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RpoE fine tunes expression of a subset of SsrB-regulated virulence factors in Salmonella enterica serovar Typhimurium.

Osborne SE, Coombes BK - BMC Microbiol. (2009)

Bottom Line: The survival of Salmonella enterica within the intracellular host niche requires highly co-ordinated expression of virulence effectors predominantly regulated by the SsrAB two-component regulatory system.Mutants lacking the gene encoding the alternative sigma factor sigmaE (rpoE) are also highly attenuated for intracellular survival, pointing to a potential connection with the SsrAB regulatory system.In this study we demonstrate that RpoE is involved in fine-tuning the expression of a subset of SsrB-regulated genes found in the Salmonella pathogenicity island-2 (SPI-2) genetic locus that encodes a horizontally acquired type III secretion system, and unlinked genes integrated into this regulon that are required for virulence in host animals.

View Article: PubMed Central - HTML - PubMed

Affiliation: Michael G DeGroote Institute for Infectious Disease Research, Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, Canada. osborns@mcmaster.ca

ABSTRACT

Background: The survival of Salmonella enterica within the intracellular host niche requires highly co-ordinated expression of virulence effectors predominantly regulated by the SsrAB two-component regulatory system. S. enterica serovar Typhimurium mutants lacking the ssrAB genes are avirulent in mice, highlighting the importance of this regulatory system in vivo. Mutants lacking the gene encoding the alternative sigma factor sigmaE (rpoE) are also highly attenuated for intracellular survival, pointing to a potential connection with the SsrAB regulatory system.

Results: In this study we demonstrate that RpoE is involved in fine-tuning the expression of a subset of SsrB-regulated genes found in the Salmonella pathogenicity island-2 (SPI-2) genetic locus that encodes a horizontally acquired type III secretion system, and unlinked genes integrated into this regulon that are required for virulence in host animals.

Conclusion: These data point to a potential connection between the virulence phenotype of strains lacking ssrB and rpoE, and highlight new transcriptional regulation that might be essential for appropriate temporal and spatial control of the virulence-associated type III secretion system during host infection.

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Model for σE-dependent regulation of the SsrB regulon. Membrane-targeting host defences including reactive oxygen stress and antimicrobial peptides cause an accumulation of unfolded outer membrane proteins (OMPs) and stimulate the cleavage of the anti-sigma factor RseA consequently releasing σE into the cytoplasm where it directs RNA polymerase to a subset of SPI-2 promoters. RpoE can positively or negatively regulate SsrB-regulated genes including integrated virulence genes unlinked with SPI-2 but has no effect on some effector genes such as sseL. This regulatory pathway may have evolved to coordinate virulence gene expression with host infection by responding to host-specific defence pathways that perturb the bacterial outer membrane.
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Figure 4: Model for σE-dependent regulation of the SsrB regulon. Membrane-targeting host defences including reactive oxygen stress and antimicrobial peptides cause an accumulation of unfolded outer membrane proteins (OMPs) and stimulate the cleavage of the anti-sigma factor RseA consequently releasing σE into the cytoplasm where it directs RNA polymerase to a subset of SPI-2 promoters. RpoE can positively or negatively regulate SsrB-regulated genes including integrated virulence genes unlinked with SPI-2 but has no effect on some effector genes such as sseL. This regulatory pathway may have evolved to coordinate virulence gene expression with host infection by responding to host-specific defence pathways that perturb the bacterial outer membrane.

Mentions: When we examined the -10 and -35 positions of the promoters studied here relative to the transcriptional start sites identified previously [24], these promoters did not appear to contain σE consensus sequences. Instead they appeared to have consensus sites for σ70. Although a bioinformatics screen identified σE consensus sequences upstream of the SPI-2 genes ssaU, ssaJ, sscA and ssaC [26], these genes were not tested for σE-dependence in the present study because the identified consensus sites are in coding sequence within operons, and as a result may not be directly relevant. Due to the high degree of conservation in σ factor binding sequences, σE may not be directly regulating SsrB-dependent promoters. The lack of a canonical σE sequence at these promoters suggests that another regulatory gene may be epistatic to σE or that these promoters encode functional, but non-canonical σE-binding sites due to their horizontal acquisition and gradual integration into the σE regulatory network. This integration may help Salmonella coordinate expression of the virulence-associated T3SS in response to host factors that compromise bacterial membrane integrity (Figure 4). This mechanism would activate a restorative σE pathway, which is consistent with the enhanced susceptibility of rpoE mutants to oxidative stress and antimicrobial peptides [13,15,16], both of which perturb membrane integrity in vivo. Although there is no evidence that σE can directly repress transcription, the negative effect on two promoters observed here might be due to an intermediate RpoE-regulated repressor or compensatory effect where loss of rpoE increases the relative abundance of another sigma factor that can directly activate the ssaG and srfN promoters. Future work will be required to resolve these possibilities.


