SlyA and HilD Counteract H-NS-Mediated Repression on the ssrAB Virulence Operon of Salmonella enterica Serovar Typhimurium and Thus Promote Its Activation by OmpR

Abstract

H-NS-mediated repression of acquired genes and the subsequent adaptation of regulatory mechanisms that counteract this repression have played a central role in the Salmonella pathogenicity evolution. The Salmonella pathogenicity island 2 (SPI-2) is an acquired chromosomal region containing genes necessary for Salmonella enterica to colonize and replicate in different niches of hosts. The ssrAB operon, located in SPI-2, encodes the two-component system SsrA-SsrB, which positively controls the expression of the SPI-2 genes but also other many genes located outside SPI-2. Several regulators have been involved in the expression of ssrAB, such as the ancestral regulators SlyA and OmpR, and the acquired regulator HilD. In this study, we show how SlyA, HilD, and OmpR coordinate to induce the expression of ssrAB under different growth conditions. We found that when Salmonella enterica serovar Typhimurium is grown in nutrient-rich lysogeny broth (LB), SlyA and HilD additively counteract H-NS-mediated repression on ssrAB, whereas in N-minimal medium (N-MM), SlyA antagonizes H-NS-mediated repression on ssrAB independently of HilD. Interestingly, our results indicate that OmpR is required for the expression of ssrAB independently of the growth conditions, even in the absence of repression by H-NS. Therefore, our data support two mechanisms adapted for the expression of ssrAB under different growth conditions. One involves the additive action of SlyA and HilD, whereas the other involves SlyA, but not HilD, to counteract H-NS-mediated repression on ssrAB, thus favoring in both cases the activation of ssrAB by OmpR. IMPORTANCE The global regulator H-NS represses the expression of acquired genes and thus avoids possible detrimental effects on bacterial fitness. Regulatory mechanisms are adapted to induce expression of the acquired genes in particular niches to obtain a benefit from the information encoded in the foreign DNA, as for pathogenesis. Here, we show two mechanisms that were integrated for the expression of virulence genes in Salmonella Typhimurium. One involves the additive action of the regulators SlyA and HilD, whereas the other involves SlyA, but not HilD, to counteract H-NS-mediated repression on the ssrAB operon, thus favoring its activation by the OmpR regulator. To our knowledge, this is the first report involving the coordinated action of two regulators to counteract H-NS-mediated repression.

HilD induces expression of a novel Salmonella Typhimurium invasion factor, YobH, through a regulatory cascade involving SprB

“…Previous studies indicate that HilD can also control expression of sprB by acting on the hilC gene, located upstream of sprB ; a hilC-sprB transcript was detected in a previous study 28 and direct regulation of hilC by HilD is well documented 28,30,38,48,51 . We show that HilD induces expression of sprB by directly displacing the repressor H-NS from the regulatory region upstream of this gene; a mechanism that HilD follows to induce expression of other target genes 36,37,41–43 . H-NS represses expression of hilC 43,53,69,70 , which suggest that HilD induces expression of the hilC-sprB transcript also by antagonizing H-NS mediated repression.…”

“…HilD induces expression of a high number of target genes by acting directly or through distinct regulators, in growth conditions that somehow resemble the intestinal environment (SPI-1-inducing conditions), such as those that we assessed in this study 2 . At present, three different regulatory cascades headed by HilD have been well characterized: the HilD-HilA-InvF, HilD-SsrA/SsrB and HilD-FlhDC cascades 2,25,27,32,33,36,37 . Additionally, HilD forms a feed-forward positive loop with HilC and RtsA, which amplifies the activation of the HilD-HilA-InvF cascade 30,31 , and probably also the activation of the other regulatory cascades and genes controlled directly by HilD.…”

“…On another hand, HilD positively controls expression of the FlhD 4 C 2 complex, the master positive transcriptional regulator of the flagellar/chemotaxis genes, which are also required for the invasion phenotype of Salmonella 32–35 . Likewise, HilD induces expression of the SPI-2 genes, which are mainly required for replication of Salmonella within host cells, by acting on the ssrAB operon that codes for the SsrA/SsrB two-component system, the master regulator of SPI-2 25,36,37 . Additionally, HilD directly controls expression of several other virulence genes 28,38–40 .…”

“…Additionally, HilD directly controls expression of several other virulence genes 28,38–40 . For all cases characterized up to now, HilD induces expression of target genes by directly antagonizing repression mediated by the histone-like protein H-NS on the respective promoters 36,37,41–43 .…”

Horizontally acquired regulatory gene activates ancestral regulatory system to promote Salmonella virulence

“…SsrB is a DNA-binding transcriptional regulator activated by the sensor SpiR (sometimes referred to as SsrA) ( 15 , 16 ). Transcription of the horizontally acquired ssrB and spiR genes is regulated by several ancestral regulators, including SlyA ( 20 ), OmpR ( 6 ), and PhoP ( 7 ).…”