Genome-Scale Mapping of Escherichia coli σ54 Reveals Widespread, Conserved Intragenic Binding

Multiple regulatory inputs including cell envelope stress orchestrate expression of the Escherichia coli rpoN operon

The rpoN operon, an important regulatory hub in Enterobacteriaceae, includes rpoN encoding sigma factor σ54, hpf involved in ribosome hibernation, rapZ regulating glucosamine-6-phosphate levels, and two genes encoding proteins of the nitrogen-related phosphotransferase system. Little is known about regulatory mechanisms controlling the abundance of these proteins. This study employs transposon mutagenesis and chemical screens to dissect the complex expression of the rpoN operon. We find that envelope stress conditions trigger read-through transcription into the rpoN operon from a promoter located upstream of the preceding lptA-lptB locus. This promoter is controlled by the envelope stress sigma factor E and response regulator PhoP is required for its full response to a subset of stress signals. σE also stimulates ptsN-rapZ-npr expression using an element downstream of rpoN, presumably by interfering with mRNA processing by RNase E. Additionally, we identify a novel promoter in the 3' end of rpoN that directs transcription of the distal genes in response to ethanol. Finally, we show that translation of hpf and ptsN is individually regulated by the RNA chaperone Hfq, perhaps involving small RNAs. Collectively, our work demonstrates that the rpoN operon is subject to complex regulation, integrating signals related to envelope stress and carbon source quality.

RpoN (sigma factor 54) contributes to bacterial fitness during tracheal colonization of Bordetella bronchiseptica

The Gram-negative pathogenic bacterium Bordetella bronchiseptica is a respiratory pathogen closely related to Bordetella pertussis, the causative agent of whooping cough. Despite sharing homologous virulence factors, B. bronchiseptica infects a broad range of mammalian hosts, including some experimental animals, whereas B. pertussis is strictly adapted to humans. Therefore, B. bronchiseptica is often used as a representative model to explore the pathogenicity of Bordetella in infection experiments with laboratory animals. Although Bordetella virulence factors, including toxins and adhesins have been studied well, our recent study implied that unknown virulence factors are involved in tracheal colonization and infection. Here, we investigated bacterial genes contributing to tracheal colonization by high-throughput transposon sequencing (Tn-seq). After the screening, we picked up 151 candidate genes of various functions and found that a rpoN-deficient mutant strain was defective in tracheal colonization when co-inoculated with the wild-type strain. rpoN encodes σ54 , a sigma factor that regulates the transcription of various genes, implying its contribution to various bacterial activities. In fact, we found RpoN of B. bronchiseptica is involved in bacterial motility and initial biofilm formation. From these results, we propose that RpoN supports bacterial colonization by regulating various bacteriological functions.

σE of Streptomyces coelicolor can function both as a direct activator or repressor of transcription

σ factors are considered as positive regulators of gene expression. Here we reveal the opposite, inhibitory role of these proteins. We used a combination of molecular biology methods and computational modeling to analyze the regulatory activity of the extracytoplasmic σE factor from Streptomyces coelicolor. The direct activator/repressor function of σE was then explored by experimental analysis of selected promoter regions in vivo. Additionally, the σE interactome was defined. Taken together, the results characterize σE, its regulation, regulon, and suggest its direct inhibitory function (as a repressor) in gene expression, a phenomenon that may be common also to other σ factors and organisms.

Experimental promoter identification of a foodborne pathogen Salmonella enterica subsp. enterica serovar Typhimurium with near single base-pair resolution

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a common foodborne pathogen which is frequently used as the reference strain for Salmonella. Investigating the sigma factor network and protomers is crucial to understand the genomic and transcriptomic properties of the bacterium. Its promoters were identified using various methods such as dRNA-seq, ChIP-chip, or ChIP-Seq. However, validation using ChIP-exo, which exhibits higher-resolution performance compared to conventional ChIP, has not been conducted to date. In this study, using the representative strain S. Typhimurium LT2 (LT2), the ChIP-exo experiment was conducted to accurately determine the binding sites of catalytic RNA polymerase subunit RpoB and major sigma factors (RpoD, RpoN, RpoS, and RpoE) during exponential phase. Integrated with the results of RNA-Seq, promoters and sigmulons for the sigma factors and their association with RpoB have been discovered. Notably, the overlapping regions among binding sites of each alternative sigma factor were found. Furthermore, comparative analysis with Escherichia coli str. K-12 substr. MG1655 (MG1655) revealed conserved binding sites of RpoD and RpoN across different species. In the case of small RNAs (sRNAs), 50 sRNAs observed their expression during the exponential growth of LT2. Collectively, the integration of ChIP-exo and RNA-Seq enables genome-scale promoter mapping with high resolution and facilitates the characterization of binding events of alternative sigma factors, enabling a comprehensive understanding of the bacterial sigma factor network and condition-specific active promoters.

Genome-Wide Mapping of the Escherichia coli PhoB Regulon Reveals Many Transcriptionally Inert, Intragenic Binding Sites

Genome-scale analyses have revealed many transcription factor binding sites within, rather than upstream of, genes, raising questions as to the function of these binding sites. Here, we use complementary approaches to map the regulon of the Escherichia coli transcription factor PhoB, a response regulator that controls transcription of genes involved in phosphate homeostasis. Strikingly, the majority of PhoB binding sites are located within genes, but these intragenic sites are not associated with detectable transcription regulation and are not evolutionarily conserved. Many intragenic PhoB sites are located in regions bound by H-NS, likely due to shared sequence preferences of PhoB and H-NS. However, these PhoB binding sites are not associated with transcription regulation even in the absence of H-NS. We propose that for many transcription factors, including PhoB, binding sites not associated with promoter sequences are transcriptionally inert and hence are tolerated as genomic "noise." IMPORTANCE Recent studies have revealed large numbers of transcription factor binding sites within the genes of bacteria. The function, if any, of the vast majority of these binding sites has not been investigated. Here, we map the binding of the transcription factor PhoB across the Escherichia coli genome, revealing that the majority of PhoB binding sites are within genes. We show that PhoB binding sites within genes are not associated with regulation of the overlapping genes. Indeed, our data suggest that bacteria tolerate the presence of large numbers of nonregulatory, intragenic binding sites for transcription factors and that these binding sites are not under selective pressure.

