T7 phage protein Gp2 inhibits the Escherichia coli RNA polymerase by antagonizing stable DNA strand separation near the transcription start site

Pseudomonas aeruginosa as a model bacterium in antiphage defense research

Bacteriophages, or phages, depend on their bacterial hosts for proliferation, leading to a coevolutionary relationship characterized by on-going arms races, where bacteria evolve diverse antiphage defense systems. The development of in silico methods and high-throughput screening techniques has dramatically expanded our understanding of bacterial antiphage defense systems, enormously increasing the known repertoire of the distinct mechanisms across various bacterial species. These advances have revealed that bacterial antiphage defense systems exhibit a remarkable level of complexity, ranging from highly conserved to specialized mechanisms, underscoring the intricate nature of bacterial antiphage defense systems. In this review, we provide a concise snapshot of antiphage defense research highlighting two preponderantly commandeered approaches and classification of the known antiphage defense systems. A special focus is placed on the model bacterial pathogen, Pseudomonas aeruginosa in antiphage defense research. We explore the complexity and adaptability of these systems, which play crucial roles in genome evolution and adaptation of P. aeruginosa in response to an arsenal of diverse phage strains, emphasizing the importance of this organism as a key emerging model bacterium in recent antiphage defense research.

Structural basis of phage transcriptional regulation

Phages are the most prevalent and diverse entities in the biosphere and represent the simplest systems that are capable of self-replication. Many fundamental concepts of transcriptional regulation were revealed through phage studies. The replication of phages within bacteria entails the hijacking of the host transcription machinery. Typically, this is accomplished through proteins and RNAs encoded by the phage genome that bind to the host RNA polymerase and modify its characteristics. Understanding these processes offers valuable insights into the mechanisms of bacterial transcription itself. Historically, X-ray crystallography has been the major tool for elucidating the structural basis of phage transcriptional regulation. In recent years, the application of cryoelectron microscopy has not only allowed the exploration of protein-protein and protein-nucleic acid interactions at near-atomic resolution but also captured transient intermediate states, further expanding our mechanistic understanding of phage transcriptional regulation.

Membrane platform protein PulF of the Klebsiella type II secretion system forms a trimeric ion channel essential for endopilus assembly and protein secretion

IMPORTANCE

Type IV pili and type II secretion systems are members of the widespread type IV filament (T4F) superfamily of nanomachines that assemble dynamic and versatile surface fibers in archaea and bacteria. The assembly and retraction of T4 filaments with diverse surface properties and functions require the plasma membrane platform proteins of the GspF/PilC superfamily. Generally considered dimeric, platform proteins are thought to function as passive transmitters of the mechanical energy generated by the ATPase motor, to somehow promote insertion of pilin subunits into the nascent pilus fibers. Here, we generate and experimentally validate structural predictions that support the trimeric state of a platform protein PulF from a type II secretion system. The PulF trimers form selective proton or sodium channels which might energize pilus assembly using the membrane potential. The conservation of the channel sequence and structural features implies a common mechanism for all T4F assembly systems. We propose a model of the oligomeric PulF-PulE ATPase complex that provides an essential framework to investigate and understand the pilus assembly mechanism.

How to Shut Down Transcription in Archaea during Virus Infection

Multisubunit RNA polymerases (RNAPs) carry out transcription in all domains of life; during virus infection, RNAPs are targeted by transcription factors encoded by either the cell or the virus, resulting in the global repression of transcription with distinct outcomes for different host–virus combinations. These repressors serve as versatile molecular probes to study RNAP mechanisms, as well as aid the exploration of druggable sites for the development of new antibiotics. Here, we review the mechanisms and structural basis of RNAP inhibition by the viral repressor RIP and the crenarchaeal negative regulator TFS4, which follow distinct strategies. RIP operates by occluding the DNA-binding channel and mimicking the initiation factor TFB/TFIIB. RIP binds tightly to the clamp and locks it into one fixed position, thereby preventing conformational oscillations that are critical for RNAP function as it progresses through the transcription cycle. TFS4 engages with RNAP in a similar manner to transcript cleavage factors such as TFS/TFIIS through the NTP-entry channel; TFS4 interferes with the trigger loop and bridge helix within the active site by occlusion and allosteric mechanisms, respectively. The conformational changes in RNAP described above are universally conserved and are also seen in inactive dimers of eukaryotic RNAPI and several inhibited RNAP complexes of both bacterial and eukaryotic RNA polymerases, including inactive states that precede transcription termination. A comparison of target sites and inhibitory mechanisms reveals that proteinaceous repressors and RNAP-specific antibiotics use surprisingly common ways to inhibit RNAP function.

RNA Management During T7 Infection

Post-transcriptional regulation (PTR) determines the fate of RNA in the cell and represents an important control point in the flow of genetic information and thus underpins many, if not all, aspects of cell function. Host takeover by phages through misappropriation of the bacterial transcription machinery is a relatively advanced area of research. However, several phages encode small regulatory RNAs, which are major mediators of PTR, and produce specific proteins to manipulate bacterial enzymes involved in RNA degradation.1-4 However, PTR during phage development still represents an understudied area of phage-bacteria interaction biology. In this study, we discuss the potential role PTR could play in determining the fate of RNA during the lifecycle of the prototypic phage T7 in Escherichia coli.

Mycobacteriophage D29 induced association of Mycobacterial RNA polymerase with ancillary factors leads to increased transcriptional activity

Mycobacteriophage D29 infects species belonging to the genus Mycobacterium including the deadly pathogen Mycobacterium tuberculosis. D29 is a lytic phage, although, related to the lysogenic mycobacteriophage L5. This phage is unable to lysogenize in mycobacteria as it lacks the gene encoding the phage repressor. Infection by many mycobacteriophages cause various changes in the host that ultimately leads to inactivation of the latter. One of the host targets often modified in the process is RNA polymerase. During our investigations with phage D29 infected Mycobacterium smegmatis (Msm) we observed that the promoters from both phage, and to a lesser extent those of the host were found to be more active in cells that were exposed to D29, as compared to the unexposed. Further experiments indicate that the RNA polymerase purified from phage infected cells possessed higher affinity for promoters particularly those that were phage derived. Comparison of the purified RNA polymerase preparations from infected and uninfected cells showed that several ancillary transcription factors, Sigma factor F, Sigma factor H, CarD and RbpA are prominently associated with the RNA polymerase from infected cells. Based on our observations we conclude that the higher activity of RNA polymerase observed in D29 infected cells is due to its increased association with ancillary transcription factors.

