Comprehensive mapping of the Escherichia coli flagellar regulatory network

Genomic Analysis of the Endophytic Stenotrophomonas Strain 169 Reveals Features Related to Plant-Growth Promotion and Stress Tolerance

Plant-associated Stenotrophomonas isolates have great potential for plant growth promotion, especially under stress conditions, due to their ability to promote tolerance to abiotic stresses such as salinity or drought. The endophytic strain Stenotrophomonas sp. 169, isolated from a field-grown poplar, increased the growth of inoculated in vitro plants, with a particular effect on root development, and was able to stimulate the rooting of poplar cuttings in the greenhouse. The strain produced high amounts of the plant growth-stimulating hormone auxin under in vitro conditions. The comparison of the 16S rRNA gene sequences and the phylogenetic analysis of the core genomes showed a close relationship to Stenotrophomonas chelatiphaga and a clear separation from Stenotrophomonas maltophilia. Whole genome sequence analysis revealed functional genes potentially associated with attachment and plant colonization, growth promotion, and stress protection. In detail, an extensive set of genes for twitching motility, chemotaxis, flagella biosynthesis, and the ability to form biofilms, which are connected with host plant colonization, could be identified in the genome of strain 169. The production of indole-3-acetic acid and the presence of genes for auxin biosynthesis pathways and the spermidine pathway could explain the ability to promote plant growth. Furthermore, the genome contained genes encoding for features related to the production of different osmoprotective molecules and enzymes mediating the regulation of stress tolerance and the ability of bacteria to quickly adapt to changing environments. Overall, the results of physiological tests and genome analysis demonstrated the capability of endophytic strain 169 to promote plant growth. In contrast to related species, strain 169 can be considered non-pathogenic and suitable for biotechnology applications.

Roles of OmpX, an Outer Membrane Protein, on Virulence and Flagellar Expression in Uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) is a major pathogen that causes urinary tract infection (UTI). This bacterium adheres to and internalizes within urinary tract cells, where it aggregates and subsequently forms biofilm-like multicellular colonies that protect UPEC from antimicrobial agents and the host's immune system. Here, we show that OmpX, an outer membrane protein, plays a role in the pathogenesis of UPEC in renal cells. Deletion of ompX decreased bacterial internalization and aggregation within kidney epithelial cells and also impaired the colonization of mouse urinary tracts, but the ompX mutant still adhered to the epithelial cells at a level similar to that of the parent strain. FlhD, the master regulator of flagellum-related genes, had a low expression level in the ompX mutant compared to the parent strain, and the ompX mutant exhibited defective motility due to lower flagellar production than the parent strain. The fliC mutant, which lacks flagella, exhibited lower levels of bacterial internalization and aggregation than the parent strain. Additional deletion of ompX in the fliC mutant did not further decrease bacterial internalization. These combined results suggest that OmpX contributes to flagellar production in UPEC and then sustains UPEC virulence associated with bacterial internalization and aggregation within urinary tract cells and colonization in the urinary tract.

Improved Yield of Recombinant Protein via Flagella Regulator Deletion in Escherichia coli

Protein production requires a significant amount of intracellular energy. Eliminating the flagella has been proposed to help Escherichia coli improve protein production by reducing energy consumption. In this study, the gene encoding a subunit of FlhC, a master regulator of flagella assembly, was deleted to reduce the expression of flagella-related genes. FlhC knockout in the ptsG-deleted strain triggered significant growth retardation with increased ATP levels and a higher NADPH/NADP⁺ ratio. Metabolic flux analysis using a ¹³C-labeled carbon substrate showed increased fluxes toward the pentose phosphate and tricarboxylic acid cycle pathways in the flhC- and ptsG-deleted strains. Introduction of a high copy number plasmid or overexpression of the recombinant protein in this strain restored growth rate without increasing glucose consumption. These results suggest that the metabolic burden caused by flhC deletion was resolved by recombinant protein production. The recombinant enhanced green fluorescent protein yield per glucose consumption increased 1.81-fold in the flhC mutant strain. Thus, our study demonstrates that high-yield production of the recombinant protein was achieved with reduced flagella formation.

High-throughput transcriptome sequencing and comparative analysis of Escherichia coli and Schizosaccharomyces pombe in respiratory and fermentative growth

In spite of increased complexity in eukaryotes compared to prokaryotes, several basic metabolic and regulatory processes are conserved. Here we explored analogies in the eubacteria Escherichia coli and the unicellular fission yeast Schizosaccharomyces pombe transcriptomes under two carbon sources: 2% glucose; or a mix of 2% glycerol and 0.2% sodium acetate using the same growth media and growth phase. Overall, twelve RNA-seq libraries were constructed. A total of 593 and 860 genes were detected as differentially expressed for E. coli and S. pombe, respectively, with a log2 of the Fold Change ≥ 1 and False Discovery Rate ≤ 0.05. In aerobic glycolysis, most of the expressed genes were associated with cell proliferation in both organisms, including amino acid metabolism and glycolysis. In contrast in glycerol/acetate condition, genes related to flagellar assembly and membrane proteins were differentially expressed such as the general transcription factors fliA, flhD, flhC, and flagellum assembly genes were detected in E. coli, whereas in S. pombe genes for hexose transporters, integral membrane proteins, galactose metabolism, and ncRNAs related to cellular stress were overexpressed. In general, our study shows that a conserved "foraging behavior" response is observed in these eukaryotic and eubacterial organisms in gluconeogenic carbon sources.

The σ54 system directly regulates bacterial natural product genes

Bacterial-derived polyketide and non-ribosomal peptide natural products are crucial sources of therapeutics and yet little is known about the conditions that favor activation of natural product genes or the regulatory machinery controlling their transcription. Recent findings suggest that the σ⁵⁴ system, which includes σ⁵⁴-loaded RNA polymerase and transcriptional activators called enhancer binding proteins (EBPs), might be a common regulator of natural product genes. Here, we explored this idea by analyzing a selected group of putative σ⁵⁴ promoters identified in Myxococcus xanthus natural product gene clusters. We show that mutations in putative σ⁵⁴-RNA polymerase binding regions and in putative Nla28 EBP binding sites dramatically reduce in vivo promoter activities in growing and developing cells. We also show in vivo promoter activities are reduced in a nla28 mutant, that Nla28 binds to wild-type fragments of these promoters in vitro, and that in vitro binding is lost when the Nla28 binding sites are mutated. Together, our results indicate that M. xanthus uses σ⁵⁴ promoters for transcription of at least some of its natural product genes. Interestingly, the vast majority of experimentally confirmed and putative σ⁵⁴ promoters in M. xanthus natural product loci are located within genes and not in intergenic sequences.

Synergistic Activity of Equol and Meropenem against Carbapenem-Resistant Escherichia coli

The emergence of carbapenem-resistant Enterobacterales (CRE) seriously limits treatment options for bacterial infections. Combined drugs are an effective strategy to treat these resistant strains. This study aimed to evaluate the synergistic effect of equol and meropenem against carbapenem-resistant Escherichia coli. First, this study investigated the antibacterial activity of carbapenems on clinically isolated E. coli strains by analyzing the minimum inhibitory concentrations (MICs). The E. coli strains were all resistant to carbapenem antibiotics. Therefore, we confirmed the cause of carbapenem resistance by detecting bla_KPC and bla_OXA-48 among the carbapenemase genes using polymerase chain reaction (PCR) analysis. Checkerboard and time-kill analyses confirmed that equol restored the susceptibility of carbapenem-resistant E. coli to meropenem. Also, the transcription levels of specific carbapenemase genes in E. coli were significantly suppressed by equol. The study also evaluated the anti-virulence effects of equol on bacterial biofilm and motility through phenotypic and genotypic analyses. In conclusion, our results revealed that equol had a synergistic effect with meropenem on carbapenem-resistant E. coli. Therefore, this study suggests that equol is a promising antibiotic adjuvant that prevents the expression of carbapenemases and virulence factors in carbapenem-resistant E. coli.

