The Small RNA MicC Downregulates hilD Translation To Control the Salmonella Pathogenicity Island 1 Type III Secretion System in Salmonella enterica Serovar Typhimurium.

Salmonella enterica serovar Typhimurium invades the intestinal epithelium and induces inflammatory diarrhea using the Salmonella pathogenicity island 1 (SPI1) type III secretion system (T3SS). Expression of the SPI1 T3SS is controlled by three AraC-like regulators, HilD, HilC, and RtsA, which form a feed-forward regulatory loop that leads to activation of hilA, encoding the main transcriptional regulator of the T3SS structural genes. This complex system is affected by numerous regulatory proteins and environmental signals, many of which act at the level of hilD mRNA translation or HilD protein function. Here, we show that the sRNA MicC blocks translation of the hilD mRNA by base pairing near the ribosome binding site. MicC does not induce degradation of the hilD message. Our data indicate that micC is transcriptionally activated by SlyA, and SlyA feeds into the SPI1 regulatory network solely through MicC. Transcription of micC is negatively regulated by the OmpR/EnvZ two-component system, but this regulation is dependent on SlyA. OmpR/EnvZ control SPI1 expression partially through MicC but also affect expression through other pathways, including an EnvZ-dependent, OmpR-independent mechanism. MicC-mediated regulation plays a role during infection, as evidenced by an SPI1 T3SS-dependent increase in Salmonella fitness in the intestine in the micC deletion mutant. These results further elucidate the complex regulatory network controlling SPI1 expression and add to the list of sRNAs that control this primary virulence factor. IMPORTANCE The Salmonella pathogenicity island 1 (SPI1) type III secretion system (T3SS) is the primary virulence factor required for causing intestinal disease and initiating systemic infection. The system is regulated in response to a large variety of environmental and physiological factors such that the T3SS is expressed at only the appropriate time and place in the host during infection. Here, we show how the sRNA MicC affects expression of the system. This work adds to our detailed mechanistic studies aimed at a complete understanding of the regulatory circuit.

Two Additional Connections between the Transcriptional Programs Controlling Invasion and Intracellular Replication of Salmonella: HilD-SprB Positively Regulates phoP and slyA.

Salmonella virulence relies on the ability of this bacterium to invade the intestinal epithelium and to replicate inside macrophages, which are functions mainly encoded in Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2), respectively. Complex regulatory programs control the expression of SPI-1 and SPI-2 and functionally related genes, involving the integration of ancestral regulators and regulators that Salmonella has acquired during its evolution. Interestingly, some previous studies have revealed cross talk between the regulatory programs for SPI-1 and SPI-2. Here, we report two additional connections between the regulatory programs controlling the expression of genes for invasion and intracellular replication. Our results show that the acquired regulators HilD and SprB, both encoded in SPI-1, induce, in a cascade fashion, the expression of PhoP and SlyA, two ancestral regulators that activate the expression of SPI-2 and other genes required for intracellular replication. We provide evidence supporting that the regulation of phoP and slyA by HilD-SprB was adapted during the divergence of Salmonella from its closer species, Escherichia coli, with the acquisition of SPI-1 and thus the gain of HilD and SprB, as well as through cis-regulatory evolution of phoP and slyA. Therefore, our study further expands the knowledge about the intricate regulatory network controlling the expression of virulence genes in Salmonella. IMPORTANCE Bacteria have developed diverse regulatory mechanisms to control genetic expression, in the case of pathogenic bacteria, to induce the expression of virulence genes in particular niches during host infection. In Salmonella, an intricate regulatory network has been determined, which controls the spatiotemporal expression of the SPI-1 and SPI-2 gene clusters that mediate the invasion to and the replication inside host cells, respectively. In this study, we report two additional pathways of cross talk between the transcriptional programs for SPI-1 and SPI-2. Additionally, our results support that these additional regulatory pathways were adapted during the divergence of Salmonella from its closer species, Escherichia coli. This study further expands the knowledge about the mechanisms determining the Salmonella virulence.

