RNA Phosphorothioate Modification in Prokaryotes and Eukaryotes

RNA modifications in physiology and pathology: Progressing towards application in clinical settings

The potential to modify individual nucleotides through chemical means in order to impact the electrostatic charge, hydrophobic properties, and base pairing of RNA molecules is harnessed in the medical application of stable synthetic RNAs like mRNA vaccines and synthetic small RNA molecules. These modifications are used to either increase or decrease the production of therapeutic proteins. Additionally, naturally occurring biochemical alterations of nucleotides play a role in regulating RNA metabolism and function, thereby modulating essential cellular processes. Research elucidating the mechanisms through which RNA modifications govern fundamental cellular functions in multicellular organisms has enhanced our comprehension of how irregular RNA modification profiles can lead to human diseases. Collectively, these fundamental scientific findings have unveiled the molecular and cellular functions of RNA modifications, offering new opportunities for therapeutic intervention and paving the way for a variety of innovative clinical strategies.

Interrogating the Presence of RNA Phosphorothioate in Nature by a Highly Selective and Sensitive Fluorescence Method

In nature, DNA phosphorothioate (PT) is found in the genomic materials of some prokaryotes. In contrast, whether there is natural RNA PT is still a question under debate. A groundbreaking study reported the discovery of RNA PT in cellular RNA samples from both prokaryotes and eukaryotes at contents of >100 PT per million nucleotides (PPM-nt) according to liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) analysis. However, this finding was challenged by a later work showing that other RNA modifications, such as 2'-O-methylation, could give almost the same LC-MS/MS signal patterns as RNA PT. As the LC-MS/MS technique led to contradicting conclusions, another independent method is thus needed to interrogate the presence of RNA PT in nature. In this work, we have developed a highly selective and sensitive fluorescence method for RNA PT quantification based on a new RNA PT-specific conversion reaction. It can detect as low as 2.8 PPM-nt in RNA without interference from RNA thiobases or protein cysteines. We measured the total RNA samples from some bacteria and human cells using this method. None of these samples gave any RNA PT signal above the detection limit (2.8 PPM-nt), suggesting that the widespread presence of natural RNA PT at the 100 PPM-nt level or above is highly unlikely. Nevertheless, due to the limited number of cell species tested in this work, the possible existence of natural RNA PT cannot be excluded. The fluorescence method reported here is simple and low-cost; therefore, it should be an ideal assay for broadly screening various types of cells to search for the clue of RNA PT in nature.

Characterization of m6A Modifiers and RNA Modifications in Uterine Fibroids

Uterine leiomyoma or fibroids are prevalent noncancerous tumors of the uterine muscle layer, yet their origin and development remain poorly understood. We analyzed RNA expression profiles of 15 epigenetic mediators in uterine fibroids compared to myometrium using publicly available RNA sequencing (RNA-seq) data. To validate our findings, we performed RT-qPCR on a separate cohort of uterine fibroids targeting these modifiers confirming our RNA-seq data. We then examined protein profiles of key N6-methyladenosine (m6A) modifiers in fibroids and their matched myometrium, showing no significant differences in concordance with our RNA expression profiles. To determine RNA modification abundance, mRNA and small RNA from fibroids and matched myometrium were analyzed by ultra-high performance liquid chromatography-mass spectrometry identifying prevalent m6A and 11 other known modifiers. However, no aberrant expression in fibroids was detected. We then mined a previously published dataset and identified differential expression of m6A modifiers that were specific to fibroid genetic subtype. Our analysis also identified m6A consensus motifs on genes previously identified to be dysregulated in uterine fibroids. Overall, using state-of-the-art mass spectrometry, RNA expression, and protein profiles, we characterized and identified differentially expressed m6A modifiers in relation to driver mutations. Despite the use of several different approaches, we identified limited differential expression of RNA modifiers and associated modifications in uterine fibroids. However, considering the highly heterogenous genomic and cellular nature of fibroids, and the possible contribution of single molecule m6A modifications to fibroid pathology, there is a need for greater in-depth characterization of m6A marks and modifiers in a larger and diverse patient cohort.