Regulatory Cross Talk between Motility and Interbacterial Communication in Salmonella enterica Serovar Typhimurium

FliA is a broadly conserved σ factor that directs transcription of genes involved in flagellar motility. We previously identified FliA-transcribed genes in Escherichia coli and Salmonella enterica serovar Typhimurium, and we showed that E. coli FliA transcribes many unstable, noncoding RNAs from intragenic promoters. Here, we show that FliA in S Typhimurium also directs the transcription of large numbers of unstable, noncoding RNAs from intragenic promoters, and we identify two previously unreported FliA-transcribed protein-coding genes. One of these genes, sdiA, encodes a transcription factor that responds to quorum-sensing signals produced by other bacteria. We show that FliA-dependent transcription of sdiA is required for SdiA activity, highlighting a regulatory link between flagellar motility and intercellular communication.IMPORTANCE Initiation of bacterial transcription requires association of a σ factor with the core RNA polymerase to facilitate sequence-specific recognition of promoter elements. FliA is a widely conserved σ factor that directs transcription of genes involved in flagellar motility. We previously showed that Escherichia coli FliA transcribes many unstable, noncoding RNAs from promoters within genes. Here, we demonstrate the same phenomenon in Salmonella Typhimurium. We also show that S Typhimurium FliA directs transcription of the sdiA gene, which encodes a transcription factor that responds to quorum-sensing signals produced by other bacteria. FliA-dependent transcription of sdiA is required for transcriptional control of SdiA target genes, highlighting a regulatory link between flagellar motility and intercellular communication.

Restoration of wild-type motility to flagellin-knockout Escherichia coli by varying promoter, copy number and induction strength in plasmid-based expression of flagellin

Flagellin is the major constituent of the flagellar filament and faithful restoration of wild-type motility to flagellin mutants may be beneficial for studies of flagellar biology and biotechnological exploitation of the flagellar system. However, gene complementation studies often fail to report whether true wild-type motility was restored by expressing flagellin from a plasmid. Therefore, we explored the restoration of motility by flagellin expressed from a variety of combinations of promoter, plasmid copy number and induction strength. Motility was only partially (~50%) restored using the tightly regulated rhamnose promoter due to weak flagellin gene expression, but wild-type motility was regained with the T5 promoter, which, although leaky, allowed titration of induction strength. The endogenous E. coli flagellin promoter also restored wild-type motility. However, flagellin gene transcription levels increased 3.1–27.9-fold when wild-type motility was restored, indicating disturbances in the flagellar regulatory mechanisms. Motility was little affected by plasmid copy number when dependent on inducible promoters. However, plasmid copy number was important when expression was controlled by the native E. coli flagellin promoter. Motility was poorly correlated with flagellin transcription levels, but strongly correlated with the amount of flagellin associated with the flagellar filament, suggesting that excess monomers are either not exported or not assembled into filaments. This study provides a useful reference for further studies of flagellar function and a simple blueprint for similar studies with other proteins.

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Restoration of motility to flagellin-knockout E. coli depends on choice of promoter.

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Plasmid copy number is important when using the natural flagellin promoter.

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For inducible promoters, induction strength is more important than copy number.

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Large increase in flagellin transcription but not flagella-associated protein.

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Plasmid-based expression interrupts flagellin expression control mechanisms.

The transcription regulator and c-di-GMP phosphodiesterase PdeL represses motility in Escherichia coli

PdeL is a transcription regulator and catalytically active c-di-GMP phosphodiesterases (PDE) in Escherichia coli PdeL has been shown to be a transcription autoregulator, while no other target genes have been identified so far. Here, we show that PdeL represses transcription of the flagella class II operon, fliFGHIJK, and activates sslE encoding an extracellular anchored metalloprotease, among additional loci. DNA-binding studies and expression analyses using plasmidic reporters suggest that regulation of the fliF and sslE promoters by PdeL is direct. Transcription repression of the fliFGHIJK operon, encoding protein required for assembly of the flagellar basal body, results in inhibition of motility on soft agar plates and reduction of flagella assembly, as shown by fluorescence staining of the flagella hook protein FlgE. PdeL-mediated repression of motility is independent of its phosphodiesterase activity. Thus, in motility control the transcription regulator function of PdeL reducing the number of assembled flagella is apparently epistatic to its phosphodiesterase function, which can indirectly promote the activity of the flagellar motor by lowering the c-di-GMP concentration.Bacteria adopt different lifestyles depending on their environment and physiological condition. In Escherichia coli and other enteric bacteria the transition between the motile and the sessile state is controlled at multiple levels from the regulation of gene expression to the modulation of various processes by the second messenger c-di-GMP as signaling molecule. The significance of our research is in identifying PdeL, a protein of dual function that hydrolyzes c-di-GMP and that regulates transcription of genes, as a repressor of Flagella gene expression and an inhibitor of motility, which adds an additional regulatory switch to the control of motility.

Designing next generation recombinant protein expression platforms by modulating the cellular stress response in Escherichia coli

BACKGROUND

A cellular stress response (CSR) is triggered upon recombinant protein synthesis which acts as a global feedback regulator of protein expression. To remove this key regulatory bottleneck, we had previously proposed that genes that are up-regulated post induction could be part of the signaling pathways which activate the CSR. Knocking out some of these genes which were non-essential and belonged to the bottom of the E. coli regulatory network had provided higher expression of GFP and L-asparaginase.

RESULTS

We chose the best performing double knockout E. coli BW25113ΔelaAΔcysW and demonstrated its ability to enhance the expression of the toxic Rubella E1 glycoprotein by 2.5-fold by tagging it with sfGFP at the C-terminal end to better quantify expression levels. Transcriptomic analysis of this hyper-expressing mutant showed that a significantly lower proportion of genes got down-regulated post induction, which included genes for transcription, translation, protein folding and sorting, ribosome biogenesis, carbon metabolism, amino acid and ATP synthesis. This down-regulation which is a typical feature of the CSR was clearly blocked in the double knockout strain leading to its enhanced expression capability. Finally, we supplemented the expression of substrate uptake genes glpK and glpD whose down-regulation was not prevented in the double knockout, thus ameliorating almost all the negative effects of the CSR and obtained a further doubling in recombinant protein yields.

CONCLUSION

The study validated the hypothesis that these up-regulated genes act as signaling messengers which activate the CSR and thus, despite having no casual connection with recombinant protein synthesis, can improve cellular health and protein expression capabilities. Combining gene knockouts with supplementing the expression of key down-regulated genes can counter the harmful effects of CSR and help in the design of a truly superior host platform for recombinant protein expression.

Transcription termination and antitermination of bacterial CRISPR arrays

A hallmark of CRISPR-Cas immunity systems is the CRISPR array, a genomic locus consisting of short, repeated sequences ('repeats') interspersed with short, variable sequences ('spacers'). CRISPR arrays are transcribed and processed into individual CRISPR RNAs that each include a single spacer, and direct Cas proteins to complementary sequences in invading nucleic acid. Most bacterial CRISPR array transcripts are unusually long for untranslated RNA, suggesting the existence of mechanisms to prevent premature transcription termination by Rho, a conserved bacterial transcription termination factor that rapidly terminates untranslated RNA. We show that Rho can prematurely terminate transcription of bacterial CRISPR arrays, and we identify a widespread antitermination mechanism that antagonizes Rho to facilitate complete transcription of CRISPR arrays. Thus, our data highlight the importance of transcription termination and antitermination in the evolution of bacterial CRISPR-Cas systems.

Diverse Mechanisms and Circuitry for Global Regulation by the RNA-Binding Protein CsrA

The carbon storage regulator (Csr) or repressor of stationary phase metabolites (Rsm) system of Gammaproteobacteria is among the most complex and best-studied posttranscriptional regulatory systems. Based on a small RNA-binding protein, CsrA and homologs, it controls metabolism, physiology, and bacterial lifestyle decisions by regulating gene expression on a vast scale. Binding of CsrA to sequences containing conserved GGA motifs in mRNAs can regulate translation, RNA stability, riboswitch function, and transcript elongation. CsrA governs the expression of dozens of transcription factors and other regulators, further expanding its influence on cellular physiology, and these factors can participate in feedback to the Csr system. Expression of csrA itself is subject to autoregulation via translational inhibition and indirect transcriptional activation. CsrA activity is controlled by small noncoding RNAs (sRNAs), CsrB and CsrC in Escherichia coli, which contain multiple high affinity CsrA binding sites that compete with those of mRNA targets. Transcription of CsrB/C is induced by certain nutrient limitations, cellular stresses, and metabolites, while these RNAs are targeted for degradation by the presence of a preferred carbon source. Consistent with these findings, CsrA tends to activate pathways and processes that are associated with robust growth and repress stationary phase metabolism and stress responses. Regulatory loops between Csr components affect the signaling dynamics of the Csr system. Recently, systems-based approaches have greatly expanded our understanding of the roles played by CsrA, while reinforcing the notion that much remains to be learned about the Csr system.

