Structural basis of ECF-σ-factor-dependent transcription initiation

Structural basis for regulation of a CBASS-CRISPR-Cas defense island by a transmembrane anti-σ factor and its ECF σ partner

How CRISPR-Cas and cyclic oligonucleotide-based antiphage signaling systems (CBASS) are coordinately deployed against invaders remains unclear. We show that a locus containing two CBASS and one type III-B CRISPR-Cas system, regulated by the transmembrane anti-σ DdvA and its cognate extracytoplasmic function (ECF) σ DdvS, can defend Myxococcus xanthus against a phage. Cryo-electron microscopy reveals DdvA-DdvS pairs assemble as arrow-shaped transmembrane dimers. Each DdvA periplasmic domain adopts a separase/craspase-type tetratricopeptide repeat (TPR)-caspase HetF-associated with TPR (TPR-CHAT) architecture with an incomplete His-Cys active site, lacking three α-helices conserved among CHAT domains. Each active site faces the dimer interface, raising the possibility that signal-induced caspase-like DdvA autoproteolysis in trans precedes RseP-mediated intramembrane proteolysis and DdvS release. Nuclear magnetic resonance reveals a DdvA cytoplasmic CHCC-type zinc-bound three-helix bundle that binds to DdvS σ2 and σ4 domains, undergoing σ4-induced helix extension to trap DdvS. Altogether, we provide structural-mechanistic insights into membrane anti-σ-ECF σ regulation of an antiviral CBASS-CRISPR-Cas defense island.

Design of a tunable bacterial gene expression system using engineered σ factors

Extracytoplasmic function (ECF) σ factors selectively upregulate expression of specific genes in bacteria. These σ factors, belonging to the σ70 family, are much smaller than the primary, housekeeping σ factor with two helical domains that interact with the Pribnow box and the -35 element of the promoter DNA. Structural studies reveal that promoter specificity in a σ factor is determined by the interactions between a loop (L3) and the Pribnow box element. Similarly, the efficiency of transcription initiation is governed by the polypeptide linker between the two promoter-binding domains. Both these polypeptide segments are dynamic and poorly conserved among ECF σ factor homologs. This feature hitherto limited insights from protein-DNA interactions to be correlated with transcription initiation efficiency. Here, we describe an approach to characterize these features that govern the dynamic range of gene expression using chimeric Escherichia coli σE. The L3 loop and linker polypeptides in these σE chimeras were replaced by the corresponding segments from 10 annotated and functional Mycobacterium tuberculosis ECF σ's. In vitro and in vivo measurements to determine the effect of these polypeptide replacements provided an experimentally validated σE chimera- gene expression level data set. We illustrate the utility of this chimeric σE library in improving the efficiency of a biosynthetic pathway in E. coli. In a two-enzyme step, unaffected by feedback inhibition and substrate concentration, we show an increase in desired product levels by altering the relative intracellular levels of the target enzymes using this library of σ factors. The chimeric σE library thus demonstrates the feasibility of engineering σ factors to achieve bespoke expression levels of target genes for diverse applications in synthetic microbiology.

IMPORTANCE

The synthesis of organic compounds involves the action of multiple enzymes in a biosynthetic pathway. Incorporating such biosynthetic pathways into microbes often leads to substantial cellular and metabolic stress resulting in low titers of the target compound. This limitation can be offset, in part, by optimizing enzyme efficiency and cellular enzyme concentration. The former involves significant efforts to achieve improvements in catalytic efficiency with the caveat that the metabolic load on a microbial cell imposed by the overexpression of the exogenous enzyme could result in reduced cell fitness. Here, we demonstrate the feasibility of engineered σ factors to modulate gene expression levels without significant genetic engineering. We note that changing the sequence of two flexible polypeptide loops without any changes to the structural scaffold of the transcription initiation factor σE could modulate the expression levels of the target genes. This ability provides a route to improve the efficiency of a biosynthetic pathway without altering the overall genomic makeup. The σE chimera library thus provides an avenue for pre-determined conditional gene expression of specific genes in Escherichia coli.

