Isolation and Characterization of Phages That Bypass the Requirement for RNA-Mediated Antitermination

Introduction

The rpoCY75N mutation in the zinc-binding domain of the β' subunit of Escherichia coli RNA polymerase blocks the RNA-based mechanism of transcription antitermination utilized by bacteriophage HK022.

Materials and Methods

Mutant phages that overcome the block imposed by the rpoCY75N mutation are described. These phages, designated "orc" (overcomes rpoC), carry mutations that create new promoters. Promoter activity was assessed by cloning the respective regions from the wild-type and orc phages into a promoterless lacZ reporter vector.

Results

Reporter assays showed that the sequence originating from orc phages had significant promoter activity when compared with the equivalent sequence cloned from the parental phage.

Conclusions

The newly created promoters facilitate the expression of phage genes that are essential for growth on the rpoCY75N strain by bypassing transcription terminators. The small plaque phenotype of orc phages, when grown on the mutant host, suggests that suppression of the rpoCY75N mutation is incomplete.

NusG, an Ancient Yet Rapidly Evolving Transcription Factor

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

Origins and Molecular Evolution of the NusG Paralog RfaH

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

Universal Actuator for Efficient Silencing of Escherichia coli Genes Based on Convergent Transcription Resistant to Rho-Dependent Termination

Dynamic control is a distinguished strategy in modern metabolic engineering, in which inducible convergent transcription is an attractive approach for conditional gene silencing. Instead of a simple strong "reverse" (r-) promoter, a three-component actuator has been developed for constitutive genes silencing. These actuators, consisting of r-promoters with different strengths, the ribosomal transcription antitermination-inducing sequence rrnG-AT, and the RNase III processing site, were inserted into the 3'-UTR of three E. coli metabolic genes. Second and third actuator components were important to improve the effectiveness and robustness of the approach. The maximal silencing folds achieved for gltA, pgi, and ppc were approximately 7, 11, and >100, respectively. Data were analyzed using a simple model that considered RNA polymerase (RNAP) head-on collisions as the unique reason for gene silencing and continued transcription after collision with only one of two molecules. It was previously established that forward (f-) RNAP with a trailing ribosome was approximately 13-times more likely to continue transcription after head-on collision than untrailed r-RNAP which is sensitive to Rho-dependent transcription termination (RhoTT). According to the current results, this bias in complex stabilities decreased to no more than (3.0-5.7)-fold if r-RNAP became resistant to RhoTT. Therefore, the developed constitutive actuator could be considered as an improved tool for controlled gene expression mainly due to the transfer of r-transcription into a state that is resistant to potential termination and used as the basis for the design of tightly regulated actuators for the achievement of conditional silencing.

Posttranscription Initiation Control of Gene Expression Mediated by Bacterial RNA-Binding Proteins

RNA-binding proteins play vital roles in regulating gene expression and cellular physiology in all organisms. Bacterial RNA-binding proteins can regulate transcription termination via attenuation or antitermination mechanisms, while others can repress or activate translation initiation by affecting ribosome binding. The RNA targets for these proteins include short repeated sequences, longer single-stranded sequences, RNA secondary or tertiary structure, and a combination of these features. The activity of these proteins can be influenced by binding of metabolites, small RNAs, or other proteins, as well as by phosphorylation events. Some of these proteins regulate specific genes, while others function as global regulators. As the regulatory mechanisms, components, targets, and signaling circuitry surrounding RNA-binding proteins have become better understood, in part through rapid advances provided by systems approaches, a sense of the true nature of biological complexity is becoming apparent, which we attempt to capture for the reader of this review.

