Characterization of the RcsC sensor kinase from Erwinia amylovora and other Enterobacteria

Novel Single Nucleotide Variations Alter Pathogenicity in Korean Isolates of Erwinia amylovora

Erwinia amylovora, the causal agent of fire blight disease, has become a serious threat to the pome fruit industry in Korea since 2015. In this study, we showed that two new isolates of E. amylovora, Ea17-2187 and Ea19-7, obtained from pear orchards in Anseong, Korea, exhibited unique pathogenicity compared with other isolates thus far. Both were nonpathogenic to immature apple fruits but occasionally caused disease on immature pear fruits at varying reduced rates. Bioinformatic analyses revealed that their genomes are highly similar to those of the type strains TS3128 and ATCC49946 but have different mutations in essential virulence regulatory genes. Ea17-2187 has a single nucleotide substitution in rcsC, which encodes the core components of the Rcs system that activates the exopolysaccharide amylovoran production. In contrast, Ea19-7 contains a single nucleotide insertion in hrpL, which encodes a master regulator of the type III secretion system. In both cases, the mutation can cause premature termination and production of truncated gene products, disrupting virulence regulation. Introduction of the nonmutated rcsC and hrpL genes into Ea17-2187 and Ea19-7, respectively, fully recovered pathogenicity, comparable with that of TS3128; hence, these mutations were responsible for the altered pathogenicity observed. Interestingly, virulence assays on immature pear fruits showed that the hrpL mutant of Ea19-7 was still pathogenic, although its virulence level was markedly reduced. Taken together, these results suggest that the two new isolates might act as opportunistic pathogens or cheaters and that some Korean isolates might have evolved to acquire alternative pathways for activating pathogenicity factors.

A Novel Signaling Pathway Connects Thiamine Biosynthesis, Bacterial Respiration, and Production of the Exopolysaccharide Amylovoran in Erwinia amylovora

Erwinia amylovora is a plant pathogen causing necrotrophic fire blight disease of apple, pear, and other rosaceous plants. This bacterium colonizes host vascular tissues via the production of exopolysaccharides (EPSs) including amylovoran. It is well-established that the nearly ubiquitous plasmid pEA29 of E. amylovora is an essential virulence factor, but the underlying mechanism remains uncharacterized. Here, we demonstrated that pEA29 was required for E. amylovora to produce amylovoran and to form a biofilm, and this regulation was dependent on the thiamine biosynthesis operon thiOSGF. We then conducted carbohydrate and genetic analyses demonstrating that the thiamine-mediated effect on amylovoran production was indirect, as cells lacking thiOSGF produced an EPS that did not contain glucuronic acid, one of the key components of amylovoran, whereas the transcriptional activity and RNA levels of the amylovoran biosynthesis genes were not altered. Alternatively, addition of exogenous thiamine restored amylovoran production in the pEA29-cured strain of E. amylovora and positively impacted amylovoran production in a dose-dependent manner. Individual deletion of several chromosomal thiamine biosynthesis genes also affected amylovoran production, implying that a complete thiamine biosynthesis pathway is required for the thiamine-mediated effect on amylovoran production in E. amylovora. Finally, we determined that an imbalanced tricarboxylic acid cycle negatively affected amylovoran production, which was restored by addition of exogenous thiamine or overexpression of the thiOSGF operon. In summary, our report revealed a novel signaling pathway that impacts E. amylovora virulence in which thiamine biosynthesis enhances bacterial respiration that provides energetic requirements for the biosynthesis of EPS amylovoran.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genetic Dissection of the Erwinia amylovora Disease Cycle

Fire blight, caused by the bacterial phytopathogen Erwinia amylovora, is an economically important and mechanistically complex disease that affects apple and pear production in most geographic production hubs worldwide. We compile, assess, and present a genetic outlook on the progression of an E. amylovora infection in the host. We discuss the key aspects of type III secretion-mediated infection and systemic movement, biofilm formation in xylem, and pathogen dispersal via ooze droplets, a concentrated suspension of bacteria and exopolysaccharide components. We present an overall outlook on the genetic elements contributing to E. amylovora pathogenesis, including an exploration of the impact of floral microbiomes on E. amylovora colonization, and summarize the current knowledge of host responses to an incursion and how this response stimulates further infection and systemic spread. We hope to facilitate the identification of new, unexplored areas of research in this pathosystem that can help identify evolutionarily susceptible genetic targets to ultimately aid in the design of sustainable strategies for fire blight disease mitigation.

