Recent studies of T7 RNA polymerase mechanism

Transcription-Based Amplified Colorimetric Thrombin Sensor Using Non-Crosslinking Aggregation of DNA-Modified Gold Nanoparticles

Gold nanoparticles (AuNPs) have been employed as colorimetric biosensors due to the color difference between their dispersed (red) and aggregated (blue) states. Although signal amplification reactions triggered by structural changes of the ligands on AuNPs have been widely used to improve measurement sensitivity, the use of ligands is limited. In this study, we designed a AuNP-based signal-amplifying sandwich biosensor, which does not require a conformational change in the ligands. Thrombin was used as a model target, which is recognized by two different probes. In the presence of the target, an extension reaction occurs as a result of hybridization of the two probes. Then RNA synthesis is started by RNA polymerase activation due to RNA promoter duplex formation. The amplified RNA drives aggregation or dispersion of the AuNPs, and a difference of the color if the AuNP solution is observed. As this detection system does not require a conformational change in the ligand, it can be generically applied to a wide range ligands.

Thermostability, Tunability, and Tenacity of RNA as Rubbery Anionic Polymeric Materials in Nanotechnology and Nanomedicine-Specific Cancer Targeting with Undetectable Toxicity

RNA nanotechnology is the bottom-up self-assembly of nanometer-scale architectures, resembling LEGOs, composed mainly of RNA. The ideal building material should be (1) versatile and controllable in shape and stoichiometry, (2) spontaneously self-assemble, and (3) thermodynamically, chemically, and enzymatically stable with a long shelf life. RNA building blocks exhibit each of the above. RNA is a polynucleic acid, making it a polymer, and its negative-charge prevents nonspecific binding to negatively charged cell membranes. The thermostability makes it suitable for logic gates, resistive memory, sensor set-ups, and NEM devices. RNA can be designed and manipulated with a level of simplicity of DNA while displaying versatile structure and enzyme activity of proteins. RNA can fold into single-stranded loops or bulges to serve as mounting dovetails for intermolecular or domain interactions without external linking dowels. RNA nanoparticles display rubber- and amoeba-like properties and are stretchable and shrinkable through multiple repeats, leading to enhanced tumor targeting and fast renal excretion to reduce toxicities. It was predicted in 2014 that RNA would be the third milestone in pharmaceutical drug development. The recent approval of several RNA drugs and COVID-19 mRNA vaccines by FDA suggests that this milestone is being realized. Here, we review the unique properties of RNA nanotechnology, summarize its recent advancements, describe its distinct attributes inside or outside the body and discuss potential applications in nanotechnology, medicine, and material science.

Development of Host-Orthogonal Genetic Systems for Synthetic Biology

The construction of a host-orthogonal genetic system can not only minimize the impact of host-specific nuances on fine-tuning of gene expression, but also expand cellular functions such as in vivo continuous evolution of genes based on an error-prone DNA polymerase. It represents an emerging powerful approach for making biology easier to engineer. In this review, the recent advances are described on the design of genetic systems that can be stably inherited in the host cells and are responsible for important biological processes including DNA replication, RNA transcription, protein translation, and gene regulation. Their applications in synthetic biology are summarized and the future challenges and opportunities are discussed in developing such systems.

An integrative approach identifies direct targets of the late viral transcription complex and an expanded promoter recognition motif in Kaposi's sarcoma-associated herpesvirus

The structural proteins of DNA viruses are generally encoded by late genes, whose expression relies on recruitment of the host transcriptional machinery only after the onset of viral genome replication. β and γ-herpesviruses encode a unique six-member viral pre-initiation complex (vPIC) for this purpose, although how the vPIC directs specific activation of late genes remains largely unknown. The specificity underlying late transcription is particularly notable given that late gene promoters are unusually small, with a modified TATA-box being the only recognizable element. Here, we explored the basis for this specificity using an integrative approach to evaluate vPIC-dependent gene expression combined with promoter occupancy during Kaposi's sarcoma-associated herpesvirus (KSHV) infection. This approach distinguished the direct and indirect targets of the vPIC, ultimately revealing a novel promoter motif critical for KSHV vPIC binding. Additionally, we found that the KSHV vPIC component ORF24 is required for efficient viral DNA replication and identified a ORF24 binding element in the origin of replication that is necessary for late gene promoter activation. Together, these results identify an elusive element that contributes to vPIC specificity and suggest novel links between KSHV DNA replication and late transcription.

