A Hydrophobic Patch on the Flap-tip Helix of E.coli RNA Polymerase Mediates σ70 Region 4 Function

Structural Insight into the Mechanism of σ32-Mediated Transcription Initiation of Bacterial RNA Polymerase

Bacterial RNA polymerases (RNAP) form distinct holoenzymes with different σ factors to initiate diverse gene expression programs. In this study, we report a cryo-EM structure at 2.49 Å of RNA polymerase transcription complex containing a temperature-sensitive bacterial σ factor, σ³² (σ³²-RPo). The structure of σ³²-RPo reveals key interactions essential for the assembly of E. coli σ³²-RNAP holoenzyme and for promoter recognition and unwinding by σ³². Specifically, a weak interaction between σ³² and -35/-10 spacer is mediated by T128 and K130 in σ³². A histidine in σ³², rather than a tryptophan in σ⁷⁰, acts as a wedge to separate the base pair at the upstream junction of the transcription bubble, highlighting the differential promoter-melting capability of different residue combinations. Structure superimposition revealed relatively different orientations between βFTH and σ₄ from other σ-engaged RNAPs and biochemical data suggest that a biased σ₄-βFTH configuration may be adopted to modulate binding affinity to promoter so as to orchestrate the recognition and regulation of different promoters. Collectively, these unique structural features advance our understanding of the mechanism of transcription initiation mediated by different σ factors.

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

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

Structural basis for activation of Swi2/Snf2 ATPase RapA by RNA polymerase

RapA is a bacterial RNA polymerase (RNAP)-associated Swi2/Snf2 ATPase that stimulates RNAP recycling. The ATPase activity of RapA is autoinhibited by its N-terminal domain (NTD) but activated with RNAP bound. Here, we report a 3.4-Å cryo-EM structure of Escherichia coli RapA-RNAP elongation complex, in which the ATPase active site of RapA is structurally remodeled. In this process, the NTD of RapA is wedged open by RNAP β' zinc-binding domain (ZBD). In addition, RNAP β flap tip helix (FTH) forms extensive hydrophobic interactions with RapA ATPase core domains. Functional assay demonstrates that removing the ZBD or FTH of RNAP significantly impairs its ability to activate the ATPase activity of RapA. Our results provide the structural basis of RapA ATPase activation by RNAP, through the active site remodeling driven by the ZBD-buttressed large-scale opening of NTD and the direct interactions between FTH and ATPase core domains.

RNA polymerases in strict endosymbiont bacteria with extreme genome reduction show distinct erosions that might result in limited and differential promoter recognition

Strict endosymbiont bacteria present high degree genome reduction, retain smaller proteins, and in some instances, lack complete functional domains compared to free-living counterparts. Until now, the mechanisms underlying these genetic reductions are not well understood. In this study, the conservation of RNA polymerases, the essential machinery for gene expression, is analyzed in endosymbiont bacteria with extreme genome reductions. We analyzed the RNA polymerase subunits to identify and define domains, subdomains, and specific amino acids involved in precise biological functions known in Escherichia coli. We also perform phylogenetic analysis and three-dimensional models over four lineages of endosymbiotic proteobacteria with the smallest genomes known to date: Candidatus Hodgkinia cicadicola, Candidatus Tremblaya phenacola, Candidatus Tremblaya Princeps, Candidatus Nasuia deltocephalinicola, and Candidatus Carsonella ruddii. We found that some Hodgkinia strains do not encode for the RNA polymerase α subunit. The rest encode genes for α, β, β’, and σ subunits to form the RNA polymerase. However, 16% shorter, on average, respect their orthologous in E. coli. In the α subunit, the amino-terminal domain is the most conserved. Regarding the β and β’ subunits, both the catalytic core and the assembly domains are the most conserved. However, they showed compensatory amino acid substitutions to adapt to changes in the σ subunit. Precisely, the most erosive diversity occurs within the σ subunit. We identified broad amino acid substitution even in those recognizing and binding to the -10-box promoter element. In an overall conceptual image, the RNA polymerase from Candidatus Nasuia conserved the highest similarity with Escherichia coli RNA polymerase and their σ⁷⁰. It might be recognizing the two main promoter elements (-10 and -35) and the two promoter accessory elements (-10 extended and UP-element). In Candidatus Carsonella, the RNA polymerase could recognize all the promoter elements except the -10-box extended. In Candidatus Tremblaya and Hodgkinia, due to the α carboxyl-terminal domain absence, they might not recognize the UP-promoter element. We also identified the lack of the β flap-tip helix domain in most Hodgkinia’s that suggests the inability to bind the -35-box promoter element.

Structural insights into the functional divergence of WhiB-like proteins in Mycobacterium tuberculosis

WhiB7 represents a distinct subclass of transcription factors in the WhiB-Like (Wbl) family, a unique group of iron-sulfur (4Fe-4S] cluster-containing proteins exclusive to the phylum of Actinobacteria. In Mycobacterium tuberculosis (Mtb), WhiB7 interacts with domain 4 of the primary sigma factor (σ^A₄) in the RNA polymerase holoenzyme and activates genes involved in multiple drug resistance and redox homeostasis. Here, we report crystal structures of the WhiB7:σ^A₄ complex alone and bound to its target promoter DNA at 1.55-Å and 2.6-Å resolution, respectively. These structures show how WhiB7 regulates gene expression by interacting with both σ^A₄ and the AT-rich sequence upstream of the -35 promoter DNA via its C-terminal DNA-binding motif, the AT-hook. By combining comparative structural analysis of the two high-resolution σ^A₄-bound Wbl structures with molecular and biochemical approaches, we identify the structural basis of the functional divergence between the two distinct subclasses of Wbl proteins in Mtb.

Region 4 of the RNA polymerase σ subunit counteracts pausing during initial transcription

All cellular genetic information is transcribed into RNA by multisubunit RNA polymerases (RNAPs). The basal transcription initiation factors of cellular RNAPs stimulate the initial RNA synthesis via poorly understood mechanisms. Here, we explored the mechanism employed by the bacterial factor σ in promoter-independent initial transcription. We found that the RNAP holoenzyme lacking the promoter-binding domain σ4 is ineffective in de novo transcription initiation and displays high propensity to pausing upon extension of RNAs 3 to 7 nucleotides in length. The nucleotide at the RNA 3' end determines the pause lifetime. The σ4 domain stabilizes short RNA:DNA hybrids and suppresses pausing by stimulating RNAP active-center translocation. The antipausing activity of σ4 is modulated by its interaction with the β subunit flap domain and by the σ remodeling factors AsiA and RbpA. Our results suggest that the presence of σ4 within the RNA exit channel compensates for the intrinsic instability of short RNA:DNA hybrids by increasing RNAP processivity, thus favoring productive transcription initiation. This "RNAP boosting" activity of the initiation factor is shaped by the thermodynamics of RNA:DNA interactions and thus, should be relevant for any factor-dependent RNAP.

