Fur Negatively Regulates hns and Is Required for the Expression of HilA and Virulence in Salmonella enterica Serovar Typhimurium

Abstract

Iron is an essential element for the survival of living cells. However, excess iron is toxic, and its uptake is exquisitely regulated by the ferric uptake regulator, Fur. In Salmonella, the Salmonella pathogenicity island 1 (SPI-1) encodes a type three secretion system, which is required for invasion of host epithelial cells in the small intestine. A major activator of SPI-1 is HilA, which is encoded within SPI-1. One known regulator of hilA is Fur. The mechanism of hilA regulation by Fur is unknown. We report here that Fur is required for virulence in Salmonella enterica serovar Typhimurium and that Fur is required for the activation of hilA, as well as of other HilA-dependent genes, invF and sipC. The Fur-dependent regulation of hilA was independent of PhoP, a known repressor of hilA. Instead, the expression of the gene coding for the histone-like protein, hns, was significantly derepressed in the fur mutant. Indeed, the activation of hilA by Fur was dependent on 28 nucleotides located upstream of hns. Moreover, we used chromatin immunoprecipitation to show that Fur bound, in vivo, to the upstream region of hns in a metal-dependent fashion. Finally, deletion of fur in an hns mutant resulted in Fur-independent activation of hilA. In conclusion, Fur activates hilA by repressing the expression of hns.

The lipopolysaccharide modification regulator PmrA limitsSalmonellavirulence by repressing the type three-secretion system Spi/Ssa

“…By contrast, the ferric uptake regulator (Fur) does not appear to be involved in this process because an increase in iron would promote Fur dimerization and repression of its target promoters. Because one of Fur's target promoters is that corresponding to the hns gene (53), and H-NS is known to silence expression of SPI-2 genes (22), the increase in iron should increase expression of SPI-2 genes; however, we observed decreased expression (Fig. 3A).…”

Role of the Salmonella enterica serovar Typhi Fur regulator and small RNAs RfrA and RfrB in iron homeostasis and interaction with host cells

“…Recently, Fur was shown to be involved in regulation of genes encoding regulators of SPI-1, a group of genes encoding all the machinery of a type three secretion system essential to the invasion of epithelial cells. Fur positively regulates HilA, the master regulator of SPI-1, via HilD (Ellermeier & Slauch, 2008;Teixidó et al, 2011) or via the repression of the histone-like protein H-NS (Troxell et al, 2011b). qPCR data confirm that hilA expression was lower in the S. Typhi fur mutant in SPI-1-inducing conditions (data not shown).…”

“…It was previously shown that an S. Typhimurium fur mutant did not have a survival defect in murine macrophages (Garciadel Portillo et al, 1993), so it is possible that Fur has a different impact in virulence between these two serovars. However, Fur is required for the virulence of S. Typhimurium in mice (Curtiss et al, 2009;Garcia-del Portillo et al, 1993;Riesenberg-Wilmes et al, 1996;Troxell et al, 2011b;Velayudhan et al, 2007).…”

“…Indeed, Fur is important for the acid tolerance response of Salmonella (Foster, 1991;Hall & Foster, 1996). Fur also plays a role in virulence of S. Typhimurium, as it is important for the establishment of a systemic infection in mice and some studies have shown that a fur mutant had a lower level of invasion in epithelial cells (Curtiss et al, 2009;Garcia-del Portillo et al, 1993;Riesenberg-Wilmes et al, 1996;Troxell et al, 2011b;Velayudhan et al, 2007). Recently, a link between Fur and the Salmonella pathogenicity island 1 (SPI-1) genes necessary for invasion has been demonstrated (Ellermeier & Slauch, 2008;Teixidó et al, 2011;Troxell et al, 2011b).…”

“…Fur also plays a role in virulence of S. Typhimurium, as it is important for the establishment of a systemic infection in mice and some studies have shown that a fur mutant had a lower level of invasion in epithelial cells (Curtiss et al, 2009;Garcia-del Portillo et al, 1993;Riesenberg-Wilmes et al, 1996;Troxell et al, 2011b;Velayudhan et al, 2007). Recently, a link between Fur and the Salmonella pathogenicity island 1 (SPI-1) genes necessary for invasion has been demonstrated (Ellermeier & Slauch, 2008;Teixidó et al, 2011;Troxell et al, 2011b). Many iron-and fur-responsive elements were identified in Salmonella (Bäumler et al, 1996;Bjarnason et al, 2003;Foster & Hall, 1992;Troxell et al, 2011a;Tsolis et al, 1995), but characterization of the regulators of iron homeostasis is missing.…”