C, Da LT, Yu J. A Viral T7 RNA Polymerase Ratcheting Along DNA With Fidelity Control

RNA polymerase (RNAP) from bacteriophage T7 is a representative single-subunit viral RNAP that can transcribe with high promoter activities without assistances from transcription factors. We accordingly studied this small transcription machine computationally as a model system to understand underlying mechanisms of mechano-chemical coupling and fidelity control in the RNAP transcription elongation. Here we summarize our computational work from several recent publications to demonstrate first how T7 RNAP translocates via Brownian alike motions along DNA right after the catalytic product release. Then we show how the backward translocation motions are prevented at post-translocation upon successful nucleotide incorporation, which is also subject to stepwise nucleotide selection and acts as a pawl for "selective ratcheting". The structural dynamics and energetics features revealed from our atomistic molecular dynamics (MD) simulations and related analyses on the single-subunit T7 RNAP thus provided detailed and quantitative characterizations on the Brownian-ratchet working scenario of a prototypical transcription machine with sophisticated nucleotide selectivity for fidelity control. The presented mechanisms can be more or less general for structurally similar viral or mitochondrial RNAPs and some of DNA polymerases, or even for the RNAP engine of the more complicated transcription machinery in higher organisms.

Utilization of biodiesel by-product as substrate for high-production of β-farnesene via relatively balanced mevalonate pathway in Escherichia coli

Farnesene has been identified as suitable jet fuel substitutes and metabolic engineering for microbial production of farnesene is an alternative and attractive route. In this study, due to accumulation of toxic intermediate isopentenyl pyrophosphate (IPP), an engineered Escherichia coli strain harboring heterologous mevalonate pathway produced only 4.11mg/L β-farnesene. Through higher-level expression of isopentenyl diphosphate isomerase and farnesyl diphosphate synthase to minimize the accumulated IPP, another engineered strain with relatively balanced mevalonate pathway was constructed and had the highest production of β-farnesene to date (8.74g/L) by Escherichia coli in a lab bioreactor. Furthermore, this is the first report on utilization of biodiesel by-product (simple purification) as substrate for high-production of β-farnesene by the engineered strain optimized and β-farnesene concentration reached 2.83g/L in a lab bioreactor. Therefore, the engineered strain optimized could be used as a platform host for high-production of other terpenoids using biodiesel by-product as substrate.

Interaction Analysis of T7 RNA Polymerase with Heparin and Its Low Molecular Weight Derivatives - An In Silico Approach

The single subunit T7 RNA polymerase (T7RNAP) is a model enzyme for studying the transcription process and for various biochemical and biophysical studies. Heparin is a commonly used inhibitor against T7RNAP and other RNA polymerases. However, exact interaction between heparin and T7RNAP is still not completely understood. In this work, we analyzed the binding pattern of heparin by docking heparin and few of its low molecular weight derivatives to T7RNAP, which helps in better understanding of T7RNAP inhibition mechanism. The efficiency of the compounds was calculated by docking the selected compounds and post-docking molecular mechanics/generalized Born surface area analysis. Evaluation of the simulation trajectories and binding free energies of the complexes after simulation showed enoxaparin to be the best among low molecular weight heparins. Binding free energy analysis revealed that van der Waals interactions and polar solvation energy provided the substantial driving force for the binding process. Furthermore, per-residue free energy decomposition analysis revealed that the residues Asp 471, Asp 506, Asp 537, Tyr 571, Met 635, Asp 653, Pro 780, and Asp 812 are important for heparin interaction. Apart from these residues, most favorable contribution in all the three complexes came from Asp 506, Tyr 571, Met 635, Glu 652, and Asp 653, which can be essential for binding of heparin-like structures with T7RNAP. The results obtained from this study will be valuable for the future rational design of novel and potent inhibitors against T7RNAP and related proteins.

Fluorescent methods to study transcription initiation and transition into elongation

The DNA-dependent RNA polymerases induce specific conformational changes in the promoter DNA during transcription initiation. Fluorescence spectroscopy sensitively monitors these DNA conformational changes in real time and at equilibrium providing powerful ways to estimate interactions in transcriptional complexes and to assess how transcription is regulated by the promoter DNA sequence, transcription factors, and small ligands. Ensemble fluorescence methods described here probe the individual steps of promoter binding, bending, opening, and transition into the elongation using T7 phage and mitochondrial transcriptional systems as examples.

