Analysis of the Salmonella regulatory network suggests involvement of SsrB and H-NS in σE-regulated SPI-2 gene expression

Li, Jie; Overall, Christopher C.; Nakayasu, Ernesto S.; Kidwai, Afshan S.; Jones, Marcus B.; Johnson, Rudd; Nguyen, Nhu T.; McDermott, Jason E.; Ansong, Charles; Heffron, Fred; Cambronne, Eric; Adkins, Joshua N.

doi:10.3389/fmicb.2015.00027

Title: Analysis of the Salmonella regulatory network suggests involvement of SsrB and H-NS in σ^E-regulated SPI-2 gene expression

Journal Article · Tue Feb 10 00:00:00 EST 2015 · Frontiers in Microbiology

DOI:https://doi.org/10.3389/fmicb.2015.00027· OSTI ID:1184934

Li, Jie; Overall, Christopher C.; Nakayasu, Ernesto S.; Kidwai, Afshan S.; Jones, Marcus B.; Johnson, Rudd; Nguyen, Nhu T.; McDermott, Jason E.; Ansong, Charles; Heffron, Fred; Cambronne, Eric; Adkins, Joshua N.

The extracytoplasmic functioning sigma factor σ^E is known to play an essential role for Salmonella enterica serovar Typhimurium to survive and proliferate in macrophages and mice. However, its regulatory network is not well characterized, especially during infection. Here we used microarray to identify genes regulated by σ^E in Salmonella grown in three conditions: a nutrient-rich condition and two others that mimic early and late intracellular infection. We found that in each condition σ^E regulated different sets of genes, and notably, several global regulators. When comparing nutrient-rich and infection-like conditions, large changes were observed in the expression of genes involved in Salmonella pathogenesis island (SPI)-1 type-three secretion system (TTSS), SPI-2 TTSS, protein synthesis, and stress responses. In total, the expression of 58% of Salmonella genes was affected by σ^E in at least one of the three conditions. An important finding is that σ^E up-regulates SPI-2 genes, which are essential for Salmonella intracellular survival, by up-regulating SPI-2 activator ssrB expression at the early stage of infection and down-regulating SPI-2 repressor hns expression at a later stage. Moreover, σ^E is capable of countering the silencing of H-NS, releasing the expression of SPI-2 genes. This connection between σ^E and SPI-2 genes, combined with the global regulatory effect of σ^E, may account for the lethality of rpoE-deficient Salmonella in murine infection.

View Accepted Manuscript (DOE)

Cite

Export

Save

Research Organization:: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Organization:: USDOE

Grant/Contract Number:: AC05-76RL01830

OSTI ID:: 1184934

Report Number(s):: PNNL-SA-101010

Journal Information:: Frontiers in Microbiology, Vol. 6; ISSN 1664-302X

Publisher:: Frontiers Research FoundationCopyright Statement

Country of Publication:: United States

Language:: English

Citation Metrics:

Cited by: 18 works

Citation information provided by
Web of Science

References (64)

