Thermoregulation in Yersinia enterocolitica is coincident with changes in DNA supercoiling

Pathogenicity and virulence of Yersinia

The genus Yersinia includes human, animal, insect, and plant pathogens as well as many symbionts and harmless bacteria. Within this genus are Yersinia enterocolitica and the Yersinia pseudotuberculosis complex, with four human pathogenic species that are highly related at the genomic level including the causative agent of plague, Yersinia pestis. Extensive laboratory, field work, and clinical research have been conducted to understand the underlying pathogenesis and zoonotic transmission of these pathogens. There are presently more than 500 whole genome sequences from which an evolutionary footprint can be developed that details shared and unique virulence properties. Whereas the virulence of Y. pestis now seems in apparent homoeostasis within its flea transmission cycle, substantial evolutionary changes that affect transmission and disease severity continue to ndergo apparent selective pressure within the other Yersiniae that cause intestinal diseases. In this review, we will summarize the present understanding of the virulence and pathogenesis of Yersinia, highlighting shared mechanisms of virulence and the differences that determine the infection niche and disease severity.

Comparison of Yersinia enterocolitica DNA Methylation at Ambient and Host Temperatures

Pathogenic bacteria recognize environmental cues to vary gene expression for host adaptation. Moving from ambient to host temperature, Yersinia enterocolitica responds by immediately repressing flagella synthesis and inducing the virulence plasmid (pYV)-encoded type III secretion system. In contrast, shifting from host to ambient temperature requires 2.5 generations to restore motility, suggesting a link to the cell cycle. We hypothesized that differential DNA methylation contributes to temperature-regulated gene expression. We tested this hypothesis by comparing single-molecule real-time (SMRT) sequencing of Y. enterocolitica DNA from cells growing exponentially at 22 °C and 37 °C. The inter-pulse duration ratio rather than the traditional QV scoring was the kinetic metric to compare DNA from cells grown at each temperature. All 565 YenI restriction sites were fully methylated at both temperatures. Among the 27,118 DNA adenine methylase (Dam) sites, 42 had differential methylation patterns, while 17 remained unmethylated regardless of the temperature. A subset of the differentially methylated Dam sites localized to promoter regions of predicted regulatory genes including LysR-type and PadR-like transcriptional regulators and a cyclic-di-GMP phosphodiesterase. The unmethylated Dam sites localized with a bias to the replication terminus, suggesting they were protected from Dam methylase. No cytosine methylation was detected at Dcm sites.

RNA thermometer-coordinated assembly of the Yersinia injectisome

The type III secretion system (T3SS) is indispensable for successful host cell infection by many Gram-negative pathogens. The molecular syringe delivers effector proteins that suppress the host immune response. Synthesis of T3SS components in Yersinia pseudotuberculosis relies on host body temperature, which induces the RNA thermometer (RNAT)-controlled translation of lcrF coding for a virulence master regulator that activates transcription of the T3SS regulon. The assembly of the secretion machinery follows a strict coordinated succession referred to as outside-in assembly, in which the membrane ring complex and the export apparatus represent the nucleation points. Two components essential for the initial assembly are YscJ and YscT. While YscJ connects the membrane ring complex with the export apparatus in the inner membrane, YscT is required for a functional export apparatus. Previous transcriptome-wide RNA structuromics data suggested the presence of unique intercistronic RNATs upstream of yscJ and yscT. Here, we show by reporter gene fusions that both upstream regions confer translational control. Moreover, we demonstrate the temperature-induced opening of the Shine-Dalgarno region, which facilitates ribosome binding, by in vitro structure probing and toeprinting methods. Rationally designed thermostable RNAT variants of the yscJ and yscT thermometers confirmed their physiological relevance with respect to T3SS assembly and host infection. Since we have shown in a recent study that YopN, the gatekeeper of type III secretion, also is under RNAT control, it appears that the synthesis, assembly and functionality of the Yersinia T3S machinery is coordinated by RNA-based temperature sensors at multiple levels.

The gatekeeper of Yersinia type III secretion is under RNA thermometer control

Many bacterial pathogens use a type III secretion system (T3SS) as molecular syringe to inject effector proteins into the host cell. In the foodborne pathogen Yersinia pseudotuberculosis, delivery of the secreted effector protein cocktail through the T3SS depends on YopN, a molecular gatekeeper that controls access to the secretion channel from the bacterial cytoplasm. Here, we show that several checkpoints adjust yopN expression to virulence conditions. A dominant cue is the host body temperature. A temperature of 37°C is known to induce the RNA thermometer (RNAT)-dependent synthesis of LcrF, a transcription factor that activates expression of the entire T3SS regulon. Here, we uncovered a second layer of temperature control. We show that another RNAT silences translation of the yopN mRNA at low environmental temperatures. The long and short 5’-untranslated region of both cellular yopN isoforms fold into a similar secondary structure that blocks ribosome binding. The hairpin structure with an internal loop melts at 37°C and thereby permits formation of the translation initiation complex as shown by mutational analysis, in vitro structure probing and toeprinting methods. Importantly, we demonstrate the physiological relevance of the RNAT in the faithful control of type III secretion by using a point-mutated thermostable RNAT variant with a trapped SD sequence. Abrogated YopN production in this strain led to unrestricted effector protein secretion into the medium, bacterial growth arrest and delayed translocation into eukaryotic host cells. Cumulatively, our results show that substrate delivery by the Yersinia T3SS is under hierarchical surveillance of two RNATs.

Temperature serves as reliable external cue for pathogenic bacteria to recognize the entry into or exit from a warm-blooded host. At the molecular level, a temperature of 37°C induces various virulence-related processes that manipulate host cell physiology. Here, we demonstrate the temperature-dependent synthesis of the secretion regulator YopN in the foodborne pathogen Yersinia pseudotuberculosis, a close relative of Yersinia pestis. YopN blocks secretion of effector proteins through the type III secretion system unless host cell contact is established. Temperature-specific regulation relies on an RNA structure in the 5’-untranslated region of the yopN mRNA, referred to as RNA thermometer, which allows ribosome binding and thus translation initiation only at an infection-relevant temperature of 37°C. A mutated variant of the thermosensor resulting in a closed conformation prevented synthesis of the molecular gatekeeper YopN and led to permanent secretion and defective translocation of virulence factors into host cells. We suggest that the RNA thermometer plays a critical role in adjusting the optimal cellular concentration of a surveillance factor that maintains the controlled translocation of virulence factors.

DNA Supercoiling: an Ancestral Regulator of Gene Expression in Pathogenic Bacteria?