Noncanonical structures and their thermodynamics of DNA and RNA under molecular crowding: beyond the Watson-Crick double helix

How does molecular crowding affect the stability of nucleic acid structures inside cells? Water is the major solvent component in living cells, and the properties of water in the highly crowded media inside cells differ from that in buffered solution. As it is difficult to measure the thermodynamic behavior of nucleic acids in cells directly and quantitatively, we recently developed a cell-mimicking system using cosolutes as crowding reagents. The influences of molecular crowding on the structures and thermodynamics of various nucleic acid sequences have been reported. In this chapter, we discuss how the structures and thermodynamic properties of nucleic acids differ under various conditions such as highly crowded environments, compartment environments, and in the presence of ionic liquids, and the major determinants of the crowding effects on nucleic acids are discussed. The effects of molecular crowding on the activities of ribozymes and riboswitches on noncanonical structures of DNA- and RNA-like quadruplexes that play important roles in transcription and translation are also described.

Mitochondrial transcription: how does it end?

The structure of the mitochondrial transcription termination factor (MTERF1) provides novel insight into the mechanism of binding, recognition of the termination sequence and the conformational changes involved in mediating termination. Besides its functional implications, this structure provides a framework to understand the consequences of numerous diseases associated with mitochondrial DNA mutations.

A small post-translocation energy bias aids nucleotide selection in T7 RNA polymerase transcription

The RNA polymerase (RNAP) of bacteriophage T7 is a single subunit enzyme that can transcribe DNA to RNA in the absence of additional protein factors. In this work, we present a model of T7 RNAP translocation during elongation. Based on structural information and experimental data from single-molecule force measurements, we show that a small component of facilitated translocation or power stroke coexists with the Brownian-ratchet-driven motions, and plays a crucial role in nucleotide selection at pre-insertion. The facilitated translocation is carried out by the conserved Tyr(639) that moves its side chain into the active site, pushing aside the 3'-end of the RNA, and forming a locally stabilized post-translocation intermediate. Pre-insertion of an incoming nucleotide into this stabilized intermediate state ensures that Tyr(639) closely participates in selecting correct nucleotides. A similar translocation mechanism has been suggested for multi-subunit RNAPs involving the bridge-helix bending. Nevertheless, the bent bridge-helix sterically prohibits nucleotide binding in the post-transolocation intermediate analog; moreover, the analog is not stabilized unless an inhibitory protein factor binds to the enzyme. Using our scheme, we also compared the efficiencies of different strategies for nucleotide selection, and examined effects of facilitated translocation on forward tracking.

Hitting the brakes: termination of mitochondrial transcription

Deficiencies in mitochondrial protein production are associated with human disease and aging. Given the central role of transcription in gene expression, recent years have seen a renewed interest in understanding the molecular mechanisms controlling this process. In this review, we have focused on the mostly uncharacterized process of transcriptional termination. We review how several recent breakthroughs have provided insight into our understanding of the termination mechanism, the protein factors that mediate termination, and the functional relevance of different termination events. Furthermore, the identification of termination defects resulting from a number of mtDNA mutations has led to the suggestion that this could be a common mechanism influencing pathogenesis in a number of mitochondrial diseases, highlighting the importance of understanding the processes that regulate transcription in human mitochondria. We discuss how these recent findings set the stage for future studies on this important regulatory mechanism. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Biochemical characterization of enzyme fidelity of influenza A virus RNA polymerase complex

BACKGROUND

It is widely accepted that the highly error prone replication process of influenza A virus (IAV), together with viral genome assortment, facilitates the efficient evolutionary capacity of IAV. Therefore, it has been logically assumed that the enzyme responsible for viral RNA replication process, influenza virus type A RNA polymerase (IAV Pol), is a highly error-prone polymerase which provides the genomic mutations necessary for viral evolution and host adaptation. Importantly, however, the actual enzyme fidelity of IAV RNA polymerase has never been characterized.

PRINCIPAL FINDINGS

Here we established new biochemical assay conditions that enabled us to assess both polymerase activity with physiological NTP pools and enzyme fidelity of IAV Pol. We report that IAV Pol displays highly active RNA-dependent RNA polymerase activity at unbiased physiological NTP substrate concentrations. With this robust enzyme activity, for the first time, we were able to compare the enzyme fidelity of IAV Pol complex with that of bacterial phage T7 RNA polymerase and the reverse transcriptases (RT) of human immunodeficiency virus (HIV-1) and murine leukemia virus (MuLV), which are known to be low and high fidelity enzymes, respectively. We observed that IAV Pol displayed significantly higher fidelity than HIV-1 RT and T7 RNA polymerase and equivalent or higher fidelity than MuLV RT. In addition, the IAV Pol complex showed increased fidelity at lower temperatures. Moreover, upon replacement of Mg(++) with Mn(++), IAV Pol displayed increased polymerase activity, but with significantly reduced processivity, and misincorporation was slightly elevated in the presence of Mn(++). Finally, when the IAV nucleoprotein (NP) was included in the reactions, the IAV Pol complex exhibited enhanced polymerase activity with increased fidelity.

