The elongation factor RfaH and the initiation factor sigma bind to the same site on the transcription elongation complex

Metamorphic proteins under a computational microscope: Lessons from a fold-switching RfaH protein

Metamorphic proteins constitute unexpected paradigms of the protein folding problem, as their sequences encode two alternative folds, which reversibly interconvert within biologically relevant timescales to trigger different cellular responses. Once considered a rare aberration, metamorphism may be common among proteins that must respond to rapidly changing environments, exemplified by NusG-like proteins, the only transcription factors present in every domain of life. RfaH, a specialized paralog of bacterial NusG, undergoes an all-α to all-β domain switch to activate expression of virulence and conjugation genes in many animal and plant pathogens and is the quintessential example of a metamorphic protein. The dramatic nature of RfaH structural transformation and the richness of its evolutionary history makes for an excellent model for studying how metamorphic proteins switch folds. Here, we summarize the structural and functional evidence that sparked the discovery of RfaH as a metamorphic protein, the experimental and computational approaches that enabled the description of the molecular mechanism and refolding pathways of its structural interconversion, and the ongoing efforts to find signatures and general properties to ultimately describe the protein metamorphome.

NusG, an Ancient Yet Rapidly Evolving Transcription Factor

Timely and accurate RNA synthesis depends on accessory proteins that instruct RNA polymerase (RNAP) where and when to start and stop transcription. Among thousands of transcription factors, NusG/Spt5 stand out as the only universally conserved family of regulators. These proteins interact with RNAP to promote uninterrupted RNA synthesis and with diverse cellular partners to couple transcription to RNA processing, modification or translation, or to trigger premature termination of aberrant transcription. NusG homologs are present in all cells that utilize bacterial-type RNAP, from endosymbionts to plants, underscoring their ancient and essential function. Yet, in stark contrast to other core RNAP components, NusG family is actively evolving: horizontal gene transfer and sub-functionalization drive emergence of NusG paralogs, such as bacterial LoaP, RfaH, and UpxY. These specialized regulators activate a few (or just one) operons required for expression of antibiotics, capsules, secretion systems, toxins, and other niche-specific macromolecules. Despite their common origin and binding site on the RNAP, NusG homologs differ in their target selection, interacting partners and effects on RNA synthesis. Even among housekeeping NusGs from diverse bacteria, some factors promote pause-free transcription while others slow the RNAP down. Here, we discuss structure, function, and evolution of NusG proteins, focusing on unique mechanisms that determine their effects on gene expression and enable bacterial adaptation to diverse ecological niches.

Origins and Molecular Evolution of the NusG Paralog RfaH

The only universally conserved family of transcription factors comprises housekeeping regulators and their specialized paralogs, represented by well-studied NusG and RfaH. Despite their ubiquity, little information is available on the evolutionary origins, functions, and gene targets of the NusG family members. We built a hidden Markov model profile of RfaH and identified its homologs in sequenced genomes. While NusG is widespread among bacterial phyla and coresides with genes encoding RNA polymerase and ribosome in all except extremely reduced genomes, RfaH is mostly limited to Proteobacteria and lacks common gene neighbors. RfaH activates only a few xenogeneic operons that are otherwise silenced by NusG and Rho. Phylogenetic reconstructions reveal extensive duplications and horizontal transfer of rfaH genes, including those borne by plasmids, and the molecular evolution pathway of RfaH, from "early" exclusion of the Rho terminator and tightened RNA polymerase binding to "late" interactions with the ops DNA element and autoinhibition, which together define the RfaH regulon. Remarkably, NusG is not only ubiquitous in Bacteria but also common in plants, where it likely modulates the transcription of plastid genes.IMPORTANCE In all domains of life, NusG-like proteins make contacts similar to those of RNA polymerase and promote pause-free transcription yet may play different roles, defined by their divergent interactions with nucleic acids and accessory proteins, in the same cell. This duality is illustrated by Escherichia coli NusG and RfaH, which silence and activate xenogenes, respectively. We combined sequence analysis and recent functional and structural insights to envision the evolutionary transformation of NusG, a core regulator that we show is present in all cells using bacterial RNA polymerase, into a virulence factor, RfaH. Our results suggest a stepwise conversion of a NusG duplicate copy into a sequence-specific regulator which excludes NusG from its targets but does not compromise the regulation of housekeeping genes. We find that gene duplication and lateral transfer give rise to a surprising diversity within the only ubiquitous family of transcription factors.

Complex Multilevel Control of Hemolysin Production by Uropathogenic Escherichia coli

Uropathogenic E. coli (UPEC) is the major cause of urinary tract infections and a frequent cause of sepsis. Nearly half of all UPEC strains produce the potent cytotoxin hemolysin, and its expression is associated with enhanced virulence. In this study, we explored hemolysin variation within the globally dominant UPEC ST131 clone, finding that strains from the ST131 sublineage with the greatest multidrug resistance also possess the strongest hemolytic activity. We also employed an innovative forward genetic screen to define the set of genes required for hemolysin production. Using this approach, and subsequent targeted mutagenesis and complementation, we identified new hemolysin-controlling elements involved in LPS inner core biosynthesis and cytoplasmic chaperone activity, and we show that mechanistically they are required for hemolysin secretion. These original discoveries substantially enhance our understanding of hemolysin regulation, secretion and function.

