A mutant T7 RNA polymerase as a DNA polymerase

Synthetase and Hydrolase Specificity Collectively Excludes 2'-Deoxyguanosine from Bacterial Alarmone

In response to starvation, virtually all bacteria pyrophosphorylate the 3'-hydroxy group of GTP or GDP to produce two messenger nucleotides collectively denoted as (p)ppGpp. Also known as alarmones, (p)ppGpp reprograms bacterial physiology to arrest growth and promote survival. Intriguingly, although cellular concentration of dGTP is two orders of magnitude lower than that of GTP, alarmone synthetases are highly selective against using 2'-deoxyguanosine (2dG) nucleotides as substrates. We thus hypothesize that production of 2dG alarmone, (p)pp(dG)pp, is highly deleterious, which drives a strong negative selection to exclude 2dG nucleotides from alarmone signaling. In this work, we show that the B. subtilis SasB synthetase prefers GDP over dGDP with 65,000-fold higher kcat/Km, a specificity stricter than RNA polymerase selecting against 2'-deoxynucleotides. Using comparative chemical proteomics, we found that although most known alarmone-binding proteins in Escherichia coli cannot distinguish ppGpp from pp(dG)pp, hydrolysis of pp(dG)pp by the essential hydrolase, SpoT, is 1,000-fold slower. This inability to degrade 2'-deoxy-3'-pyrophosphorylated substrate is a common feature of the alarmone hydrolase family. We further show that SpoT is a binuclear metallopyrophoshohydrolase and that hydrolysis of ppGpp and pp(dG)pp shares the same metal dependence. Our results support a model in which 2'-OH directly coordinates the Mn2+ at SpoT active center to stabilize the hydrolysis-productive conformation of ppGpp. Taken together, our study reveals a vital role of 2'-OH in alarmone degradation, provides new insight on the catalytic mechanism of alarmone hydrolases, and leads to the conclusion that 2dG nucleotides must be strictly excluded from alarmone synthesis because bacteria lack the key machinery to down-regulate such products.

RNA nanostructures for targeted drug delivery and imaging

The RNA molecule plays a pivotal role in many biological processes by relaying genetic information, regulating gene expression, and serving as molecular machines and catalyzers. This inherent versatility of RNA has fueled significant advancements in the field of RNA nanotechnology, driving the engineering of complex nanoscale architectures toward biomedical applications, including targeted drug delivery and bioimaging. RNA polymers, serving as building blocks, offer programmability and predictability of Watson-Crick base pairing, as well as non-canonical base pairing, for the construction of nanostructures with high precision and stoichiometry. Leveraging the ease of chemical modifications to protect the RNA from degradation, researchers have developed highly functional and biocompatible RNA architectures and integrated them into preclinical studies for the delivery of payloads and imaging agents. This review offers an educational introduction to the use of RNA as a biopolymer in the design of multifunctional nanostructures applied to targeted delivery in vivo, summarizing physical and biological barriers along with strategies to overcome them. Furthermore, we highlight the most recent progress in the development of both small and larger RNA nanostructures, with a particular focus on imaging reagents and targeted cancer therapeutics in pre-clinical models and provide insights into the prospects of this rapidly evolving field.

Post-transcriptional capping generates coenzyme A-linked RNA

NAD can be inserted co-transcriptionally via non-canonical initiation to form NAD-RNA. However, that mechanism is unlikely for CoA-linked RNAs due to low intracellular concentration of the required initiator nucleotide, 3'-dephospho-CoA (dpCoA). We report here that phosphopantetheine adenylyltransferase (PPAT), an enzyme of CoA biosynthetic pathway, accepts RNA transcripts as its acceptor substrate and transfers 4'-phosphopantetheine to yield CoA-RNA post-transcriptionally. Synthetic natural (RNAI) and small artificial RNAs were used to identify the features of RNA that are needed for it to serve as PPAT substrate. RNAs with 4-10 unpaired nucleotides at the 5' terminus served as PPAT substrates, but RNAs having <4 unpaired nucleotides did not undergo capping. No capping was observed when the +1A was changed to G or when 5' triphosphate was removed by RNA pyrophosphohydrolase (RppH), suggesting the enzyme recognizes pppA-RNA as an ATP analog. PPAT binding affinities were equivalent for transcripts with +1A, +1 G, or 5'OH (+1A), indicating that productive enzymatic recognition is driven more by local positioning effects than by overall binding affinity. Capping rates were independent of the number of unpaired nucleotides in the range of 4-10 nucleotides. Capping was strongly inhibited by ATP, reducing CoA-RNA production ~70% when equimolar ATP and substrate RNA were present. Dual bacterial expression of candidate RNAs with different 5' structures followed by CoA-RNA CaptureSeq revealed 12-fold enrichment of the better PPAT substrate, consistent with in vivo CoA-capping of RNA transcripts by PPAT. These results suggest post-transcriptional RNA capping as a possible mechanism for the biogenesis of CoA-RNAs in bacteria.

Differentiation SELEX approach identifies RNA aptamers with different specificities for HIV-1 capsid assembly forms

The HIV-1 capsid protein (CA) assumes distinct assembly forms during replication, each presenting unique, solvent-accessible surfaces that facilitate multifaceted functions and host factor interactions. However, contributions of individual CA assemblies remain unclear, as the evaluation of CA in cells presents several technical challenges. To address this need, we sought to identify CA assembly form-specific aptamers. Aptamer subsets with different specificities emerged from within a highly converged, pre-enriched aptamer library previously selected to bind the CA hexamer lattice. Subsets were either highly specific for CA lattice or bound both CA lattice and CA hexamer. We further evaluated four representatives to reveal aptamer structural features required for binding, highlighting interesting features and challenges in aptamer structure determination. Importantly, our aptamers bind biologically relevant forms of CA and we demonstrate aptamer-mediated affinity purification of CA from cell lysates without virus or host modification. Thus, we have identified CA assembly form-specific aptamers that represent exciting new tools for the study of CA.

Modified nucleic acid aptamers: development, characterization, and biological applications

Aptamers are single-stranded oligonucleotides that bind to their targets via specific structural interactions. To improve the properties and performance of aptamers, modified nucleotides are incorporated during or after a selection process such as systematic evolution of ligands by exponential enrichment (SELEX). We summarize the latest modified nucleotides and strategies used in modified (mod)-SELEX and post-SELEX to develop modified aptamers, highlight the methods used to characterize aptamer-target interactions, and present recent progress in modified aptamers that recognize different targets. We discuss the challenges and perspectives in further advancing the methodologies and toolsets to accelerate the discovery of modified aptamers, improve the throughput of aptamer-target characterization, and expand the functional diversity and complexity of modified aptamers.