A general mechanism for transcription bubble nucleation in bacteria

Bacterial transcription initiation requires σ factors for nucleation of the transcription bubble. The canonical housekeeping σ factor, σ70, nucleates DNA melting via recognition of conserved bases of the promoter -10 motif, which are unstacked and captured in pockets of σ70. By contrast, the mechanism of transcription bubble nucleation and formation during the unrelated σN-mediated transcription initiation is poorly understood. Herein, we combine structural and biochemical approaches to establish that σN, like σ70, captures a flipped, unstacked base in a pocket formed between its N-terminal region I (RI) and extra-long helix features. Strikingly, RI inserts into the nascent bubble to stabilize the nucleated bubble prior to engagement of the obligate ATPase activator. Our data suggest a general paradigm of transcription initiation that requires σ factors to nucleate an early melted intermediate prior to productive RNA synthesis.

Deep-learning optimized DEOCSU suite provides an iterable pipeline for accurate ChIP-exo peak calling

Recognizing binding sites of DNA-binding proteins is a key factor for elucidating transcriptional regulation in organisms. ChIP-exo enables researchers to delineate genome-wide binding landscapes of DNA-binding proteins with near single base-pair resolution. However, the peak calling step hinders ChIP-exo application since the published algorithms tend to generate false-positive and false-negative predictions. Here, we report the development of DEOCSU (DEep-learning Optimized ChIP-exo peak calling SUite), a novel machine learning-based ChIP-exo peak calling suite. DEOCSU entails the deep convolutional neural network model which was trained with curated ChIP-exo peak data to distinguish the visualized data of bona fide peaks from false ones. Performance validation of the trained deep-learning model indicated its high accuracy, high precision and high recall of over 95%. Applying the new suite to both in-house and publicly available ChIP-exo datasets obtained from bacteria, eukaryotes and archaea revealed an accurate prediction of peaks containing canonical motifs, highlighting the versatility and efficiency of DEOCSU. Furthermore, DEOCSU can be executed on a cloud computing platform or the local environment. With visualization software included in the suite, adjustable options such as the threshold of peak probability, and iterable updating of the pre-trained model, DEOCSU can be optimized for users' specific needs.

Purine catabolism by enterobacteria

Purines are abundant among organic nitrogen sources and have high nitrogen content. Accordingly, microorganisms have evolved different pathways to catabolize purines and their metabolic products such as allantoin. Enterobacteria from the genera Escherichia, Klebsiella and Salmonella have three such pathways. First, the HPX pathway, found in the genus Klebsiella and very close relatives, catabolizes purines during aerobic growth, extracting all four nitrogen atoms in the process. This pathway includes several known or predicted enzymes not previously observed in other purine catabolic pathways. Second, the ALL pathway, found in strains from all three species, catabolizes allantoin during anaerobic growth in a branched pathway that also includes glyoxylate assimilation. This allantoin fermentation pathway originally was characterized in a gram-positive bacterium, and therefore is widespread. Third, the XDH pathway, found in strains from Escherichia and Klebsiella spp., at present is ill-defined but likely includes enzymes to catabolize purines during anaerobic growth. Critically, this pathway may include an enzyme system for anaerobic urate catabolism, a phenomenon not previously described. Documenting such a pathway would overturn the long-held assumption that urate catabolism requires oxygen. Overall, this broad capability for purine catabolism during either aerobic or anaerobic growth suggests that purines and their metabolites contribute to enterobacterial fitness in a variety of environments.

Genome-wide mapping of the Escherichia coli PhoB regulon reveals many transcriptionally inert, intragenic binding sites

Genome-scale analyses have revealed many transcription factor binding sites within, rather than upstream of genes, raising questions as to the function of these binding sites. Here, we use complementary approaches to map the regulon of the Escherichia coli transcription factor PhoB, a response regulator that controls transcription of genes involved in phosphate homeostasis. Strikingly, the majority of PhoB binding sites are located within genes, but these intragenic sites are not associated with detectable transcription regulation and are not evolutionarily conserved. Many intragenic PhoB sites are located in regions bound by H-NS, likely due to shared sequence preferences of PhoB and H-NS. However, these PhoB binding sites are not associated with transcription regulation even in the absence of H-NS. We propose that for many transcription factors, including PhoB, binding sites not associated with promoter sequences are transcriptionally inert, and hence are tolerated as genomic "noise".

IMPORTANCE

Recent studies have revealed large numbers of transcription factor binding sites within the genes of bacteria. The function, if any, of the vast majority of these binding sites has not been investigated. Here, we map the binding of the transcription factor PhoB across the Escherichia coli genome, revealing that the majority of PhoB binding sites are within genes. We show that PhoB binding sites within genes are not associated with regulation of the overlapping genes. Indeed, our data suggest that bacteria tolerate the presence of large numbers of non-regulatory, intragenic binding sites for transcription factors, and that these binding sites are not under selective pressure.

RpoN Regulon in Erwinia amylovora Revealed by Transcriptional Profiling and In Silico Binding Site Analysis

Erwinia amylovora causes a devastating fire blight disease in apples and pears. One of the main virulence determinants in E. amylovora is the hypersensitive response (HR) and pathogenicity (hrp)-type III secretion system (T3SS), which is activated by the RpoN-HrpL sigma factor cascade. However, the RpoN regulon in E. amylovora has not been investigated. In this study, we determined the RpoN regulon in E. amylovora by combining RNA-seq transcriptomic analysis with in silico binding site analysis. RNA-seq revealed that 262 genes, approximately 7.5% genes in the genome of E. amylovora, were differentially transcribed in the rpoN mutant as compared with the wild type. Specifically, genes associated with virulence, motility, nitrogen assimilation, the PspF system, stress response, and arginine biosynthesis are positively regulated by RpoN, whereas genes associated with biosynthesis of amino acids and sorbitol transport are negatively regulated by RpoN. In silico binding site analysis identified 46 potential target genes with a putative RpoN binding site, and the upstream sequences of six, three, and three genes also contain putative GlnG, PspF, and YfhA binding sites, respectively. Overall, RpoN directly regulates genes associated with virulence, nitrogen assimilation, the PspF system, motility and the YfhA/YfhK two-component regulatory system.