Role of the Two-Component Regulator CpxAR in the Virulence of Salmonella enterica Serotype Typhimurium Humphreys, Sue; Rowley, Gary; Stevenson, Andrew Infection and Immunity, Vol. 72, Issue 8 https://doi.org/10.1128/iai.72.8.4654-4661.2004	journal	August 2004
Salmonella enterica Response Regulator SsrB Relieves H-NS Silencing by Displacing H-NS Bound in Polymerization Mode and Directly Activates Transcription Walthers, Don; Li, You; Liu, Yingjie Journal of Biological Chemistry, Vol. 286, Issue 3 https://doi.org/10.1074/jbc.m110.164962	journal	November 2010
Escherichia coli RNA Polymerase Recognition of a σ70-Dependent Promoter Requiring a −35 DNA Element and an Extended −10 TGn Motif Hook-Barnard, India; Johnson, Xanthia B.; Hinton, Deborah M. Journal of Bacteriology, Vol. 188, Issue 24 https://doi.org/10.1128/jb.00853-06	journal	September 2006
Conserved and Variable Functions of the σE Stress Response in Related Genomes Rhodius, Virgil A.; Suh, Won Chul; Nonaka, Gen PLoS Biology, Vol. 4, Issue 1 https://doi.org/10.1371/journal.pbio.0040002	journal	December 2005
limma: Linear Models for Microarray Data Smyth, G. K. Bioinformatics and Computational Biology Solutions Using R and Bioconductor https://doi.org/10.1007/0-387-29362-0_23	book	January 2005
Identification of the E regulon of Salmonella enterica serovar Typhimurium Skovierova, H. Microbiology, Vol. 152, Issue 5 https://doi.org/10.1099/mic.0.28744-0	journal	May 2006
Selective Silencing of Foreign DNA with Low GC Content by the H-NS Protein in Salmonella Navarre, W. W. Science, Vol. 313, Issue 5784 https://doi.org/10.1126/science.1128794	journal	July 2006
Hfq-dependent regulation of OmpA synthesis is mediated by an antisense RNA Udekwu, K. I. Genes & Development, Vol. 19, Issue 19 https://doi.org/10.1101/gad.354405	journal	October 2005
The phage shock protein PspA facilitates divalent metal transport and is required for virulence of Salmonella enterica sv. Typhimurium: Phage shock protein A in Salmonella Karlinsey, Joyce E.; Maguire, Michael E.; Becker, Lynne A. Molecular Microbiology, Vol. 78, Issue 3 https://doi.org/10.1111/j.1365-2958.2010.07357.x	journal	September 2010
The response regulator SsrB activates expression of diverse Salmonella pathogenicity island 2 promoters and counters silencing by the nucleoid-associated protein H-NS Walthers, Don; Carroll, Ronan K.; Navarre, William Wiley Molecular Microbiology, Vol. 65, Issue 2 https://doi.org/10.1111/j.1365-2958.2007.05800.x	journal	July 2007
Dual regulation by phospho-OmpR of ssrA/B gene expression in Salmonella pathogenicity island 2: SsrA and SsrB regulation are uncoupled Feng, Xiuhong; Oropeza, Ricardo; Kenney, Linda J. Molecular Microbiology, Vol. 48, Issue 4 https://doi.org/10.1046/j.1365-2958.2003.03502.x	journal	May 2003
Role of the alternative sigma factors σ E and σ S in survival of Salmonella enterica serovar Typhimurium during starvation, refrigeration and osmotic shock McMeechan, Alisdair; Roberts, Mark; Cogan, Tristan A. Microbiology, Vol. 153, Issue 1 https://doi.org/10.1099/mic.0.29235-0	journal	January 2007
The response to extracytoplasmic stress in Escherichia coli is controlled by partially overlapping pathways Connolly, L.; Penas, A. D. L.; Alba, B. M. Genes & Development, Vol. 11, Issue 15 https://doi.org/10.1101/gad.11.15.2012	journal	August 1997
Characterization of the Escherichia coli ς ^E Regulon Dartigalongue, Claire; Missiakas, Dominique; Raina, Satish Journal of Biological Chemistry, Vol. 276, Issue 24 https://doi.org/10.1074/jbc.M100464200	journal	March 2001
The PhoP/PhoQ system controls the intramacrophage type three secretion system of Salmonella enterica: SPI-2 regulation by the PhoP/PhoQ system Bijlsma, Jetta J. E.; Groisman, Eduardo A. Molecular Microbiology, Vol. 57, Issue 1 https://doi.org/10.1111/j.1365-2958.2005.04668.x	journal	May 2005
A connecter-like factor, CacA, links RssB/RpoS and the CpxR/CpxA two-component system in Salmonella Kato, Akinori; Hayashi, Hironori; Nomura, Wataru BMC Microbiology, Vol. 12, Issue 1 https://doi.org/10.1186/1471-2180-12-224	journal	January 2012
Salmonella typhimurium activates virulence gene transcription within acidified macrophage phagosomes. Alpuche Aranda, C. M.; Swanson, J. A.; Loomis, W. P. Proceedings of the National Academy of Sciences, Vol. 89, Issue 21 https://doi.org/10.1073/pnas.89.21.10079	journal	November 1992
The activity of sigma E, an Escherichia coli heat-inducible sigma-factor, is modulated by expression of outer membrane proteins. Mecsas, J.; Rouviere, P. E.; Erickson, J. W. Genes & Development, Vol. 7, Issue 12b https://doi.org/10.1101/gad.7.12b.2618	journal	December 1993
New insights into transcriptional regulation by H-NS Fang, Ferric C.; Rimsky, Sylvie Current Opinion in Microbiology, Vol. 11, Issue 2 https://doi.org/10.1016/j.mib.2008.02.011	journal	April 2008
The transcriptional landscape and small RNAs of Salmonella enterica serovar Typhimurium Kroger, C.; Dillon, S. C.; Cameron, A. D. S. Proceedings of the National Academy of Sciences, Vol. 109, Issue 20 https://doi.org/10.1073/pnas.1201061109	journal	April 2012
Regulation of Escherichia coli hchA, a stress-inducible gene encoding molecular chaperone Hsp31 Mujacic, Mirna; Baneyx, Francois Molecular Microbiology, Vol. 60, Issue 6 https://doi.org/10.1111/j.1365-2958.2006.05207.x	journal	June 2006
