Ribozyme speed limits

Impact of mixing and shaking on mRNA-LNP drug product quality characteristics

Since the COVID-19 pandemic, the interest in RNA-lipid nanoparticle (LNP) based drug products has increased drastically. While one RNA-LNP drug product, Onpattro, was already on the market in 2018, high volume manufacturing was only initiated end of 2020 with the approval of the mRNA-LNP vaccines, Comirnaty and Spikevax. As such, deep product knowledge for RNA-LNPs is continuously increasing. In this article the effect of large-scale mixing and lab-scale shaking on mRNA-LNP drug product quality characteristics is investigated. It is shown that mixing and shaking can have a profound impact on both LNP size distribution and mRNA encapsulation, suggesting a direct correlation between both quality characteristics, and further supported by a proposed underlying mechanism. An in-depth investigation of different drug product (DP) presentations reveals a consistent effect of headspace volume and LNP content on the shaking stress sensitivity. Results reported in this study are of utter importance for both small- and large-scale manufacturers but also for care givers and patients using these products.

Assessment of Nucleobase Protomeric and Tautomeric States in Nucleic Acid Structures for Interaction Analysis and Structure-Based Ligand Design

With increasing interest in RNA as a therapeutic and a potential target, the role of RNA structures has become more important. Even slight changes in nucleobases, such as modifications or protomeric and tautomeric states, can have a large impact on RNA structure and function, while local environments in turn affect protonation and tautomerization. In this work, the application of empirical tools for pKa and tautomer prediction for RNA modifications was elucidated and compared with ab initio quantum mechanics (QM) methods and expanded toward macromolecular RNA structures, where QM is no longer feasible. In this regard, the Protonate3D functionality within the molecular operating environment (MOE) was expanded for nucleobase protomer and tautomer predictions and applied to reported examples of altered protonation states depending on the local environment. Overall, observations of nonstandard protomers and tautomers were well reproduced, including structural C+G:C(A) and A+GG motifs, several mismatches, and protonation of adenosine or cytidine as the general acid in nucleolytic ribozymes. Special cases, such as cobalt hexamine-soaked complexes or the deprotonation of guanosine as the general base in nucleolytic ribozymes, proved to be challenging. The collected set of examples shall serve as a starting point for the development of further RNA protonation prediction tools, while the presented Protonate3D implementation already delivers reasonable protonation predictions for RNA and DNA macromolecules. For cases where higher accuracy is needed, like following catalytic pathways of ribozymes, incorporation of QM-based methods can build upon the Protonate3D-generated starting structures. Likewise, this protonation prediction can be used for structure-based RNA-ligand design approaches.

Energy landscapes of homopolymeric RNAs revealed by deep unsupervised learning

Conformational dynamics of RNA plays important roles in a variety of cellular functions such as transcriptional regulation, catalysis, scaffolding, and sensing. Recently, RNAs with low-complexity sequences have been shown to phase separate and form condensate phases similar to lowcomplexity protein domains. The affinity for phase separation and the material characteristics of RNA condensates are strongly dependent on sequence composition and patterning. We hypothesize that differences in the affinities for RNA phase separation can be uncovered by studying sequence-dependent conformational dynamics of single RNA chains. To this end, we have employed atomistic simulations and deep dimensionality reduction techniques to map temperature-dependent conformational free energy landscapes for 20 base-long homopolymeric RNA sequences: poly(U), poly(G), poly(C), and poly(A). The energy landscapes of homopolymeric RNAs reveal a plethora of metastable states with qualitatively different populations stemming from differences in base chemistry. Through detailed analysis of base, phosphate, and sugar interactions, we show that experimentally observed temperature-driven shifts in metastable state populations align with experiments on RNA phase transitions. Specifically, we find that the thermodynamics of unfolding of homopolymeric RNA follows the poly(G) > poly(A) > poly(C) > poly(U) order of stability, mirroring the propensity of RNA to form condensates. To conclude, this work shows that at least for homopolymeric RNA sequences the single-chain conformational dynamics contains sufficient information for predicting and quantifying condensate forming affinities of RNAs. Thus, we anticipate that atomically detailed studies of temeprature -dependent energy landscapes of RNAs will be a useful guide for understanding the propensity of various RNA molecules to form condensates.

RNA targeting and cleavage by the type III-Dv CRISPR effector complex

CRISPR-Cas are adaptive immune systems in bacteria and archaea that utilize CRISPR RNA-guided surveillance complexes to target complementary RNA or DNA for destruction1-5. Target RNA cleavage at regular intervals is characteristic of type III effector complexes6-8. Here, we determine the structures of the Synechocystis type III-Dv complex, an apparent evolutionary intermediate from multi-protein to single-protein type III effectors9,10, in pre- and post-cleavage states. The structures show how multi-subunit fusion proteins in the effector are tethered together in an unusual arrangement to assemble into an active and programmable RNA endonuclease and how the effector utilizes a distinct mechanism for target RNA seeding from other type III effectors. Using structural, biochemical, and quantum/classical molecular dynamics simulation, we study the structure and dynamics of the three catalytic sites, where a 2'-OH of the ribose on the target RNA acts as a nucleophile for in line self-cleavage of the upstream scissile phosphate. Strikingly, the arrangement at the catalytic residues of most type III complexes resembles the active site of ribozymes, including the hammerhead, pistol, and Varkud satellite ribozymes. Our work provides detailed molecular insight into the mechanisms of RNA targeting and cleavage by an important intermediate in the evolution of type III effector complexes.