The RNA Polymerase α Subunit Recognizes the DNA Shape of the Upstream Promoter Element

We demonstrate here that the α subunit C-terminal domain of Escherichia coli RNA polymerase (αCTD) recognizes the upstream promoter (UP) DNA element via its characteristic minor groove shape and electrostatic potential. In two compositionally distinct crystallized assemblies, a pair of αCTD subunits bind in tandem to the UP element consensus A-tract that is 6 bp in length (A₆-tract), each with their arginine 265 guanidinium group inserted into the minor groove. The A₆-tract minor groove is significantly narrowed in these crystal structures, as well as in computationally predicted structures of free and bound DNA duplexes derived by Monte Carlo and molecular dynamics simulations, respectively. The negative electrostatic potential of free A₆-tract DNA is substantially enhanced compared to that of generic DNA. Shortening the A-tract by 1 bp is shown to "knock out" binding of the second αCTD through widening of the minor groove. Furthermore, in computationally derived structures with arginine 265 mutated to alanine in either αCTD, either with or without the "knockout" DNA mutation, contact with the DNA is perturbed, highlighting the importance of arginine 265 in achieving αCTD-DNA binding. These results demonstrate that the importance of the DNA shape in sequence-dependent recognition of DNA by RNA polymerase is comparable to that of certain transcription factors.

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.

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

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

Novel Sequence Feature of SecA Translocase Protein Unique to the Thermophilic Bacteria: Bioinformatics Analyses to Investigate Their Potential Roles

SecA is an evolutionarily conserved protein that plays an indispensable role in the secretion of proteins across the bacterial cell membrane. Comparative analyses of SecA homologs have identified two large conserved signature inserts (CSIs) that are unique characteristics of thermophilic bacteria. A 50 aa conserved insert in SecA is exclusively present in the SecA homologs from the orders Thermotogales and Aquificales, while a 76 aa insert in SecA is specific for the order Thermales and Hydrogenibacillus schlegelii. Phylogenetic analyses on SecA sequences show that the shared presence of these CSIs in unrelated groups of thermophiles is not due to lateral gene transfers, but instead these large CSIs have likely originated independently in these lineages due to their advantageous function. Both of these CSIs are located in SecA protein in a surface exposed region within the ATPase domain. To gain insights into the functional significance of the 50 aa CSI in SecA, molecular dynamics (MD) simulations were performed at two different temperatures using ADP-bound SecA from Thermotoga maritima. These analyses have identified a conserved network of water molecules near the 50 aa insert in which the Glu185 residue from the CSI is found to play a key role towards stabilizing these interactions. The results provide evidence for the possible role of the 50 aa CSI in stabilizing the binding interaction of ADP/ATP, which is required for SecA function. Additionally, the surface-exposed CSIs in SecA, due to their potential to make novel protein-protein interactions, could also contribute to the thermostability of SecA from thermophilic bacteria.

Mutagenesis Induced by Sub-Lethal Doses of Ciprofloxacin: Genotypic and Phenotypic Differences Between the Pseudomonas aeruginosa Strain PA14 and Clinical Isolates

Bacterial resistance is a severe threat to global public health. Exposure to sub-lethal concentrations has been considered a major driver of mutagenesis leading to antibiotic resistance in clinical settings. Ciprofloxacin is broadly used to treat infections caused by Pseudomonas aeruginosa, whereas increased mutagenesis induced by sub-lethal concentrations of ciprofloxacin has been reported for the reference strain, PAO1, in vitro. In this study we report increased mutagenesis induced by sub-lethal concentrations of ciprofloxacin for another reference strain, PA14-UCBPP, and lower mutagenesis for clinical isolates when compared to the reference strain. This unexpected result may be associated with missense mutations in imuB and recX, involved in adaptive responses, and the presence of Pyocin S2, which were found in all clinical isolates but not in the reference strain genome. The genetic differences between clinical isolates of P. aeruginosa and the reference PA14-UCBPP, often used to study P. aeruginosa phenotypes in vitro, may be involved in reduced mutagenesis under sub-lethal concentrations of CIP, a scenario that should be further explored for the understanding of bacterial fitness in hospital environments. Moreover, we highlight the presence of a complete umuDC operon in a P. aeruginosa clinical isolate. Even though the presence of umuDC did not contribute to a significant increase in mutagenesis, it highlights the dynamic exchange of genetic material between bacterial species in the hospital environment.

Phylogenetic analysis of the diacylglycerol kinase family of proteins and identification of multiple highly-specific conserved inserts and deletions within the catalytic domain that are distinctive characteristics of different classes of DGK homologs

Diacylglycerol kinase (DGK) family of proteins, which phosphorylates diacylglycerol into phosphatidic acid, play important role in controlling diverse cellular processes in eukaryotic organisms. Most vertebrate species contain 10 different DGK isozymes, which are grouped into 5 different classes based on the presence or absence of specific functional domains. However, the relationships among different DGK isozymes or how they have evolved from a common ancestor is unclear. The catalytic domain constitutes the single largest sequence element within the DGK proteins that is commonly and uniquely shared by all family members, but there is limited understanding of the overall function of this domain. In this work, we have used the catalytic domain sequences to construct a phylogenetic tree for the DGK family members from representatives of the main vertebrate classes and have also examined the distributions of various DGK isozymes in eukaryotic phyla. In a tree based on catalytic domain sequences, the DGK homologs belonging to different classes formed strongly supported clusters which were separated by long branches, and the different isozymes within each class also generally formed monophyletic groupings. Further, our analysis of the sequence alignments of catalytic domains has identified >10 novel sequence signatures consisting of conserved signature indels (inserts or deletions, CSIs) that are distinctive characteristics of either particular classes of DGK isozymes, or are commonly shared by members of two or more classes of DGK isozymes. The conserved indels in protein sequences are known to play important functional roles in the proteins/organisms where they are found. Thus, our identification of multiple highly specific CSIs that are distinguishing characteristics of different classes of DGK homologs points to the existence of important differences in the catalytic domain function among the DGK isozymes. The identified CSIs in conjunction with the results of blast searches on species distribution of DGK isozymes also provide useful insights into the evolutionary relationships among the DGK family of proteins.

Ribonucleotide Reductases from Bifidobacteria Contain Multiple Conserved Indels Distinguishing Them from All Other Organisms: In Silico Analysis of the Possible Role of a 43 aa Bifidobacteria-Specific Insert in the Class III RNR Homolog

Bifidobacteria comprises an important group/order of bacteria whose members have widespread usage in the food and health industry due to their health-promoting activity in the human gastrointestinal tract. However, little is known about the underlying molecular properties that are responsible for the probiotic effects of these bacteria. The enzyme ribonucleotide reductase (RNR) plays a key role in all organisms by reducing nucleoside di- or tri- phosphates into corresponding deoxyribose derivatives required for DNA synthesis, and RNR homologs belonging to classes I and III are present in either most or all Bifidobacteriales. Comparative analyses of these RNR homologs have identified several novel sequence features in the forms of conserved signature indels (CSIs) that are exclusively found in bifidobacterial RNRs. Specifically, in the large subunit of the aerobic class Ib RNR, three CSIs have been identified that are uniquely found in the Bifidobacteriales homologs. Similarly, the large subunit of the anaerobic class III RNR contains five CSIs that are also distinctive characteristics of bifidobacteria. Phylogenetic analyses indicate that these CSIs were introduced in a common ancestor of the Bifidobacteriales and retained by all descendants, likely due to their conferring advantageous functional roles. The identified CSIs in the bifidobacterial RNR homologs provide useful tools for further exploration of the novel functional aspects of these important enzymes that are exclusive to these bacteria. We also report here the results of homology modeling studies, which indicate that most of the bifidobacteria-specific CSIs are located within the surface loops of the RNRs, and of these, a large 43 amino acid insert in the class III RNR homolog forms an extension of the allosteric regulatory site known to be essential for protein function. Preliminary docking studies suggest that this large CSI may be playing a role in enhancing the stability of the RNR dimer complex. The possible significances of the identified CSIs, as well as the distribution of RNR homologs in the Bifidobacteriales, are discussed.