The molecular basis of selective promoter activation by the ? ^S subunit of RNA polymerase Typas, Athanasios; Becker, Gisela; Hengge, Regine Molecular Microbiology, Vol. 63, Issue 5 https://doi.org/10.1111/j.1365-2958.2007.05601.x	journal	March 2007
The Alternative Sigma Factor, ςE, Is Critically Important for the Virulence of Salmonella typhimurium Humphreys, Sue; Stevenson, Andrew; Bacon, Andrew Infection and Immunity, Vol. 67, Issue 4 https://doi.org/10.1128/iai.67.4.1560-1568.1999	journal	April 1999
TM4: A Free, Open-Source System for Microarray Data Management and Analysis Saeed, A. I.; Sharov, V.; White, J. BioTechniques, Vol. 34, Issue 2 https://doi.org/10.2144/03342mt01	journal	February 2003
The RpoS-Mediated General Stress Response in Escherichia coli Battesti, Aurelia; Majdalani, Nadim; Gottesman, Susan Annual Review of Microbiology, Vol. 65, Issue 1 https://doi.org/10.1146/annurev-micro-090110-102946	journal	October 2011
Acid stress activation of the σ ^E stress response in Salmonella enterica serovar Typhimurium Muller, Cécile; Bang, Iel-Soo; Velayudhan, Jyoti Molecular Microbiology, Vol. 71, Issue 5 https://doi.org/10.1111/j.1365-2958.2009.06597.x	journal	March 2009
Signal Transduction and Regulatory Mechanisms Involved in Control of the σS (RpoS) Subunit of RNA Polymerase Hengge-Aronis, Regine Microbiology and Molecular Biology Reviews, Vol. 66, Issue 3 https://doi.org/10.1128/MMBR.66.3.373-395.2002	journal	September 2002
Discovery of Novel Secreted Virulence Factors from Salmonella enterica Serovar Typhimurium by Proteomic Analysis of Culture Supernatants Niemann, George S.; Brown, Roslyn N.; Gustin, Jean K. Infection and Immunity, Vol. 79, Issue 1 https://doi.org/10.1128/iai.00771-10	journal	October 2010
The starvation-stress response of Salmonella enterica serovar Typhimurium requires σE-, but not CpxR-regulated extracytoplasmic functions Kenyon, William J.; Sayers, D. Geary; Humphreys, Sue Microbiology, Vol. 148, Issue 1 https://doi.org/10.1099/00221287-148-1-113	journal	January 2002
The RNA chaperone Hfq is essential for the virulence of Salmonella typhimurium Sittka, Alexandra; Pfeiffer, Verena; Tedin, Karsten Molecular Microbiology, Vol. 63, Issue 1 https://doi.org/10.1111/j.1365-2958.2006.05489.x	journal	January 2007
Global Systems-Level Analysis of Hfq and SmpB Deletion Mutants in Salmonella: Implications for Virulence and Global Protein Translation Ansong, Charles; Yoon, Hyunjin; Porwollik, Steffen PLoS ONE, Vol. 4, Issue 3 https://doi.org/10.1371/journal.pone.0004809	journal	March 2009
Overcoming H-NS-mediated Transcriptional Silencing of Horizontally Acquired Genes by the PhoP and SlyA Proteins in Salmonella enterica Perez, J. Christian; Latifi, Tammy; Groisman, Eduardo A. Journal of Biological Chemistry, Vol. 283, Issue 16 https://doi.org/10.1074/jbc.m709843200	journal	February 2008
Linear Models and Empirical Bayes Methods for Assessing Differential Expression in Microarray Experiments Smyth, Gordon K. Statistical Applications in Genetics and Molecular Biology, Vol. 3, Issue 1 https://doi.org/10.2202/1544-6115.1027	journal	January 2004
OmpR Regulates the Two-Component System SsrA-SsrB in Salmonella Pathogenicity Island 2 Lee, A. K.; Detweiler, C. S.; Falkow, S. Journal of Bacteriology, Vol. 182, Issue 3 https://doi.org/10.1128/JB.182.3.771-781.2000	journal	February 2000
Coordinated Regulation of Virulence during Systemic Infection of Salmonella enterica Serovar Typhimurium Yoon, Hyunjin; McDermott, Jason E.; Porwollik, Steffen PLoS Pathogens, Vol. 5, Issue 2 https://doi.org/10.1371/journal.ppat.1000306	journal	February 2009
Small RNAs endow a transcriptional activator with essential repressor functions for single-tier control of a global stress regulon Gogol, E. B.; Rhodius, V. A.; Papenfort, K. Proceedings of the National Academy of Sciences, Vol. 108, Issue 31 https://doi.org/10.1073/pnas.1109379108	journal	July 2011
Cell lysis directed by σ E in early stationary phase and effect of induction of the rpoE gene on global gene expression in Escherichia coli Kabir, Md. Shahinur; Yamashita, Daisuke; Koyama, Satoshi Microbiology, Vol. 151, Issue 8 https://doi.org/10.1099/mic.0.28004-0	journal	August 2005
Regulation of Alternative Sigma Factor Use Österberg, Sofia; Peso-Santos, Teresa del; Shingler, Victoria Annual Review of Microbiology, Vol. 65, Issue 1 https://doi.org/10.1146/annurev.micro.112408.134219	journal	October 2011
*New components of protein folding in extracytoplasmic compartments of Escherichia coli* SurA, FkpA and Skp/OmpH** Missiakas, Dominique; Betton, Jean-Michel; Raina, Satish Molecular Microbiology, Vol. 21, Issue 4 https://doi.org/10.1046/j.1365-2958.1996.561412.x	journal	August 1996
Alternative sigma factor interactions in Salmonella : σ ^E and σ ^H promote antioxidant defences by enhancing σ ^S levels: Alternative sigma factor interactions Bang, Iel-Soo; Frye, Jonathan G.; McClelland, Michael Molecular Microbiology, Vol. 56, Issue 3 https://doi.org/10.1111/j.1365-2958.2005.04580.x	journal	March 2005
Genome-wide analysis of Fis binding in Escherichia coli indicates a causative role for A-/AT-tracts Cho, B. -K.; Knight, E. M.; Barrett, C. L. Genome Research, Vol. 18, Issue 6 https://doi.org/10.1101/gr.070276.107	journal	May 2008
The roles of SsrA-SsrB and OmpR-EnvZ in the regulation of genes encoding the Salmonella typhimurium SPI-2 type III secretion system Garmendia, J. Microbiology, Vol. 149, Issue 9 https://doi.org/10.1099/mic.0.26397-0	journal	September 2003