Rapid molecular imaging of active thrombi in vivo using aptamer-antidote probes

Pathological blood clotting, or thrombosis, limits vital blood flow to organs; such deprivation can lead to catastrophic events including myocardial infarction, pulmonary embolism, and ischemic stroke. Prompt restoration of blood flow greatly improves outcomes. We explored whether aptamers could serve as molecular imaging probes to rapidly detect thrombi. An aptamer targeting thrombin, Tog25t, was found to rapidly localize to and visualize pre-existing clots in the femoral and jugular veins of mice using fluorescence imaging and, when circulating, was able to image clots as they form. Since free aptamer is quickly cleared from circulation, contrast is rapidly developed, allowing clot visualization within minutes. Moreover, administration of an antidote oligonucleotide further enhanced contrast development, causing the unbound aptamer to clear within 5min while impacting the clot-bound aptamer more slowly. These findings suggest that aptamers can serve as imaging agents for rapid detection of thrombi in acute care and perioperative settings.

N2 modified dinucleotide cap analogs as a potent tool for mRNA engineering

mRNA-based vaccines are relatively new technologies that have been in the field of interest of research centers and pharmaceutical companies in recent years. Such therapeutics are an attractive alternative for DNA-based vaccines since they provide material that can be used with no risk of genomic integration. Additionally, mRNA can be quite easily engineered to introduce modifications for different applications or to modulate its properties, for example, to increase translational efficiency or stability, which is not available for DNA vectors. Here, we describe the use of N2 modified dinucleotide cap analogs as components of mRNA transcripts. The compounds obtained showed very promising biological properties while incorporated into mRNA. The presented N2-guanine modifications within the cap structure ensure proper attachment of the dinucleotide to the transcripts in the IVT reaction, guarantees their incorporation only in the correct orientation, and enables highly efficient translation of mRNA both in the in vitro translation system and in human HEK293 cells.

Aptamer nucleotide analog drug conjugates in the targeting therapy of cancers

Aptamers are short single-strand oligonucleotides that can form secondary and tertiary structures, fitting targets with high affinity and specificity. They are so-called "chemical antibodies" and can target specific biomarkers in both diagnostic and therapeutic applications. Systematic evolution of ligands by exponential enrichment (SELEX) is usually used for the enrichment and selection of aptamers, and the targets could be metal ions, small molecules, nucleotides, proteins, cells, or even tissues or organs. Due to the high specificity and distinctive binding affinity of aptamers, aptamer-drug conjugates (ApDCs) have demonstrated their potential role in drug delivery for cancer-targeting therapies. Compared with antibodies which are produced by a cell-based bioreactor, aptamers are chemically synthesized molecules that can be easily conjugated to drugs and modified; however, the conventional ApDCs conjugate the aptamer with an active drug using a linker which may add more concerns to the stability of the ApDC, the drug-releasing efficiency, and the drug-loading capacity. The function of aptamer in conventional ApDC is just as a targeting moiety which could not fully perform the advantages of aptamers. To address these drawbacks, scientists have started using active nucleotide analogs as the cargoes of ApDCs, such as clofarabine, ara-guanosine, gemcitabine, and floxuridine, to replace all or part of the natural nucleotides in aptamer sequences. In turn, these new types of ApDCs, aptamer nucleotide analog drug conjugates, show the strength for targeting efficacy but avoid the complex drug linker designation and improve the synthetic efficiency. More importantly, these classic nucleotide analog drugs have been used for many years, and aptamer nucleotide analog drug conjugates would not increase any unknown druggability risk but improve the target tumor accumulation. In this review, we mainly summarized aptamer-conjugated nucleotide analog drugs in cancer-targeting therapies.

The Landscape of Nucleic-Acid-Based Aptamers for Treatment of Hematologic Malignancies: Challenges and Future Directions

Hematologic malignancies, including leukemia, lymphoma, myeloproliferative disorder and plasma cell neoplasia, are genetically heterogeneous and characterized by an uncontrolled proliferation of their corresponding cell lineages in the bone marrow, peripheral blood, tissues or plasma. Although there are many types of therapeutic drugs (e.g., TKIs, chemotherapy drugs) available for treatment of different malignancies, the relapse, drug resistance and severe side effects due to the lack of selectivity seriously limit their clinical application. Currently, although antibody–drug conjugates have been well established as able to target and deliver highly potent chemotherapy agents into cancer cells for the reduction of damage to healthy cells and have achieved success in leukemia treatment, they still also have shortcomings such as high cost, high immunogenicity and low stability. Aptamers are ssDNA or RNA oligonucleotides that can also precisely deliver therapeutic agents into cancer cells through specifically recognizing the membrane protein on cancer cells, which is similar to the capabilities of monoclonal antibodies. Aptamers exhibit higher binding affinity, lower immunogenicity and higher thermal stability than antibodies. Therefore, in this review we comprehensively describe recent advances in the development of aptamer–drug conjugates (ApDCs) with cytotoxic payload through chemical linkers or direct incorporation, as well as further introduce the latest promising aptamers-based therapeutic strategies such as aptamer–T cell therapy and aptamer–PROTAC, clarifying their bright application, development direction and challenges in the treatment of hematologic malignancies.

From polymerase engineering to semi-synthetic life: artificial expansion of the central dogma

Nucleic acids have been extensively modified in different moieties to expand the scope of genetic materials in the past few decades. While the development of unnatural base pairs (UBPs) has expanded the genetic information capacity of nucleic acids, the production of synthetic alternatives of DNA and RNA has increased the types of genetic information carriers and introduced novel properties and functionalities into nucleic acids. Moreover, the efforts of tailoring DNA polymerases (DNAPs) and RNA polymerases (RNAPs) to be efficient unnatural nucleic acid polymerases have enabled broad application of these unnatural nucleic acids, ranging from production of stable aptamers to evolution of novel catalysts. The introduction of unnatural nucleic acids into living organisms has also started expanding the central dogma in vivo. In this article, we first summarize the development of unnatural nucleic acids with modifications or alterations in different moieties. The strategies for engineering DNAPs and RNAPs are then extensively reviewed, followed by summarization of predominant polymerase mutants with good activities for synthesizing, reverse transcribing, or even amplifying unnatural nucleic acids. Some recent application examples of unnatural nucleic acids with their polymerases are then introduced. At the end, the approaches of introducing UBPs and synthetic genetic polymers into living organisms for the creation of semi-synthetic organisms are reviewed and discussed.

