Structural and mechanistic basis of σ-dependent transcriptional pausing.

In σ-dependent transcriptional pausing, the transcription initiation factor σ, translocating with RNA polymerase (RNAP), makes sequence-specific protein­DNA interactions with a promoter-like sequence element in the transcribed region, inducing pausing. It has been proposed that, in σ-dependent pausing, the RNAP active center can access off-pathway "backtracked" states that are substrates for the transcript-cleavage factors of the Gre family and on-pathway "scrunched" states that mediate pause escape. Here, using site-specific protein­DNA photocrosslinking to define positions of the RNAP trailing and leading edges and of σ relative to DNA at the λPR' promoter, we show directly that σ-dependent pausing in the absence of GreB in vitro predominantly involves a state backtracked by 2­4 bp, and σ-dependent pausing in the presence of GreB in vitro and in vivo predominantly involves a state scrunched by 2­3 bp. Analogous experiments with a library of 47 (∼16,000) transcribed-region sequences show that the state scrunched by 2­3 bp­and only that state­is associated with the consensus sequence, T−3N−2Y−1G+1, (where −1 corresponds to the position of the RNA 3' end), which is identical to the consensus for pausing in initial transcription and which is related to the consensus for pausing in transcription elongation. Experiments with heteroduplex templates show that sequence information at position T−3 resides in the DNA nontemplate strand. A cryoelectron microscopy structure of a complex engaged in σ-dependent pausing reveals positions of DNA scrunching on the DNA nontemplate and template strands and suggests that position T−3 of the consensus sequence exerts its effects by facilitating scrunching.

Structural and mechanistic basis of reiterative transcription initiation.

Reiterative transcription initiation, observed at promoters that contain homopolymeric sequences at the transcription start site, generates RNA products having 5' sequences noncomplementary to the DNA template. Here, using crystallography and cryoelectron microscopy to define structures, protein-DNA photocrosslinking to map positions of RNAP leading and trailing edges relative to DNA, and single-molecule DNA nanomanipulation to assess RNA polymerase (RNAP)-dependent DNA unwinding, we show that RNA extension in reiterative transcription initiation 1) occurs without DNA scrunching; 2) involves a short, 2- to 3-bp, RNA-DNA hybrid; and 3) generates RNA that exits RNAP through the portal by which scrunched nontemplate-strand DNA exits RNAP in standard transcription initiation. The results establish that, whereas RNA extension in standard transcription initiation proceeds through a scrunching mechanism, RNA extension in reiterative transcription initiation proceeds through a slippage mechanism, with slipping of RNA relative to DNA within a short RNA-DNA hybrid, and with extrusion of RNA from RNAP through an alternative RNA exit.

Promoter-sequence determinants and structural basis of primer-dependent transcription initiation in Escherichia coli.

Chemical modifications of RNA 5'-ends enable "epitranscriptomic" regulation, influencing multiple aspects of RNA fate. In transcription initiation, a large inventory of substrates compete with nucleoside triphosphates for use as initiating entities, providing an ab initio mechanism for altering the RNA 5'-end. In Escherichia coli cells, RNAs with a 5'-end hydroxyl are generated by use of dinucleotide RNAs as primers for transcription initiation, "primer-dependent initiation." Here, we use massively systematic transcript end readout (MASTER) to detect and quantify RNA 5'-ends generated by primer-dependent initiation for ∼410 (∼1,000,000) promoter sequences in E. coli The results show primer-dependent initiation in E. coli involves any of the 16 possible dinucleotide primers and depends on promoter sequences in, upstream, and downstream of the primer binding site. The results yield a consensus sequence for primer-dependent initiation, YTSS-2NTSS-1NTSSWTSS+1, where TSS is the transcription start site, NTSS-1NTSS is the primer binding site, Y is pyrimidine, and W is A or T. Biochemical and structure-determination studies show that the base pair (nontemplate-strand base:template-strand base) immediately upstream of the primer binding site (Y:RTSS-2, where R is purine) exerts its effect through the base on the DNA template strand (RTSS-2) through interchain base stacking with the RNA primer. Results from analysis of a large set of natural, chromosomally encoded Ecoli promoters support the conclusions from MASTER. Our findings provide a mechanistic and structural description of how TSS-region sequence hard-codes not only the TSS position but also the potential for epitranscriptomic regulation through primer-dependent transcription initiation.