Bacteriophage protein Gp46 is a cross-species inhibitor of nucleoid-associated HU proteins

Histone-like protein from Escherichia coli strain U93 (HU) protein is the most abundant nucleoid-associated protein in bacteria, which plays a fundamental role in chromosomal compaction and organization. It is essential for most bacteria as well as Apicomplexans, thus an important target for the development of antimicrobial and antimalaria drugs. We report Gp46 as a phage protein HU inhibitor. It inhibits HU of Bacillus subtilis by occupying its DNA binding site, thus preventing chromosome segregation during cell division. As key residues for the interaction are highly conserved, Gp46 interacts with HUs of a broad range of pathogens, including many pathogenic bacteria and Apicomplexan parasites like Plasmodium falciparum. Thus, this cross-species property could benefit antibiotic and antimalaria drug development that targets HU.

The architectural protein histone-like protein from Escherichia coli strain U93 (HU) is the most abundant bacterial DNA binding protein and highly conserved among bacteria and Apicomplexan parasites. It not only binds to double-stranded DNA (dsDNA) to maintain DNA stability but also, interacts with RNAs to regulate transcription and translation. Importantly, HU is essential to cell viability for many bacteria; hence, it is an important antibiotic target. Here, we report that Gp46 from bacteriophage SPO1 of Bacillus subtilis is an HU inhibitor whose expression prevents nucleoid segregation and causes filamentous morphology and growth defects in bacteria. We determined the solution structure of Gp46 and revealed a striking negatively charged surface. An NMR-derived structural model for the Gp46–HU complex shows that Gp46 occupies the DNA binding motif of the HU and therefore, occludes DNA binding, revealing a distinct strategy for HU inhibition. We identified the key residues responsible for the interaction that are conserved among HUs of bacteria and Apicomplexans, including clinically significant Mycobacterium tuberculosis, Acinetobacter baumannii, and Plasmodium falciparum, and confirm that Gp46 can also interact with these HUs. Our findings provide detailed insight into a mode of HU inhibition that provides a useful foundation for the development of antibacteria and antimalaria drugs.

De novo developed protein binders mimicking Interferon lambda signaling

We hereby describe the process of design and selection of nonantibody protein binders mimicking cytokine signaling. We chose to mimic signaling of IFN-λ1, type 3 interferon (also known as IL-29) for its novelty and the importance of its biological functions. All four known interferons λ signal through binding to the extracellular domains of IL-28 receptor 1 (IL-28R1) and IL-10 receptor 2 (IL-10R2). Our binders were therefore trained to bind both receptors simultaneously. The bifunctional binder molecules were developed by yeast display, a method of directed evolution. The signaling capacity of the bivalent binders was tested by measuring phosphorylation of the JAK/STAT signaling pathway and production of mRNA of six selected genes naturally induced by IFN- λ1 in human cell lines. The newly developed bivalent binders offer opportunities to study cytokine-related biological functions and modulation of the cell behavior by receptor activation on the cell surfaces alternative to the use of natural IFN-λ.

Decoupling Protein Production from Cell Growth Enhances the Site-Specific Incorporation of Noncanonical Amino Acids in E. coli

The site-specific incorporation of noncanonical amino acids (ncAAs) into proteins by amber stop codon suppression has become a routine method in academic laboratories. This approach requires an amber suppressor tRNA_CUA to read the amber codon and an aminoacyl-tRNA synthetase to charge the tRNA_CUA with the ncAA. However, a major drawback is the low yield of the mutant protein in comparison to the wild type. This effect primarily results from the competition of release factor 1 with the charged suppressor tRNA_CUA for the amber codon at the A-site of the ribosome. A number of laboratories have attempted to improve the incorporation efficiency of ncAAs with moderate results. We aimed at increasing the efficiency to produce high yields of ncAA-functionalized proteins in a scalable setting for industrial application. To do this, we inserted an ncAA into the enhanced green fluorescent protein and an antibody mimetic molecule using an industrial E. coli strain, which produces recombinant proteins independent of cell growth. The controlled decoupling of recombinant protein production from cell growth considerably increased the incorporation of the ncAA, producing substantially higher protein yields versus the reference E. coli strain BL21(DE3). The target proteins were expressed at high levels, and the ncAA was efficiently incorporated with excellent fidelity while the protein function was preserved.

Phage-based target discovery and its exploitation towards novel antibacterial molecules

The deeply intertwined evolutionary history between bacteriophages and bacteria has endowed phages with highly specific mechanisms to hijack bacterial cell metabolism for their propagation. Here, we present a comprehensive, phage-driven strategy to reveal novel antibacterial targets by the exploitation of phage-bacteria interactions. This strategy will enable the design of small molecules, which mimic the inhibitory phage proteins, and allow the subsequent hit-to-lead development of these antimicrobial compounds. This proposed small molecule approach is distinct from phage therapy and phage enzyme-based antimicrobials and may produce a more sustainable generation of new antibiotics that exploit novel bacterial targets and act in a pathogen-specific manner.

The Phage-Encoded N-Acetyltransferase Rac Mediates Inactivation of Pseudomonas aeruginosa Transcription by Cleavage of the RNA Polymerase Alpha Subunit

In this study, we describe the biological function of the phage-encoded protein RNA polymerase alpha subunit cleavage protein (Rac), a predicted Gcn5-related acetyltransferase encoded by phiKMV-like viruses. These phages encode a single-subunit RNA polymerase for transcription of their late (structure- and lysis-associated) genes, whereas the bacterial RNA polymerase is used at the earlier stages of infection. Rac mediates the inactivation of bacterial transcription by introducing a specific cleavage in the α subunit of the bacterial RNA polymerase. This cleavage occurs within the flexible linker sequence and disconnects the C-terminal domain, required for transcription initiation from most highly active cellular promoters. To achieve this, Rac likely taps into a novel post-translational modification (PTM) mechanism within the host Pseudomonas aeruginosa. From an evolutionary perspective, this novel phage-encoded regulation mechanism confirms the importance of PTMs in the prokaryotic metabolism and represents a new way by which phages can hijack the bacterial host metabolism.

Novel Escherichia coli RNA Polymerase Binding Protein Encoded by Bacteriophage T5

The Escherichia coli bacteriophage T5 has three temporal classes of genes (pre-early, early, and late). All three classes are transcribed by host RNA polymerase (RNAP) containing the σ⁷⁰ promoter specificity subunit. Molecular mechanisms responsible for the switching of viral transcription from one class to another remain unknown. Here, we find the product of T5 gene 026 (gpT5.026) in RNAP preparations purified from T5-infected cells and demonstrate in vitro its tight binding to E. coli RNAP. While proteins homologous to gpT5.026 are encoded by all T5-related phages, no similarities to proteins with known functions can be detected. GpT5.026 binds to two regions of the RNAP β subunit and moderately inhibits RNAP interaction with the discriminator region of σ⁷⁰-dependent promoters. A T5 mutant with disrupted gene 026 is viable, but the host cell lysis phase is prolongated and fewer virus particles are produced. During the mutant phage infection, the number of early transcripts increases, whereas the number of late transcripts decreases. We propose that gpT5.026 is part of the regulatory cascade that orchestrates a switch from early to late bacteriophage T5 transcription.