Structural Analysis of Monomeric RNA-Dependent Polymerases Revisited

In the past few years, our understanding of the RNA virosphere has changed dramatically due to the growth and spurt of metagenomics, exponentially increasing the number of RNA viral sequences, and providing a better understanding of their range of potential hosts. As of today, the only conserved protein among RNA viruses appears to be the monomeric RNA-dependent RNA polymerase. This enzyme belongs to the right-hand DNA-and RNA polymerases, which also includes reverse transcriptases and eukaryotic replicative DNA polymerases. The ubiquity of this protein in RNA viruses makes it a unique evolutionary marker and an appealing broad-spectrum antiviral target. In this work pairwise structural comparisons of viral RdRps and RTs were performed, including tertiary structures that have been obtained in the last few years. The resulting phylogenetic tree shows that the RdRps from (+)ss- and dsRNA viruses might have been recruited several times throughout the evolution of mobile genetic elements. RTs also display multiple evolutionary routes. We have identified a structural core comprising the entire palm, a large moiety of the fingers and the N-terminal helices of the thumb domain, comprising over 300 conserved residues, including two regions that we have named the “knuckles” and the “hypothenar eminence”. The conservation of an helix bundle in the region preceding the polymerase domain confirms that (−)ss and dsRNA Reoviruses’ polymerases share a recent ancestor. Finally, the inclusion of DNA polymerases into our structural analyses suggests that monomeric RNA-dependent polymerases might have diverged from B-family polymerases.

Nucleic Acids and Their Analogues for Biomedical Applications

Nucleic acids are emerging as powerful and functional biomaterials due to their molecular recognition ability, programmability, and ease of synthesis and chemical modification. Various types of nucleic acids have been used as gene regulation tools or therapeutic agents for the treatment of human diseases with genetic disorders. Nucleic acids can also be used to develop sensing platforms for detecting ions, small molecules, proteins, and cells. Their performance can be improved through integration with other organic or inorganic nanomaterials. To further enhance their biological properties, various chemically modified nucleic acid analogues can be generated by modifying their phosphodiester backbone, sugar moiety, nucleobase, or combined sites. Alternatively, using nucleic acids as building blocks for self-assembly of highly ordered nanostructures would enhance their biological stability and cellular uptake efficiency. In this review, we will focus on the development and biomedical applications of structural and functional natural nucleic acids, as well as the chemically modified nucleic acid analogues over the past ten years. The recent progress in the development of functional nanomaterials based on self-assembled DNA-based platforms for gene regulation, biosensing, drug delivery, and therapy will also be presented. We will then summarize with a discussion on the advanced development of nucleic acid research, highlight some of the challenges faced and propose suggestions for further improvement.

Optimization of RNA Aptamer SELEX Methods: Improved Aptamer Transcript 3'-End Homogeneity, PAGE Purification Yield, and Target-Bound Aptamer RNA Recovery

Since its inception in the early 1990s, SELEX remains the gold standard for discovering RNA aptamers specific for proteins and small molecules. The SELEX process has undergone countless modifications and now encompasses a breadth of innovative selection schemes to pare an aptamer library toward target-specific aptamers. Common to all these RNA aptamer SELEX processes are the steps for the preparation of DNA template and in vitro transcription of aptamer RNA. These steps have remained mostly unchanged over the past three decades and would benefit from optimization. We focused on three key areas: improving the homogeneity of in vitro transcribed aptamer RNA, increasing the efficiency of in vitro transcribed aptamer RNA purification by PAGE, and improving the quality of target-bound aptamer RNA recovered during SELEX. Together, these optimizations contribute toward a more efficient SELEX process and are applicable to both protein-based and cell-based RNA aptamer selections.

Selection and identification of an RNA aptamer that specifically binds the HIV-1 capsid lattice and inhibits viral replication

The HIV-1 capsid core participates in several replication processes. The mature capsid core is a lattice composed of capsid (CA) monomers thought to assemble first into CA dimers, then into ∼250 CA hexamers and 12 CA pentamers. CA assembly requires conformational flexibility of each unit, resulting in the presence of unique, solvent-accessible surfaces. Significant advances have improved our understanding of the roles of the capsid core in replication; however, the contributions of individual CA assembly forms remain unclear and there are limited tools available to evaluate these forms in vivo. Here, we have selected aptamers that bind CA lattice tubes. We describe aptamer CA15-2, which selectively binds CA lattice, but not CA monomer or CA hexamer, suggesting that it targets an interface present and accessible only on CA lattice. CA15-2 does not compete with PF74 for binding, indicating that it likely binds a non-overlapping site. Furthermore, CA15-2 inhibits HIV-1 replication when expressed in virus producer cells, but not target cells, suggesting that it binds a biologically-relevant site during virus production that is either not accessible during post-entry replication steps or is accessible but unaltered by aptamer binding. Importantly, CA15-2 represents the first aptamer that specifically recognizes the HIV-1 CA lattice.

Aptamers: an emerging navigation tool of therapeutic agents for targeted cancer therapy

Chemotherapeutic agents have been used for the treatment of numerous cancers, but due to poor selectivity and severe systemic side effects, their clinical application is limited. Single-stranded DNA (ssDNA) or RNA aptamers could conjugate with highly toxic chemotherapy drugs, toxins, therapeutic RNAs or other molecules as novel aptamer-drug conjugates (ApDCs), which are capable of significantly improving the therapeutic efficacy and reducing the systemic toxicity of drugs and have great potential in clinics for targeted cancer therapy. In this review, we have comprehensively discussed and summarized the current advances in the screening approaches of aptamers for specific cancer biomarker targeting and development of the aptamer-drug conjugate strategy for targeted drug delivery. Moreover, considering the huge progress in artificial intelligence (AI) for protein and RNA structure predictions, automatic design of aptamers using deep/machine learning techniques could be a powerful approach for rapid and precise construction of biopharmaceutics (i.e., ApDCs) for application in cancer targeted therapy.

Comparative study on tertiary contacts and folding of RNase P RNAs from a psychrophilic, a mesophilic/radiation-resistant, and a thermophilic bacterium

In most bacterial type A RNase P RNAs (P RNAs), two major loop-helix tertiary contacts (L8-P4 and L18-P8) help to orient the two independently folding S- and C-domains for concerted recognition of precursor tRNA substrates. Here, we analyze the effects of mutations in these tertiary contacts in P RNAs from three different species: (i) the psychrophilic bacterium Pseudoalteromonas translucida (Ptr), (ii) the mesophilic radiation-resistant bacterium Deinococcus radiodurans (Dra), and (iii) the thermophilic bacterium Thermus thermophilus (Tth). We show by UV melting experiments that simultaneous disruption of these two interdomain contacts has a stabilizing effect on all three P RNAs. This can be inferred from reduced RNA unfolding at lower temperatures and a more concerted unfolding at higher temperatures. Thus, when the two domains tightly interact via the tertiary contacts, one domain facilitates structural transitions in the other. P RNA mutants with disrupted interdomain contacts showed severe kinetic defects that were most pronounced upon simultaneous disruption of the L8-P4 and L18-P8 contacts. At 37°C, the mildest effects were observed for the thermostable Tth RNA. A third interdomain contact, L9-P1, makes only a minor contribution to P RNA tertiary folding. Furthermore, D. radiodurans RNase P RNA forms an additional pseudoknot structure between the P9 and P12 of its S-domain. This interaction was found to be particularly crucial for RNase P holoenzyme activity at near-physiological Mg²⁺ concentrations (2 mM). We further analyzed an exceptionally stable folding trap of the G,C-rich Tth P RNA.