M. tuberculosis Transcription Machinery: A Review on the Mycobacterial RNA Polymerase and Drug Discovery Efforts

Mycobacterium tuberculosis (MTB) is the main source of tuberculosis (TB), one of the oldest known diseases in the human population. Despite the drug discovery efforts of past decades, TB is still one of the leading causes of mortality and claimed more than 1.5 million lives worldwide in 2020. Due to the emergence of drug-resistant strains and patient non-compliance during treatments, there is a pressing need to find alternative therapeutic agents for TB. One of the important areas for developing new treatments is in the inhibition of the transcription step of gene expression; it is the first step to synthesize a copy of the genetic material in the form of mRNA. This further translates to functional protein synthesis, which is crucial for the bacteria living processes. MTB contains a bacterial DNA-dependent RNA polymerase (RNAP), which is the key enzyme for the transcription process. MTB RNAP has been targeted for designing and developing antitubercular agents because gene transcription is essential for the mycobacteria survival. Initiation, elongation, and termination are the three important sequential steps in the transcription process. Each step is complex and highly regulated, involving multiple transcription factors. This review is focused on the MTB transcription machinery, especially in the nature of MTB RNAP as the main enzyme that is regulated by transcription factors. The mechanism and conformational dynamics that occur during transcription are discussed and summarized. Finally, the current progress on MTB transcription inhibition and possible drug target in mycobacterial RNAP are also described to provide insight for future antitubercular drug design and development.

Automated model-predictive design of synthetic promoters to control transcriptional profiles in bacteria

Transcription rates are regulated by the interactions between RNA polymerase, sigma factor, and promoter DNA sequences in bacteria. However, it remains unclear how non-canonical sequence motifs collectively control transcription rates. Here, we combine massively parallel assays, biophysics, and machine learning to develop a 346-parameter model that predicts site-specific transcription initiation rates for any σ70 promoter sequence, validated across 22132 bacterial promoters with diverse sequences. We apply the model to predict genetic context effects, design σ70 promoters with desired transcription rates, and identify undesired promoters inside engineered genetic systems. The model provides a biophysical basis for understanding gene regulation in natural genetic systems and precise transcriptional control for engineering synthetic genetic systems.

Genome-wide protein-DNA interaction site mapping in bacteria using a double-stranded DNA-specific cytosine deaminase

DNA-protein interactions are central to fundamental cellular processes, yet widely implemented technologies for measuring these interactions on a genome scale in bacteria are laborious and capture only a snapshot of binding events. We devised a facile method for mapping DNA-protein interaction sites in vivo using the double-stranded DNA-specific cytosine deaminase toxin DddA. In 3D-seq (DddA-sequencing), strains containing DddA fused to a DNA-binding protein of interest accumulate characteristic mutations in DNA sequence adjacent to sites occupied by the DNA-bound fusion protein. High-depth sequencing enables detection of sites of increased mutation frequency in these strains, yielding genome-wide maps of DNA-protein interaction sites. We validated 3D-seq for four transcription regulators in two bacterial species, Pseudomonas aeruginosa and Escherichia coli. We show that 3D-seq offers ease of implementation, the ability to record binding event signatures over time and the capacity for single-cell resolution.

The role of nitrogen-responsive regulators in controlling inorganic polyphosphate synthesis in Escherichia coli

Inorganic polyphosphate (polyP) is synthesized by bacteria under stressful environmental conditions and acts by a variety of mechanisms to promote cell survival. While the kinase that synthesizes polyP (PPK, encoded by the ppk gene) is well known, ppk transcription is not activated by environmental stress and little is understood about how environmental stress signals lead to polyP accumulation. Previous work has shown that the transcriptional regulators DksA, RpoN (σ54) and RpoE (σ24) positively regulate polyP production, but not ppk transcription, in Escherichia coli. In this work, we examine the role of the alternative sigma factor RpoN and nitrogen starvation stress response pathways in controlling polyP synthesis. We show that the RpoN enhancer binding proteins GlnG and GlrR impact polyP production, and uncover a new role for the nitrogen phosphotransferase regulator PtsN (EIIANtr) as a positive regulator of polyP production, acting upstream of DksA, downstream of RpoN and apparently independently of RpoE. However, neither these regulatory proteins nor common nitrogen metabolites appear to act directly on PPK, and the precise mechanism(s) by which polyP production is modulated after stress remain(s) unclear. Unexpectedly, we also found that the genes that impact polyP production vary depending on the composition of the rich media in which the cells were grown before exposure to polyP-inducing stress. These results constitute progress towards deciphering the regulatory networks driving polyP production under stress, and highlight the remarkable complexity of this regulation and its connections to a broad range of stress-sensing pathways.

A Role for the RNA Polymerase Gene Specificity Factor σ54 in the Uniform Colony Growth of Uropathogenic Escherichia coli

The canonical function of a bacterial sigma (σ) factor is to determine the gene specificity of the RNA polymerase (RNAP). In several diverse bacterial species, the σ⁵⁴ factor uniquely confers distinct functional and regulatory properties on the RNAP. A hallmark feature of the σ⁵⁴-RNAP is the obligatory requirement for an activator ATPase to allow transcription initiation. Different activator ATPases couple diverse environmental cues to the σ⁵⁴-RNAP to mediate adaptive changes in gene expression. Hence, the genes that rely upon σ⁵⁴ for their transcription have a wide range of different functions suggesting that the repertoire of functions performed by genes, directly or indirectly affected by σ⁵⁴, is not yet exhaustive. By comparing the growth patterns of prototypical enteropathogenic, uropathogenic, and nonpathogenic Escherichia coli strains devoid of σ⁵⁴, we uncovered that the absence of σ⁵⁴ results in two differently sized colonies that appear at different times specifically in the uropathogenic E. coli (UPEC) strain. Notably, UPEC bacteria devoid of individual activator ATPases of the σ⁵⁴-RNAP do not phenocopy the σ⁵⁴ mutant strain. Thus, it seems that σ⁵⁴’s role as a determinant of uniform colony appearance in UPEC bacteria represents a putative non-canonical function of σ⁵⁴ in regulating genetic information flow.