The essential activities of the bacterial sigma factor

Transcription is the first and most heavily regulated step in gene expression. Sigma (σ) factors are general transcription factors that reversibly bind RNA polymerase (RNAP) and mediate transcription of all genes in bacteria. σ Factors play 3 major roles in the RNA synthesis initiation process: they (i) target RNAP holoenzyme to specific promoters, (ii) melt a region of double-stranded promoter DNA and stabilize it as a single-stranded open complex, and (iii) interact with other DNA-binding transcription factors to contribute complexity to gene expression regulation schemes. Recent structural studies have demonstrated that when σ factors bind promoter DNA, they capture 1 or more nucleotides that are flipped out of the helical DNA stack and this stabilizes the promoter open-complex intermediate that is required for the initiation of RNA synthesis. This review describes the structure and function of the σ⁷⁰ family of σ proteins and the essential roles they play in the transcription process.

Characterization of a protein-protein interaction within the SigO-RsoA two-subunit σ factor: the σ70 region 2.3-like segment of RsoA mediates interaction with SigO

σ factors are single subunit general transcription factors that reversibly bind core RNA polymerase and mediate gene-specific transcription in bacteria. Previously, an atypical two-subunit σ factor was identified that activates transcription from a group of related promoters in Bacillus subtilis. Both of the subunits, named SigO and RsoA, share primary sequence similarity with the canonical σ70 family of σ factors and interact with each other and with RNA polymerase subunits. Here we show that the σ70 region 2.3-like segment of RsoA is unexpectedly sufficient for interaction with the amino-terminus of SigO and the β' subunit. A mutational analysis of RsoA identified aromatic residues conserved amongst all RsoA homologues, and often amongst canonical σ factors, that are particularly important for the SigO-RsoA interaction. In a canonical σ factor, region 2.3 amino acids bind non-template strand DNA, trapping the promoter in a single-stranded state required for initiation of transcription. Accordingly, we speculate that RsoA region 2.3 protein-binding activity likely arose from a motif that, at least in its ancestral protein, participated in DNA-binding interactions.

Visualizing the phage T4 activated transcription complex of DNA and E. coli RNA polymerase

The ability of RNA polymerase (RNAP) to select the right promoter sequence at the right time is fundamental to the control of gene expression in all organisms. However, there is only one crystallized structure of a complete activator/RNAP/DNA complex. In a process called σ appropriation, bacteriophage T4 activates a class of phage promoters using an activator (MotA) and a co-activator (AsiA), which function through interactions with the σ⁷⁰ subunit of RNAP. We have developed a holistic, structure-based model for σ appropriation using multiple experimentally determined 3D structures (Escherichia coli RNAP, the Thermus aquaticus RNAP/DNA complex, AsiA /σ⁷⁰ Region 4, the N-terminal domain of MotA [MotA^NTD], and the C-terminal domain of MotA [MotA^CTD]), molecular modeling, and extensive biochemical observations indicating the position of the proteins relative to each other and to the DNA. Our results visualize how AsiA/MotA redirects σ, and therefore RNAP activity, to T4 promoter DNA, and demonstrate at a molecular level how the tactful interaction of transcriptional factors with even small segments of RNAP can alter promoter specificity. Furthermore, our model provides a rational basis for understanding how a mutation within the β subunit of RNAP (G1249D), which is far removed from AsiA or MotA, impairs σ appropriation.

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.

Functional reconstitution of an unusual Firmicutes σ factor into a Gram-negative heterologous host

Sigma (σ) factors are single-subunit proteins that reversibly bind RNA polymerase and play an important role in the transcription initiation process. An unusual 2-subunit σ factor, consisting of proteins SigO and RsoA, activates transcription from a group of related promoters in Bacillus subtilis. These 2 proteins specifically interact with each other and with RNA polymerase subunits. This system is widespread among species in several Bacillus-related genera, but otherwise appears restricted to the Firmicutes. Here, we reconstituted SigO-RsoA, and a cognate promoter, into the distantly related heterologous host Escherichia coli to examine whether this system can function in bacteria outside of the Firmicutes. We show that these proteins can productively associate with E. coli RNA polymerase and activate transcription, demonstrating that there are no structural barriers to function. In parallel, we tested a wide array of protein-protein interaction mutations and promoter mutations that impact SigO-RsoA function in both B. subtilis and E. coli and conclude that the SigO-RsoA system behaves, in most instances, similarly in both genetic backgrounds. These data raise the possibility of genetically isolating the system in this heterologous host and away from unknown B. subtilis factors that may also be playing a role in SigO-RsoA regulatory pathways, thus facilitating further study of the system. As a result of this work, we also provide a comprehensive mutational analysis of a SigO-RsoA promoter and report the preliminary identification of amino acids in SigO that play a role in mediating the SigO-RsoA protein-protein interaction.

Structural analysis of sigma E interactions with core RNA polymerase and its cognate P-hsp20 promoter of Mycobacterium tuberculosis

Alternate sigma factor plays an important role for the survival of Mycobacterium tuberculosis in adverse environmental condition. Stress-induced sigma factors are major cause for expression of genes involved in pathogenesis, dormancy and various unusual environmental conditions. In the present work, an attempt has been made to characterize one of such M. tuberculosis (Mtb) sigma factor, SigE. The structures of Mtb-SigE and Mtb-β have been predicted using comparative modelling techniques and validated. Effort has also been implied to understand the nature of interaction of SigE with the core RNA polymerase subunits which have well identified the amino acid residues in the binding interface and prompted the fact that Mtb-β' and Mtb-β interact with domain 2 and domain 4 of Mtb-SigE, respectively. Furthermore, intermolecular docking study predicted the interface between the Mtb-SigE and its putative promoter P-hsp20. The report confers the probable amino acid residues and the nitrogenous bases involved in the recognition of P-hsp20 by the sigma factor to initiate the transcription process.

The crystal structure of the anti-σ factor CnrY in complex with the σ factor CnrH shows a new structural class of anti-σ factors targeting extracytoplasmic function σ factors

Gene expression in bacteria is regulated at the level of transcription initiation, a process driven by σ factors. The regulation of σ factor activity proceeds from the regulation of their cytoplasmic availability, which relies on specific inhibitory proteins called anti-σ factors. With anti-σ factors regulating their availability according to diverse cues, extracytoplasmic function σ factors (σ(ECF)) form a major signal transduction system in bacteria. Here, structure:function relationships have been characterized in an emerging class of minimal-size transmembrane anti-σ factors, using CnrY from Cupriavidus metallidurans CH34 as a model. This study reports the 1.75-Å-resolution structure of CnrY cytosolic domain in complex with CnrH, its cognate σ(ECF), and identifies a small hydrophobic knob in CnrY as the major determinant of this interaction in vivo. Unsuspected structural similarity with the molecular switch regulating the general stress response in α-proteobacteria unravels a new class of anti-σ factors targeting σ(ECF). Members of this class carry out their function via a 30-residue stretch that displays helical propensity but no canonical structure on its own.