Virus detection via programmable Type III-A CRISPR-Cas systems

Among the currently available virus detection assays, those based on the programmable CRISPR-Cas enzymes have the advantage of rapid reporting and high sensitivity without the requirement of thermocyclers. Type III-A CRISPR-Cas system is a multi-component and multipronged immune effector, activated by viral RNA that previously has not been repurposed for disease detection owing in part to the complex enzyme reconstitution process and functionality. Here, we describe the construction and application of a virus detection method, based on an in vivo-reconstituted Type III-A CRISPR-Cas system. This system harnesses both RNA- and transcription-activated dual nucleic acid cleavage activities as well as internal signal amplification that allow virus detection with high sensitivity and at multiple settings. We demonstrate the use of the Type III-A system-based method in detection of SARS-CoV-2 that reached 2000 copies/μl sensitivity in amplification-free and 60 copies/μl sensitivity via isothermal amplification within 30 min and diagnosed SARS-CoV-2-infected patients in both settings. The high sensitivity, flexible reaction conditions, and the small molecular-driven amplification make the Type III-A system a potentially unique nucleic acid detection method with broad applications.

Isotope-Labeled RNA Building Blocks for NMR Structure and Dynamics Studies

RNA structural research lags behind that of proteins, preventing a robust understanding of RNA functions. NMR spectroscopy is an apt technique for probing the structures and dynamics of RNA molecules in solution at atomic resolution. Still, RNA analysis by NMR suffers from spectral overlap and line broadening, both of which worsen for larger RNAs. Incorporation of stable isotope labels into RNA has provided several solutions to these challenges. In this review, we summarize the benefits and limitations of various methods used to obtain isotope-labeled RNA building blocks and how they are used to prepare isotope-labeled RNA for NMR structure and dynamics studies.

Enzymatic Synthesis of Chimeric DNA Oligonucleotides by in Vitro Transcription with dTTP, dCTP, dATP, and 2'-Fluoro Modified dGTP

Efficient ways to produce single-stranded DNA are of great interest for diverse applications in molecular biology and nanotechnology. In the present study, we selected T7 RNA polymerase mutants with reduced substrate specificity to employ an in vitro transcription reaction for the synthesis of chimeric DNA oligonucleotides, either individually or in pools. We performed in vitro evolution based on fluorescence-activated droplet sorting and identified mutations V783M, V783L, V689Q, and G555L as novel variants leading to relaxed substrate discrimination. Transcribed chimeric oligonucleotides were tested in PCR, and the quality of amplification products as well as fidelity of oligonucleotide synthesis were assessed by NGS. We concluded that enzymatically produced chimeric DNA transcripts contain significantly fewer deletions and insertions compared to chemically synthesized counterparts and can successfully serve as PCR primers, making the evolved enzymes superior for simple and cheap one-pot synthesis of multiple chimeric DNA oligonucleotides in parallel using a plethora of premixed templates.

Directed Evolution of 2'-Fluoro-Modified, RNA-Supported Carbohydrate Clusters That Bind Tightly to HIV Antibody 2G12

Carbohydrate binding proteins (CBPs) are attractive targets in medicine and biology. Multivalency, with several glycans binding to several binding pockets in the CBP, is important for high-affinity interactions. Herein, we describe a novel platform for design of multivalent carbohydrate cluster ligands by directed evolution, in which serum-stable 2'-fluoro modified RNA (F-RNA) backbones evolve to present the glycan in optimal clusters. We have validated this method by the selection of oligomannose (Man₉) glycan clusters from a sequence pool of ∼10¹³ that bind to broadly neutralizing HIV antibody 2G12 with 13 to 36 nM affinities.

Multivalent Aptamer-Functionalized Single-Strand RNA Origami as Effective, Target-Specific Anticoagulants with Corresponding Reversal Agents

Anticoagulants are commonly utilized during surgeries and to treat thrombotic diseases like stroke and deep vein thrombosis. However, conventional anticoagulants have serious side-effects, narrow therapeutic windows, and lack safe reversal agents (antidotes). Here, an alternative RNA origami displaying RNA aptamers as target-specific anticoagulant is described. Improved design and construction techniques for self-folding, single-molecule RNA origami as a platform for displaying pre-selected RNA aptamers with precise orientational and spatial control are reported. Nuclease resistance is added using 2'-fluoro-modified pyrimidines during in vitro transcription. When four aptamers are displayed on the RNA origami platform, the measured thrombin inhibition and anticoagulation activity is higher than observed for free aptamers, ssRNA-linked RNA aptamers, and RNA origami displaying fewer aptamers. Importantly, thrombin inhibition is immediately switched off by addition of specific reversal agents. Results for single-stranded DNA (ssDNA) and single-stranded peptide nucleic acid (PNA) antidotes show restoration of 63% and 95% coagulation activity, respectively. To demonstrate potential for practical, long-term storage for clinical use, RNA origami is freeze-dried, and stored at room temperature. Freshly produced and freeze-dried RNA show identical levels of activity in coagulation assays. Compared to current commercial intravenous anticoagulants, RNA origami-based molecules show promise as safer alternatives with rapid activity switching for future therapeutic applications.

The mechanism of the nucleo-sugar selection by multi-subunit RNA polymerases

RNA polymerases (RNAPs) synthesize RNA from NTPs, whereas DNA polymerases synthesize DNA from 2'dNTPs. DNA polymerases select against NTPs by using steric gates to exclude the 2'OH, but RNAPs have to employ alternative selection strategies. In single-subunit RNAPs, a conserved Tyr residue discriminates against 2'dNTPs, whereas selectivity mechanisms of multi-subunit RNAPs remain hitherto unknown. Here, we show that a conserved Arg residue uses a two-pronged strategy to select against 2'dNTPs in multi-subunit RNAPs. The conserved Arg interacts with the 2'OH group to promote NTP binding, but selectively inhibits incorporation of 2'dNTPs by interacting with their 3'OH group to favor the catalytically-inert 2'-endo conformation of the deoxyribose moiety. This deformative action is an elegant example of an active selection against a substrate that is a substructure of the correct substrate. Our findings provide important insights into the evolutionary origins of biopolymers and the design of selective inhibitors of viral RNAPs.