IMPORTANCE RNA synthesis is the first step of gene expression. The multisubunit RNA polymerase (RNAP) is the central enzyme responsible for RNA synthesis in bacteria. The dissociable sigma (σ) factor subunit directs the RNAP to different sets of genes to allow their expression in response to various cellular needs. Of the seven σ factors in Escherichia coli and related bacteria, σ⁵⁴ exists in a class of its own. This study has uncovered that σ⁵⁴ is a determinant of the uniform growth of uropathogenic E. coli on solid media. This finding suggests a role for this σ⁵⁴ in gene regulation that extends beyond its known function as an RNAP gene specificity factor.

Methylation Motifs in Promoter Sequences May Contribute to the Maintenance of a Conserved m5C Methyltransferase in Helicobacter pylori

DNA methylomes of Helicobacter pylori strains are complex due to the large number of DNA methyltransferases (MTases) they possess. H. pylori J99 M.Hpy99III is a 5-methylcytosine (^m5C) MTase that converts GCGC motifs to G^m5CGC. Homologs of M.Hpy99III are found in essentially all H. pylori strains. Most of these homologs are orphan MTases that lack a cognate restriction endonuclease, and their retention in H. pylori strains suggest they have roles in gene regulation. To address this hypothesis, green fluorescent protein (GFP) reporter genes were constructed with six putative promoters that had a GCGC motif in the extended −10 region, and the expression of the reporter genes was compared in wild-type H. pylori G27 and a mutant lacking the M.Hpy99III homolog (M.HpyGIII). The expression of three of the GFP reporter genes was decreased significantly in the mutant lacking M.HpyGIII. In addition, the growth rate of the H. pylori G27 mutant lacking M.HpyGIII was reduced markedly compared to that of the wild type. These findings suggest that the methylation of the GCGC motif in many H. pylori GCGC-containing promoters is required for the robust expression of genes controlled by these promoters, which may account for the universal retention of M.Hpy99III homologs in H. pylori strains.

Whole set of constitutive promoters for RpoN sigma factor and the regulatory role of its enhancer protein NtrC in Escherichia coli K-12

The promoter selectivity of Escherichia coli RNA polymerase (RNAP) is determined by its promoter-recognition sigma subunit. The model prokaryote E. coli K-12 contains seven species of the sigma subunit, each recognizing a specific set of promoters. Using genomic SELEX (gSELEX) screening in vitro, we identified the whole set of ‘constitutive’ promoters recognized by the reconstituted RNAP holoenzyme alone, containing RpoD (σ⁷⁰), RpoS (σ³⁸), RpoH (σ³²), RpoF (σ²⁸) or RpoE (σ²⁴), in the absence of other supporting regulatory factors. In contrast, RpoN sigma (σ⁵⁴), involved in expression of nitrogen-related genes and also other cellular functions, requires an enhancer (or activator) protein, such as NtrC, for transcription initiation. In this study, a series of gSELEX screenings were performed to search for promoters recognized by the RpoN RNAP holoenzyme in the presence and absence of the major nitrogen response enhancer NtrC, the best-characterized enhancer. Based on the RpoN holoenzyme-binding sites, a total of 44 to 61 putative promoters were identified, which were recognized by the RpoN holoenzyme alone. In the presence of the enhancer NtrC, the recognition target increased to 61–81 promoters. Consensus sequences of promoters recognized by RpoN holoenzyme in the absence and presence of NtrC were determined. The promoter activity of a set of NtrC-dependent and -independent RpoN promoters was verified in vivo under nitrogen starvation, in the presence and absence of RpoN and/or NtrC. The promoter activity of some RpoN-recognized promoters increased in the absence of RpoN or NtrC, supporting the concept that the promoter-bound NtrC-enhanced RpoN holoenzyme functions as a repressor against RpoD holoenzyme. Based on our findings, we propose a model in which the RpoN holoenzyme fulfils the dual role of repressor and transcriptase for the same set of genes. We also propose that the promoter recognized by RpoN holoenzyme in the absence of enhancers is the ‘repressive’ promoter. The presence of high-level RpoN sigma in growing E. coli K-12 in rich medium may be related to the repression role of a set of genes needed for the utilization of ammonia as a nitrogen source in poor media. The list of newly identified regulatory targets of RpoN provides insight into E. coli survival under nitrogen-depleted conditions in nature.

The alternative sigma factors, rpoN1 and rpoN2 are required for mycophagous activity of Burkholderia gladioli strain NGJ1

Bacteria utilize RpoN, an alternative sigma factor (σ54) to grow in diverse habitats, including nitrogen-limiting conditions. Here, we report that a rice-associated mycophagous bacterium Burkholderia gladioli strain NGJ1 encodes two paralogues of rpoN viz. rpoN1 and rpoN2. Both of them are upregulated during 24 h of mycophagous interaction with Rhizoctonia solani, a polyphagous fungal pathogen. Disruption of either one of rpoNs renders the mutant NGJ1 bacterium defective in mycophagy, whereas ectopic expression of respective rpoN genes restores mycophagy in the complementing strains. NGJ1 requires rpoN1 and rpoN2 for efficient biocontrol to prevent R. solani to establish disease in rice and tomato. Further, we have identified 17 genes having RpoN regulatory motif in NGJ1, majority of them encode potential type III secretion system (T3SS) effectors, nitrogen assimilation, and cellular transport-related functions. Several of these RpoN regulated genes as well as certain previously reported T3SS apparatus (hrcC and hrcN) and effector (Bg_9562 and endo-β-1,3-glucanase) encoding genes are upregulated in NGJ1 but not in ΔrpoN1 or ΔrpoN2 mutant bacterium, during mycophagous interaction with R. solani. This highlights that RpoN1 and RpoN2 modulate T3SS, nitrogen assimilation as well as cellular transport systems in NGJ1 and thereby promote bacterial mycophagy.