Structure of the transcription open complex of distinct σI factors

Bacterial σI factors of the σ70-family are widespread in Bacilli and Clostridia and are involved in the heat shock response, iron metabolism, virulence, and carbohydrate sensing. A multiplicity of σI paralogues in some cellulolytic bacteria have been shown to be responsible for the regulation of the cellulosome, a multienzyme complex that mediates efficient cellulose degradation. Here, we report two structures at 3.0 Å and 3.3 Å of two transcription open complexes formed by two σI factors, SigI1 and SigI6, respectively, from the thermophilic, cellulolytic bacterium, Clostridium thermocellum. These structures reveal a unique, hitherto-unknown recognition mode of bacterial transcriptional promoters, both with respect to domain organization and binding to promoter DNA. The key characteristics that determine the specificities of the σI paralogues were further revealed by comparison of the two structures. Consequently, the σI factors represent a distinct set of the σ70-family σ factors, thus highlighting the diversity of bacterial transcription.

Past, Present, and Future of Extracytoplasmic Function σ Factors: Distribution and Regulatory Diversity of the Third Pillar of Bacterial Signal Transduction

Responding to environmental cues is a prerequisite for survival in the microbial world. Extracytoplasmic function σ factors (ECFs) represent the third most abundant and by far the most diverse type of bacterial signal transduction. While archetypal ECFs are controlled by cognate anti-σ factors, comprehensive comparative genomics efforts have revealed a much higher abundance and regulatory diversity of ECF regulation than previously appreciated. They have also uncovered a diverse range of anti-σ factor-independent modes of controlling ECF activity, including fused regulatory domains and phosphorylation-dependent mechanisms. While our understanding of ECF diversity is comprehensive for well-represented and heavily studied bacterial phyla-such as Proteobacteria, Firmicutes, and Actinobacteria (phylum Actinomycetota)-our current knowledge about ECF-dependent signaling in the vast majority of underrepresented phyla is still far from complete. In particular, the dramatic extension of bacterial diversity in the course of metagenomic studies represents both a new challenge and an opportunity in expanding the world of ECF-dependent signal transduction.

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.

The structure and activities of the archaeal transcription termination factor Eta detail vulnerabilities of the transcription elongation complex

Transcription must be properly regulated to ensure dynamic gene expression underlying growth, development, and response to environmental cues. Regulation is imposed throughout the transcription cycle, and while many efforts have detailed the regulation of transcription initiation and early elongation, the termination phase of transcription also plays critical roles in regulating gene expression. Transcription termination can be driven by only a few proteins in each domain of life. Detailing the mechanism(s) employed provides insight into the vulnerabilities of transcription elongation complexes (TECs) that permit regulated termination to control expression of many genes and operons. Here, we describe the biochemical activities and crystal structure of the superfamily 2 helicase Eta, one of two known factors capable of disrupting archaeal transcription elongation complexes. Eta retains a twin-translocase core domain common to all superfamily 2 helicases and a well-conserved C terminus wherein individual amino acid substitutions can critically abrogate termination activities. Eta variants that perturb ATPase, helicase, single-stranded DNA and double-stranded DNA translocase and termination activities identify key regions of the C terminus of Eta that, when combined with modeling Eta-TEC interactions, provide a structural model of Eta-mediated termination guided in part by structures of Mfd and the bacterial TEC. The susceptibility of TECs to disruption by termination factors that target the upstream surface of RNA polymerase and potentially drive termination through forward translocation and allosteric mechanisms that favor opening of the clamp to release the encapsulated nucleic acids emerges as a common feature of transcription termination mechanisms.