Structural basis of transcription inhibition by the DNA mimic protein Ocr of bacteriophage T7

Bacteriophage T7 infects Escherichia coli and evades the host restriction/modification system. The Ocr protein of T7 was shown to exist as a dimer mimicking DNA and to bind to host restriction enzymes, thus preventing the degradation of the viral genome by the host. Here we report that Ocr can also inhibit host transcription by directly binding to bacterial RNA polymerase (RNAP) and competing with the recruitment of RNAP by sigma factors. Using cryo electron microscopy, we determined the structures of Ocr bound to RNAP. The structures show that an Ocr dimer binds to RNAP in the cleft, where key regions of sigma bind and where DNA resides during transcription synthesis, thus providing a structural basis for the transcription inhibition. Our results reveal the versatility of Ocr in interfering with host systems and suggest possible strategies that could be exploited in adopting DNA mimicry as a basis for forming novel antibiotics.

Conformational heterogeneity and bubble dynamics in single bacterial transcription initiation complexes

Transcription initiation is a major step in gene regulation for all organisms. In bacteria, the promoter DNA is first recognized by RNA polymerase (RNAP) to yield an initial closed complex. This complex subsequently undergoes conformational changes resulting in DNA strand separation to form a transcription bubble and an RNAP-promoter open complex; however, the series and sequence of conformational changes, and the factors that influence them are unclear. To address the conformational landscape and transitions in transcription initiation, we applied single-molecule Förster resonance energy transfer (smFRET) on immobilized Escherichia coli transcription open complexes. Our results revealed the existence of two stable states within RNAP–DNA complexes in which the promoter DNA appears to adopt closed and partially open conformations, and we observed large-scale transitions in which the transcription bubble fluctuated between open and closed states; these transitions, which occur roughly on the 0.1 s timescale, are distinct from the millisecond-timescale dynamics previously observed within diffusing open complexes. Mutational studies indicated that the σ⁷⁰ region 3.2 of the RNAP significantly affected the bubble dynamics. Our results have implications for many steps of transcription initiation, and support a bend-load-open model for the sequence of transitions leading to bubble opening during open complex formation.

Constrained Combinatorial Libraries of Gp2 Proteins Enhance Discovery of PD-L1 Binders

Engineered protein ligands are used for molecular therapy, diagnostics, and industrial biotechnology. The Gp2 domain is a 45-amino acid scaffold that has been evolved for specific, high-affinity binding to multiple targets by diversification of two solvent-exposed loops. Inspired by sitewise enrichment of select amino acids, including cysteine pairs, in earlier Gp2 discovery campaigns, we hypothesized that the breadth and efficiency of de novo Gp2 discovery will be aided by sitewise amino acid constraint within combinatorial library design. We systematically constructed eight libraries and comparatively evaluated their efficacy for binder discovery via yeast display against a panel of targets. Conservation of a cysteine pair at the termini of the first diversified paratope loop increased binder discovery 16-fold ( p < 0.001). Yet two other libraries with conserved cysteine pairs, within the second loop or an interloop pair, did not aid discovery thereby indicating site-specific impact. Via a yeast display protease resistance assay, Gp2 variants from the loop one cysteine pair library were 3.3 ± 2.1-fold ( p = 0.005) more stable than nonconstrained variants. Sitewise constraint of noncysteine residues-guided by previously evolved binders, natural Gp2 homology, computed stability, and structural analysis-did not aid discovery. A panel of binders to programmed death ligand 1 (PD-L1), a key target in cancer immunotherapy, were discovered from the loop 1 cysteine constraint library. Affinity maturation via loop walking resulted in strong, specific cellular PD-L1 affinity ( K_d = 6-9 nM).

T7 phage factor required for managing RpoS in Escherichia coli

T7 development in Escherichia coli requires the inhibition of the housekeeping form of the bacterial RNA polymerase (RNAP), Eσ⁷⁰, by two T7 proteins: Gp2 and Gp5.7. Although the biological role of Gp2 is well understood, that of Gp5.7 remains to be fully deciphered. Here, we present results from functional and structural analyses to reveal that Gp5.7 primarily serves to inhibit Eσ^S, the predominant form of the RNAP in the stationary phase of growth, which accumulates in exponentially growing E. coli as a consequence of the buildup of guanosine pentaphosphate [(p)ppGpp] during T7 development. We further demonstrate a requirement of Gp5.7 for T7 development in E. coli cells in the stationary phase of growth. Our finding represents a paradigm for how some lytic phages have evolved distinct mechanisms to inhibit the bacterial transcription machinery to facilitate phage development in bacteria in the exponential and stationary phases of growth.

Engineered Charge Redistribution of Gp2 Proteins through Guided Diversity for Improved PET Imaging of Epidermal Growth Factor Receptor

The Gp2 domain is a protein scaffold for synthetic ligand engineering. However, the native protein function results in a heterogeneous distribution of charge on the conserved surface, which may hinder further development and utility. We aim to modulate charge, without diminishing function, which is challenging in small proteins where each mutation is a significant fraction of protein structure. We constructed rationally guided combinatorial libraries with charge-neutralizing or charge-flipping mutations and sorted them, via yeast display and flow cytometry, for stability and target binding. Deep sequencing of functional variants revealed effective mutations both in clone-dependent contexts and broadly across binders to epidermal growth factor receptor (EGFR), insulin receptor, and immunoglobulin G. Functional mutants averaged 4.3 charge neutralizing mutations per domain while maintaining net negative charge. We evolved an EGFR-targeted Gp2 mutant that reduced charge density by 33%, maintained net charge, and improved charge distribution homogeneity while elevating thermal stability ( Tm = 87 ± 1 °C), improving binding specificity, and maintaining affinity ( Kd = 8.8 ± 0.6 nM). This molecule was conjugated with 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid for 64Cu chelation and evaluated for physiological distribution in mice with xenografted A431 (EGFRhigh) and MDA-MB-435 (EGFRlow) tumors. Excised tissue gamma counting and positron emission tomography/computed tomography imaging revealed good EGFRhigh tumor signal (4.7 ± 0.5%ID/g) at 2 h post-injection and molecular specificity evidenced by low uptake in EGFRlow tumors (0.6 ± 0.1%ID/g, significantly lower than for non-charge-modified Gp2, p = 0.01). These results provide charge mutations for an improved Gp2 framework, validate an effective approach to charge engineering, and advance performance of physiological EGFR targeting for molecular imaging.