The Rcs Stress Response System Regulator GumB Modulates Serratia marcescens-Induced Inflammation and Bacterial Proliferation in a Rabbit Keratitis Model and Cytotoxicity In Vitro

In this study, we tested the hypothesis that the conserved bacterial IgaA-family protein, GumB, mediates microbial pathogenesis associated with Serratia marcescens ocular infections through regulation of the Rcs stress response system. The role of the Rcs system and bacterial stress response systems for microbial keratitis is not known, and the role of IgaA proteins in mammalian pathogenesis models has only been tested with partial-function allele variants of Salmonella. Here, we observed that an Rcs-activated gumB mutant had a >50-fold reduction in proliferation compared to the wild type within rabbit corneas at 48 h and demonstrated a notable reduction in inflammation based on inflammatory signs, including the absence of hypopyons, and proinflammatory markers measured at the RNA and protein levels. The gumB mutant phenotypes could be complemented by wild-type gumB on a plasmid. We observed that bacteria with an inactivated Rcs stress response system induced high levels of ocular inflammation and restored corneal virulence to the gumB mutant. The high virulence of the ΔrcsB mutant was dependent upon the ShlA cytolysin transporter ShlB. Similar results were found for testing the cytotoxic effects of wild-type and mutant bacteria on a human corneal epithelial cell line in vitro. Together, these data indicate that GumB regulates virulence factor production through the Rcs system, and this overall stress response system is a key mediator of a bacterium's ability to induce vision-threatening keratitis.

Conservation of Erwinia amylovora pathogenicity-relevant genes among Erwinia genomes

The Erwinia genus comprises species that are plant pathogens, non-pathogen, epiphytes, and opportunistic human pathogens. Within the genus, Erwinia amylovora ranks among the top 10 plant pathogenic bacteria. It causes the fire blight disease and is a global threat to commercial apple and pear production. We analyzed the presence/absence of the E. amylovora genes reported to be important for pathogenicity towards Rosaceae within various Erwinia strains genomes. This simple bottom-up approach, allowed us to correlate the analyzed genes to pathogenicity, host specificity, and make useful considerations to drive targeted studies.

Bacterial hybrid histidine kinases in plant-bacteria interactions

Two-component signal transduction systems are essential for many bacteria to maintain homeostasis and adapt to environmental changes. Two-component signal transduction systems typically involve a membrane-bound histidine kinase that senses stimuli, autophosphorylates in the transmitter region and then transfers the phosphoryl group to the receiver domain of a cytoplasmic response regulator that mediates appropriate changes in bacterial physiology. Although usually found on distinct proteins, the transmitter and receiver modules are sometimes fused into a so-called hybrid histidine kinase (HyHK). Such structure results in multiple phosphate transfers that are believed to provide extra-fine-tuning mechanisms and more regulatory checkpoints than classical phosphotransfers. HyHK-based regulation may be crucial for finely tuning gene expression in a heterogeneous environment such as the rhizosphere, where intricate plant-bacteria interactions occur. In this review, we focus on roles fulfilled by bacterial HyHKs in plant-associated bacteria, providing recent findings on the mechanistic of their signalling properties. Recent insights into understanding additive regulatory properties fulfilled by the tethered receiver domain of HyHKs are also addressed.