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.

Genome-Scale Mapping Reveals Complex Regulatory Activities of RpoN in Yersinia pseudotuberculosis

RpoN, an alternative sigma factor commonly known as σ54, is implicated in persistent stages of Yersinia pseudotuberculosis infections in which genes associated with this regulator are upregulated. We here combined phenotypic and genomic assays to provide insight into its role and function in this pathogen. RpoN was found essential for Y. pseudotuberculosis virulence in mice, and in vitro functional assays showed that it controls biofilm formation and motility. Mapping genome-wide associations of Y. pseudotuberculosis RpoN using chromatin immunoprecipitation coupled with next-generation sequencing identified an RpoN binding motif located at 103 inter- and intragenic sites on both sense and antisense strands. Deletion of rpoN had a large impact on gene expression, including downregulation of genes encoding proteins involved in flagellar assembly, chemotaxis, and quorum sensing. There were also clear indications of cross talk with other sigma factors, together with indirect effects due to altered expression of other regulators. Matching differential gene expression with locations of the binding sites implicated around 130 genes or operons potentially activated or repressed by RpoN. Mutagenesis of selected intergenic binding sites confirmed both positive and negative regulatory effects of RpoN binding. Corresponding mutations of intragenic sense sites had less impact on associated gene expression. Surprisingly, mutating intragenic sites on the antisense strand commonly reduced expression of genes carried by the corresponding sense strand.IMPORTANCE The alternative sigma factor RpoN (σ54), which is widely distributed in eubacteria, has been implicated in controlling gene expression of importance for numerous functions including virulence. Proper responses to host environments are crucial for bacteria to establish infection, and regulatory mechanisms involved are therefore of high interest for development of future therapeutics. Little is known about the function of RpoN in the intestinal pathogen Y. pseudotuberculosis, and we therefore investigated its regulatory role in this pathogen. This regulator was indeed found to be critical for establishment of infection in mice, likely involving its requirement for motility and biofilm formation. The RpoN regulon involved both activating and suppressive effects on gene expression which could be confirmed with mutagenesis of identified binding sites. This is the first study of its kind of RpoN in Y. pseudotuberculosis, revealing complex regulation of gene expression involving both productive and silent effects of its binding to DNA, providing important information about RpoN regulation in enterobacteria.

Redefining fundamental concepts of transcription initiation in bacteria

Despite enormous progress in understanding the fundamentals of bacterial gene regulation, our knowledge remains limited when compared with the number of bacterial genomes and regulatory systems to be discovered. Derived from a small number of initial studies, classic definitions for concepts of gene regulation have evolved as the number of characterized promoters has increased. Together with discoveries made using new technologies, this knowledge has led to revised generalizations and principles. In this Expert Recommendation, we suggest precise, updated definitions that support a logical, consistent conceptual framework of bacterial gene regulation, focusing on transcription initiation. The resulting concepts can be formalized by ontologies for computational modelling, laying the foundation for improved bioinformatics tools, knowledge-based resources and scientific communication. Thus, this work will help researchers construct better predictive models, with different formalisms, that will be useful in engineering, synthetic biology, microbiology and genetics.

Evolution of Regulated Transcription

The genomes of all organisms abound with various cis-regulatory elements, which control gene activity. Transcriptional enhancers are a key group of such elements in eukaryotes and are DNA regions that form physical contacts with gene promoters and precisely orchestrate gene expression programs. Here, we follow gradual evolution of this regulatory system and discuss its features in different organisms. In eubacteria, an enhancer-like element is often a single regulatory element, is usually proximal to the core promoter, and is occupied by one or a few activators. Activation of gene expression in archaea is accompanied by the recruitment of an activator to several enhancer-like sites in the upstream promoter region. In eukaryotes, activation of expression is accompanied by the recruitment of activators to multiple enhancers, which may be distant from the core promoter, and the activators act through coactivators. The role of the general DNA architecture in transcription control increases in evolution. As a whole, it can be seen that enhancers of multicellular eukaryotes evolved from the corresponding prototypic enhancer-like regulatory elements with the gradually increasing genome size of organisms.

A Novel Eukaryote-Like CRISPR Activation Tool in Bacteria: Features and Capabilities

CRISPR (clustered regularly interspaced short palindromic repeats) activation (CRISPRa) in bacteria is an attractive method for programmable gene activation. Recently, a eukaryote-like, σ⁵⁴ -dependent CRISPRa system has been reported. It exhibits high dynamic ranges and permits flexible target site selection. Here, an overview of the existing strategies of CRISPRa in bacteria is presented, and the characteristics and design principles of the CRISPRa system are introduced. Possible scenarios for applying the eukaryote-like CRISPRa system is discussed with corresponding suggestions for performance optimization and future functional expansion. The authors envision the new eukaryote-like CRISPRa system enabling novel designs in multiplexed gene regulation and promoting research in the σ⁵⁴ -dependent gene regulatory networks among a variety of biotechnology relevant or disease-associated bacterial species.