Uropathogenic Escherichia coli (UPEC) is the major cause of urinary tract infections. Nearly half of all UPEC strains secrete hemolysin, a cytotoxic pore-forming toxin. Here, we show that the prevalence of the hemolysin toxin gene (hlyA) is highly variable among the most common 83 E. coli sequence types (STs) represented on the EnteroBase genome database. To explore this diversity in the context of a defined monophyletic lineage, we contextualized sequence variation of the hlyCABD operon within the genealogy of the globally disseminated multidrug-resistant ST131 clone. We show that sequence changes in hlyCABD and its newly defined 1.616-kb-long leader sequence correspond to phylogenetic designation, and that ST131 strains with the strongest hemolytic activity belong to the most extensive multidrug-resistant sublineage (clade C2). To define the set of genes involved in hemolysin production, the clade C2 strain S65EC was completely sequenced and subjected to a genome-wide screen by combining saturated transposon mutagenesis and transposon-directed insertion site sequencing with the capacity to lyse red blood cells. Using this approach, and subsequent targeted mutagenesis and complementation, 13 genes were confirmed to be specifically required for production of active hemolysin. New hemolysin-controlling elements included discrete sets of genes involved in lipopolysaccharide (LPS) inner core biosynthesis (waaC, waaF, waaG, and rfaE) and cytoplasmic chaperone activity (dnaK and dnaJ), and we show these are required for hemolysin secretion. Overall, this work provides a unique description of hemolysin sequence diversity in a single clonal lineage and describes a complex multilevel system of regulatory control for this important toxin.

Structure and Function of RNA Polymerases and the Transcription Machineries

In all living organisms, the flow of genetic information is a two-step process: first DNA is transcribed into RNA, which is subsequently used as template for protein synthesis during translation. In bacteria, archaea and eukaryotes, transcription is carried out by multi-subunit RNA polymerases (RNAPs) sharing a conserved architecture of the RNAP core. RNAPs catalyse the highly accurate polymerisation of RNA from NTP building blocks, utilising DNA as template, being assisted by transcription factors during the initiation, elongation and termination phase of transcription. The complexity of this highly dynamic process is reflected in the intricate network of protein-protein and protein-nucleic acid interactions in transcription complexes and the substantial conformational changes of the RNAP as it progresses through the transcription cycle.In this chapter, we will first briefly describe the early work that led to the discovery of multisubunit RNAPs. We will then discuss the three-dimensional organisation of RNAPs from the bacterial, archaeal and eukaryotic domains of life, highlighting the conserved nature, but also the domain-specific features of the transcriptional apparatus. Another section will focus on transcription factors and their role in regulating the RNA polymerase throughout the different phases of the transcription cycle. This includes a discussion of the molecular mechanisms and dynamic events that govern transcription initiation, elongation and termination.

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.

Locking the nontemplate DNA to control transcription

Universally conserved NusG/Spt5 factors reduce RNA polymerase pausing and arrest. In a widely accepted model, these proteins bridge the RNA polymerase clamp and lobe domains across the DNA channel, inhibiting the clamp opening to promote pause-free RNA synthesis. However, recent structures of paused transcription elongation complexes show that the clamp does not open and suggest alternative mechanisms of antipausing. Among these mechanisms, direct contacts of NusG/Spt5 proteins with the nontemplate DNA in the transcription bubble have been proposed to prevent unproductive DNA conformations and thus inhibit arrest. We used Escherichia coli RfaH, whose interactions with DNA are best characterized, to test this idea. We report that RfaH stabilizes the upstream edge of the transcription bubble, favoring forward translocation, and protects the upstream duplex DNA from exonuclease cleavage. Modeling suggests that RfaH loops the nontemplate DNA around its surface and restricts the upstream DNA duplex mobility. Strikingly, we show that RfaH-induced DNA protection and antipausing activity can be mimicked by shortening the nontemplate strand in elongation complexes assembled on synthetic scaffolds. We propose that remodeling of the nontemplate DNA controls recruitment of regulatory factors and R-loop formation during transcription elongation across all life.

Reading of the non-template DNA by transcription elongation factors

Unlike transcription initiation and termination, which have easily discernable signals, such as promoters and terminators, elongation is regulated through a dynamic network involving RNA/DNA pause signals and states-rather than sequence-specific protein interactions. A report by Nedialkov et al. () provides experimental evidence for sequence-specific recruitment of elongation factor RfaH to transcribing RNA polymerase (RNAP) and outlines the mechanism of gene expression regulation by restraint ('locking') of the DNA non-template strand. According to this model, the elongation complex pauses at the so called 'operon polarity sequence' (found in some long bacterial operons coding for virulence genes), when the usually flexible non-template DNA strand adopts a distinct hairpin-loop conformation on the surface of transcribing RNAP. Sequence-specific binding of RfaH to this DNA segment facilitates conversion of RfaH from its inactive closed to its active open conformation. The interaction network formed between RfaH, non-template DNA and RNAP locks DNA in a conformation that renders RNAP resistant to pausing and termination. The effects of such locking on elongation can be mimicked by restraint of the non-template strand due to its shortening. This work advances our understanding of transcription regulation and has important implications for the action of general elongation factors, such as NusG, which lack apparent sequence-specificity, as well as for the mechanisms of other linked processes, such as transcription-coupled DNA repair.

Structural Basis for Transcript Elongation Control by NusG Family Universal Regulators

NusG/RfaH/Spt5 transcription elongation factors are the only transcription regulators conserved across all life. Bacterial NusG regulates RNA polymerase (RNAP) elongation complexes (ECs) across most genes, enhancing elongation by suppressing RNAP backtracking and coordinating ρ-dependent termination and translation. The NusG paralog RfaH engages the EC only at operon polarity suppressor (ops) sites and suppresses both backtrack and hairpin-stabilized pausing. We used single-particle cryoelectron microscopy (cryo-EM) to determine structures of ECs at ops with NusG or RfaH. Both factors chaperone base-pairing of the upstream duplex DNA to suppress backtracking, explaining stimulation of elongation genome-wide. The RfaH-opsEC structure reveals how RfaH confers operon specificity through specific recognition of an ops hairpin in the single-stranded nontemplate DNA and tighter binding to the EC to exclude NusG. Tight EC binding by RfaH sterically blocks the swiveled RNAP conformation necessary for hairpin-stabilized pausing. The universal conservation of NusG/RfaH/Spt5 suggests that the molecular mechanisms uncovered here are widespread.