Beyond antibody engineering: directed evolution of alternative binding scaffolds and enzymes using yeast surface display

Pioneered exactly 20 years ago, yeast surface display (YSD) continues to take a major role in protein engineering among the high-throughput display methodologies that have been developed to date. The classical yeast display technology relies on tethering an engineered protein to the cell wall by genetic fusion to one subunit of a dimeric yeast-mating agglutination receptor complex. This method enables an efficient genotype-phenotype linkage while exploiting the benefits of a eukaryotic expression machinery. Over the past two decades, a plethora of protein engineering efforts encompassing conventional antibody Fab and scFv fragments have been reported. In this review, we will focus on the versatility of YSD beyond conventional antibody engineering and, instead, place the focus on alternative scaffold proteins and enzymes which have successfully been tailored for purpose with regard to improving binding, activity or specificity.

Phenotypic consequences of RNA polymerase dysregulation in Escherichia coli

Many bacterial adaptive responses to changes in growth conditions due to biotic and abiotic factors involve reprogramming of gene expression at the transcription level. The bacterial RNA polymerase (RNAP), which catalyzes transcription, can thus be considered as the major mediator of cellular adaptive strategies. But how do bacteria respond if a stress factor directly compromises the activity of the RNAP? We used a phage-derived small protein to specifically perturb bacterial RNAP activity in exponentially growing Escherichia coli. Using cytological profiling, tracking RNAP behavior at single-molecule level and transcriptome analysis, we reveal that adaptation to conditions that directly perturb bacterial RNAP performance can result in a biphasic growth behavior and thereby confer the 'adapted' bacterial cells an enhanced ability to tolerate diverse antibacterial stresses. The results imply that while synthetic transcriptional rewiring may confer bacteria with the intended desirable properties, such approaches may also collaterally allow them to acquire undesirable traits.

Full shut-off of Escherichia coli RNA-polymerase by T7 phage requires a small phage-encoded DNA-binding protein

Infection of Escherichia coli by the T7 phage leads to rapid and selective inhibition of the bacterial RNA polymerase (RNAP) by the 7 kDa T7 protein Gp2. We describe the identification and functional and structural characterisation of a novel 7 kDa T7 protein, Gp5.7, which adopts a winged helix-turn-helix-like structure and specifically represses transcription initiation from host RNAP-dependent promoters on the phage genome via a mechanism that involves interaction with DNA and the bacterial RNAP. Whereas Gp2 is indispensable for T7 growth in E. coli, we show that Gp5.7 is required for optimal infection outcome. Our findings provide novel insights into how phages fine-tune the activity of the host transcription machinery to ensure both successful and efficient phage progeny development.

Exploring the potential of T7 bacteriophage protein Gp2 as a novel inhibitor of mycobacterial RNA polymerase

Over the past six decades, there has been a decline in novel therapies to treat tuberculosis, while the causative agent of this disease has become increasingly resistant to current treatment regimens. Bacteriophages (phages) are able to kill bacterial cells and understanding this process could lead to novel insights for the treatment of mycobacterial infections. Phages inhibit bacterial gene transcription through phage-encoded proteins which bind to RNA polymerase (RNAP), thereby preventing bacterial transcription. Gp2, a T7 phage protein which binds to the beta prime (β') subunit of RNAP in Escherichia coli, has been well characterized in this regard. Here, we aimed to determine whether Gp2 is able to inhibit RNAP in Mycobacterium tuberculosis as this may provide new possibilities for inhibiting the growth of this deadly pathogen. Results from an electrophoretic mobility shift assay and in vitro transcription assay revealed that Gp2 binds to mycobacterial RNAP and inhibits transcription; however to a much lesser degree than in E. coli. To further understand the molecular basis of these results, a series of in silico techniques were used to assess the interaction between mycobacterial RNAP and Gp2, providing valuable insight into the characteristics of this protein-protein interaction.

The bacterial enhancer-dependent RNA polymerase

Transcription initiation is highly regulated in bacterial cells, allowing adaptive gene regulation in response to environment cues. One class of promoter specificity factor called sigma54 enables such adaptive gene expression through its ability to lock the RNA polymerase down into a state unable to melt out promoter DNA for transcription initiation. Promoter DNA opening then occurs through the action of specialized transcription control proteins called bacterial enhancer-binding proteins (bEBPs) that remodel the sigma54 factor within the closed promoter complexes. The remodelling of sigma54 occurs through an ATP-binding and hydrolysis reaction carried out by the bEBPs. The regulation of bEBP self-assembly into typically homomeric hexamers allows regulated gene expression since the self-assembly is required for bEBP ATPase activity and its direct engagement with the sigma54 factor during the remodelling reaction. Crystallographic studies have now established that in the closed promoter complex, the sigma54 factor occupies the bacterial RNA polymerase in ways that will physically impede promoter DNA opening and the loading of melted out promoter DNA into the DNA-binding clefts of the RNA polymerase. Large-scale structural re-organizations of sigma54 require contact of the bEBP with an amino-terminal glutamine and leucine-rich sequence of sigma54, and lead to domain movements within the core RNA polymerase necessary for making open promoter complexes and synthesizing the nascent RNA transcript.

Mechanism of duplex DNA destabilization by RNA-guided Cas9 nuclease during target interrogation

The prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)-associated 9 (Cas9) endonuclease cleaves double-stranded DNA sequences specified by guide RNA molecules and flanked by a protospacer adjacent motif (PAM) and is widely used for genome editing in various organisms. The RNA-programmed Cas9 locates the target site by scanning genomic DNA. We sought to elucidate the mechanism of initial DNA interrogation steps that precede the pairing of target DNA with guide RNA. Using fluorometric and biochemical assays, we studied Cas9/guide RNA complexes with model DNA substrates that mimicked early intermediates on the pathway to the final Cas9/guide RNA-DNA complex. The results show that Cas9/guide RNA binding to PAM favors separation of a few PAM-proximal protospacer base pairs allowing initial target interrogation by guide RNA. The duplex destabilization is mediated, in part, by Cas9/guide RNA affinity for unpaired segments of nontarget strand DNA close to PAM. Furthermore, our data indicate that the entry of double-stranded DNA beyond a short threshold distance from PAM into the Cas9/single-guide RNA (sgRNA) interior is hindered. We suggest that the interactions unfavorable for duplex DNA binding promote DNA bending in the PAM-proximal region during early steps of Cas9/guide RNA-DNA complex formation, thus additionally destabilizing the protospacer duplex. The mechanism that emerges from our analysis explains how the Cas9/sgRNA complex is able to locate the correct target sequence efficiently while interrogating numerous nontarget sequences associated with correct PAMs.