Bacterial Enhancer Binding Proteins-AAA+ Proteins in Transcription Activation

Bacterial enhancer-binding proteins (bEBPs) are specialised transcriptional activators. bEBPs are hexameric AAA⁺ ATPases and use ATPase activities to remodel RNA polymerase (RNAP) complexes that contain the major variant sigma factor, σ⁵⁴ to convert the initial closed complex to the transcription competent open complex. Earlier crystal structures of AAA⁺ domains alone have led to proposals of how nucleotide-bound states are sensed and propagated to substrate interactions. Recently, the structure of the AAA⁺ domain of a bEBP bound to RNAP-σ⁵⁴-promoter DNA was revealed. Together with structures of the closed complex, an intermediate state where DNA is partially loaded into the RNAP cleft and the open promoter complex, a mechanistic understanding of how bEBPs use ATP to activate transcription can now be proposed. This review summarises current structural models and the emerging understanding of how this special class of AAA⁺ proteins utilises ATPase activities to allow σ⁵⁴-dependent transcription initiation.

Small RNA-binding protein RapZ mediates cell envelope precursor sensing and signaling in Escherichia coli

The RNA‐binding protein RapZ cooperates with small RNAs (sRNAs) GlmY and GlmZ to regulate the glmS mRNA in Escherichia coli. Enzyme GlmS synthesizes glucosamine‐6‐phosphate (GlcN6P), initiating cell envelope biosynthesis. GlmZ activates glmS expression by base‐pairing. When GlcN6P is ample, GlmZ is bound by RapZ and degraded through ribonuclease recruitment. Upon GlcN6P depletion, the decoy sRNA GlmY accumulates through a previously unknown mechanism and sequesters RapZ, suppressing GlmZ decay. This circuit ensures GlcN6P homeostasis and thereby envelope integrity. In this work, we identify RapZ as GlcN6P receptor. GlcN6P‐free RapZ stimulates phosphorylation of the two‐component system QseE/QseF by interaction, which in turn activates glmY expression. Elevated GlmY levels sequester RapZ into stable complexes, which prevents GlmZ decay, promoting glmS expression. Binding of GlmY also prevents RapZ from activating QseE/QseF, generating a negative feedback loop limiting the response. When GlcN6P is replenished, GlmY is released from RapZ and rapidly degraded. We reveal a multifunctional sRNA‐binding protein that dynamically engages into higher‐order complexes for metabolite signaling.

Plastic Circuits: Regulatory Flexibility in Fine Tuning Pathogen Success

Bacterial pathogens employ diverse fitness and virulence mechanisms to gain an advantage in competitive niches. These lifestyle-specific traits require integration into the regulatory network of the cell and are often controlled by pre-existing transcription factors. In this review, we highlight recent advances that have been made in characterizing this regulatory flexibility in prominent members of the Enterobacteriaceae. We focus on the direct global interactions between transcription factors and their target genes in pathogenic Escherichia coli and Salmonella revealed using chromatin immunoprecipitation coupled with next-generation sequencing. Furthermore, the implications and advantages of such regulatory adaptations in benefiting distinct pathogenic lifestyles are discussed.

Ribosome Hibernation

Protein synthesis consumes a large fraction of available resources in the cell. When bacteria encounter unfavorable conditions and cease to grow, specialized mechanisms are in place to ensure the overall reduction of costly protein synthesis while maintaining a basal level of translation. A number of ribosome-associated factors are involved in this regulation; some confer an inactive, hibernating state of the ribosome in the form of 70S monomers (RaiA; this and the following are based on Escherichia coli nomenclature) or 100S dimers (RMF and HPF homologs), and others inhibit translation at different stages in the translation cycle (RsfS, YqjD and paralogs, SRA, and EttA). Stationary phase cells therefore exhibit a complex array of different ribosome subpopulations that adjusts the translational capacity of the cell to the encountered conditions and ensures efficient reactivation of translation when conditions improve. Here, we review the current state of research regarding stationary phase-specific translation factors, in particular ribosome hibernation factors and other forms of translational regulation in response to stress conditions.

In silico simulations of occurrence of transcription factor binding sites in bacterial genomes

Background

Interactions between transcription factors and their specific binding sites are a key component of regulation of gene expression. Until recently, it was generally assumed that most bacterial transcription factor binding sites are located at or near promoters. However, several recent works utilizing high-throughput technology to detect transcription factor binding sites in bacterial genomes found a large number of binding sites in unexpected locations, particularly inside genes, as opposed to known or expected promoter regions. While some of these intragenic binding sites likely have regulatory functions, an alternative scenario is that many of these binding sites arise by chance in the absence of selective constraints. The latter possibility was supported by in silico simulations for σ⁵⁴ binding sites in Salmonella.

Results

In this work, we extend these simulations to more than forty transcription factors from E. coli and other bacteria. The results suggest that binding sites for all analyzed transcription factors are likely to arise throughout the genome by random genetic drift and many transcription factor binding sites found in genomes may not have specific regulatory functions. In addition, when comparing observed and expected patterns of occurrence of binding sites in genomes, we observed distinct differences among different transcription factors.

Conclusions

We speculate that transcription factor binding sites randomly occurring throughout the genome could be beneficial in promoting emergence of new regulatory interactions and thus facilitating evolution of gene regulatory networks.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1381-8) contains supplementary material, which is available to authorized users.

Regulation of Escherichia coli Pathogenesis by Alternative Sigma Factor N

σ^N (also σ⁵⁴) is an alternative sigma factor subunit of the RNA polymerase complex that regulates the expression of genes from many different ontological groups. It is broadly conserved in the Eubacteria with major roles in nitrogen metabolism, membrane biogenesis, and motility. σ^N is encoded as the first gene of a five-gene operon including rpoN (σ^N), ptsN, hpf, rapZ, and npr that has been genetically retained among species of Escherichia, Shigella, and Salmonella. In an increasing number of bacteria, σ^N has been implicated in the control of genes essential to pathogenic behavior, including those involved in adherence, secretion, immune subversion, biofilm formation, toxin production, and resistance to both antimicrobials and biological stressors. For most pathogens how this is achieved is unknown. In enterohemorrhagic Escherichia coli (EHEC) O157, Salmonella enterica, and Borrelia burgdorferi, regulation of virulence by σ^N requires another alternative sigma factor, σ^S, yet the model by which σ^N-σ^S virulence regulation is predicted to occur is varied in each of these pathogens. In this review, the importance of σ^N to bacterial pathogenesis is introduced, and common features of σ^N-dependent virulence regulation discussed. Emphasis is placed on the molecular mechanisms underlying σ^N virulence regulation in E. coli O157. This includes a review of the structure and function of regulatory pathways connecting σ^N to virulence expression, predicted input signals for pathway stimulation, and the role for cognate σ^N activators in initiation of gene systems determining pathogenic behavior.