Ancient Transcription Factors in the News

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

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

Competitive folding of RNA structures at a termination-antitermination site

Antitermination is a regulatory process based on the competitive folding of terminator-antiterminator structures that can form in the leader region of nascent transcripts. In the case of the Bacillus subtilis licS gene involved in β-glucosides utilization, the binding of the antitermination protein LicT to a short RNA hairpin (RAT) prevents the formation of an overlapping terminator and thereby allows transcription to proceed. Here, we monitored in vitro the competition between termination and antitermination by combining bulk and single-molecule fluorescence-based assays using labeled RNA oligonucleotide constructs of increasing length that mimic the progressive transcription of the terminator invading the antiterminator hairpin. Although high affinity binding is abolished as soon as the antiterminator basal stem is disrupted by the invading terminator, LicT can still bind and promote closing of the partially unfolded RAT hairpin. However, binding no longer occurs once the antiterminator structure has been disrupted by the full-length terminator. Based on these findings, we propose a kinetic competition model for the sequential events taking place at the termination-antitermination site, where LicT needs to capture its RAT target before completion of the terminator to remain tightly bound during RNAP pausing, before finally dissociating irreversibly from the elongated licS transcript.

sRNA-Mediated Control of Transcription Termination in E. coli

Bacterial small RNAs (sRNAs) have been implicated in various aspects of post-transcriptional gene regulation. Here, we demonstrate that sRNAs also act at the level of transcription termination. We use the rpoS gene, which encodes a general stress sigma factor σ(S), as a model system, and show that sRNAs DsrA, ArcZ, and RprA bind the rpoS 5'UTR to suppress premature Rho-dependent transcription termination, both in vitro and in vivo. sRNA-mediated antitermination markedly stimulates transcription of rpoS during the transition to the stationary phase of growth, thereby facilitating a rapid adjustment of bacteria to global metabolic changes. Next generation RNA sequencing and bioinformatic analysis indicate that Rho functions as a global "attenuator" of transcription, acting at the 5'UTR of hundreds of bacterial genes, and that its suppression by sRNAs is a widespread mode of bacterial gene regulation.

An RNA motif advances transcription by preventing Rho-dependent termination

The transcription termination factor Rho associates with most nascent bacterial RNAs as they emerge from RNA polymerase. However, pharmacological inhibition of Rho derepresses only a small fraction of these transcripts. What, then, determines the specificity of Rho-dependent transcription termination? We now report the identification of a Rho-antagonizing RNA element (RARE) that hinders Rho-dependent transcription termination. We establish that RARE traps Rho in an inactive complex but does not prevent Rho binding to its recruitment sites. Although translating ribosomes normally block Rho access to an mRNA, inefficient translation of an open reading frame in the leader region of the Salmonella mgtCBR operon actually enables transcription of its associated coding region by favoring an RNA conformation that sequesters RARE. The discovery of an RNA element that inactivates Rho signifies that the specificity of nucleic-acid binding proteins is defined not only by the sequences that recruit these proteins but also by sequences that antagonize their activity.

Non-Conserved Residues in Clostridium acetobutylicum tRNA(Ala) Contribute to tRNA Tuning for Efficient Antitermination of the alaS T Box Riboswitch

The T box riboswitch regulates expression of amino acid-related genes in Gram-positive bacteria by monitoring the aminoacylation status of a specific tRNA, the binding of which affects the folding of the riboswitch into mutually exclusive terminator or antiterminator structures. Two main pairing interactions between the tRNA and the leader RNA have been demonstrated to be necessary, but not sufficient, for efficient antitermination. In this study, we used the Clostridium acetobutylicum alaS gene, which encodes alanyl-tRNA synthetase, to investigate the specificity of the tRNA response. We show that the homologous C. acetobutylicum tRNA^Ala directs antitermination of the C. acetobutylicum alaS gene in vitro, but the heterologous Bacillus subtilis tRNA^Ala (with the same anticodon and acceptor end) does not. Base substitutions at positions that vary between these two tRNAs revealed synergistic and antagonistic effects. Variation occurs primarily at positions that are not conserved in tRNA^Ala species, which indicates that these non-conserved residues contribute to optimal antitermination of the homologous alaS gene. This study suggests that elements in tRNA^Ala may have coevolved with the homologous alaS T box leader RNA for efficient antitermination.