Identification of the HrpS binding site in the hrpL promoter and effect of the RpoN binding site of HrpS on the regulation of the type III secretion system in Erwinia amylovora

The type III secretion system (T3SS) is a key pathogenicity factor in Erwinia amylovora. Previous studies have demonstrated that the T3SS in E. amylovora is transcriptionally regulated by an RpoN-HrpL sigma factor cascade, which is activated by the bacterial alarmone (p)ppGpp. In this study, the binding site of HrpS, an enhancer binding protein, was identified for the first time in plant-pathogenic bacteria. Complementation of the hrpL mutant with promoter deletion constructs of the hrpL gene and promoter activity analyses using various lengths of the hrpL promoter fused to a promoter-less green fluorescent protein (gfp) reporter gene delineated the upstream region for HrpS binding. Sequence analysis revealed a dyad symmetry sequence between -138 and -125 nucleotides (TGCAA-N4-TTGCA) as the potential HrpS binding site, which is conserved in the promoter of the hrpL gene among plant enterobacterial pathogens. Results of quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) and electrophoresis mobility shift assay coupled with site-directed mutagenesis (SDM) analysis showed that the intact dyad symmetry sequence was essential for HrpS binding, full activation of T3SS gene expression and virulence. In addition, the role of the GAYTGA motif (RpoN binding site) of HrpS in the regulation of T3SS gene expression in E. amylovora was characterized by complementation of the hrpS mutant using mutant variants generated by SDM. Results showed that a Y100F substitution of HrpS complemented the hrpS mutant, whereas Y100A and Y101A substitutions did not. These results suggest that tyrosine (Y) and phenylalanine (F) function interchangeably in the conserved GAYTGA motif of HrpS in E. amylovora.

A comparative proteome analysis reveals flagellin, chemotaxis regulated proteins and amylovoran to be involved in virulence differences between Erwinia amylovora strains

Erwinia amylovora is a Gram-negative bacterium that causes the destructive disease fire blight affecting most members of the Rosaceae family, of which apple and pear are economically the most important hosts. E. amylovora has been considered as a homogeneous species in whole, although significant differences in virulence patterns have been observed. However, the underlying causes of the differences in virulence remain to be discovered. In a first-time comparative proteomic approach using E. amylovora, 2D differential in-gel electrophoresis (DIGE) was used to identify proteins that could explain the gradual difference in virulence between four different strains. Two important proteins were identified, FliC and CheY, both involved in flagella structure, motility and chemotaxis, which were more abundant in the least virulent strain. In the highly virulent strains the protein GalF, involved in amylovoran production, was more abundant, which was consistent with the higher expression of the gene and the higher amylovoran content in this strain in vitro. Together, these results confirm the involvement of amylovoran in virulence, but also imply an indirect role of flagellin in virulence as elicitor of plant defence.

BIOLOGICAL SIGNIFICANCE

This research provides new insights into our current understanding of the virulence of Erwinia amylovora. This plant-pathogen is considered a homogeneous species although different strains show differences in virulence. Despite the efforts made on the genomic level which resulted in the discovery of virulence factors, the reason for the different virulence patterns between strains has not yet been identified. In our lab we used a comparative proteomic approach, which has never been published before, to identify proteins involved in these differences between strains and hereby possibly involved in virulence. Our results provide interesting insights in virulence and present us with the opportunity to glance into the proteome of E. amylovora.

The bacterial alarmone (p)ppGpp activates the type III secretion system in Erwinia amylovora

The hypersensitive response and pathogenicity (hrp) type III secretion system (T3SS) is a key pathogenicity factor in Erwinia amylovora. Previous studies have demonstrated that the T3SS in E. amylovora is transcriptionally regulated by a sigma factor cascade. In this study, the role of the bacterial alarmone ppGpp in activating the T3SS and virulence of E. amylovora was investigated using ppGpp mutants generated by Red recombinase cloning. The virulence of a ppGpp-deficient mutant (ppGpp(0)) as well as a dksA mutant of E. amylovora was completely impaired, and bacterial growth was significantly reduced, suggesting that ppGpp is required for full virulence of E. amylovora. Expression of T3SS genes was greatly downregulated in the ppGpp(0) and dksA mutants. Western blotting showed that accumulations of the HrpA protein in the ppGpp(0) and dksA mutants were about 10 and 4%, respectively, of that in the wild-type strain. Furthermore, higher levels of ppGpp resulted in a reduced cell size of E. amylovora. Moreover, serine hydroxamate and α-methylglucoside, which induce amino acid and carbon starvation, respectively, activated hrpA and hrpL promoter activities in hrp-inducing minimal medium. These results demonstrated that ppGpp and DksA play central roles in E. amylovora virulence and indicated that E. amylovora utilizes ppGpp as an internal messenger to sense environmental/nutritional stimuli for regulation of the T3SS and virulence.