DNA supercoiling acts as a global and ancestral regulator of bacterial gene expression. In this review, we advocate that it plays a pivotal role in host-pathogen interactions by transducing environmental signals to the bacterial chromosome and coordinating its transcriptional response. We present available evidence that DNA supercoiling is modulated by environmental stress conditions relevant to the infection process according to ancestral mechanisms, in zoopathogens as well as phytopathogens. We review the results of transcriptomics studies obtained in widely distant bacterial species, showing that such structural transitions of the chromosome are associated to a complex transcriptional response affecting a large fraction of the genome. Mechanisms and computational models of the transcriptional regulation by DNA supercoiling are then discussed, involving both basal interactions of RNA Polymerase with promoter DNA, and more specific interactions with regulatory proteins. A final part is specifically focused on the regulation of virulence genes within pathogenicity islands of several pathogenic bacterial species.

DNA supercoiling is a fundamental regulatory principle in the control of bacterial gene expression

Although it has become routine to consider DNA in terms of its role as a carrier of genetic information, it is also an important contributor to the control of gene expression. This regulatory principle arises from its structural properties. DNA is maintained in an underwound state in most bacterial cells and this has important implications both for DNA storage in the nucleoid and for the expression of genetic information. Underwinding of the DNA through reduction in its linking number potentially imparts energy to the duplex that is available to drive DNA transactions, such as transcription, replication and recombination. The topological state of DNA also influences its affinity for some DNA binding proteins, especially in DNA sequences that have a high A + T base content. The underwinding of DNA by the ATP-dependent topoisomerase DNA gyrase creates a continuum between metabolic flux, DNA topology and gene expression that underpins the global response of the genome to changes in the intracellular and external environments. These connections describe a fundamental and generalised mechanism affecting global gene expression that underlies the specific control of transcription operating through conventional transcription factors. This mechanism also provides a basal level of control for genes acquired by horizontal DNA transfer, assisting microbial evolution, including the evolution of pathogenic bacteria.

DNA supercoiling is a fundamental regulatory principle in the control of bacterial gene expression

Although it has become routine to consider DNA in terms of its role as a carrier of genetic information, it is also an important contributor to the control of gene expression. This regulatory principle arises from its structural properties. DNA is maintained in an underwound state in most bacterial cells and this has important implications both for DNA storage in the nucleoid and for the expression of genetic information. Underwinding of the DNA through reduction in its linking number potentially imparts energy to the duplex that is available to drive DNA transactions, such as transcription, replication and recombination. The topological state of DNA also influences its affinity for some DNA binding proteins, especially in DNA sequences that have a high A + T base content. The underwinding of DNA by the ATP-dependent topoisomerase DNA gyrase creates a continuum between metabolic flux, DNA topology and gene expression that underpins the global response of the genome to changes in the intracellular and external environments. These connections describe a fundamental and generalised mechanism affecting global gene expression that underlies the specific control of transcription operating through conventional transcription factors. This mechanism also provides a basal level of control for genes acquired by horizontal DNA transfer, assisting microbial evolution, including the evolution of pathogenic bacteria.

Yersinia adhesins: An arsenal for infection

The Yersiniae are a group of Gram-negative coccobacilli inhabiting a wide range of habitats. The genus harbors three recognized human pathogens: Y. enterocolitica and Y. pseudotuberculosis, which both cause gastrointestinal disease, and Y. pestis, the causative agent of plague. These three organisms have served as models for a number of aspects of infection biology, including adhesion, immune evasion, evolution of pathogenic traits, and retracing the course of ancient pandemics. The virulence of the pathogenic Yersiniae is heavily dependent on a number of adhesin molecules. Some of these, such as the Yersinia adhesin A and invasin of the enteropathogenic species, and the pH 6 antigen of Y. pestis, have been extensively studied. However, genomic sequencing has uncovered a host of other adhesins present in these organisms, the functions of which are only starting to be investigated. Here, we review the current state of knowledge on the adhesin molecules present in the Yersiniae, and their functions and putative roles in the infection process.

Environmental Regulation of Yersinia Pathophysiology

Hallmarks of Yersinia pathogenesis include the ability to form biofilms on surfaces, the ability to establish close contact with eukaryotic target cells and the ability to hijack eukaryotic cell signaling and take over control of strategic cellular processes. Many of these virulence traits are already well-described. However, of equal importance is knowledge of both confined and global regulatory networks that collaborate together to dictate spatial and temporal control of virulence gene expression. This review has the purpose to incorporate historical observations with new discoveries to provide molecular insight into how some of these regulatory mechanisms respond rapidly to environmental flux to govern tight control of virulence gene expression by pathogenic Yersinia.

Yersinia Type III Secretion System Master Regulator LcrF

Many Gram-negative pathogens express a type III secretion (T3SS) system to enable growth and survival within a host. The three human-pathogenic Yersinia species, Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica, encode the Ysc T3SS, whose expression is controlled by an AraC-like master regulator called LcrF. In this review, we discuss LcrF structure and function as well as the environmental cues and pathways known to regulate LcrF expression. Similarities and differences in binding motifs and modes of action between LcrF and the Pseudomonas aeruginosa homolog ExsA are summarized. In addition, we present a new bioinformatics analysis that identifies putative LcrF binding sites within Yersinia target gene promoters.

Regulatory principles governing Salmonella and Yersinia virulence

Enteric pathogens such as Salmonella and Yersinia evolved numerous strategies to survive and proliferate in different environmental reservoirs and mammalian hosts. Deciphering common and pathogen-specific principles for how these bacteria adjust and coordinate spatiotemporal expression of virulence determinants, stress adaptation, and metabolic functions is fundamental to understand microbial pathogenesis. In order to manage sudden environmental changes, attacks by the host immune systems and microbial competition, the pathogens employ a plethora of transcriptional and post-transcriptional control elements, including transcription factors, sensory and regulatory RNAs, RNAses, and proteases, to fine-tune and control complex gene regulatory networks. Many of the contributing global regulators and the molecular mechanisms of regulation are frequently conserved between Yersinia and Salmonella. However, the interplay, arrangement, and composition of the control elements vary between these closely related enteric pathogens, which generate phenotypic differences leading to distinct pathogenic properties. In this overview we present common and different regulatory networks used by Salmonella and Yersinia to coordinate the expression of crucial motility, cell adhesion and invasion determinants, immune defense strategies, and metabolic adaptation processes. We highlight evolutionary changes of the gene regulatory circuits that result in different properties of the regulatory elements and how this influences the overall outcome of the infection process.

Thermal control of virulence factors in bacteria: a hot topic

Pathogenic bacteria sense environmental cues, including the local temperature, to control the production of key virulence factors. Thermal regulation can be achieved at the level of DNA, RNA or protein and although many virulence factors are subject to thermal regulation, the exact mechanisms of control are yet to be elucidated in many instances. Understanding how virulence factors are regulated by temperature presents a significant challenge, as gene expression and protein production are often influenced by complex regulatory networks involving multiple transcription factors in bacteria. Here we highlight some recent insights into thermal regulation of virulence in pathogenic bacteria. We focus on bacteria which cause disease in mammalian hosts, which are at a significantly higher temperature than the outside environment. We outline the mechanisms of thermal regulation and how understanding this fundamental aspect of the biology of bacteria has implications for pathogenesis and human health.

