Regulation of transcriptional pausing through the secondary channel of RNA polymerase

Suppressor mutations in Escherichia coli RNA polymerase alter transcription initiation but do not affect translesion RNA synthesis in vitro

Bacterial RNA polymerase (RNAP) coordinates transcription with DNA repair and replication. Many RNAP mutations have pleiotropic phenotypes with profound effects on transcription-coupled processes. One class of RNAP mutations (rpo*) has been shown to suppress mutations in regulatory factors responsible for changes in gene expression during stationary phase or starvation, as well as in factors involved in the restoration of replication forks after DNA damage. These mutations were suggested to affect the ability of RNAP to transcribe damaged DNA and to decrease the stability of transcription complexes, thus facilitating their dislodging during DNA replication and repair, although this was not explicitly demonstrated. Here, we obtained nine mutations of this class located around the DNA/RNA binding cleft of E. coli RNAP and analyzed their transcription properties in vitro. We found that these mutations decreased promoter complex stability to varying degrees and all decreased the activity of rRNA promoters. However, they did not have strong effects on elongation complex stability. Some mutations were shown to stimulate transcriptional pauses or decrease intrinsic RNA cleavage by RNAP, but none altered the ability of RNAP to transcribe DNA templates containing damaged nucleotides. Thus, we conclude that the suppressor phenotypes of the mutations are unlikely to result from direct effects on DNA lesion recognition by RNAP but may be primarily explained by changes in transcription initiation. Further analysis of the effects of these mutations on the genomic distribution of RNAP and its interactions with regulatory factors will be essential for understanding their diverse phenotypes in vivo.

Beyond the approved: target sites and inhibitors of bacterial RNA polymerase from bacteria and fungi

Covering: 2016 to 2022RNA polymerase (RNAP) is the central enzyme in bacterial gene expression representing an attractive and validated target for antibiotics. Two well-known and clinically approved classes of natural product RNAP inhibitors are the rifamycins and the fidaxomycins. Rifampicin (Rif), a semi-synthetic derivative of rifamycin, plays a crucial role as a first line antibiotic in the treatment of tuberculosis and a broad range of bacterial infections. However, more and more pathogens such as Mycobacterium tuberculosis develop resistance, not only against Rif and other RNAP inhibitors. To overcome this problem, novel RNAP inhibitors exhibiting different target sites are urgently needed. This review includes recent developments published between 2016 and today. Particular focus is placed on novel findings concerning already known bacterial RNAP inhibitors, the characterization and development of new compounds isolated from bacteria and fungi, and providing brief insights into promising new synthetic compounds.

OUP accepted manuscript

Cellular DNA is continuously transcribed into RNA by multisubunit RNA polymerases (RNAPs). The continuity of transcription can be disrupted by DNA lesions that arise from the activities of cellular enzymes, reactions with endogenous and exogenous chemicals or irradiation. Here, we review available data on translesion RNA synthesis by multisubunit RNAPs from various domains of life, define common principles and variations in DNA damage sensing by RNAP, and consider existing controversies in the field of translesion transcription. Depending on the type of DNA lesion, it may be correctly bypassed by RNAP, or lead to transcriptional mutagenesis, or result in transcription stalling. Various lesions can affect the loading of the templating base into the active site of RNAP, or interfere with nucleotide binding and incorporation into RNA, or impair RNAP translocation. Stalled RNAP acts as a sensor of DNA damage during transcription-coupled repair. The outcome of DNA lesion recognition by RNAP depends on the interplay between multiple transcription and repair factors, which can stimulate RNAP bypass or increase RNAP stalling, and plays the central role in maintaining the DNA integrity. Unveiling the mechanisms of translesion transcription in various systems is thus instrumental for understanding molecular pathways underlying gene regulation and genome stability.