The universally-conserved transcription factor RfaH is recruited to a hairpin structure of the non-template DNA strand

RfaH, a transcription regulator of the universally conserved NusG/Spt5 family, utilizes a unique mode of recruitment to elongating RNA polymerase to activate virulence genes. RfaH function depends critically on an ops sequence, an exemplar of a consensus pause, in the non-template DNA strand of the transcription bubble. We used structural and functional analyses to elucidate the role of ops in RfaH recruitment. Our results demonstrate that ops induces pausing to facilitate RfaH binding and establishes direct contacts with RfaH. Strikingly, the non-template DNA forms a hairpin in the RfaH:ops complex structure, flipping out a conserved T residue that is specifically recognized by RfaH. Molecular modeling and genetic evidence support the notion that ops hairpin is required for RfaH recruitment. We argue that both the sequence and the structure of the non-template strand are read out by transcription factors, expanding the repertoire of transcriptional regulators in all domains of life.

Possible roles of σ-dependent RNA polymerase pausing in transcription regulation

The σ subunit of bacterial RNA polymerase is required for promoter recognition during transcription initiation but may also regulate transcription elongation. The principal σ⁷⁰ subunit of Escherichia coli was shown to travel with RNA polymerase and induce transcriptional pausing at promoter-like motifs, with potential regulatory output. We recently demonstrated that an alternative σ³⁸ subunit can also induce RNA polymerase pausing. Here, we outline proposed regulatory roles of σ-dependent pausing in bacteria and discuss possible interplay between alternative σ variants and regulatory factors during transcription elongation.

σ38-dependent promoter-proximal pausing by bacterial RNA polymerase

Transcription initiation by bacterial RNA polymerase (RNAP) requires a variable σ subunit that directs it to promoters for site-specific priming of RNA synthesis. The principal σ subunit responsible for expression of house-keeping genes can bind the transcription elongation complex after initiation and induce RNAP pausing through specific interactions with promoter-like motifs in transcribed DNA. We show that the stationary phase and stress response σ³⁸ subunit can also induce pausing by Escherichia coli RNAP on DNA templates containing promoter-like motifs in the transcribed regions. The pausing depends on σ³⁸ contacts with the DNA template and RNAP core enzyme and results in formation of backtracked transcription elongation complexes, which can be reactivated by Gre factors that induce RNA cleavage by RNAP. Our data suggest that σ³⁸ can bind the transcription elongation complex in trans but likely acts in cis during transcription initiation, by staying bound to RNAP and recognizing promoter-proximal pause signals. Analysis of σ³⁸-dependent promoters reveals that a substantial fraction of them contain potential pause-inducing motifs, suggesting that σ³⁸-depended pausing may be a common phenomenon in bacterial transcription.

Structure determination of transient transcription complexes

The determination of detailed 3D structures of large and transient multicomponent complexes remains challenging. Here I describe the approaches that were used and developed by our laboratory to achieve structure solution of eukaryotic transcription complexes. I hope this collection serves as a resource for structural biologists seeking solutions for difficult structure determination projects.

Evolutionary Origins of Two-Barrel RNA Polymerases and Site-Specific Transcription Initiation

Evolution-related multisubunit RNA polymerases (RNAPs) carry out RNA synthesis in all domains life. Although their catalytic cores and fundamental mechanisms of transcription elongation are conserved, the initiation stage of the transcription cycle differs substantially in bacteria, archaea, and eukaryotes in terms of the requirements for accessory factors and details of the molecular mechanisms. This review focuses on recent insights into the evolution of the transcription apparatus with regard to (a) the surprisingly pervasive double-Ψ β-barrel active-site configuration among different nucleic acid polymerase families, (b) the origin and phylogenetic distribution of TBP, TFB, and TFE transcription factors, and

A global analysis of transcription reveals two modes of Spt4/5 recruitment to archaeal RNA polymerase

The archaeal transcription apparatus is closely related to the eukaryotic RNA polymerase (RNAP) II system, while archaeal genomes are more similar to bacteria with densely packed genes organized in operons. This makes understanding transcription in archaea vital, both in terms of molecular mechanisms and evolution. Very little is known about how archaeal cells orchestrate transcription on a systems level. We have characterized the genome-wide occupancy of the Methanocaldococcus jannaschii transcription machinery and its transcriptome. Our data reveal how the TATA and BRE promoter elements facilitate recruitment of the essential initiation factors TATA-binding protein and transcription factor B, respectively, which in turn are responsible for the loading of RNAP into the transcription units. The occupancies of RNAP and Spt4/5 strongly correlate with each other and with RNA levels. Our results show that Spt4/5 is a general elongation factor in archaea as its presence on all genes matches RNAP. Spt4/5 is recruited proximal to the transcription start site on the majority of transcription units, while on a subset of genes, including rRNA and CRISPR loci, Spt4/5 is recruited to the transcription elongation complex during early elongation within 500 base pairs of the transcription start site and akin to its bacterial homologue NusG.

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.

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.

Structural and biochemical insights into the DNA-binding mode of MjSpt4p:Spt5 complex at the exit tunnel of RNAPII

Spt5 (NusG in bacteria) is the only RNA polymerase-associated factor known to be conserved in all three domains of life. In archaea and eukaryotes, Spt5 associates with Spt4, an elongation factor that is absent in bacteria, to form a functional heterodimeric complex. Previous studies suggest that the Spt4:Spt5 complex interacts directly with DNA at the double-stranded DNA exit tunnel of RNA polymerase to regulate gene transcription. In this study, the DNA-binding ability of Spt4:Spt5 from the archaeon Methanocaldococcus jannaschii was confirmed via nuclear magnetic resonance chemical shift perturbation and fluorescence polarization assays. Crystallographic analysis of the full-length MjSpt4:Spt5 revealed two distinct conformations of the C-terminal KOW domain of Spt5. A similar alkaline region was found on the Spt4:Spt5 surface in both crystal forms, and identified as double-stranded DNA binding patch through mutagenesis-fluorescence polarization assays. Based on these structural and biochemical data, the Spt4:Spt5-DNA binding model was built for the first time.

