The identification and characterization of a selected SAM-dependent methyltransferase ribozyme that is present in natural sequences

RNA catalysis moving towards metabolic reactions: progress with ribozyme catalyzed alkyl transfer

The RNA world hypothesis proposes that the early stages of the emergence of life on Earth comprised primitive cells in which RNA acted both to store genetic information and catalyze chemical reactions as RNA enzymes (ribozymes). Most contemporary ribozymes catalyze phosphoryl transfer reactions, but early ribozymes would have been required to catalyze a broader range of metabolic interconversions. None has been found in modern cells, yet ribozymes have been generated by in vitro evolution to catalyze several different chemical reactions, providing proof of principle of RNA-catalyzed metabolism. Recently, several different ribozymes that accelerate methyl or alkyl transfer have been isolated. As we discuss here, one of these, MTR1, uses a remarkably sophisticated catalytic mechanism involving nucleobase-mediated general acid catalysis.

Ribozyme-Catalyzed Site-Specific Labeling of RNA Using O6-alkylguanine SNAP-Tag Substrates

Site-specific modification of RNAs with functional handles enables studies of RNA structure, fate, function, and interactions. Ribozymes provide an elegant way to covalently modify RNA of interest (ROI). Here, we report that the methyltransferase ribozyme MTR1 can be employed as a versatile tool for RNA modification and labeling. Using O6-alkylguanine cofactors, designed in analogy to SNAP-tag substrates for protein labeling, MTR1 installs various bioorthogonal functional groups at N1 of a specific adenosine in the RNA target. In this application of ribozyme-catalyzed RNA labeling, MTR1 is now called SNAPR. In contrast to the self-labeling SNAP-tag, which is appended to the protein of interest, SNAPR is a truly intermolecular RNA catalyst (active in trans). SNAPR assembles with the ROI to the active ribozyme, allowing for the transfer of clickable tags, such as azide and alkyne moieties, as well as photolabile groups or cross-linkers from the guanine cofactor to the ROI. Moreover, we demonstrate a two-step approach to attach labels at N6 of the target adenosine: first, SNAPR generates N1A-modified RNA, followed by preparative Dimroth rearrangement to produce N6A-modified RNA. We demonstrate this strategy with p-azidobenzyl groups as photocrosslinker to generate covalent RNA-protein conjugates. Overall, this work expands the toolbox for site-specific RNA modification.

Site-Selective Modification and Labeling of Native RNA

Ribonucleic acid (RNA) plays a pivotal role in regulating biological processes within living systems, with modified nucleosides serving as critical modulators of various aspects of biological functions. Therefore, the development of efficient methodologies for late-stage, site-selective RNA modification is of considerable interest, as it facilitates the functional exploration of RNA chemical modifications and their implications for therapeutic applications. Precise RNA modification holds significant promise for the treatment of genetic diseases by enabling the correction of mutated nucleobases to their wild-type forms. Additionally, the site-selective incorporation of synthetic labeling groups into RNA provides invaluable tools for structural and functional studies, thereby uncovering previously hidden dimensions of RNA's role in biological systems. In this review, we provide a comprehensive overview of three principal approaches to site-selective, late-stage RNA modifications: enzyme-mediated strategies, catalytic nucleic acid-based techniques, and chemical methodologies. These approaches predominantly target the nucleobase or the 2'-hydroxyl (2'-OH) group of RNA nucleosides. We evaluate the advantages and limitations of each strategy and discuss future directions for advancing this field of research.

Structure and catalytic activity of the SAM-utilizing ribozyme SAMURI

Ribozymes that catalyze site-specific RNA modification have recently gained increasing interest for their ability to mimic methyltransferase enzymes and for their application to install molecular tags. Recently, we reported SAMURI as a site-specific alkyltransferase ribozyme using S-adenosylmethionine (SAM) or a stabilized analog to transfer a methyl or propargyl group to N3 of an adenosine. Here, we report the crystal structures of SAMURI in the postcatalytic state. The structures reveal a three-helix junction with the catalytic core folded into four stacked layers, harboring the cofactor and the modified nucleotide. Detailed structure-activity analyses explain the cofactor scope and the structural basis for site selectivity. A structural comparison of SAMURI with SAM riboswitches sheds light on how the synthetic ribozyme overcomes the strategies of natural riboswitches to avoid self-methylation. Our results suggest that SAM and its analogs may serve as substrates for various RNA-catalyzed reactions, for which the corresponding ribozymes remain to be identified.