The Impact of SlyA on Cell Metabolism of Salmonella typhimurium: A Joint Study of Transcriptomics and Metabolomics.

SlyA is an important transcriptional regulator in Salmonella typhimurium (S. typhimurium). Numerous reports have indicated the impact of SlyA on the virulence of S. typhimurium. Less information regarding the role of SlyA in the cell metabolism of S. typhimurium is available. To close this gap, we compared the growth kinetics of an S. typhimurium wild-type strain to a slyA deletion mutant strain. The data suggested that the cell growth of S. typhimurium was impaired when slyA abolished, indicating that SlyA might affect the cell metabolism of S. typhimurium. To determine the role of SlyA in cell metabolism, we analyzed the metabolite profiles of S. typhimurium in the presence or absence of slyA using gas chromatography coupled with tandem mass spectrometry (GC-MS-MS). With the aim of appropriately interpreting the results obtained from metabolomics, a transcriptomic analysis on both the wild-type S. typhimurium and the slyA deletion mutant was performed. The metabolome data indicated that several glycolysis and lipid metabolism-associated pathways, including the turnover of glycerolipid, pyruvate, butanoate, and glycerophospholipid, were affected in the absence of slyA. In addition, the mRNA levels of several genes associated with glycolysis and lipid turnover were downregulated when slyA was deleted, including pagP, fadL, mgtB, iacp, and yciA. Collectively, these evidence suggested that SlyA affects the glycolysis and lipid turnover of S. typhimurium at a transcriptional level. The raw data of metabolomics is available in the MetaboLights database with an access number of MTBLS1858. The raw data of transcriptome is available in the Sequence Read Archive (SRA) database with an access number of PRJNA656165.

Regulatory Effect of SlyA on rcsB Expression in Salmonella enterica Serovar Typhimurium.

The Salmonella enterica serovar Typhimurium RcsCDB system regulates the synthesis of colanic acid and the flagellum as well as the expression of virulence genes. We previously demonstrated that the rcsC11 mutant, which constitutively activates the RcsB regulator, attenuates Salmonella virulence in an animal model. This attenuated phenotype was also produced by deletion of the slyA gene. In this work, we investigated if this antagonistic behavior is produced by modulating the expression of both regulator-encoding genes. We demonstrated that SlyA overproduction negatively regulates rcsB transcription. A bioinformatics analysis enabled us to identify putative SlyA binding sites on both promoters, P rcsDB and P rcsB , which control rcsB transcriptional levels. We also determined that SlyA is able to recognize and bind to these predicted sites to modulate the activity of both rcsB promoters. According to these results, SlyA represses rcsB transcription by direct binding to specific sites located on the rcsB promoters, thus accounting for the attenuated/virulence antagonistic behaviors. Moreover, we showed that the opposite effect between both regulators also physiologically affects the Salmonella motility phenotype. In this sense, we observed that under SlyA overproduction, P rcsB is repressed, and consequently, bacterial motility is increased. On the basis of these results, we suggest that during infection, the different RcsB levels produced act as a switch between the virulent and attenuated forms of Salmonella Thereby, we propose that higher concentrations of RcsB tilt the balance toward the attenuated form, while absence or low concentrations resulting from SlyA overproduction tilt the balance toward the virulent form.IMPORTANCE The antagonistic behavior of RcsB and SlyA on virulence gene expression led us to hypothesize that there is interplay between both regulators in a regulatory network and these could be considered coordinators of this process. Here, we report that the SlyA virulence factor influences motility behavior by controlling rcsB transcription from the P rcsB promoter. We also demonstrate that SlyA negatively affects the expression of the rcsB gene by direct binding to P rcsDB and P rcsB promoters. We suggest that different levels of RcsB act as a switch between the virulent and attenuated forms of Salmonella, where high concentrations of the regulator tend to tilt the balance toward the attenuated form and low concentrations or its absence tilt it toward the virulent form.

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.

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.

Generation and evaluation of Salmonella entericaserovar Choleraesuis mutant strains as a potential live-attenuated vaccine.