Substrate Specificity of T7 RNA Polymerase toward Hypophosphoric Analogues of ATP

Modified nucleotides are commonly used in molecular biology as substrates or inhibitors for several enzymes but also as tools for the synthesis of modified DNA and RNA fragments. Introduction of modification into RNA, such as phosphorothioate (PS), has been demonstrated to provide higher stability, more effective transport, and enhanced activity of potential therapeutic molecules. Hence, in order to achieve widespread use of RNA molecules in medicine, it is crucial to continuously refine the techniques that enable the effective introduction of modifications into RNA strands. Numerous analogues of nucleotides have been tested for their substrate activity with the T7 RNA polymerase and therefore in the context of their utility for use in in vitro transcription. In the present studies, the substrate preferences of the T7 RNA polymerase toward β,γ-hypophospho-modified ATP derivatives for the synthesis of unmodified RNA and phosphorothioate RNA (PS) are presented. The performed studies revealed the stereoselectivity of this enzyme for α-thio-β,γ-hypo-ATP derivatives, similar to that for α-thio-ATP. Additionally, it is demonstrated herein that hypodiphosphoric acid may inhibit in vitro transcription catalyzed by T7 RNA polymerase.

RNA modifications in physiology and disease: towards clinical applications

The ability of chemical modifications of single nucleotides to alter the electrostatic charge, hydrophobic surface and base pairing of RNA molecules is exploited for the clinical use of stable artificial RNAs such as mRNA vaccines and synthetic small RNA molecules - to increase or decrease the expression of therapeutic proteins. Furthermore, naturally occurring biochemical modifications of nucleotides regulate RNA metabolism and function to modulate crucial cellular processes. Studies showing the mechanisms by which RNA modifications regulate basic cell functions in higher organisms have led to greater understanding of how aberrant RNA modification profiles can cause disease in humans. Together, these basic science discoveries have unravelled the molecular and cellular functions of RNA modifications, have provided new prospects for therapeutic manipulation and have led to a range of innovative clinical approaches.

Pitfalls in RNA Modification Quantification Using Nucleoside Mass Spectrometry

In recent years, there has been a high interest in researching RNA modifications, as they are involved in many cellular processes and in human diseases. A substantial set of enzymes within the cell, called RNA writers, place RNA modifications selectively and site-specifically. Another set of enzymes, called readers, recognize these modifications which guide the fate of the modified RNA. Although RNA is a transient molecule and RNA modification could be removed by RNA degradation, a subclass of enzymes, called RNA erasers, remove RNA modifications selectively and site-specifically to alter the characteristics of the RNA. The detection of RNA modifications can be done by various methods including second and next generation sequencing but also mass spectrometry. An approach capable of both qualitative and quantitative RNA modification analysis is liquid chromatography coupled to mass spectrometry of enzymatic hydrolysates of RNA into nucleosides. However, for successful detection and quantification, various factors must be considered to avoid biased identification and inaccurate quantification. In this Account, we identify three classes of errors that may distort the analysis. These classes comprise (I) errors related to chemical instabilities, (II) errors revolving around enzymatic hydrolysis to nucleosides, and (III) errors arising from issues with chromatographic separation and/or subsequent mass spectrometric analysis.A prominent example for class 1 is Dimroth rearrangement of m1A to m6A, but class 1 also comprises hydrolytic reactions and reactions with buffer components. Here, we also present the conversion of m3C to m3U under mild alkaline conditions and propose a practical solution to overcome these instabilities. Class 2 errors-such as contaminations in hydrolysis reagents or nuclease specificities-have led to erroneous discoveries of nucleosides in the past and possess the potential for misquantification of nucleosides. Impurities in the samples may also lead to class 3 errors: For instance, issues with chromatographic separation may arise from residual organic solvents, and salt adducts may hamper mass spectrometric quantification. This Account aims to highlight various errors connected to mass spectrometry analysis of nucleosides and presents solutions for how to overcome or circumnavigate those issues. Therefore, the authors anticipate that many scientists, but especially those who plan on doing nucleoside mass spectrometry, will benefit from the collection of data presented in this Account as a raised awareness, toward the variety of potential pitfalls, may further enhance the quality of data.