Key features of σS required for specific recognition by Crl, a transcription factor promoting assembly of RNA polymerase holoenzyme

Bacteria use multiple sigma factors to coordinate gene expression in response to environmental perturbations. In Escherichia coli and other γ-proteobacteria, the transcription factor Crl stimulates σ(S)-dependent transcription during times of cellular stress by promoting the association of σ(S) with core RNA polymerase. The molecular basis for specific recognition of σ(S) by Crl, rather than the homologous and more abundant primary sigma factor σ(70), is unknown. Here we use bacterial two-hybrid analysis in vivo and p-benzoyl-phenylalanine cross-linking in vitro to define the features in σ(S) responsible for specific recognition by Crl. We identify residues in σ(S) conserved domain 2 (σ(S)2) that are necessary and sufficient to allow recognition of σ(70) conserved domain 2 by Crl, one near the promoter-melting region and the other at the position where a large nonconserved region interrupts the sequence of σ(70). We then use luminescence resonance energy transfer to demonstrate directly that Crl promotes holoenzyme assembly using these specificity determinants on σ(S). Our results explain how Crl distinguishes between sigma factors that are largely homologous and activates discrete sets of promoters even though it does not bind to promoter DNA.

Peptide-based investigation of the Escherichia coli RNA polymerase σ(70):core interface as target site

The number of bacterial strains that are resistant against antibiotics increased dramatically during the past decades. This fact stresses the urgent need for the development of new antibacterial agents with novel modes of action targeting essential enzymes such as RNA polymerase (RNAP). Bacterial RNAP is a large multi-subunit complex consisting of a core enzyme (subunits: α(2)ββ'ω) and a dissociable sigma factor (σ(70); holo enzyme: α(2)ββ'ωσ(70)) that is responsible for promoter recognition and transcription initiation. The interface between core RNAP and σ(70) represents a promising binding site. Nevertheless, detailed studies investigating its druggability are rare. Compounds binding to this region could inhibit this protein-protein interaction and thus holo enzyme formation, resulting in inhibition of transcription initiation. Sixteen peptides covering different regions of the Escherichia coli σ(70):core interface were designed; some of them-all derived from σ(70) 2.2 region-led to a strong RNAP inhibition. Indeed, an ELISA-based experiment confirmed the most active peptide P07 to inhibit the σ(70):core interaction. Furthermore, an abortive transcription assay revealed that P07 impedes transcription initiation. In order to study the mechanism of action of P07 in more detail, molecular dynamics simulations and a rational amino acid replacement study were performed, leading to the conclusion that P07 binds to the coiled-coil region in β' and that its flexible N-terminus inhibits the enzyme by interaction with the β' lid-rudder-system (LRS). This work revisits the β' coiled-coil as a hot spot for the protein-protein interaction inhibition and expands it by introduction of the LRS as target site.

Crystal structure of the bacteriophage T4 late-transcription coactivator gp33 with the β-subunit flap domain of Escherichia coli RNA polymerase

Activated transcription of the bacteriophage T4 late genes, which is coupled to concurrent DNA replication, is accomplished by an initiation complex containing the host RNA polymerase associated with two phage-encoded proteins, gp55 (the basal promoter specificity factor) and gp33 (the coactivator), as well as the DNA-mounted sliding-clamp processivity factor of the phage T4 replisome (gp45, the activator). We have determined the 3.0 Å-resolution X-ray crystal structure of gp33 complexed with its RNA polymerase binding determinant, the β-flap domain. Like domain 4 of the promoter specificity σ factor (σ(4)), gp33 interacts with RNA polymerase primarily by clamping onto the helix at the tip of the β-flap domain. Nevertheless, gp33 and σ(4) are not structurally related. The gp33/β-flap structure, combined with biochemical, biophysical, and structural information, allows us to generate a structural model of the T4 late promoter initiation complex. The model predicts protein/protein interactions within the complex that explain the presence of conserved patches of surface-exposed residues on gp33, and provides a structural framework for interpreting and designing future experiments to functionally characterize the complex.

Bacteriophage T4 MotA activator and the β-flap tip of RNA polymerase target the same set of σ70 carboxyl-terminal residues

Sigma factors, the specificity subunits of RNA polymerase, are involved in interactions with promoter DNA, the core subunits of RNA polymerase, and transcription factors. The bacteriophage T4-encoded activator, MotA, is one such factor, which engages the C terminus of the Escherichia coli housekeeping sigma factor, σ(70). MotA functions in concert with a phage-encoded co-activator, AsiA, as a molecular switch. This process, termed sigma appropriation, inhibits host transcription while activating transcription from a class of phage promoters. Previous work has demonstrated that MotA contacts the C terminus of σ(70), H5, a region that is normally bound within RNA polymerase by its interaction with the β-flap tip. To identify the specific σ(70) residues responsible for interacting with MotA and the β-flap tip, we generated single substitutions throughout the C terminus of σ(70). We find that MotA targets H5 residues that are normally engaged by the β-flap. In two-hybrid assays, the interaction of σ(70) with either the β-flap tip or MotA is impaired by alanine substitutions at residues Leu-607, Arg-608, Phe-610, Leu-611, and Asp-613. Transcription assays identify Phe-610 and Leu-611 as the key residues for MotA/AsiA-dependent transcription. Phe-610 is a crucial residue in the H5/β-flap tip interaction using promoter clearance assays with RNA polymerase alone. Our results show how the actions of small transcriptional factors on a defined local region of RNA polymerase can fundamentally change the specificity of polymerase.

The structure of a transcription activation subcomplex reveals how σ(70) is recruited to PhoB promoters

PhoB is a two-component response regulator that activates transcription by interacting with the σ(70) subunit of the E. coli RNA polymerase in promoters in which the -35 σ(70)-recognition element is replaced by the pho box. The crystal structure of a transcription initiation subcomplex that includes the σ(4) domain of σ(70) fused with the RNA polymerase β subunit flap tip helix, the PhoB effector domain and the pho box DNA reveals how σ(4) recognizes the upstream pho box repeat. As with the -35 element, σ(4) achieves this recognition through the N-terminal portion of its DNA recognition helix, but contact with the DNA major groove is less extensive. Unexpectedly, the same recognition helix contacts the transactivation loop and helices α2 and α3 of PhoB. This result shows a simple and elegant mechanism for polymerase recruitment to pho box promoters in which the lost -35 element contacts are compensated by new ones with the activator. In addition, σ(4) is reoriented, thereby suggesting a remodelling mechanism for transcription initiation.