BACKGROUND: Salmonella entericaserovar Choleraesuis (S.C) is a swine enteric pathogen causing paratyphoid fever, enterocolitis, and septicemia in piglets. S. C is mainly transmitted through the fecal-oral route. Vaccination is an effective strategy for preventing and controlling Salmonella infection. RESULTS: Herein, we used CRISPR-Cas9 technology to knockout the virulence regulatory genes, rpoS, and slyA of S. C and constructed the ∆rpoS, ∆slyA, and ∆rpoS ∆slyA strains. The phenotypic characteristics of the mutant strains remained unchanged compared with the parental wild-type strain. In vivo study, unlike the wild-type strain, the ∆slyA and ∆rpoS ∆slyA strains alleviated splenomegaly, colon atrophy, and lower bacterial loads in the spleen, liver, ileum, and colon. These mutant strains survived in Peyer's patches (PPs) and mesenteric lymph nodes (MLN) for up to 15 days post-infection. Furthermore, the immunization of the ∆rpoS ∆slyA strain induced robust humoral and cellular immune responses. CONCLUSIONS: Consequently, vaccination with ∆rpoS ∆slyA conferred a high percentage of protection against lethal invasive Salmonella, specifically S. C, in mice. This study provided novel insights into the development of live-attenuated vaccines against the infection of S. C.

Regulation of Escherichia coli Group 2 Capsule Gene Expression: A Mini Review and Update.

The expression of a group 2 capsule (K antigen), such as the K1 or K5 antigen, is a key virulence factor of Escherichia coli responsible for extra-intestinal infections. Capsule expression confers resistance to innate host defenses and plays a critical role in invasive disease. Capsule expression is temperature-dependent being expressed at 37°C but not at 20°C when outside the host. Group 2 capsule gene expression involves two convergent promoters PR1 and PR3, the regulation of which is critical to capsule expression. Temperature-dependent expression is controlled at transcriptional level directly by the binding of H-NS to PR1 and PR3 and indirectly through BipA with additional input from IHF and SlyA. More recently, other regulatory proteins, FNR, Fur, IHF, MprA, and LrhA, have been implicated in regulating capsule gene expression in response to other environmental stimuli and there is merging data for the growth phase-dependent regulation of the PR1 and PR3 promoters. The aim of the present Mini Review is to provide a unified update on the latest data on how the expression of group 2 capsules is regulated in response to a number of stimuli and the growth phase something that has not to date been addressed.

Proteomics analysis reveals the importance of transcriptional regulator slyA in regulation of several physiological functions in Aeromonas hydrophila.

SlyA is a well-known transcription factor that plays important roles in the regulation of diverse physiological functions including virulence and stress response in various bacterial species. The biological effects of slyA have species-specific characteristics. In this study, a phenotype assay showed that slyA gene deletion in Aeromonas hydrophila (ahslyA) decreased biofilm formation capability but did not affect bacterial hemolytic activity or acid stress response. The differentially expressed proteins between ΔahslyA and wild-type strains were compared by label-free quantitative proteomics to further understand the effects of AhSlyA on biological functions. Bioinformatics assays showed that ΔahslyA may be involved in the regulation of several intracellular metabolic pathways such as galactose metabolism, arginine biosynthesis, and sulfur metabolism. A further phenotypic assay confirmed that AhSlyA plays an important role in the regulation of sulfur and phosphate metabolism. Moreover, ahslyA also directly or indirectly regulated at least eight outer membrane proteins involved in the maintenance of cell permeability. Overall, the results provide insights into the functions of ahslyA and demonstrate its importance in A. hydrophila. BIOLOGICAL SIGNIFICANCE: In this study, we compared the DEPs between the transcriptional regulator slyA-deleted and the wild-type A. hydrophila strains using a label-free quantitative proteomics method. The bioinformatics analysis showed that slyA may be involved in the regulation of several metabolic pathways. Subsequent phenotype and growth assays confirmed that ΔahslyA affected sulfur and phosphate metabolism, and OM permeability. Finally, a ChIP-PCR assay further confirmed that AhSlyA directly binds to the promoters of several candidate genes, including sulfur metabolism-related genes. These results indicated that slyA plays an important regulatory role in pleiotropic physiological functions of A. hydrophila, and these functions may be different from those identified in previous reports from other bacterial species.