RpoE fine tunes expression of a subset of SsrB-regulated virulence factors in Salmonella enterica serovar Typhimurium.

Osborne SE, Coombes BK - BMC Microbiol. (2009)

Model for σE-dependent regulation of the SsrB regulon. Membrane-targeting host defences including reactive oxygen stress and antimicrobial peptides cause an accumulation of unfolded outer membrane proteins (OMPs) and stimulate the cleavage of the anti-sigma factor RseA consequently releasing σE into the cytoplasm where it directs RNA polymerase to a subset of SPI-2 promoters. RpoE can positively or negatively regulate SsrB-regulated genes including integrated virulence genes unlinked with SPI-2 but has no effect on some effector genes such as sseL. This regulatory pathway may have evolved to coordinate virulence gene expression with host infection by responding to host-specific defence pathways that perturb the bacterial outer membrane.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2651894&req=5

Figure 4: Model for σE-dependent regulation of the SsrB regulon. Membrane-targeting host defences including reactive oxygen stress and antimicrobial peptides cause an accumulation of unfolded outer membrane proteins (OMPs) and stimulate the cleavage of the anti-sigma factor RseA consequently releasing σE into the cytoplasm where it directs RNA polymerase to a subset of SPI-2 promoters. RpoE can positively or negatively regulate SsrB-regulated genes including integrated virulence genes unlinked with SPI-2 but has no effect on some effector genes such as sseL. This regulatory pathway may have evolved to coordinate virulence gene expression with host infection by responding to host-specific defence pathways that perturb the bacterial outer membrane.
Mentions: When we examined the -10 and -35 positions of the promoters studied here relative to the transcriptional start sites identified previously [24], these promoters did not appear to contain σE consensus sequences. Instead they appeared to have consensus sites for σ70. Although a bioinformatics screen identified σE consensus sequences upstream of the SPI-2 genes ssaU, ssaJ, sscA and ssaC [26], these genes were not tested for σE-dependence in the present study because the identified consensus sites are in coding sequence within operons, and as a result may not be directly relevant. Due to the high degree of conservation in σ factor binding sequences, σE may not be directly regulating SsrB-dependent promoters. The lack of a canonical σE sequence at these promoters suggests that another regulatory gene may be epistatic to σE or that these promoters encode functional, but non-canonical σE-binding sites due to their horizontal acquisition and gradual integration into the σE regulatory network. This integration may help Salmonella coordinate expression of the virulence-associated T3SS in response to host factors that compromise bacterial membrane integrity (Figure 4). This mechanism would activate a restorative σE pathway, which is consistent with the enhanced susceptibility of rpoE mutants to oxidative stress and antimicrobial peptides [13,15,16], both of which perturb membrane integrity in vivo. Although there is no evidence that σE can directly repress transcription, the negative effect on two promoters observed here might be due to an intermediate RpoE-regulated repressor or compensatory effect where loss of rpoE increases the relative abundance of another sigma factor that can directly activate the ssaG and srfN promoters. Future work will be required to resolve these possibilities.

Bottom Line: The survival of Salmonella enterica within the intracellular host niche requires highly co-ordinated expression of virulence effectors predominantly regulated by the SsrAB two-component regulatory system.Mutants lacking the gene encoding the alternative sigma factor sigmaE (rpoE) are also highly attenuated for intracellular survival, pointing to a potential connection with the SsrAB regulatory system.In this study we demonstrate that RpoE is involved in fine-tuning the expression of a subset of SsrB-regulated genes found in the Salmonella pathogenicity island-2 (SPI-2) genetic locus that encodes a horizontally acquired type III secretion system, and unlinked genes integrated into this regulon that are required for virulence in host animals.

View Article: PubMed Central - HTML - PubMed

Affiliation: Michael G DeGroote Institute for Infectious Disease Research, Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON, Canada. osborns@mcmaster.ca

ABSTRACT

Background: The survival of Salmonella enterica within the intracellular host niche requires highly co-ordinated expression of virulence effectors predominantly regulated by the SsrAB two-component regulatory system. S. enterica serovar Typhimurium mutants lacking the ssrAB genes are avirulent in mice, highlighting the importance of this regulatory system in vivo. Mutants lacking the gene encoding the alternative sigma factor sigmaE (rpoE) are also highly attenuated for intracellular survival, pointing to a potential connection with the SsrAB regulatory system.

Results: In this study we demonstrate that RpoE is involved in fine-tuning the expression of a subset of SsrB-regulated genes found in the Salmonella pathogenicity island-2 (SPI-2) genetic locus that encodes a horizontally acquired type III secretion system, and unlinked genes integrated into this regulon that are required for virulence in host animals.

Conclusion: These data point to a potential connection between the virulence phenotype of strains lacking ssrB and rpoE, and highlight new transcriptional regulation that might be essential for appropriate temporal and spatial control of the virulence-associated type III secretion system during host infection.

Show MeSH
Related in: MedlinePlus