Enzymatic Reactions of S-Adenosyl-L-Methionine: Synthesis and Applications

S-adenosyl-L-methionine (SAM, AdoMet) is a ubiquitous biomolecule present in all living organisms, playing a central role in a wide array of biochemical reactions and intracellular regulatory pathways. It is the second most common participant in enzymatic reactions in living systems, following adenosine triphosphate (ATP). This review provides a comprehensive analysis of enzymatic reactions involving SAM, whether as a product, a reactant (cosubstrate), or as a non-consumable enzyme cofactor. The discussion encompasses various methods for SAM synthesis, including biotechnological, chemical, and enzymatic approaches. Particular emphasis is placed on the biochemical reactions where SAM functions as a cosubstrate, notably in trans-alkylation reactions, where it acts as a key methyl group donor. Beyond methylation, SAM also serves as a precursor for the synthesis of other molecular building blocks, which are explored in a dedicated section. The review also addresses the role of SAM as a non-consumable cofactor in enzymatic processes, highlighting its function as a prosthetic group for certain protein enzymes and its ability to form complexes with ribozymes. In addition, bioorthogonal systems involving SAM analogues are discussed. These systems employ engineered enzyme-cofactor pairs designed to enable highly selective interactions between target SAM analogues and specific enzymes, facilitating precise reactions even in the presence of other SAM-dependent enzymes. The concluding section explores practical applications of SAM analogues, including their use as selective inhibitors in clinical medicine and as components of reporter systems.

The Role of General Acid Catalysis in the Mechanism of an Alkyl Transferase Ribozyme

MTR1 is an in vitro-selected alkyl transferase ribozyme that transfers an alkyl group from O 6-alkylguanine to N1 of the target adenine in the RNA substrate (A63). The structure of the ribozyme suggested a mechanism in which a cytosine (C10) acts as a general acid to protonate O 6-alkylguanine N1. Here, we have analyzed the role of the C10 general acid and the A63 nucleophile by atomic mutagenesis and computation. C10 was substituted by n1c and n1c, c5n variants. The n1c variant has an elevated pK a (11.4 as the free nucleotide) and leads to a 104-fold lower activity that is pH-independent. Addition of the second c5n substitution with a lower pK a restored both the rate and pH dependence of alkyl transfer. Quantum mechanical calculations indicate that protonation of O 6-alkylguanine lowers the barrier to alkyl transfer and that there is a significantly elevated barrier to proton transfer for the n1c single substitution. The calculated pK a values are in good agreement with the apparent values from measured rates. Increasing the pK a of the nucleophile by A63 n7c substitution led to a 6-fold higher rate. The increased reactivity of the nucleophile corresponds to a βnuc of ∼0.5, indicating significant C-N bond formation in the transition state. Taken together, these results are consistent with a two-step mechanism comprising protonation of the O 6-alkylguanine followed by alkyl transfer.

Cofactor binding triggers rapid conformational remodelling of the active site of a methyltransferase ribozyme

The methyltransferase ribozyme SMRZ-1 utilizes S-adenosyl-methionine (SAM) and Cu (II) ions to methylate RNA. A comparison of the SAM-bound and unbound RNA structures has shown a conformational change in the RNA. However, the contribution of specific interactions and the role of a pseudo-triplex motif in the catalytic center on the methylation reaction is not completely understood. In this study, we have used atomic substitutions and mutational analysis to investigate the reaction specificity and the key interactions required for catalysis. Substitution of the fluorescent nucleotide 2-aminopurine within the active ribozyme enabled the conformational dynamics of the RNA upon co-factor binding to be explored using fluorescence spectroscopy. We show that fast co-factor binding (t1/2 ∼ 0.7 s) drives a conformational change in the RNA to facilitate methyl group transfer. The importance of stacking interactions at the pseudo-triplex motif and chelation of the Cu (II) ion were shown to be essential for SAM binding.