The primary σ factor in Escherichia coli can access the transcription elongation complex from solution in vivo

The σ subunit of bacterial RNA polymerase (RNAP) confers on the enzyme the ability to initiate promoter-specific transcription. Although σ factors are generally classified as initiation factors, σ can also remain associated with, and modulate the behavior of, RNAP during elongation. Here we establish that the primary σ factor in Escherichia coli, σ⁷⁰, can function as an elongation factor in vivo by loading directly onto the transcription elongation complex (TEC) in trans. We demonstrate that σ⁷⁰ can bind in trans to TECs that emanate from either a σ⁷⁰-dependent promoter or a promoter that is controlled by an alternative σ factor. We further demonstrate that binding of σ⁷⁰ to the TEC in trans can have a particularly large impact on the dynamics of transcription elongation during stationary phase. Our findings establish a mechanism whereby the primary σ factor can exert direct effects on the composition of the entire transcriptome, not just that portion that is produced under the control of σ⁷⁰-dependent promoters.

DOI:http://dx.doi.org/10.7554/eLife.10514.001

Proteins are made following instructions that are encoded by sections of DNA called genes. In the first step of protein production, an enzyme called RNA polymerase uses the gene as a template to make molecules of messenger ribonucleic acid (mRNA). This process—known as transcription—starts when RNA polymerase binds to a site at the start of a gene. The enzyme then moves along the DNA, assembling the mRNA as it goes. This stage of transcription is known as elongation and continues until the RNA polymerase reaches the end of the gene.

In bacteria, RNA polymerase needs a family of proteins called sigma factors to help it identify and bind to the start sites associated with the genes that will be transcribed. In the well studied bacterium known as E. coli, the primary sigma factor that is required for transcription initiation on most genes is called sigma 70. Recent research has shown that sigma 70 also influences the activity of RNA polymerase during elongation. During this stage, the RNA polymerase and several other proteins interact to form a complex called the transcription elongation complex (or TEC for short). However, it is not clear how sigma 70 gains access to this complex: does it simply remain with RNA polymerase after transcription starts, or is it freshly incorporated into the TEC during elongation?

Goldman, Nair et al. found that sigma 70 is able to incorporate into TECs during elongation and causes them to pause at specific sites in the gene. Sigma 70 can even incorporate into TECs on genes where transcription was initiated by a different sigma factor. These findings indicate that sigma 70 can directly influence the transcription of all genes, not just the genes with start sites that are recognized by this sigma factor.

Goldman et al. also observed that in cells that were growing and dividing rapidly, the pauses that occurred due to sigma 70 associating with TECs were of shorter duration than those in cells that were growing slowly. This implies that the growth status of the cells modulates the pausing of RNA polymerase during transcription. In the future, it will be important to understand how much influence the primary sigma factor has on RNA polymerase during elongation in E. coli and other bacteria.

DOI:http://dx.doi.org/10.7554/eLife.10514.002

Fluorescently labeled recombinant RNAP system to probe archaeal transcription initiation

The transcriptional apparatus is one of the most complex cellular machineries and in order to fully appreciate the behavior of these protein-nucleic acid assemblies one has to understand the molecular details of the system. In addition to classical biochemical and structural studies, fluorescence-based techniques turned out as an important--and sometimes the critical--tool to obtain information about the molecular mechanisms of transcription. Fluorescence is not only a multi-modal parameter that can report on molecular interactions, environment and oligomerization status. Measured on the single-molecule level it also informs about the heterogeneity of the system and gives access to distances and distance changes in the molecular relevant nanometer regime. A pre-requisite for fluorescence-based measurements is the site-specific incorporation of one or multiple fluorescent dyes. In this respect, the archaeal transcription system is ideally suited as it is available in a fully recombinant form and thus allows for site-specific modification via sophisticated labeling schemes. The application of fluorescence based approaches to the archaeal transcription apparatus changed our understanding of the molecular mechanisms and dynamics that drive archaeal transcription and unraveled the architecture of transcriptional complexes not amenable to structural interrogation.

Ubiquitous transcription factors display structural plasticity and diverse functions: NusG proteins - Shifting shapes and paradigms

Numerous accessory factors modulate RNA polymerase response to regulatory signals and cellular cues and establish communications with co-transcriptional RNA processing. Transcription regulators are astonishingly diverse, with similar mechanisms arising via convergent evolution. NusG/Spt5 elongation factors comprise the only universally conserved and ancient family of regulators. They bind to the conserved clamp helices domain of RNA polymerase, which also interacts with non-homologous initiation factors in all domains of life, and reach across the DNA channel to form processivity clamps that enable uninterrupted RNA chain synthesis. In addition to this ubiquitous function, NusG homologs exert diverse, and sometimes opposite, effects on gene expression by competing with each other and other regulators for binding to the clamp helices and by recruiting auxiliary factors that facilitate termination, antitermination, splicing, translation, etc. This surprisingly diverse range of activities and the underlying unprecedented structural changes make studies of these "transformer" proteins both challenging and rewarding.

Transcriptional control of RfaH on polysialic and colanic acid synthesis by Escherichia coli K92

The transcriptional antiterminator RfaH promotes transcription of long operons encoding surface cell components important for the virulence of Escherichiacoli pathogens. In this paper, we show that RfaH enhanced kps expression for the synthesis of group 2 polysialic acid capsule in E. coli K92. In addition, we demonstrate for the first time that RfaH promotes cps expression for the synthesis of colanic acid, a cell wall component with apparently no role on pathogenicity. Finally, we show a novel RfaH requirement for growth at low temperatures.

Interdomain contacts control folding of transcription factor RfaH

Escherichia coli RfaH activates gene expression by tethering the elongating RNA polymerase to the ribosome. This bridging action requires a complete refolding of the RfaH C-terminal domain (CTD) from an α-helical hairpin, which binds to the N-terminal domain (NTD) in the free protein, to a β-barrel, which interacts with the ribosomal protein S10 following RfaH recruitment to its target operons. The CTD forms a β-barrel when expressed alone or proteolytically separated from the NTD, indicating that the α-helical state is trapped by the NTD, perhaps co-translationally. Alternatively, the interdomain contacts may be sufficient to drive the formation of the α-helical form. Here, we use functional and NMR analyses to show that the denatured RfaH refolds into the native state and that RfaH in which the order of the domains is reversed is fully functional in vitro and in vivo. Our results indicate that all information necessary to determine its fold is encoded within RfaH itself, whereas accessory factors or sequential folding of NTD and CTD during translation are dispensable. These findings suggest that universally conserved RfaH homologs may change folds to accommodate diverse interaction partners and that context-dependent protein refolding may be widespread in nature.