Compensatory role of PspA, a member of the phage shock protein operon, in rpoE mutant Salmonella enterica serovar Typhimurium: Compensatory role of PspA in rpoE mutant Salmonella Becker, Lynne A.; Bang, Iel-Soo; Crouch, Marie-Laure Molecular Microbiology, Vol. 56, Issue 4 https://doi.org/10.1111/j.1365-2958.2005.04604.x	journal	March 2005
DNA looping-mediated repression by histone-like protein H-NS: specific requirement of E 70 as a cofactor for looping Shin, M. Genes & Development, Vol. 19, Issue 19 https://doi.org/10.1101/gad.1316305	journal	October 2005
SipA, SopA, SopB, SopD, and SopE2 Contribute to Salmonella enterica Serotype Typhimurium Invasion of Epithelial Cells Raffatellu, Manuela; Wilson, R. Paul; Chessa, Daniela Infection and Immunity, Vol. 73, Issue 1 https://doi.org/10.1128/iai.73.1.146-154.2005	journal	January 2005
RpoE fine tunes expression of a subset of SsrB-regulated virulence factors in Salmonella enterica serovar Typhimurium Osborne, Suzanne E.; Coombes, Brian K. BMC Microbiology, Vol. 9, Issue 1 https://doi.org/10.1186/1471-2180-9-45	journal	January 2009
ChIP-Seq and the Complexity of Bacterial Transcriptional Regulation Galagan, James; Lyubetskaya, Anna; Gomes, Antonio Systems Biology https://doi.org/10.1007/82_2012_257	book	January 2012
The alternative sigma factor σ ^E is required for resistance of Salmonella enterica serovar Typhimurium to anti-microbial peptides: rpoE is required for Salmonella resistance to anti-microbial peptides Crouch, Marie-Laure; Becker, Lynne A.; Bang, Iel-Soo Molecular Microbiology, Vol. 56, Issue 3 https://doi.org/10.1111/j.1365-2958.2005.04578.x	journal	March 2005
Regulon and promoter analysis of the E. coli heat-shock factor, σ ³² , reveals a multifaceted cellular response to heat stress Nonaka, Gen; Blankschien, Matthew; Herman, Christophe Genes & Development, Vol. 20, Issue 13 https://doi.org/10.1101/gad.1428206	journal	July 2006
SigmaE is an essential sigma factor in Escherichia coli. De Las Peñas, A.; Connolly, L.; Gross, C. A. Journal of bacteriology, Vol. 179, Issue 21 https://doi.org/10.1128/jb.179.21.6862-6864.1997	journal	January 1997
Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens Rowley, Gary; Spector, Michael; Kormanec, Jan Nature Reviews Microbiology, Vol. 4, Issue 5 https://doi.org/10.1038/nrmicro1394	journal	April 2006
SseL Is a Salmonella-Specific Translocated Effector Integrated into the SsrB-Controlled Salmonella Pathogenicity Island 2 Type III Secretion System Coombes, B. K.; Lowden, M. J.; Bishop, J. L. Infection and Immunity, Vol. 75, Issue 2 https://doi.org/10.1128/iai.00985-06	journal	December 2006
A comparison of normalization methods for high density oligonucleotide array data based on variance and bias Bolstad, B. M.; Irizarry, R. A.; Astrand, M. Bioinformatics, Vol. 19, Issue 2 https://doi.org/10.1093/bioinformatics/19.2.185	journal	January 2003
The σE and Cpx regulatory pathways: Overlapping but distinct envelope stress responses Raivio, Tracy L.; Silhavy, Thomas J. Current Opinion in Microbiology, Vol. 2, Issue 2 https://doi.org/10.1016/s1369-5274(99)80028-9	journal	April 1999
Bioconductor : open software development for computational biology and bioinformatics C., Gentleman, Robert; J., Carey, Vincent; M., Bates, Douglas BioMed Central https://doi.org/10.21256/zhaw-1907	text	January 2004
? ^E -dependent small RNAs of Salmonella respond to membrane stress by accelerating global omp mRNA decay Papenfort, Kai; Pfeiffer, Verena; Mika, Franziska Molecular Microbiology, Vol. 62, Issue 6 https://doi.org/10.1111/j.1365-2958.2006.05524.x	journal	December 2006
The chaperone-assisted membrane release and folding pathway is sensed by two signal transduction systems Jones, C. H. The EMBO Journal, Vol. 16, Issue 21 https://doi.org/10.1093/emboj/16.21.6394	journal	November 1997
Deep Sequencing Analysis of Small Noncoding RNA and mRNA Targets of the Global Post-Transcriptional Regulator, Hfq Sittka, Alexandra; Lucchini, Sacha; Papenfort, Kai PLoS Genetics, Vol. 4, Issue 8 https://doi.org/10.1371/journal.pgen.1000163	journal	August 2008
Hfq and its constellation of RNA Vogel, Jörg; Luisi, Ben F. Nature Reviews Microbiology, Vol. 9, Issue 8 https://doi.org/10.1038/nrmicro2615	journal	August 2011
Identification of amino acid residues of Salmonella SlyA that are critical for transcriptional regulation Okada, N.; Oi, Y.; Takeda-Shitaka, M. Microbiology, Vol. 153, Issue 2 https://doi.org/10.1099/mic.0.29259-0	journal	February 2007
New members of the Escherichia coli Ï ^E regulon identified by a two-plasmid system Rezuchova, Bronislava; Miticka, Henrieta; Homerova, Dagmar FEMS Microbiology Letters, Vol. 225, Issue 1 https://doi.org/10.1016/S0378-1097(03)00480-4	journal	August 2003
Transcriptional analysis of the rpoE gene encoding extracytoplasmic stress response sigma factor Ï ^E in Salmonella enterica serovar Typhimurium Miticka, Henrieta; Rowley, Gary; Rezuchova, Bronislava FEMS Microbiology Letters, Vol. 226, Issue 2 https://doi.org/10.1016/s0378-1097(03)00600-1	journal	September 2003
Bioconductor: open software development for computational biology and bioinformatics Gentleman, Robert C.; Carey, Vincent J.; Bates, Douglas M. Genome Biology, Vol. 5, Issue 10, p. R80 https://doi.org/10.1186/gb-2004-5-10-r80	journal	September 2004
Purification and Properties of C55-Isoprenoid Alcohol Phosphokinase from Staphylococcus aureus Sandermann, Heinrich; Strominger, Jack L. Journal of Biological Chemistry, Vol. 247, Issue 16 https://doi.org/10.1016/S0021-9258(19)44948-X	journal	August 1972