The archetypal gene transfer agent RcGTA is regulated via direct interaction with the enigmatic RNA polymerase omega subunit

Gene transfer agents (GTAs) are small virus-like particles that indiscriminately package and transfer any DNA present in their host cell, with clear implications for bacterial evolution. The first transcriptional regulator that directly controls GTA expression, GafA, was recently discovered, but its mechanism of action has remained elusive. Here, we demonstrate that GafA controls GTA gene expression via direct interaction with the RNA polymerase omega subunit (Rpo-ω) and also positively autoregulates its own expression by an Rpo-ω-independent mechanism. We show that GafA is a modular protein with distinct DNA and protein binding domains. The functional domains we observe in Rhodobacter GafA also correspond to two-gene operons in Hyphomicrobiales pathogens. These data allow us to produce the most complete regulatory model for a GTA and point toward an atypical mechanism for RNA polymerase recruitment and specific transcriptional activation in the Alphaproteobacteria.

An Extracytoplasmic Function Sigma Factor Required for Full Virulence in Xanthomonas citri pv. citri

The genus Xanthomonas includes more than 30 phytopathogenic species that infect a wide range of plants and cause severe diseases that greatly impact crop productivity. These bacteria are highly adapted to the soil and plant environment, being found in decaying material, as epiphytes, and colonizing the plant mesophyll. Signal transduction mechanisms involved in the responses of Xanthomonas to environmental changes are still poorly characterized. Xanthomonad genomes typically encode several representatives of the extracytoplasmic function σ (σ^ECF) factors, whose physiological roles remain elusive. In this work, we functionally characterized the Xanthomonas citri pv. citri EcfL, a σ^ECF factor homologous to members of the iron-responsive FecI-like group. We show that EcfL is not required or induced during iron starvation, despite presenting the common features of other FecI-like σ^ECF factors. EcfL positively regulates one operon composed of three genes that encode a TonB-dependent receptor involved in cell surface signaling, an acid phosphatase, and a lectin-domain containing protein. Furthermore, we demonstrate that EcfL is required for full virulence in citrus, and its regulon is induced inside the plant mesophyll and in response to acid stress. Together, our study suggests a role for EcfL in the adaptation of X. citri to the plant environment, in this way contributing to its ability to cause citrus canker disease. IMPORTANCE The Xanthomonas genus comprises a large number of phytopathogenic species that infect a wide variety of economically important plants worldwide. Bacterial adaptation to the plant and soil environment relies on their repertoire of signal transduction pathways, including alternative sigma factors of the extracytoplasmic function family (σ^ECF). Here, we describe a new σ^ECF factor found in several Xanthomonas species, demonstrating its role in Xanthomonas citri virulence to citrus plants. We show that EcfL regulates a single operon containing three genes, which are also conserved in other Xanthomonas species. This study further expands our knowledge on the functions of the widespread family of σ^ECF factors in phytopathogenic bacteria.

Reconstructing the transcription regulatory network to optimize resource allocation for robust biosynthesis

An ideal microbial cell factory (MCF) should deliver maximal resources to production, which conflicts with the microbe's native growth-oriented resource allocation strategy and can therefore lead to early termination of the high-yield period. Reallocating resources from growth to production has become a critical factor in constructing robust MCFs. Instead of strengthening specific biosynthetic pathways, emerging endeavors are focused on rearranging the gene regulatory network to fundamentally reprogram the resource allocation pattern. Combining this idea with transcriptional regulation within the hierarchical regulatory network, this review discusses recent engineering strategies targeting the transcription machinery, module networks, regulatory edges, and bottom network layer. This global view will help to construct a production-oriented phenotype that fully harnesses the potential of MCFs.