Filtering input fluctuations in intensity and in time underlies stochastic transcriptional pulses without feedback

Stochastic pulsatile dynamics have been observed in an increasing number of biological circuits with known mechanism involving feedback control and bistability. Surprisingly, recent single-cell experiments in Escherichia coli flagellar synthesis showed that flagellar genes are activated in stochastic pulses without the means of feedback. However, the mechanism for pulse generation in these feedbackless circuits has remained unclear. Here, by developing a system-level stochastic model constrained by a large set of single-cell E. coli flagellar synthesis data from different strains and mutants, we identify the general underlying design principles for generating stochastic transcriptional pulses without feedback. Our study shows that an inhibitor (YdiV) of the transcription factor (FlhDC) creates a monotonic ultrasensitive switch that serves as a digital filter to eliminate small-amplitude FlhDC fluctuations. Furthermore, we find that the high-frequency (fast) fluctuations of FlhDC are filtered out by integration over a timescale longer than the timescale of the input fluctuations. Together, our results reveal a filter-and-integrate design for generating stochastic pulses without feedback. This filter-and-integrate mechanism enables a general strategy for cells to avoid premature activation of the expensive downstream gene expression by filtering input fluctuations in both intensity and time so that the system only responds to input signals that are both strong and persistent.

A Multi-Scale Approach to Modeling E. coli Chemotaxis

The degree to which we can understand the multi-scale organization of cellular life is tied to how well our models can represent this organization and the processes that drive its evolution. This paper uses Vivarium-an engine for composing heterogeneous computational biology models into integrated, multi-scale simulations. Vivarium's approach is demonstrated by combining several sub-models of biophysical processes into a model of chemotactic E. coli that exchange molecules with their environment, express the genes required for chemotaxis, swim, grow, and divide. This model is developed incrementally, highlighting cross-compartment mechanisms that link E. coli to its environment, with models for: (1) metabolism and transport, with transport moving nutrients across the membrane boundary and metabolism converting them to useful metabolites, (2) transcription, translation, complexation, and degradation, with stochastic mechanisms that read real gene sequence data and consume base pairs and ATP to make proteins and complexes, and (3) the activity of flagella and chemoreceptors, which together support navigation in the environment.

Engineering regulatory networks for complex phenotypes in E. coli

Regulatory networks describe the hierarchical relationship between transcription factors, associated proteins, and their target genes. Regulatory networks respond to environmental and genetic perturbations by reprogramming cellular metabolism. Here we design, construct, and map a comprehensive regulatory network library containing 110,120 specific mutations in 82 regulators expected to perturb metabolism. We screen the library for different targeted phenotypes, and identify mutants that confer strong resistance to various inhibitors, and/or enhanced production of target compounds. These improvements are identified in a single round of selection, showing that the regulatory network library is universally applicable and is convenient and effective for engineering targeted phenotypes. The facile construction and mapping of the regulatory network library provides a path for developing a more detailed understanding of global regulation in E. coli, with potential for adaptation and use in less-understood organisms, expanding toolkits for future strain engineering, synthetic biology, and broader efforts.

Interactions between DksA and Stress-Responsive Alternative Sigma Factors Control Inorganic Polyphosphate Accumulation in Escherichia coli

Bacteria synthesize inorganic polyphosphate (polyP) in response to a variety of different stress conditions. polyP protects bacteria by acting as a protein-stabilizing chaperone, metal chelator, or regulator of protein function, among other mechanisms. However, little is known about how stress signals are transmitted in the cell to lead to increased polyP accumulation. Previous work in the model enterobacterium Escherichia coli has indicated that the RNA polymerase-binding regulatory protein DksA is required for polyP synthesis in response to nutrient limitation stress. In this work, I set out to characterize the role of DksA in polyP regulation in more detail. I found that overexpression of DksA increases cellular polyP content (explaining the long-mysterious phenotype of dksA overexpression rescuing growth of a dnaK mutant at high temperatures) and characterized the roles of known functional residues of DksA in this process, finding that binding to RNA polymerase is required but that none of the other functions of DksA appear to be necessary. Transcriptomics revealed genome-wide transcriptional changes upon nutrient limitation, many of which were affected by DksA, and follow-up experiments identified complex interactions between DksA and the stress-sensing alternative sigma factors FliA, RpoN, and RpoE that impact polyP production, indicating that regulation of polyP synthesis is deeply entwined in the multifactorial stress response network of E. coli IMPORTANCE Inorganic polyphosphate (polyP) is an evolutionarily ancient, widely conserved biopolymer required for stress resistance and pathogenesis in diverse bacteria, but we do not understand how its synthesis is regulated. In this work, I gained new insights into this process by characterizing the role of the transcriptional regulator DksA in polyP regulation in Escherichia coli and identifying previously unknown links between polyP synthesis and the stress-responsive alternative sigma factors FliA, RpoN, and RpoE.

Structural basis of bacterial σ28 -mediated transcription reveals roles of the RNA polymerase zinc-binding domain

In bacteria, σ²⁸ is the flagella-specific sigma factor that targets RNA polymerase (RNAP) to control the expression of flagella-related genes involving bacterial motility and chemotaxis. However, the structural mechanism of σ²⁸ -dependent promoter recognition remains uncharacterized. Here, we report cryo-EM structures of E. coli σ²⁸ -dependent transcribing complexes on a complete flagella-specific promoter. These structures reveal how σ²⁸ -RNAP recognizes promoter DNA through strong interactions with the -10 element, but weak contacts with the -35 element, to initiate transcription. In addition, we observed a distinct architecture in which the β' zinc-binding domain (ZBD) of RNAP stretches out from its canonical position to interact with the upstream non-template strand. Further in vitro and in vivo assays demonstrate that this interaction has the overall effect of facilitating closed-to-open isomerization of the RNAP-promoter complex by compensating for the weak interaction between σ4 and -35 element. This suggests that ZBD relocation may be a general mechanism employed by σ⁷⁰ family factors to enhance transcription from promoters with weak σ4/-35 element interactions.

Transcriptional profiling of Vibrio cholerae O1 following exposure to human anti- lipopolysaccharide monoclonal antibodies

Following an episode of cholera, a rapidly dehydrating, watery diarrhea caused by the Gram-negative bacterium, Vibrio cholerae O1, humans mount a robust anti-lipopolysaccharide (LPS) antibody response that is associated with immunity to subsequent re-infection. In neonatal mouse and rabbit models of cholera, passively administered anti-LPS polyclonal and monoclonal (MAb) antibodies reduce V. cholerae colonization of the intestinal epithelia by inhibiting bacterial motility and promoting vibrio agglutination. Here we demonstrate that human anti-LPS IgG MAbs also arrest V. cholerae motility and induce bacterial paralysis. A subset of those MAbs also triggered V. cholerae to secrete an extracellular matrix (ECM). To identify changes in gene expression that accompany antibody exposure and that may account for motility arrest and ECM production, we subjected V. cholerae O1 El Tor to RNA-seq analysis after treatment with ZAC-3 IgG, a high affinity MAb directed against the core/lipid A region of LPS. We identified > 160 genes whose expression was altered following ZAC-3 IgG treatment, although canonical outer membrane stress regulons were not among them. ompS (VCA1028), a porin associated with virulence and indirectly regulated by ToxT, and norR (VCA0182), a σ54-dependent transcription factor involved in late stages of infection, were two upregulated genes worth noting.