Structural basis of RNA polymerase inhibition by viral and host factors

RNA polymerase inhibition plays an important role in the regulation of transcription in response to environmental changes and in the virus-host relationship. Here we present the high-resolution structures of two such RNAP-inhibitor complexes that provide the structural bases underlying RNAP inhibition in archaea. The Acidianus two-tailed virus encodes the RIP factor that binds inside the DNA-binding channel of RNAP, inhibiting transcription by occlusion of binding sites for nucleic acid and the transcription initiation factor TFB. Infection with the Sulfolobus Turreted Icosahedral Virus induces the expression of the host factor TFS4, which binds in the RNAP funnel similarly to eukaryotic transcript cleavage factors. However, TFS4 allosterically induces a widening of the DNA-binding channel which disrupts trigger loop and bridge helix motifs. Importantly, the conformational changes induced by TFS4 are closely related to inactivated states of RNAP in other domains of life indicating a deep evolutionary conservation of allosteric RNAP inhibition.

Gre factors prevent thermal and mechanical stresses induced by terahertz irradiation during transcription

During transcription in cells, the transcription complex consisting of RNA polymerase, DNA and nascent RNA is exposed to fluctuating temperature and pressure. However, little is known about the mechanism of transcriptional homeostasis under fluctuating physical parameters. In this study, we generated these fluctuating parameters using pulsed local heating and acoustic waves in the reaction system of transcription by Escherichia coli RNA polymerase, using a terahertz free-electron laser. We demonstrated that transcription processes, including abortive initiation and elongation pausing, and the fidelity of elongation are significantly affected by the laser-based local perturbations. We also found that all these functional alternations in the transcription process are almost completely mitigated by the presence of Gre proteins. It is well known that Gre proteins enhance RNA cleavage of polymerase by binding to the pore structure termed secondary channel. Recently, the chaperone activities have also been proposed for Gre proteins, yet the details directly associated with transcription are largely unknown. Our finding indicates that Gre proteins are necessary for maintaining transcriptional homeostasis under thermal and mechanical stresses.

Role of the trigger loop in translesion RNA synthesis by bacterial RNA polymerase

DNA lesions can severely compromise transcription and block RNA synthesis by RNA polymerase (RNAP), leading to subsequent recruitment of DNA repair factors to the stalled transcription complex. Recent structural studies have uncovered molecular interactions of several DNA lesions within the transcription elongation complex. However, little is known about the role of key elements of the RNAP active site in translesion transcription. Here, using recombinantly expressed proteins, in vitro transcription, kinetic analyses, and in vivo cell viability assays, we report that point amino acid substitutions in the trigger loop, a flexible element of the active site involved in nucleotide addition, can stimulate translesion RNA synthesis by Escherichia coli RNAP without altering the fidelity of nucleotide incorporation. We show that these substitutions also decrease transcriptional pausing and strongly affect the nucleotide addition cycle of RNAP by increasing the rate of nucleotide addition but also decreasing the rate of translocation. The secondary channel factors DksA and GreA modulated translesion transcription by RNAP, depending on changes in the trigger loop structure. We observed that although the mutant RNAPs stimulate translesion synthesis, their expression is toxic in vivo, especially under stress conditions. We conclude that the efficiency of translesion transcription can be significantly modulated by mutations affecting the conformational dynamics of the active site of RNAP, with potential effects on cellular stress responses and survival.

Four paralogous Gfh factors in the extremophilic bacterium Deinococcus peraridilitoris have distinct effects on various steps of transcription

Gre factors are ubiquitous transcription regulators that stimulate co-transcriptional RNA cleavage by bacterial RNA polymerase (RNAP). Here, we show that the stress-resistant bacterium Deinococcus peraridilitoris encodes four Gre factor homologs, Gfh proteins, that have distinct effects on transcription by RNAP. Two of the factors, Gfh1α and Gfh2β inhibit transcription initiation, and one of them, Gfh1α can also regulate transcription elongation. We show that this factor strongly stimulates transcriptional pausing and intrinsic termination in the presence of manganese ions but has no effect on RNA cleavage. Thus, some Gfh factors encoded by Deinococci serve as lineage-specific transcription inhibitors that may play a role in stress resistance, while the functions of others remain to be discovered.