XACT-Seq Comprehensively Defines the Promoter-Position and Promoter-Sequence Determinants for Initial-Transcription Pausing.

Pausing by RNA polymerase (RNAP) during transcription elongation, in which a translocating RNAP uses a "stepping" mechanism, has been studied extensively, but pausing by RNAP during initial transcription, in which a promoter-anchored RNAP uses a "scrunching" mechanism, has not. We report a method that directly defines the RNAP-active-center position relative to DNA with single-nucleotide resolution (XACT-seq; "crosslink-between-active-center-and-template sequencing"). We apply this method to detect and quantify pausing in initial transcription at 411 (∼4,000,000) promoter sequences in vivo in Escherichia coli. The results show initial-transcription pausing can occur in each nucleotide addition during initial transcription, particularly the first 4 to 5 nucleotide additions. The results further show initial-transcription pausing occurs at sequences that resemble the consensus sequence element for transcription-elongation pausing. Our findings define the positional and sequence determinants for initial-transcription pausing and establish initial-transcription pausing is hard coded by sequence elements similar to those for transcription-elongation pausing.

Stepwise Promoter Melting by Bacterial RNA Polymerase.

Transcription initiation requires formation of the open promoter complex (RPo). To generate RPo, RNA polymerase (RNAP) unwinds the DNA duplex to form the transcription bubble and loads the DNA into the RNAP active site. RPo formation is a multi-step process with transient intermediates of unknown structure. We use single-particle cryoelectron microscopy to visualize seven intermediates containing Escherichia coli RNAP with the transcription factor TraR en route to forming RPo. The structures span the RPo formation pathway from initial recognition of the duplex promoter in a closed complex to the final RPo. The structures and supporting biochemical data define RNAP and promoter DNA conformational changes that delineate steps on the pathway, including previously undetected transient promoter-RNAP interactions that contribute to populating the intermediates but do not occur in RPo. Our work provides a structural basis for understanding RPo formation and its regulation, a major checkpoint in gene expression throughout evolution.

The mechanism of variability in transcription start site selection.

During transcription initiation, RNA polymerase (RNAP) binds to promoter DNA, unwinds promoter DNA to form an RNAP-promoter open complex (RPo) containing a single-stranded 'transcription bubble,' and selects a transcription start site (TSS). TSS selection occurs at different positions within the promoter region, depending on promoter sequence and initiating-substrate concentration. Variability in TSS selection has been proposed to involve DNA 'scrunching' and 'anti-scrunching,' the hallmarks of which are: (i) forward and reverse movement of the RNAP leading edge, but not trailing edge, relative to DNA, and (ii) expansion and contraction of the transcription bubble. Here, using in vitro and in vivo protein-DNA photocrosslinking and single-molecule nanomanipulation, we show bacterial TSS selection exhibits both hallmarks of scrunching and anti-scrunching, and we define energetics of scrunching and anti-scrunching. The results establish the mechanism of TSS selection by bacterial RNAP and suggest a general mechanism for TSS selection by bacterial, archaeal, and eukaryotic RNAP.

6S RNA Mimics B-Form DNA to Regulate Escherichia coli RNA Polymerase.