Bacterial MerR family transcription regulators: activationby distortion

Transcription factors (TFs) modulate gene expression by regulating the accessibility of promoter DNA to RNA polymerases (RNAPs) in bacteria. The MerR family TFs are a large class of bacterial proteins unique in their physiological functions and molecular action: they function as transcription repressors under normal circumstances, but rapidly transform to transcription activators under various cellular triggers, including oxidative stress, imbalance of cellular metal ions, and antibiotic challenge. The promoters regulated by MerR TFs typically contain an abnormal long spacer between the -35 and -10 elements, where MerR TFs bind and regulate transcription activity through unique mechanisms. In this review, we summarize the function, ligand reception, DNA recognition, and molecular mechanism of transcription regulation of MerR-family TFs.

Promoter Architecture Differences among Alphaproteobacteria and Other Bacterial Taxa

Much of our knowledge of bacterial transcription initiation has been derived from studying the promoters of Escherichia coli and Bacillus subtilis. Given the expansive diversity across the bacterial phylogeny, it is unclear how much of this knowledge can be applied to other organisms. Here, we report on bioinformatic analyses of promoter sequences of the primary σ factor (σ70) by leveraging publicly available transcription start site (TSS) sequencing data sets for nine bacterial species spanning five phyla. This analysis identifies previously unreported differences in the -35 and -10 elements of σ70-dependent promoters in several groups of bacteria. We found that Actinobacteria and Betaproteobacteria σ70-dependent promoters lack the TTG triad in their -35 element, which is predicted to be conserved across the bacterial phyla. In addition, the majority of the Alphaproteobacteria σ70-dependent promoters analyzed lacked the thymine at position -7 that is highly conserved in other phyla. Bioinformatic examination of the Alphaproteobacteria σ70-dependent promoters identifies a significant overrepresentation of essential genes and ones encoding proteins with common cellular functions downstream of promoters containing an A, C, or G at position -7. We propose that transcription of many σ70-dependent promoters in Alphaproteobacteria depends on the transcription factor CarD, which is an essential protein in several members of this phylum. Our analysis expands the knowledge of promoter architecture across the bacterial phylogeny and provides new information that can be used to engineer bacteria for use in medical, environmental, agricultural, and biotechnological processes. IMPORTANCE Transcription of DNA to RNA by RNA polymerase is essential for cells to grow, develop, and respond to stress. Understanding the process and control of transcription is important for health, disease, the environment, and biotechnology. Decades of research on a few bacteria have identified promoter DNA sequences that are recognized by the σ subunit of RNA polymerase. We used bioinformatic analyses to reveal previously unreported differences in promoter DNA sequences across the bacterial phylogeny. We found that many Actinobacteria and Betaproteobacteria promoters lack a sequence in their -35 DNA recognition element that was previously assumed to be conserved and that Alphaproteobacteria lack a thymine residue at position -7, also previously assumed to be conserved. Our work reports important new information about bacterial transcription, illustrates the benefits of studying bacteria across the phylogenetic tree, and proposes new lines of future investigation.

Extracytoplasmic Function σ Factors as Tools for Coordinating Stress Responses

The ability of bacterial core RNA polymerase (RNAP) to interact with different σ factors, thereby forming a variety of holoenzymes with different specificities, represents a powerful tool to coordinately reprogram gene expression. Extracytoplasmic function σ factors (ECFs), which are the largest and most diverse family of alternative σ factors, frequently participate in stress responses. The classification of ECFs in 157 different groups according to their phylogenetic relationships and genomic context has revealed their diversity. Here, we have clustered 55 ECF groups with experimentally studied representatives into two broad classes of stress responses. The remaining 102 groups still lack any mechanistic or functional insight, representing a myriad of systems yet to explore. In this work, we review the main features of ECFs and discuss the different mechanisms controlling their production and activity, and how they lead to a functional stress response. Finally, we focus in more detail on two well-characterized ECFs, for which the mechanisms to detect and respond to stress are complex and completely different: Escherichia coli RpoE, which is the best characterized ECF and whose structural and functional studies have provided key insights into the transcription initiation by ECF-RNAP holoenzymes, and the ECF15-type EcfG, the master regulator of the general stress response in Alphaproteobacteria.