SIGNIFICANCE

Our study indicates that IAV Pol is a high fidelity enzyme. We envision that the high fidelity nature of IAV Pol may be important to counter-balance the multiple rounds of IAV genome amplification per infection cycle, which provides IAV Pol with ample opportunities to generate and amplify genomic founder mutations, and thus achieve optimal viral mutagenesis for its evolution.

Transchip: single-molecule detection of transcriptional elongation complexes

A new single-molecule system, Transchip, was developed for analysis of transcription products at their genomic origins. The bacteriophage T7 RNA polymerase and its promoters were used in a model system, and resultant RNAs were imaged and detected at their positions along single template DNA molecules. The Transchip system has drawn from critical aspects of Optical Mapping, a single-molecule system that enables the construction of high-resolution ordered restriction maps of whole genomes from single DNA molecules. Through statistical analysis of hundreds of single-molecule template/transcript complexes, Transchip enables analysis of the locations and strength of promoters, the direction and processivity of transcription reactions, and the termination of transcription. These novel results suggest that the new system may serve as a high-throughput platform to investigate transcriptional events on a large genome-wide scale.

Baculovirus Late Expression Factors

Autographa californica nuclear polyhedrosis virus, or AcMNPV, is the type member of the baculoviruses, a family of double-stranded DNA viruses with large circular genomes. The successive and concomitant expression of an assortment of early, late and very late genes is instrumental for successful baculovirus infection, and requires a switch from early dependence on a host cell-derived polymerase II to a novel virus-encoded RNA polymerase that is required for transcription later on in infection. A series of repetitive and highly conserved sequences known as homologous regions, or hrs, function both as origins of DNA replication as well as transcriptional enhancers of late gene expression. An array of AcMNPV genes produced early on in infection, known as late expression factors, or LEFs, are essential for both replication and late gene expression. In this review, an overview of baculovirus LEFs and their roles in viral replication and late gene expression is presented. The role of LEFs in determining baculovirus host range is described. Finally, we compare baculovirus replication and transcription machinery with other viral systems.

Effect of equatorial ligands of dirhodium(II,II) complexes on the efficiency and mechanism of transcription inhibition in vitro

The nature of the equatorial ligands spanning the dirhodium core was shown to affect the ability and mechanism of various lantern-type complexes to inhibit transcription in vitro. The inhibition of transcription by Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and [Rh(2)(mu-O(2)CCH(3))(2)(CH(3)CN)(6)](2+) appears to proceed predominantly via binding of the complexes to T7-RNA polymerase (T7-RNAP) and is dependent on the concentration of enzyme and Mg(2+) ions in solution. The concentrations of the aforementioned complexes required to inhibit 50% of the transcription, C(inh)(50), are similar to that measured for activated cisplatin, whereas a significantly higher concentration of Rh(2)(mu-HNCOCH(3))(4) is required to effect similar inhibition; the inhibition induced by Rh(2)(mu-HNCOCH(3))(4) does not involve binding to T7-RNAP. The spectral changes observed for each complex upon addition of enzyme are consistent with Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and [Rh(2)(mu-O(2)CCH(3))(2)(CH(3)CN)(6)](2+) binding to the enzyme and may involve partial displacement of the equatorial (eq) groups by the Lewis basic sites of T7-RNAP. In contrast, addition of enzyme to solutions of Rh(2)(mu-HNCOCH(3))(4) does not result in significant spectral changes, a finding consistent with lack of enzyme dependence in the transcription inhibition. These differences in reactivity and transcription inhibition mechanism among complexes with different bridging ligands are explained by variations of the Lewis acidity of the axial (ax) sites in the series of complexes Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and Rh(2)(mu-HNCOCH(3))(4). The Lewis acidity of the ax sites is expected to affect the initial interaction of the complexes with the biomolecules, followed by their rearrangement to eq positions if the bridging ligands are labile.