Characterization of m6A modifiers and RNA modifications in uterine fibroids

Uterine leiomyoma or fibroids are the most common prevalent noncancerous tumors of the uterine muscle layer. Common symptoms associated with fibroids include pelvic pain, heavy menstrual bleeding, anemia, and pelvic pressure. These tumors are a leading cause of gynecological care but lack long-term therapy as the origin and development of fibroids are not well understood. Several next-generation sequencing technologies have been performed to identify the underlying genetic and epigenetic basis of fibroids. However, there remains a systemic gap in our understanding of molecular and biological process that define uterine fibroids. Recent epitranscriptomics studies have unraveled RNA modifications that are associated with all forms of RNA and are thought to influence both normal physiological functions and the progression of diseases. We quantified RNA expression profiles by analyzing publicly available RNA-seq data for 15 known epigenetic mediators to identify their expression profile in uterine fibroids compared to myometrium. To validate our findings, we performed RT-qPCR on a separate cohort of uterine fibroids targeting these modifiers confirming our RNA-seq data. We then examined protein profiles of key m6A modifiers in fibroids and their matched myometrium. In concordance with our RNA expression profiles, no significant differences were observed in these proteins in uterine fibroids compared to myometrium. To determine abundance of RNA modifications, mRNA and small RNA from fibroids and matched myometrium were analyzed by UHPLC MS/MS. In addition to the prevalent N6-methyladenosine (m6A), we identified 11 other known modifiers but did not identify any aberrant expression in fibroids. We then mined a previously published dataset and identified differential expression of m6A modifiers that were specific to fibroid genetic sub-type. Our analysis also identified m6A consensus motifs on genes previously identified to be dysregulated in uterine fibroids. Overall, using state-of-the-art mass spectrometry, RNA expression and protein profiles, we characterized and identified differentially expressed m6A modifiers in relation to driver mutations. Despite the use of several different approaches, we identified limited differential expression of RNA modifiers and associated modifications in uterine fibroids. However, considering the highly heterogenous genomic and cellular nature of fibroids, and the possible contribution of single molecule m6A modifications to fibroid pathology, there is a need for greater in-depth characterization of m6A marks and modifiers in a larger and varied patient cohort.

Modified Nucleotides for Chemical and Enzymatic Synthesis of Therapeutic RNA

In recent years, RNA has emerged as a medium with a broad spectrum of therapeutic potential, however, for years, a group of short RNA fragments was studied and considered therapeutic molecules. In nature, RNA plays both functions, with coding and non-coding potential. For RNA, like any other therapeutic, to be used clinically, certain barriers must be crossed. Among them, there are biocompatibility, relatively low toxicity, bioavailability, increased stability, target efficiency and low off-target effects. In the case of RNA, most of these obstacles can be overcome by incorporating modified nucleotides into its structure. This may be achieved by both, in vitro and in vivo biosynthetic methods, as well as chemical synthesis. Some advantages and disadvantages of each approach are summarized here. The wide range of nucleotide analogues has been tested for their utility as monomers for RNA synthesis. Many of them have been successfully implemented, and a lot of pre-clinical and clinical studies involving modified RNA have been carried out. Some of these medications have already been introduced into clinics. After the huge success of RNA-based vaccines that were introduced into widespread use in 2020, and the introduction to the market of some RNA-based drugs, RNA therapeutics containing modified nucleotides appear to be the future of medicine.