Universal functions of the σ finger in alternative σ factors during transcription initiation by bacterial RNA polymerase

The bacterial σ factor plays the central role in promoter recognition by RNA polymerase (RNAP). The primary σ factor, involved in transcription of housekeeping genes, was also shown to participate in the initiation of RNA synthesis and promoter escape by RNAP. In the open promoter complex, the σ finger formed by σ region 3.2 directly interacts with the template DNA strand upstream of the transcription start site. Here, we analysed the role of the σ finger in transcription initiation by four alternative σ factors in Escherichia coli, σ³⁸, σ³², σ²⁸ and σ²⁴. We found that deletions of the σ finger to various extent compromise the activity of RNAP holoenzymes containing alternative σ factors, especially at low NTP concentrations. All four σs are able to utilize NADH as a noncanonical priming substrate but it has only mild effects on the efficiency of transcription initiation. The mediators of the stringent response, transcription factor DksA and the alarmone ppGpp decrease RNAP activity and promoter complex stability for all four σ factors on tested promoters. For all σs except σ³⁸, deletions of the σ finger conversely increase the stability of promoter complexes and decrease their sensitivity to DksA and ppGpp. The result suggests that the σ finger plays a universal role in transcription initiation by alternative σ factors and sensitizes promoter complexes to the action of global transcription regulators DksA and ppGpp by modulating promoter complex stability.

Expansion and re-classification of the extracytoplasmic function (ECF) σ factor family

Extracytoplasmic function σ factors (ECFs) represent one of the major bacterial signal transduction mechanisms in terms of abundance, diversity and importance, particularly in mediating stress responses. Here, we performed a comprehensive phylogenetic analysis of this protein family by scrutinizing all proteins in the NCBI database. As a result, we identified an average of ∼10 ECFs per bacterial genome and 157 phylogenetic ECF groups that feature a conserved genetic neighborhood and a similar regulation mechanism. Our analysis expands previous classification efforts ∼50-fold, enriches many original ECF groups with previously unclassified proteins and identifies 22 entirely new ECF groups. The ECF groups are hierarchically related to each other and are further composed of subgroups with closely related sequences. This two-tiered classification allows for the accurate prediction of common promoter motifs and the inference of putative regulatory mechanisms across subgroups composing an ECF group. This comprehensive, high-resolution description of the phylogenetic distribution of the ECF family, together with the massive expansion of classified ECF sequences and an openly accessible data repository called ‘ECF Hub’ (https://www.computational.bio.uni-giessen.de/ecfhub), will serve as a powerful hypothesis-generator to guide future research in the field.

PG1659 functions as anti-sigma factor to extracytoplasmic function sigma factor RpoE in Porphyromonas gingivalis W83

Anti-sigma factors play a critical role in regulating the expression of sigma factors in response to environmental stress signals. PG1659 is cotranscribed with an upstream gene PG1660 (rpoE), which encodes for a sigma factor that plays an important role in oxidative stress resistance and the virulence regulatory network of P. gingivalis. PG1659, which is annotated as a hypothetical gene, is evaluated in this study. PG1659, composed of 130 amino acids, is predicted to be transmembrane protein with a single calcium (Ca²⁺ ) binding site. In P. gingivalis FLL358 (ΔPG1659::ermF), the rpoE gene was highly upregulated compared to the wild-type W83 strain. RpoE-induced genes were also upregulated in the PG1659-defective isogenic mutant. Both protein-protein pull-down and bacterial two-hybrid assays revealed that the PG1659 protein could interact with/bind RpoE. The N-terminal domain of PG1659, representing the cytoplasmic fragment of the protein, is critical for interaction with RpoE. In the presence of PG1659, the initiation of transcription by the RpoE sigma factor was inhibited. Taken together, our data suggest that PG1659 is an anti-sigma factor which plays an important regulatory role in the modulation of the sigma factor RpoE in P. gingivalis.