Elucidating the Regulon of a Fur-like Protein in Mycobacterium avium subsp. paratuberculosis (MAP)

Intracellular iron concentration is tightly regulated to maintain cell viability. Iron plays important roles in electron transport, nucleic acid synthesis, and oxidative stress. A Mycobacterium avium subsp. paratuberculosis (MAP)-specific genomic island carries a putative metal transport operon that includes MAP3773c, which encodes a Fur-like protein. Although well characterized as a global regulator of iron homeostasis in multiple bacteria, the function of Fur (ferric uptake regulator) in MAP is unknown as this organism also carries IdeR (iron dependent regulator), a native iron regulatory protein specific to mycobacteria. Computational analysis using PRODORIC identified 23 different pathways involved in respiration, metabolism, and virulence that were likely regulated by MAP3773c. Thus, chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) was performed to confirm the putative regulon of MAP3773c (Fur-like protein) in MAP. ChIP-Seq revealed enriched binding to 58 regions by Fur under iron-replete and -deplete conditions, located mostly within open reading frames (ORFs). Three ChIP peaks were identified in genes that are directly related to iron regulation: MAP3638c (hemophore-like protein), MAP3736c (Fur box), and MAP3776c (ABC transporter). Fur box consensus sequence was identified, and binding specificity and dependence on Mn2+ availability was confirmed by a chemiluminescent electrophoresis mobility shift assay (EMSA). The results confirmed that MAP3773c is a Fur ortholog that recognizes a 19 bp DNA sequence motif (Fur box) and it is involved in metal homeostasis. This work provides a regulatory network of MAP Fur binding sites during iron-replete and -deplete conditions, highlighting unique properties of Fur regulon in MAP.

A LuxR-type regulator, AcrR, regulates flagellar assembly and contributes to virulence, motility, biofilm formation, and growth ability of Acidovorax citrulli

LuxR-type regulators regulate many bacterial processes and play important roles in bacterial motility and virulence. Acidovorax citrulli is a seedborne bacterial pathogen responsible for bacterial fruit blotch, which causes great losses in melon and watermelon worldwide. We identified a LuxR-type, nonquorum sensing-related regulator, AcrR, in the group II strain Aac-5 of A. citrulli. We found that the acrR mutant lost twitching and swimming motilities, and flagellar formation. It also showed reduced virulence, but increased biofilm formation and growth ability. Transcriptomic analysis revealed that 394 genes were differentially expressed in the acrR mutant of A. citrulli, including 33 genes involved in flagellar assembly. Our results suggest that AcrR may act as a global regulator affecting multiple important biological functions of A. citrulli.

Linking bacterial growth, survival, and multicellularity - small signaling molecules as triggers and drivers

An overarching theme of cellular regulation in bacteria arises from the trade-off between growth and stress resilience. In addition, the formation of biofilms contributes to stress survival, since these dense multicellular aggregates, in which cells are embedded in an extracellular matrix of self-produced polymers, represent a self-constructed protective and homeostatic 'niche'. As shown here for the model bacterium Escherichia coli, the inverse coordination of bacterial growth with survival and the transition to multicellularity is achieved by a highly integrated regulatory network with several sigma subunits of RNA polymerase and a small number of transcriptional hubs as central players. By conveying information about the actual (micro)environments, nucleotide second messengers such as cAMP, (p)ppGpp, and in particular c-di-GMP are the key triggers and drivers that promote either growth or stress resistance and organized multicellularity in a world of limited resources.

Genome Insights of the Plant-Growth Promoting Bacterium Cronobacter muytjensii JZ38 With Volatile-Mediated Antagonistic Activity Against Phytophthora infestans

Salinity stress is a major challenge to agricultural productivity and global food security in light of a dramatic increase of human population and climate change. Plant growth promoting bacteria can be used as an additional solution to traditional crop breeding and genetic engineering. In the present work, the induction of plant salt tolerance by the desert plant endophyte Cronobacter sp. JZ38 was examined on the model plant Arabidopsis thaliana using different inoculation methods. JZ38 promoted plant growth under salinity stress via contact and emission of volatile compounds. Based on the 16S rRNA and whole genome phylogenetic analysis, fatty acid analysis and phenotypic identification, JZ38 was identified as Cronobacter muytjensii and clearly separated and differentiated from the pathogenic C. sakazakii. Full genome sequencing showed that JZ38 is composed of one chromosome and two plasmids. Bioinformatic analysis and bioassays revealed that JZ38 can grow under a range of abiotic stresses. JZ38 interaction with plants is correlated with an extensive set of genes involved in chemotaxis and motility. The presence of genes for plant nutrient acquisition and phytohormone production could explain the ability of JZ38 to colonize plants and sustain plant growth under stress conditions. Gas chromatography-mass spectrometry analysis of volatiles produced by JZ38 revealed the emission of indole and different sulfur volatile compounds that may play a role in contactless plant growth promotion and antagonistic activity against pathogenic microbes. Indeed, JZ38 was able to inhibit the growth of two strains of the phytopathogenic oomycete Phytophthora infestans via volatile emission. Genetic, transcriptomic and metabolomics analyses, combined with more in vitro assays will provide a better understanding the highlighted genes' involvement in JZ38's functional potential and its interaction with plants. Nevertheless, these results provide insight into the bioactivity of C. muytjensii JZ38 as a multi-stress tolerance promoting bacterium with a potential use in agriculture.

Small RNAs OmrA and OmrB promote class III flagellar gene expression by inhibiting the synthesis of anti-Sigma factor FlgM

Bacteria can move by a variety of mechanisms, the best understood being flagella-mediated motility. Flagellar genes are organized in a three-tiered cascade allowing for temporally regulated expression that involves both transcriptional and post-transcriptional control. The class I operon encodes the master regulator FlhDC that drives class II gene transcription. Class II genes include fliA and flgM, which encode the Sigma factor σ²⁸, required for class III transcription, and the anti-Sigma factor FlgM, which inhibits σ²⁸ activity, respectively. The flhDC mRNA is regulated by several small regulatory RNAs (sRNAs). Two of these, the sequence-related OmrA and OmrB RNAs, inhibit FlhD synthesis. Here, we report on a second layer of sRNA-mediated control downstream of FhlDC in the flagella pathway. By mutational analysis, we confirm that a predicted interaction between the conserved 5ʹ seed sequences of OmrA/B and the early coding sequence in flgM mRNA reduces FlgM expression. Regulation is dependent on the global RNA-binding protein Hfq. In vitro experiments support a canonical mechanism: binding of OmrA/B prevents ribosome loading and decreases FlgM protein synthesis. Simultaneous inhibition of both FlhD and FlgM synthesis by OmrA/B complicated an assessment of how regulation of FlgM alone impacts class III gene transcription. Using a combinatorial mutation strategy, we were able to uncouple these two targets and demonstrate that OmrA/B-dependent inhibition of FlgM synthesis liberates σ²⁸ to ultimately promote higher expression of the class III flagellin gene fliC.

Stochastic transcriptional pulses orchestrate flagellar biosynthesis in Escherichia coli

The classic picture of flagellum biosynthesis in Escherichia coli, inferred from population measurements, depicts a deterministic program where promoters are sequentially up-regulated and are maintained steadily active throughout exponential growth. However, complex regulatory dynamics at the single-cell level can be masked by bulk measurements. Here, we discover that in individual E. coli cells, flagellar promoters are stochastically activated in pulses. These pulses are coordinated within specific classes of promoters and comprise "on" and "off" states, each of which can span multiple generations. We demonstrate that in this pulsing program, the regulatory logic of flagellar assembly dictates which promoters skip pulses. Surprisingly, pulses do not require specific transcriptional or translational regulation of the flagellar master regulator, FlhDC, but instead appears to be essentially governed by an autonomous posttranslational circuit. Our results suggest that even topologically simple transcriptional networks can generate unexpectedly rich temporal dynamics and phenotypic heterogeneities.

Limits to a classic paradigm: most transcription factors in E. coli regulate genes involved in multiple biological processes

Transcription factors (TFs) are important drivers of cellular decision-making. When bacteria encounter a change in the environment, TFs alter the expression of a defined set of genes in order to adequately respond. It is commonly assumed that genes regulated by the same TF are involved in the same biological process. Examples of this are methods that rely on coregulation to infer function of not-yet-annotated genes. We have previously shown that only 21% of TFs involved in metabolism regulate functionally homogeneous genes, based on the proximity of the gene products’ catalyzed reactions in the metabolic network. Here, we provide more evidence to support the claim that a 1-TF/1-process relationship is not a general property. We show that the observed functional heterogeneity of regulons is not a result of the quality of the annotation of regulatory interactions, nor the absence of protein–metabolite interactions, and that it is also present when function is defined by Gene Ontology terms. Furthermore, the observed functional heterogeneity is different from the one expected by chance, supporting the notion that it is a biological property. To further explore the relationship between transcriptional regulation and metabolism, we analyzed five other types of regulatory groups and identified complex regulons (i.e. genes regulated by the same combination of TFs) as the most functionally homogeneous, and this is supported by coexpression data. Whether higher levels of related functions exist beyond metabolism and current functional annotations remains an open question.