Structure and DNA damage-dependent derepression mechanism for the XRE family member DG-DdrO

DdrO is an XRE family transcription repressor that, in coordination with the metalloprotease PprI, is critical in the DNA damage response of Deinococcus species. Here, we report the crystal structure of Deinococcus geothermalis DdrO. Biochemical and structural studies revealed the conserved recognizing α-helix and extended dimeric interaction of the DdrO protein, which are essential for promoter DNA binding. Two conserved oppositely charged residues in the HTH motif of XRE family proteins form salt bridge interactions that are essential for promoter DNA binding. Notably, the C-terminal domain is stabilized by hydrophobic interactions of leucine/isoleucine-rich helices, which is critical for DdrO dimerization. Our findings suggest that DdrO is a novel XRE family transcriptional regulator that forms a distinctive dimer. The structure also provides insight into the mechanism of DdrO-PprI-mediated DNA damage response in Deinococcus.

The diversity and commonalities of the radiation-resistance mechanisms of Deinococcus and its up-to-date applications

Deinococcus is an extremophilic microorganism found in a wide range of habitats, including hot springs, radiation-contaminated areas, Antarctic soils, deserts, etc., and shows some of the highest levels of resistance to ionizing radiation known in nature. The highly efficient radiation-protection mechanisms of Deinococcus depend on a combination of passive and active defense mechanisms, including self-repair of DNA damage (homologous recombination, MMR, ER and ESDSA), efficient cellular damage clearance mechanisms (hydrolysis of damaged proteins, overexpression of repair proteins, etc.), and effective clearance of reactive oxygen species (ROS). Due to these mechanisms, Deinococcus cells are highly resistant to oxidation, radiation and desiccation, which makes them potential chassis cells for wide applications in many fields. This article summarizes the latest research on the radiation-resistance mechanisms of Deinococcus and prospects its biotechnological application potentials.

Gre-family factors modulate DNA damage sensing by Deinococcus radiodurans RNA polymerase

Deinococcus radiodurans is a highly stress resistant bacterium that encodes universal as well as lineage-specific factors involved in DNA transcription and repair. However, the effects of DNA lesions on RNA synthesis by D. radiodurans RNA polymerase (RNAP) have never been studied. We investigated the ability of this RNAP to transcribe damaged DNA templates and demonstrated that various lesions significantly affect the efficiency and fidelity of RNA synthesis. DNA modifications that disrupt correct base-pairing can strongly inhibit transcription and increase nucleotide misincorporation by D. radiodurans RNAP. The universal transcription factor GreA and Deinococcus-specific factor Gfh1 stimulate RNAP stalling at various DNA lesions, depending on the type of the lesion and the presence of Mn²⁺ ions, abundant divalent cations in D. radiodurans. Furthermore, Gfh1 stimulates the action of the Mfd translocase, which removes transcription elongation complexes paused at the sites of DNA lesions. Thus, Gre-family factors in D. radiodurans might have evolved to increase the efficiency of DNA damage recognition by the transcription and repair machineries in this bacterium.

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.

How Acts of Infidelity Promote DNA Break Repair: Collision and Collusion Between DNA Repair and Transcription

Transcription is a fundamental cellular process and the first step in gene regulation. Although RNA polymerase (RNAP) is highly processive, in growing cells the progression of transcription can be hindered by obstacles on the DNA template, such as damaged DNA. The authors recent findings highlight a trade-off between transcription fidelity and DNA break repair. While a lot of work has focused on the interaction between transcription and nucleotide excision repair, less is known about how transcription influences the repair of DNA breaks. The authors suggest that when the cell experiences stress from DNA breaks, the control of RNAP processivity affects the balance between preserving transcription integrity and DNA repair. Here, how the conflict between transcription and DNA double-strand break (DSB) repair threatens the integrity of both RNA and DNA are discussed. In reviewing this field, the authors speculate on cellular paradigms where this equilibrium is well sustained, and instances where the maintenance of transcription fidelity is favored over genome stability.