Rewiring the specificity of extracytoplasmic function sigma factors

Bacterial genomes are being sequenced at an exponentially increasing rate, but our inability to decipher their transcriptional wiring limits our ability to derive new biology from these sequences. De novo determination of regulatory interactions requires accurate prediction of regulators' DNA binding and precise determination of biologically significant binding sites. Here we address these challenges by solving the DNA-specificity code of extracytoplasmic function sigma factors (ECF σs), a major family of bacterial regulators, and determining their putative regulons. We generated an aligned collection of ECF σs and their promoters by leveraging the autoregulatory nature of ECF σs as a means of promoter discovery and analyzed it to identify and characterize the conserved amino acid-nucleotide interactions that determine promoter specificity. This enabled de novo prediction of ECF σ specificity, which we combined with a statistically rigorous phylogenetic footprinting pipeline based on precomputed orthologs to predict the direct targets of ∼67% of ECF σs. This global survey indicated that some ECF σs are conserved global regulators controlling many genes throughout the genome, which are important under many conditions, while others are local regulators, controlling a few closely linked genes in response to specific stimuli in select species. This analysis reveals important organizing principles of bacterial gene regulation and presents a conceptual and computational framework for deciphering gene regulatory networks.

The novel ECF56 SigG1-RsfG system modulates morphological differentiation and metal-ion homeostasis in Streptomyces tsukubaensis

Extracytoplasmic function (ECF) sigma factors are key transcriptional regulators that prokaryotes have evolved to respond to environmental challenges. Streptomyces tsukubaensis harbours 42 ECFs to reprogram stress-responsive gene expression. Among them, SigG1 features a minimal conserved ECF σ2-σ4 architecture and an additional C-terminal extension that encodes a SnoaL_2 domain, which is characteristic for ECF σ factors of group ECF56. Although proteins with such domain organisation are widely found among Actinobacteria, the functional role of ECFs with a fused SnoaL_2 domain remains unknown. Our results show that in addition to predicted self-regulatory intramolecular amino acid interactions between the SnoaL_2 domain and the ECF core, SigG1 activity is controlled by the cognate anti-sigma protein RsfG, encoded by a co-transcribed sigG1-neighbouring gene. Characterisation of ∆sigG1 and ∆rsfG strains combined with RNA-seq and ChIP-seq experiments, suggests the involvement of SigG1 in the morphological differentiation programme of S. tsukubaensis. SigG1 regulates the expression of alanine dehydrogenase, ald and the WhiB-like regulator, wblC required for differentiation, in addition to iron and copper trafficking systems. Overall, our work establishes a model in which the activity of a σ factor of group ECF56, regulates morphogenesis and metal-ions homeostasis during development to ensure the timely progression of multicellular differentiation.

Diverse and unified mechanisms of transcription initiation in bacteria

Transcription of DNA is a fundamental process in all cellular organisms. The enzyme responsible for transcription, RNA polymerase, is conserved in general architecture and catalytic function across the three domains of life. Diverse mechanisms are used among and within the different branches to regulate transcription initiation. Mechanistic studies of transcription initiation in bacteria are especially amenable because the promoter recognition and melting steps are much less complicated than in eukaryotes or archaea. Also, bacteria have critical roles in human health as pathogens and commensals, and the bacterial RNA polymerase is a proven target for antibiotics. Recent biophysical studies of RNA polymerases and their inhibition, as well as transcription initiation and transcription factors, have detailed the mechanisms of transcription initiation in phylogenetically diverse bacteria, inspiring this Review to examine unifying and diverse themes in this process.