Site-specific N-alkylation of DNA oligonucleotide nucleobases by DNAzyme-catalyzed reductive amination

DNA and RNA nucleobase modifications are biologically relevant and valuable in fundamental biochemical and biophysical investigations of nucleic acids. However, directly introducing site-specific nucleobase modifications into long unprotected oligonucleotides is a substantial challenge. In this study, we used in vitro selection to identify DNAzymes that site-specifically N-alkylate the exocyclic nucleobase amines of particular cytidine, guanosine, and adenosine (C, G and A) nucleotides in DNA substrates, by reductive amination using a 5'-benzaldehyde oligonucleotide as the reaction partner. The new DNAzymes each require one or more of Mg2+, Mn2+, and Zn2+ as metal ion cofactors and have kobs from 0.04 to 0.3 h-1, with rate enhancement as high as ∼104 above the splinted background reaction. Several of the new DNAzymes are catalytically active when an RNA substrate is provided in place of DNA. Similarly, several new DNAzymes function when a small-molecule benzaldehyde compound replaces the 5'-benzaldehyde oligonucleotide. These findings expand the scope of DNAzyme catalysis to include nucleobase N-alkylation by reductive amination. Further development of this new class of DNAzymes is anticipated to facilitate practical covalent modification and labeling of DNA and RNA substrates.

Comprehensive review on lipid metabolism and RNA methylation: Biological mechanisms, perspectives and challenges

Adipose tissue plays a crucial role in maintaining energy balance, regulating hormones, and promoting metabolic health. To address disorders related to obesity and develop effective therapies, it is essential to have a deep understanding of adipose tissue biology. In recent years, RNA methylation has emerged as a significant epigenetic modification involved in various cellular functions and metabolic pathways. Particularly in the realm of adipogenesis and lipid metabolism, extensive research is ongoing to uncover the mechanisms and functional importance of RNA methylation. Increasing evidence suggests that RNA methylation plays a regulatory role in adipocyte development, metabolism, and lipid utilization across different organs. This comprehensive review aims to provide an overview of common RNA methylation modifications, their occurrences, and regulatory mechanisms, focusing specifically on their intricate connections to fat metabolism. Additionally, we discuss the research methodologies used in studying RNA methylation and highlight relevant databases that can aid researchers in this rapidly advancing field.

Chemical Synthesis of Modified RNA

Ribonucleic acids (RNAs) play a vital role in living organisms. Many of their cellular functions depend critically on chemical modification. Methods to modify RNA in a controlled manner-both in vitro and in vivo-are thus essential to evaluate and understand RNA biology at the molecular and mechanistic levels. The diversity of modifications, combined with the size and uniformity of RNA (made up of only 4 nucleotides) makes its site-specific modification a challenging task that needs to be addressed by complementary approaches. One such approach is solid-phase RNA synthesis. We discuss recent developments in this field, starting with new protection concepts in the ongoing effort to overcome current size limitations. We continue with selected modifications that have posed significant challenges for their incorporation into RNA. These include deazapurine bases required for atomic mutagenesis to elucidate mechanistic aspects of catalytic RNAs, and RNA containing xanthosine, N4-acetylcytidine, 5-hydroxymethylcytidine, 3-methylcytidine, 2'-OCF3, and 2'-N3 ribose modifications. We also discuss the all-chemical synthesis of 5'-capped mRNAs and the enzymatic ligation of chemically synthesized oligoribonucleotides to obtain long RNA with multiple distinct modifications, such as those needed for single-molecule FRET studies. Finally, we highlight promising developments in RNA-catalyzed RNA modification using cofactors that transfer bioorthogonal functionalities.

S-adenosyl-l-methionine interaction signatures in methyltransferases

The switching on or off of methylation, a change from a normal methylation to hyper or hypo methylation is implicated in many diseases that include cancers, infectious, neurodegenerative diseases and others. Methyltransferases are one of the most sought targets that have diversified for the methylation of a variety of substrates. However, without S-adenosyl-l-methionine (SAM), the universal methyl donor, the majority of the methyltransferases remain functionally inactive. In this article, we did a comprehensive analysis of all available SAM-receptor crystal structures at atom, moiety and structure levels to gain deeper insights into the structure and function of SAM. SAM demonstrated flexibility in binding to a variety of receptors irrespective of the size of the binding pockets. Further analysis of the binding pockets resulted in all SAM conformations clustering into four natural shapes. The conserved interaction analysis provides an unambiguous orientation of SAM binding to receptors which has been elusive till now. SAM peptide moiety (SPM) and SAM nucleobase moiety (SNM) show up to 89% interactions with receptors whereas only 11% interactions with SAM ribose moiety (SRM). It is found that SPM and SNM terminal atoms anchor to the highly conserved receptor subsites creating a workbench for catalysis. It is seen that every interacting atom and its position is crucial in the methyl transfer phenomenon. A very unique observation is that the methyl group of SAM does not have even one interaction with the receptor. The deep insights gained help in the design and development of novel drugs against the methyltransferases.Communicated by Ramaswamy H. Sarma.