Involvement of rppH in thermoregulation in Pseudomonas syringae

Temperature, among other environmental factors, influences the incidence and severity of many plant diseases. Likewise, numerous traits, including the expression of virulence factors, are regulated by temperature. Little is known about the underlying genetic determinants of thermoregulation in plant-pathogenic bacteria. Previously, we showed that the expression of both fliC (encoding flagellin) and syfA (encoding a nonribosomal polypeptide synthetase) was suppressed at high temperatures in Pseudomonas syringae. In this work, we used a high-throughput screen to identify mutations that conferred overexpression of syfA at elevated temperatures (28°C compared to 20°C). Two genes, Psyr_2474, encoding an acyl-coenzyme A (CoA) dehydrogenase, and Psyr_4843, encoding an ortholog of RppH, which in Escherichia coli mediates RNA turnover, contribute to thermoregulation of syfA. To assess the global role of rppH in thermoregulation in P. syringae, RNA sequencing was used to compare the transcriptomes of an rppH deletion mutant and the wild-type strain incubated at 20°C and 30°C. The disruption of rppH had a large effect on the temperature-dependent transcriptome of P. syringae, affecting the expression of 569 genes at either 20°C or 30°C but not at both temperatures. Intriguingly, RppH is involved in the thermoregulation of ribosome-associated proteins, as well as of RNase E, suggesting a prominent role of rppH on the proteome in addition to its effect on the transcriptome.

LcrQ blocks the role of LcrF in regulating the Ysc-Yop type III secretion genes in Yersinia pseudotuberculosis

Pathogenic Yersinia species employ the Ysc-Yop type III secretion system (T3SS) encoded by a highly conserved pYV virulence plasmid to export the virulence effectors into host cells. The Ysc-Yop T3SS is tightly regulated by multiple contributing proteins that function at different levels. However, systematic transcriptional regulation analysis of Ysc-Yop T3SS is lacking and the detailed mechanism under this regulation process is still elusive. Aimed at systematically characterizing transcriptional regulations of all T3SS genes in Y. pseudotuberculosis, we amplified 97 non-coding fragments from the pYV plasmid and analyzed transcriptional responses of the T3SS genes under different growth conditions. Transcriptions of T3SS genes were induced at 37°C and genes encoding T3SS effectors were highly induced by further depletion of Ca2+. The temperature induced gene transcription process is mediated by modules encoded on the chromosome, while the Ca2+ depletion-induced process is controlled by the positive regulatory protein LcrF as well as the negative regulatory protein LcrQ. In this process, LcrQ shares the same targets with LcrF and the effect of LcrQ is dependent on the presence of LcrF. Furthermore, over-expression of LcrF showed the same phenotype as that of the lcrQ mutant strain and intracellular amount balance of LcrQ and LcrF is important in T3SS regulation. When the expression level of LcrF exceeds LcrQ, expression of the Ysc-Yop T3SS genes is activated and vice versa. Together, these data support a model in which LcrQ blocks the activation role of LcrF in regulating the transcription of T3SS genes in Yersinia.

YmoA negatively controls the expression of insecticidal genes in Yersinia enterocolitica

Yersinia enterocolitica is toxic towards invertebrates due to the presence of the toxin complex (tc) genes that are activated by the thermolabile regulator TcaR2. In the search for further regulatory factors involved in insecticidal gene expression, the modulator of yersinial virulence, YmoA, was identified to silence all tc genes of the Y. enterocolitica strain W22703 (biovar 2, serovar O:9). Using promoter fusions with the luciferase reporter, we found that the deletion of ymoA results in elevated transcription of tcaR1, tcaR2, tcaA, tcaB, tcaC, tccC1 and tccC2 at both 15 °C and 37 °C. Complementation by episomal ymoA significantly reduced tc gene expression, thus validating the inhibitory activity of YmoA on the production of insecticidal proteins. YmoA contributes to the binding properties of H-NS to the tc promoters by forming a complex with this nucleoid-associated protein, and this complex not only binds to the upstream regions of all tc genes, but also to intragenic sites of tcaA and tcaB that play an important role in controlling the expression of both genes. At low temperature, the intracellular amount of thermostable YmoA is not reduced, but the repressor is less functional. These data point to H-NS/YmoA as an antagonist of the inducer TcaR2.

Thermosensing to adjust bacterial virulence in a fluctuating environment

The lifecycle of most microbial pathogens can be divided into two states: existence outside and inside their hosts. The sudden temperature upshift experienced upon entry from environmental or vector reservoirs into a warm-blooded host is one of the most crucial signals informing the pathogens to adjust virulence gene expression and their host-stress survival program. This article reviews the plethora of sophisticated strategies that bacteria have evolved to sense temperature, and outlines the molecular signal transduction mechanisms used to modulate synthesis of crucial virulence determinants. The molecular details of thermal control through conformational changes of DNA, RNA and proteins are summarized, complex and diverse thermosensing principles are introduced and their potential as drug targets or synthetic tools are discussed.

Thermo-regulation of genes mediating motility and plant interactions in Pseudomonas syringae

Pseudomonas syringae is an important phyllosphere colonist that utilizes flagellum-mediated motility both as a means to explore leaf surfaces, as well as to invade into leaf interiors, where it survives as a pathogen. We found that multiple forms of flagellum-mediated motility are thermo-suppressed, including swarming and swimming motility. Suppression of swarming motility occurs between 28° and 30°C, which coincides with the optimal growth temperature of P. syringae. Both fliC (encoding flagellin) and syfA (encoding a non-ribosomal peptide synthetase involved in syringafactin biosynthesis) were suppressed with increasing temperature. RNA-seq revealed 1440 genes of the P. syringae genome are temperature sensitive in expression. Genes involved in polysaccharide synthesis and regulation, phage and IS elements, type VI secretion, chemosensing and chemotaxis, translation, flagellar synthesis and motility, and phytotoxin synthesis and transport were generally repressed at 30°C, while genes involved in transcriptional regulation, quaternary ammonium compound metabolism and transport, chaperone/heat shock proteins, and hypothetical genes were generally induced at 30°C. Deletion of flgM, a key regulator in the transition from class III to class IV gene expression, led to elevated and constitutive expression of fliC regardless of temperature, but did not affect thermo-regulation of syfA. This work highlights the importance of temperature in the biology of P. syringae, as many genes encoding traits important for plant-microbe interactions were thermo-regulated.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Concerted actions of a thermo-labile regulator and a unique intergenic RNA thermosensor control Yersinia virulence