Synthesis and hybridizing properties of P-stereodefined chimeric [PS]-{DNA:RNA} and [PS]-{DNA:(2′-OMe)-RNA} oligomers

Oxathiaphospholane derivatives of 2′-OMe-ribonucleosides and 2′-O-TBDMS-ribonucleosides (^MN-OTP and ^TN-OTP, respectively; nucleobase protected) were synthesized and separated into pure P-diastereomers. X-ray analysis showed the R_P absolute configuration of the phosphorus atom in the fast-eluting diastereomer of ^TA-OTP. The fast- and slow-eluting P-diastereomers of ^MN-OTP and ^TN-OTP were used in the solid-phase synthesis of phosphorothioate dinucleotides (^MN_PST and N_PST, respectively), which were subsequently hydrolyzed with R_P-selective phosphodiesterase svPDE and S_P-selective nuclease P1 to determine the absolute configuration of the phosphorus atoms. P-Stereodefined phosphorothioate ([PS]) 10-mer chimeric oligomers [PS]-{DNA:(2′-OMe)-RNA} and isosequential [PS]-{DNA:RNA} containing two ^MN_PS or N_PS units were synthesized. Melting experiments performed for their complexes with Watson–Crick paired DNA matrix showed that ^MN_PS or N_PS units decrease the thermal stability of the duplexes (ΔT_m = −0.5 ÷ −5.5 °C per modification) regardless of the absolute configuration of the P-atoms. When the (2′-OMe)-RNA matrix was used an increase in T_m was noted in all cases (ΔT_m = +1 ÷ +7 °C per modification). The changes in thermal stability of the duplexes formed by [PS]-chimeras with DNA and (2′-OMe)-RNA matrices do not correlate with the absolute configuration of the phosphorus atoms.

P-Diastereomers 1 were used in the stereoretentive synthesis of chimeric [PS]-{DNA:(2′-OMe)-RNA/RNA} oligomers and the stability of their duplexes did not correlate with the P-stereochemistry.

Phosphorothioate-DNA bacterial diet reduces the ROS levels in C. elegans while improving locomotion and longevity

DNA phosphorothioation (PT) is widely distributed in the human gut microbiome. In this work, PT-diet effect on nematodes was studied with PT-bioengineering bacteria. We found that the ROS level decreased by about 20–50% and the age-related lipofuscin accumulation was reduced by 15–25%. Moreover, the PT-feeding worms were more active at all life periods, and more resistant to acute stressors. Intriguingly, their lifespans were prolonged by ~21.7%. Comparative RNA-seq analysis indicated that many gene expressions were dramatically regulated by PT-diet, such as cysteine-rich protein (scl-11/12/13), sulfur-related enzyme (cpr-2), longevity gene (jnk-1) and stress response (sod-3/5, gps-5/6, gst-18/20, hsp-12.6). Both the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis suggested that neuroactivity pathways were upregulated, while phosphoryl transfer and DNA-repair pathways were down-regulated in good-appetite young worms. The findings pave the way for pro-longevity of multicellular organisms by PT-bacterial interference.

Qiang Huang et al. fed C. elegans with E. coli containing phosphorothioate (PT) DNA or a control strain and evaluated the impact on animal physiology. They observed that worms fed PT( + ) diets exhibited low reactive oxygen species, more active movement, and a longer lifespan compared to controls, suggesting that PT-DNA may have a positive effect on animal health.

Synthesis and Purification of N3 -Methylcytidine (m3 C) Modified RNA Oligonucleotides

This protocol describes a step-by-step chemical synthesis approach to prepare N³ -methylcytidine (m³ C) and its phosphoramidite. The method for synthesizing m³ C starts from commercially available cytidine, and proceeds via N³ -methylation in the presence of MeI, which generates the N³ -methylcytidine (m³ C) nucleoside, followed by the installation of several protecting groups at sites that include the 5'-hydroxyl group (4,4'-dimethoxytrityl protection), the 4-amino group (benzoyl protection), and the 2'-hydroxyl group (tert-butyldimethylsilyl, TBDMS, protection). Standard phosphoramidite chemistry is applied to prepare the final m³ C phosphoramidite for solid-phase synthesis of a series of RNA oligonucleotides. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Synthesis of N³ -methylcytidine (m³ C) and its phosphoramidite Basic Protocol 2: Automated synthesis of m³ C modified RNA oligonucleotides.