Advantage of the F2:A1:B- IncF Pandemic Plasmid over IncC Plasmids in In Vitro Acquisition and Evolution of bla CTX-M Gene-Bearing Plasmids in Escherichia coli

Despite a fitness cost imposed on bacterial hosts, large conjugative plasmids play a key role in the diffusion of resistance determinants, such as CTX-M extended-spectrum β-lactamases. Among the large conjugative plasmids, IncF plasmids are the most predominant group, and an F2:A1:B- IncF-type plasmid encoding a CTX-M-15 variant was recently described as being strongly associated with the emerging worldwide Escherichia coli sequence type 131 (ST131)-O25b:H4 H30Rx/C2 sublineage.

Despite a fitness cost imposed on bacterial hosts, large conjugative plasmids play a key role in the diffusion of resistance determinants, such as CTX-M extended-spectrum β-lactamases. Among the large conjugative plasmids, IncF plasmids are the most predominant group, and an F2:A1:B- IncF-type plasmid encoding a CTX-M-15 variant was recently described as being strongly associated with the emerging worldwide Escherichia coli sequence type 131 (ST131)-O25b:H4 H30Rx/C2 sublineage. In this context, we investigated the fitness cost of narrow-range F-type plasmids, including the F2:A1:B- IncF-type CTX-M-15 plasmid, and of broad-range C-type plasmids in the K-12-like J53-2 E. coli strain. Although all plasmids imposed a significant fitness cost to the bacterial host immediately after conjugation, we show, using an experimental-evolution approach, that a negative impact on the fitness of the host strain was maintained throughout 1,120 generations with the IncC-IncR plasmid, regardless of the presence or absence of cefotaxime, in contrast to the F2:A1:B- IncF plasmid, whose cost was alleviated. Many chromosomal and plasmid rearrangements were detected after conjugation in transconjugants carrying the IncC plasmids but not in transconjugants carrying the F2:A1:B- IncF plasmid, except for insertion sequence (IS) mobilization from the fliM gene leading to the restoration of motility of the recipient strains. Only a few mutations occurred on the chromosome of each transconjugant throughout the experimental-evolution assay. Our findings indicate that the F2:A1:B- IncF CTX-M-15 plasmid is well adapted to the E. coli strain studied, contrary to the IncC-IncR CTX-M-15 plasmid, and that such plasmid-host adaptation could participate in the evolutionary success of the CTX-M-15-producing pandemic E. coli ST131-O25b:H4 lineage.

Rapid Accumulation of Motility-Activating Mutations in Resting Liquid Culture of Escherichia coli

Expression of motility genes is a potentially beneficial but costly process in bacteria. Interestingly, many isolate strains of Escherichia coli possess motility genes but have lost the ability to activate them under conditions in which motility is advantageous, raising the question of how they respond to these situations. Through transcriptome profiling of strains in the E. coli single-gene knockout Keio collection, we noticed drastic upregulation of motility genes in many of the deletion strains compared to levels in their weakly motile parent strain (BW25113). We show that this switch to a motile phenotype is not a direct consequence of the genes deleted but is instead due to a variety of secondary mutations that increase the expression of the major motility regulator, FlhDC. Importantly, we find that this switch can be reproduced by growing poorly motile E. coli strains in nonshaking liquid medium overnight but not in shaking liquid medium. Individual isolates after the nonshaking overnight incubations acquired distinct mutations upstream of the flhDC operon, including different insertion sequence (IS) elements and, to a lesser extent, point mutations. The rapidity with which genetic changes sweep through the populations grown without shaking shows that poorly motile strains can quickly adapt to a motile lifestyle by genetic rewiring.IMPORTANCE The ability to tune gene expression in times of need outside preordained regulatory networks is an essential evolutionary process that allows organisms to survive and compete. Here, we show that upon overnight incubation in liquid medium without shaking, populations of largely nonmotile Escherichia coli bacteria can rapidly accumulate mutants that have constitutive motility. This effect contributes to widespread secondary mutations in the single-gene knockout library, the Keio collection. As a result, 49/71 (69%) of the Keio strains tested exhibited various degrees of motility, whereas their parental strain is poorly motile. These observations highlight the plasticity of gene expression even in the absence of preexisting regulatory programs and should raise awareness of procedures for handling laboratory strains of E. coli.

Diversity, versatility and complexity of bacterial gene regulation mechanisms: opportunities and drawbacks for applications in synthetic biology

Gene expression occurs in two essential steps: transcription and translation. In bacteria, the two processes are tightly coupled in time and space, and highly regulated. Tight regulation of gene expression is crucial. It limits wasteful consumption of resources and energy, prevents accumulation of potentially growth inhibiting reaction intermediates, and sustains the fitness and potential virulence of the organism in a fluctuating, competitive and frequently stressful environment. Since the onset of studies on regulation of enzyme synthesis, numerous distinct regulatory mechanisms modulating transcription and/or translation have been discovered. Mostly, various regulatory mechanisms operating at different levels in the flow of genetic information are used in combination to control and modulate the expression of a single gene or operon. Here, we provide an extensive overview of the very diverse and versatile bacterial gene regulatory mechanisms with major emphasis on their combined occurrence, intricate intertwinement and versatility. Furthermore, we discuss the potential of well-characterized basal expression and regulatory elements in synthetic biology applications, where they may ensure orthogonal, predictable and tunable expression of (heterologous) target genes and pathways, aiming at a minimal burden for the host.

Essential Role of σ Factor RpoF in Flagellar Biosynthesis and Flagella-Mediated Motility of Acidithiobacillus caldus

Acidithiobacillaceae, an important family of acidophilic and chemoautotrophic sulfur or iron oxidizers, participate in geobiochemical circulation of the elements and drive the release of heavy metals in mining associated habitats. Because of their environmental adaptability and energy metabolic systems, Acidithiobacillus spp. have become the dominant bacteria used in bioleaching for heavy metal recovery. Flagella-driven motility is associated with bacterial chemotaxis and bacterial responses to environmental stimuli. However, little is known about how the flagellum of Acidithiobacillus spp. is regulated and how the flagellum affects the growth of these chemoautotrophic bacteria. In this study, we analyzed the flagellar gene clusters in Acidithiobacillus strains and uncovered the close relationship between flagella and the sulfur-oxidizing systems (Sox system). The σ²⁸ gene (rpoF) knockout and overexpression strains of Acidithiobacillus caldus were constructed. Scanning electron microscopy shows that A. caldus ΔrpoF cells lacked flagella, indicating the essential role of RpoF in regulating flagella synthesis in these chemoautotrophic bacteria. Motility analysis suggests that the deletion of rpoF resulted in the reduction of swarming capability, while this capability was enhanced in the rpoF overexpression strain. Both static cultivation and low concentration of energy substrates (elemental sulfur or tetrathionate) led to weak growth of A. caldus ΔrpoF cells. The deletion of rpoF promoted bacterial attachment to the surface of elemental sulfur in static cultivation. The absence of RpoF caused an obvious change in transcription profile, including genes in flagellar cluster and those involved in biofilm formation. These results provide an understanding on the regulation of flagellar hierarchy and the flagellar function in these sulfur or iron oxidizers.

Mycobacterial SigA and SigB Cotranscribe Essential Housekeeping Genes during Exponential Growth

Mycobacterial σ^B belongs to the group II family of sigma factors, which are widely considered to transcribe genes required for stationary-phase survival and the response to stress. Here we explored the mechanism underlying the observed hypersensitivity of ΔsigB deletion mutants of Mycobacterium smegmatis, M. abscessus, and M. tuberculosis to rifampin (RIF) and uncovered an additional constitutive role of σ^B during exponential growth of mycobacteria that complements the function of the primary sigma factor, σ^A Using chromatin immunoprecipitation sequencing (ChIP-Seq), we show that during exponential phase, σ^B binds to over 200 promoter regions, including those driving expression of essential housekeeping genes, like the rRNA gene. ChIP-Seq of ectopically expressed σ^A-FLAG demonstrated that at least 61 promoter sites are recognized by both σ^A and σ^B These results together suggest that RNA polymerase holoenzymes containing either σ^A or σ^B transcribe housekeeping genes in exponentially growing mycobacteria. The RIF sensitivity of the ΔsigB mutant possibly reflects a decrease in the effective housekeeping holoenzyme pool, which results in susceptibility of the mutant to lower doses of RIF. Consistent with this model, overexpression of σ^A restores the RIF tolerance of the ΔsigB mutant to that of the wild type, concomitantly ruling out a specialized role of σ^B in RIF tolerance. Although the properties of mycobacterial σ^B parallel those of Escherichia coli σ³⁸ in its ability to transcribe a subset of housekeeping genes, σ^B presents a clear departure from the E. coli paradigm, wherein the cellular levels of σ³⁸ are tightly controlled during exponential growth, such that the transcription of housekeeping genes is initiated exclusively by a holoenzyme containing σ⁷⁰ (E.σ⁷⁰).IMPORTANCE All mycobacteria encode a group II sigma factor, σ^B, closely related to the group I principal housekeeping sigma factor, σ^A Group II sigma factors are widely believed to play specialized roles in the general stress response and stationary-phase transition in the bacteria that encode them. Contrary to this widely accepted view, we show an additional housekeeping function of σ^B that complements the function of σ^A in logarithmically growing cells. These findings implicate a novel and dynamic partnership between σ^A and σ^B in maintaining the expression of housekeeping genes in mycobacteria and can perhaps be extended to other bacterial species that possess multiple group II sigma factors.