Expression of all Yersinia pathogenicity factors encoded on the virulence plasmid, including the yop effector and the ysc type III secretion genes, is controlled by the transcriptional activator LcrF in response to temperature. Here, we show that a protein- and RNA-dependent hierarchy of thermosensors induce LcrF synthesis at body temperature. Thermally regulated transcription of lcrF is modest and mediated by the thermo-sensitive modulator YmoA, which represses transcription from a single promoter located far upstream of the yscW-lcrF operon at moderate temperatures. The transcriptional response is complemented by a second layer of temperature-control induced by a unique cis-acting RNA element located within the intergenic region of the yscW-lcrF transcript. Structure probing demonstrated that this region forms a secondary structure composed of two stemloops at 25°C. The second hairpin sequesters the lcrF ribosomal binding site by a stretch of four uracils. Opening of this structure was favored at 37°C and permitted ribosome binding at host body temperature. Our study further provides experimental evidence for the biological relevance of an RNA thermometer in an animal model. Following oral infections in mice, we found that two different Y. pseudotuberculosis patient isolates expressing a stabilized thermometer variant were strongly reduced in their ability to disseminate into the Peyer's patches, liver and spleen and have fully lost their lethality. Intriguingly, Yersinia strains with a destabilized version of the thermosensor were attenuated or exhibited a similar, but not a higher mortality. This illustrates that the RNA thermometer is the decisive control element providing just the appropriate amounts of LcrF protein for optimal infection efficiency.

Insertional mutagenesis of Listeria monocytogenes 568 reveals genes that contribute to enhanced thermotolerance

The objectives of this study were to identify molecular mechanisms of thermotolerance using transposon mutants of Listeria monocytogenes 568, serotype 1/2a, and to compare their thermal death kinetics at 52, 56 and 60 degrees C. Sixteen Tn917 transposon mutants with enhanced heat resistance were acquired from a library of 4300 mutants following a multi-step screening process. Genetic regions with Tn917 insertions encompassed a broad range of functionalities including; transport, metabolism, replication and repair, general stress, and structural properties. Modeling of the heat inactivation data using the Geeraerd et al. and Whiting (Fermi) models showed that the mutants' enhanced thermal resistance was manifested mostly through a significant (p<or=0.05) extension of the lag period on the thermal death curve. This new knowledge impacts our understanding of molecular mechanisms affecting the kinetics of thermally induced cell death and enables the development of safer thermal processes.

YtxR acts as an overriding transcriptional off switch for the Yersinia enterocolitica Ysc-Yop type 3 secretion system

The Yersinia enterocolitica YtxR protein is a LysR-type transcriptional regulator that induces expression of the ytxAB locus, which encodes a putative ADP-ribosylating toxin. The ytxR and ytxAB genes are not closely linked in the Y. enterocolitica chromosome, and whereas ytxR is present in all sequenced Yersinia spp., the ytxAB locus is not. These observations suggested that there might be other YtxR-regulon members besides ytxAB and prompted us to investigate coregulated genes and gene products by using transcriptional and proteomic approaches. Microarray and reverse transcription-PCR analysis showed that YtxR strongly activates expression of the yts2 locus, which encodes a putative type 2 secretion system, as well as several uncharacterized genes predicted to encode extracytoplasmic proteins. Strikingly, we also discovered that under Ysc-Yop type 3 secretion system-inducing conditions, YtxR prevented the appearance of Yop proteins in the culture supernatant. Microarray and lacZ operon fusion analysis showed that this was due to specific repression of ysc-yop gene expression. YtxR was also able to repress VirF-dependent Phi(yopE-lacZ) and Phi(yopH-lacZ) expression in a strain lacking the virulence plasmid, which suggested a direct repression mechanism. This was supported by DNase I footprinting, which showed that YtxR interacted with the yopE and yopH control regions. Therefore, YtxR is a newly identified regulator of the ysc-yop genes that can act as an overriding off switch for this critical virulence system.

The SOS response affects thermoregulation of colicin K synthesis

Temperature is one of the key environmental parameters affecting bacterial gene expression. This study investigated the effect of temperature on synthesis of Escherichia coli colicins E1, K, N and E7 as well as the molecular basis underlying thermoregulation of the colicin K activity gene cka. The results of our study show that synthesis of the investigated colicins is higher at 37 degrees C than at 22 degrees C and that temperature regulates cka expression at the level of transcription. We propose that the SOS response indirectly regulates thermoregulation of colicin K (and possibly of the other examined colicins). Two LexA dimers bind cooperatively with high affinity to the two overlapping LexA boxes in a temperature-independent manner. At 22 degrees C the relative degree of repression is higher as a result of less LexA cleavage due to a slower growth rate, while at 37 degrees C the extent of LexA cleavage is higher due to a higher growth rate. Thermoregulation of colicin synthesis is an additional example of the connection between the SOS regulon and cell physiology.

Presence of a functional flagellar cluster Flag-2 and low-temperature expression of flagellar genes in Yersinia enterocolitica W22703

Twelve Yersinia enterocolitica mutants carrying luxCDABE-transposon insertions in motility and chemotaxis genes were isolated on the basis of strong low-temperature induction. Two transposons were located within an 11.2 kb enteric flagellar cluster 2 (Flag-2) of Y. enterocolitica biotype 2, serotype O : 9 strain W22703. The Flag-2 gene cluster is absent from the corresponding genomic location of the sequenced strain Y. enterocolitica biotype 1B, serotype O : 8 strain 8081. Evidence for the functionality of the O : 9 Flag-2 genes, probably located within the plasticity zone of the genome, is provided by swarming assays. PCR analysis of 49 strains revealed the presence of Flag-2 genes in biotypes 2-5, but not in biotypes 1A or 1B. Bioluminescence, measured between 6 and 37 degrees C, showed that the expression of all genes located in Flag-2 and in the known flagellar cluster, Flag-1, was highest at approximately 20 degrees C, and that expression of two Flag-2 genes is FlhC-dependent. In a motility assay, a non-motile and a hyper-motile phenotype resulted from knockout mutations of the Flag-1 genes fliS1 and fliT, respectively. Complemented strains validated these results, confirming the regulatory role of FliT.

Pleiotropic phenotypes of a Yersinia enterocolitica flhD mutant include reduced lethality in a chicken embryo model

Background

The Yersinia enterocolitica flagellar master regulator FlhD/FlhC affects the expression levels of non-flagellar genes, including 21 genes that are involved in central metabolism. The sigma factor of the flagellar system, FliA, has a negative effect on the expression levels of seven plasmid-encoded virulence genes in addition to its positive effect on the expression levels of eight of the flagellar operons. This study investigates the phenotypes of flhD and fliA mutants that result from the complex gene regulation.