A Broccoli aptamer chimera yields a fluorescent K+ sensor spanning physiological concentrations

The RNA aptamer Broccoli accepts 2'fluorinated (2'F) pyrimidine nucleotide incorporation without perturbation of structure or fluorescence in the presence of potassium and DFHBI. However, the modification decreases Broccoli's apparent affinity for K⁺ >30-fold. A chimera of Broccoli RNAs with mixed chemistries displays linear fluorescent gain spanning physiological K⁺ concentrations, yielding an effective RNA-based fluorescent K⁺ sensor.

Cryo-EM Structures Reveal Transcription Initiation Steps by Yeast Mitochondrial RNA Polymerase

Mitochondrial RNA polymerase (mtRNAP) is crucial in cellular energy production, yet understanding of mitochondrial DNA transcription initiation lags that of bacterial and nuclear DNA transcription. We report structures of two transcription initiation intermediate states of yeast mtRNAP that explain promoter melting, template alignment, DNA scrunching, abortive synthesis, and transition into elongation. In the partially melted initiation complex (PmIC), transcription factor MTF1 makes base-specific interactions with flipped non-template (NT) nucleotides "AAGT" at -4 to -1 positions of the DNA promoter. In the initiation complex (IC), the template in the expanded 7-mer bubble positions the RNA and NTP analog UTPαS, while NT scrunches into an NT loop. The scrunched NT loop is stabilized by the centrally positioned MTF1 C-tail. The IC and PmIC states coexist in solution, revealing a dynamic equilibrium between two functional states. Frequent scrunching/unscruching transitions and the imminent steric clashes of the inflating NT loop and growing RNA:DNA with the C-tail explain abortive synthesis and transition into elongation.

2'-fluoro-modified pyrimidines enhance affinity of RNA oligonucleotides to HIV-1 reverse transcriptase

Nucleic acid aptamers can be chemically modified to enhance function, but modifying previously selected aptamers can have nontrivial structural and functional consequences. We present a reselection strategy to evaluate the impact of several modifications on preexisting aptamer pools. RNA aptamer libraries with affinity to HIV-1 reverse transcriptase (RT) were retranscribed with 2'-F, 2'-OMe, or 2'-NH₂ pyrimidines and subjected to three additional selection cycles. RT inhibition was observed for representative aptamers from several structural families identified by high-throughput sequencing when transcribed with their corresponding modifications. Thus, reselection identified specialized subsets of aptamers that tolerated chemical modifications from unmodified preenriched libraries. Inhibition was the strongest with the 2'-F-pyrimidine (2'-FY) RNAs, as compared to inhibition by the 2'-OMeY and 2'-NH₂Y RNAs. Unexpectedly, a diverse panel of retroviral RTs were strongly inhibited by all 2'-FY-modified transcripts, including sequences that do not inhibit those RTs as unmodified RNA. The magnitude of promiscuous RT inhibition was proportional to mole fraction 2'-FY in the transcript. RT binding affinity by 2'-FY transcripts was more sensitive to salt concentration than binding by unmodified transcripts, indicating that interaction with retroviral RTs is more ionic in character for 2'-FY RNA than for unmodified 2'-OH RNA. These surprising features of 2'-FY-modified RNA may have general implications for applied aptamer technologies.

Development of a Novel Class of Self-Assembling dsRNA Cancer Therapeutics: A Proof-of-Concept Investigation

Cancer has proven to be an extremely difficult challenge to treat. Several fundamental issues currently underlie cancer treatment, including differentiating self from nonself, functional coupling of the recognition and therapeutic components of various therapies, and the propensity of cancerous cells to develop resistance to common treatment modalities via evolutionary pressure. Given these limitations, there is an increasing need to develop an all-encompassing therapeutic that can uniquely target malignant cells, decouple recognition from treatment, and overcome evolutionarily driven cancer resistance. We describe herein a new class of programmable self-assembling double-stranded RNA (dsRNA)-based cancer therapeutics that uniquely targets aberrant genetic sequences and in a functionally decoupled manner, undergoes oncogenic RNA-activated displacement (ORAD), initiating a therapeutic cascade that induces apoptosis and immune activation. As a proof of concept, we show that RNA strands targeting the EWS/Fli1 fusion gene in Ewing sarcoma cells that are end blocked with phosphorothioate bonds and additionally sealed with a 2'-deoxyuridine (2'-U)-modified DNA protector can be used to induce specific and potent killing of cells containing the target oncogenic sequence but not wild type.

PfKsgA1 functions as a transcription initiation factor and interacts with the N-terminal region of the mitochondrial RNA polymerase of Plasmodium falciparum

The small mitochondrial genome (mtDNA) of the malaria parasite is known to transcribe its genes polycistonically, although promoter element(s) have not yet been identified. An unusually large Plasmodium falciparum candidate mitochondrial phage-like RNA polymerase (PfmtRNAP) with an extended N-terminal region is encoded by the parasite nuclear genome. Using specific antibodies against the enzyme, we established that PfmtRNAP was targeted exclusively to the mitochondrion and interacted with mtDNA. Phylogenetic analysis showed that it is part of a separate apicomplexan clade. A search for PfmtRNAP-associated transcription initiation factors using sequence homology and in silico protein-protein interaction network analysis identified PfKsgA1. PfKsgA1 is a dual cytosol- and mitochondrion-targeted protein that functions as a small subunit rRNA dimethyltransferase in ribosome biogenesis. Chromatin immunoprecipitation showed that PfKsgA1 interacts with mtDNA, and in vivo crosslinking and pull-down experiments confirmed PfmtRNAP-PfKsgA1 interaction. The ability of PfKsgA1 to serve as a transcription initiation factor was demonstrated by complementation of yeast mitochondrial transcription factor Mtf1 function in Rpo41-driven in vitro transcription. Pull-down experiments using PfKsgA1 and PfmtRNAP domains indicated that the N-terminal region of PfmtRNAP interacts primarily with the PfKsgA1 C-terminal domain with some contacts being made with the linker and N-terminal domain of PfKsgA1. In the absence of full-length recombinant PfmtRNAP, solution structures of yeast mitochondrial RNA polymerase Rpo41 complexes with Mtf1 or PfKsgA1 were determined by small-angle X-ray scattering. Protein interaction interfaces thus identified matched with those reported earlier for Rpo41-Mtf1 interaction and overlaid with the PfmtRNAP-interfacing region identified experimentally for PfKsgA1. Our results indicate that in addition to a role in mitochondrial ribosome biogenesis, PfKsgA1 has an independent function as a transcription initiation factor for PfmtRNAP.