Inefficient Secretion of Anti-sigma Factor FlgM Inhibits Bacterial Motility at High Temperature

Temperature is one of the key cues that enable microorganisms to adjust their physiology in response to environmental changes. Here we show that motility is the major cellular function of Escherichia coli that is differentially regulated between growth at normal host temperature of 37°C and the febrile temperature of 42°C. Expression of both class II and class III flagellar genes is reduced at 42°C because of lowered level of the upstream activator FlhD. Class III genes are additionally repressed because of the destabilization and malfunction of secretion apparatus at high temperature, which prevents secretion of the anti-sigma factor FlgM. This mechanism of repression apparently accelerates loss of motility at 42°C. We hypothesize that E. coli perceives high temperature as a sign of inflammation, downregulating flagella to escape detection by the immune system of the host. Secretion-dependent coupling of gene expression to the environmental temperature is likely common among many bacteria.

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E. coli motility is tightly turned off at febrile temperature (42°C)

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Repression of motility is achieved at two levels of hierarchical gene regulation

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Lowered FlhD level reduces expression of all flagellar genes

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Impaired FlgM secretion tightens repression of class III genes

Physiological roles of antisense RNAs in prokaryotes

Prokaryotes encounter constant and often brutal modifications to their environment. In order to survive, they need to maintain fitness, which includes adapting their protein expression patterns. Many factors control gene expression but this review focuses on just one, namely antisense RNAs (asRNAs), a class of non-coding RNAs (ncRNAs) characterized by their location in cis and their perfect complementarity with their targets. asRNAs were considered for a long time to be trivial and only to be found on mobile genetic elements. However, recent advances in methodology have revealed that their abundance and potential activities have been underestimated. This review aims to illustrate the role of asRNA in various physiologically crucial functions in both archaea and bacteria, which can be regrouped in three categories: cell maintenance, horizontal gene transfer and virulence. A literature survey of asRNAs demonstrates the difficulties to characterize and assign a role to asRNAs. With the aim of facilitating this task, we describe recent technological advances that could be of interest to identify new asRNAs and to discover their function.

Using RegulonDB, the Escherichia coli K-12 Gene Regulatory Transcriptional Network Database

In RegulonDB, for over 25 years, we have been gathering knowledge by manual curation from original scientific literature on the regulation of transcription initiation and genome organization in transcription units of the Escherichia coli K-12 genome. This unit describes six basic protocols that can serve as a guiding introduction to the main content of the current version (v9.4) of this electronic resource. These protocols include general navigation as well as searching for specific objects such as genes, gene products, transcription units, promoters, transcription factors, coexpression, and genetic sensory response units or GENSOR Units. In these protocols, the user will find an initial introduction to the concepts pertinent to the protocol, the content obtained when performing the given navigation, and the necessary resources for carrying out the protocol. This easy-to-follow presentation should help anyone interested in quickly seeing all that is currently offered in RegulonDB, including position weight matrices of transcription factors, coexpression values based on published microarrays, and the GENSOR Units unique to RegulonDB that offer regulatory mechanisms in the context of their signals and metabolic consequences. © 2018 by John Wiley & Sons, Inc.

Probing chemotaxis activity in Escherichia coli using fluorescent protein fusions

Bacterial chemotaxis signaling may be interesting for the development of rapid biosensor assays, but is difficult to quantify. Here we explore two potential fluorescent readouts of chemotactically active Escherichia coli cells. In the first, we probed interactions between the chemotaxis signaling proteins CheY and CheZ by fusing them individually with non-fluorescent parts of stable or unstable ‘split’-Green Fluorescent Protein. Wild-type chemotactic cells but not mutants lacking the CheA kinase produced distinguishable fluorescence foci, two-thirds of which localize at the cell poles with the chemoreceptors and one-third at motor complexes. Fluorescent foci based on stable split-eGFP displayed small fluctuations in cells exposed to attractant or repellent, but those based on an unstable ASV-tagged eGFP showed a higher dynamic behaviour both in the foci intensity changes and the number of foci per cell. For the second readout, we expressed the pH-sensitive fluorophore pHluorin in the cyto- and periplasm of chemotactically active E. coli. Calibrations of pHluorin fluorescence as a function of pH demonstrated that cells accumulating near a chemo-attractant temporally increase cytoplasmic pH while decreasing periplasmic pH. Both readouts thus show promise for biosensor assays based on bacterial chemotaxis activity.

MrpJ Directly Regulates Proteus mirabilis Virulence Factors, Including Fimbriae and Type VI Secretion, during Urinary Tract Infection

Proteus mirabilis is a leading cause of catheter-associated urinary tract infections (CAUTIs) and urolithiasis. The transcriptional regulator MrpJ inversely modulates two critical aspects of P. mirabilis UTI progression: fimbria-mediated attachment and flagellum-mediated motility. Transcriptome data indicated a network of virulence-associated genes under MrpJ's control. Here, we identify the direct gene regulon of MrpJ and its contribution to P. mirabilis pathogenesis, leading to the discovery of novel virulence targets. Chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) was used for the first time in a CAUTI pathogen to probe for in vivo direct targets of MrpJ. Selected MrpJ-regulated genes were mutated and assessed for their contribution to UTI using a mouse model. ChIP-seq revealed a palindromic MrpJ binding sequence and 78 MrpJ-bound regions, including binding sites upstream of genes involved in motility, fimbriae, and a type VI secretion system (T6SS). A combinatorial mutation approach established the contribution of three fimbriae (fim8A, fim14A, and pmpA) to UTI and a new pathogenic role for the T6SS in UTI progression. In conclusion, this study (i) establishes the direct gene regulon and an MrpJ consensus binding site and (ii) led to the discovery of new virulence genes in P. mirabilis UTI, which could be targeted for therapeutic intervention of CAUTI.

A Multiscale Agent-Based Model for the Investigation of E. coli K12 Metabolic Response During Biofilm Formation

Bacterial biofilm formation is an organized collective response to biochemical cues that enables bacterial colonies to persist and withstand environmental insults. We developed a multiscale agent-based model that characterizes the intracellular, extracellular, and cellular scale interactions that modulate Escherichia coli MG1655 biofilm formation. Each bacterium's intracellular response and cellular state were represented as an outcome of interactions with the environment and neighboring bacteria. In the intracellular model, environment-driven gene expression and metabolism were captured using statistical regression and Michaelis-Menten kinetics, respectively. In the cellular model, growth, death, and type IV pili- and flagella-dependent movement were based on the bacteria's intracellular state. We implemented the extracellular model as a three-dimensional diffusion model used to describe glucose, oxygen, and autoinducer 2 gradients within the biofilm and bulk fluid. We validated the model by comparing simulation results to empirical quantitative biofilm profiles, gene expression, and metabolic concentrations. Using the model, we characterized and compared the temporal metabolic and gene expression profiles of sessile versus planktonic bacterial populations during biofilm formation and investigated correlations between gene expression and biofilm-associated metabolites and cellular scale phenotypes. Based on our in silico studies, planktonic bacteria had higher metabolite concentrations in the glycolysis and citric acid cycle pathways, with higher gene expression levels in flagella and lipopolysaccharide-associated genes. Conversely, sessile bacteria had higher metabolite concentrations in the autoinducer 2 pathway, with type IV pili, autoinducer 2 export, and cellular respiration genes upregulated in comparison with planktonic bacteria. Having demonstrated results consistent with in vitro static culture biofilm systems, our model enables examination of molecular phenomena within biofilms that are experimentally inaccessible and provides a framework for future exploration of how hypothesized molecular mechanisms impact bulk community behavior.