Results

Phenotypes relating to central metabolism were investigated with Phenotype MicroArrays. Compared to the wild-type strain, isogenic flhD and fliA mutants exhibited increased growth on purines and reduced growth on N-acetyl-D-glucosamine and D-mannose, when used as a sole carbon source. Both mutants grew more poorly on pyrimidines and L-histidine as sole nitrogen source. Several intermediates of the tricarboxylic acid and the urea cycle, as well as several dipeptides, provided differential growth conditions for the two mutants. Gene expression was determined for selected genes and correlated with the observed phenotypes. Phenotypes relating to virulence were determined with the chicken embryo lethality assay. The assay that was previously established for Escherichia coli strains was modified for Y. enterocolitica. The flhD mutant caused reduced chicken embryo lethality when compared to wild-type bacteria. In contrast, the fliA mutant caused wild-type lethality. This indicates that the virulence phenotype of the flhD mutant might be due to genes that are regulated by FlhD/FlhC but not FliA, such as those that encode the flagellar type III secretion system.

Conclusion

Phenotypes of flhD and fliA mutants are related to central metabolism and virulence and correlate with gene regulation.

A rationale for repression and/or loss of motility by pathogenic Yersinia in the mammalian host

Pathogenic yersiniae either repress flagella expression under host conditions (Yersinia enterocolitica and Yersinia pseudotuberculosis) or have permanently lost this capability by mutation (Yersinia pestis). The block in flagella synthesis for the enteropathogenic Yersinia centers on fliA (sigmaF) repression. This repression ensures the downstream repression of flagellin structural genes which can be cross-recognized and secreted by virulence type III secretion systems. Y. pestis carries several flagellar mutations including a frame shift mutation in flhD, part of the flagellar master control operon. Repression of flagellins in the host environment may be critical because they are potent inducers of innate immunity. Artificial expression of flagellin in Y. enterocolitica completely attenuates virulence, supporting the hypothesis that motility is a liability in the mammalian host.

Quorum sensing by enteric pathogens

PURPOSE OF REVIEW

This review presents advances in our understanding of how pathogenic, enteric bacteria use quorum sensing to regulate several traits that allow them to establish and maintain infection in their host, including motility, biofilm formation, and virulence-specific genes.

RECENT FINDINGS

Quorum sensing in enteric bacteria has been elusive for a long time. Recent data indicate that enteric bacteria use several quorum-sensing mechanisms including the LuxR-I quorum-sensing system, the LuxS/AI-2 system, and the AI-3/epinephrine/norepinephrine system to assess their environment and to recognize the host environment. These systems allow bacteria to communicate across species boundaries, and the AI-3/epinephrine/norepinephrine system is involved in interkingdom signaling.

SUMMARY

Recent developments in our understanding of the molecular and biochemical mechanisms involved in quorum sensing as well as the chemical signal(s) to which bacteria respond and their corresponding physiological responses will improve our understanding of bacterial pathogenesis and microbial flora-host interactions, and potentially lead to novel strategies for combating infection.

Altered Ca(2+) regulation of Yop secretion in Yersinia enterocolitica after DNA adenine methyltransferase overproduction is mediated by Clp-dependent degradation of LcrG

DNA methylation by the DNA adenine methyltransferase (Dam) interferes with the coordinated expression of virulence functions in an increasing number of pathogens. While analyzing the effect of Dam on the virulence of the human pathogen Yersinia enterocolitica, we observed type III secretion of Yop effector proteins under nonpermissive conditions. Dam alters the Ca(2+) regulation of Yop secretion but does not affect the temperature regulation of Yop/Ysc expression. The phenotype is different from that of classical "Ca(2+)-blind" mutants of Yersinia, as Dam-overproducing (Dam(OP)) strains still translocate Yops polarly into eukaryotic cells. Although transcription of the lcrGV and yopN-tyeA operons is slightly upregulated, LcrG is absent from lysates of Dam(OP) bacteria, while the amounts of YopN and TyeA are not changed. We present evidence that clpXP expression increases after Dam overproduction and that the ClpP protease then degrades LcrG, thereby releasing a block in type III secretion. This is the first example of posttranslational regulation of type III secretion by the Clp protease and adds a new flavor to the complex regulatory mechanisms underlying the controlled release of effector proteins from bacterial cells.

Roles for DNA supercoiling and the Fis protein in modulating expression of virulence genes during intracellular growth of Salmonella enterica serovar Typhimurium

Adaptation of bacterial pathogens to an intracellular environment requires resetting of the expression levels of a wide range of both virulence and housekeeping genes. We investigated the possibility that changes in DNA supercoiling could modulate the expression of genes known to be important in the intracellular growth of the pathogen Salmonella enterica serovar Typhimurium. Our data show that DNA becomes relaxed when Salmonella grows in murine macrophage but not in epithelial cells, indicating that DNA supercoiling plays a role in discrimination between two types of intracellular environment. The ssrA regulatory gene within the SPI-2 pathogenicity island that is required for survival in macrophage was found to be upregulated by DNA relaxation. This enhancement of expression also required the Fis nucleoid-associated protein. Manipulating the level of the Fis protein modulated both the level of DNA supercoiling and ssrA transcription. We discuss a model of bacterial intracellular adaptation in which Fis and DNA supercoiling collaborate to fine-tune virulence gene expression.

Growth studies of plasmid bearing and plasmid cured Yersinia enterocolitica GER O:3 in the presence of cefsulodin, irgasan and novobiocin at 25 and 37oC

AIM

In this study, the growth characteristics of Yersinia enterocolitica biotype 4, GER O:3 plasmid bearing (P+) and plasmid cured (P-) strain types were evaluated in brain heart infusion broth supplemented with cefsulodin, irgasan, and novobiocin alone or in combination.

METHODS AND RESULTS

Growth curves were obtained for the two strain types in broth supplemented with selective agents at 25 or 37 degrees C for 32 h to obtain data on the lag phase durations and growth rates of the strains. Generally, the lag times and growth rates of the P+ and P- strains were similar for cultures incubated at 25 degrees C regardless of the selective agent added and where plasmid replication and expression were not under any significant burden. However, where the lag times and growth rates of the strains were examined at 37 degrees C, significant differences were observed in the lag phase durations of the plasmid bearing strain type compared the plasmid cured strain, an effect that was due to the burden of the plasmid and the influence of selective agents. Generally, when two or more agents were present, lag phase durations were longer for the plasmid bearing strain. Some exceptions noted where in the presence of irgasan or full selective agent (CIN) the opposite case was observed. When growth rates were compared, the plasmidless strain type was typically faster than the plasmid bearing strain in the presence of most selective agents at 37 degrees C and the growth rates of both strain types at 25 degrees C were similar where the temperature appeared to negate the effects of plasmid.

CONCLUSIONS

The data obtained in these studies suggest that selective agents (in particular irgasan) and incubation temperature play a significant role in influencing the growth characteristics of plasmid bearing and plasmid cured strains of Y. enterocolitica.

SIGNIFICANCE AND IMPACT OF THE STUDY

This data presented in this study has significant implications for enrichment methods used in the detection or recovery of plasmid bearing Y. enterocolitica strains from food, environmental or clinical samples.