Enzymatically synthesized 2'-fluoro-modified Dicer-substrate siRNA swarms against herpes simplex virus demonstrate enhanced antiviral efficacy and low cytotoxicity

Chemical modifications of small interfering (si)RNAs are used to enhance their stability and potency, and to reduce possible off-target effects, including immunogenicity. We have earlier introduced highly effective antiviral siRNA swarms against herpes simplex virus (HSV), targeting 653 bp of the essential UL29 viral gene. Here, we report a method for enzymatic production and antiviral use of 2′-fluoro-modified siRNA swarms. Utilizing the RNA-dependent RNA polymerase from bacteriophage phi6, we produced 2′-F-siRNA swarms containing either all or a fraction of modified adenosine, cytidine or uridine residues in the antisense strand of the UL29 target. The siRNA containing modified pyrimidines demonstrated high resistance to RNase A and the antiviral potency of all the UL29-specific 2′-F-siRNA swarms was 100-fold in comparison with the unmodified counterpart, without additional cytotoxicity. Modest stimulation of innate immunity signaling, including induced expression of both type I and type III interferons, as well as interferon-stimulated gene 54, by 2′-F-cytidine and 2′-F-uridine modified siRNA swarms occurred at early time points after transfection while the 2′-F-adenosine-containing siRNA was similar to the unmodified antiviral siRNA swarm in this respect. The antiviral efficacy of the 2′-F-siRNA swarms and the elicited cellular innate responses did not correlate suggesting that innate immunity pathways do not significantly contribute to the observed enhanced antiviral activity of the modified siRNAs. The results support further applications of enzymatically produced siRNA molecules with incorporated adenosine nucleotides, carrying fluoro-modification on ribose C2′ position, for further antiviral studies in vitro and in vivo.

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Phage phi6 polymerase can use 2′-F-dNTP substrates to produce 2′-F-modified dsRNA.

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SiRNAs containing 2′-F-modified pyrimidine nucleotides demonstrate resistance to RNase A.

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Enzymatically produced 2′-F-siRNA swarms display low cytotoxicity.

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Antiviral activity of 2′-F-siRNAs against HSV exceeds that of the unmodified siRNAs.

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Innate immunity induction by 2′-F-siRNAs is similar to that of unmodified siRNAs.

Genetic Code Expansion Facilitates Position-Selective Modification of Nucleic Acids and Proteins

Transcription and translation obey to the genetic code of four nucleobases and 21 amino acids evolved over billions of years. Both these processes have been engineered to facilitate the use of non-natural building blocks in both nucleic acids and proteins, enabling researchers with a decent toolbox for structural and functional analyses. Here, we review the most common approaches for how labeling of both nucleic acids as well as proteins in a site-selective fashion with either modifiable building blocks or spectroscopic probes can be facilitated by genetic code expansion. We emphasize methodological approaches and how these can be adapted for specific modifications, both during as well as after biomolecule synthesis. These modifications can facilitate, for example, a number of different spectroscopic analysis techniques and can under specific circumstances even be used in combination.

Molecular crowding induces primer extension by RNA polymerase through base stacking beyond Watson–Crick rules

The polymerisation of nucleic acids is essential for copying genetic information correctly to the next generations, whereas mispolymerisation could promote genetic diversity. It is possible that in the prebiotic era, polymerases might have used mispolymerisation to accelerate the diversification of genetic information. Even in the current era, polymerases of RNA viruses frequently cause mutations. In this study, primer extension under different molecular crowding conditions was measured using T7 RNA polymerase as a model for the reaction in the prebiotic world. Interestingly, molecular crowding using 20 wt% poly(ethylene glycol) 2000 preferentially promoted the primer extensions with ATP and GTP by T7 RNA polymerase, regardless of Watson–Crick base-pairing rules. This indicates that molecular crowding decreases the dielectric constants in solution, resulting in enhancement of stacking interactions between the primer and an incorporated nucleotide. These findings suggest that molecular crowding could accelerate genetic diversity in the prebiotic world and may promote transcription error of RNA viruses in the current era.

Primer extension by T7 RNA polymerase showed preference of monomer through base stacking beyond Watson–Crick rules under molecular crowding condition.

Aptamers: A Review of Their Chemical Properties and Modifications for Therapeutic Application

Aptamers are short, single-stranded oligonucleotides that bind to specific target molecules. The shape-forming feature of single-stranded oligonucleotides provides high affinity and excellent specificity toward targets. Hence, aptamers can be used as analogs of antibodies. In December 2004, the US Food and Drug Administration approved the first aptamer-based therapeutic, pegaptanib (Macugen), targeting vascular endothelial growth factor, for the treatment of age-related macular degeneration. Since then, however, no aptamer medication for public health has appeared. During these relatively silent years, many trials and improvements of aptamer therapeutics have been performed, opening multiple novel directions for the therapeutic application of aptamers. This review summarizes the basic characteristics of aptamers and the chemical modifications available for aptamer therapeutics.

Klebsiella Phage KP34 RNA Polymerase and Its Use in RNA Synthesis

We have characterized the single subunit RNA polymerase from Klebsiella phage KP34. The enzyme is unique among known bacteriophage RNA polymerases in that it recognizes two unrelated promoter sequences, which provided clues for the evolution of phage single-subunit RNA polymerases. As the first representative enzyme from the “phiKMV-like viruses” cluster, its use in run-off RNA synthesis was investigated. RNA-Seq analysis revealed that the KP34 RNA polymerase does not possess the undesired self-templated RNA terminus extension known for T7 RNA polymerase and is suitable to synthesize RNAs with structured 3′ termini such as sgRNAs. A KP34 RNA polymerase Y603F mutant is engineered to incorporate deoxy- and 2′-fluoro ribonucleotide into RNA.