An optimized purification protocol for enzymatically synthesized S-adenosyl-L-methionine (SAM) for applications in solution state infrared spectroscopic studies

S-adenosyl-L-methionine (SAM) is an abundant biomolecule used by methyltransferases to regulate a wide range of essential cellular processes such as gene expression, cell signaling, protein functions, and metabolism. Despite considerable effort, there remain many specificity challenges associated with designing small molecule inhibitors for methyltransferases, most of which exhibit off-target effects. Interestingly, NMR evidence suggests that SAM undergoes conformeric exchange between several states when free in solution. Infrared spectroscopy can detect different conformers of molecules if present in appreciable populations. When SAM is noncovalently bound within enzyme active sites, the nature and the number of different conformations of the molecule are likely to be altered from when it is free in solution. If there are unique structures or different numbers of conformers between different methyltransferase active sites, solution-state information may provide promising structural leads to increase inhibitor specificity for a particular methyltransferase. Toward this goal, frequencies measured in SAM's infrared spectra must be assigned to the motions of specific atoms via isotope incorporation at discrete positions. The incorporation of isotopes into SAM's structure can be accomplished via an established enzymatic synthesis using isotopically labeled precursors. However, published protocols produced an intense and highly variable IR signal which overlapped with many of the signals from SAM rendering comparison between isotopes challenging. We observed this intense absorption to be from co-purifying salts and the SAM counterion, producing a strong, broad signal at 1100 cm-1. Here, we report a revised SAM purification protocol that mitigates the contaminating salts and present the first IR spectra of isotopically labeled CD3-SAM. These results provide a foundation for isotopic labeling experiments of SAM that will define which atoms participate in individual molecular vibrations, as a means to detect specific molecular conformations.

Overexpression of HbGRF4 or HbGRF4-HbGIF1 Chimera Improves the Efficiency of Somatic Embryogenesis in Hevea brasiliensis

Transgenic technology is a crucial tool for gene functional analysis and targeted genetic modification in the para rubber tree (Hevea brasiliensis). However, low efficiency of plant regeneration via somatic embryogenesis remains a bottleneck of successful genetic transformation in H. brasiliensis. Enhancing expression of GROWTH-REGULATING FACTOR 4 (GRF4)-GRF-INTERACTING FACTOR 1 (GIF1) has been reported to significantly improve shoot and embryo regeneration in multiple crops. Here, we identified endogenous HbGRF4 and HbGIF1 from the rubber clone Reyan7-33-97, the expressions of which dramatically increased along with somatic embryo (SE) production. Intriguingly, overexpression of HbGRF4 or HbGRF4-HbGIF1 markedly enhanced the efficiency of embryogenesis in two H. brasiliensis callus lines with contrasting rates of SE production. Transcriptional profiling revealed that the genes involved in jasmonic acid response were up-regulated, whereas those in ethylene biosynthesis and response as well as the S-adenosylmethionine-dependent methyltransferase activity were down-regulated in HbGRF4- and HbGRF4-HbGIF1-overexpressing H. brasiliensis embryos. These findings open up a new avenue for improving SE production in rubber tree, and help to unravel the underlying mechanisms of HbGRF4-enhanced somatic embryogenesis.

