The NusA N-terminal domain is necessary and sufficient for enhancement of transcriptional pausing via interaction with the RNA exit channel of RNA polymerase

Bacteroides expand the functional versatility of a universal transcription factor and transcribed DNA to program capsule diversity

Human gut Bacteroides species encode numerous (eight or more) tightly regulated capsular polysaccharides (CPS). Specialized paralogs of the universal transcription elongation factor NusG, called UpxY (Y), and an anti-Y UpxZ (Z) are encoded by the first two genes of each CPS operon. The Y-Z regulators combine with promoter inversions to limit CPS transcription to a single operon in most cells. Y enhances transcript elongation whereas Z inhibits noncognate Ys. How Y distinguishes among cognate CPS operons and how Z inhibits only noncognate Ys are unknown. Using in-vivo nascent-RNA sequencing and promoter-less in vitro transcription (PIVoT), we establish that Y recognizes a paused RNA polymerase via sequences in both the exposed non-template DNA and the upstream duplex DNA. Y association is aided by novel 'pause-then-escape' nascent RNA hairpins. Z binds non-cognate Ys to directly inhibit Y association. This Y-Z hierarchical regulatory program allows Bacteroides to create CPS subpopulations for optimal fitness.

A nascent riboswitch helix orchestrates robust transcriptional regulation through signal integration

Widespread manganese-sensing transcriptional riboswitches effect the dependable gene regulation needed for bacterial manganese homeostasis in changing environments. Riboswitches - like most structured RNAs - are believed to fold co-transcriptionally, subject to both ligand binding and transcription events; yet how these processes are orchestrated for robust regulation is poorly understood. Through a combination of single-molecule and bulk approaches, we discover how a single Mn2+ ion and the transcribing RNA polymerase (RNAP), paused immediately downstream by a DNA template sequence, are coordinated by the bridging switch helix P1.1 in the representative Lactococcus lactis riboswitch. This coordination achieves a heretofore-overlooked semi-docked global conformation of the nascent RNA, P1.1 base pair stabilization, transcription factor NusA ejection, and RNAP pause extension, thereby enforcing transcription readthrough. Our work demonstrates how a central, adaptable RNA helix functions analogous to a molecular fulcrum of a first-class lever system to integrate disparate signals for finely balanced gene expression control.

A nascent riboswitch helix orchestrates robust transcriptional regulation through signal integration

Widespread manganese-sensing transcriptional riboswitches effect the dependable gene regulation needed for bacterial manganese homeostasis in changing environments. Riboswitches - like most structured RNAs - are believed to fold co-transcriptionally, subject to both ligand binding and transcription events; yet how these processes are orchestrated for robust regulation is poorly understood. Through a combination of single molecule and bulk approaches, we discovered how a single Mn 2+ ion and the transcribing RNA polymerase (RNAP), paused immediately downstream by a DNA template sequence, are coordinated by the bridging switch helix P1.1 in the paradigmatic Lactococcus lactis riboswitch. This coordination achieves a heretofore-overlooked semi-docked global conformation of the nascent RNA, P1.1 base pair stabilization, transcription factor NusA ejection, and RNAP pause extension, thereby enforcing transcription readthrough. Our work demonstrates how a central, adaptable RNA helix functions analogous to a molecular fulcrum of a first-class lever system to integrate disparate signals for finely balanced gene expression control.

Correlative Escherichia coli Transcription Rate and Bubble Conformation Remodeled by NusA and NusG

Transcription is highly regulated by a variety of transcription factors, among which NusA and NusG act contradictorily in Escherichia coli (E. coli) that NusA stabilizes a paused RNA polymerase (RNAP) and NusG suppresses it. The mechanism of the NusA and NusG regulations on RNAP transcription has been addressed, but their effect on the conformational changes of the transcription bubble correlated with transcription kinetics remains elusive. By using single-molecule magnetic trap, we identify a reduction in the transcription rate of ∼40% events by NusA. Although the rest ∼60% of transcription events exhibit unaffected transcription rates, a NusA-enhanced standard deviation of the transcription rate is observed. NusA remodeling also increases the extent of DNA unwinding in the transcription bubble by 1-2 base pairs, which can be reduced by NusG. The NusG remodeling is more significant on the RNAP molecules with reduced transcription rates rather than those without. Our results provide a quantitative view on the mechanisms of transcriptional regulation by NusA and NusG factors.

Structural basis for intrinsic transcription termination

Efficient and accurate termination is required for gene transcription in all living organisms^1,2. Cellular RNA polymerases in both bacteria and eukaryotes can terminate their transcription through a factor-independent termination pathway^3,4-called intrinsic termination transcription in bacteria-in which RNA polymerase recognizes terminator sequences, stops nucleotide addition and releases nascent RNA spontaneously. Here we report a set of single-particle cryo-electron microscopy structures of Escherichia coli transcription intrinsic termination complexes representing key intermediate states of the event. The structures show how RNA polymerase pauses at terminator sequences, how the terminator RNA hairpin folds inside RNA polymerase, and how RNA polymerase rewinds the transcription bubble to release RNA and then DNA. These macromolecular snapshots define a structural mechanism for bacterial intrinsic termination and a pathway for RNA release and DNA collapse that is relevant for factor-independent termination by all RNA polymerases.

Factor-stimulated intrinsic termination: getting by with a little help from some friends

Transcription termination is known to occur via two mechanisms in bacteria, intrinsic termination (also frequently referred to as Rho-independent or factor-independent termination) and Rho-dependent termination. Based primarily on in vitro studies using Escherichia coli RNA polymerase, it was generally assumed that intrinsic termination and Rho-dependent termination are distinct mechanisms and that the signals required for intrinsic termination are present primarily within the nucleic acids. In this review, we detail recent findings from studies in Bacillus subtilis showing that intrinsic termination in this organism is highly stimulated by NusA, NusG, and even Rho. In NusA-stimulated intrinsic termination, NusA facilitates the formation of weak terminator hairpins and compensates for distal U-rich tract interruptions. In NusG-stimulated intrinsic termination, NusG stabilizes a sequence-dependent pause at the point of termination, which extends the time frame for RNA hairpins with weak terminal base pairs to form in either a NusA-stimulated or a NusA-independent fashion. In Rho-stimulated intrinsic termination, Rho prevents the formation of antiterminator-like RNA structures that could otherwise compete with the terminator hairpin. Combined, NusA, NusG, and Rho stimulate approximately 97% of all intrinsic terminators in B. subtilis. Thus, the general view that intrinsic termination is primarily a factor-independent process needs to be revised to account for recent findings. Moreover, the historical distinction between Rho-dependent and intrinsic termination is overly simplistic and needs to be modernized.