The Xp10 Bacteriophage Protein P7 Inhibits Transcription by the Major and Major Variant Forms of the Host RNA Polymerase via a Common Mechanism

The σ factor is a functionally obligatory subunit of the bacterial transcription machinery, the RNA polymerase. Bacteriophage-encoded small proteins that either modulate or inhibit the bacterial RNAP to allow the temporal regulation of bacteriophage gene expression often target the activity of the major bacterial σ factor, σ⁷⁰. Previously, we showed that during Xanthomonas oryzae phage Xp10 infection, the phage protein P7 inhibits the host RNAP by preventing the productive engagement with the promoter and simultaneously displaces the σ⁷⁰ factor from the RNAP. In this study, we demonstrate that P7 also inhibits the productive engagement of the bacterial RNAP containing the major variant bacterial σ factor, σ⁵⁴, with its cognate promoter. The results suggest for the first time that the major variant form of the host RNAP can also be targeted by bacteriophage-encoded transcription regulatory proteins. Since the major and major variant σ factor interacting surfaces in the RNAP substantially overlap, but different regions of σ⁷⁰ and σ⁵⁴ are used for binding to the RNAP, our results further underscore the importance of the σ–RNAP interface in bacterial RNAP function and regulation and potentially for intervention by antibacterials.

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Xp10 phage transcription regulator P7 inhibits transcription by RNAP containing σ⁵⁴.

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P7 prevents the productive engagement of the σ⁵⁴–RNAP with the promoter DNA.

• P7 disrupts preformed σ⁵⁴–RNAP-promoter complexes.

A 45-Amino-Acid Scaffold Mined from the PDB for High-Affinity Ligand Engineering

Small protein ligands can provide superior physiological distribution compared with antibodies, and improved stability, production, and specific conjugation. Systematic evaluation of the PDB identified a scaffold to push the limits of small size and robust evolution of stable, high-affinity ligands: 45-residue T7 phage gene 2 protein (Gp2) contains an α helix opposite a β sheet with two adjacent loops amenable to mutation. De novo ligand discovery from 10(8) mutants and directed evolution toward four targets yielded target-specific binders with affinities as strong as 200 ± 100 pM, Tms from 65 °C ± 3 °C to 80°C ± 1 °C, and retained activity after thermal denaturation. For cancer targeting, a Gp2 domain for epidermal growth factor receptor was evolved with 18 ± 8 nM affinity, receptor-specific binding, and high thermal stability with refolding. The efficiency of evolving new binding function and the size, affinity, specificity, and stability of evolved domains render Gp2 a uniquely effective ligand scaffold.

E. coli RNA Polymerase Determinants of Open Complex Lifetime and Structure

In transcription initiation by Escherichia coli RNA polymerase (RNAP), initial binding to promoter DNA triggers large conformational changes, bending downstream duplex DNA into the RNAP cleft and opening 13bp to form a short-lived open intermediate (I2). Subsequent conformational changes increase lifetimes of λPR and T7A1 open complexes (OCs) by >10(5)-fold and >10(2)-fold, respectively. OC lifetime is a target for regulation. To characterize late conformational changes, we determine effects on OC dissociation kinetics of deletions in RNAP mobile elements σ(70) region 1.1 (σ1.1), β' jaw and β' sequence insertion 3 (SI3). In very stable OC formed by the wild type WT RNAP with λPR (RPO) and by Δσ1.1 RNAP with λPR or T7A1, we conclude that downstream duplex DNA is bound to the jaw in an assembly with SI3, and bases -4 to +2 of the nontemplate strand discriminator region are stably bound in a positively charged track in the cleft. We deduce that polyanionic σ1.1 destabilizes OC by competing for binding sites in the cleft and on the jaw with the polyanionic discriminator strand and downstream duplex, respectively. Examples of σ1.1-destabilized OC are the final T7A1 OC and the λPR I3 intermediate OC. Deleting σ1.1 and either β' jaw or SI3 equalizes OC lifetimes for λPR and T7A1. DNA closing rates are similar for both promoters and all RNAP variants. We conclude that late conformational changes that stabilize OC, like early ones that bend the duplex into the cleft, are primary targets of regulation, while the intrinsic DNA opening/closing step is not.

Small things considered: the small accessory subunits of RNA polymerase in Gram-positive bacteria

The DNA-dependent RNA polymerase core enzyme in Gram-positive bacteria consists of seven subunits. Whilst four of them (α2ββ(')) are essential, three smaller subunits, δ, ε and ω (∼9-21.5 kDa), are considered accessory. Both δ and ω have been viewed as integral components of RNAP for several decades; however, ε has only recently been described. Functionally these three small subunits carry out a variety of tasks, imparting important, supportive effects on the transcriptional process of Gram-positive bacteria. While ω is thought to have a wide range of roles, reaching from maintaining structural integrity of RNAP to σ factor recruitment, the only suggested function for ε thus far is in protecting cells from phage infection. The third subunit, δ, has been shown to have distinct influences in maintaining transcriptional specificity, and thus has a key role in cellular fitness. Collectively, all three accessory subunits, although dispensable under laboratory conditions, are often thought to be crucial for proper RNAP function. Herein we provide an overview of the available literature on each subunit, summarizing landmark findings that have deepened our understanding of these proteins and their function, and outline future challenges in understanding the role of these small subunits in the transcriptional process.

Robotic high-throughput purification of affinity-tagged recombinant proteins

Affinity purification of recombinant proteins has become the method of choice to obtain good quantities and qualities of proteins for a variety of downstream biochemical applications. While manual or FPLC-assisted purification techniques are generally time-consuming and labor-intensive, the advent of high-throughput technologies and liquid handling robotics has simplified and accelerated this process significantly. Additionally, without the human factor as a potential source of error, automated purification protocols allow for the generation of large numbers of proteins simultaneously and under directly comparable conditions. The delivered material is ideal for activity comparisons of different variants of the same protein. Here, we present our strategy for the simultaneous purification of up to 24 affinity-tagged proteins for activity measurements in biochemical assays. The protocol described is suitable for the scale typically required in individual research laboratories.

Coupling of downstream RNA polymerase-promoter interactions with formation of catalytically competent transcription initiation complex

Bacterial RNA polymerase (RNAP) makes extensive contacts with duplex DNA downstream of the transcription bubble in initiation and elongation complexes. We investigated the role of downstream interactions in formation of catalytically competent transcription initiation complex by measuring initiation activity of stable RNAP complexes with model promoter DNA fragments whose downstream ends extend from +3 to +21 relative to the transcription start site at +1. We found that DNA downstream of position +6 does not play a significant role in transcription initiation when RNAP-promoter interactions upstream of the transcription start site are strong and promoter melting region is AT rich. Further shortening of downstream DNA dramatically reduces efficiency of transcription initiation. The boundary of minimal downstream DNA duplex needed for efficient transcription initiation shifted further away from the catalytic center upon increasing the GC content of promoter melting region or in the presence of bacterial stringent response regulators DksA and ppGpp. These results indicate that the strength of RNAP-downstream DNA interactions has to reach a certain threshold to retain the catalytically competent conformation of the initiation complex and that establishment of contacts between RNAP and downstream DNA can be coupled with promoter melting. The data further suggest that RNAP interactions with DNA immediately downstream of the transcription bubble are particularly important for initiation of transcription. We hypothesize that these active center-proximal contacts stabilize the DNA template strand in the active center cleft and/or position the RNAP clamp domain to allow RNA synthesis.