The transcript cleavage factor paralogue TFS4 is a potent RNA polymerase inhibitor

TFIIS-like transcript cleavage factors enhance the processivity and fidelity of archaeal and eukaryotic RNA polymerases. Sulfolobus solfataricus TFS1 functions as a bona fide cleavage factor, while the paralogous TFS4 evolved into a potent RNA polymerase inhibitor. TFS4 destabilises the TBP–TFB–RNAP pre-initiation complex and inhibits transcription initiation and elongation. All inhibitory activities are dependent on three lysine residues at the tip of the C-terminal zinc ribbon of TFS4; the inhibition likely involves an allosteric component and is mitigated by the basal transcription factor TFEα/β. A chimeric variant of yeast TFIIS and TFS4 inhibits RNAPII transcription, suggesting that the molecular basis of inhibition is conserved between archaea and eukaryotes. TFS4 expression in S. solfataricus is induced in response to infection with the Sulfolobus turreted icosahedral virus. Our results reveal a compelling functional diversification of cleavage factors in archaea, and provide novel insights into transcription inhibition in the context of the host–virus relationship.

Transcript cleavage factors such as eukaryotic TFIIS assist the resumption of transcription following RNA pol II backtracking. Here the authors find that one of the Sulfolobus solfataricus TFIIS homolog—TFS4—has evolved into a potent RNA polymerase inhibitor potentially involved in antiviral defense.

Multisubunit RNA Polymerase Cleavage Factors Modulate the Kinetics and Energetics of Nucleotide Incorporation: An RNA Polymerase I Case Study

All cellular RNA polymerases are influenced by protein factors that stimulate RNA polymerase-catalyzed cleavage of the nascent RNA. Despite divergence in amino acid sequence, these so-called "cleavage factors" appear to share a common mechanism of action. Cleavage factors associate with the polymerase through a conserved structural element of the polymerase known as the secondary channel or pore. This mode of association enables the cleavage factor to reach through the secondary channel into the polymerase active site to reorient the active site divalent metal ions. This reorientation converts the polymerase active site into a nuclease active site. Interestingly, eukaryotic RNA polymerases I and III (Pols I and III, respectively) have incorporated their cleavage factors as bona fide subunits known as A12.2 and C11, respectively. Although it is clear that A12.2 and C11 dramatically stimulate the polymerase's cleavage activity, it is not known if or how these subunits affect the polymerization mechanism. In this work we have used transient-state kinetic techniques to characterize a Pol I isoform lacking A12.2. Our data clearly demonstrate that the A12.2 subunit profoundly affects the kinetics and energetics of the elementary steps of Pol I-catalyzed nucleotide incorporation. Given the high degree of conservation between polymerase-cleavage factor interactions, these data indicate that cleavage factor-modulated nucleotide incorporation mechanisms may be common to all cellular RNA polymerases.

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.

Regulation of transcription initiation by Gfh factors from Deinococcus radiodurans

Transcription factors of the Gre family bind within the secondary channel of bacterial RNA polymerase (RNAP) directly modulating its catalytic activities. Universally conserved Gre factors activate RNA cleavage by RNAP, by chelating catalytic metal ions in the RNAP active site, and facilitate both promoter escape and transcription elongation. Gfh factors are Deinococcus/Thermus-specific homologues of Gre factors whose transcription functions remain poorly understood. Recently, we found that Gfh1 and Gfh2 proteins from Deinococcus radiodurans dramatically stimulate RNAP pausing during transcription elongation in the presence of Mn²⁺, but not Mg²⁺, ions. In contrast, we show that Gfh1 and Gfh2 moderately inhibit transcription initiation in the presence of either Mg²⁺ or Mn²⁺ ions. By using a molecular beacon assay, we demonstrate that Gfh1 and Gfh2 do not significantly change promoter complex stability or the rate of promoter escape by D. radiodurans RNAP. At the same time, Gfh factors significantly increase the apparent K_M value for the 5'-initiating nucleotide, without having major effects on the affinity of metal ions for the RNAP active site. Similar inhibitory effects of Gfh factors are observed for transcription initiation on promoters recognized by the principal and an alternative σ factor. In summary, our data suggest that D. radiodurans Gfh factors impair the binding of initiating substrates independently of the metal ions bound in the RNAP active site, but have only mild overall effects on transcription initiation. Thus the mechanisms of modulation of RNAP activity by these factors are different for various steps of transcription.