SlyA Transcriptional Regulator Is Not Directly Affected by ppGpp Levels.

The SlyA transcriptional regulator controls the expression of genes involved in virulence and production of surface components in S. Typhimurium and E. coli. Its mode of action is mainly explained by its antagonism with the H-NS repressor for the same DNA binding regions. Interestingly, it has been reported that the alarmone ppGpp promotes SlyA dimerization and DNA binding at the promoter of pagC, enhancing the expression of this gene in Salmonella. A recurring problem in the field of stringent response has been to find a way of following ppGpp levels in vivo in real time. We thought that SlyA, as a ppGpp responsive ligand, was a perfect candidate for the development of a specific ppGpp biosensor. Therefore, we decided to characterize in depth this SlyA control by ppGpp. However, using various genes whose expression is activated by SlyA, as reporters, we showed that ppGpp does not affect SlyA regulation in vivo. In addition, modulating ppGpp levels did not affect SlyA dimerization in vivo, and did not impact its binding to DNA in vitro. We finally showed that ppGpp is required for the expression of hlyE in E. coli, a gene also activated by SlyA, and propose that both regulators are independently required for hlyE expression. The initial report of ppGpp action on SlyA might be explained by a similar action of SlyA and ppGpp on pagC expression, and the complexity of promoters controlled by several global regulators, such as the promoters of pagC in Salmonella or hlyE in E. coli.

The Evolution of SlyA/RovA Transcription Factors from Repressors to Countersilencers in Enterobacteriaceae.

Gene duplication and subsequent evolutionary divergence have allowed conserved proteins to develop unique roles. The MarR family of transcription factors (TFs) has undergone extensive duplication and diversification in bacteria, where they act as environmentally responsive repressors of genes encoding efflux pumps that confer resistance to xenobiotics, including many antimicrobial agents. We have performed structural, functional, and genetic analyses of representative members of the SlyA/RovA lineage of MarR TFs, which retain some ancestral functions, including repression of their own expression and that of divergently transcribed multidrug efflux pumps, as well as allosteric inhibition by aromatic carboxylate compounds. However, SlyA and RovA have acquired the ability to countersilence horizontally acquired genes, which has greatly facilitated the evolution of Enterobacteriaceae by horizontal gene transfer. SlyA/RovA TFs in different species have independently evolved novel regulatory circuits to provide the enhanced levels of expression required for their new role. Moreover, in contrast to MarR, SlyA is not responsive to copper. These observations demonstrate the ability of TFs to acquire new functions as a result of evolutionary divergence of both cis-regulatory sequences and in trans interactions with modulatory ligands.IMPORTANCE Bacteria primarily evolve via horizontal gene transfer, acquiring new traits such as virulence and antibiotic resistance in single transfer events. However, newly acquired genes must be integrated into existing regulatory networks to allow appropriate expression in new hosts. This is accommodated in part by the opposing mechanisms of xenogeneic silencing and countersilencing. An understanding of these mechanisms is necessary to understand the relationship between gene regulation and bacterial evolution. Here we examine the functional evolution of an important lineage of countersilencers belonging to the ancient MarR family of classical transcriptional repressors. We show that although members of the SlyA lineage retain some ancestral features associated with the MarR family, their cis-regulatory sequences have evolved significantly to support their new function. Understanding the mechanistic requirements for countersilencing is critical to understanding the pathoadaptation of emerging pathogens and also has practical applications in synthetic biology.

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.

Identification of Two Regulators of Virulence That Are Conserved in Klebsiella pneumoniae Classical and Hypervirulent Strains.