IMPORTANCE

The type III secretion system (T3SS) is a key pathogenicity factor in Gram-negative bacteria. Fully elucidating how the T3SS is activated is crucial for comprehensively understanding the function of the T3SS, bacterial pathogenesis, and survival under stress conditions. In this study, we present the first evidence that the bacterial alarmone ppGpp-mediated stringent response activates the T3SS through a sigma factor cascade, indicating that ppGpp acts as an internal messenger to sense environmental/nutritional stimuli for the regulation of the T3SS and virulence in plant-pathogenic bacteria. Furthermore, the recovery of an spoT null mutant, which displayed very unique phenotypes, suggested that small proteins containing a single ppGpp hydrolase domain are functional.

Conserved aspartate and lysine residues of RcsB are required for amylovoran biosynthesis, virulence, and DNA binding in Erwinia amylovora

In Erwinia amylovora, the Rcs phosphorelay system is essential for amylovoran production and virulence. To further understand the role of conserved aspartate residue (D56) in the phosphor receiver (PR) domain and lysine (K180) residue in the function domain of RcsB, amino acid substitutions of RcsB mutant alleles were generated by site-directed mutagenesis and complementation of various rcs mutants were performed. A D56E substitution of RcsB, which mimics the phosphorylation state of RcsB, complemented the rcsB mutant, resulting in increased amylovoran production and gene expression, reduced swarming motility, and restored pathogenicity. In contrast, D56N and K180A or K180Q substitutions of RcsB did not complement the rcsB mutant. Electrophoresis mobility shift assays showed that D56E, but not D56N, K180Q and K180A substitutions of RcsB bound to promoters of amsG and flhD, indicating that both D56 and K180 are required for DNA binding. Interestingly, the RcsBD56E allele could also complement rcsAB, rcsBC and rcsABCD mutants with restored virulence and increased amylovoran production, indicating that RcsB phosphorylation is essential for virulence of E. amylovora. In addition, mutations of T904 and A905, but not phosphorylation mimic mutation of D876 in the PR domain of RcsC, constitutively activate the Rcs system, suggesting that phosphor transfer is required for activating the Rcs system and indicating both A905 and T904 are required for the phosphatase activity of RcsC. Our results demonstrated that RcsB phosphorylation and dephosphorylation, phosphor transfer from RcsC are essential for the function of the Rcs system, and also suggested that constitutive activation of the Rcs system could reduce the fitness of E. amylovora.

Cloning, purification, crystallization and 1.57 Å resolution X-ray data analysis of AmsI, the tyrosine phosphatase controlling amylovoran biosynthesis in the plant pathogen Erwinia amylovora

The Gram-negative bacterium Erwinia amylovora is a destructive pathogen of plants belonging to the Rosaceae family. Amongst its pathogenicity factors, E. amylovora produces the exopolysaccharide amylovoran, which contributes to the occlusion of plant vessels, causing wilting of shoots and eventually resulting in plant death. Amylovoran biosynthesis requires the presence of 12 genes (from amsA to amsL) clustered in the ams region of the E. amylovora genome. They mostly encode glycosyl transferases (AmsG, AmsB, AmsD, AmsE, AmsJ and AmsK), proteins involved in amylovoran translocation and assembly (AmsH, AmsL and AmsC), and also a tyrosine kinase (AmsA) and a tyrosine phosphatase (AmsI), which are both involved in the regulation of amylovoran biosynthesis. The low-molecular-weight protein tyrosine phosphatase AmsI was overexpressed as a His6-tagged protein in Escherichia coli, purified and crystallized. X-ray diffraction data were collected to a maximum resolution of 1.57 Å in space group P3121.