Inhibition of Bacterial Gene Transcription with an RpoN-Based Stapled Peptide

In response to environmental and other stresses, the σ⁵⁴ subunit of bacterial RNA polymerase (RNAP) controls expression of several genes that play a significant role in the virulence of both plant and animal pathogens. Recruitment of σ⁵⁴ to RNAP initiates promoter-specific transcription via the double-stranded DNA denaturation mechanism of the cofactor. The RpoN box, a recognition helix found in the C-terminal region of σ⁵⁴, has been identified as the component necessary for major groove insertion at the -24 position of the promoter. We employed the hydrocarbon stapled peptide methodology to design and synthesize stapled σ⁵⁴ peptides capable of penetrating Gram-negative bacteria, binding the σ⁵⁴ promoter, and blocking the interaction between endogenous σ⁵⁴ and its target DNA sequence, thereby reducing transcription and activation of σ⁵⁴ response genes.

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.

The sigma factor σ54 is required for the long-term survival of Leptospira biflexa in water

Leptospira spp. comprise both pathogenic and free-living saprophytic species. Little is known about the environmental adaptation and survival mechanisms of Leptospira. Alternative sigma factor, σ54 (RpoN) is known to play an important role in environmental and host adaptation in many bacteria. In this study, we constructed an rpoN mutant by allele exchange, and the complemented strain in saprophytic L. biflexa. Transcriptome analysis revealed that expression of several genes involved in nitrogen uptake and metabolism, including amtB1, glnB-amtB2, ntrX and narK, were controlled by σ54 . While wild-type L. biflexa could not grow under nitrogen-limiting conditions but was able to survive under such conditions and recover rapidly, the rpoN mutant was not. The rpoN mutant also had dramatically reduced ability to survive long-term in water. σ54 appears to regulate expression of amtB1, glnK-amtB2, ntrX and narK in an indirect manner. However, we identified a novel nitrogen-related gene, LEPBI_I1011, whose expression was directly under the control of σ54 (herein renamed as rcfA for RpoN-controlled factor A). Taken together, our data reveal that the σ54 regulatory network plays an important role in the long-term environmental survival of Leptospira spp.

Both widespread PEP-CTERM proteins and exopolysaccharides are required for floc formation of Zoogloea resiniphila and other activated sludge bacteria

Bacterial floc formation plays a central role in the activated sludge (AS) process, which has been widely utilized for sewage and wastewater treatment. The formation of AS flocs has long been known to require exopolysaccharide biosynthesis. This study demonstrates an additional requirement for a PEP-CTERM protein in Zoogloea resiniphila, a dominant AS bacterium harboring a large exopolysaccharide biosynthesis gene cluster. Two members of a wide-spread family of high copy number-per-genome PEP-CTERM genes, transcriptionally regulated by the RpoN sigma factor and accessory PrsK-PrsR two-component system and at least one of these, pepA, must be expressed for Zoogloea to build the floc structures that allow gravitational sludge settling and recycling. Without PrsK or PrsR, Zoogloea cells were planktonic rather than flocculated and secreted exopolysaccharides were released into the growth broth in soluble form. Overexpression of PepA could circumvent the requirement of rpoN, prsK and prsR for the floc-forming phenotype by fixing the exopolysaccharides to bacterial cells. However, overexpression of PepA, which underwent post-translational modifications, could not rescue the long-rod morphology of the rpoN mutant. Consistently, PEP-CTERM genes and exopolysaccharide biosynthesis gene cluster are present in the genome of the floc-forming Nitrospira comammox and Mitsuaria strain as well as many other AS bacteria.

Novel DNA Binding and Regulatory Activities for σ54 (RpoN) in Salmonella enterica Serovar Typhimurium 14028s

The variable sigma (σ) subunit of the bacterial RNA polymerase (RNAP) holoenzyme, which is responsible for promoter specificity and open complex formation, plays a strategic role in the response to environmental changes. Salmonella enterica serovar Typhimurium utilizes the housekeeping σ⁷⁰ and five alternative sigma factors, including σ⁵⁴ The σ⁵⁴-RNAP differs from other σ-RNAP holoenzymes in that it forms a stable closed complex with the promoter and requires ATP hydrolysis by an activated cognate bacterial enhancer binding protein (bEBP) to transition to an open complex and initiate transcription. In S. Typhimurium, σ⁵⁴-dependent promoters normally respond to one of 13 different bEBPs, each of which is activated under a specific growth condition. Here, we utilized a constitutively active, promiscuous bEBP to perform a genome-wide identification of σ⁵⁴-RNAP DNA binding sites and the transcriptome of the σ⁵⁴ regulon of S. Typhimurium. The position and context of many of the identified σ⁵⁴ RNAP DNA binding sites suggest regulatory roles for σ⁵⁴-RNAP that connect the σ⁵⁴ regulon to regulons of other σ factors to provide a dynamic response to rapidly changing environmental conditions.IMPORTANCE The alternative sigma factor σ⁵⁴ (RpoN) is required for expression of genes involved in processes with significance in agriculture, bioenergy production, bioremediation, and host-microbe interactions. The characterization of the σ⁵⁴ regulon of the versatile pathogen S. Typhimurium has expanded our understanding of the scope of the σ⁵⁴ regulon and how it links to other σ regulons within the complex regulatory network for gene expression in bacteria.