Strategies to Avoid Artifacts in Mass Spectrometry-Based Epitranscriptome Analyses

In this report, we perform structure validation of recently reported RNA phosphorothioate (PT) modifications, a new set of epitranscriptome marks found in bacteria and eukaryotes including humans. By comparing synthetic PT-containing diribonucleotides with native species in RNA hydrolysates by high-resolution mass spectrometry (MS), metabolic stable isotope labeling, and PT-specific iodine-desulfurization, we disprove the existence of PTs in RNA from E. coli, S. cerevisiae, human cell lines, and mouse brain. Furthermore, we discuss how an MS artifact led to the initial misidentification of 2'-O-methylated diribonucleotides as RNA phosphorothioates. To aid structure validation of new nucleic acid modifications, we present a detailed guideline for MS analysis of RNA hydrolysates, emphasizing how the chosen RNA hydrolysis protocol can be a decisive factor in discovering and quantifying RNA modifications in biological samples.

Sulfur modification in natural RNA and therapeutic oligonucleotides

Sulfur modifications have been discovered on both DNA and RNA. Sulfur substitution of oxygen atoms at nucleobase or backbone locations in the nucleic acid framework led to a wide variety of sulfur-modified nucleosides and nucleotides. While the discovery, regulation and functions of DNA phosphorothioate (PS) modification, where one of the non-bridging oxygen atoms is replaced by sulfur on the DNA backbone, are important topics, this review focuses on the sulfur modification in natural cellular RNAs and therapeutic nucleic acids. The sulfur modifications on RNAs exhibit diversity in terms of modification location and cellular function, but the various sulfur modifications share common biosynthetic strategies across RNA species, cell types and domains of life. The first section reviews the post-transcriptional sulfur modifications on nucleobases with an emphasis on thiouridine on tRNA and phosphorothioate modification on RNA backbones, as well as the functions of the sulfur modifications on different species of cellular RNAs. The second section reviews the biosynthesis of different types of sulfur modifications and summarizes the general strategy for the biosynthesis of sulfur-containing RNA residues. One of the main goals of investigating sulfur modifications is to aid the genomic drug development pipeline and enhance our understandings of the rapidly growing nucleic acid-based gene therapies. The last section of the review focuses on the current drug development strategies employing sulfur substitution of oxygen atoms in therapeutic RNAs.

Quantification and mapping of DNA modifications

Apart from the four canonical nucleobases, DNA molecules carry a number of natural modifications. Substantial evidence shows that DNA modifications can regulate diverse biological processes. Dynamic and reversible modifications of DNA are critical for cell differentiation and development. Dysregulation of DNA modifications is closely related to many human diseases. The research of DNA modifications is a rapidly expanding area and has been significantly stimulated by the innovations of analytical methods. With the recent advances in methods and techniques, a series of new DNA modifications have been discovered in the genomes of prokaryotes and eukaryotes. Deciphering the biological roles of DNA modifications depends on the sensitive detection, accurate quantification, and genome-wide mapping of modifications in genomic DNA. This review provides an overview of the recent advances in analytical methods and techniques for both the quantification and genome-wide mapping of natural DNA modifications. We discuss the principles, advantages, and limitations of these developed methods. It is anticipated that new methods and techniques will resolve the current challenges in this burgeoning research field and expedite the elucidation of the functions of DNA modifications.

Chemical synthesis of stimuli-responsive guide RNA for conditional control of CRISPR-Cas9 gene editing

CRISPR-Cas9 promotes changes in identity or abundance of nucleic acids in live cells and is a programmable modality of broad biotechnological and therapeutic interest. To reduce off-target effects, tools for conditional control of CRISPR-Cas9 functions are under active research, such as stimuli-responsive guide RNA (gRNA). However, the types of physiologically relevant stimuli that can trigger gRNA are largely limited due to the lack of a versatile synthetic approach in chemistry to introduce diverse labile modifications into gRNA. In this work, we developed such a general method to prepare stimuli-responsive gRNA based on site-specific derivatization of 2′-O-methylribonucleotide phosphorothioate (PS-2′-OMe). We demonstrated CRISPR-Cas9-mediated gene editing in human cells triggered by oxidative stress and visible light, respectively. Our study tackles the synthetic challenge and paves the way for chemically modified RNA to play more active roles in gene therapy.