Genome-wide identification of Hfq-regulated small RNAs in the fire blight pathogen Erwinia amylovora discovered small RNAs with virulence regulatory function

BACKGROUND

Erwinia amylovora is a phytopathogenic bacterium and causal agent of fire blight disease in apples and pears. Although many virulence factors have been characterized, the coordination of expression of these virulence factors in E. amylovora is still not clear. Regulatory small RNAs (sRNAs) are important post-transcriptional regulatory components in bacteria. A large number of sRNAs require the RNA chaperone Hfq for both stability and functional activation. In E. amylovora, Hfq was identified as a major regulator of virulence and various virulence traits. However, information is still lacking about Hfq-dependent sRNAs on a genome scale, including the virulence regulatory functions of these sRNAs in E. amylovora.

RESULTS

Using both an RNA-seq analysis and a Rho-independent terminator search, 40 candidate Hfq-dependent sRNAs were identified in E. amylovora. The expression and sizes of 12 sRNAs and the sequence boundaries of seven sRNAs were confirmed by Northern blot and 5' RACE assay respectively. Sequence conservation analysis identified sRNAs conserved only in the Erwinia genus as well as E. amylovora species-specific sRNAs. In addition, a dynamic re-patterning of expression of Hfq-dependent sRNAs was observed at 6 and 12 hours after induction in Hrp-inducing minimal medium. Furthermore, sRNAs that control virulence traits were characterized, among which ArcZ positively controls the type III secretion system (T3SS), amylovoran exopolysaccahride production, biofilm formation, and motility, and negatively modulates attachment while RmaA (Hrs6) and OmrAB both negatively regulate amylovoran production and positively regulate motility.

CONCLUSIONS

This study has significantly enhanced our understanding of the Hfq-dependent sRNAs in E. amylovora at the genome level. The identification of multiple virulence-regulating sRNAs also suggests that post-transcriptional regulation by sRNAs may play a role in the deployment of virulence factors needed during varying stages of pathogenesis during host invasion by E. amylovora.

Co-regulation of polysaccharide production, motility, and expression of type III secretion genes by EnvZ/OmpR and GrrS/GrrA systems in Erwinia amylovora

The EnvZ/OmpR and GrrS/GrrA systems, two widely distributed two-component systems in gamma-Proteobacteria, negatively control amylovoran biosynthesis in Erwinia amylovora, and the two systems regulate motility in an opposing manner. In this study, we examined the interplay of EnvZ/OmpR and GrrS/GrrA systems in controlling various virulence traits in E. amylovora. Results showed that amylovoran production was significantly higher when both systems were inactivated, indicating that the two systems act as negative regulators and their combined effect on amylovoran production appears to be enhanced. In contrast, reduced motility was observed when both systems were deleted as compared to that of grrA/grrS mutants and WT strain, indicating that the two systems antagonistically regulate motility in E. amylovora. In addition, glycogen accumulation was much higher in envZ/ompR and two triple mutants than that of grrS/grrA mutants and WT strain, suggesting that EnvZ/OmpR plays a dominant role in regulating glycogen accumulation, whereas levan production was significantly lower in the grrS/grrA and two triple mutants as compared with that of WT and envZ/ompR mutants, indicating that GrrS/GrrA system dominantly controls levan production. Furthermore, both systems negatively regulated expression of three type III secretion (T3SS) genes and their combined negative effect on hrp-T3SS gene expression increased when both systems were deleted. These results demonstrated that EnvZ/OmpR and GrrS/GrrA systems co-regulate various virulence factors in E. amylovora by still unknown mechanisms or through different target genes, sRNAs, or proteins, indicating that a complex regulatory network may be involved, which needs to be further explored.