HldE Is Important for Virulence Phenotypes in Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is one of the most common causes of diarrheal illness in third world countries and it especially affects children and travelers visiting these regions. ETEC causes disease by adhering tightly to the epithelial cells in a concerted effort by adhesins, flagella, and other virulence-factors. When attached ETEC secretes toxins targeting the small intestine host-cells, which ultimately leads to osmotic diarrhea. HldE is a bifunctional protein that catalyzes the nucleotide-activated heptose precursors used in the biosynthesis of lipopolysaccharide (LPS) and in post-translational protein glycosylation. Both mechanisms have been linked to ETEC virulence: Lipopolysaccharide (LPS) is a major component of the bacterial outer membrane and is needed for transport of heat-labile toxins to the host cells, and ETEC glycoproteins have been shown to play an important role for bacterial adhesion to host epithelia. Here, we report that HldE plays an important role for ETEC virulence. Deletion of hldE resulted in markedly reduced binding to the human intestinal cells due to reduced expression of colonization factor CFA/I on the bacterial surface. Deletion of hldE also affected ETEC motility in a flagella-dependent fashion. Expression of both colonization factors and flagella was inhibited at the level of transcription. In addition, the hldE mutant displayed altered growth, increased biofilm formation and clumping in minimal growth medium. Investigation of an orthogonal LPS-deficient mutant combined with mass spectrometric analysis of protein glycosylation indicated that HldE exerts its role on ETEC virulence both through protein glycosylation and correct LPS configuration. These results place HldE as an attractive target for the development of future antimicrobial therapeutics.

The evolutionary impact of intragenic FliA promoters in proteobacteria

In Escherichia coli, one sigma factor recognizes the majority of promoters, and six 'alternative' sigma factors recognize specific subsets of promoters. The alternative sigma factor FliA (σ28 ) recognizes promoters upstream of many flagellar genes. We previously showed that most E. coli FliA binding sites are located inside genes. However, it was unclear whether these intragenic binding sites represent active promoters. Here, we construct and assay transcriptional promoter-lacZ fusions for all 52 putative FliA promoters previously identified by ChIP-seq. These experiments, coupled with integrative analysis of published genome-scale transcriptional datasets, strongly suggest that most intragenic FliA binding sites are active promoters that transcribe highly unstable RNAs. Additionally, we show that widespread intragenic FliA-dependent transcription may be a conserved phenomenon, but that specific promoters are not themselves conserved. We conclude that intragenic FliA-dependent promoters and the resulting RNAs are unlikely to have important regulatory functions. Nonetheless, one intragenic FliA promoter is broadly conserved and constrains evolution of the overlapping protein-coding gene. Thus, our data indicate that intragenic regulatory elements can influence bacterial protein evolution and suggest that the impact of intragenic regulatory sequences on genome evolution should be considered more broadly.

Determining the Specificity of Cascade Binding, Interference, and Primed Adaptation In Vivo in the Escherichia coli Type I-E CRISPR-Cas System

In clustered regularly interspaced short palindromic repeat (CRISPR)-Cas (CRISPR-associated) immunity systems, short CRISPR RNAs (crRNAs) are bound by Cas proteins, and these complexes target invading nucleic acid molecules for degradation in a process known as interference. In type I CRISPR-Cas systems, the Cas protein complex that binds DNA is known as Cascade. Association of Cascade with target DNA can also lead to acquisition of new immunity elements in a process known as primed adaptation. Here, we assess the specificity determinants for Cascade-DNA interaction, interference, and primed adaptation in vivo, for the type I-E system of Escherichia coli Remarkably, as few as 5 bp of crRNA-DNA are sufficient for association of Cascade with a DNA target. Consequently, a single crRNA promotes Cascade association with numerous off-target sites, and the endogenous E. coli crRNAs direct Cascade binding to >100 chromosomal sites. In contrast to the low specificity of Cascade-DNA interactions, >18 bp are required for both interference and primed adaptation. Hence, Cascade binding to suboptimal, off-target sites is inert. Our data support a model in which the initial Cascade association with DNA targets requires only limited sequence complementarity at the crRNA 5' end whereas recruitment and/or activation of the Cas3 nuclease, a prerequisite for interference and primed adaptation, requires extensive base pairing.IMPORTANCE Many bacterial and archaeal species encode CRISPR-Cas immunity systems that protect against invasion by foreign DNA. In the Escherichia coli CRISPR-Cas system, a protein complex, Cascade, binds 61-nucleotide (nt) CRISPR RNAs (crRNAs). The Cascade complex is directed to invading DNA molecules through base pairing between the crRNA and target DNA. This leads to recruitment of the Cas3 nuclease, which destroys the invading DNA molecule and promotes acquisition of new immunity elements. We made the first in vivo measurements of Cascade binding to DNA targets. Thus, we show that Cascade binding to DNA is highly promiscuous; endogenous E. coli crRNAs can direct Cascade binding to >100 chromosomal locations. In contrast, we show that targeted degradation and acquisition of new immunity elements require highly specific association of Cascade with DNA, limiting CRISPR-Cas function to the appropriate targets.

Molecular Prediction of the O157:H-Negative Phenotype Prevalent in Australian Shiga Toxin-Producing Escherichia coli Cases Improves Concordance of In Silico Serotyping with Phenotypic Motility

Shiga toxin-producing Escherichia coli (STEC) is a foodborne pathogen, and serotype O157:H7 is typically associated with severe disease. Australia is unique in its STEC epidemiology, as severe cases are typically associated with non-O157 serogroups, and locally acquired O157 isolates are H-negative/nonmotile. The H-negative phenotype and reduced severity of disease compared to that associated with H7/motile strains are distinct features of Australian O157 strains, but the molecular mechanism behind this phenotype has not been reported. Accurate characterization of the H-negative phenotype is important in epidemiological surveillance of STEC. Serotyping is moving away from phenotype-based methods, as next generation sequencing allows rapid extrapolation of serotype through in silico detection of the O-antigen processing genes, wzx, wzy, wzm, and wzt, and the H-antigen gene, fliC The detection and genotyping of fliC alone is unable to determine the motility of the strain. Typically, most Australian O157:H-negative strains carry an H7 genotype yet phenotypically are nonmotile; thus, many are mischaracterized as H7 strains by in silico serotyping tools. Comparative genomic analysis of flagellar genes between Australian and international isolates was performed and an insertion at nucleotide (nt) 125 in the flgF gene was identified in H-negative isolates. Chi-square results showed that this insertion was significantly associated with the H-negative phenotype (P < 0.0001). Phylogenetic analysis was also completed and showed that the Australian H-negative isolates with the insertion in flgF represent a clade within the O157 serogroup, distinct from O157:H7 serotypes. This study provides a genetic target for inferring the nonmotile phenotype of Australian O157 STEC, which increases the predictive value of in silico serotyping.

Transcriptional responses of Escherichia coli during recovery from inorganic or organic mercury exposure

Background

The protean chemical properties of mercury have long made it attractive for diverse applications, but its toxicity requires great care in its use, disposal, and recycling. Mercury occurs in multiple chemical forms, and the molecular basis for the distinct toxicity of its various forms is only partly understood. Global transcriptomics applied over time can reveal how a cell recognizes a toxicant and what cellular subsystems it marshals to repair and recover from the damage. The longitudinal effects on the transcriptome of exponential phase E. coli were compared during sub-acute exposure to mercuric chloride (HgCl₂) or to phenylmercuric acetate (PMA) using RNA-Seq.

Results

Differential gene expression revealed common and distinct responses to the mercurials throughout recovery. Cultures exhibited growth stasis immediately after each mercurial exposure but returned to normal growth more quickly after PMA exposure than after HgCl₂ exposure. Correspondingly, PMA rapidly elicited up-regulation of a large number of genes which continued for 30 min, whereas fewer genes were up-regulated early after HgCl₂ exposure only some of which overlapped with PMA up-regulated genes. By 60 min gene expression in PMA-exposed cells was almost indistinguishable from unexposed cells, but HgCl₂ exposed cells still had many differentially expressed genes. Relative expression of energy production and most metabolite uptake pathways declined with both compounds, but nearly all stress response systems were up-regulated by one or the other mercurial during recovery.