Structural basis of AlpA-dependent transcription antitermination

AlpA positively regulates a programmed cell death pathway linked to the virulence of Pseudomonas aeruginosa by recognizing an AlpA binding element within the promoter, then binding RNA polymerase directly and allowing it to bypass an intrinsic terminator positioned downstream. Here, we report the single-particle cryo-electron microscopy structures of both an AlpA-loading complex and an AlpA-loaded complex. These structures indicate that the C-terminal helix-turn-helix motif of AlpA binds to the AlpA binding element and that the N-terminal segment of AlpA forms a narrow ring inside the RNA exit channel. AlpA was also revealed to render RNAP resistant to termination signals by prohibiting RNA hairpin formation in the RNA exit channel. Structural analysis predicted that AlpA, 21Q, λQ and 82Q share the same mechanism of transcription antitermination.

Identification of putative essential protein domains from high-density transposon insertion sequencing

A first clue to gene function can be obtained by examining whether a gene is required for life in certain standard conditions, that is, whether a gene is essential. In bacteria, essential genes are usually identified by high-density transposon mutagenesis followed by sequencing of insertion sites (Tn-seq). These studies assign the term "essential" to whole genes rather than the protein domain sequences that encode the essential functions. However, genes can code for multiple protein domains that evolve their functions independently. Therefore, when essential genes code for more than one protein domain, only one of them could be essential. In this study, we defined this subset of genes as "essential domain-containing" (EDC) genes. Using a Tn-seq data set built-in Burkholderia cenocepacia K56-2, we developed an in silico pipeline to identify EDC genes and the essential protein domains they encode. We found forty candidate EDC genes and demonstrated growth defect phenotypes using CRISPR interference (CRISPRi). This analysis included two knockdowns of genes encoding the protein domains of unknown function DUF2213 and DUF4148. These putative essential domains are conserved in more than two hundred bacterial species, including human and plant pathogens. Together, our study suggests that essentiality should be assigned to individual protein domains rather than genes, contributing to a first functional characterization of protein domains of unknown function.

Dynamic competition between a ligand and transcription factor NusA governs riboswitch-mediated transcription regulation

Cotranscriptional RNA folding is widely assumed to influence the timely control of gene expression, but our understanding remains limited. In bacteria, the fluoride (F^-)-sensing riboswitch is a transcriptional control element essential to defend against toxic F^- levels. Using this model riboswitch, we find that its ligand F^- and essential bacterial transcription factor NusA compete to bind the cotranscriptionally folding RNA, opposing each other's modulation of downstream pausing and termination by RNA polymerase. Single-molecule fluorescence assays probing active transcription elongation complexes discover that NusA unexpectedly binds highly reversibly, frequently interrogating the complex for emerging, cotranscriptionally folding RNA duplexes. NusA thus fine-tunes the transcription rate in dependence of the ligand-responsive higher-order structure of the riboswitch. At the high NusA concentrations found intracellularly, this dynamic modulation is expected to lead to adaptive bacterial transcription regulation with fast response times.

Transcription complexes as RNA chaperones

To exert their functions, RNAs adopt diverse structures, ranging from simple secondary to complex tertiary and quaternary folds. In vivo, RNA folding starts with RNA transcription, and a wide variety of processes are coupled to co-transcriptional RNA folding events, including the regulation of fundamental transcription dynamics, gene regulation by mechanisms like attenuation, RNA processing or ribonucleoprotein particle formation. While co-transcriptional RNA folding and associated co-transcriptional processes are by now well accepted as pervasive regulatory principles in all organisms, investigations into the role of the transcription machinery in co-transcriptional folding processes have so far largely focused on effects of the order in which RNA regions are produced and of transcription kinetics. Recent structural and structure-guided functional analyses of bacterial transcription complexes increasingly point to an additional role of RNA polymerase and associated transcription factors in supporting co-transcriptional RNA folding by fostering or preventing strategic contacts to the nascent transcripts. In general, the results support the view that transcription complexes can act as RNA chaperones, a function that has been suggested over 30 years ago. Here, we discuss transcription complexes as RNA chaperones based on recent examples from bacterial transcription.

Macromolecular assemblies supporting transcription-translation coupling

Coordination between the molecular machineries that synthesize and decode prokaryotic mRNAs is an important layer of gene expression control known as transcription-translation coupling. While it has long been known that translation can regulate transcription and vice-versa, recent structural and biochemical work has shed light on the underlying mechanistic basis. Complexes of RNA polymerase linked to a trailing ribosome (expressomes) have been structurally characterized in a variety of states at near-atomic resolution, and also directly visualized in cells. These data are complemented by recent biochemical and biophysical analyses of transcription-translation systems and the individual components within them. Here, we review our improved understanding of the molecular basis of transcription-translation coupling. These insights are discussed in relation to our evolving understanding of the role of coupling in cells.

Transcriptional Pausing as a Mediator of Bacterial Gene Regulation

Cellular life depends on transcription of DNA by RNA polymerase to express genetic information. RNA polymerase has evolved not just to read information from DNA and write it to RNA but also to sense and process information from the cellular and extracellular environments. Much of this information processing occurs during transcript elongation, when transcriptional pausing enables regulatory decisions. Transcriptional pauses halt RNA polymerase in response to DNA and RNA sequences and structures at locations and times that help coordinate interactions with small molecules and transcription factors important for regulation. Four classes of transcriptional pause signals are now evident after decades of study: elemental pauses, backtrack pauses, hairpin-stabilized pauses, and regulator-stabilized pauses. In this review, I describe current understanding of the molecular mechanisms of these four classes of pause signals, remaining questions about how RNA polymerase responds to pause signals, and the many exciting directions now open to understand pausing and the regulation of transcript elongation on a genome-wide scale. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

β-N-Methylamino-L-Alanine (BMAA) Causes Severe Stress in Nostoc sp. PCC 7120 Cells under Diazotrophic Conditions: A Proteomic Study