The RPB2 flap loop of human RNA polymerase II is dispensable for transcription initiation and elongation

The flap domain of multisubunit RNA polymerases (RNAPs), also called the wall, forms one side of the RNA exit channel. In bacterial RNAP, the mobile part of the flap is called the flap tip and makes essential contacts with initiation and elongation factors. Cocrystal structures suggest that the orthologous part of eukaryotic RNAPII, called the flap loop, contacts transcription factor IIB (TFIIB), but the function of the flap loop has not been assessed. We constructed and tested a deletion of the flap loop in human RNAPII (subunit RPB2 Δ873-884) that removes the flap loop interaction interface with TFIIB. Genome-wide analysis of the distribution of the RNAPII with the flap loop deletion expressed in a human embryonic kidney cell line (HEK 293) revealed no effect of the flap loop on global transcription initiation, RNAPII occupancy within genes, or the efficiency of promoter escape and productive elongation. In vitro, the flap loop deletion had no effect on promoter binding, abortive initiation or promoter escape, TFIIS-stimulated transcript cleavage, or inhibition of transcript elongation by the complex of negative elongation factor (NELF) and 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) sensitivity-inducing factor (DSIF). A modest effect on transcript elongation and pausing was suppressed by TFIIF. Although similar to the flap tip of bacterial RNAP, the RNAPII flap loop is not equivalently essential.

The epitope for the polyol-responsive monoclonal antibody 8RB13 is in the flap-domain of the beta-subunit of bacterial RNA polymerase and can be used as an epitope tag for immunoaffinity chromatography

Polyol-responsive monoclonal antibodies (PR-mAbs) are useful for the purification of proteins in an easy, one step immunoaffinity step. These antibodies allow for gentle purification of proteins and protein complexes using a combination of a low molecular weight polyhydroxylated compound (polyol) and a nonchaotrophic salt in the eluting buffer. mAb 8RB13 has been characterized as one of these PR-mAbs and has been used to purify RNA polymerase from five species of bacteria. Here the epitope for 8RB13 has been identified as PEEKLLRAIFGEKAS, a sequence that is highly conserved in the β-subunit of bacterial RNA polymerase. This sequence is located in the "beta-flap" domain of RNA polymerase (and essentially comprises the "flap-tip helix"), an important binding site for sigma70. This location explains why only the core RNAP is purified using this mAb. This amino acid sequence has been developed into an epitope tag that can be used to purify a target protein from either bacterial or eukaryotic cells when genetically fused to a protein of interest.

Genetic evidence for a novel interaction between transcriptional activator SoxS and region 4 of the σ(70) subunit of RNA polymerase at class II SoxS-dependent promoters in Escherichia coli

Escherichia coli SoxS activates transcription of the genes of the soxRS regulon, which provide the cell's defense against oxidative stress. In response to this stress, SoxS is synthesized de novo. Because the DNA binding site of SoxS is highly degenerate, SoxS efficiently activates transcription by the mechanism of prerecruitment. In prerecruitment, newly synthesized SoxS first forms binary complexes with RNA polymerase. These complexes then scan the chromosome for class I and II SoxS-dependent promoters, using the specific DNA-recognition properties of SoxS and σ(70) to distinguish SoxS-dependent promoters from the vast excess of sequence-equivalent soxboxes that do not reside in promoters. Previously, we determined that SoxS interacts with RNA polymerase in two ways: by making protein-protein interactions with the DNA-binding determinant of the α subunit and by interacting with σ(70) region 4 (σ(70) R4) both "on-DNA" and "off-DNA." Here, we address the question of how SoxS and σ(70) R4 coexist at class II promoters, where the binding site for SoxS either partially or completely overlaps the -35 region of the promoter, which is usually bound by σ(70) R4. To do so, we created a tri-alanine scanning library that covers all of σ(70) R4. We determined that interactions between σ(70) R4 and the DNA in the promoter's -35 region are required for activation of class I promoters, where the binding site lies upstream of the -35 hexamer, but they are not required at class II promoters. In contrast, specific three-amino-acid stretches are required for activation of class I (lac) and class II (galP1) cyclic AMP receptor protein-dependent promoters. We conclude from these data that SoxS and σ(70) R4 interact with each other in a novel way at class II SoxS-dependent promoters such that the two proteins do not accommodate one another in the -35 region but instead SoxS binding there occludes the binding of σ(70) R4.

Crl binds to domain 2 of σ(S) and confers a competitive advantage on a natural rpoS mutant of Salmonella enterica serovar Typhi

The RpoS sigma factor (σ(S)) is the master regulator of the bacterial response to a variety of stresses. Mutants in rpoS arise in bacterial populations in the absence of stress, probably as a consequence of a subtle balance between self-preservation and nutritional competence. We characterized here one natural rpoS mutant of Salmonella enterica serovar Typhi (Ty19). We show that the rpoS allele of Ty19 (rpoS(Ty19)) led to the synthesis of a σ(S)(Ty19) protein carrying a single glycine-to-valine substitution at position 282 in σ(S) domain 4, which was much more dependent than the wild-type σ(S) protein on activation by Crl, a chaperone-like protein that increases the affinity of σ(S) for the RNA polymerase core enzyme (E). We used the bacterial adenylate cyclase two-hybrid system to demonstrate that Crl bound to residues 72 to 167 of σ(S) domain 2 and that G282V substitution did not directly affect Crl binding. However, this substitution drastically reduced the ability of σ(S)(Ty19) to bind E in a surface plasmon resonance assay, a defect partially rescued by Crl. The modeled structure of the Eσ(S) holoenzyme suggested that substitution G282V could directly disrupt a favorable interaction between σ(S) and E. The rpoS(Ty19) allele conferred a competitive fitness when the bacterial population was wild type for crl but was outcompeted in Δcrl populations. Thus, these results indicate that the competitive advantage of the rpoS(Ty19) mutant is dependent on Crl and suggest that crl plays a role in the appearance of rpoS mutants in bacterial populations.

A mutation within the β subunit of Escherichia coli RNA polymerase impairs transcription from bacteriophage T4 middle promoters

During infection of Escherichia coli, bacteriophage T4 usurps the host transcriptional machinery, redirecting it to the expression of early, middle, and late phage genes. Middle genes, whose expression begins about 1 min postinfection, are transcribed both from the extension of early RNA into middle genes and by the activation of T4 middle promoters. Middle-promoter activation requires the T4 transcriptional activator MotA and coactivator AsiA, which are known to interact with σ(70), the specificity subunit of RNA polymerase. T4 motA amber [motA(Am)] or asiA(Am) phage grows poorly in wild-type E. coli. However, previous work has found that T4 motA(Am)does not grow in the E. coli mutant strain TabG. We show here that the RNA polymerase in TabG contains two mutations within its β-subunit gene: rpoB(E835K) and rpoB(G1249D). We find that the G1249D mutation is responsible for restricting the growth of either T4 motA(Am)or asiA(Am) and for impairing transcription from MotA/AsiA-activated middle promoters in vivo. With one exception, transcription from tested T4 early promoters is either unaffected or, in some cases, even increases, and there is no significant growth phenotype for the rpoB(E835K G1249D) strain in the absence of T4 infection. In reported structures of thermophilic RNA polymerase, the G1249 residue is located immediately adjacent to a hydrophobic pocket, called the switch 3 loop. This loop is thought to aid in the separation of the RNA from the DNA-RNA hybrid as RNA enters the RNA exit channel. Our results suggest that the presence of MotA and AsiA may impair the function of this loop or that this portion of the β subunit may influence interactions among MotA, AsiA, and RNA polymerase.