Conclusions

Sub-acute exposure influenced expression of ~45% of all genes with many distinct responses for each compound, reflecting differential biochemical damage by each mercurial and the corresponding resources available for repair. This study is the first global, high-resolution view of the transcriptional responses to any common toxicant in a prokaryotic model system from exposure to recovery of active growth. The responses provoked by these two mercurials in this model bacterium also provide insights about how higher organisms may respond to these ubiquitous metal toxicants.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4413-z) contains supplementary material, which is available to authorized users.

Controlling Bdellovibrio bacteriovorus Gene Expression and Predation Using Synthetic Riboswitches

Bdellovibrio bacteriovorus is a predatory bacterium that feeds on Gram-negative bacteria including a wide range of pathogens and thus has potential applications as a biocontrol agent. Owing to its unique life cycle, however, there are limited tools that enable genetic manipulation of B. bacteriovorus. This work describes our first steps toward engineering the predatory bacterium for practical applications by developing basic genetic parts to control gene expression. Specifically, we evaluated four robust promoters that are active during the attack phase of B. bacteriovorus. Subsequently, we tested several synthetic riboswitches that have been reported to function in Escherichia coli, and identified theophylline-activated riboswitches that function in B. bacteriovorus. Finally, we inserted the riboswitch into the bacterial chromosome to regulate expression of the flagellar sigma factor fliA, which was previously predicted to be essential for predation, and observed that the engineered strain shows a faster predation kinetics in the presence of theophylline.

SalmoNet, an integrated network of ten Salmonella enterica strains reveals common and distinct pathways to host adaptation

Salmonella enterica is a prominent bacterial pathogen with implications on human and animal health. Salmonella serovars could be classified as gastro-intestinal or extra-intestinal. Genome-wide comparisons revealed that extra-intestinal strains are closer relatives of gastro-intestinal strains than to each other indicating a parallel evolution of this trait. Given the complexity of the differences, a systems-level comparison could reveal key mechanisms enabling extra-intestinal serovars to cause systemic infections. Accordingly, in this work, we introduce a unique resource, SalmoNet, which combines manual curation, high-throughput data and computational predictions to provide an integrated network for Salmonella at the metabolic, transcriptional regulatory and protein-protein interaction levels. SalmoNet provides the networks separately for five gastro-intestinal and five extra-intestinal strains. As a multi-layered, multi-strain database containing experimental data, SalmoNet is the first dedicated network resource for Salmonella. It comprehensively contains interactions between proteins encoded in Salmonella pathogenicity islands, as well as regulatory mechanisms of metabolic processes with the option to zoom-in and analyze the interactions at specific loci in more detail. Application of SalmoNet is not limited to strain comparisons as it also provides a Salmonella resource for biochemical network modeling, host-pathogen interaction studies, drug discovery, experimental validation of novel interactions, uncovering new pathological mechanisms from emergent properties and epidemiological studies. SalmoNet is available at http://salmonet.org.

Building a complete image of genome regulation in the model organism Escherichia coli

The model organism, Escherichia coli, contains a total of more than 4,500 genes, but the total number of RNA polymerase (RNAP) core enzyme or the transcriptase is only about 2,000 molecules per genome. The regulatory targets of RNAP are, however, modulated by changing its promoter selectivity through two-steps of protein-protein interplay with 7 species of the sigma factor in the first step, and then 300 species of the transcription factor (TF) in the second step. Scientists working in the field of prokaryotic transcription in Japan have made considerable contributions to the elucidation of genetic frameworks and regulatory modes of the genome transcription in E. coli K-12. This review summarizes the findings by this group, first focusing on three sigma factors, the stationary-phase sigma RpoS, the heat-shock sigma RpoH, and the flagellar-chemotaxis sigma RpoF, as examples. It also presents an overview of the current state of the systematic research being carried out to identify the regulatory functions of all TFs from a single and the same bacterium E. coli K-12, using the genomic SELEX and PS-TF screening systems. All these studies have been undertaken with the aim of understanding the genome regulation in E. coli K-12 as a whole.

The whole set of the constitutive promoters recognized by four minor sigma subunits of Escherichia coli RNA polymerase

The promoter selectivity of Escherichia coli RNA polymerase (RNAP) is determined by the sigma subunit. The model prokaryote Escherichia coli K-12 contains seven species of the sigma subunit, each recognizing a specific set of promoters. For identification of the "constitutive promoters" that are recognized by each RNAP holoenzyme alone in the absence of other supporting factors, we have performed the genomic SELEX screening in vitro for their binding sites along the E. coli K-12 W3110 genome using each of the reconstituted RNAP holoenzymes and a collection of genome DNA segments of E. coli K-12. The whole set of constitutive promoters for each RNAP holoenzyme was then estimated based on the location of RNAP-binding sites. The first successful screening of the constitutive promoters was achieved for RpoD (σ70), the principal sigma for transcription of growth-related genes. As an extension, we performed in this study the screening of constitutive promoters for four minor sigma subunits, stationary-phase specific RpoS (σ38), heat-shock specific RpoH (σ32), flagellar-chemotaxis specific RpoF (σ28) and extra-cytoplasmic stress-response RpoE (σ24). The total number of constitutive promoters were: 129~179 for RpoS; 101~142 for RpoH; 34~41 for RpoF; and 77~106 for RpoE. The list of constitutive promoters were compared with that of known promoters identified in vivo under various conditions and using varieties of E. coli strains, altogether allowing the estimation of "inducible promoters" in the presence of additional supporting factors.

Global Gene-expression Analysis of the Response of Salmonella Enteritidis to Egg White Exposure Reveals Multiple Egg White-imposed Stress Responses

Chicken egg white protects the embryo from bacterial invaders by presenting an assortment of antagonistic activities that combine together to both kill and inhibit growth. The key features of the egg white anti-bacterial system are iron restriction, high pH, antibacterial peptides and proteins, and viscosity. Salmonella enterica serovar Enteritidis is the major pathogen responsible for egg-borne infection in humans, which is partly explained by its exceptional capacity for survival under the harsh conditions encountered within egg white. However, at temperatures up to 42°C, egg white exerts a much stronger bactericidal effect on S. Enteritidis than at lower temperatures, although the mechanism of egg white-induced killing is only partly understood. Here, for the first time, the impact of exposure of S. Enteritidis to egg white under bactericidal conditions (45°C) is explored by global-expression analysis. A large-scale (18.7% of genome) shift in transcription is revealed suggesting major changes in specific aspects of S. Enteritidis physiology: induction of egg white related stress-responses (envelope damage, exposure to heat and alkalinity, and translation shutdown); shift in energy metabolism from respiration to fermentation; and enhanced micronutrient provision (due to iron and biotin restriction). Little evidence of DNA damage or redox stress was obtained. Instead, data are consistent with envelope damage resulting in cell death by lysis. A surprise was the high degree of induction of hexonate/hexuronate utilization genes, despite no evidence indicating the presence of these substrates in egg white.

Identification and Characterization of Differentially-Regulated Type IVb Pilin Genes Necessary for Predation in Obligate Bacterial Predators

Bdellovibrio bacteriovorus is an obligate predator of bacteria that grows and divides within the periplasm of its prey. Functions involved in the early steps of predation have been identified and characterized, but mediators of prey invasion are still poorly detailed. By combining omics data available for Bdellovibrio and like organisms (BALO’s), we identified 43 genes expressed in B. bacteriovorus during the early interaction with prey. These included genes in a tight adherence (TAD) operon encoding for two type IVb fimbriae-like pilin proteins (flp1 and flp2), and their processing and export machinery. Two additional flp genes (flp3 and flp4) were computationally identified at other locations along the chromosome, defining the largest and most diverse type IVb complement known in bacteria to date. Only flp1, flp2 and flp4 were expressed; their respective gene knock-outs resulted in a complete loss of the predatory ability without losing the ability to adhere to prey cells. Additionally, we further demonstrate differential regulation of the flp genes as the TAD operon of BALOs with different predatory strategies is controlled by a flagellar sigma factor FliA, while flp4 is not. Finally, we show that FliA, a known flagellar transcriptional regulator in other bacteria, is an essential Bdellovibrio gene.