DNAzyme-Catalyzed Site-Specific N-Acylation of DNA Oligonucleotide Nucleobases

We used in vitro selection to identify DNAzymes that acylate the exocyclic nucleobase amines of cytidine, guanosine, and adenosine in DNA oligonucleotides. The acyl donor was the 2,3,5,6-tetrafluorophenyl ester (TFPE) of a 5'-carboxyl oligonucleotide. Yields are as high as >95 % in 6 h. Several of the N-acylation DNAzymes are catalytically active with RNA rather than DNA oligonucleotide substrates, and eight of nine DNAzymes for modifying C are site-specific (>95 %) for one particular substrate nucleotide. These findings expand the catalytic ability of DNA to include site-specific N-acylation of oligonucleotide nucleobases. Future efforts will investigate the DNA and RNA substrate sequence generality of DNAzymes for oligonucleotide nucleobase N-acylation, toward a universal approach for site-specific oligonucleotide modification.

RNA-Guided RNA Modifications: Biogenesis, Functions, and Applications

Post-transcriptional modifications are ubiquitous in both protein-coding and noncoding RNAs (ncRNAs), playing crucial functional roles in diverse biological processes across all kingdoms of life. These RNA modifications can be achieved through two distinct mechanisms: RNA-independent and RNA-guided (also known as RNA-dependent). In the RNA-independent mechanism, modifications are directly introduced onto RNA molecules by enzymes without the involvement of other RNA molecules, while the cellular RNA-guided RNA modification system exists in the form of RNA-protein complexes, wherein one guide RNA collaborates with a set of proteins, including the modifying enzyme. The primary function of guide RNAs lies in their ability to bind to complementary regions within the target RNAs, orchestrating the installation of specific modifications. Both mechanisms offer unique advantages and are critical to the diverse and dynamic landscape of RNA modifications. RNA-independent modifications provide rapid and direct modification of RNA molecules, while RNA-guided mechanisms offer precise and programmable means to introduce modifications at specific RNA sites. Recently, emerging evidence has shed light on RNA-guided RNA modifications as a captivating area of research, providing precise and programmable control over RNA sequences and functions.In this Account, we focus on RNA modifications synthesized in an RNA-guided manner, including 2'-O-methylated nucleotides (Nm), pseudouridine (Ψ), N4-acetylcytidine (ac4C), and inosine (I). This Account sheds light on the intricate processes of biogenesis and elucidates the regulatory roles of these modifications in RNA metabolism. These roles include pivotal functions such as RNA stability, translation, and splicing, where each modification contributes to the diverse and finely tuned regulatory landscape of RNA biology. In addition to elucidating the biogenesis and functions of these modifications, we also provide an overview of high-throughput methods and their underlying biochemical principles used for the transcriptome-wide investigation of these modifications and their fundamental interactions in RNA-guided systems. This includes exploring RNA-protein interactions and RNA-RNA interactions, which play crucial roles in the dynamic regulatory networks of RNA-guided modifications. The ever-advancing methodologies have greatly enhanced our understanding of the dynamic and widespread nature of RNA-guided RNA modifications and their regulatory functions. Furthermore, the applications of RNA-guided RNA modifications are discussed, illuminating their potential in diverse fields. From basic research to gene therapy, the programmable nature of RNA-guided modifications presents exciting opportunities for manipulating gene expression and developing innovative therapeutic strategies.