Cited By (4)

ChIP-Seq Analysis of the σE Regulon of Salmonella enterica Serovar Typhimurium Reveals New Genes Implicated in Heat Shock and Oxidative Stress Response Li, Jie; Overall, Christopher C.; Johnson, Rudd C. PLOS ONE, Vol. 10, Issue 9 https://doi.org/10.1371/journal.pone.0138466	journal	September 2015
Preliminary Transcriptome Analysis of Mature Biofilm and Planktonic Cells of Salmonella Enteritidis Exposure to Acid Stress Jia, Kun; Wang, Guangyu; Liang, Lijiao Frontiers in Microbiology, Vol. 8 https://doi.org/10.3389/fmicb.2017.01861	journal	September 2017
Combination of berberine and ciprofloxacin reduces multi-resistant Salmonella strain biofilm formation by depressing mRNA expressions of luxS, rpoE, and ompR Shi, Chenxi; Li, Minmin; Muhammad, Ishfaq Journal of Veterinary Science, Vol. 19, Issue 6 https://doi.org/10.4142/jvs.2018.19.6.808	journal	January 2018
HldE Is Important for Virulence Phenotypes in Enterotoxigenic Escherichia coli Maigaard Hermansen, Grith M.; Boysen, Anders; Krogh, Thøger J. Frontiers in Cellular and Infection Microbiology, Vol. 8 https://doi.org/10.3389/fcimb.2018.00253	journal	August 2018