Klebsiella pneumoniae is widely recognized as a pathogen with a propensity for acquiring antibiotic resistance. It is capable of causing a range of hospital-acquired infections (urinary tract infections [UTI], pneumonia, sepsis) and community-acquired invasive infections. The genetic heterogeneity of K. pneumoniae isolates complicates our ability to understand the virulence of K. pneumoniae Characterization of virulence factors conserved between strains as well as strain-specific factors will improve our understanding of this important pathogen. The MarR family of regulatory proteins is widely distributed in bacteria and regulates cellular processes such as antibiotic resistance and the expression of virulence factors. Klebsiella encodes numerous MarR-like proteins, and they likely contribute to the ability of K. pneumoniae to respond to and survive under a wide variety of environmental conditions, including those present in the human body. We tested loss-of-function mutations in all the marR homologues in a murine pneumonia model and found that two (kvrA and kvrB) significantly impacted the virulence of K1 and K2 capsule type hypervirulent (hv) strains and that kvrA affected the virulence of a sequence type 258 (ST258) classical strain. In the hv strains, kvrA and kvrB mutants displayed phenotypes associated with reduced capsule production, mucoviscosity, and transcription from galF and manC promoters that drive expression of capsule synthesis genes. In contrast, kvrA and kvrB mutants in the ST258 strain had no effect on capsule gene expression or capsule-related phenotypes. Thus, KvrA and KvrB affect virulence in classical and hv strains but the effect on virulence may not be exclusively due to effects on capsule production.IMPORTANCE In addition to having a reputation as the causative agent for hospital-acquired infections as well as community-acquired invasive infections, Klebsiella pneumoniae has gained widespread attention as a pathogen with a propensity for acquiring antibiotic resistance. Due to the rapid emergence of carbapenem resistance among K. pneumoniae strains, a better understanding of virulence mechanisms and identification of new potential drug targets are needed. This study identified two novel regulators (KvrA and KvrB) of virulence in K. pneumoniae and demonstrated that their effect on virulence in invasive strains is likely due in part to effects on capsule production (a major virulence determinant) and hypermucoviscosity. KvrA also impacts the virulence of classical strains but does not appear to affect capsule gene expression in this strain. KvrA and KvrB are conserved among K. pneumoniae strains and thus could regulate capsule expression and virulence in diverse strains regardless of capsule type.

The transcription factor SlyA from Salmonella Typhimurium regulates genes in response to hydrogen peroxide and sodium hypochlorite.

Salmonella Typhimurium is an intracellular pathogen that is capable of generating systemic fever in a murine model. Over the course of the infection, Salmonella faces different kinds of stressors, including harmful reactive oxygen species (ROS). Various defence mechanisms enable Salmonella to successfully complete the infective process in the presence of such stressors. The transcriptional factor SlyA is involved in the oxidative stress response and invasion of murine macrophages. We evaluated the role of SlyA in response to H2O2 and NaOCl and found an increase of slyA expression upon exposure to these toxics. However, the SlyA target genes and the molecular mechanisms by which they influence the infective process are unknown. We hypothesised that SlyA regulates the expression of genes required for ROS resistance, metabolism, or virulence under oxidative stress conditions. Transcriptional profiling in wild type and ΔslyA strains confirmed that SlyA regulates the expression of several genes involved in virulence [sopD (STM14_3550), sopE2 (STM14_2244), hilA (STM14_3475)] and central metabolism [kgtP (STM14_3252), fruK (STM14_2722), glpA (STM14_2819)] in response to H2O2 and NaOCl. These findings were corroborated by functional assay and transcriptional fusion assays using GFP. DNA-protein interaction assays showed that SlyA regulates these genes through direct interaction with their promoter regions.

Shigellaflexneri Regulator SlyA Controls Bacterial Acid Resistance by Directly Activating the Glutamate Decarboxylation System.