RNA polymerase II-TFIIB structure and mechanism of transcription initiation

To initiate gene transcription, RNA polymerase II (Pol II) requires the transcription factor IIB (B). Here we present the crystal structure of the complete Pol II-B complex at 4.3 A resolution, and complementary functional data. The results indicate the mechanism of transcription initiation, including the transition to RNA elongation. Promoter DNA is positioned over the Pol II active centre cleft with the 'B-core' domain that binds the wall at the end of the cleft. DNA is then opened with the help of the 'B-linker' that binds the Pol II rudder and clamp coiled-coil at the edge of the cleft. The DNA template strand slips into the cleft and is scanned for the transcription start site with the help of the 'B-reader' that approaches the active site. Synthesis of the RNA chain and rewinding of upstream DNA displace the B-reader and B-linker, respectively, to trigger B release and elongation complex formation.

A two-subunit bacterial sigma-factor activates transcription in Bacillus subtilis

The sigma-like factor YvrI and coregulator YvrHa activate transcription from a small set of conserved promoters in Bacillus subtilis. We report here that these two proteins independently contribute sigma-region 2 and sigma-region 4 functions to a holoenzyme-promoter DNA complex. YvrI binds RNA polymerase (RNAP) through a region 4 interaction with the beta-subunit flap domain and mediates specific promoter recognition but cannot initiate DNA melting at the -10 promoter element. Conversely, YvrHa possesses sequence similarity to a conserved core-binding motif in sigma-region 2 and binds to the N-terminal coiled-coil element in the RNAP beta'-subunit previously implicated in interaction with region 2 of sigma-factors. YvrHa plays an essential role in stabilizing the open complex and interacts specifically with the N-terminus of YvrI. Based on these results, we propose that YvrHa is situated in the transcription complex proximal to the -10 element of the promoter, whereas YvrI is responsible for -35 region recognition. This system presents an unusual example of a two-subunit bacterial sigma-factor.

A regulator from Chlamydia trachomatis modulates the activity of RNA polymerase through direct interaction with the beta subunit and the primary sigma subunit

The obligate intracellular human pathogen Chlamydia trachomatis undergoes a complex developmental program involving transition between two forms: the infectious elementary body (EB), and the rapidly dividing reticulate body (RB). However, the regulators controlling this development have not been identified. To uncover potential regulators of transcription in C. trachomatis, we screened a C. trachomatis genomic library for sequences encoding proteins that interact with RNA polymerase (RNAP). We report the identification of one such protein, CT663, which interacts with the beta and sigma subunits of RNAP. Specifically, we show that CT663 interacts with the flap domain of the beta subunit (beta-flap) and conserved region 4 of the primary sigma subunit (sigma(66) in C. trachomatis). We find that CT663 inhibits sigma(66)-dependent (but not sigma(28)-dependent) transcription in vitro, and we present evidence that CT663 exerts this effect as a component of the RNAP holoenzyme. The analysis of C. trachomatis-infected cells reveals that CT663 begins to accumulate at the commencement of the RB-to-EB transition. Our findings suggest that CT663 functions as a negative regulator of sigma(66)-dependent transcription, facilitating a global change in gene expression. The strategy used here is generally applicable in cases where genetic tools are unavailable.

6S RNA binding to Esigma(70) requires a positively charged surface of sigma(70) region 4.2

6S RNA is a small, non-coding RNA that interacts with sigma(70)-RNA polymerase and downregulates transcription at many promoters during stationary phase. When bound to sigma(70)-RNA polymerase, 6S RNA is engaged in the active site of sigma(70)-RNA polymerase in a manner similar enough to promoter DNA that the RNA can serve as a template for RNA synthesis. It has been proposed that 6S RNA mimics the conformation of DNA during transcription initiation, suggesting contacts between RNA polymerase and 6S RNA or DNA may be similar. Here we demonstrate that region 4.2 of sigma(70) is critical for the interaction between 6S RNA and RNA polymerase. We define an expanded binding surface that encompasses positively charged residues throughout the recognition helix of the helix-turn-helix motif in region 4.2, in contrast to DNA binding that is largely focused on the N-terminal region of this helix. Furthermore, negatively charged residues in region 4.2 weaken binding to 6S RNA but do not similarly affect DNA binding. We propose that the binding sites for promoter DNA and 6S RNA on region 4.2 of sigma(70) are overlapping but distinct, raising interesting possibilities for how core promoter elements contribute to defining promoters that are sensitive to 6S RNA regulation.

The bacteriophage T4 AsiA protein contacts the beta-flap domain of RNA polymerase

To initiate transcription from specific promoters, the bacterial RNA polymerase (RNAP) core enzyme must associate with the initiation factor sigma, which contains determinants that allow sequence-specific interactions with promoter DNA. Most bacteria contain several sigma factors, each of which directs recognition of a distinct set of promoters. A large and diverse family of proteins known as "anti-sigma factors" regulates promoter utilization by targeting specific sigma factors. The founding member of this family is the AsiA protein of bacteriophage T4. AsiA specifically targets the primary sigma factor in Escherichia coli, sigma(70), and inhibits transcription from the major class of sigma(70)-dependent promoters. AsiA-dependent transcription inhibition has been attributed to a well-documented interaction between AsiA and conserved region 4 of sigma(70). Here, we establish that efficient AsiA-dependent transcription inhibition also requires direct protein-protein contact between AsiA and the RNAP core. In particular, we demonstrate that AsiA contacts the flap domain of the RNAP beta-subunit (the beta-flap). Our findings support the emerging view that the beta-flap is a target site for regulatory proteins that affect RNAP function during all stages of the transcription cycle.

Mutagenesis of region 4 of sigma 28 from Chlamydia trachomatis defines determinants for protein-protein and protein-DNA interactions

Transcription factor sigma(28) in Chlamydia trachomatis (sigma(28)(Ct)) plays a role in the regulation of genes that are important for late-stage morphological differentiation. In vitro mutational and genetic screening in Salmonella enterica serovar Typhimurium was performed in order to identify mutants with mutations in region 4 of sigma(28)(Ct) that were defective in sigma(28)-specific transcription. Specially, the previously undefined but important interactions between sigma(28)(Ct) region 4 and the flap domain of the RNA polymerase beta subunit (beta-flap) or the -35 element of the chlamydial hctB promoter were examined. Our results indicate that amino acid residues E206, Y214, and E222 of sigma(28)(Ct) contribute to an interaction with the beta-flap when sigma(28)(Ct) associates with the core RNA polymerase. These residues function in contacts with the beta-flap similarly to their counterpart residues in Escherichia coli sigma(70). Conversely, residue Q236 of sigma(28)(Ct) directly binds the chlamydial hctB -35 element. The conserved counterpart residue in E. coli sigma(70) has not been reported to interact with the -35 element of the sigma(70) promoter. Observed functional disparity between sigma(28)(Ct) and sigma(70) region 4 is consistent with their divergent properties in promoter recognition. This work provides new insight into understanding the molecular basis of gene regulation controlled by sigma(28)(Ct) in C. trachomatis.