Non-proteinogenic neurotoxic amino acid β-N-methylamino-L-alanine (BMAA) is synthesized by cyanobacteria, diatoms, and dinoflagellates, and is known to be a causative agent of human neurodegenerative diseases. Different phytoplankton organisms' ability to synthesize BMAA could indicate the importance of this molecule in the interactions between microalgae in nature. We were interested in the following: what kinds of mechanisms underline BMAA's action on cyanobacterial cells in different nitrogen supply conditions. Herein, we present a proteomic analysis of filamentous cyanobacteria Nostoc sp. PCC 7120 cells that underwent BMAA treatment in diazotrophic conditions. In diazotrophic growth conditions, to survive, cyanobacteria can use only biological nitrogen fixation to obtain nitrogen for life. Note that nitrogen fixation is an energy-consuming process. In total, 1567 different proteins of Nostoc sp. PCC 7120 were identified by using LC-MS/MS spectrometry. Among them, 123 proteins belonging to different functional categories were selected-due to their notable expression differences-for further functional analysis and discussion. The presented proteomic data evidences that BMAA treatment leads to very strong (up to 80%) downregulation of α (NifD) and β (NifK) subunits of molybdenum-iron protein, which is known to be a part of nitrogenase. This enzyme is responsible for catalyzing nitrogen fixation. The genes nifD and nifK are under transcriptional control of a global nitrogen regulator NtcA. In this study, we have found that BMAA impacts in a total of 22 proteins that are under the control of NtcA. Moreover, BMAA downregulates 18 proteins that belong to photosystems I or II and light-harvesting complexes; BMAA treatment under diazotrophic conditions also downregulates five subunits of ATP synthase and enzyme NAD(P)H-quinone oxidoreductase. Therefore, we can conclude that the disbalance in energy and metabolite amounts leads to severe intracellular stress that induces the upregulation of stress-activated proteins, such as starvation-inducible DNA-binding protein, four SOS-response enzymes, and DNA repair enzymes, nine stress-response enzymes, and four proteases. The presented data provide new leads into the ecological impact of BMAA on microalgal communities that can be used in future investigations.

A Peek Inside the Machines of Bacterial Nucleotide Excision Repair

Double stranded DNA (dsDNA), the repository of genetic information in bacteria, archaea and eukaryotes, exhibits a surprising instability in the intracellular environment; this fragility is exacerbated by exogenous agents, such as ultraviolet radiation. To protect themselves against the severe consequences of DNA damage, cells have evolved at least six distinct DNA repair pathways. Here, we review recent key findings of studies aimed at understanding one of these pathways: bacterial nucleotide excision repair (NER). This pathway operates in two modes: a global genome repair (GGR) pathway and a pathway that closely interfaces with transcription by RNA polymerase called transcription-coupled repair (TCR). Below, we discuss the architecture of key proteins in bacterial NER and recent biochemical, structural and single-molecule studies that shed light on the lesion recognition steps of both the GGR and the TCR sub-pathways. Although a great deal has been learned about both of these sub-pathways, several important questions, including damage discrimination, roles of ATP and the orchestration of protein binding and conformation switching, remain to be addressed.

Clusters of bacterial RNA polymerase are biomolecular condensates that assemble through liquid-liquid phase separation

Bacterial cells are small and were long thought to have little to no internal structure. However, advances in microscopy have revealed that bacteria do indeed contain subcellular compartments. But how these compartments form has remained a mystery. Recent progress in larger, more complex eukaryotic cells has identified a novel mechanism for intracellular organization known as liquid–liquid phase separation. This process causes certain types of molecules to concentrate within distinct compartments inside the cell. Here, we demonstrate that the same process also occurs in bacteria. This work, together with a growing body of literature, suggests that liquid–liquid phase separation is a common mechanism for intracellular organization in both eukaryotic and prokaryotic cells.

Once described as mere “bags of enzymes,” bacterial cells are in fact highly organized, with many macromolecules exhibiting nonuniform localization patterns. Yet the physical and biochemical mechanisms that govern this spatial heterogeneity remain largely unknown. Here, we identify liquid–liquid phase separation (LLPS) as a mechanism for organizing clusters of RNA polymerase (RNAP) in Escherichia coli. Using fluorescence imaging, we show that RNAP quickly transitions from a dispersed to clustered localization pattern as cells enter log phase in nutrient-rich media. RNAP clusters are sensitive to hexanediol, a chemical that dissolves liquid-like compartments in eukaryotic cells. In addition, we find that the transcription antitermination factor NusA forms droplets in vitro and in vivo, suggesting that it may nucleate RNAP clusters. Finally, we use single-molecule tracking to characterize the dynamics of cluster components. Our results indicate that RNAP and NusA molecules move inside clusters, with mobilities faster than a DNA locus but slower than bulk diffusion through the nucleoid. We conclude that RNAP clusters are biomolecular condensates that assemble through LLPS. This work provides direct evidence for LLPS in bacteria and demonstrates that this process can serve as a mechanism for intracellular organization in prokaryotes and eukaryotes alike.

NusA directly interacts with antitermination factor Q from phage λ

Antitermination (AT) is a ubiquitous principle in the regulation of bacterial transcription to suppress termination signals. In phage λ antiterminator protein Q controls the expression of the phage’s late genes with loading of λQ onto the transcription elongation complex halted at a σ-dependent pause requiring a specific DNA element. The molecular basis of λQ-dependent AT and its dependence on N-utilization substance (Nus) A is so far only poorly understood. Here we used solution-state nuclear magnetic resonance spectroscopy to show that the solution structure of λQ is in agreement with the crystal structure of an N-terminally truncated variant and that the 60 residues at the N-terminus are unstructured. We also provide evidence that multidomain protein NusA interacts directly with λQ via its N-terminal domain (NTD) and the acidic repeat (AR) 2 domain, with the λQ:NusA-AR2 interaction being able to release NusA autoinhibition. The binding sites for NusA-NTD and NusA-AR2 on λQ overlap and the interactions are mutually exclusive with similar affinities, suggesting distinct roles during λQ-dependent AT, e.g. the λQ:NusA-NTD interaction might position NusA-NTD in a way to suppress termination, making NusA-NTD repositioning a general scheme in AT mechanisms.