Conditional control of CRISPR-Cas9 activity by reactive oxygen species and visible light is achieved using stimuli-responsive guide RNA synthesized by a general method based on RNA 2′-O-methylribonucleotide phosphorothioate.

Microbial Arsenal of Antiviral Defenses - Part I

Bacteriophages or phages are viruses that infect bacterial cells (for the scope of this review we will also consider viruses that infect Archaea). Constant threat of phage infection is a major force that shapes evolution of the microbial genomes. To withstand infection, bacteria had evolved numerous strategies to avoid recognition by phages or to directly interfere with phage propagation inside the cell. Classical molecular biology and genetic engineering have been deeply intertwined with the study of phages and host defenses. Nowadays, owing to the rise of phage therapy, broad application of CRISPR-Cas technologies, and development of bioinformatics approaches that facilitate discovery of new systems, phage biology experiences a revival. This review describes variety of strategies employed by microbes to counter phage infection, with a focus on novel systems discovered in recent years. First chapter covers defense associated with cell surface, role of small molecules, and innate immunity systems relying on DNA modification.

Beyond ribose and phosphate: Selected nucleic acid modifications for structure-function investigations and therapeutic applications

Over the past 25 years, the acceleration of achievements in the development of oligonucleotide-based therapeutics has resulted in numerous new drugs making it to the market for the treatment of various diseases. Oligonucleotides with alterations to their scaffold, prepared with modified nucleosides and solid-phase synthesis, have yielded molecules with interesting biophysical properties that bind to their targets and are tolerated by the cellular machinery to elicit a therapeutic outcome. Structural techniques, such as crystallography, have provided insights to rationalize numerous properties including binding affinity, nuclease stability, and trends observed in the gene silencing. In this review, we discuss the chemistry, biophysical, and structural properties of a number of chemically modified oligonucleotides that have been explored for gene silencing.

Structural Insights into the Interaction of Clinically Relevant Phosphorothioate mRNA Cap Analogs with Translation Initiation Factor 4E Reveal Stabilization via Electrostatic Thio-Effect

mRNA-based therapies and vaccines constitute a disruptive technology with the potential to revolutionize modern medicine. Chemically modified 5′ cap structures have provided access to mRNAs with superior translational properties that could benefit the currently flourishing mRNA field. Prime examples of compounds that enhance mRNA properties are antireverse cap analog diastereomers that contain an O-to-S substitution within the β-phosphate (β-S-ARCA D1 and D2), where D1 is used in clinically investigated mRNA vaccines. The compounds were previously found to have high affinity for eukaryotic translation initiation factor 4E (eIF4E) and augment translation in vitro and in vivo. However, the molecular basis for the beneficial “thio-effect” remains unclear. Here, we employed multiple biophysical techniques and captured 11 cap analog-eIF4E crystallographic structures to investigate the consequences of the β-O-to-S or -Se substitution on the interaction with eIF4E. We determined the S_P/R_P configurations of β-S-ARCA and related compounds and obtained structural insights into the binding. Unexpectedly, in both stereoisomers, the β-S/Se atom occupies the same binding cavity between Lys162 and Arg157, indicating that the key driving force for complex stabilization is the interaction of negatively charged S/Se with positively charged amino acids. This was observed for all structural variants of the cap and required significantly different conformations of the triphosphate for each diastereomer. This finding explains why both β-S-ARCA diastereomers have higher affinity for eIF4E than unmodified caps. Binding affinities determined for di-, tri-, and oligonucleotide cap analogs suggested that the “thio-effect” was preserved in longer RNAs. Our observations broaden the understanding of thiophosphate biochemistry and enable the rational design of translationally active mRNAs and eIF4E-targeting drugs.

