the system has not been identified. The phoPQ and phoBR regulons are both two-component systems that negatively affect hilA expression, although the mechanism of action has not been determined. Our results show that PhoBR is capable of inducing fimZ expression, whereas PhoPQ does not affect fimZ expression but does upregulate hilE in an FimZ-dependent manner. Therefore, phosphate (sensed by PhoBR) and magnesium (sensed by PhoPQ) levels are important in controlling hilA expression levels when Salmonella is in the intestinal environment. Salmonellae have caused disease for many years. These Gramnegative bacteria can be transmitted through meat, dairy products, or eggs, from animals through the fecal-oral route, and indirectly via fecally contaminated water (1). The CDC tracks two forms of the disease, salmonellosis and typhoid fever. Salmonellosis is a mild form of disease that is typically confined to the gastrointestinal tract. It produces symptoms of fever, abdominal cramps, nausea, and diarrhea (2). A common feature of gastroenteritis or typhoid fever is the ability of Salmonella to invade host cells.Salmonella contains an island known as pathogenicity island 1 (SPI-1). This island is responsible for encoding both the structural proteins necessary for creating a type III secretion needle complex as well as some of the secreted effectors responsible for the manipulation of host cells (3, 4). Central to the control of this island is the transcriptional activator hilA, which needs to be upregulated for invasion to occur. This upregulation leads to the increased expression of all the other genes contained within SPI-1 (5, 6). Many different activators and repressors of hilA have been identified. These activators respond to a myriad of environmental signals, specifically, osmolarity, oxygen, pH, growth state, shortchain fatty acids, bile, and temperature (6-12), leading to precise control of hilA expression. The transcriptional activators HilD and HilC play an important role in controlling hilA expression. Work has shown that both genes are encode...

PagR mediates the precise regulation of Salmonella pathogenicity island 2 gene expression in response to magnesium and phosphate signals in Salmonella Typhimurium

“…We proposed a model for the PagR-dependent regulatory pathway (Figure 8). Before entry into the macrophages, pagR PhoP/Q and PhoB/R also regulate hilA and SPI-1 gene expression in opposition to SPI-2 (Baxter & Jones, 2015), indicating the regulatory system mediated by low Mg 2+ and low P i is also involved in the repression of SPI-1 genes after cell invasion, which has been reported to be essential for avoiding innate macrophage immunity and intracellular replication (Takaya et al, 2005;Zhao & Shao, 2015).…”

“…To investigate whether PhoB-independent regulation exists, we also compared the expression levels of slyA in 2, 8, 16, and 24 hpi, the cells were lysed and luminescence and CFU counts were determined. PhoP/Q and PhoB/R also regulate hilA and SPI-1 gene expression in opposition to SPI-2 (Baxter & Jones, 2015), indicating the regulatory system mediated by low Mg 2+ and low P i is also involved in the repression of SPI-1 genes after cell invasion, which has been reported to be essential for avoiding innate macrophage immunity and intracellular replication (Takaya et al, 2005;Zhao & Shao, 2015). P values were determined using Student's t test ( * P < .05; ** P < .01; *** P < .001; ns, not significant) the phoB mutant and pagR/phoB double mutant strains.…”

The Hcp-like protein HilE inhibits homodimerization and DNA binding of the virulence-associated transcriptional regulator HilD in Salmonella

“…The hilE gene resides in a region of the Salmonella chromosome similar to a pathogenicity island (38), supporting that it was acquired by horizontal transfer. The expression of hilE is positively regulated by the PhoPQ and PhoBR two-component systems, as well as by FimZ and LeuO (39,40), whereas this is negatively regulated by Mlc and the small RNA IsmR (41,42), which provides additional inputs controlling the activity of HilD.…”

Salmonella Fimbriae: What is the Clue to Their Hairdo?

“…FimZ also downregulates flhDC, genes implicated in flagella expression [75]. Flagellar gene fliZ also represses fimZ on a post-transcriptional manner, reinforcing the fact that there is an alternated expression of flagella (motility) and fimbriae (adhesion) [51] and confirming a regulation network between SPI-1 (invasion) and fimbrial expression [76]. Thus, a combination of factors directly implicated in fimbrial genes regulation can also impact on other virulence systems of Salmonella (Figure 4).…”