Noncoding RNAs (ncRNAs) regulate gene expression in all organisms. Bacterial 6S RNAs globally regulate transcription by binding RNA polymerase (RNAP) holoenzyme and competing with promoter DNA. Escherichia coli (Eco) 6S RNA interacts specifically with the housekeeping σ70-holoenzyme (Eσ70) and plays a key role in the transcriptional reprogramming upon shifts between exponential and stationary phase. Inhibition is relieved upon 6S RNA-templated RNA synthesis. We report here the 3.8 Å resolution structure of a complex between 6S RNA and Eσ70 determined by single-particle cryo-electron microscopy and validation of the structure using footprinting and crosslinking approaches. Duplex RNA segments have A-form C3' endo sugar puckers but widened major groove widths, giving the RNA an overall architecture that mimics B-form promoter DNA. Our results help explain the specificity of Eco 6S RNA for Eσ70 and show how an ncRNA can mimic B-form DNA to directly regulate transcription by the DNA-dependent RNAP.

Open complex DNA scrunching: A key to transcription start site selection and promoter escape.

Bacterial RNA polymerase-promoter open complexes can exist in a range of states in which the leading edge of the enzyme moves but the trailing edge does not, a phenomenon we refer to as "open complex scrunching." Here we describe how open complex scrunching can determine the position of the transcription start site for some promoters, modulate the level of expression, and potentially could be targeted by factors to regulate transcription. We suggest that open complex scrunching at the extraordinarily active ribosomal RNA promoters might have evolved to initiate transcription at an unusual position relative to the core promoter elements in order to maximize the rate of promoter escape.

Multiplexed protein-DNA cross-linking: Scrunching in transcription start site selection.

In bacterial transcription initiation, RNA polymerase (RNAP) selects a transcription start site (TSS) at variable distances downstream of core promoter elements. Using next-generation sequencing and unnatural amino acid-mediated protein-DNA cross-linking, we have determined, for a library of 4(10) promoter sequences, the TSS, the RNAP leading-edge position, and the RNAP trailing-edge position. We find that a promoter element upstream of the TSS, the "discriminator," participates in TSS selection, and that, as the TSS changes, the RNAP leading-edge position changes, but the RNAP trailing-edge position does not change. Changes in the RNAP leading-edge position, but not the RNAP trailing-edge position, are a defining hallmark of the "DNA scrunching" that occurs concurrent with RNA synthesis in initial transcription. We propose that TSS selection involves DNA scrunching prior to RNA synthesis.

Open complex scrunching before nucleotide addition accounts for the unusual transcription start site of E. coli ribosomal RNA promoters.

Most Escherichia coli promoters initiate transcription with a purine 7 or 8 nt downstream from the -10 hexamer, but some promoters, including the ribosomal RNA promoter rrnB P1, start 9 nt from the -10 element. We identified promoter and RNA polymerase determinants of this noncanonical rrnB P1 start site using biochemical and genetic approaches including mutational analysis of the promoter, Fe(2+) cleavage assays to monitor template strand positions near the active-site, and Bpa cross-linking to map the path of open complex DNA at amino acid and nucleotide resolution. We find that mutations in several promoter regions affect transcription start site (TSS) selection. In particular, we show that the absence of strong interactions between the discriminator region and σ region 1.2 and between the extended -10 element and σ region 3.0, identified previously as a determinant of proper regulation of rRNA promoters, is also required for the unusual TSS. We find that the DNA in the single-stranded transcription bubble of the rrnB P1 promoter complex expands and is "scrunched" into the active site channel of RNA polymerase, similar to the situation in initial transcribing complexes. However, in the rrnB P1 open complex, scrunching occurs before RNA synthesis begins. We find that the scrunched open complex exhibits reduced abortive product synthesis, suggesting that scrunching and unusual TSS selection contribute to the extraordinary transcriptional activity of rRNA promoters by increasing promoter escape, helping to offset the reduction in promoter activity that would result from the weak interactions with σ.

TRANSCRIPTION. Structures of the RNA polymerase-σ54 reveal new and conserved regulatory strategies.