Nucleic acid binding by SAMHD1 contributes to the antiretroviral activity and is enhanced by the GpsN modification

SAMHD1 impedes infection of myeloid cells and resting T lymphocytes by retroviruses, and the enzymatic activity of the protein-dephosphorylation of deoxynucleotide triphosphates (dNTPs)-implicates enzymatic dNTP depletion in innate antiviral immunity. Here we show that the allosteric binding sites of the enzyme are plastic and can accommodate oligonucleotides in place of the allosteric activators, GTP and dNTP. SAMHD1 displays a preference for oligonucleotides containing phosphorothioate bonds in the Rp configuration located 3' to G nucleotides (GpsN), the modification pattern that occurs in a mechanism of antiviral defense in prokaryotes. In the presence of GTP and dNTPs, binding of GpsN-containing oligonucleotides promotes formation of a distinct tetramer with mixed occupancy of the allosteric sites. Mutations that impair formation of the mixed-occupancy complex abolish the antiretroviral activity of SAMHD1, but not its ability to deplete dNTPs. The findings link nucleic acid binding to the antiretroviral activity of SAMHD1, shed light on the immunomodulatory effects of synthetic phosphorothioated oligonucleotides and raise questions about the role of nucleic acid phosphorothioation in human innate immunity.

Phosphorothioate Substitutions in RNA Structure Studied by Molecular Dynamics Simulations, QM/MM Calculations, and NMR Experiments

Phosphorothioates (PTs) are important chemical modifications of the RNA backbone where a single nonbridging oxygen of the phosphate is replaced with a sulfur atom. PT can stabilize RNAs by protecting them from hydrolysis and is commonly used as a tool to explore their function. It is, however, unclear what basic physical effects PT has on RNA stability and electronic structure. Here, we present molecular dynamics (MD) simulations, quantum mechanical (QM) calculations, and NMR spectroscopy measurements, exploring the effects of PT modifications in the structural context of the neomycin-sensing riboswitch (NSR). The NSR is the smallest biologically functional riboswitch with a well-defined structure stabilized by a U-turn motif. Three of the signature interactions of the U-turn: an H-bond, an anion-π interaction, and a potassium binding site; are formed by RNA phosphates, making the NSR an ideal model for studying how PT affects RNA structure and dynamics. By comparing with high-level QM calculations, we reveal the distinct physical properties of the individual interactions facilitated by the PT. The sulfur substitution, besides weakening the direct H-bond interaction, reduces the directionality of H-bonding while increasing its dispersion and induction components. It also reduces the induction and increases the dispersion component of the anion-π stacking. The sulfur force-field parameters commonly employed in the literature do not reflect these distinctions, leading to the unsatisfactory description of PT in simulations of the NSR. We show that it is not possible to accurately describe the PT interactions using one universal set of van der Waals sulfur parameters and provide suggestions for improving the force-field performance.

Base Pairing and Functional Insights into N3-Methylcytidine (m3C) in RNA

N³-methylcytidine (m³C) is present in both eukaryotic tRNA and mRNA and plays critical roles in many biological processes. We report the synthesis of the m³C phosphoramidite building block and its containing RNA oligonucleotides. The base-pairing stability and specificity studies show that the m³C modification significantly disrupts the stability of the Watson-Crick C:G pair. Further m³C decreases the base pairing discrimination between C:G and the other mismatched C:A, C:U, and C:C pairs. Our molecular dynamic simulation study further reveals the detailed structural insights into the m³C:G base pairing pattern in an RNA duplex. More importantly, the biochemical investigation of m³C using reverse transcription in vitro shows that N³-methylation specifies the C:A pair and induces a G to A change using HIV-1-RT, MMLV-RT, and MutiScribe-RT enzymes, all with relatively low replication fidelity. For other reverse transcriptases with higher fidelity like AMV-RT, the methylation could completely shut down DNA synthesis. Our work provides detailed insights into the thermostability of m³C in RNA and a foundation for developing new molecular tools for mapping m³C in different RNA contexts and exploring the biochemical and biomedical potentials of m³C in the design and development of RNA based therapeutics.