Codon-Anticodon Recognition in the Bacillus subtilis glyQS T Box Riboswitch: RNA-DEPENDENT CODON SELECTION OUTSIDE THE RIBOSOME

Many amino acid-related genes in Gram-positive bacteria are regulated by the T box riboswitch. The leader RNA of genes in the T box family controls the expression of downstream genes by monitoring the aminoacylation status of the cognate tRNA. Previous studies identified a three-nucleotide codon, termed the "Specifier Sequence," in the riboswitch that corresponds to the amino acid identity of the downstream genes. Pairing of the Specifier Sequence with the anticodon of the cognate tRNA is the primary determinant of specific tRNA recognition. This interaction mimics codon-anticodon pairing in translation but occurs in the absence of the ribosome. The goal of the current study was to determine the effect of a full range of mismatches for comparison with codon recognition in translation. Mutations were individually introduced into the Specifier Sequence of the glyQS leader RNA and tRNA(Gly) anticodon to test the effect of all possible pairing combinations on tRNA binding affinity and antitermination efficiency. The functional role of the conserved purine 3' of the Specifier Sequence was also verifiedin this study. We found that substitutions at the Specifier Sequence resulted in reduced binding, the magnitude of which correlates well with the predicted stability of the RNA-RNA pairing. However, the tolerance for specific mismatches in antitermination was generally different from that during decoding, which reveals a unique tRNA recognition pattern in the T box antitermination system.

Regulation of Transcript Elongation

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

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

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

Translational coupling of nasST expression in Azotobacter vinelandii prevents overexpression of the nasT gene

The nasST operon encodes the transcriptional regulators of assimilatory nitrate reductase operons in phylogenetically diverse bacteria. NasT is a RNA-binding antiterminator and helps RNA polymerase read through the regulatory terminator sequences upstream of the structural genes. NasS senses nitrate and nitrite and regulates the activity of NasT through stoichiometric interaction. In this study, we analyzed the nasST sequence in Azotobacter vinelandii and revealed that the nasS and nasT genes overlap by 19 nucleotides. Our genetic analyses suggested that translational initiation of NasT was coupled with NasS translation, a regulatory mechanism that prevents overproduction of NasT. The significance of tight control of nasT expression was demonstrated in a nasT-overexpression strain, where expression of the assimilatory nitrate reductase operon was deregulated.

Two-codon T-box riboswitch binding two tRNAs

T-box riboswitches control transcription of downstream genes through the tRNA-binding formation of terminator or antiterminator structures. Previously reported T-boxes were described as single-specificity riboswitches that can bind specific tRNA anticodons through codon-anticodon interactions with the nucleotide triplet of their specifier loop (SL). However, the possibility that T-boxes might exhibit specificity beyond a single tRNA had been overlooked. In Clostridium acetobutylicum, the T-box that regulates the operon for the essential tRNA-dependent transamidation pathway harbors a SL with two potential overlapping codon positions for tRNA(Asn) and tRNA(Glu). To test its specificity, we performed extensive mutagenic, biochemical, and chemical probing analyses. Surprisingly, both tRNAs can efficiently bind the SL in vitro and in vivo. The dual specificity of the T-box is allowed by a single base shift on the SL from one overlapping codon to the next. This feature allows the riboswitch to sense two tRNAs and balance the biosynthesis of two amino acids. Detailed genomic comparisons support our observations and suggest that "flexible" T-box riboswitches are widespread among bacteria, and, moreover, their specificity is dictated by the metabolic interconnection of the pathways under control. Taken together, our results support the notion of a genome-dependent codon ambiguity of the SLs. Furthermore, the existence of two overlapping codons imposes a unique example of tRNA-dependent regulation at the transcriptional level.

The antitermination activity of bacteriophage lambda N protein is controlled by the kinetics of an RNA-looping-facilitated interaction with the transcription complex

