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

To establish systemic infections, Salmonella enterica serovar Typhimurium (S. Typhimurium) requires Salmonella pathogenicity island 2 (SPI-2) to survive and replicate within macrophages. High expression of many SPI-2 genes during the entire intracellular growth period within macrophages is essential, as it contributes to the formation of Salmonella-containing vacuole and bacterial replication. However, the regulatory mechanisms underlying the sustained induction of SPI-2 within macrophages are not fully understood. Here, we revealed a time-dependent regulation of SPI-2 expression mediated by a novel regulator PagR (STM2345) in response to the low Mg2+ and low phosphate (Pi ) signals, which ensured the high induction of SPI-2 during the entire intramacrophage growth period. Deletion of pagR results in reduced bacterial replication in macrophages and attenuation of systemic virulence in mice. The effects of pagR on virulence are dependent on upregulating the expression of slyA, a regulator of SPI-2. At the early (0-4 hr) and later (after 4 hr) stage post-infection of macrophages, pagR is induced by the low Pi via PhoB/R two-component systems and low Mg2+ via PhoP/Q systems, respectively. Collectively, our findings revealed that the PagR-mediated regulatory mechanism contributes to the precise and sustained activation of SPI-2 genes within macrophages, which is essential for S. Typhimurium systemic virulence.

Jiang L ，Wang P ，Li X ，Lv R ，Wang L ，Yang B ，Huang D ，Feng L ，Liu B ... -  《-》

In Salmonella enterica, the pathogenicity island 2 (SPI-2) regulator PagR regulates its own expression and the expression of a five-gene operon that encodes transketolase C.

The enteropathogen Salmonella enterica subsp. enterica sv. Typhimurium str. LT2 (hereafter S. Typhimurium) utilizes a cluster of genes encoded within the pathogenicity island 2 (SPI-2) of its genome to proliferate inside macrophages. The expression of SPI-2 is controlled by a complex network of transcriptional regulators and environmental cues, which now include a recently characterized DNA-binding protein named PagR. Growth of S. Typhimurium in low-phosphate, low-magnesium medium mimics conditions inside macrophages. Under such conditions, PagR ensures SPI-2 induction by upregulating the transcription of slyA, which encodes a known activator of SPI-2. Here, we report that PagR represses the expression of a divergently transcribed polycistronic operon that encodes the two subunits of transketolase TktC (i.e., tktD, tktE) of this bacterium. Transketolases contribute to the nonredox rearrangements of phosphorylated sugars of the pentose phosphate pathway, which provide building blocks for amino acids, nucleotides, cofactors, etc. We also demonstrate that PagR represses the expression of its own gene and define two PagR-binding sites between stm2344 and pagR.

Parks AR ，McCormick RD ，Byrne JA ，Escalante-Semerena JC ... -  《-》

A Dopamine-Responsive Signal Transduction Controls Transcription of Salmonella enterica Serovar Typhimurium Virulence Genes.