Incorporating a Thiophosphate Modification into a Common RNA Tetraloop Motif Causes an Unanticipated Stability Boost

GNRA (N = A, C, G, or U; R = A or G) tetraloops are common RNA secondary structural motifs and feature a phosphate stacked atop a nucleobase. The rRNA sarcin/ricin loop (SRL) is capped by GApGA, and the phosphate p stacks on G. We recently found that regiospecific incorporation of a single dithiophosphate (PS2) but not a monothiophosphate (PSO) instead of phosphate in the backbone of RNA aptamers dramatically increases the binding affinity for their targets. In the RNA:thrombin complex, the key contribution to the 1000-fold tighter binding stems from an edge-on contact between PS2 and a phenylalanine ring. Here we investigated the consequences of replacing the SRL phosphate engaged in a face-on interaction with guanine with either PS2 or PSO for stability. We found that PS2···G and Rp-PSO···G contacts stabilize modified SRLs compared to the parent loop to unexpected levels: up to 6.3 °C in melting temperature T_m and -4.7 kcal/mol in ΔΔG°. Crystal structures demonstrate that the vertical distance to guanine for the closest sulfur is just 0.05 Å longer on average compared to that of oxygen despite the larger van der Waals radius of the former (1.80 Å for S vs 1.52 Å for O). The higher stability is enthalpy-based, and the negative charge as assessed by a neutral methylphosphonate modification plays only a minor role. Quantum mechanical/molecular mechanical calculations are supportive of favorable dispersion attraction interactions by sulfur making the dominant contribution. A stacking interaction between phosphate and guanine (SRL) or uracil (U-turn) is also found in newly classified RNA tetraloop families besides GNRA.

Phosphodiester modifications in mRNA poly(A) tail prevent deadenylation without compromising protein expression

Chemical modifications enable preparation of mRNAs with augmented stability and translational activity. In this study, we explored how chemical modifications of 5',3'-phosphodiester bonds in the mRNA body and poly(A) tail influence the biological properties of eukaryotic mRNA. To obtain modified and unmodified in vitro transcribed mRNAs, we used ATP and ATP analogs modified at the α-phosphate (containing either O-to-S or O-to-BH3 substitutions) and three different RNA polymerases-SP6, T7, and poly(A) polymerase. To verify the efficiency of incorporation of ATP analogs in the presence of ATP, we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantitative assessment of modification frequency based on exhaustive degradation of the transcripts to 5'-mononucleotides. The method also estimated the average poly(A) tail lengths, thereby providing a versatile tool for establishing a structure-biological property relationship for mRNA. We found that mRNAs containing phosphorothioate groups within the poly(A) tail were substantially less susceptible to degradation by 3'-deadenylase than unmodified mRNA and were efficiently expressed in cultured cells, which makes them useful research tools and potential candidates for future development of mRNA-based therapeutics.

Detection and Quantification of RNA Phosphorothioate Modifications Using Mass Spectrometry

This article describes a protocol for detecting and quantifying RNA phosphorothioate modifications in cellular RNA samples. Starting from solid-phase synthesis of phosphorothioate RNA dinucleotides, followed by purification with reversed-phase HPLC, phosphorothioate RNA dinucleotide standards are prepared for UPLC-MS and LC-MS/MS methods. RNA samples are extracted from cells using TRIzol reagent, then digested with a nuclease mixture and analyzed by mass spectrometry. UPLC-MS is employed first to identify RNA phosphorothioate modifications. An optimized LC-MS/MS method is then employed to quantify the frequency of RNA phosphorothioate modifications in a series of model cells. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Synthesis, purification, and characterization of RNA phosphorothioate dinucleotides Basic Protocol 2: Digestion of RNA samples extracted from cells Basic Protocol 3: Detection and quantification of RNA phosphorothioate modifications by mass spectrometry.