Structural basis for the function of SuhB as a transcription factor in ribosomal RNA synthesis

Ribosomal RNA synthesis in Escherichia coli involves a transcription complex, in which RNA polymerase is modified by a signal element on the transcript, Nus factors A, B, E and G, ribosomal protein S4 and inositol mono-phosphatase SuhB. This complex is resistant to ρ-dependent termination and facilitates ribosomal RNA folding, maturation and subunit assembly. The functional contributions of SuhB and their structural bases are presently unclear. We show that SuhB directly binds the RNA signal element and the C-terminal AR2 domain of NusA, and we delineate the atomic basis of the latter interaction by macromolecular crystallography. SuhB recruitment to a ribosomal RNA transcription complex depends on the RNA signal element but not on the NusA AR2 domain. SuhB in turn is required for stable integration of the NusB/E dimer into the complex. In vitro transcription assays revealed that SuhB is crucial for delaying or suppressing ρ-dependent termination, that SuhB also can reduce intrinsic termination, and that SuhB-AR2 contacts contribute to these effects. Together, our results reveal functions of SuhB during ribosomal RNA synthesis and delineate some of the underlying molecular interactions.

mTERF5 Acts as a Transcriptional Pausing Factor to Positively Regulate Transcription of Chloroplast psbEFLJ

RNA polymerase transcriptional pausing represents a major checkpoint in transcription in bacteria and metazoans, but it is unknown whether this phenomenon occurs in plant organelles. Here, we report that transcriptional pausing occurs in chloroplasts. We found that mTERF5 specifically and positively regulates the transcription of chloroplast psbEFLJ in Arabidopsis thaliana that encodes four key subunits of photosystem II. We found that mTERF5 causes the plastid-encoded RNA polymerase (PEP) complex to pause at psbEFLJ by binding to the +30 to +51 region of double-stranded DNA. Moreover, we revealed that mTERF5 interacts with pTAC6, an essential subunit of the PEP complex, although pTAC6 is not involved in the transcriptional pausing at psbEFLJ. We showed that mTERF5 recruits additional pTAC6 to the transcriptionally paused region of psbEFLJ, and the recruited pTAC6 proteins could be assembled into the PEP complex to regulate psbEFLJ transcription. Taken together, our findings shed light on the role of transcriptional pausing in chloroplast transcription in plants.

Role of a hairpin-stabilized pause in the Escherichia coli thiC riboswitch function

Transcriptional pauses have been reported in bacterial riboswitches and, in some cases, their specific positioning has been shown to be important for gene regulation. Here, we show that a hairpin structure in the Escherichia coli thiamin pyrophosphate (TPP) thiC riboswitch is involved in transcriptional pausing and ligand sensitivity. Using in vitro transcription kinetic experiments, we show that all three major transcriptional pauses in the thiC riboswitch are affected by NusA, a transcriptional factor known to stimulate hairpin-stabilized pauses. Using a truncated region of the riboswitch, we isolated the hairpin structure responsible for stabilization of the most upstream pause. Destabilization of this structure led to a weaker pause and a decreased NusA effect. In the context of the full-length riboswitch, this same mutation also led to a weaker pause, as well as a decreased TPP binding affinity. Our work suggests that RNA structures involved in transcriptional pausing in riboswitches are important for ligand sensitivity, most likely by increasing the time allowed to the ligand for binding to the riboswitch.

Mechanisms of Transcriptional Pausing in Bacteria

Pausing by RNA polymerase (RNAP) during transcription regulates gene expression in all domains of life. In this review, we recap the history of transcriptional pausing discovery, summarize advances in our understanding of the underlying causes of pausing since then, and describe new insights into the pausing mechanisms and pause modulation by transcription factors gained from structural and biochemical experiments. The accumulated evidence to date suggests that upon encountering a pause signal in the nucleic-acid sequence being transcribed, RNAP rearranges into an elemental, catalytically inactive conformer unable to load NTP substrate. The conformation, and as a consequence lifetime, of an elemental paused RNAP is modulated by backtracking, nascent RNA structure, binding of transcription regulators, or a combination of these mechanisms. We conclude the review by outlining open questions and directions for future research in the field of transcriptional pausing.

The Mechanisms of Substrate Selection, Catalysis, and Translocation by the Elongating RNA Polymerase

Multi-subunit DNA-dependent RNA polymerases synthesize all classes of cellular RNAs, ranging from short regulatory transcripts to gigantic messenger RNAs. RNA polymerase has to make each RNA product in just one try, even if it takes millions of successive nucleotide addition steps. During each step, RNA polymerase selects a correct substrate, adds it to a growing chain, and moves one nucleotide forward before repeating the cycle. However, RNA synthesis is anything but monotonous: RNA polymerase frequently pauses upon encountering mechanical, chemical and torsional barriers, sometimes stepping back and cleaving off nucleotides from the growing RNA chain. A picture in which these intermittent dynamics enable processive, accurate, and controllable RNA synthesis is emerging from complementary structural, biochemical, computational, and single-molecule studies. Here, we summarize our current understanding of the mechanism and regulation of the on-pathway transcription elongation. We review the details of substrate selection, catalysis, proofreading, and translocation, focusing on rate-limiting steps, structural elements that modulate them, and accessory proteins that appear to control RNA polymerase translocation.

Ancient Transcription Factors in the News

In every cell from bacteria to mammals, NusG-like proteins bind transcribing RNA polymerase to modulate the rate of nascent RNA synthesis and to coordinate it with numerous cotranscriptional processes that ultimately determine the transcript fate. Housekeeping NusG factors regulate expression of the bulk of the genome, whereas their highly specialized paralogs control just a few targets.

In every cell from bacteria to mammals, NusG-like proteins bind transcribing RNA polymerase to modulate the rate of nascent RNA synthesis and to coordinate it with numerous cotranscriptional processes that ultimately determine the transcript fate. Housekeeping NusG factors regulate expression of the bulk of the genome, whereas their highly specialized paralogs control just a few targets. In Escherichia coli, NusG stimulates silencing of horizontally acquired genes, while its paralog RfaH counters NusG action by activating a subset of these genes. Acting alone or as part of regulatory complexes, NusG factors can promote uninterrupted RNA synthesis, bring about transcription pausing or premature termination, modulate RNA processing, and facilitate translation. Recent structural and mechanistic studies of NusG homologs from all domains of life reveal molecular details of multifaceted interactions that underpin their unexpectedly diverse regulatory roles. NusG proteins share conserved binding sites on RNA polymerase and many effects on the transcription elongation complex but differ in their mechanisms of recruitment, interactions with nucleic acids and secondary partners, and regulatory outcomes. Strikingly, some can alternate between autoinhibited and activated states that possess dramatically different secondary structures to achieve exquisite target specificity.

