Repression of Intracellular Virulence Factors in Salmonella by the Hha and YdgT Nucleoid-Associated Proteins

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223

The acquisition of new genetic traits by horizontal gene transfer and their incorporation into preexisting regulatory networks have been essential events in the evolution of bacterial pathogens. An example of successful assimilation of virulence traits is Salmonella enterica, which acquired, at distinct evolutionary times, Salmonella pathogenicity island 1 (SPI-1), required for efficient invasion of the intestinal epithelium and intestinal disease, and SPI-2, essential for Salmonella replication and survival within macrophages and the progression of a systemic infection. A positive regulatory cascade mainly composed of HilD, HilA, and InvF, encoded in SPI-1, controls the expression of SPI-1 genes, whereas the two-component regulatory system SsrA/B, encoded in SPI-2, controls expression of SPI-2 genes. In this study, we report a previously undescribed transcriptional cross-talk between SPI-1 and SPI-2, where the SPI-1-encoded regulator HilD is essential for the activation of both the SPI-1 and SPI-2 regulons but at different times during the stationary phase of growth in Luria-Bertani medium. Our data indicate that HilD counteracts the H-NS-mediated repression exerted on the OmpRdependent activation of the ssrAB operon by specifically interacting with its regulatory region. In contrast, HilD is not required for SPI-2 regulon expression under the in vitro growth conditions that are thought to resemble the intracellular environment. Our results suggest that two independent SPI-2 activation pathways evolved to take advantage of the SPI-2-encoded information at different niches and, in consequence, in response to different growth conditions.H-NS ͉ OmpR ͉ microbial pathogenesis ͉ transcriptional regulation ͉ salmonella

Section: Discussionmentioning

confidence: 99%

HilD-mediated transcriptional cross-talk between SPI-1 and SPI-2

Bustamante

Martínez

Santana

et al. 2008

146

223

“…Recently, transcription of SPI-2 genes was reported to be negatively controlled by small regulatory proteins, such as YdgT and H-NS (7). Under noninducing conditions, these proteins are likely to be associated with regulatory regions of SPI-2 to silence gene transcription (12,13). When activated within macrophages, the SsrB protein promotes transcription of SPI-2, possibly by counteracting the H-NS-and YdgT-mediated repression (7).…”

Section: Discussionmentioning

confidence: 99%

“…Another small protein, YdgT, also functions as a negative regulator of SPI-2 expression; deletion of the ydgT gene increases transcription levels of the SsrB-regulated SPI-2 genes (12). The ydgT mutant strain is attenuated for virulence in mice, suggesting that the maintenance of appropriate SPI-2 expression level may be another determinant of Salmonella pathogenesis (12,13).…”

mentioning

confidence: 99%

Salmonella pathogenicity island 2 expression negatively controlled by EIIA ^Ntr –SsrB interaction is required for Salmonella virulence

Choi

Shin

Yoon

et al. 2010

SsrA/SsrB is a primary two-component system that mediates the survival and replication of Salmonella within host cells. When activated, the SsrB response regulator directly promotes the transcription of multiple genes within Salmonella pathogenicity island 2 (SPI-2). As expression of the SsrB protein is promoted by several transcription factors, including SsrB itself, the expression of SPI-2 genes can increase to undesirable levels under activating conditions. Here, we report that Salmonella can avoid the hyperactivation of SPI-2 genes by using ptsN-encoded EIIA Ntr , a component of the nitrogen-metabolic phosphotransferase system. Under SPI-2-inducing conditions, the levels of SsrB-regulated gene transcription increased abnormally in a ptsN deletion mutant, whereas they decreased in a strain overexpressing EIIA Ntr . We found that EIIA Ntr controls SPI-2 genes by acting on the SsrB protein at the posttranscriptional level. EIIA Ntr interacted directly with SsrB, which prevented the SsrB protein from binding to its target promoter. Finally, the Salmonella strain, either lacking the ptsN gene or overexpressing EIIA Ntr , was unable to replicate within macrophages, and the ptsN deletion mutant was attenuated for virulence in mice. These results indicated that normal SPI-2 gene expression maintained by an EIIA Ntr -SsrB interaction is another determinant of Salmonella virulence.virulence gene regulation | nitrogen-metabolic phosphotransferase system (PTS) | protein-protein interaction

“…4 and 5). Deletion of the genes specifying the nucleoid-associated protein attenuates Salmonella's virulence in mice (48,49), whereas preventing PmrA-mediated negative regulation of SsrB increases virulence (Fig. 1).…”

Section: Discussionmentioning

confidence: 99%

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

Choi

Groisman

2013

The regulatory protein PmrA controls expression of lipopolysaccharide (LPS) modification genes in Salmonella enterica serovar Typhimurium, the etiologic agent of human gastroenteritis and murine typhoid fever. PmrA-dependent LPS modifications confer resistance to serum, Fe 3+ , and several antimicrobial peptides, suggesting that the pmrA gene is required for Salmonella virulence. We now report that, surprisingly, a pmrA null mutant is actually hypervirulent when inoculated i.p. into C3H/HeN mice. We establish that the PmrA protein binds to the promoter and represses transcription of ssrB , a virulence regulatory gene required for expression of the Spi/Ssa type three-secretion system inside macrophages. The pmrA mutant displayed heightened expression of SsrB-dependent genes and faster Spi/Ssa-dependent macrophage killing than wild-type Salmonella . A mutation in the ssrB promoter that abolished repression by the PmrA protein rendered Salmonella as hypervirulent as the pmrA null mutant. The antivirulence function of the PmrA protein may limit the acute phase of Salmonella infection, thereby enhancing pathogen persistence in host tissues.