ε, a new subunit of RNA polymerase found in gram-positive bacteria

RNA polymerase in bacteria is a multisubunit protein complex that is essential for gene expression. We have identified a new subunit of RNA polymerase present in the high-A+T Firmicutes phylum of Gram-positive bacteria and have named it ε. Previously ε had been identified as a small protein (ω1) that copurified with RNA polymerase. We have solved the structure of ε by X-ray crystallography and show that it is not an ω subunit. Rather, ε bears remarkable similarity to the Gp2 family of phage proteins involved in the inhibition of host cell transcription following infection. Deletion of ε shows no phenotype and has no effect on the transcriptional profile of the cell. Determination of the location of ε within the assembly of RNA polymerase core by single-particle analysis suggests that it binds toward the downstream side of the DNA binding cleft. Due to the structural similarity of ε with Gp2 and the fact they bind similar regions of RNA polymerase, we hypothesize that ε may serve a role in protection from phage infection.

Molecular basis of RNA polymerase promoter specificity switch revealed through studies of Thermus bacteriophage transcription regulator

Transcription initiation is the central point of gene expression regulation. Understanding of molecular mechanism of transcription regulation requires, ultimately, the structural understanding of consequences of transcription factors binding to DNA-dependent RNA polymerase (RNAP), the enzyme of transcription. We recently determined a structure of a complex between transcription factor gp39 encoded by a Thermus bacteriophage and Thermus RNAP holoenzyme. In this addendum to the original publication, we highlight structural insights that explain the ability of gp39 to act as an RNAP specificity switch which inhibits transcription initiation from a major class of bacterial promoters, while allowing transcription from a minor promoter class to continue.

Structural basis for promoter specificity switching of RNA polymerase by a phage factor

Transcription of DNA to RNA by DNA-dependent RNA polymerase (RNAP) is the first step of gene expression and a major regulation point. Bacteriophages hijack their host's transcription machinery and direct it to serve their needs. The gp39 protein encoded by Thermus thermophilus phage P23-45 binds the host's RNAP and inhibits transcription initiation from its major "-10/-35" class promoters. Phage promoters belonging to the minor "extended -10" class are minimally inhibited. We report the crystal structure of the T. thermophilus RNAP holoenzyme complexed with gp39, which explains the mechanism for RNAP promoter specificity switching. gp39 simultaneously binds to the RNAP β-flap domain and the C-terminal domain of the σ subunit (region 4 of the σ subunit [σ4]), thus relocating the β-flap tip and σ4. The ~45 Å displacement of σ4 is incompatible with its binding to the -35 promoter consensus element, thus accounting for the inhibition of transcription from -10/-35 class promoters. In contrast, this conformational change is compatible with the recognition of extended -10 class promoters. These results provide the structural bases for the conformational modulation of the host's RNAP promoter specificity to switch gene expression toward supporting phage development for gp39 and, potentially, other phage proteins, such as T4 AsiA.

The sabotage of the bacterial transcription machinery by a small bacteriophage protein

Many bacteriophages produce small proteins that specifically interfere with the bacterial host transcription machinery and thus contribute to the acquisition of the bacterial cell by the bacteriophage. We recently described how a small protein, called P7, produced by the Xp10 bacteriophage inhibits bacterial transcription initiation by causing the dissociation of the promoter specificity sigma factor subunit from the host RNA polymerase holoenzyme. In this addendum to the original publication, we present the highlights of that research.

A bacteriophage transcription regulator inhibits bacterial transcription initiation by σ-factor displacement

Bacteriophages (phages) appropriate essential processes of bacterial hosts to benefit their own development. The multisubunit bacterial RNA polymerase (RNAp) enzyme, which catalyses DNA transcription, is targeted by phage-encoded transcription regulators that selectively modulate its activity. Here, we describe the structural and mechanistic basis for the inhibition of bacterial RNAp by the transcription regulator P7 encoded by Xanthomonas oryzae phage Xp10. We reveal that P7 uses a two-step mechanism to simultaneously interact with the catalytic β and β' subunits of the bacterial RNAp and inhibits transcription initiation by inducing the displacement of the σ(70)-factor on initial engagement of RNAp with promoter DNA. The new mode of interaction with and inhibition mechanism of bacterial RNAp by P7 underscore the remarkable variety of mechanisms evolved by phages to interfere with host transcription.

Phage T7 Gp2 inhibition of Escherichia coli RNA polymerase involves misappropriation of σ70 domain 1.1

Bacteriophage T7 encodes an essential inhibitor of the Escherichia coli host RNA polymerase (RNAP), the product of gene 2 (Gp2). We determined a series of X-ray crystal structures of E. coli RNAP holoenzyme with or without Gp2. The results define the structure and location of the RNAP σ(70) subunit domain 1.1(σ(1.1)(70)) inside the RNAP active site channel, where it must be displaced by the DNA upon formation of the open promoter complex. The structures and associated data, combined with previous results, allow for a complete delineation of the mechanism for Gp2 inhibition of E. coli RNAP. In the primary inhibition mechanism, Gp2 forms a protein-protein interaction with σ(1.1)(70), preventing the normal egress of σ(1.1)(70) from the RNAP active site channel. Gp2 thus misappropriates a domain of the RNAP holoenzyme, σ(1.1)(70), to inhibit the function of the enzyme.

A non-bacterial transcription factor inhibits bacterial transcription by a multipronged mechanism

The process of transcription initiation is the major target for regulation of gene expression in bacteria and is performed by a multi-subunit RNA polymerase enzyme (RNAp). A complex network of regulatory elements controls the activity of the RNAp to fine-tune transcriptional output. Thus, RNAp is a nexus for controlling bacterial gene expression at the transcription level. Many bacteriophages, viruses that infect bacteria, encode transcription factors that specifically target and modulate the activity of the host RNAp and, thereby, facilitate the acquisition of the host bacteria by the phage. Here, we describe the modus operandi of a T7 bacteriophage-encoded small protein called Gp2 and define Gp2 as a non-bacterial regulator of bacterial transcription.