Transcriptional Fidelity of Mitochondrial RNA Polymerase RpoTm from Arabidopsis thaliana

Fidelity of RNA synthesis is essential for the faithful transfer of information from DNA to RNA. A comprehensive analysis of the nucleotide selectivity by the mitochondrial RNA polymerase (RNAP) RpoTm, from Arabidopsis thaliana, has been carried out. The kinetic parameters for the incorporation of cognate, noncognate, and oxidized bases have been determined. The results establish high fidelity of mitochondrial transcription resembling those of replicative polymerases in the absence of repair. In addition, RpoTm incorporates oxidized nucleotides with similar efficiency compared with mismatches and is capable of extending the RNA beyond the insertion of the oxidized base. Furthermore, lesion bypass study on RpoTm demonstrates that the enzyme bypasses 8-oxo-guanine by insertion of adenine leading to C to A mutations in RNA. Homology modeling of RpoTm elongation complex allows delineation of the residues necessary for stabilizing the incoming NTP substrate and for posing the template nucleotide residue. Substitution of these residues leads to compromise in the activity of the enzyme corroborating their importance in RNA synthesis. Comparison of the data with T7 RNAPs indicates that low efficiency of misincorporation is a universal strategy used by single-subunit RNAPs for maintaining high fidelity in the absence of proofreading and repair activity in mitochondria.

Development of a Monomeric Inhibitory RNA Aptamer Specific for FGFR3 that Acts as an Activator When Dimerized

There have been limited options for people who suffer from fibroblast growth factor receptor (FGFR) signaling disorders. In this study, we developed RNA aptamers specific for FGFR3 as potential therapeutic agents. Using a structured aptamer library, we performed ten rounds of SELEX (systematic evolution of ligands by exponential enrichment) against mouse FGFR3c protein. Using an engineered BaF3 cell line, one aptamer clone from round 6 of the selection inhibited FGF-dependent cell growth with a concentration at which 50% of growth is observed (IC₅₀) of ∼260 nM and bound both mouse and human FGFR3 but not FGFR1 or FGFR2. This inhibitor of FGFR3 signaling (iR3), when dimerized using a template-driven approach, resulted in a functional activator of FGFR3 (aR3). We validated the activity and specificity of iR3 and aR3 on engineered BaF3 cell lines, mouse and human FGFR protein, and primary cultures of neuroepithelial precursor cells.

T7 RNA Polymerase Discriminates Correct and Incorrect Nucleoside Triphosphates by Free Energy

RNA polymerase (RNAP) is the primary machine responsible for transcription. Its ability to distinguish between correct (cognate) and incorrect (noncognate) nucleoside triphosphates (NTPs) is important for fidelity control in transcription. In this work, we investigated the substrate selection mechanism of T7 RNAP from the perspective of energetics. The dissociation free energies were determined for matched and unmatched base pairs in the preinsertion complex using the umbrella sampling method. A clear hydrogen-bond-rupture peak is observed in the potential of mean force curve for a matched base pair, whereas no such peaks are present in the position of mean force profiles for unmatched ones. The free-energy barrier could prevent correct substrates from being separated from the active site. Therefore, when NTPs diffuse into the active site, correct ones will stay for chemistry once they establish effective base pairing contacts with the template nucleotide, whereas incorrect ones will be withdrawn from the active site and rejected back to solution. This result provides an important energy evidence for the substrate selection mechanism of RNAP. Then we elucidated energetics and molecular details for correct NTP binding to the active site of the insertion complex. Our observations reveal that strong interactions act on the triphosphate of NTP to constrain its movement, whereas relatively weak interactions serve to position the base in the correct conformation. Triple interactions, hydrophobic contacts from residues M635 and Y639, base stacking from the 3' RNA terminal nucleotide, and base pairing from the template nucleotide act together to position the NTP base in a catalytically competent conformation. At last, we observed that incorrect NTPs cannot be as well-stabilized as the correct one in the active site when they are misincorporated in the insertion site. It is expected that our work can be helpful for comprehensively understanding details of this basic step in genetic transcription.

Determining selection free energetics from nucleotide pre-insertion to insertion in viral T7 RNA polymerase transcription fidelity control

An elongation cycle of a transcribing RNA polymerase (RNAP) usually consists of multiple kinetics steps, so there exist multiple kinetic checkpoints where non-cognate nucleotides can be selected against. We conducted comprehensive free energy calculations on various nucleotide insertions for viral T7 RNAP employing all-atom molecular dynamics simulations. By comparing insertion free energy profiles between the non-cognate nucleotide species (rGTP and dATP) and a cognate one (rATP), we obtained selection free energetics from the nucleotide pre-insertion to the insertion checkpoints, and further inferred the selection energetics down to the catalytic stage. We find that the insertion of base mismatch rGTP proceeds mainly through an off-path along which both pre-insertion screening and insertion inhibition play significant roles. In comparison, the selection against dATP is found to go through an off-path pre-insertion screening along with an on-path insertion inhibition. Interestingly, we notice that two magnesium ions switch roles of leave and stay during the dATP on-path insertion. Finally, we infer that substantial selection energetic is still required to catalytically inhibit the mismatched rGTP to achieve an elongation error rate ∼10-4 or lower; while no catalytic selection seems to be further needed against dATP to obtain an error rate ∼10-2.

Robotic assisted generation of 2'-deoxy-2'-fluoro-modifed RNA aptamers - High performance enabling strategies in aptamer selection

Aptamer selection is a laborious procedure, requiring expertise and significant resources. These characteristics limit the accessibility of researchers to these molecular tools. We describe a selection procedure, making use of a robotic system that allows the fully automated selection of RNA and 2'deoxy-2'-fluoro pyrimidine RNA aptamers. The platform offers a rapid access to aptamers for basic research and development, therefore opening the path to aptamer-based systemic analysis of proteomes in biological settings.

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.

A ribose modification of Spinach aptamer accelerates lead(ii) cation association in vitro

Spinach aptamer fluorescence requires formation of a tripartite complex composed of folded RNA, a GFP-like fluorophore, and selective cation coordination. 2'F pyrimidine modified Spinach has retained fluorescence, increased chemical stability, and accelerated cation association via increased G-quadruplex dynamics, thereby reducing readout time and enhancing Spinach utility for aqueous Pb²⁺ detection.

Chemical methods for the modification of RNA

RNA is often considered as being the vector for the transmission of genetic information from DNA to the protein synthesis machinery. However, besides translation RNA participates in a broad variety of fundamental biological roles such as gene expression and regulation, protein synthesis, and even catalysis of chemical reactions. This variety of function combined with intricate three-dimensional structures and the discovery of over 100 chemical modifications in natural RNAs require chemical methods for the modification of RNAs in order to investigate their mechanism, location, and exact biological roles. In addition, numerous RNA-based tools such as ribozymes, aptamers, or therapeutic oligonucleotides require the presence of additional chemical functionalities to strengthen the nucleosidic backbone against degradation or enhance the desired catalytic or binding properties. Herein, the two main methods for the chemical modification of RNA are presented: solid-phase synthesis using phosphoramidite precursors and the enzymatic polymerization of nucleoside triphosphates. The different synthetic and biochemical steps required for each method are carefully described and recent examples of practical applications based on these two methods are discussed.