Global gene regulation in Yersinia enterocolitica: effect of FliA on the expression levels of flagellar and plasmid-encoded virulence genes

This study describes the involvement of the sigma factor of the flagellar system, FliA, in global gene regulation of Yersinia enterocolitica. In addition to exhibiting a positive effect upon the expression levels of eight class III flagellar operons, FliA also exhibited a negative effect upon the expression levels of four virulence operons that are located on the pYV virulence plasmid. These are yadA, virC, yopQ, and the insertion element ISYen1. While the positive effect on class III flagellar operons by FliA is most likely direct, the negative effect on the virulence operons appears to require the known transcriptional activator of these genes, VirF. This was determined using microarray analysis, quantitative PCR and a search for putative binding sites for FliA. In addition to the FliA regulation of flagellar and plasmid-encoded virulence genes, we studied temperature regulation of these genes. While wild-type cells exhibited increased expression levels of flagellar genes and decreased expression levels of plasmid-encoded virulence genes at 25 degrees C (as compared to 37 degrees C), temperature dependence of gene expression was much reduced in the fliA mutants. We conclude that FliA contributes to the inverse temperature regulation of flagellar and plasmid-encoded virulence genes. We present a network of transcriptional regulation around FlhD/FlhC and FliA.

Transcriptional organization of the temperature-sensitive transfer system from the IncHI1 plasmid R27

One of the characteristic features of IncHI1 plasmids is a thermosensitive process of conjugation, which is optimal between 22 °C and 30 °C but inhibited at 37 °C. R27, the prototypical IncHI1 plasmid, contains transfer genes clustered in two regions of the plasmid, Tra1 and Tra2. In the present study, transcriptional analyses of thetragenes were undertaken at both 30 °C and 37 °C. Screening of 38tragenes showed thattragenes are transcriptionally linked in six operons, three in each Tra region. RT-PCR analysis showed that gene expression was reduced at 37 °C relative to that observed at 30 °C. The transcription start sites of the six transcripts were identified, promoters and upstream regions were cloned, and transcription was tested at both temperatures. In cells grown at 37 °C, in the presence of R27, the promoters were inhibited, except for promoters of the H operon and AN operon. Conditions that influenced DNA topology, such as osmolarity, anaerobiosis, quorum sensing and acidity, showed no significant influence on transfer frequency. These results should facilitate future understanding of the basis of temperature-sensitive transfer in this large conjugative plasmid.

H-NS is a part of a thermally controlled mechanism for bacterial gene regulation

Temperature is a primary environmental stress to which micro-organisms must be able to adapt and respond rapidly. Whereas some bacteria are restricted to specific niches and have limited abilities to survive changes in their environment, others, such as members of the Enterobacteriaceae, can withstand wide fluctuations in temperature. In addition to regulating cellular physiology, pathogenic bacteria use temperature as a cue for activating virulence gene expression. This work confirms that the nucleoid-associated protein H-NS (histone-like nucleoid structuring protein) is an essential component in thermoregulation of Salmonella. On increasing the temperature from 25 to 37 degrees C, more than 200 genes from Salmonella enterica serovar Typhimurium showed H-NS-dependent up-regulation. The thermal activation of gene expression is extremely rapid and change in temperature affects the DNA-binding properties of H-NS. The reduction in gene repression brought about by the increase in temperature is concomitant with a conformational change in the protein, resulting in the decrease in size of high-order oligomers and the appearance of increasing concentrations of discrete dimers of H-NS. The present study addresses one of the key complex mechanisms by which H-NS regulates gene expression.

The ATP-dependent ClpXP and Lon proteases regulate expression of the Yersinia pestis type III secretion system via regulated proteolysis of YmoA, a small histone-like protein

The Yersinia pestis plasmid pCD1-encoded type III secretion system (T3SS) is essential for the pathogenicity of Y. pestis in mammalian hosts. T3SS-associated genes are maximally expressed at 37 degrees C in the absence of extracellular calcium. Expression of T3SS genes requires LcrF, an AraC-like transcriptional activator, and is repressed by YmoA, a small histone-like protein. The mechanism by which temperature regulates T3SS gene expression has not been determined; however, changes in DNA topology have been implicated in this process. We report here that a Y. pestis strain deficient in production of the ClpXP and Lon proteases does not express a functional T3SS partly because of high cytosolic levels of YmoA. YmoA is rapidly degraded at 37 degrees C in wild-type Y. pestis, but remains stable in a clpXPlon deletion mutant. The stability of YmoA in wild-type Y. pestis increased as the growth temperature of the culture decreased; in contrast, YmoA was stable at all temperatures examined in the clpXPlon deletion mutant. These results indicate that the ClpXP and Lon proteases contribute to the environmental regulation of the Y. pestis T3SS system through regulated proteolysis of YmoA.

Stress-induced DNA duplex destabilization (SIDD) in the E. coli genome: SIDD sites are closely associated with promoters

We present the first analysis of stress-induced DNA duplex destabilization (SIDD) in a complete chromosome, the Escherichia coli K12 genome. We used a newly developed method to calculate the locations and extents of stress-induced destabilization to single-base resolution at superhelix density sigma = -0.06. We find that SIDD sites in this genome show a statistically highly significant tendency to avoid coding regions. And among intergenic regions, those that either contain documented promoters or occur between divergently transcribing coding regions, and hence may be inferred to contain promoters, are associated with strong SIDD sites in a statistically highly significant manner. Intergenic regions located between convergently transcribing genes, which are inferred not to contain promoters, are not significantly enriched for destabilized sites. Statistical analysis shows that a strongly destabilized intergenic region has an 80% chance of containing a promoter, whereas an intergenic region that does not contain a strong SIDD site has only a 24% chance. We describe how these observations may illuminate specific mechanisms of regulation, and assist in the computational identification of promoter locations in prokaryotes.

Molecualar survival strategies of the Lyme disease spirochete Borrelia burgdorferi

Lyme disease is a tick-transmitted disease caused by the spirochete Borrelia burgdorferi. The bacterium adopts different strategies for its survival inside the immunocompetent host from the time of infection until dissemination in different parts of body tissues. The success of this spirochete depends on its ability to colonise the host tissues and counteract the host's defence mechanisms. During this process borrelia seems to maintain its vitality to ensure long-term survival in the host. Borrelia's proteins are encoded by plasmid and chromosomal genes. These genes are differentially regulated and expressed by different environmental factors in ticks as well as in the mammalian host during infection. In addition, antigenic diversity enables the spirochete to escape host defence mechanisms and maintain infection. In this review we focus on the differential expression of proteins and genes, and further molecular mechanisms used by borrelia to maintain its survival in the host. In light of these pathogenetic mechanisms, further studies on spirochete host interaction are needed to understand the complex interplay that finally lead to host autoimmunity.