RNA Polymerase Accommodates a Pause RNA Hairpin by Global Conformational Rearrangements that Prolong Pausing

Sequence-specific pausing by RNA polymerase (RNAP) during transcription plays crucial and diverse roles in gene expression. In bacteria, RNA structures are thought to fold within the RNA exit channel of the RNAP and can increase pause lifetimes significantly. The biophysical mechanism of pausing is uncertain. We used single-particle cryo-EM to determine structures of paused complexes, including a 3.8-Å structure of an RNA hairpin-stabilized, paused RNAP that coordinates RNA folding in the his operon attenuation control region of E. coli. The structures revealed a half-translocated pause state (RNA post-translocated, DNA pre-translocated) that can explain transcriptional pausing and a global conformational change of RNAP that allosterically inhibits trigger loop folding and can explain pause hairpin action. Pause hairpin interactions with the RNAP RNA exit channel suggest how RNAP guides the formation of nascent RNA structures.

Experimental evolution of diverse Escherichia coli metabolic mutants identifies genetic loci for convergent adaptation of growth rate

Cell growth is determined by substrate availability and the cell’s metabolic capacity to assimilate substrates into building blocks. Metabolic genes that determine growth rate may interact synergistically or antagonistically, and can accelerate or slow growth, depending on genetic background and environmental conditions. We evolved a diverse set of Escherichia coli single-gene deletion mutants with a spectrum of growth rates and identified mutations that generally increase growth rate. Despite the metabolic differences between parent strains, mutations that enhanced growth largely mapped to core transcription machinery, including the β and β’ subunits of RNA polymerase (RNAP) and the transcription elongation factor, NusA. The structural segments of RNAP that determine enhanced growth have been previously implicated in antibiotic resistance and in the control of transcription elongation and pausing. We further developed a computational framework to characterize how the transcriptional changes that occur upon acquisition of these mutations affect growth rate across strains. Our experimental and computational results provide evidence for cases in which RNAP mutations shift the competitive balance between active transcription and gene silencing. This study demonstrates that mutations in specific regions of RNAP are a convergent adaptive solution that can enhance the growth rate of cells from distinct metabolic states.

The loss of a metabolic function caused by gene deletion can be compensated, in certain cases, by the concurrent mutation of a second gene. Whether such gene pairs share a local chemical or regulatory relationship or interact via a non-local mechanism has implications for the co-evolution of genetic changes, development of alternatives to gene therapy, and the design of combination antimicrobial therapies that select against resistance. Yet, we lack a comprehensive knowledge of adaptive responses to metabolic mutations, and our understanding of the mechanisms underlying genetic rescue remains limited. We present results of a laboratory evolution approach that has the potential to address both challenges, showing that mutations in specific regions of RNA polymerase enhance growth rates of distinct mutant strains of Escherichia coli with a spectrum of growth defects. Several of these adaptive mutations are deleterious when engineered directly into the original wild-type strain under alternative cultivation conditions, and thus have epistatic rescue properties when paired with the corresponding primary metabolic gene deletions. Our combination of adaptive evolution, directed genetic engineering, and mathematical analysis of transcription and growth rate distinguishes between rescue interactions that are specific or non-specific to a particular deletion. Our study further supports a model for RNA polymerase as a locus of convergent adaptive evolution from different sub-optimal metabolic starting points.

Structural Basis for NusA Stabilized Transcriptional Pausing

Transcriptional pausing by RNA polymerases (RNAPs) is a key mechanism to regulate gene expression in all kingdoms of life and is a prerequisite for transcription termination. The essential bacterial transcription factor NusA stimulates both pausing and termination of transcription, thus playing a central role. Here, we report single-particle electron cryo-microscopy reconstructions of NusA bound to paused E. coli RNAP elongation complexes with and without a pause-enhancing hairpin in the RNA exit channel. The structures reveal four interactions between NusA and RNAP that suggest how NusA stimulates RNA folding, pausing, and termination. An asymmetric translocation intermediate of RNA and DNA converts the active site of the enzyme into an inactive state, providing a structural explanation for the inhibition of catalysis. Comparing RNAP at different stages of pausing provides insights on the dynamic nature of the process and the role of NusA as a regulatory factor.

Architecture of a transcribing-translating expressome

DNA transcription is functionally coupled to messenger RNA (mRNA) translation in bacteria, but how this is achieved remains unclear. Here we show that RNA polymerase (RNAP) and the ribosome of Escherichia coli can form a defined transcribing and translating "expressome" complex. The cryo-electron microscopic structure of the expressome reveals continuous protection of ~30 nucleotides of mRNA extending from the RNAP active center to the ribosome decoding center. The RNAP-ribosome interface includes the RNAP subunit α carboxyl-terminal domain, which is required for RNAP-ribosome interaction in vitro and for pronounced cell growth defects upon translation inhibition in vivo, consistent with its function in transcription-translation coupling. The expressome structure can only form during transcription elongation and explains how translation can prevent transcriptional pausing, backtracking, and termination.

Transcriptional precision and accuracy in development: from measurements to models and mechanisms

During development, genes are transcribed at specific times, locations and levels. In recent years, the emergence of quantitative tools has significantly advanced our ability to measure transcription with high spatiotemporal resolution in vivo. Here, we highlight recent studies that have used these tools to characterize transcription during development, and discuss the mechanisms that contribute to the precision and accuracy of the timing, location and level of transcription. We attempt to disentangle the discrepancies in how physicists and biologists use the term ‘precision' to facilitate interactions using a common language. We also highlight selected examples in which the coupling of mathematical modeling with experimental approaches has provided important mechanistic insights, and call for a more expansive use of mathematical modeling to exploit the wealth of quantitative data and advance our understanding of animal transcription.

Summary: This Review highlights how high-resolution quantitative tools and theoretical models have formed our current view of the mechanisms determining precision and accuracy in the timing, location and level of transcription in the Drosophila embryo.