Structural Insights into Transcription Initiation from De Novo RNA Synthesis to Transitioning into Elongation

In bacteria, the dissociable σ subunit of the RNA polymerase (RNAP) is responsible for initiating RNA synthesis from specific DNA sites. As nascent RNA grows, downstream DNA unwinds and is pulled into the RNAP, causing stress accumulation and initiation complex destabilization. Processive transcription elongation requires at least partial separation of the σ factor from the RNAP core enzyme. Here, we present a series of transcription complexes captured between the early initiation and elongation phases via in-crystal RNA synthesis and cleavage. Crystal structures of these complexes indicate that stress accumulation during transcription initiation is not due to clashing of the growing nascent RNA with the σ3.2 loop, but results from scrunching of the template strand DNA that is contained inside the RNAP by the σ3 domain. Our results shed light on how scrunching of template-strand DNA drives both abortive initiation and σ-RNAP core separation to transition transcription from initiation to elongation.

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.

The C-terminal domain of M. tuberculosis ECF sigma factor I (SigI) interferes in SigI-RNAP interaction

Mycobacterium tuberculosis is equipped with diversified ECF sigma factors that are generally expressed under adverse environmental conditions. Mtb-SigI belongs to the ECF41 family of sigma factor, and no information is available about their expression during stringent response. This study provides the structural insight of Mtb-SigI and the characterization of its C-terminal polypeptide extension. C-terminal site of Mtb-SigI is truncated in two ways: (a) conserved region of C-terminal extension is preserved while the rest of the portion is deleted and (b) complete deletion of C-terminal extension. Each of the wild-type and truncated Mtb-SigI is docked with a β subunit of core RNA polymerase and simulated for 100 ns. Relative binding strength calculated from trajectory analysis reflects that the complete deletion of the C-terminal extension of Mtb-SigI favors interaction with core RNA polymerase. It can be implicated that the C-terminal domain in the wild-type docked complex help flipping of domain 4 of Mtb-SigI and thereby impaired holoenzyme formation. When the C-terminal extension is partially deleted, such flipping of domain 4 of Mtb-SigI diminishes and complete deletion of C-terminal extension promotes holoenzyme formation. In the absence of any sigma factor antagonist, the C-terminal extension of Mtb-SigI might behave as a complex player in transcription regulation. Graphical abstract Role of Mtb-SigI in transcription regulation.

RNA extension drives a stepwise displacement of an initiation-factor structural module in initial transcription

All organisms-bacteria, archaea, and eukaryotes-have a transcription initiation factor that contains a structural module that binds within the RNA polymerase (RNAP) active-center cleft and interacts with template-strand single-stranded DNA (ssDNA) in the immediate vicinity of the RNAP active center. This transcription initiation-factor structural module preorganizes template-strand ssDNA to engage the RNAP active center, thereby facilitating binding of initiating nucleotides and enabling transcription initiation from initiating mononucleotides. However, this transcription initiation-factor structural module occupies the path of nascent RNA and thus presumably must be displaced before or during initial transcription. Here, we report four sets of crystal structures of bacterial initially transcribing complexes that demonstrate and define details of stepwise, RNA-extension-driven displacement of the "σ-finger" of the bacterial transcription initiation factor σ. The structures reveal that-for both the primary σ-factor and extracytoplasmic (ECF) σ-factors, and for both 5'-triphosphate RNA and 5'-hydroxy RNA-the "σ-finger" is displaced in stepwise fashion, progressively folding back upon itself, driven by collision with the RNA 5'-end, upon extension of nascent RNA from ∼5 nt to ∼10 nt.