Protein N of bacteriophage lambda activates the lytic phase of phage development in infected Escherichia coli cells by suppressing the activity of transcriptional terminators that prevent the synthesis of essential phage proteins. N binds tightly to the boxB RNA hairpin located near the 5' end of the nascent pL and pR transcripts and induces an antitermination response in the RNA polymerase (RNAP) of elongation complexes located at terminators far downstream. Here we test an RNA looping model for this N-dependent "action at a distance" by cleaving the nascent transcript between boxB and RNAP during transcript elongation. Cleavage decreases antitermination, showing that an intact RNA transcript is required to stabilize the interaction of boxB-bound N with RNAP during transcription. In contrast, an antitermination complex that also contains Nus factors retains N-dependent activity after transcript cleavage, suggesting that these host factors further stabilize the N-RNAP interaction. Thus, the binding of N alone to RNAP is controlled by an RNA looping equilibrium, but after formation of the initial RNA loop and in the presence of Nus factors the system no longer equilibrates on the transcription time scale, meaning that the "range" of antitermination activity along the template in the full antitermination system is kinetically controlled by the dissociation rate of the stabilized N-RNAP complex. Theoretical calculations of nucleic acid end-to-end contact probabilities are used to estimate the local concentrations of boxB-bound N at elongation complexes poised at terminators, and are combined with N activity measurements at various boxB-to-terminator distances to obtain an intrinsic affinity (K(d)) of approximately 2 x 10(-5) M for the N-RNAP interaction. This RNA looping approach is extended to include the effects of N binding at nonspecific RNA sites on the transcript and the implications for transcription control in other regulatory systems are discussed.

Comparative genomic analysis of T-box regulatory systems in bacteria

T-box antitermination is one of the main mechanisms of regulation of genes involved in amino acid metabolism in Gram-positive bacteria. T-box regulatory sites consist of conserved sequence and RNA secondary structure elements. Using a set of known T-box sites, we constructed the common pattern and used it to scan available bacterial genomes. New T-boxes were found in various Gram-positive bacteria, some Gram-negative bacteria (delta-proteobacteria), and some other bacterial groups (Deinococcales/Thermales, Chloroflexi, Dictyoglomi). The majority of T-box-regulated genes encode aminoacyl-tRNA synthetases. Two other groups of T-box-regulated genes are amino acid biosynthetic genes and transporters, as well as genes with unknown function. Analysis of candidate T-box sites resulted in new functional annotations. We assigned the amino acid specificity to a large number of candidate amino acid transporters and a possible function to amino acid biosynthesis genes. We then studied the evolution of the T-boxes. Analysis of the constructed phylogenetic trees demonstrated that in addition to the normal evolution consistent with the evolution of regulated genes, T-boxes may be duplicated, transferred to other genes, and change specificity. We observed several cases of recent T-box regulon expansion following the loss of a previously existing regulatory system, in particular, arginine regulon in Clostridium difficile and methionine regulon in Lactobacillaceae. Finally, we described a new structural class of T-boxes containing duplicated terminator-antiterminator elements and unusual reduced T-boxes regulating initiation of translation in the Actinobacteria.

A quantitative description of the binding states and in vitro function of antitermination protein N of bacteriophage lambda

The N protein of bacteriophage lambda activates transcription of genes that lie downstream of termination sequences by suppressing transcription termination. N binds to specific (boxB) and non-specific sites on the transcript RNA and contacts RNA polymerase via cis-RNA looping, resulting in "antitermination" of transcription. To find the effect of N-boxB binding on antitermination, we quantitatively relate binding measurements made in isolation to in vitro antitermination activity. We measure binding of N to boxB RNA, non-specific single-stranded RNA, and non-specific double-stranded DNA fluorimetrically, and use an equilibrium model to describe quantitatively the binding of N to nucleic acids of Escherichia coli transcription elongation complexes. We then test the model by comparison with in vitro N antitermination activity measured in reactions containing these same elongation complexes. We find that binding of N protein to the nucleic acid components of transcription elongation complexes can quantitatively predict antitermination activity, suggesting that antitermination in vitro is determined by a nucleic acid binding equilibrium with one molecule of N protein per RNA transcript being sufficient for antitermination. Elongation complexes contain numerous overlapping non-specific RNA and DNA-binding sites for N; the large number of sites compensates for the low N binding affinity, so multiple N proteins are expected to bind to elongation complexes. The occupancy/activity of these proteins is described by a binomial distribution of proteins on transcripts containing multiple non-specific sites. The contribution of specific (boxB) binding to activity also depends on this distribution. Specificity is not measured accurately by measurements made in the presence and in the absence of boxB. We find that antitermination is inhibited by non-productive binding of N to non-specific sites on template DNA, and that NusA protein covers RNA sites on the transcript, limiting N access and activity. The activity and specificity of regulatory proteins that loop from high-affinity binding sites are likely modulated by multiple non-specific binding events; in vivo activity may also be regulated by the modulation of non-specific binding.