The global regulator Ncb2 escapes from the core promoter and impacts transcription in response to drug stress in Candida albicans

Ncb2, the β subunit of NC2 complex, a heterodimeric regulator of transcription was earlier shown to be involved in the activated transcription of CDR1 gene in azole resistant isolate (AR) of Candida albicans. This study examines its genome-wide role by profiling Ncb2 occupancy between genetically matched pair of azole sensitive (AS) and AR clinical isolates. A comparison of Ncb2 recruitment between the two isolates displayed that 29 genes had higher promoter occupancy of Ncb2 in the AR isolate. Additionally, a host of genes exhibited exclusive occupancy of Ncb2 at promoters of either AR or AS isolate. The analysis also divulged new actors of multi-drug resistance, whose transcription was activated owing to the differential occupancy of Ncb2. The conditional, sequence-specific positional escape of Ncb2 from the core promoter in AS isolate and its preferential recruitment to the core promoter of certain genes in AR isolates was most noteworthy means of transcription regulation. Together, we show that positional rearrangement of Ncb2 resulting in either activation or repression of gene expression in response to drug-induced stress, represents a novel regulatory mechanism that opens new opportunities for therapeutic intervention to prevent development of drug tolerance in C. albicans cells.

Genome-Wide Transcriptional Response to Varying RpoS Levels in Escherichia coli K-12

The alternative sigma factor RpoS is a central regulator of many stress responses in Escherichia coli The level of functional RpoS differs depending on the stress. The effect of these differing concentrations of RpoS on global transcriptional responses remains unclear. We investigated the effect of RpoS concentration on the transcriptome during stationary phase in rich media. We found that 23% of genes in the E. coli genome are regulated by RpoS, and we identified many RpoS-transcribed genes and promoters. We observed three distinct classes of response to RpoS by genes in the regulon: genes whose expression changes linearly with increasing RpoS level, genes whose expression changes dramatically with the production of only a little RpoS ("sensitive" genes), and genes whose expression changes very little with the production of a little RpoS ("insensitive"). We show that sequences outside the core promoter region determine whether an RpoS-regulated gene is sensitive or insensitive. Moreover, we show that sensitive and insensitive genes are enriched for specific functional classes and that the sensitivity of a gene to RpoS corresponds to the timing of induction as cells enter stationary phase. Thus, promoter sensitivity to RpoS is a mechanism to coordinate specific cellular processes with growth phase and may also contribute to the diversity of stress responses directed by RpoS.IMPORTANCE The sigma factor RpoS is a global regulator that controls the response to many stresses in Escherichia coli Different stresses result in different levels of RpoS production, but the consequences of this variation are unknown. We describe how changing the level of RpoS does not influence all RpoS-regulated genes equally. The cause of this variation is likely the action of transcription factors that bind the promoters of the genes. We show that the sensitivity of a gene to RpoS levels explains the timing of expression as cells enter stationary phase and that genes with different RpoS sensitivities are enriched for specific functional groups. Thus, promoter sensitivity to RpoS is a mechanism that coordinates specific cellular processes in response to stresses.

Dynamics of the Escherichia coli proteome in response to nitrogen starvation and entry into the stationary phase

Nitrogen is needed for the biosynthesis of biomolecules including proteins and nucleic acids. In the absence of fixed nitrogen prokaryotes such as E. coli immediately ceases growth. Ammonium is the preferred nitrogen source for E. coli supporting the fastest growth rates. Under conditions of ammonium limitation, E. coli can use alternative nitrogen sources to supply ammonium ions and this reprogramming is led by the induction of the NtrC regulon. Here we used label free proteomics to determine the dynamics of E. coli proteins expression in response to ammonium starvation in both the short (30min) and the longer (60min) starvation. Protein abundances and post-translational modifications confirmed that activation of the NtrC regulon acts as the first line of defense against nitrogen starvation. The ribosome inactivating protein Rmf was induced shortly after ammonium exhaustion and this was preceded by induction of other ribosome inactivating proteins such as Hpf and RaiA supporting the hypothesis that ribosome shut-down is a key process during nitrogen limitation stress. The proteomic data revealed that growth arrest due to nitrogen starvation correlates with the accumulation of proteins involved in DNA condensation, RNA and protein catabolism and ribosome hibernation. Collectively, these proteome adaptations will result in metabolic inactive cells which are likely to exhibit multidrug tolerance.

Modulating Salmonella Typhimurium's Response to a Changing Environment through Bacterial Enhancer-Binding Proteins and the RpoN Regulon

Transcription sigma factors direct the selective binding of RNA polymerase holoenzyme (Eσ) to specific promoters. Two families of sigma factors determine promoter specificity, the σ(70) (RpoD) family and the σ(54) (RpoN) family. In transcription controlled by σ(54), the Eσ(54)-promoter closed complex requires ATP hydrolysis by an associated bacterial enhancer-binding protein (bEBP) for the transition to open complex and transcription initiation. Given the wide host range of Salmonella enterica serovar Typhimurium, it is an excellent model system for investigating the roles of RpoN and its bEBPs in modulating the lifestyle of bacteria. The genome of S. Typhimurium encodes 13 known or predicted bEBPs, each responding to a unique intracellular or extracellular signal. While the regulons of most alternative sigma factors respond to a specific environmental or developmental signal, the RpoN regulon is very diverse, controlling genes for response to nitrogen limitation, nitric oxide stress, availability of alternative carbon sources, phage shock/envelope stress, toxic levels of zinc, nucleic acid damage, and other stressors. This review explores how bEBPs respond to environmental changes encountered by S. Typhimurium during transmission/infection and influence adaptation through control of transcription of different components of the S. Typhimurium RpoN regulon.

The unexpected complexity of bacterial genomes

Gene organization and control are described by models conceived in the 1960s. These models explain basic gene regulatory mechanisms and underpin current genome annotation. However, such models struggle to explain recent genome-scale observations. For example, accounts of RNA synthesis initiating within genes, widespread antisense transcription and non-canonical DNA binding by gene regulatory proteins are difficult to reconcile with traditional thinking. As a result, unexpected observations have often been dismissed and downstream consequences ignored. In this paper I will argue that, to fully understand the biology of bacterial chromosomes, we must embrace their hidden layers of complexity.