Similar Records

Global Analysis of Salmonella Alternative Sigma Factor E on Protein Translation

Journal Article · Mon Feb 16 00:00:00 EST 2015 · Journal of Proteome Research · OSTI ID:1184934

Li, Jie; Nakayasu, Ernesto S.; Overall, Christopher C.; +7 more

Discovery of Novel Secreted Virulence Factors from Salmonella enterica Serovar Typhimurium by Proteomic Analysis of Culture Supernatants

Journal Article · Sat Jan 01 00:00:00 EST 2011 · Infection and Immunity, 79(1):33-43 · OSTI ID:1184934

Niemann, George; Brown, Roslyn N.; Gustin, Jean K.; +9 more

Discovery of Salmonella Virulence Factors Translocated via Outer Membrane Vesicles to Murine Macrophages.

Journal Article · Wed Jun 01 00:00:00 EDT 2011 · Infection and Immunity, 79(6):2182-92 · OSTI ID:1184934

Yoon, Hyunjin; Ansong, Charles; Adkins, Joshua N; +1 more

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
Salmonella Typhimurium
bacterial pathogenesis
systems biology
transcriptomics
regulatory network
bioinformatics
computational biology

Title: Analysis of the Salmonella regulatory network suggests involvement of SsrB and H-NS in σE-regulated SPI-2 gene expression

Citation Formats

References (64)

Cited By (4)

Similar Records

Related Subjects

Title: Analysis of the Salmonella regulatory network suggests involvement of SsrB and H-NS in σ^E-regulated SPI-2 gene expression