We have shown that the ligand-responsive MarR family member SlyA plays an important role in transcription activation of multiple virulence genes in Salmonella enterica serovar Typhimurium by responding to guanosine tetraphosphate (ppGpp). Here, we demonstrate that another MarR family member, EmrR, is required for virulence of S. Typhimurium and another enteric bacterium, Yersinia pestis EmrR is found to activate transcription of an array of virulence determinants, including Salmonella pathogenicity island 2 (SPI-2) genes and several divergent operons, which have been shown to be activated by SlyA and the PhoP/PhoQ two-component system. We studied the regulatory effect of EmrR on one of these genetic loci, i.e., the pagC-pagD divergent operon, and characterized a catecholamine neurotransmitter, dopamine, as an EmrR-sensed signal. Dopamine acts on EmrR to reduce its ability to bind to the target promoters, thus functioning as a negative signal to downregulate this EmrR-activated transcription. EmrR can bind to AT-rich sequences, which particularly overlap the SlyA and PhoP binding sites in the pagC-pagD divergent promoter. EmrR is a priming transcription regulator that binds its target promoters prior to successive transcription activators, by which it displaces universal silencer H-NS from these promoters and facilitates successive regulators to bind these regions. Regulation of the Salmonella-specific gene in Escherichia coli and Y. pestis reveals that EmrR-dependent regulation is conserved in enteric bacteria. These observations suggest that EmrR is a transcription activator to control the expression of virulence genes, including the SPI-2 genes. Dopamine can act on the EmrR-mediated signal transduction, thus downregulating expression of these virulence factors.IMPORTANCE In this study, MarR family regulator EmrR is identified as a novel virulence factor of enteric bacteria, here exemplified by Salmonella enterica serovar Typhimurium and Yersinia pestis EmrR exerts an essential effect as a transcription activator for expression of virulence determinants, including Salmonella pathogenicity island 2 genes and a set of horizontally acquired genetic loci that formed divergent operons. EmrR senses the neurotransmitter dopamine and is subsequently released from target promoters, resulting in downregulation of the virulence gene expression. Through this action on EmrR, dopamine can weaken Salmonella resistance against host defense mechanisms. This provides an explanation for the previous observation that dopamine inhibits bacterial infection in animal gastrointestinal tracts. Our findings provide evidence that this neurotransmitter can modulate bacterial gene expression through interaction with virulence regulator EmrR.

Yang D ，Kong Y ，Sun W ，Kong W ，Shi Y ... -  《mBio》

LoiA directly represses lon gene expression to activate the expression of Salmonella pathogenicity island-1 genes.

Salmonella enterica serovar Typhimurium uses virulence factors encoded by Salmonella pathogenicity island 1 (SPI-1) to invade the intestinal epithelium. The low oxygen-induced transcriptional regulator LoiA is established to be a positive regulator of SPI-1 genes and can also repress the expression of the ATP-dependent Lon protease, a negative regulator of SPI-1 genes. Whether the repression of lon by LoiA is associated with its regulation of SPI-1 genes, as well as the regulatory mechanism involved, is unknown. In this study, we showed that LoiA directly represses the expression of the lon gene during invasion by binding to the promoter region of lon. The activation of S. Typhimurium SPI-1 gene expression by LoiA is associated with its repression of lon. Moreover, through invasion and mouse infection assays, we found that the repression of lon by LoiA contributes to S. Typhimurium invasion of intestinal epithelial cells and virulence in mice. However, LoiA does not influence lon expression during S. Typhimurium growth in macrophages, which may be associated with the low expression level of loiA in macrophages. Collectively, these findings establish the direct regulation of lon by LoiA and describe a novel mechanism by which LoiA regulates SPI-1 gene expression.

Jiang L ，Li X ，Lv R ，Feng L ... -  《-》

The transcriptional regulator VarN contributes to Salmonella Typhimurium growth in macrophages and virulence in mice.

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a major intracellular pathogen of humans and animals and its survival and growth in macrophages is essential for its pathogenicity. The S. Typhimurium genome encodes more than 50 putative regulatory proteins, but their involvement in pathogenicity and their regulatory roles are largely unknown. In this study, we investigated the biological function of the S. Typhimurium STM4320 gene (named varN), which encodes a putative MerR family transcriptional regulator. We found that varN is upregulated 2.6- to 6.8-fold after S. Typhimurium enters murine macrophages. A varN mutant reduced S. Typhimurium growth in murine macrophages and attenuated virulence in mice. Moreover, we showed that deletion of varN decreased the transcription of Salmonella pathogenicity island-2 (SPI-2) genes, which are required for S. Typhimurium growth in macrophages, indicative of the positive regulation of SPI-2 by VarN. We confirmed that the virulence defects of the varN mutant are entirely dependent on its regulation of SPI-2. Thus, our results revealed that VarN is a novel activator of the expression of SPI-2 genes and contributes to S. Typhimurium growth in macrophages and virulence in mice. Our findings provide a significant example of how a putative regulatory protein facilitates S. Typhimurium pathogenicity.

Jiang X ，Li X ，Sun S ，Jiang L ... -  《-》