The bacteriophage lambda Q antiterminator protein contacts the beta-flap domain of RNA polymerase

The multisubunit RNA polymerase (RNAP) in bacteria consists of a catalytically active core enzyme (alpha(2)beta beta'omega) complexed with a sigma factor that is required for promoter-specific transcription initiation. During early elongation the stability of interactions between sigma and core decreases, in part because of the nascent RNA-mediated destabilization of an interaction between region 4 of sigma and the flap domain of the beta-subunit (beta-flap). The nascent RNA-mediated destabilization of the sigma region 4/beta-flap interaction is required for the bacteriophage lambda Q antiterminator protein (lambdaQ) to engage the RNAP holoenzyme. Here, we provide an explanation for this requirement by showing that lambdaQ establishes direct contact with the beta-flap during the engagement process, thus competing with sigma(70) region 4 for access to the beta-flap. We also show that lambdaQ's affinity for the beta-flap is calibrated to ensure that lambdaQ activity is restricted to the lambda late promoter P(R'). Specifically, we find that strengthening the lambdaQ/beta-flap interaction allows lambdaQ to bypass the requirement for specific cis-acting sequence elements, a lambdaQ-DNA binding site and a RNAP pause-inducing element, that normally ensure lambdaQ is recruited exclusively to transcription complexes associated with P(R'). Our findings demonstrate that the beta-flap can serve as a direct target for regulators of elongation.

Poising of Escherichia coli RNA polymerase and its release from the sigma 38 C-terminal tail for osmY transcription

Bacteria must adapt their transcription to overcome the osmotic stress associated with the gastrointestinal tract of their host. This requires the sigma 38 (rpoS) form of RNA polymerase. Here, chromatin immunoprecipitation experiments show that activation is associated with a poise-and-release mechanism in vivo. A C-terminal tail unique among sigma factors is shown to be required for in vivo recruitment of RNA polymerase to the promoter region prior to osmotic shock. C-terminal domain tail-dependent transcription in vivo can be mimicked by using the intracellular signaling molecule potassium glutamate in vitro. Following signaling, the barrier to elongation into the gene body is overcome and RNA polymerase is released to produce osmY mRNA.

Promoter specificity for 6S RNA regulation of transcription is determined by core promoter sequences and competition for region 4.2 of sigma70

6S RNA binds sigma70-RNA polymerase and downregulates transcription at many sigma70-dependent promoters, but others escape regulation even during stationary phase when the majority of the transcription machinery is bound by the RNA. We report that core promoter elements determine this promoter specificity; a weak -35 element allows a promoter to be 6S RNA sensitive, and an extended -10 element similarly determines 6S RNA inhibition except when a consensus -35 element is present. These two features together predicted that hundreds of mapped Escherichia coli promoters might be subject to 6S RNA dampening in stationary phase. Microarray analysis confirmed 6S RNA-dependent downregulation of expression from 68% of the predicted genes, which corresponds to 49% of the expressed genes containing mapped E. coli promoters and establishes 6S RNA as a global regulator in stationary phase. We also demonstrate a critical role for region 4.2 of sigma70 in RNA polymerase interactions with 6S RNA. Region 4.2 binds the -35 element during transcription initiation; therefore we propose one mechanism for 6S RNA regulation of transcription is through competition for binding region 4.2 of sigma70.

Crystal structure of the Escherichia coli regulator of sigma70, Rsd, in complex with sigma70 domain 4

The Escherichia coli Rsd protein binds tightly and specifically to the RNA polymerase (RNAP) sigma(70) factor. Rsd plays a role in alternative sigma factor-dependent transcription by biasing the competition between sigma(70) and alternative sigma factors for the available core RNAP. Here, we determined the 2.6 A-resolution X-ray crystal structure of Rsd bound to sigma(70) domain 4 (sigma(70)(4)), the primary determinant for Rsd binding within sigma(70). The structure reveals that Rsd binding interferes with the two primary functions of sigma(70)(4), core RNAP binding and promoter -35 element binding. Interestingly, the most highly conserved Rsd residues form a network of interactions through the middle of the Rsd structure that connect the sigma(70)(4)-binding surface with three cavities exposed on distant surfaces of Rsd, suggesting functional coupling between sigma(70)(4) binding and other binding surfaces of Rsd, either for other proteins or for as yet unknown small molecule effectors. These results provide a structural basis for understanding the role of Rsd, as well as its ortholog, AlgQ, a positive regulator of Pseudomonas aeruginosa virulence, in transcription regulation.

A sigma-core interaction of the RNA polymerase holoenzyme that enhances promoter escape

The sigma subunit of bacterial RNA polymerase (RNAP) is required for promoter-specific transcription initiation and can also participate in downstream events. Several functionally important intersubunit interactions between Escherichia coli sigma(70) and the core enzyme (alpha(2)betabeta'omega) have been defined. These include an interaction between conserved region 2 of sigma(70) (sigma(2)) and the coiled-coil domain of beta' (beta' coiled-coil) that is required for sequence-specific interaction between sigma(2) and the DNA during both promoter open complex formation and sigma(70)-dependent early elongation pausing. Here, we describe a previously uncharacterized interaction between a region of sigma(70) adjacent to sigma(2) called the nonconserved region (sigma(70) NCR) and a region in the N-terminal portion of beta' that appears to functionally antagonize the sigma(2)/beta' coiled-coil interaction. Specifically, we show that the sigma(70) NCR/beta' interaction facilitates promoter escape and hinders early elongation pausing, in contrast to the sigma(2)/beta' coiled-coil interaction, which has opposite effects. We also demonstrate that removal of the sigma(70) NCR results in a severe growth defect; we suggest that its importance for growth may reflect its role in promoter escape.

Mutational analysis of sigma70 region 4 needed for appropriation by the bacteriophage T4 transcription factors AsiA and MotA

Transcriptional activation of bacteriophage T4 middle promoters requires sigma70-containing Escherichia coli RNA polymerase, the T4 activator MotA, and the T4 co-activator AsiA. T4 middle promoters contain the sigma70 -10 DNA element. However, these promoters lack the sigma70 -35 element, having instead a MotA box centered at -30, which is bound by MotA. Previous work has indicated that AsiA and MotA interact with region 4 of sigma70, the C-terminal portion that normally contacts -35 DNA and the beta-flap structure in core. AsiA binding prevents the sigma70/beta-flap and sigma70/-35 DNA interactions, inhibiting transcription from promoters that require a -35 element. To test the importance of residues within sigma70 region 4 for MotA and AsiA function, we investigated how sigma70 region 4 mutants interact with AsiA, MotA, and the beta-flap and function in transcription assays in vitro. We find that alanine substitutions at residues 584-588 (region 4.2) do not impair the interaction of region 4 with the beta-flap or MotA, but they eliminate the interaction with AsiA and prevent AsiA inhibition and MotA/AsiA activation. In contrast, alanine substitutions at 551-552, 554-555 (region 4.1) eliminate the region 4/beta-flap interaction, significantly impair the AsiA/sigma70 interaction, and eliminate AsiA inhibition. However, the 4.1 mutant sigma70 is still fully competent for activation if both MotA and AsiA are present. A previous NMR structure shows AsiA binding to sigma70 region 4, dramatically distorting regions 4.1 and 4.2 and indirectly changing the conformation of the MotA interaction site at the sigma70 C terminus. Our analyses provide biochemical relevance for the sigma70 residues identified in the structure, indicate that the interaction of AsiA with sigma70 region 4.2 is crucial for activation, and support the idea that AsiA binding facilitates an interaction between MotA and the far C terminus of sigma70.