Archaeology of RNA polymerase: factor swapping during the transcription cycle

All RNAPs (RNA polymerases) repeatedly make use of their DNA template by progressing through the transcription cycle multiple times. During transcription initiation and elongation, distinct sets of transcription factors associate with multisubunit RNAPs and modulate their nucleic-acid-binding and catalytic properties. Between the initiation and elongation phases of the cycle, the factors have to be exchanged by a largely unknown mechanism. We have shown that the binding sites for initiation and elongation factors are overlapping and that the binding of the factors to RNAP is mutually exclusive. This ensures an efficient exchange or 'swapping' of factors and could furthermore assist RNAP during promoter escape, enabling robust transcription. A similar mechanism applies to the bacterial RNAP system. The elongation factors are evolutionarily conserved between the bacterial (NusG) and archaeo-eukaryotic (Spt5) systems; however, the initiation factors [σ and TBP (TATA-box-binding protein)/TF (transcription factor) B respectively] are not. Therefore we propose that this factor-swapping mechanism, operating in all three domains of life, is the outcome of convergent evolution.

Pause, play, repeat: CDKs push RNAP II's buttons

Cyclin-dependent kinases (CDKs) play a central role in governing eukaryotic cell division. It is becoming clear that the transcription cycle of RNA polymerase II (RNAP II) is also regulated by CDKs; in metazoans, the cell cycle and transcriptional CDK networks even share an upstream activating kinase, which is itself a CDK. From recent chemical-genetic analyses we know that CDKs and their substrates control events both early in transcription (the transition from initiation to elongation) and late (3' end formation and transcription termination). Moreover, mutual dependence on CDK activity might couple the "beginning" and "end" of the cycle, to ensure the fidelity of mRNA maturation and the efficient recycling of RNAP II from sites of termination to the transcription start site (TSS). As is the case for CDKs involved in cell cycle regulation, different transcriptional CDKs act in defined sequence on multiple substrates. These phosphorylations are likely to influence gene expression by several mechanisms, including direct, allosteric effects on the transcription machinery, co-transcriptional recruitment of proteins needed for mRNA-capping, splicing and 3' end maturation, dependent on multisite phosphorylation of the RNAP II C-terminal domain (CTD) and, perhaps, direct regulation of RNA-processing or histone-modifying machinery. Here we review these recent advances, and preview the emerging challenges for transcription-cycle research.

High-mobility-group a-like CarD binds to a DNA site optimized for affinity and position and to RNA polymerase to regulate a light-inducible promoter in Myxococcus xanthus

The CarD-CarG complex controls various cellular processes in the bacterium Myxococcus xanthus including fruiting body development and light-induced carotenogenesis. The CarD N-terminal domain, which defines the large CarD_CdnL_TRCF protein family, binds to CarG, a zinc-associated protein that does not bind DNA. The CarD C-terminal domain resembles eukaryotic high-mobility-group A (HMGA) proteins, and its DNA binding AT hooks specifically recognize the minor groove of appropriately spaced AT-rich tracts. Here, we investigate the determinants of the only known CarD binding site, the one crucial in CarD-CarG regulation of the promoter of the carQRS operon (P(QRS)), a light-inducible promoter dependent on the extracytoplasmic function (ECF) σ factor CarQ. In vitro, mutating either of the 3-bp AT tracts of this CarD recognition site (TTTCCAGAGCTTT) impaired DNA binding, shifting the AT tracts relative to P(QRS) had no effect or marginally lowered DNA binding, and replacing the native site by the HMGA1a binding one at the human beta interferon promoter (with longer AT tracts) markedly enhanced DNA binding. In vivo, however, all of these changes deterred P(QRS) activation in wild-type M. xanthus, as well as in a strain with the CarD-CarG pair replaced by the Anaeromyxobacter dehalogenans CarD-CarG (CarD(Ad)-CarG(Ad)). CarD(Ad)-CarG(Ad) is functionally equivalent to CarD-CarG despite the lower DNA binding affinity in vitro of CarD(Ad), whose C-terminal domain resembles histone H1 rather than HMGA. We show that CarD physically associates with RNA polymerase (RNAP) specifically via interactions with the RNAP β subunit. Our findings suggest that CarD regulates a light-inducible, ECF σ-dependent promoter by coupling RNAP recruitment and binding to a specific DNA site optimized for affinity and position.

The Spt4-Spt5 complex: a multi-faceted regulator of transcription elongation

In all domains of life, elongating RNA polymerases require the assistance of accessory factors to maintain their processivity and regulate their rate. Among these elongation factors, the Spt5/NusG factors stand out. Members of this protein family appear to be the only transcription accessory proteins that are universally conserved across all domains of life. In archaea and eukaryotes, Spt5 associates with a second protein, Spt4. In addition to regulating elongation, the eukaryotic Spt4-Spt5 complex appears to couple chromatin modification states and RNA processing to transcription elongation. This review discusses the experimental bases for our current understanding of Spt4-Spt5 function and recent studies that are beginning to elucidate the structure of Spt4-Spt5/RNA polymerase complexes and mechanism of Spt4-Spt5 action. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

Characteristics of σ-dependent pausing by RNA polymerases from Escherichia coli and Thermus aquaticus

The σ(70) subunit of RNA polymerase (RNAP) is the major transcription initiation factor in Escherichia coli. During transcription initiation, conserved region 2 of the σ(70) subunit interacts with the -10 promoter element and plays a key role in DNA melting around the starting point of transcription. During transcription elongation, the σ(70) subunit can induce pauses in RNA synthesis owing to interactions of region 2 with DNA regions similar to the -10 promoter element. We demonstrated that the major σ subunit from Thermus aquaticus (σ(A)) is also able to induce transcription pausing by T. aquaticus RNAP. However, hybrid RNAP containing the σ(A) subunit and E. coli core RNAP is unable to form pauses during elongation, while being able to recognize promoters and initiate transcription. Inability of the σ(A) subunit to induce pausing by E. coli RNAP is explained by the substitutions of non-conserved amino acids in region 2, in the subregions interacting with the RNAP core enzyme. Thus, changes in the structure of region 2 of the σ(70) subunit have stronger effects on transcription pausing than on promoter recognition, likely by weakening the interactions of the σ subunit with the core RNAP during transcription elongation.