Transcription by RNA polymerase (RNAP) in bacteria requires specific promoter recognition by σ factors. The major variant σ factor (σ(54)) initially forms a transcriptionally silent complex requiring specialized adenosine triphosphate-dependent activators for initiation. Our crystal structure of the 450-kilodalton RNAP-σ(54) holoenzyme at 3.8 angstroms reveals molecular details of σ(54) and its interactions with RNAP. The structure explains how σ(54) targets different regions in RNAP to exert its inhibitory function. Although σ(54) and the major σ factor, σ(70), have similar functional domains and contact similar regions of RNAP, unanticipated differences are observed in their domain arrangement and interactions with RNAP, explaining their distinct properties. Furthermore, we observe evolutionarily conserved regulatory hotspots in RNAPs that can be targeted by a diverse range of mechanisms to fine tune transcription.

Crosslink Mapping at Amino Acid-Base Resolution Reveals the Path of Scrunched DNA in Initial Transcribing Complexes.

RNA polymerase binds tightly to DNA to recognize promoters with high specificity but then releases these contacts during the initial stage of transcription. We report a site-specific crosslinking approach to map the DNA path in bacterial transcription intermediates at amino acid and nucleotide resolution. After validating the approach by showing that the DNA path in open complexes (RPO) is the same as in high-resolution X-ray structures, we define the path following substrate addition in "scrunched" complexes (RPITC). The DNA bulges that form within the transcription bubble in RPITC are positioned differently on the two strands. Our data suggest that the non-template strand bulge is extruded into solvent in complexes containing a 5-mer RNA, whereas the template strand bulge remains within the template strand tunnel, exerting stress on interactions between the ß flap, ß' clamp, and σ3.2. We propose that this stress contributes to σ3.2 displacement from the RNA exit channel, facilitating promoter escape.

RemA is a DNA-binding protein that activates biofilm matrix gene expression in Bacillus subtilis.

Biofilm formation in Bacillus subtilis requires expression of the eps and tapA-sipW-tasA operons to synthesize the extracellular matrix components, extracellular polysaccharide and TasA amyloid proteins, respectively. Expression of both operons is inhibited by the DNA-binding protein master regulator of biofilm formation SinR and activated by the protein RemA. Here we show that RemA is a DNA-binding protein that binds to multiple sites upstream of the promoters of both operons and is both necessary and sufficient for transcriptional activation in vivo and in vitro. We further show that SinR negatively regulates eps operon expression by occluding RemA binding and thus for the P(eps) promoter SinR functions as an anti-activator. Finally, transcriptional profiling indicated that RemA was primarily a regulator of the extracellular matrix genes, but it also activated genes involved in osmoprotection, leading to the identification of another direct target, the opuA operon.

RemA (YlzA) and RemB (YaaB) regulate extracellular matrix operon expression and biofilm formation in Bacillus subtilis.

Biofilms are multicellular aggregates stabilized by an extracellular matrix. In Bacillus subtilis, the biofilm matrix is composed of an extracellular polysaccharide and the secreted protein TasA. Expression of both of the matrix components is repressed by the DNA-binding master regulator, SinR. Here we identify two small protein regulators of the extracellular matrix: RemA (formerly YlzA) and RemB (formerly YaaB). Mutation of RemA or RemB impairs pellicle formation, complex colony architecture, and motility inhibition in a sinR mutant background. Both proteins are required for the activation of the matrix biosynthesis operons and appear to act in parallel to SinR and two other known biofilm regulators, AbrB and DegU.

A molecular clutch disables flagella in the Bacillus subtilis biofilm.

Biofilms are multicellular aggregates of sessile bacteria encased by an extracellular matrix and are important medically as a source of drug-resistant microbes. In Bacillus subtilis, we found that an operon required for biofilm matrix biosynthesis also encoded an inhibitor of motility, EpsE. EpsE arrested flagellar rotation in a manner similar to that of a clutch, by disengaging motor force-generating elements in cells embedded in the biofilm matrix. The clutch is a simple, rapid, and potentially reversible form of motility control.