Regulation of the cytotoxic effects of Pseudomonas fluorescens by growth temperature

We had previously shown that the psychrotrophic bacterium Pseudomonas fluorescens can act as a pathogen, inducing apoptosis and necrosis in neurons and glial cells. In the present study, we investigated the influence of the growth temperature of P. fluorescens on its infectious potential. Adherence of P. fluorescens to glial cells was found to be maximal with bacteria grown at a low temperature (8 degrees C). At that temperature the swimming behaviour was markedly reduced. An increase in the growth temperature to 19, 28 or 32 degrees C strongly diminished the binding of bacteria to host cells. Thus, the adhesion phenotype of P. fluorescens appears to be independent of the motility of the bacteria. The apoptotic effect of P. fluorescens, determined by morphological (nuclear condensation) and biochemical (induction of nitric oxide synthase activity) indicators, correlated well with its binding activity on glial cells. In contrast, there was a clear dissociation between maximum binding and maximal necrotic action (measured by the release of lactate dehydrogenase) observed with bacteria grown at 19 degrees C. As suggested by capillary electrophoresis analysis, the differences in apoptotic effects may be related to variations in the molecular structure of LPS originating from bacteria grown at low and high temperatures, whereas the necrotic effect, which was maximal at the optimum temperature for the secretion of exoenzymes, could reflect variations in the metabolic activity of bacteria.

Regulation cascade of flagellar expression in Gram-negative bacteria

Flagellar motility helps bacteria to reach the most favourable environments and to successfully compete with other micro-organisms. These complex organelles also play an important role in adhesion to substrates, biofilm formation and virulence process. In addition, because their synthesis and functioning are very expensive for the cell (about 2% of biosynthetic energy expenditure in Escherichia coli) and may induce a strong immune response in the host organism, the expression of flagellar genes is highly regulated by environmental conditions. In the past few years, many data have been published about the regulation of motility in polarly and laterally flagellated bacteria. However, the mechanism of motility control by environmental factors and by some regulatory proteins remains largely unknown. In this respect, recent experimental data suggest that the master regulatory protein-encoding genes at the first level of the cascade are the main target for many environmental factors. This mechanism might require DNA topology alterations of their regulatory regions. Finally, despite some differences the polar and lateral flagellar cascades share many functional similarities, including a similar hierarchical organisation of flagellar systems. The remarkable parallelism in the functional organisation of flagellar systems suggests an evolutionary conservation of regulatory mechanisms in Gram-negative bacteria.

Transcriptional regulation of the ospAB and ospC promoters from Borrelia burgdorferi

OspA, OspB and OspC are the major outer surface proteins of Borrelia burgdorferi that are differentially synthesized in response to environmental conditions, including culture temperature. We found that DNA was more negatively supercoiled in B. burgdorferi cultures grown at 23 degrees C compared with cultures grown at 35-37 degrees C. We examined the regulation of ospAB and ospC transcription by temperature and DNA supercoiling. DNA supercoiling was relaxed by adding coumermycin A1, an antibiotic that inhibits DNA gyrase. Syntheses of the major outer surface proteins, expression of the ospA and ospC genes and the activities of the ospAB operon and ospC gene promoters were assayed. ospA product levels decreased, whereas ospC product levels increased after shifting from 23 degrees C to 35 degrees C or after adding coumermycin A1. In addition, OspC synthesis was higher in a gyrB mutant than in wild-type B. burgdorferi. Promoter activity was quantified using cat reporter fusions. Increasing temperature or relaxing supercoiled DNA resulted in a decrease in ospAB promoter activity in B. burgdorferi, but not in Escherichia coli, as well as an increase in ospC promoter activity in both bacteria. ospC promoter activity was increased in an E. coli gyrB mutant with an attenuated DNA supercoiling phenotype. These results suggest that B. burgdorferi senses environmental changes in temperature by altering the level of DNA supercoiling, which then affects the expression of the ospAB operon and the ospC gene. This implies that DNA supercoiling acts as a signal transducer for environmental regulation of outer surface protein synthesis.

A microarray-based antibiotic screen identifies a regulatory role for supercoiling in the osmotic stress response of Escherichia coli

Changes in DNA supercoiling are induced by a wide range of environmental stresses in Escherichia coli, but the physiological significance of these responses remains unclear. We now demonstrate that an increase in negative supercoiling is necessary for transcriptional activation of a large subset of osmotic stress-response genes. Using a microarray-based approach, we have characterized supercoiling-dependent gene transcription by expression profiling under conditions of high salt, in conjunction with the microbial antibiotics novobiocin, pefloxacin, and chloramphenicol. Algorithmic clustering and statistical measures for gauging cellular function show that this subset is enriched for genes critical in osmoprotectant transport/synthesis and rpoS-driven stationary phase adaptation. Transcription factor binding site analysis also supports regulation by the global stress sigma factor rpoS. In addition, these studies implicate 60 uncharacterized genes in the osmotic stress regulon, and offer evidence for a broader role for supercoiling in the control of stress-induced transcription.

Simultaneous coexpression of Borrelia burgdorferi Erp proteins occurs through a specific, erp locus-directed regulatory mechanism

An individual Borrelia burgdorferi bacterium can encode as many as 13 different Erp (OspE/F-related) proteins from mono-and bicistronic loci that are carried on up to 10 separate plasmids. We demonstrate through multilabel immunofluorescence analyses that individual bacteria simultaneously coexpress their entire Erp protein repertoire. While it has been proposed that B. burgdorferi controls expression of Erp and other plasmid-encoded proteins through changes in DNA topology, we observed regulated Erp expression in the absence of detectable differences in DNA supercoiling. Likewise, inhibition of DNA gyrase had no detectable effect on Erp expression. Furthermore, expression of loci physically adjacent to erp loci was observed to be independently regulated. It is concluded that Erp expression is regulated by a mechanism(s) directed at erp loci and not by a global, plasmid-wide mechanism.

Up-regulation of the Yersinia enterocolitica yop regulon by deletion of the flagellum master operon flhDC

The Yop virulon enables extracellularly located Yersinia, in close contact with a eukaryotic target cell, to inject bacterial toxic proteins directly into the cytosol of this cell. Several Ysc proteins, forming the Yop secretion apparatus, display homology with proteins of the flagellar basal body. To determine whether this relationship could extend to the regulatory pathways, we analyzed the influence of flhDC, the master regulatory operon of the flagellum, on the yop regulon. In an flhDC mutant, the yop regulon was up-regulated. The transcription of virF and the steady-state level of the transcriptional activator VirF were enhanced. yop transcription was increased at 37 degrees C and could also be detected at a low temperature. Yop secretion was increased at 37 degrees C and occurred even at a low temperature. The Ysc secretion machinery was thus functional at room temperature in the absence of flagella, implying that in wild-type bacteria, FlhD and/or FlhC, or the product of a gene downstream of flhDC, represses the yop regulon. In agreement with this notion, increased expression of flhDC in wild-type bacteria resulted in the oversecretion of flagellins at room temperature and in decreased Yop secretion at 37 degrees C.