Shigella flexneri is an important foodborne bacterial pathogen with infectious dose as low as 10-100 cells. SlyA, a transcriptional regulator of the MarR family, has been shown to regulate virulence in a closely related bacterial pathogen, Salmonella Typhimurium. However, the regulatory role of SlyA in S. flexneri is less understood. Here we applied unbiased proteomic profiling to define the SlyA regulon in S. flexneri. We found that the genetic ablation of slyA led to the alteration of 18 bacterial proteins among over 1400 identifications. Intriguingly, most down-regulated proteins (whose expression is SlyA-dependent) were associated with bacterial acid resistance such as the glutamate decarboxylation system. We further demonstrated that SlyA directly regulates the expression of GadA, a glutamate decarboxylase, by binding to the promotor region of its coding gene. Importantly, overexpression of GadA was able to rescue the survival defect of the ΔslyA mutant under acid stress. Therefore, our study highlights a major role of SlyA in controlling S. flexneri acid resistance and provides a molecular mechanism underlying such regulation as well.

SlyA regulates phytotoxin production and virulence in Dickeya zeae EC1.

Dickeya zeae is a causal agent of rice root rot disease. The pathogen is known to produce a range of virulence factors, including phytotoxic zeamines and extracellular enzymes, but the mechanisms of virulence regulation remain vague. In this study, we identified a SlyA/MarR family transcription factor SlyA in D. zeae strain EC1. Disruption of slyA significantly decreased zeamine production, enhanced swimming and swarming motility, reduced biofilm formation and significantly decreased pathogenicity on rice. Quantitative polymerase chain reaction (qPCR) analysis confirmed the role of SlyA in transcriptional modulation of a range of genes associated with bacterial virulence. In trans expression of slyA in expI mutants recovered the phenotypes of motility and biofilm formation, suggesting that SlyA is downstream of the acylhomoserine lactone-mediated quorum sensing pathway. Taken together, the findings from this study unveil a key transcriptional regulatory factor involved in the modulation of virulence factor production and overall pathogenicity of D. zeae EC1.

SlyA regulates motA and motB, virulence and stress-related genes under conditions induced by the PhoP-PhoQ system in Dickeya dadantii 3937.

We previously showed that SlyA of Dickeya dadantii 3937 plays an important role in virulence toward plants, and that the ΔslyA mutant is hypermotile, whereas flagellum synthesis and flagellin production are indistinguishable from the wild type. Here we show that motility factors, including the distance of continuous directed movement, time for that movement and speed, were significantly higher in the ΔslyA mutant than in the wild type. Remarkably, transcription levels of motA and motB, that are involved in flagellar rotation, were elevated in the ΔslyA mutant, suggesting that the mutant's hypermotility was due to an increase in flagellar rotation. In low (10 µM) magnesium medium that activates the PhoP-PhoQ system, growth and virulence of the ΔslyA mutant were much lower than for the wild type; expression of motA, motB, mgtA, pelA, pelB, pelC, pelD, pelE, pelI, indA, tolC, sodC, acsA and hrpN were also reduced in the mutant. Interestingly, motA, motB, pelD, pelE, pelI, sodC and indA were also reduced in phoP and phoQ mutants. Because the SlyA protein directly binds to the promoter region of PhoP, SlyA regulates virulence by controlling multiple pathogenicity-related genes directly and/or at least by controlling PhoP in D. dadantii 3937 when magnesium is low.

Transcriptional engineering of Escherichia coli K4 for fructosylated chondroitin production.

The capsule polysaccharide (CPS) of Escherichia coli K4 (K4CPS) is identical to fructosylated chondroitin, which can be modified to chondroitin sulfate, a commercially valuable biopolymer commonly used in pharmaceutical applications. In this study, we homologously overexpressed the transcriptional regulator SlyA to enhance the expression of K4 capsule gene cluster and production of CPS. The iTRAQ quantificaton of proteomics revealed 77 up-regulated proteins and 143 down-regulated proteins in E. coli THslyA. Most enzymes of glycolysis and citrate cycle pathway were weakened, while proteins associated with K4CPS synthesis were up-regulated, showing a shift of carbon flux from cell growth to K4CPS production. Further, the production of K4CPS by the recombinant strain was 1 and 2.6 g/L in a shake flask and 7-L batch bioreactor, which was 1.85- and 1.53-fold higher than that of the wild-type strain, respectively. Thus, this study provides a viable strategy for improving the production of K4CPS through a transcriptional-level manipulation.