A nexus for gene expression-molecular mechanisms of Spt5 and NusG in the three domains of life

Evolutionary related multisubunit RNA polymerases (RNAPs) transcribe the genomes of all living organisms. Whereas the core subunits of RNAPs are universally conserved in all three domains of life—indicative of a common evolutionary descent—this only applies to one RNAP-associated transcription factor—Spt5, also known as NusG in bacteria. All other factors that aid RNAP during the transcription cycle are specific for the individual domain or only conserved between archaea and eukaryotes. Spt5 and its bacterial homologue NusG regulate gene expression in several ways by (i) modulating transcription processivity and promoter proximal pausing, (ii) coupling transcription and RNA processing or translation, and (iii) recruiting termination factors and thereby silencing laterally transferred DNA and protecting the genome against double-stranded DNA breaks. This review discusses recent discoveries that identify Spt5-like factors as evolutionary conserved nexus for the regulation and coordination of the machineries responsible for information processing in the cell.

A novel phage-encoded transcription antiterminator acts by suppressing bacterial RNA polymerase pausing

Gp39, a small protein encoded by Thermus thermophilus phage P23–45, specifically binds the host RNA polymerase (RNAP) and inhibits transcription initiation. Here, we demonstrate that gp39 also acts as an antiterminator during transcription through intrinsic terminators. The antitermination activity of gp39 relies on its ability to suppress transcription pausing at poly(U) tracks. Gp39 also accelerates transcription elongation by decreasing RNAP pausing and backtracking but does not significantly affect the rates of catalysis of individual reactions in the RNAP active center. We mapped the RNAP-gp39 interaction site to the β flap, a domain that forms a part of the RNA exit channel and is also a likely target for λ phage antiterminator proteins Q and N, and for bacterial elongation factor NusA. However, in contrast to Q and N, gp39 does not depend on NusA or other auxiliary factors for its activity. To our knowledge, gp39 is the first characterized phage-encoded transcription factor that affects every step of the transcription cycle and suppresses transcription termination through its antipausing activity.

Competing for the clamp: promoting RNA polymerase processivity and managing the transition from initiation to elongation

Transcription elongation factor NusG/Spt5 spans the central cleft of RNA polymerase and functionally competes with transcription initiation factors. This work highlights the RNA polymerase clamp as a target for regulation and points to dynamic interactions between initiation and elongation machineries.

The β subunit gate loop is required for RNA polymerase modification by RfaH and NusG

In all organisms, RNA polymerase (RNAP) relies on accessory factors to complete synthesis of long RNAs. These factors increase RNAP processivity by reducing pausing and termination, but their molecular mechanisms remain incompletely understood. We identify the β gate loop as an RNAP element required for antipausing activity of a bacterial virulence factor RfaH, a member of the universally conserved NusG family. Interactions with the gate loop are necessary for suppression of pausing and termination by RfaH, but are dispensable for RfaH binding to RNAP mediated by the β' clamp helices. We hypothesize that upon binding to the clamp helices and the gate loop RfaH bridges the gap across the DNA channel, stabilizing RNAP contacts with nucleic acid and disfavoring isomerization into a paused state. We show that contacts with the gate loop are also required for antipausing by NusG and propose that most NusG homologs use similar mechanisms to increase RNAP processivity.

The initiation factor TFE and the elongation factor Spt4/5 compete for the RNAP clamp during transcription initiation and elongation

TFIIE and the archaeal homolog TFE enhance DNA strand separation of eukaryotic RNAPII and the archaeal RNAP during transcription initiation by an unknown mechanism. We have developed a fluorescently labeled recombinant M. jannaschii RNAP system to probe the archaeal transcription initiation complex, consisting of promoter DNA, TBP, TFB, TFE, and RNAP. We have localized the position of the TFE winged helix (WH) and Zinc ribbon (ZR) domains on the RNAP using single-molecule FRET. The interaction sites of the TFE WH domain and the transcription elongation factor Spt4/5 overlap, and both factors compete for RNAP binding. Binding of Spt4/5 to RNAP represses promoter-directed transcription in the absence of TFE, which alleviates this effect by displacing Spt4/5 from RNAP. During elongation, Spt4/5 can displace TFE from the RNAP elongation complex and stimulate processivity. Our results identify the RNAP “clamp” region as a regulatory hot spot for both transcription initiation and transcription elongation.

Structural transitions in the transcription elongation complexes of bacterial RNA polymerase during σ-dependent pausing

A transcription initiation factor, the σ⁷⁰ subunit of Escherichia coli RNA polymerase (RNAP) induces transcription pausing through the binding to a promoter-like pause-inducing sequence in the DNA template during transcription elongation. Here, we investigated the mechanism of σ-dependent pausing using reconstituted transcription elongation complexes which allowed highly efficient and precisely controlled pause formation. We demonstrated that, following engagement of the σ subunit to the pause site, RNAP continues RNA synthesis leading to formation of stressed elongation complexes, in which the nascent RNA remains resistant to Gre-induced cleavage while the transcription bubble and RNAP footprint on the DNA template extend in downstream direction, likely accompanied by DNA scrunching. The stressed complexes can then either break σ-mediated contacts and continue elongation or isomerize to a backtracked conformation. Suppressing of the RNAP backtracking decreases pausing and increases productive elongation. On the contrary, core RNAP mutations that impair RNAP interactions with the downstream part of the DNA template stimulate pausing, presumably by destabilizing the stressed complexes. We propose that interplay between DNA scrunching and RNAP backtracking may have an essential role in transcription pausing and its regulation in various systems.