Global small RNA chaperone Hfq and regulatory small RNAs are important virulence regulators in Erwinia amylovora

Hfq is a global small RNA (sRNA) chaperone that interacts with Hfq-regulated sRNAs and functions in the posttranscriptional regulation of gene expression. In this work, we identified Hfq to be a virulence regulator in the Gram-negative fire blight pathogen Erwinia amylovora. Deletion of hfq in E. amylovora Ea1189 significantly reduced bacterial virulence in both immature pear fruits and apple shoots. Analysis of virulence determinants in strain Ea1189Δhfq showed that Hfq exerts pleiotropic regulation of amylovoran exopolysaccharide production, biofilm formation, motility, and the type III secretion system (T3SS). Further characterization of biofilm regulation by Hfq demonstrated that Hfq limits bacterial attachment to solid surfaces while promoting biofilm maturation. Characterization of T3SS regulation by Hfq revealed that Hfq positively regulates the translocation and secretion of the major type III effector DspE and negatively controls the secretion of the putative translocator HrpK and the type III effector Eop1. Lastly, 10 Hfq-regulated sRNAs were identified using a computational method, and two of these sRNAs, RprA and RyhA, were found to be required for the full virulence of E. amylovora.

AmyR is a novel negative regulator of amylovoran production in Erwinia amylovora

In this study, we attempted to understand the role of an orphan gene amyR in Erwinia amylovora, a functionally conserved ortholog of ybjN in Escherichia coli, which has recently been characterized. Amylovoran, a high molecular weight acidic heteropolymer exopolysaccharide, is a virulent factor of E. amylovora. As reported earlier, amylovoran production in an amyR knockout mutant was about eight-fold higher than that in the wild type (WT) strain of E. amylovora. When a multicopy plasmid containing the amyR gene was introduced into the amyR mutant or WT strains, amylovoran production was strongly inhibited. Furthermore, amylovoran production was also suppressed in various amylovoran-over-producing mutants, such as grrSA containing multicopies of the amyR gene. Consistent with amylovoran production, an inverse correlation was observed between in vitro expression of amyR and that of amylovoran biosynthetic genes. However, both the amyR knockout mutant and over-expression strains showed reduced levan production, another exopolysaccharide produced by E. amylovora. Virulence assays demonstrated that while the amyR mutant was capable of inducing slightly greater disease severity than that of the WT strain, strains over-expressing the amyR gene did not incite disease on apple shoots or leaves, and only caused reduced disease on immature pear fruits. Microarray studies revealed that amylovoran biosynthesis and related membrane protein-encoding genes were highly expressed in the amyR mutant, but down-regulated in the amyR over-expression strains in vitro. Down-regulation of amylovoran biosynthesis genes in the amyR over-expression strain partially explained why over-expression of amyR led to non-pathogenic or reduced virulence in vivo. These results suggest that AmyR plays an important role in regulating exopolysaccharide production, and thus virulence in E. amylovora.

Multiple domains of the tobacco mosaic virus p126 protein can independently suppress local and systemic RNA silencing

Small RNA-mediated RNA silencing is a widespread antiviral mechanism in plants and other organisms. Many viruses encode suppressors of RNA silencing for counter-defense. The p126 protein encoded by Tobacco mosaic virus (TMV) has been reported to be a suppressor of RNA silencing but the mechanism of its function remains unclear. This protein is unique among the known plant viral silencing suppressors because of its large size and multiple domains. Here, we report that the methyltransferase, helicase, and nonconserved region II (NONII) of p126 each has silencing-suppressor function. The silencing-suppression activities of methyltransferase and helicase can be uncoupled from their enzyme activities. Specific amino acids in NONII previously shown to be crucial for viral accumulation and symptom development are also crucial for silencing suppression. These results suggest that some viral proteins have evolved to possess modular structural domains that can independently interfere with host silencing, and that this may be an effective mechanism of increasing the robustness of a virus.