RNA polymerase mutations that impair conversion to a termination-resistant complex by Q antiterminator proteins

Bacteriophage lambda Q-protein stably binds and modifies RNA polymerase (RNAP) to a termination-resistant form. We describe amino acid substitutions in RNAP that disrupt Q-mediated antitermination in vivo and in vitro. The positions of these substitutions in the modeled RNAP/DNA/RNA ternary elongation complex, and their biochemical properties, suggest that they do not define a binding site for Q in RNAP, but instead act by impairing interactions among core RNAP subunits and nucleic acids that are essential for Q modification. A specific conjecture is that Q modification stabilizes interactions of RNAP with the DNA/RNA hybrid and optimizes alignment of the nucleic acids in the catalytic site. Such changes would inhibit the activity of the RNA hairpin of an intrinsic terminator to disrupt the 5'-terminal bases of the hybrid and remove the RNA 3' terminus from the active site.

The alpha subunit of E. coli RNA polymerase activates RNA binding by NusA

The Escherichia coli NusA protein modulates pausing, termination, and antitermination by associating with the transcribing RNA polymerase core enzyme. NusA can be covalently cross-linked to nascent RNA within a transcription complex, but does not bind RNA on its own. We have found that deletion of the 79 carboxy-terminal amino acids of the 495-amino-acid NusA protein allows NusA to bind RNA in gel mobility shift assays. The carboxy-terminal domain (CTD) of the alpha subunit of RNA polymerase, as well as the bacteriophage lambda N gene antiterminator protein, bind to carboxy-terminal regions of NusA and enable full-length NusA to bind RNA. Binding of NusA to RNA in the presence of alpha or N involves an amino-terminal S1 homology region that is otherwise inactive in full-length NusA. The interaction of the alpha-CTD with full-length NusA stimulates termination. N may prevent termination by inducing NusA to interact with N utilization (nut) site RNA rather than RNA near the 3' end of the nascent transcript. Sequence analysis showed that the alpha-CTD contains a modified helix-hairpin-helix motif (HhH), which is also conserved in the carboxy-terminal regions of some eubacterial NusA proteins. These HhH motifs may mediate protein-protein interactions in NusA and the alpha-CTD.

A surface of Escherichia coli sigma 70 required for promoter function and antitermination by phage lambda Q protein

The sigma initiation factor sigma70 of Escherichia coli acts not only in promoter recognition and DNA strand opening, but also to mediate the transformation of RNA polymerase (RNAP) to an antiterminating form by the phage lambda gene Q protein. Q is able to bind and modify RNAP when alpha70, still present in the initially elongating enzyme, recognizes a repeat of the -10 promoter element and induces a transcription pause. We have isolated mutations in the rpoD gene for sigma70 that impair Q function because they reduce the ability of sigma70 to recognize the downstream pause site. These mutations identify a locus of sigma70 that is important for the formation and stability of open promoter complex, likely because it mediates protein interactions with RNAP core.

Translational repression by a transcriptional elongation factor

One of the classical positive regulators of gene expression is bacteriophage lambda N protein. N regulates the transcription of early phage genes by participating in the formation of a highly processive, terminator-resistant transcription complex and thereby stimulates the expression of genes lying downstream of transcriptional terminators. Also included in this antiterminating transcription complex are an RNA site (NUT) and host proteins (Nus). Here we demonstrate that N has an additional, hitherto unknown regulatory role, as a repressor of the translation of its own gene. N-dependent repression does not occur when NUT is deleted, demonstrating that N-mediated antitermination and translational repression both require the same cis-acting site in the RNA. In addition, we have identified one nut and several host mutations that eliminate antitermination and not translational repression, suggesting the independence of these two N-mediated mechanisms. Finally, the position of nutL with respect to the gene whose expression is repressed is important.