In-Culture Cross-Linking of Bacterial Cells Reveals Large-Scale Dynamic Protein-Protein Interactions at the Peptide Level

Identification of dynamic protein–protein interactions at the peptide level on a proteomic scale is a challenging approach that is still in its infancy. We have developed a system to cross-link cells directly in culture with the special lysine cross-linker bis(succinimidyl)-3-azidomethyl-glutarate (BAMG). We used the Gram-positive model bacterium Bacillus subtilis as an exemplar system. Within 5 min extensive intracellular cross-linking was detected, while intracellular cross-linking in a Gram-negative species, Escherichia coli, was still undetectable after 30 min, in agreement with the low permeability in this organism for lipophilic compounds like BAMG. We were able to identify 82 unique interprotein cross-linked peptides with <1% false discovery rate by mass spectrometry and genome-wide database searching. Nearly 60% of the interprotein cross-links occur in assemblies involved in transcription and translation. Several of these interactions are new, and we identified a binding site between the δ and β′ subunit of RNA polymerase close to the downstream DNA channel, providing a clue into how δ might regulate promoter selectivity and promote RNA polymerase recycling. Our methodology opens new avenues to investigate the functional dynamic organization of complex protein assemblies involved in bacterial growth. Data are available via ProteomeXchange with identifier PXD006287.

Gfh factors and NusA cooperate to stimulate transcriptional pausing and termination

Lineage-specific Gfh factors from the radioresistant bacterium Deinococcus radiodurans, which bind within the secondary channel of RNA polymerase, stimulate transcriptional pausing at a wide range of pause signals (elemental, hairpin-dependent, post-translocated, backtracking-dependent, and consensus pauses) and increase intrinsic termination. Universal bacterial factor NusA, which binds near the RNA exit channel, enhances the effects of Gfh factors on termination and hairpin-dependent pausing but do not act on other pause sites. It is proposed that NusA and Gfh target different steps in the pausing pathway and may act together to regulate transcription under stress conditions. Thus, transcription factors that interact with nascent RNA in the RNA exit channel can communicate with secondary channel regulators to modulate RNA polymerase activities.

In Vitro Transcription Assays and Their Application in Drug Discovery

In vitro transcription assays have been developed and widely used for many years to study the molecular mechanisms involved in transcription. This process requires multi-subunit DNA-dependent RNA polymerase (RNAP) and a series of transcription factors that act to modulate the activity of RNAP during gene expression. Sequencing gel electrophoresis of radiolabeled transcripts is used to provide detailed mechanistic information on how transcription proceeds and what parameters can affect it. In this paper we describe the protocol to study how the essential elongation factor NusA regulates transcriptional pausing, as well as a method to identify an antibacterial agent targeting transcription initiation through inhibition of RNAP holoenzyme formation. These methods can be used a as platform for the development of additional approaches to explore the mechanism of action of the transcription factors which still remain unclear, as well as new antibacterial agents targeting transcription which is an underutilized drug target in antibiotic research and development.

Transcription is regulated by NusA:NusG interaction

NusA and NusG are major regulators of bacterial transcription elongation, which act either in concert or antagonistically. Both bind to RNA polymerase (RNAP), regulating pausing as well as intrinsic and Rho-dependent termination. Here, we demonstrate by nuclear magnetic resonance spectroscopy that the Escherichia coli NusG amino-terminal domain forms a complex with the acidic repeat domain 2 (AR2) of NusA. The interaction surface of either transcription factor overlaps with the respective binding site for RNAP. We show that NusA-AR2 is able to remove NusG from RNAP. Our in vivo and in vitro results suggest that interaction between NusA and NusG could play various regulatory roles during transcription, including recruitment of NusG to RNAP, resynchronization of transcription:translation coupling, and modulation of termination efficiency.

Transcription Elongation Factor NusA Is a General Antagonist of Rho-dependent Termination in Escherichia coli

NusA is an essential protein that binds to RNA polymerase and also to the nascent RNA and influences transcription by inducing pausing and facilitating the process of transcription termination/antitermination. Its participation in Rho-dependent transcription termination has been perceived, but the molecular nature of this involvement is not known. We hypothesized that, because both Rho and NusA are RNA-binding proteins and have the potential to target the same RNA, the latter is likely to influence the global pattern of the Rho-dependent termination. Analyses of the nascent RNA binding properties and consequent effects on the Rho-dependent termination functions of specific NusA-RNA binding domain mutants revealed an existence of Rho-NusA direct competition for the overlappingnut(NusA-binding site) andrut(Rho-binding site) sites on the RNA. This leads to delayed entry of Rho at therutsite that inhibits the latter's RNA release process. High density tiling microarray profiles of these NusA mutants revealed that a significant number of genes, together with transcripts from intergenic regions, are up-regulated. Interestingly, the majority of these genes were also up-regulated when the Rho function was compromised. These results provide strong evidence for the existence of NusA-binding sites in different operons that are also the targets of Rho-dependent terminations. Our data strongly argue in favor of a direct competition between NusA and Rho for the access of specific sites on the nascent transcripts in different parts of the genome. We propose that this competition enables NusA to function as a global antagonist of the Rho function, which is unlike its role as a facilitator of hairpin-dependent termination.

TFE and Spt4/5 open and close the RNA polymerase clamp during the transcription cycle

Transcription is an intrinsically dynamic process and requires the coordinated interplay of RNA polymerases (RNAPs) with nucleic acids and transcription factors. Classical structural biology techniques have revealed detailed snapshots of a subset of conformational states of the RNAP as they exist in crystals. A detailed view of the conformational space sampled by the RNAP and the molecular mechanisms of the basal transcription factors E (TFE) and Spt4/5 through conformational constraints has remained elusive. We monitored the conformational changes of the flexible clamp of the RNAP by combining a fluorescently labeled recombinant 12-subunit RNAP system with single-molecule FRET measurements. We measured and compared the distances across the DNA binding channel of the archaeal RNAP. Our results show that the transition of the closed to the open initiation complex, which occurs concomitant with DNA melting, is coordinated with an opening of the RNAP clamp that is stimulated by TFE. We show that the clamp in elongation complexes is modulated by the nontemplate strand and by the processivity factor Spt4/5, both of which stimulate transcription processivity. Taken together, our results reveal an intricate network of interactions within transcription complexes between RNAP, transcription factors, and nucleic acids that allosterically modulate the RNAP during the transcription cycle.