Integrative Approach to Probe Alternative Redox Mechanisms in RNA Modifications

RNA modifications found in most RNAs, particularly in tRNAs and rRNAs, reveal an abundance of chemical alterations of nucleotides. Over 150 distinct RNA modifications are known, emphasizing a remarkable diversity of chemical moieties in RNA molecules. These modifications play pivotal roles in RNA maturation, structural integrity, and the fidelity and efficiency of translation processes. The catalysts responsible for these modifications are RNA-modifying enzymes that use a striking array of chemistries to directly influence the chemical landscape of RNA. This diversity is further underscored by instances where the same modification is introduced by distinct enzymes that use unique catalytic mechanisms and cofactors across different domains of life. This phenomenon of convergent evolution highlights the biological importance of RNA modification and the vast potential within the chemical repertoire for nucleotide alteration. While shared RNA modifications can hint at conserved enzymatic pathways, a major bottleneck is to identify alternative routes within species that possess a modified RNA but are devoid of known RNA-modifying enzymes. To address this challenge, a combination of bioinformatic and experimental strategies proves invaluable in pinpointing new genes responsible for RNA modifications. This integrative approach not only unveils new chemical insights but also serves as a wellspring of inspiration for biocatalytic applications and drug design. In this Account, we present how comparative genomics and genome mining, combined with biomimetic synthetic chemistry, biochemistry, and anaerobic crystallography, can be judiciously implemented to address unprecedented and alternative chemical mechanisms in the world of RNA modification. We illustrate these integrative methodologies through the study of tRNA and rRNA modifications, dihydrouridine, 5-methyluridine, queuosine, 8-methyladenosine, 5-carboxymethylamino-methyluridine, or 5-taurinomethyluridine, each dependent on a diverse array of redox chemistries, often involving organic compounds, organometallic complexes, and metal coenzymes. We explore how vast genome and tRNA databases empower comparative genomic analyses and enable the identification of novel genes that govern RNA modification. Subsequently, we describe how the isolation of a stable reaction intermediate can guide the synthesis of a biomimetic to unveil new enzymatic pathways. We then discuss the usefulness of a biochemical "shunt" strategy to study catalytic mechanisms and to directly visualize reactive intermediates bound within active sites. While we primarily focus on various RNA-modifying enzymes studied in our laboratory, with a particular emphasis on the discovery of a SAM-independent methylation mechanism, the strategies and rationale presented herein are broadly applicable for the identification of new enzymes and the elucidation of their intricate chemistries. This Account offers a comprehensive glimpse into the evolving landscape of RNA modification research and highlights the pivotal role of integrated approaches to identify novel enzymatic pathways.

A SAM analogue-utilizing ribozyme for site-specific RNA alkylation in living cells

Post-transcriptional RNA modification methods are in high demand for site-specific RNA labelling and analysis of RNA functions. In vitro-selected ribozymes are attractive tools for RNA research and have the potential to overcome some of the limitations of chemoenzymatic approaches with repurposed methyltransferases. Here we report an alkyltransferase ribozyme that uses a synthetic, stabilized S-adenosylmethionine (SAM) analogue and catalyses the transfer of a propargyl group to a specific adenosine in the target RNA. Almost quantitative conversion was achieved within 1 h under a wide range of reaction conditions in vitro, including physiological magnesium ion concentrations. A genetically encoded version of the SAM analogue-utilizing ribozyme (SAMURI) was expressed in HEK293T cells, and intracellular propargylation of the target adenosine was confirmed by specific fluorescent labelling. SAMURI is a general tool for the site-specific installation of the smallest tag for azide-alkyne click chemistry, which can be further functionalized with fluorophores, affinity tags or other functional probes.

Ghost authors revealed: The structure and function of human N6 -methyladenosine RNA methyltransferases

Despite the discovery of modified nucleic acids nearly 75 years ago, their biological functions are still being elucidated. N6 -methyladenosine (m6 A) is the most abundant modification in eukaryotic messenger RNA (mRNA) and has also been detected in non-coding RNAs, including long non-coding RNA, ribosomal RNA, and small nuclear RNA. In general, m6 A marks can alter RNA secondary structure and initiate unique RNA-protein interactions that can alter splicing, mRNA turnover, and translation, just to name a few. Although m6 A marks in human RNAs have been known to exist since 1974, the structures and functions of methyltransferases responsible for writing m6 A marks have been established only recently. Thus far, there are four confirmed human methyltransferases that catalyze the transfer of a methyl group from S-adenosylmethionine (SAM) to the N6 position of adenosine, producing m6 A: methyltransferase-like protein (METTL) 3/METTL14 complex, METTL16, METTL5, and zinc-finger CCHC-domain-containing protein 4. Though the methyltransferases have unique RNA targets, all human m6 A RNA methyltransferases contain a Rossmann fold with a conserved SAM-binding pocket, suggesting that they utilize a similar catalytic mechanism for methyl transfer. For each of the human m6 A RNA methyltransferases, we present the biological functions and links to human disease, RNA targets, catalytic and kinetic mechanisms, and macromolecular structures. We also discuss m6 A marks in human viruses and parasites, assigning m6 A marks in the transcriptome to specific methyltransferases, small molecules targeting m6 A methyltransferases, and the enzymes responsible for hypermodified m6 A marks and their biological functions in humans. Understanding m6 A methyltransferases is a critical steppingstone toward establishing the m6 A epitranscriptome and more broadly the RNome. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

RNA structure determination: From 2D to 3D

RNA molecules serve a wide range of functions that are closely linked to their structures. The basic structural units of RNA consist of single- and double-stranded regions. In order to carry out advanced functions such as catalysis and ligand binding, certain types of RNAs can adopt higher-order structures. The analysis of RNA structures has progressed alongside advancements in structural biology techniques, but it comes with its own set of challenges and corresponding solutions. In this review, we will discuss recent advances in RNA structure analysis techniques, including structural probing methods, X-ray crystallography, nuclear magnetic resonance, cryo-electron microscopy, and small-angle X-ray scattering. Often, a combination of multiple techniques is employed for the integrated analysis of RNA structures. We also survey important RNA structures that have been recently determined using various techniques.