2'-Fluoro-Pyrimidine-Modified RNA Aptamers Specific for Lipopolysaccharide Binding Protein (LBP)

Lipopolysaccaride binding protein (LBP), a glycosylated acute phase protein, plays an important role in the pathophysiology of sepsis. LBP binds with high affinity to the lipid part of bacterial lipopolysaccaride (LPS). Inhibition of the LPS-LBP interaction or blockage of LBP-mediated transfer of LPS monomers to CD14 may be therapeutical strategies to prevent septic shock. LBP is also of interest as a biomarker to identify septic patients at high risk for death, as LBP levels are elevated during early stages of severe sepsis. As a first step toward such potential applications, we isolated aptamers specific for murine LBP (mLBP) by in vitro selection from a library containing a 60-nucleotide randomized region. Modified RNA pools were transcribed in the presence of 2′-fluoro-modified pyrimidine nucleotides to stabilize transcripts against nuclease degradation. As verified for one aptamer experimentally, the selected aptamers adopt a “three-helix junction” architecture, presenting single-stranded 7-nt (5′-YGCTTCY) or 6-nt (5′-RTTTCY) consensus sequences in their core. The best binder (aptamer A011; K_d of 270 nM for binding to mLBP), characterized in more detail by structure probing and boundary analysis, was demonstrated to bind with high specificity to murine LBP.

A specific, promoter-independent activity of T7 RNA polymerase suggests a general model for DNA/RNA editing in single subunit RNA Polymerases

Insertional RNA editing has been observed and characterized in mitochondria of myxomycetes. The single subunit mitochondrial RNA polymerase adds nontemplated nucleotides co-transcriptionally to produce functional tRNA, rRNA and mRNAs with full genetic information. Addition of nontemplated nucleotides to the 3′ ends of RNAs have been observed in polymerases related to the mitochondrial RNA polymerase. This activity has been observed with T7 RNA polymerase (T7 RNAP), the well characterized prototype of the single subunit polymerases, as a nonspecific addition of nucleotides to the 3′ end of T7 RNAP transcripts in vitro. Here we show that this novel activity is an editing activity that can add specific ribonucleotides to 3′ ends of RNA or DNA when oligonucleotides, able to form intramolecular or intermolecular hairpin loops with recessed 3′ ends, are added to T7 RNA polymerase in the presence of at least one ribonucleotide triphosphate. Specific ribonucleotides are added to the recessed 3′ ends through Watson-Crick base pairing with the non-base paired nucleotide adjacent to the 3′ end. Optimization of this activity is obtained through alteration of the lengths of the 5′-extension, hairpin loop, and hairpin duplex. These properties define a T7 RNAP activity different from either transcriptional elongation or initiation.

Chemo-Enzymatic Synthesis of Position-Specifically Modified RNA for Biophysical Studies including Light Control and NMR Spectroscopy

The investigation of non-coding RNAs requires RNAs containing modifications at every possible position within the oligonucleotide. Here, we present the chemo-enzymatic RNA synthesis containing photoactivatable or ¹³ C,¹⁵ N-labelled nucleosides. All four ribonucleotides containing ortho-nitrophenylethyl (NPE) photocages, photoswitchable azobenzene C-nucleotides and ¹³ C,¹⁵ N-labelled nucleotides were incorporated position-specifically in high yields. We applied this approach for the synthesis of light-inducible 2'dG-sensing riboswitch variants and detected ligand-induced structural reorganization upon irradiation by NMR spectroscopy. This chemo-enzymatic method opens the possibility to incorporate a wide range of modifications at any desired position of RNAs of any lengths beyond the limits of solid-phase synthesis.

XNA Synthesis and Reverse Transcription by Engineered Thermophilic Polymerases

The B-family polymerases of hyperthermophilic archaea have proven an exceptional platform for engineering polymerases with extended substrate spectra, despite multiple mechanisms for detecting and avoiding incorporation of non-cognate substrates. These polymerases can efficiently synthesize and reverse-transcribe a number of xenonucleic acids (XNAs) that differ significantly from the canonical B-form of DNA. We present here a protocol for hexitol nucleic acid (HNA) synthesis by an engineered Thermococcus gorgonarius polymerase variant, including adaptation for large-scale synthesis and purification, and for other XNAs. We describe XNA purification and reverse transcription (with a previously reported XNA RT also based on Thermococcus gorgonarius), as well as key considerations for the characterization and optimization of XNA reactions. © 2018 by John Wiley & Sons, Inc.

Single-round isolation of diverse RNA aptamers from a random sequence pool

Aptamers are oligonucleotide ligands with specific binding affinity to target molecules. Generally, RNA aptamers are selected from an RNA pool with random sequences, using the technique termed SELEX, in which the target-binding RNA molecules are repeatedly isolated and exponentially amplified. Despite several advantages, SELEX often produces uncertain results during the iterative amplifications of the rare target-binding RNA molecules. Here, we develop a non-repeated, primer-less and target immobilization-free isolation method for generating RNA aptamers, which is robust to experimental noise. Uniquely, this method focuses on finding and removal of non-aptamer sequences from the RNA pool by RNase digestion leaving target-bound aptamer molecules, and thus is independent of aptamer types. The undigested RNA sequences remaining are so few in number that they must be mixed with a large excess of a known sequence for further manipulations and this sequence is then removed by restriction digestion followed by high-throughput sequencing analysis to identify aptamers. Using this method, we generated multiple RNA aptamers targeting α-thrombin and TGFβ1 proteins, independently. This method potentially generates thousands of sequences as aptamer candidates, which may enable us to predict a common average sequence or structural property of these aptamers that is different from input RNA.

Xenobiotic Nucleic Acid (XNA) Synthesis by Phi29 DNA Polymerase

Phi29 DNA polymerase (DNAP) is the replicative enzyme of the Bacillus subtilis bacteriophage Phi29. Its extraordinary processivity and its ability to perform isothermal amplification of DNA are central to many molecular biology applications, including high-sensitivity detection and large-scale production of DNA. We present here Phi29 DNAP as an efficient catalyst for the production of various artificial nucleic acids (XNAs) carrying backbone modifications such as 1,5-anhydrohexitol nucleic acid (HNA), 2'-deoxy-2'-fluoro-arabinonucleic acid (FANA), and 2'-fluoro-2'-deoxyribonucleic acid (2'-fluoro-DNA). A full protocol for the synthesis of HNA polymers by an exonuclease-deficient variant (D12A) of Phi29 DNAP plus a detailed guide for the design and test of novel XNA synthetase reactions performed by Phi29 DNAP are provided. © 2018 by John Wiley & Sons, Inc.