The Role of the Lid Element in Transcription by E. coli RNA Polymerase

The recently described crystal structures of multi-subunit RNA polymerases (RNAPs) reveal a conserved loop-like feature called the lid. The lid projects from the clamp domain and contacts the flap, thereby enclosing the RNA transcript in RNAP's RNA-exit channel and forming the junction between the exit channel and the main channel, which holds the RNA:DNA hybrid. In the initiating form of bacterial RNAP (holoenzyme containing sigma), the lid interacts with sigma region 3 and encloses an extended linker between sigma region 3 and sigma region 4 in place of the RNA in the exit channel. During initiation, the lid may be important for holding open the transcription bubble and may help displace the RNA from the template DNA strand. To test these ideas, we constructed and characterized a mutant RNAP from which the lid element was deleted. Deltalid RNAP exhibited dramatically reduced activity during initiation from -35-dependent and -35-independent promoters, verifying that the lid is important for stabilizing the open promoter complex during initiation. However, transcript elongation, RNA displacement, and, surprisingly, transcriptional termination all occurred normally in Deltalid RNAP. Importantly, Deltalid RNAP behaved differently from wild-type RNAP when transcribing single-stranded DNA templates where it synthesized long, persistent RNA:DNA hybrids, in contrast to efficient transcriptional arrest by wild-type RNAP.

Holoenzyme Switching and Stochastic Release of Sigma Factors from RNA Polymerase In Vivo

We investigated the binding of E. coli RNA polymerase holoenzymes bearing sigma70, sigma(S), sigma32, or sigma54 to the ribosomal RNA operons (rrn) in vivo. At the rrn promoter, we observed "holoenzyme switching" from Esigma70 to Esigma(S) or Esigma32 in response to environmental cues. We also examined if sigma factors are retained by core polymerase during transcript elongation. At the rrn operons, sigma70 translocates briefly with the elongating polymerase and is released stochastically from the core polymerase with an estimated half-life of approximately 4-7 s. Similarly, at gadA and htpG, operons that are targeted by Esigma(S) and Esigma32, respectively, we find that sigma(S) and sigma32 also dissociate stochastically, albeit more rapidly than sigma70, from the elongating core polymerase. Up to approximately 70% of Esigma70 (the major vegetative holoenzyme) in rapidly growing cells is engaged in transcribing the rrn operons. Thus, our results suggest that at least approximately 70% of cellular holoenzymes release sigma70 during transcript elongation. Release of sigma factors during each round of transcription provides a simple mechanism for rapidly reprogramming polymerase with the relevant sigma factor and is consistent with the occurrence of a "sigma cycle" in vivo.

An artificial activator that contacts a normally occluded surface of the RNA polymerase holoenzyme

Many activators of transcription are sequence-specific DNA-binding proteins that stimulate transcription initiation through interaction with RNA polymerase (RNAP). Such activators can be constructed artificially by fusing a DNA-binding protein to a protein domain that can interact with an accessible surface of RNAP. In these cases, the artificial activator is directed to a target promoter bearing a recognition site for the DNA-binding protein. Here we describe an artificial activator that functions by contacting a normally occluded surface of promoter-bound RNAP holoenzyme. This artificial activator consists of a DNA-binding protein fused to the bacteriophage T4-encoded transcription regulator AsiA. On its own, AsiA inhibits transcription by Escherichia coli RNAP because it remodels the holoenzyme, disrupting an intersubunit interaction that is required for recognition of the major class of bacterial promoters. However, when tethered to the DNA via a DNA-binding protein, AsiA can exert a strong stimulatory effect on transcription by disrupting the same intersubunit interaction, contacting an otherwise occluded surface of the holoenzyme. We show that mutations that affect the intersubunit interaction targeted by AsiA modulate the stimulatory effect of this artificial activator. Our results thus demonstrate that changes in the accessibility of a normally occluded surface of the RNAP holoenzyme can modulate the activity of a gene-specific regulator of transcription.

Mechanistic studies of the T4 DNA (gp41) replication helicase: functional interactions of the C-terminal Tails of the helicase subunits with the T4 (gp59) helicase loader protein

We compare the activities of the wild-type (gp41WT) and mutant (gp41delta C20) forms of the bacteriophage T4 replication helicase. In the gp41delta C20 mutant the helicase subunits have been genetically truncated to remove the 20 residue C-terminal tail peptide domains present in the wild-type enzyme. Here, we examine the interactions of these helicase forms with the T4 gp59 helicase loader and the gp32 single-stranded DNA binding proteins, both of which are physically and functionally coupled with the helicase in the T4 DNA replication complex. We show that the wild-type and mutant forms of the helicase do not differ in their ability to assemble into dimers and hexamers, nor in their interactions with gp61 (the T4 primase). However they do differ in their gp59-stimulated unwinding activities and in their abilities to translocate along a ssDNA strand that has been coated with gp32. We demonstrate that functional coupling between gp59 and gp41 involves direct interactions between the C-terminal tail peptides of the helicase subunits and the loading protein, and measure the energetics and kinetics of these interactions. This work helps to define a gp41-gp59 assembly pathway that involves an initial interaction between the C-terminal tails of the helicases and the gp59 loader proteins, followed by a conformational change of the helicase subunits that exposes new interaction surfaces, which can then be trapped by the gp59 protein. Our results suggest that the gp41-gp59 complex is then poised to bind ssDNA portions of the replication fork. We suggest that one of the important functions of gp59 may be to aid in the exposure of the ssDNA binding sites of the helicase subunits, which are otherwise masked and regulated by interactions with the helicase carboxy-terminal tail peptides.

The interaction between sigma70 and the beta-flap of Escherichia coli RNA polymerase inhibits extension of nascent RNA during early elongation

The sigma-subunit of bacterial RNA polymerase (RNAP) is required for promoter-specific transcription initiation. This function depends on specific intersubunit interactions that occur when sigma associates with the RNAP core enzyme to form RNAP holoenzyme. Among these interactions, that between conserved region 4 of sigma and the flap domain of the RNAP beta-subunit (beta-flap) is critical for recognition of the major class of bacterial promoters. Here, we describe the isolation of amino acid substitutions in region 4 of Escherichia coli sigma(70) that have specific effects on the sigma(70) region 4/beta-flap interaction, either weakening or strengthening it. Using these sigma(70) mutants, we demonstrate that the sigma region 4/beta-flap interaction also can affect events occurring downstream of transcription initiation during early elongation. Specifically, our results provide support for a structure-based proposal that, when bound to the beta-flap, sigma region 4 presents a barrier to the extension of the nascent RNA as it emerges from the RNA exit channel. Our findings support the view that the transition from initiation to elongation involves a staged disruption of sigma-core interactions.