Nascent RNA length dictates opposing effects of NusA on antitermination

The NusA protein is a universally conserved bacterial transcription elongation factor that binds RNA polymerase (RNAP). When functioning independently, NusA enhances intrinsic termination. Paradoxically, NusA stimulates the function of the N and Q antiterminator proteins of bacteriophage λ. The mechanistic basis for NusA's functional plasticity is poorly understood. Here we uncover an effect of nascent RNA length on the ability of NusA to collaborate with Q. Ordinarily, Q engages RNAP during early elongation when it is paused at a specific site just downstream of the phage late-gene promoter. NusA facilitates this engagement process and both proteins remain associated with the transcription elongation complex (TEC) as it escapes the pause and transcribes the late genes. We show that the λ-related phage 82 Q protein (82Q) can also engage RNAP that is paused at a promoter-distal position and thus contains a nascent RNA longer than that associated with the natively positioned TEC. However, the effect of NusA in this context is antagonistic rather than stimulatory. Moreover, cleaving the long RNA associated with the promoter-distal TEC restores NusA's stimulatory effect. Our findings reveal a critical role for nascent RNA in modulating NusA's effect on 82Q-mediated antitermination, with implications for understanding NusA's functional plasticity.

Mechanisms of Bacterial Transcription Termination: All Good Things Must End

Transcript termination is essential for accurate gene expression and the removal of RNA polymerase (RNAP) at the ends of transcription units. In bacteria, two mechanisms are responsible for proper transcript termination: intrinsic termination and Rho-dependent termination. Intrinsic termination is mediated by signals directly encoded within the DNA template and nascent RNA, whereas Rho-dependent termination relies upon the adenosine triphosphate-dependent RNA translocase Rho, which binds nascent RNA and dissociates the elongation complex. Although significant progress has been made in understanding these pathways, fundamental details remain undetermined. Among those that remain unresolved are the existence of an inactivated intermediate in the intrinsic termination pathway, the role of Rho-RNAP interactions in Rho-dependent termination, and the mechanisms by which accessory factors and nucleoid-associated proteins affect termination. We describe current knowledge, discuss key outstanding questions, and highlight the importance of defining the structural rearrangements of RNAP that are involved in the two mechanisms of transcript termination.

Transcription Elongation Factor NusA Is a General Antagonist of Rho-dependent Termination in Escherichia coli

NusA is an essential protein that binds to RNA polymerase and also to the nascent RNA and influences transcription by inducing pausing and facilitating the process of transcription termination/antitermination. Its participation in Rho-dependent transcription termination has been perceived, but the molecular nature of this involvement is not known. We hypothesized that, because both Rho and NusA are RNA-binding proteins and have the potential to target the same RNA, the latter is likely to influence the global pattern of the Rho-dependent termination. Analyses of the nascent RNA binding properties and consequent effects on the Rho-dependent termination functions of specific NusA-RNA binding domain mutants revealed an existence of Rho-NusA direct competition for the overlappingnut(NusA-binding site) andrut(Rho-binding site) sites on the RNA. This leads to delayed entry of Rho at therutsite that inhibits the latter's RNA release process. High density tiling microarray profiles of these NusA mutants revealed that a significant number of genes, together with transcripts from intergenic regions, are up-regulated. Interestingly, the majority of these genes were also up-regulated when the Rho function was compromised. These results provide strong evidence for the existence of NusA-binding sites in different operons that are also the targets of Rho-dependent terminations. Our data strongly argue in favor of a direct competition between NusA and Rho for the access of specific sites on the nascent transcripts in different parts of the genome. We propose that this competition enables NusA to function as a global antagonist of the Rho function, which is unlike its role as a facilitator of hairpin-dependent termination.

A Two-Way Street: Regulatory Interplay between RNA Polymerase and Nascent RNA Structure

The vectorial (5'-to-3' at varying velocity) synthesis of RNA by cellular RNA polymerases (RNAPs) creates a rugged kinetic landscape, demarcated by frequent, sometimes long-lived, pauses. In addition to myriad gene-regulatory roles, these pauses temporally and spatially program the co-transcriptional, hierarchical folding of biologically active RNAs. Conversely, these RNA structures, which form inside or near the RNA exit channel, interact with the polymerase and adjacent protein factors to influence RNA synthesis by modulating pausing, termination, antitermination, and slippage. Here, we review the evolutionary origin, mechanistic underpinnings, and regulatory consequences of this interplay between RNAP and nascent RNA structure. We categorize and rationalize the extensive linkage between the transcriptional machinery and its product, and provide a framework for future studies.

Bacterial Transcription as a Target for Antibacterial Drug Development

Transcription, the first step of gene expression, is carried out by the enzyme RNA polymerase (RNAP) and is regulated through interaction with a series of protein transcription factors. RNAP and its associated transcription factors are highly conserved across the bacterial domain and represent excellent targets for broad-spectrum antibacterial agent discovery. Despite the numerous antibiotics on the market, there are only two series currently approved that target transcription. The determination of the three-dimensional structures of RNAP and transcription complexes at high resolution over the last 15 years has led to renewed interest in targeting this essential process for antibiotic development by utilizing rational structure-based approaches. In this review, we describe the inhibition of the bacterial transcription process with respect to structural studies of RNAP, highlight recent progress toward the discovery of novel transcription inhibitors, and suggest additional potential antibacterial targets for rational drug design.

NusA-dependent transcription termination prevents misregulation of global gene expression

Intrinsic transcription terminators consist of an RNA hairpin followed by a U-rich tract, and these signals can trigger termination without the involvement of additional factors. Although NusA is known to stimulate intrinsic termination in vitro, the in vivo targets and global impact of NusA are not known because it is essential for viability. Using genome-wide 3' end-mapping on an engineered Bacillus subtilis NusA depletion strain, we show that weak suboptimal terminators are the principle NusA substrates. Moreover, a subclass of weak non-canonical terminators was identified that completely depend on NusA for effective termination. NusA-dependent terminators tend to have weak hairpins and/or distal U-tract interruptions, supporting a model in which NusA is directly involved in the termination mechanism. Depletion of NusA altered global gene expression directly and indirectly via readthrough of suboptimal terminators. Readthrough of NusA-dependent terminators caused misregulation of genes involved in essential cellular functions, especially DNA replication and metabolism. We further show that nusA is autoregulated by a transcription attenuation mechanism that does not rely on antiterminator structures. Instead, NusA-stimulated termination in its 5' UTR dictates the extent of transcription into the operon, thereby ensuring tight control of cellular NusA levels.