Catalytic mechanism and pH dependence of a methyltransferase ribozyme (MTR1) from computational enzymology

A methyltransferase ribozyme (MTR1) was selected in vitro to catalyze alkyl transfer from exogenous O6-methylguanine (O6mG) to a target adenine N1, and recently, high-resolution crystal structures have become available. We use a combination of classical molecular dynamics, ab initio quantum mechanical/molecular mechanical (QM/MM) and alchemical free energy (AFE) simulations to elucidate the atomic-level solution mechanism of MTR1. Simulations identify an active reactant state involving protonation of C10 that hydrogen bonds with O6mG:N1. The deduced mechanism involves a stepwise mechanism with two transition states corresponding to proton transfer from C10:N3 to O6mG:N1 and rate-controlling methyl transfer (19.4  kcal·mol-1 barrier). AFE simulations predict the pKa for C10 to be 6.3, close to the experimental apparent pKa of 6.2, further implicating it as a critical general acid. The intrinsic rate derived from QM/MM simulations, together with pKa calculations, enables us to predict an activity-pH profile that agrees well with experiment. The insights gained provide further support for a putative RNA world and establish new design principles for RNA-based biochemical tools.

Biological Catalysis and Information Storage Have Relied on N-Glycosyl Derivatives of β-D-Ribofuranose since the Origins of Life

Most naturally occurring nucleotides and nucleosides are N-glycosyl derivatives of β-d-ribose. These N-ribosides are involved in most metabolic processes that occur in cells. They are essential components of nucleic acids, forming the basis for genetic information storage and flow. Moreover, these compounds are involved in numerous catalytic processes, including chemical energy production and storage, in which they serve as cofactors or coribozymes. From a chemical point of view, the overall structure of nucleotides and nucleosides is very similar and simple. However, their unique chemical and structural features render these compounds versatile building blocks that are crucial for life processes in all known organisms. Notably, the universal function of these compounds in encoding genetic information and cellular catalysis strongly suggests their essential role in the origins of life. In this review, we summarize major issues related to the role of N-ribosides in biological systems, especially in the context of the origin of life and its further evolution, through the RNA-based World(s), toward the life we observe today. We also discuss possible reasons why life has arisen from derivatives of β-d-ribofuranose instead of compounds based on other sugar moieties.

Structure-based insights into recognition and regulation of SAM-sensing riboswitches

Riboswitches are highly conserved RNA elements that located in the 5'-UTR of mRNAs, which undergo real-time structure conformational change to achieve the regulation of downstream gene expression by sensing their cognate ligands. S-adenosylmethionine (SAM) is a ubiquitous methyl donor for transmethylation reactions in all living organisms. SAM riboswitch is one of the most abundant riboswitches that bind to SAM with high affinity and selectivity, serving as regulatory modules in multiple metabolic pathways. To date, seven SAM-specific riboswitch classes that belong to four families, one SAM/SAH riboswitch and one SAH riboswitch have been identified. Each SAM riboswitch family has a well-organized tertiary core scaffold to support their unique ligand-specific binding pocket. In this review, we summarize the current research progress on the distribution, structure, ligand recognition and gene regulation mechanism of these SAM-related riboswitch families, and further discuss their evolutionary prospects and potential applications.