Differential regulation of phenazine biosynthesis by RpeA and RpeB in Pseudomonas chlororaphis 30-84

RpeA is a two-component sensor protein that negatively controls biosynthesis of phenazines, which are required for biological control activity by Pseudomonas chlororaphis 30-84. In this study, we identified the cognate response regulator RpeB and investigated how RpeA and RpeB interact with the PhzR/PhzI quorum sensing system and other known regulatory genes to control phenazine production. Quantitative real-time PCR revealed that, in contrast with an rpeA mutant, expression of the phenazine biosynthetic genes as well as the pip and phzR genes were significantly reduced in an rpeB mutant, suggesting positive control of phenazines by RpeB. Complementation assays showed that overexpression of pip in trans rescued phenazine production in an rpeB mutant, whereas multiple copies of rpeB genes were unable to restore phenazine production in a pip or phzR mutant. These results indicate that RpeA and RpeB differentially regulate phenazine production and act upstream of Pip and PhzR in the phenazine regulatory network. The differential regulatory functions for RpeA and RpeB also affected the capacity of 30-84 for fungal inhibition. Based on these results, a model is proposed to illustrate the relationship of RpeA/RpeB to other regulatory genes controlling phenazine biosynthesis in P. chlororaphis 30-84, a regulatory hierarchy that may be conserved in other pseudomonads and may play a role in stress response.

The orphan gene ybjN conveys pleiotropic effects on multicellular behavior and survival of Escherichia coli

YbjN, encoding an enterobacteria-specific protein, is a multicopy suppressor of temperature sensitivity in the ts9 mutant strain of Escherichia coli. In this study, we further explored the role(s) of ybjN. First, we demonstrated that the ybjN transcript was about 10-fold lower in the ts9 strain compared to that of E. coli strain BW25113 (BW). Introduction of multiple copies of ybjN in the ts9 strain resulted in over-expression of ybjN by about 10-fold as compared to that of BW. These results suggested that temperature sensitivity of the ts9 mutant of E. coli may be related to expression levels of ybjN. Characterization of E. coli ybjN mutant revealed that ybjN mutation resulted in pleiotropic phenotypes, including increased motility, fimbriation (auto-aggregation), exopolysaccharide production, and biofilm formation. In contrast, over-expression of ybjN (in terms of multiple copies) resulted in reduced motility, fimbriation, exopolysaccharide production, biofilm formation and acid resistance. In addition, our results indicate that a ybjN-homolog gene from Erwinia amylovora, a plant enterobacterial pathogen, is functionally conserved with that of E. coli, suggesting similar evolution of the YbjN family proteins in enterobacteria. A microarray study revealed that the expression level of ybjN was inversely correlated with the expression of flagellar, fimbrial and acid resistance genes. Over-expression of ybjN significantly down-regulated genes involved in citric acid cycle, glycolysis, the glyoxylate shunt, oxidative phosphorylation, amino acid and nucleotide metabolism. Furthermore, over-expression of ybjN up-regulated toxin-antitoxin modules, the SOS response pathway, cold shock and starvation induced transporter genes. Collectively, these results suggest that YbjN may play important roles in regulating bacterial multicellular behavior, metabolism, and survival under stress conditions in E. coli. These results also suggest that ybjN over-expression-related temperature rescue of the ts9 mutant may be due to down-regulation of metabolic activity and activation of stress response genes in the ts9 mutant.

Comparative Genomics of Erwinia amylovora and Related Erwinia Species-What do We Learn?

Erwinia amylovora, the causal agent of fire blight disease of apples and pears, is one of the most important plant bacterial pathogens with worldwide economic significance. Recent reports on the complete or draft genome sequences of four species in the genus Erwinia, including E. amylovora, E. pyrifoliae, E. tasmaniensis, and E. billingiae, have provided us near complete genetic information about this pathogen and its closely-related species. This review describes in silico subtractive hybridization-based comparative genomic analyses of eight genomes currently available, and highlights what we have learned from these comparative analyses, as well as genetic and functional genomic studies. Sequence analyses reinforce the assumption that E. amylovora is a relatively homogeneous species and support the current classification scheme of E. amylovora and its related species. The potential evolutionary origin of these Erwinia species is also proposed. The current understanding of the pathogen, its virulence mechanism and host specificity from genome sequencing data is summarized. Future research directions are also suggested.