Bacterial RNA polymerase can retain σ70 throughout transcription

Production of a messenger RNA proceeds through sequential stages of transcription initiation and transcript elongation and termination. During each of these stages, RNA polymerase (RNAP) function is regulated by RNAP-associated protein factors. In bacteria, RNAP-associated σ factors are strictly required for promoter recognition and have historically been regarded as dedicated initiation factors. However, the primary σ factor in Escherichia coli, σ(70), can remain associated with RNAP during the transition from initiation to elongation, influencing events that occur after initiation. Quantitative studies on the extent of σ(70) retention have been limited to complexes halted during early elongation. Here, we used multiwavelength single-molecule fluorescence-colocalization microscopy to observe the σ(70)-RNAP complex during initiation from the λ PR' promoter and throughout the elongation of a long (>2,000-nt) transcript. Our results provide direct measurements of the fraction of actively transcribing complexes with bound σ(70) and the kinetics of σ(70) release from actively transcribing complexes. σ(70) release from mature elongation complexes was slow (0.0038 s(-1)); a substantial subpopulation of elongation complexes retained σ(70) throughout transcript elongation, and this fraction depended on the sequence of the initially transcribed region. We also show that elongation complexes containing σ(70) manifest enhanced recognition of a promoter-like pause element positioned hundreds of nucleotides downstream of the promoter. Together, the results provide a quantitative framework for understanding the postinitiation roles of σ(70) during transcription.

Nus Factors of Escherichia coli

The highly conserved Nus factors of bacteria were discovered as essential host proteins for the growth of temperate phage λ in Escherichia coli. Later, their essentiality and functions in transcription, translation, and, more recently, in DNA repair have been elucidated. Close involvement of these factors in various gene networks and circuits is also emerging from recent genomic studies. We have described a detailed overview of their biochemistry, structures, and various cellular functions, as well as their interactions with other macromolecules. Towards the end, we have envisaged different uncharted areas of studies with these factors, including their participation in pathogenicity.

Determination of RNA polymerase binding surfaces of transcription factors by NMR spectroscopy

In bacteria, RNA polymerase (RNAP), the central enzyme of transcription, is regulated by N-utilization substance (Nus) transcription factors. Several of these factors interact directly, and only transiently, with RNAP to modulate its function. As details of these interactions are largely unknown, we probed the RNAP binding surfaces of Escherichia coli (E. coli) Nus factors by nuclear magnetic resonance (NMR) spectroscopy. Perdeuterated factors with [¹H,¹³C]-labeled methyl groups of Val, Leu, and Ile residues were titrated with protonated RNAP. After verification of this approach with the N-terminal domain (NTD) of NusG and RNAP we determined the RNAP binding site of NusE. It overlaps with the NusE interaction surface for the NusG C-terminal domain, indicating that RNAP and NusG compete for NusE and suggesting possible roles for the NusE:RNAP interaction, e.g. in antitermination and direct transcription:translation coupling. We solved the solution structure of NusA-NTD by NMR spectroscopy, identified its RNAP binding site with the same approach we used for NusG-NTD, and here present a detailed model of the NusA-NTD:RNAP:RNA complex.

Regulation of Transcript Elongation

Bacteria lack subcellular compartments and harbor a single RNA polymerase that synthesizes both structural and protein-coding RNAs, which are cotranscriptionally processed by distinct pathways. Nascent rRNAs fold into elaborate secondary structures and associate with ribosomal proteins, whereas nascent mRNAs are translated by ribosomes. During elongation, nucleic acid signals and regulatory proteins modulate concurrent RNA-processing events, instruct RNA polymerase where to pause and terminate transcription, or act as roadblocks to the moving enzyme. Communications among complexes that carry out transcription, translation, repair, and other cellular processes ensure timely execution of the gene expression program and survival under conditions of stress. This network is maintained by auxiliary proteins that act as bridges between RNA polymerase, ribosome, and repair enzymes, blurring boundaries between separate information-processing steps and making assignments of unique regulatory functions meaningless. Understanding the regulation of transcript elongation thus requires genome-wide approaches, which confirm known and reveal new regulatory connections.

Regulation of transcription elongation and termination

This article will review our current understanding of transcription elongation and termination in E. coli. We discuss why transcription elongation complexes pause at certain template sites and how auxiliary host and phage transcription factors affect elongation and termination. The connection between translation and transcription elongation is described. Finally we present an overview indicating where progress has been made and where it has not.

N protein from lambdoid phages transforms NusA into an antiterminator by modulating NusA-RNA polymerase flap domain interactions

Interaction of the lambdoid phage N protein with the bacterial transcription elongation factor NusA is the key component in the process of transcription antitermination. A convex surface of E. coli NusA-NTD, located opposite to its RNA polymerase-binding domain (the β-flap domain), directly interacts with N in the antitermination complex. We hypothesized that this N-NusA interaction induces allosteric effects on the NusA-RNAP interaction leading to transformation of NusA into a facilitator of the antitermination process. Here we showed that mutations in β-flap domain specifically defective for N antitermination exhibited altered NusA-nascent RNA interaction and have widened RNA exit channel indicating an intricate role of flap domain in the antitermination. The presence of N reoriented the RNAP binding surface of NusA-NTD, which changed its interaction pattern with the flap domain. These changes caused significant spatial rearrangement of the β-flap as well as the β′ dock domains to form a more constricted RNA exit channel in the N-modified elongation complex (EC), which might play key role in converting NusA into a facilitator of the N antitermination. We propose that in addition to affecting the RNA exit channel and the active center of the EC, β-flap domain rearrangement is also a mechanistic component in the N antitermination process.

RNA polymerase-induced remodelling of NusA produces a pause enhancement complex

Pausing during transcription elongation is a fundamental activity in all kingdoms of life. In bacteria, the essential protein NusA modulates transcriptional pausing, but its mechanism of action has remained enigmatic. By combining structural and functional studies we show that a helical rearrangement induced in NusA upon interaction with RNA polymerase is the key to its modulatory function. This conformational change leads to an allosteric re-positioning of conserved basic residues that could enable their interaction with an RNA pause hairpin that forms in the exit channel of the polymerase. This weak interaction would stabilize the paused complex and increases the duration of the transcriptional pause. Allosteric spatial re-positioning of regulatory elements may represent a general approach used across all taxa for modulation of transcription and protein–RNA interactions.