Qbeta replicase discriminates between legitimate and illegitimate templates by having different mechanisms of initiation

Qbeta replicase (RNA-directed RNA polymerase of bacteriophage Qbeta) exponentially amplifies certain RNAs (RQ RNAs) in vitro. Here we characterize template properties of the 5' and 3' fragments obtained by cleaving one of such RNAs at an internal site. We unexpectedly found that, besides the 3' fragment, Qbeta replicase can copy the 5' fragment and a number of its variants, although they lack the initiator region of RQ RNA. This copying can occur as a 3'-terminal elongation or through de novo initiation. In contradistinction to RQ RNA and its 3' fragment, initiation on these templates occurs without regard to the 3'-terminal or internal oligo(C) clusters, is GTP-independent, and does not result in a stable replicative complex capable of elongation in the presence of aurintricarboxylic acid. The results suggest that, although Qbeta replicase can initiate and elongate on a variety of RNAs, only some of them are recognized as legitimate templates. GTP-dependent initiation on a legitimate template drives the enzyme to a "closed" conformation that may be important for keeping the template and the complementary nascent strand unannealed, without which the exponential replication is impossible. Triggering the GTP-dependent conformational transition at the initiation step could serve as a discriminative feature of legitimate templates providing for the high template specificity of Qbeta replicase.

Attenuated Salmonella enterica serovar typhi live vector with inducible chromosomal expression of the T7 RNA polymerase and its evaluation with reporter genes

Attenuated Salmonella strains with defined gene deletions have been extensively evaluated as suitable live carriers of passenger antigens. A number of strategies for antigen delivery by these strains have been attempted, ranging from plasmid-based to chromosomal integration systems. We report here the chromosomal integration of the T7 RNA polymerase gene (T7pol) in the attenuated strain Salmonella enterica serovar Typhi (Salmonella typhi) CVD908 (aroC(-), aroD(-)). The T7pol gene was amplified by PCR from Escherichia coli BL21(DE3) and cloned in the pNir3 plasmid under the control of the anaerobically inducible nirB promoter. Then it was subcloned in a pKTN701 derivative, suicide plasmid with the R6K ori, and flanked by the aroC gene. After evaluation of its functionality in E. coli SY327, the aroC-T7pol-aroC cassette was integrated into the aroC locus of S. typhi CVD908 by homologous recombination. The resulting strain, S. typhi CVD908-T7pol, was able to transcomplement two plasmids bearing the luc or the lacZ reporter genes controlled by the T7 promoter and produce luciferase and beta-galactosidase under anaerobic culture conditions. Therefore, an inducible system for recombinant antigen production in attenuated S. typhi was achieved.

In vitro transcription of a torsionally constrained template

RNA polymerase (RNAP) and the DNA template must rotate relative to each other during transcription elongation. In the cell, however, the components of the transcription apparatus may be subject to rotary constraints. For instance, the DNA is divided into topological domains that are delineated by rotary locked boundaries. Furthermore, RNAPs may be located in factories or attached to matrix sites limiting or prohibiting rotation. Indeed, the nascent RNA alone has been implicated in rotary constraining RNAP. Here we have investigated the consequences of rotary constraints during transcription of torsionally constrained DNA by free RNAP. We asked whether or not a newly synthesized RNA chain would limit transcription elongation. For this purpose we developed a method to immobilize covalently closed circular DNA to streptavidin-coated beads via a peptide nucleic acid (PNA)-biotin conjugate in principle mimicking a SAR/MAR attachment. We used this construct as a torsionally constrained template for transcription of the beta-lactamase gene by Escherichia coli RNAP and found that RNA synthesis displays similar characteristics in terms of rate of elongation whether or not the template is torsionally constrained. We conclude that transcription of a natural bacterial gene may proceed with high efficiency despite the fact that newly synthesized RNA is entangled around the template in the narrow confines of torsionally constrained supercoiled DNA.

Substrate/inhibition studies of bacteriophage T7 RNA polymerase with the 5'-triphosphate derivative of a ring-expanded ('fat') nucleoside possessing potent antiviral and anticancer activities

As part of an effort to explore the mechanism of potent, broad spectrum antiviral and anticancer activities of a number of ring-expanded ('fat') nucleosides that we recently reported, a representative 'fat' nucleoside 4,6-diamino-8-imino-8H-1-beta-D-ribofuranosylimidazo[4,5-e][1,3]di azepine (1) was converted to its 5'-triphosphate derivative (2), and biochemically screened for possible inhibition of nucleic acid polymerase activity, employing synthetic DNA templates and the bacteriophage T7 RNA polymerase as a representative polymerase. Our results suggest that 2 is a moderate inhibitor of T7 RNA polymerase, and that the 5'-triphosphate moiety of 2 appears to be essential for inhibition as nucleoside 1 alone failed to inhibit the polymerase reaction.