A Cofactor-Based Mechanism for the Origin of the Genetic Code

The origin of the genetic code is probably the central problem of the studies on the origin of life. The key question to answer is the molecular mechanism that allows the association of the amino acids with their triplet codons. We proposed that the codon-anticodon duplex located in the acceptor stem of primitive tRNAs would facilitate the chemical reactions required to synthesize cognate amino acids from simple amino acids (glycine, valine, and aspartic acid) linked to the 3' acceptor end. In our view, various nucleotide-A-derived cofactors (with reactive chemical groups) may be attached to the codon-anticodon duplex, which allows group-transferring reactions from cofactors to simple amino acids, thereby producing the final amino acid. The nucleotide-A-derived cofactors could be incorporated into the RNA duplex (helix) by docking Adenosine (cofactor) into the minor groove via an interaction similar to the A-minor motif, forming a base triple between Adenosine and one complementary base pair of the duplex. Furthermore, we propose that this codon-anticodon duplex could initially catalyze a self-aminoacylation reaction with a simple amino acid. Therefore, the sequence of bases in the codon-anticodon duplex would determine the reactions that occurred during the formation of new amino acids for selective binding of nucleotide-A-derived cofactors.

Small-Molecule-Mediated Split-Aptamer Assembly for Inducible CRISPR-dCas9 Transcription Activation

Inducible CRISPR-dCas9 transcription system has become a powerful tool for transcription regulation and sensing. Here, we develop a new concept of small-molecule-mediated split-aptamer assembly for inducible CRISPR-dCas9 transcription activation, allowing quantitative detection and imaging of S-adenosyl methionine (SAM) in live cells. This inducible transcription system is designed by integrating one fragment of a split SAM aptamer to guide RNA (gRNA) and the other to MS2 arrays. SAM-mediated reassembly of the split fragments recruits an MCP-fused transcription activator to the gRNA-dCas9 complex, activating the expression of a near-infrared fluorescent protein for imaging. We demonstrate that this inducible transcription system achieves quantitative detection of SAM with high sensitivity in live cells. Our system shows that methionine adenosyltransferase 1A (MAT1A) and MAT2A can both catalyze SAM production in live cells and the SAM levels in cancer cells can be increased via upregulation of MAT1A mRNA by epigenetic inhibitors. This split-aptamer assembly strategy could afford a new approach for controlling the CRISPR-dCas9 system, enabling conditional transcription regulation in response to endogenous metabolites in live cells.

1-Deazaguanosine-Modified RNA: The Missing Piece for Functional RNA Atomic Mutagenesis

Atomic mutagenesis is the key to advance our understanding of RNA recognition and RNA catalysis. To this end, deazanucleosides are utilized to evaluate the participation of specific atoms in these processes. One of the remaining challenges is access to RNA-containing 1-deazaguanosine (c1G). Here, we present the synthesis of this nucleoside and its phosphoramidite, allowing first time access to c1G-modified RNA. Thermodynamic analyses revealed the base pairing parameters for c1G-modified RNA. Furthermore, by NMR spectroscopy, a c1G-triggered switch of Watson-Crick into Hoogsteen pairing in HIV-2 TAR RNA was identified. Additionally, using X-ray structure analysis, a guanine-phosphate backbone interaction affecting RNA fold stability was characterized, and finally, the critical impact of an active-site guanine in twister ribozyme on the phosphodiester cleavage was revealed. Taken together, our study lays the synthetic basis for c1G-modified RNA and demonstrates the power of the completed deazanucleoside toolbox for RNA atomic mutagenesis needed to achieve in-depth understanding of RNA recognition and catalysis.

Structure and mechanism of a methyltransferase ribozyme

Known ribozymes in contemporary biology perform a limited range of chemical catalysis, but in vitro selection has generated species that catalyze a broader range of chemistry; yet, there have been few structural and mechanistic studies of selected ribozymes. A ribozyme has recently been selected that can catalyze a site-specific methyl transfer reaction. We have solved the crystal structure of this ribozyme at a resolution of 2.3 Å, showing how the RNA folds to generate a very specific binding site for the methyl donor substrate. The structure immediately suggests a catalytic mechanism involving a combination of proximity and orientation and nucleobase-mediated general acid catalysis. The mechanism is supported by the pH dependence of the rate of catalysis. A selected methyltransferase ribozyme can thus use a relatively sophisticated catalytic mechanism, broadening the range of known RNA-catalyzed chemistry.

The authors present the crystal structure of the MTR1 ribozyme that transfers the methyl group from O⁶-methylguanine to an adenine N1 in the target RNA and propose a catalytic mechanism based upon proximity, orientation and general acid catalysis.