Bacteriophage SPP1 DNA replication strategies promote viral and disable host replication in vitro

Complex viruses that encode their own initiation proteins and subvert the host’s elongation apparatus have provided valuable insights into DNA replication. Using purified bacteriophage SPP1 and Bacillus subtilis proteins, we have reconstituted a rolling circle replication system that recapitulates genetically defined protein requirements. Eleven proteins are required: phage-encoded helicase (G40P), helicase loader (G39P), origin binding protein (G38P) and G36P single-stranded DNA-binding protein (SSB); and host-encoded PolC and DnaE polymerases, processivity factor (β₂), clamp loader (τ-δ-δ′) and primase (DnaG). This study revealed a new role for the SPP1 origin binding protein. In the presence of SSB, it is required for initiation on replication forks that lack origin sequences, mimicking the activity of the PriA replication restart protein in bacteria. The SPP1 replisome is supported by both host and viral SSBs, but phage SSB is unable to support B. subtilis replication, likely owing to its inability to stimulate the PolC holoenzyme in the B. subtilis context. Moreover, phage SSB inhibits host replication, defining a new mechanism by which bacterial replication could be regulated by a viral factor.

Overexpression of Escherichia coli udk mimics the absence of T7 Gp2 function and thereby abrogates successful infection by T7 phage

Successful infection of Escherichia coli by bacteriophage T7 relies upon the transcription of the T7 genome by two different RNA polymerases (RNAps). The bacterial RNAp transcribes early T7 promoters, whereas middle and late T7 genes are transcribed by the T7 RNAp. Gp2, a T7-encoded transcription factor, is a 7 kDa product of an essential middle T7 gene 2, and is a potent inhibitor of the host RNAp. The essential biological role of Gp2 is to inhibit transcription of early T7 genes that fail to terminate efficiently in order to facilitate the coordinated usage of the T7 genome by both host and phage RNAps. Overexpression of the E. coli udk gene, which encodes a uridine/cytidine kinase, interferes with T7 infection. We demonstrate that overexpression of udk antagonizes Gp2 function in E. coli in the absence of T7 infection and thus independently of T7-encoded factors. It seems that overexpression of udk reduces Gp2 stability and functionality during T7 infection, which consequently results in inadequate inhibition of host RNAp and in the accumulation of early T7 transcripts. In other words, overexpression of udk mimics the absence of Gp2 during T7 infection. Our study suggests that the transcriptional regulation of the T7 genome is surprisingly complex and might potentially be affected at many levels by phage- and host-encoded factors.

Key roles of the downstream mobile jaw of Escherichia coli RNA polymerase in transcription initiation

Differences in kinetics of transcription initiation by RNA polymerase (RNAP) at different promoters tailor the pattern of gene expression to cellular needs. After initial binding, large conformational changes occur in promoter DNA and RNAP to form initiation-capable complexes. To understand the mechanism and regulation of transcription initiation, the nature and sequence of these conformational changes must be determined. Escherichia coli RNAP uses binding free energy to unwind and separate 13 base pairs of λP(R) promoter DNA to form the unstable open intermediate I(2), which rapidly converts to much more stable open complexes (I(3), RP(o)). Conversion of I(2) to RP(o) involves folding/assembly of several mobile RNAP domains on downstream duplex DNA. Here, we investigate effects of a 42-residue deletion in the mobile β' jaw (ΔJAW) and truncation of promoter DNA beyond +12 (DT+12) on the steps of initiation. We find that in stable ΔJAW open complexes the downstream boundary of hydroxyl radical protection shortens by 5-10 base pairs, as compared to wild-type (WT) complexes. Dissociation kinetics of open complexes formed with ΔJAW RNAP and/or DT+12 DNA resemble those deduced for the structurally uncharacterized intermediate I(3). Overall rate constants (k(a)) for promoter binding and DNA opening by ΔJAW RNAP are much smaller than for WT RNAP. Values of k(a) for WT RNAP with DT+12 and full-length λP(R) are similar, though contributions of binding and isomerization steps differ. Hence, the jaw plays major roles both early and late in RP(o) formation, while downstream DNA functions primarily as the assembly platform after DNA opening.

A dual switch controls bacterial enhancer-dependent transcription

Bacterial RNA polymerases (RNAPs) are targets for antibiotics. Myxopyronin binds to the RNAP switch regions to block structural rearrangements needed for formation of open promoter complexes. Bacterial RNAPs containing the major variant σ(54) factor are activated by enhancer-binding proteins (bEBPs) and transcribe genes whose products are needed in pathogenicity and stress responses. We show that (i) enhancer-dependent RNAPs help Escherichia coli to survive in the presence of myxopyronin, (ii) enhancer-dependent RNAPs partially resist inhibition by myxopyronin and (iii) ATP hydrolysis catalysed by bEBPs is obligatory for functional interaction of the RNAP switch regions with the transcription start site. We demonstrate that enhancer-dependent promoters contain two barriers to full DNA opening, allowing tight regulation of transcription initiation. bEBPs engage in a dual switch to (i) allow propagation of nucleated DNA melting from an upstream DNA fork junction and (ii) complete the formation of the transcription bubble and downstream DNA fork junction at the RNA synthesis start site, resulting in switch region-dependent RNAP clamp closure and open promoter complex formation.

Opening and closing of the bacterial RNA polymerase clamp

Using single-molecule fluorescence resonance energy transfer, we have defined bacterial RNA polymerase (RNAP) clamp conformation at each step in transcription initiation and elongation. We find that the clamp predominantly is open in free RNAP and early intermediates in transcription initiation but closes upon formation of a catalytically competent transcription initiation complex and remains closed during initial transcription and transcription elongation. We show that four RNAP inhibitors interfere with clamp opening. We propose that clamp opening allows DNA to be loaded into and unwound in the RNAP active-center cleft, that DNA loading and unwinding trigger clamp closure, and that clamp closure accounts for the high stability of initiation complexes and the high stability and processivity of elongation complexes.

Substitutions in the Escherichia coli RNA polymerase inhibitor T7 Gp2 that allow inhibition of transcription when the primary interaction interface between Gp2 and RNA polymerase becomes compromised

The Escherichia coli-infecting bacteriophage T7 encodes a 7 kDa protein, called Gp2, which is a potent inhibitor of the host RNA polymerase (RNAp). Gp2 is essential for T7 phage development. The interaction site for Gp2 on the E. coli RNAp is the β′ jaw domain, which is part of the DNA binding channel. The binding of Gp2 to the β′ jaw antagonizes several steps associated with interactions between the RNAp and promoter DNA, leading to inhibition of transcription at the open promoter complex formation step. In the structure of the complex formed between Gp2 and a fragment of the β′ jaw, amino acid residues in the β3 strand of Gp2 contribute to the primary interaction interface with the β′ jaw. The 7009 E. coli strain is resistant to T7 because it carries a charge reversal point mutation in the β′ jaw that prevents Gp2 binding. However, a T7 phage encoding a mutant form of Gp2, called Gp2^β, which carries triple amino acid substitutions E24K, F27Y and R56C, can productively infect this strain. By studying the molecular basis of inhibition of RNAp from the 7009 strain by Gp2^β, we provide several lines of evidence that the E24K and F27Y substitutions facilitate an interaction with RNAp when the primary interaction interface with the β′ jaw is compromised. The proposed additional interaction interface between RNAp and Gp2 may contribute to the multipronged mechanism of transcription inhibition by Gp2.