Domain interactions of the transcription–translation coupling factor Escherichia coli NusG are intermolecular and transient

The bacterial transcription factor NusG (N-utilization substance G) is suggested to act as a key coupling factor between transcription and translation [Burmann, Schweimer, Luo, Wahl, Stitt, Gottesman and Rösch (2010) Science 328, 501–504] and contributes to phage λ-mediated antitermination in Escherichia coli that enables read-through of early transcription termination sites. E. coli NusG consists of two structurally and functionally distinct domains that are connected through a flexible linker. The homologous Aquifex aeolicus NusG, with a secondary structure that is highly similar to E. coli NusG shows direct interaction between its N- and C-terminal domains in a domain-swapped dimer. In the present study, we performed NMR paramagnetic relaxation enhancement measurements and identified interdomain interactions that were concentration dependent and thus probably not only weak and transient, but also predominantly intermolecular. This notion of two virtually independent domains in a monomeric protein was supported by 15N-relaxation measurements. Thus we suggest that a regulatory role of NusG interdomain interactions is highly unlikely.

Termination and antitermination: RNA polymerase runs a stop sign

Termination signals induce rapid and irreversible dissociation of the nascent transcript from RNA polymerase. Terminators at the end of genes prevent unintended transcription into the downstream genes, whereas terminators in the upstream regulatory leader regions adjust expression of the structural genes in response to metabolic and environmental signals. Premature termination within an operon leads to potentially deleterious defects in the expression of the downstream genes, but also provides an important surveillance mechanism. This Review discusses the actions of bacterial and phage antiterminators that allow RNA polymerase to override a terminator when the circumstances demand it.

Mutations in β' subunit of Escherichia coli RNA polymerase perturb the activator polymerase functional interaction required for promoter clearance

Transcription activator C employs a unique mechanism to activate mom gene of bacteriophage Mu. The activation process involves, facilitating the recruitment of RNA polymerase (RNAP) by altering the topology of the promoter and enhancing the promoter clearance by reducing the abortive transcription. To understand the basis of this multi-step activation mechanism, we investigated the nature of the physical interaction between C and RNAP during the process. A variety of assays revealed that only DNA-bound C contacts the β' subunit of RNAP. Consistent to these results, we have also isolated RNAP mutants having mutations in the β' subunit which were compromised in C-mediated activation. Mutant RNAPs show reduced productive transcription and increased abortive initiation specifically at the C-dependent mom promoter. Positive control (pc) mutants of C, defective in interaction with RNAP, retained the property of recruiting RNAP to the promoter but were unable to enhance promoter clearance. These results strongly suggest that the recruitment of RNAP to the mom promoter does not require physical interaction with C, whereas a contact between the β' subunit and the activator, and the subsequent allosteric changes in the active site of the enzyme are essential for the enhancement of promoter clearance.

Single-molecule studies of RNA polymerase: one singular sensation, every little step it takes

Transcription is the first of many biochemical steps that turn the genetic information found in DNA into the proteins responsible for driving cellular processes. In this review, we highlight certain advantages of single-molecule techniques in the study of prokaryotic and eukaryotic transcription, and the specific ways in which these techniques complement conventional, ensemble-based biochemistry. We focus on recent literature, highlighting examples where single-molecule methods have provided fresh insights into mechanism. We also present recent technological advances and outline future directions in the field.

Architecture of the RNA polymerase-Spt4/5 complex and basis of universal transcription processivity

Spt5 and NusG play a conserved role in stimulating RNA polymerase II transcription elongation and processivity. Here, the crystal structure of Spt4/5 bound to the RNA polymerase clamp domain reveals that the factor binds above DNA and RNA in the active centre cleft preventing premature dissociation of the polymerase.

Related RNA polymerases (RNAPs) carry out cellular gene transcription in all three kingdoms of life. The universal conservation of the transcription machinery extends to a single RNAP-associated factor, Spt5 (or NusG in bacteria), which renders RNAP processive and may have arisen early to permit evolution of long genes. Spt5 associates with Spt4 to form the Spt4/5 heterodimer. Here, we present the crystal structure of archaeal Spt4/5 bound to the RNAP clamp domain, which forms one side of the RNAP active centre cleft. The structure revealed a conserved Spt5–RNAP interface and enabled modelling of complexes of Spt4/5 counterparts with RNAPs from all kingdoms of life, and of the complete yeast RNAP II elongation complex with bound Spt4/5. The N-terminal NGN domain of Spt5/NusG closes the RNAP active centre cleft to lock nucleic acids and render the elongation complex stable and processive. The C-terminal KOW1 domain is mobile, but its location is restricted to a region between the RNAP clamp and wall above the RNA exit tunnel, where it may interact with RNA and/or other factors.

Initial transcribed region sequences influence the composition and functional properties of the bacterial elongation complex

The bacterial RNA polymerase (RNAP) holoenzyme consists of a catalytic core enzyme (α(2)ββ'ω) in complex with a σ factor that is essential for promoter recognition and transcription initiation. During early elongation, the stability of interactions between σ and the remainder of the transcription complex decreases. Nevertheless, there is no mechanistic requirement for release of σ upon the transition to elongation. Furthermore, σ can remain associated with RNAP during transcription elongation and influence regulatory events that occur during transcription elongation. Here we demonstrate that promoter-like DNA sequence elements within the initial transcribed region that are known to induce early elongation pausing through sequence-specific interactions with σ also function to increase the σ content of downstream elongation complexes. Our findings establish σ-dependent pausing as a mechanism by which initial transcribed region sequences can influence the composition and functional properties of the transcription elongation complex over distances of at least 700 base pairs.