Regulation of bacterial motility in response to low pH in Escherichia coli: the role of H-NS protein

The effect of detrimental conditions on bacterial motility in Escherichia coli was investigated. Expression profiling of mutant E. coli strains by DNA arrays and analysis of phenotypic traits demonstrated that motility and low-pH resistance are coordinately regulated. Analysis of transcriptional fusions suggests that bacterial motility in response to an acidic environment is mediated via the control by H-NS of flhDC expression. Moreover, the results suggested that the presence of an extended mRNA 5' end and DNA topology are required in this process. Finally, the presence of a similar regulatory region in several Gram-negative bacteria implies that this mechanism is largely conserved.

Regulatory network of lipopolysaccharide O-antigen biosynthesis in Yersinia enterocolitica includes cell envelope-dependent signals

Lipopolysaccharide (LPS) is a glycolipid present in the outer membrane of all Gram-negative bacteria, and it is one of the signature molecules recognized by the receptors of the innate immune system. In addition to its lipid A portion (the endotoxin), its O-chain polysaccharide (the O-antigen) plays a critical role in the bacterium-host interplay and, in a number of bacterial pathogens, it is a virulence factor. We present evidence that, in Yersinia enterocolitica serotype O:8, a complex signalling network regulates O-antigen expression in response to temperature. Northern blotting and reporter fusion analyses indicated that temperature regulates the O-antigen expression at the transcriptional level. Promoter cloning showed that the O-antigen gene cluster contains two transcriptional units under the control of promoters P(wb1) and P(wb2). The activity of both promoters is under temperature regulation and is repressed in bacteria grown at 37 degrees C. We demonstrate that the RosA/RosB efflux pump/potassium antiporter system and Wzz, the O-antigen chain length determinant, are indirectly involved in the regulation mainly affecting the activity of promoter P(wb2). The rosAB transcription, under the control of P(ros), is activated at 37 degrees C, and P(wb2) is repressed through the signals generated by the RosAB system activation, i.e. decreased [K+] and increased [H+]. The wzz transcription is under the control of P(wb2), and we show that, at 37 degrees C, overexpression of Wzz downregulates slightly the P(wb1) and P(wb2) activities and more strongly the P(ros) activity, with the net result that more O-antigen is produced. Finally, we demonstrate that overexpression of Wzz causes membrane stress that activates the CpxAR two-component signal transduction system.

The rfaH gene, which affects lipopolysaccharide synthesis in Salmonella enterica serovar Typhi, is differentially expressed during the bacterial growth phase

We have cloned and sequenced the rfaH gene from Salmonella enterica serovar Typhi strain Ty2. The gene showed a high degree of similarity to the rfaH genes from Escherichia coli K-12 and S. enterica serovar Typhimurium. A rfaH mutant was constructed by site-directed mutagenesis. This mutant produced a rough lipopolysaccharide (LPS), with an incomplete core region. The defect in LPS expression that results from the rfaH mutation was corrected by a plasmid carrying the intact gene. The plasmid-borne rfaH gene also restored normal LPS synthesis in a rfaH mutant of E. coli. Reverse transcription-polymerase chain reaction analyses were performed to determine the effects of various environmental conditions on the expression of rfaH. The transcription of rfaH showed a growth-phase-dependent regulation, with maximal expression at the late exponential phase. Other environmental conditions, such as temperature or medium osmolarity, did not affect transcription of rfaH.

The level of Yop proteins secreted by Yersinia enterocolitica is changed in maltose mutants

Enteropathogenic Yersinia enterocolitica strains express a set of plasmid-encoded proteins called Yops, involved in pathogenicity. We studied the influence of the maltose system on the production of Yop proteins and found that the level of Yop proteins of Y. enterocolitica O:9 was reduced in the presence of maltose. Transposon insertion mutants impaired with the maltose transport activity showed a decreased level in the production of Yop proteins. The transcription of the yopH gene for YopH phosphatase in these maltose mutants was unchanged and revealed a maltose mutation impaired in the secretion of Yop proteins instead of their expression.

Control of bacterial motility by environmental factors in polarly flagellated and peritrichous bacteria isolated from Lake Baikal

Despite numerous studies on bacterial motility, little is known about the regulation of this process by environmental factors in natural isolates. In this study we investigated the control of bacterial motility in response to environmental parameters in two strains isolated from the natural habitat of Lake Baikal. Morphological characterization, carbon source utilization, fermentation analysis, and sequence comparison of 16S rRNA genes showed that these strains belong to two distinct genera, i.e., Enterobacter and Pseudomonas; they were named strains 22 and Y1000, respectively. Both strains swarmed at 25 degrees C and remained motile at low temperatures (4 degrees C), especially the Pseudomonas strain, which further supports the psychrotrophic characteristics of this strain. In contrast, a strong inhibition of motility was observed at above 30 degrees C and with a high NaCl concentration. The existence of flagellar regulatory proteins FlhDC and FleQ was demonstrated in Enterobacter strain 22 and Pseudomonas strain Y1000, respectively, and environmental conditions reduced the expression of the structural genes potentially located at the first level in the flagellar cascade in both organisms. Finally, as in Enterobacter strain 22, a strong reduction in the transcription of the master regulatory gene fleQ was observed in Pseudomonas strain Y1000 in the presence of novobiocin, a DNA gyrase inhibitor, suggesting a link between DNA supercoiling and motility control by environmental factors. Thus, striking similarities observed in the two organisms suggest that these processes have evolved toward a similar regulatory mechanism in polarly flagellated and laterally flagellated (peritrichous) bacteria.

Environmental control of invasin expression in Yersinia pseudotuberculosis is mediated by regulation of RovA, a transcriptional activator of the SlyA/Hor family

Invasin is the primary invasive factor of Yersinia pseudotuberculosis that allows efficient internalization into eukaryotic cells. We investigated invasin expression and found that the inv gene is regulated in response to a variety of environmental signals, such as temperature, growth phase, nutrients, osmolarity and pH, and requires the product of rovA, a member of the SlyA/Hor transcriptional activator family. The rovA gene was found by a genetic complementation strategy that restores temperature regulation of an unexpressed inv-phoA fusion in Escherichia coli K-12. RovA plays a role in the invasion of Y. pseudotuberculosis into mammalian cells and mediates the regulation of invasin in response to all environmental signals analysed. Deletion analysis of the inv promoter region revealed a DNA segment extending 207 bp upstream of the transcriptional start site, which is required for maximal RovA-induced inv transcription. Gel retardation assays showed that RovA interacts preferentially with this promoter fragment and suggested two potential RovA binding sites. Studies with chromosomal gene fusions also demonstrated that rovA follows the same pattern of regulation as invasin, indicating that environmental control of inv expression is mainly mediated by the control of RovA synthesis. Furthermore, we showed that a rovA-lacZ fusion is only slightly expressed in a rovA mutant strain, indicating that a positive autoregulatory mechanism is also involved in rovA expression.