Structural and mechanistic basis for the inhibition of Escherichia coli RNA polymerase by T7 Gp2

The T7 phage-encoded small protein Gp2 is a non-DNA-binding transcription factor that interacts with the jaw domain of the Escherichia coli (Ec) RNA polymerase (RNAp) β′ subunit and inhibits transcriptionally proficient promoter-complex (RPo) formation. Here, we describe the high-resolution solution structure of the Gp2-Ec β′ jaw domain complex and show that Gp2 and DNA compete for binding to the β′ jaw domain. We reveal that efficient inhibition of RPo formation by Gp2 requires the amino-terminal σ⁷⁰ domain region 1.1 (R1.1), and that Gp2 antagonizes the obligatory movement of R1.1 during RPo formation. We demonstrate that Gp2 inhibits RPo formation not just by steric occlusion of the RNAp-DNA interaction but also through long-range antagonistic effects on RNAp-promoter interactions around the RNAp active center that likely occur due to repositioning of R1.1 by Gp2. The inhibition of Ec RNAp by Gp2 thus defines a previously uncharacterized mechanism by which bacterial transcription is regulated by a viral factor.

The evolution of conflict resolution between plasmids and their bacterial hosts

It has recently been proposed that mobile elements may be a significant driver of cooperation in microorganisms. This may drive a potential conflict, where cooperative genes are transmitted independently of the rest of the genome, resulting in scenarios where horizontally spread cooperative genes are favored, whereas a chromosomal equivalent would not be. This can lead to the whole genome being exploited by surrounding noncooperative individuals. Given that there are costs associated with mobile elements themselves, infection with a plasmid carrying a cooperative trait may lead to a significant conflict within the host genome. Here, we model the mechanisms that allow the host to resolve this conflict, either by exhibiting complete resistance to the mobile element or by controlling its gene expression via a chromosomally based suppressor. We find that the gene suppression mechanism will be more stable than full resistance, implying that suppressing the expression of costly genes within a cell is preferable to preventing the acquisition of the mobile element, for the resolution of conflict within a genome.

Bacteriophage protein-protein interactions

Bacteriophages T7, λ, P22, and P2/P4 (from Escherichia coli), as well as ϕ29 (from Bacillus subtilis), are among the best-studied bacterial viruses. This chapter summarizes published protein interaction data of intraviral protein interactions, as well as known phage-host protein interactions of these phages retrieved from the literature. We also review the published results of comprehensive protein interaction analyses of Pneumococcus phages Dp-1 and Cp-1, as well as coliphages λ and T7. For example, the ≈55 proteins encoded by the T7 genome are connected by ≈43 interactions with another ≈15 between the phage and its host. The chapter compiles published interactions for the well-studied phages λ (33 intra-phage/22 phage-host), P22 (38/9), P2/P4 (14/3), and ϕ29 (20/2). We discuss whether different interaction patterns reflect different phage lifestyles or whether they may be artifacts of sampling. Phages that infect the same host can interact with different host target proteins, as exemplified by E. coli phage λ and T7. Despite decades of intensive investigation, only a fraction of these phage interactomes are known. Technical limitations and a lack of depth in many studies explain the gaps in our knowledge. Strategies to complete current interactome maps are described. Although limited space precludes detailed overviews of phage molecular biology, this compilation will allow future studies to put interaction data into the context of phage biology.

Molecular mechanism of transcription inhibition by phage T7 gp2 protein

Escherichia coli T7 bacteriophage gp2 protein is a potent inhibitor of host RNA polymerase (RNAP). gp2 inhibits formation of open promoter complex by binding to the β' jaw, an RNAP domain that interacts with downstream promoter DNA. Here, we used an engineered promoter with an optimized sequence to obtain and characterize a specific promoter complex containing RNAP and gp2. In this complex, localized melting of promoter DNA is initiated but does not propagate to include the point of the transcription start. As a result, the complex is transcriptionally inactive. Using a highly sensitive RNAP beacon assay, we performed quantitative real-time measurements of specific binding of the RNAP-gp2 complex to promoter DNA and various promoter fragments. In this way, the effect of gp2 on RNAP interaction with promoters was dissected. As expected, gp2 greatly decreased RNAP affinity to downstream promoter duplex. However, gp2 also inhibited RNAP binding to promoter fragments that lacked downstream promoter DNA that interacts with the β' jaw. The inhibition was caused by gp2-mediated decrease of the RNAP binding affinity to template and non-template strand segments of the transcription bubble downstream of the -10 promoter element. The inhibition of RNAP interactions with single-stranded segments of the transcription bubble by gp2 is a novel effect, which may occur via allosteric mechanism that is set in motion by the gp2 binding to the β' jaw.

Molecular insights into the control of transcription initiation at the Staphylococcus aureus agr operon

The accessory gene regulatory (agr) operon of the opportunistic human pathogen Staphylococcus aureus is a prime pathogenesis factor in this bacterium. The agr operon consists of two transcription units, RNAII and RNAIII, which are transcribed from divergent promoters, P2 and P3, respectively. RNAII encodes a quorum-sensing system, including AgrA, the master transcription activator of the agr operon. RNAIII is the effector RNA molecule that regulates the expression of many virulence genes. Owing to the atypical spacer lengths of P2 and P3, it is widely considered that transcription from P2 and P3 only occurs in a strictly AgrA-dependent manner. Here, using a fully native S. aureus in vitro transcription system, we provide the first molecular and mechanistic characterisation of the regulation of transcription initiation at the agr operon. Surprisingly, the results demonstrate that RNA polymerase (RNAp) can interact with P2 and P3 equally well in the absence of AgrA. However, formation of a transcription-competent open promoter complex (RPo) occurs more readily at P2 than at P3 when AgrA is absent. Reducing the atypical P3 spacer region length to the optimal length of 17 nucleotides significantly improves promoter activity by facilitating the isomerisation of the initial RNAp-P3 complexes to RPo, and the extended -10-like element of P3 is required for optimal promoter activity. AgrA increases the occupancy of both promoters by RNAp and thereby increases the amount of transcription initiated at P2 and P3. However, the AgrA-mediated effect on transcription initiation is more prominent at P3 that at P2. The effect of AgrA at P2 and P3 appears to be restricted to events leading to the formation of RPo. The relevance of AgrA-independent and AgrA-dependent transcription initiation at P2 and P3 is presented in the context of our current understanding of the role of the agr operon in the pathobiology of S. aureus.