Structural basis for the bacterial transcription-repair coupling factor/RNA polymerase interaction

The transcription-repair coupling factor (TRCF, the product of the mfd gene) is a widely conserved bacterial protein that mediates transcription-coupled DNA repair. TRCF uses its ATP-dependent DNA translocase activity to remove transcription complexes stalled at sites of DNA damage, and stimulates repair by recruiting components of the nucleotide excision repair pathway to the site. A protein/protein interaction between TRCF and the β-subunit of RNA polymerase (RNAP) is essential for TRCF function. CarD (also called CdnL), an essential regulator of rRNA transcription in Mycobacterium tuberculosis, shares a homologous RNAP interacting domain with TRCF and also interacts with the RNAP β-subunit. We determined the 2.9-Å resolution X-ray crystal structure of the RNAP interacting domain of TRCF complexed with the RNAP-β1 domain, which harbors the TRCF interaction determinants. The structure reveals details of the TRCF/RNAP protein/protein interface, providing a basis for the design and interpretation of experiments probing TRCF, and by homology CarD, function and interactions with the RNAP.

Functional analysis of Thermus thermophilus transcription factor NusG

Transcription elongation factors from the NusG family are ubiquitous from bacteria to humans and play diverse roles in the regulation of gene expression. These proteins consist of at least two domains. The N-terminal domains directly bind to the largest, β′ in bacteria, subunit of RNA polymerase (RNAP), whereas the C-terminal domains interact with other cellular components and serve as platforms for the assembly of large nucleoprotein complexes. Escherichia coli NusG and its paralog RfaH modify RNAP into a fast, pause-resistant state but the detailed molecular mechanism of this modification remains unclear since no high-resolution structural data are available for the E. coli system. We wanted to investigate whether Thermus thermophilus (Tth) NusG can be used as a model for structural studies of this family of regulators. Here, we show that Tth NusG slows down rather than facilitates transcript elongation by its cognate RNAP. On the other hand, similarly to the E. coli regulators, Tth NusG apparently binds near the upstream end of the transcription bubble, competes with σ^A, and favors forward translocation by RNAP. Our data suggest that the mechanism of NusG recruitment to RNAP is universally conserved even though the regulatory outcomes among its homologs may appear distinct.

E. coli NusG inhibits backtracking and accelerates pause-free transcription by promoting forward translocation of RNA polymerase

NusG is an essential transcription factor in Escherichia coli that is capable of increasing the overall rate of transcription. Transcript elongation by RNA polymerase (RNAP) is frequently interrupted by pauses of varying durations, and NusG is known to decrease the occupancy of at least some paused states. However, it has not been established whether NusG enhances transcription chiefly by (1) increasing the rate of elongation between pauses, (2) reducing the lifetimes of pauses, or (3) reducing the rate of entry into paused states. Here, we studied transcription by single molecules of RNAP under various conditions of ribonucleoside triphosphate concentration, applied load, and temperature, using an optical trapping assay capable of distinguishing pauses as brief as 1 s. We found that NusG increases the rate of elongation, that is, the pause-free velocity along the template. Because pauses are off-pathway states that compete with elongation, we observed a concomitant decrease in the rate of entry into short-lifetime, paused states. The effects on short pauses and elongation were comparatively modest, however. More dramatic was the effect of NusG on suppressing entry into long-lifetime ("stabilized") pauses. Because a significant fraction of the time required for the transcription of a typical gene may be occupied by long pauses, NusG is capable of exerting a significant modulatory effect on the rates of RNA synthesis. The observed properties of NusG were consistent with a unified model where the function of this accessory factor is to promote transcriptionally downstream motion of the enzyme along the DNA template, which has the effect of forward-biasing RNAP from the pre-translocated state toward the post-translocated state.

Functional regions of the N-terminal domain of the antiterminator RfaH

RfaH is a bacterial elongation factor that increases expression of distal genes in several long, horizontally acquired operons. RfaH is recruited to the transcription complex during RNA chain elongation through specific interactions with a DNA element called ops. Following recruitment, RfaH remains bound to RNA polymerase (RNAP) and acts as an antiterminator by reducing RNAP pausing and termination at some factor-independent and Rho-dependent signals. RfaH consists of two domains connected by a flexible linker. The N-terminal RfaH domain (RfaH^N) recognizes the ops element, binds to the RNAP and reduces pausing and termination in vitro. Functional analysis of single substitutions in this domain reported here suggests that three separate RfaH^N regions mediate these functions. We propose that a polar patch on one side of RfaH^N interacts with the non-template DNA strand during recruitment, whereas a hydrophobic surface on the opposite side of RfaH^N remains bound to the β′ subunit clamp helices domain throughout transcription of the entire operon. The third region is apparently dispensable for RfaH binding to the transcription complex but is required for the antitermination modification of RNAP.

Elongation by RNA polymerase: a race through roadblocks

Transcription is the first and most regulated step of gene expression. RNA polymerase (RNAP) is the heart of the transcription machinery and a major target for numerous regulatory pathways in living cells. The crystal structures of transcription complexes formed by bacterial RNAP in various configurations have provided a number of breakthroughs in understanding basic, universal mechanisms of transcription and have revealed regulatory 'hot spots' in RNAP that serve as targets and anchors for auxiliary transcription factors. In combination with biochemical analyses, these structures allow feasible modeling of the regulatory complexes for which experimental structural data are still missing. The available structural information suggests a number of general mechanistic predictions that provide a reference point and direction for future studies of transcription regulation.

Macromolecular micromovements: how RNA polymerase translocates

Multi-subunit DNA-dependent RNA polymerases synthesize RNA molecules thousands of nucleotides long. The reiterative reaction of nucleotide condensation occurs at rates of tens of nucleotides per second, invariably linked to the translocation of the enzyme along the DNA template, or threading of the DNA and the nascent RNA molecule through the enzyme. Reiteration of the nucleotide addition/translocation cycle without dissociation from the DNA and RNA requires both isomorphic and metamorphic conformational flexibility of a magnitude substantial enough to accommodate the requisite molecular motions. Here we review some of the more recently acquired insights into the structural flexibility and morphic fluctuations of RNA polymerases and their mechanistic implications.