Structures and mechanism of transcription initiation by bacterial ECF factors

Bacterial RNA polymerase (RNAP) forms distinct holoenzymes with extra-cytoplasmic function (ECF) σ factors to initiate specific gene expression programs. In this study, we report a cryo-EM structure at 4.0 Å of Escherichia coli transcription initiation complex comprising σE-the most-studied bacterial ECF σ factor (Ec σE-RPo), and a crystal structure at 3.1 Å of Mycobacterium tuberculosis transcription initiation complex with a chimeric σH/E (Mtb σH/E-RPo). The structure of Ec σE-RPo reveals key interactions essential for assembly of E. coli σE-RNAP holoenzyme and for promoter recognition and unwinding by E. coli σE. Moreover, both structures show that the non-conserved linkers (σ2/σ4 linker) of the two ECF σ factors are inserted into the active-center cleft and exit through the RNA-exit channel. We performed secondary-structure prediction of 27,670 ECF σ factors and find that their non-conserved linkers probably reach into and exit from RNAP active-center cleft in a similar manner. Further biochemical results suggest that such σ2/σ4 linker plays an important role in RPo formation, abortive production and promoter escape during ECF σ factors-mediated transcription initiation.

Crl activates transcription by stabilizing active conformation of the master stress transcription initiation factor

σ^S is a master transcription initiation factor that protects bacterial cells from various harmful environmental stresses including antibiotic pressure. Although its mechanism remains unclear, it is known that full activation of σ^S-mediated transcription requires a σ^S-specific activator, Crl. In this study, we determined a 3.80 Å cryo-EM structure of an Escherichia coli transcription activation complex (E. coli Crl-TAC) comprising E. coli σ^S-RNA polymerase (σ^S-RNAP) holoenzyme, Crl, and a nucleic-acid scaffold. The structure reveals that Crl interacts with domain 2 of σ^S (σ^S₂) and the RNAP core enzyme, but does not contact promoter DNA. Results from subsequent hydrogen-deuterium exchange mass spectrometry (HDX-MS) indicate that Crl stabilizes key structural motifs within σ^S₂ to promote the assembly of the σ^S-RNAP holoenzyme and also to facilitate formation of an RNA polymerase–promoter DNA open complex (RPo). Our study demonstrates a unique DNA contact-independent mechanism of transcription activation, thereby defining a previously unrecognized mode of transcription activation in cells.

Deciphering the essentiality and function of the anti-σM factors in Bacillus subtilis

Bacteria use alternative sigma factors to adapt to different growth and stress conditions. The Bacillus subtilis extracytoplasmic function sigma factor SigM regulates genes for cell wall synthesis and is crucial for maintaining cell wall homeostasis under stress conditions. The activity of SigM is regulated by its anti-sigma factor, YhdL, and the accessory protein YhdK. Here, we show that dysregulation of SigM caused by the absence of either component of the anti-sigma factor complex leads to toxic levels of SigM and severe growth defects. High SigM activity results from a dysregulated positive feedback loop, and can be suppressed by overexpression of the housekeeping sigma, SigA. Using a sigM merodiploid strain, we selected for suppressor mutations that allow survival of yhdL depletion strain. The recovered suppressor mutations map to the beta and beta-prime subunits of RNA polymerase core enzyme and selectively reduce SigM activity, and in some cases increase the activity of other alternative sigma factors. This work highlights the ability of mutations in RNA polymerase that remodel the sigma-core interface to differentially affect sigma factor activity, and thereby alter the transcriptional landscape of the cell.

Where to begin? Sigma factors and the selectivity of transcription initiation in bacteria

Transcription is the fundamental process that enables the expression of genetic information. DNA-directed RNA polymerase (RNAP) uses one strand of the DNA duplex as template to produce complementary RNA molecules that serve in translation (rRNA, tRNA), protein synthesis (mRNA) and regulation (sRNA). Although the RNAP core is catalytically competent for RNA synthesis, the selectivity of transcription initiation requires a sigma (σ) factor for promoter recognition and opening. Expression of alternative σ factors provides a powerful mechanism to control the expression of discrete sets of genes (a σ regulon) in response to specific nutritional, developmental or stress-related signals. Here, I review the key insights that led to the original discovery of σ factor 50 years ago and the subsequent discovery of alternative σ factors as a ubiquitous mechanism of bacterial gene regulation. These studies form a prelude to the more recent, genomics-enabled insights into the vast diversity of σ factors in bacteria.