A comparative survey of the influence of small self-cleaving ribozymes on gene expression in human cell culture

Self-cleaving ribozymes are versatile tools for synthetic biologists when it comes to controlling gene expression. Up to date, 12 different classes are known, and over the past decades more and more details about their structure, cleavage mechanisms and natural environments have been uncovered. However, when these motifs are applied to mammalian gene expression constructs, the outcome can often be unexpected. A variety of factors, such as surrounding sequences and positioning of the ribozyme influences the activity and hence performance of catalytic RNAs. While some information about the efficiency of individual ribozymes (each tested in specific contexts) is known, general trends obtained from standardized, comparable experiments are lacking, complicating decisions such as which ribozyme to choose and where to insert it into the target mRNA. In many cases, application-specific optimization is required, which can be very laborious. Here, we systematically compared different classes of ribozymes within the 3'-UTR of a given reporter gene. We then examined position-dependent effects of the best-performing ribozymes. Moreover, we tested additional variants of already widely used hammerhead ribozymes originating from various organisms. We were able to identify functional structures suited for aptazyme design and generated highly efficient hammerhead ribozyme variants originating from the human genome. The present dataset will aide decisions about how to apply ribozymes for affecting gene expression as well as for developing ribozyme-based switches for controlling gene expression in human cells.

Rapid Kinetics of Pistol Ribozyme: Insights into Limits to RNA Catalysis

Pistol ribozyme (Psr) is a distinct class of small endonucleolytic ribozymes, which are important experimental systems for defining fundamental principles of RNA catalysis and designing valuable tools in biotechnology. High-resolution structures of Psr, extensive structure-function studies, and computation support a mechanism involving one or more catalytic guanosine nucleobases acting as a general base and divalent metal ion-bound water acting as an acid to catalyze RNA 2'-O-transphosphorylation. Yet, for a wide range of pH and metal ion concentrations, the rate of Psr catalysis is too fast to measure manually and the reaction steps that limit catalysis are not well understood. Here, we use stopped-flow fluorescence spectroscopy to evaluate Psr temperature dependence, solvent H/D isotope effects, and divalent metal ion affinity and specificity unconstrained by limitations due to fast kinetics. The results show that Psr catalysis is characterized by small apparent activation enthalpy and entropy changes and minimal transition state H/D fractionation, suggesting that one or more pre-equilibrium steps rather than chemistry is rate limiting. Quantitative analyses of divalent ion dependence confirm that metal aquo ion pK_a correlates with higher rates of catalysis independent of differences in ion binding affinity. However, ambiguity regarding the rate-limiting step and similar correlation with related attributes such as ionic radius and hydration free energy complicate a definitive mechanistic interpretation. These new data provide a framework for further interrogation of Psr transition state stabilization and show how thermal instability, metal ion insolubility at optimal pH, and pre-equilibrium steps such as ion binding and folding limit the catalytic power of Psr suggesting potential strategies for further optimization.

QDπ: A Quantum Deep Potential Interaction Model for Drug Discovery

We report QDπ-v1.0 for modeling the internal energy of drug molecules containing H, C, N, and O atoms. The QDπ model is in the form of a quantum mechanical/machine learning potential correction (QM/Δ-MLP) that uses a fast third-order self-consistent density-functional tight-binding (DFTB3/3OB) model that is corrected to a quantitatively high-level of accuracy through a deep-learning potential (DeepPot-SE). The model has the advantage that it is able to properly treat electrostatic interactions and handle changes in charge/protonation states. The model is trained against reference data computed at the ωB97X/6-31G* level (as in the ANI-1x data set) and compared to several other approximate semiempirical and machine learning potentials (ANI-1x, ANI-2x, DFTB3, MNDO/d, AM1, PM6, GFN1-xTB, and GFN2-xTB). The QDπ model is demonstrated to be accurate for a wide range of intra- and intermolecular interactions (despite its intended use as an internal energy model) and has shown to perform exceptionally well for relative protonation/deprotonation energies and tautomers. An example application to model reactions involved in RNA strand cleavage catalyzed by protein and nucleic acid enzymes illustrates QDπ has average errors less than 0.5 kcal/mol, whereas the other models compared have errors over an order of magnitude greater. Taken together, this makes QDπ highly attractive as a potential force field model for drug discovery.

Dissociative Transition State in Hepatitis Delta Virus Ribozyme Catalysis

Ribonucleases and small nucleolytic ribozymes are both able to catalyze RNA strand cleavage through 2'-O-transphosphorylation, provoking the question of whether protein and RNA enzymes facilitate mechanisms that pass through the same or distinct transition states. Here, we report the primary and secondary 18O kinetic isotope effects for hepatitis delta virus ribozyme catalysis that reveal a dissociative, metaphosphate-like transition state in stark contrast to the late, associative transition states observed for reactions catalyzed by specific base, Zn2+ ions, or ribonuclease A. This new information provides evidence for a discrete ribozyme active site design that modulates the RNA cleavage pathway to pass through an altered transition state.

Scaling Catalytic Contributions of Small Self-Cleaving Ribozymes

Nucleolytic ribozymes utilize general acid-base catalysis to perform phosphodiester cleavage. In most ribozyme classes, a conserved active site guanosine is positioned to act as general base, thereby activating the 2'-OH group to attack the scissile phosphate (γ-catalysis). Here, we present an atomic mutagenesis study for the pistol ribozyme class. Strikingly, "general base knockout" by replacement of the guanine N1 atom by carbon results in only 2.7-fold decreased rate. Therefore, the common view that γ-catalysis critically depends on the N1 moiety becomes challenged. For pistol ribozymes we found that γ-catalysis is subordinate in overall catalysis, made up by two other catalytic factors (α and δ). Our approach allows scaling of the different catalytic contributions (α, β, γ, δ) with unprecedented precision and paves the way for a thorough mechanistic understanding of nucleolytic ribozymes with active site guanines.

Scaling Catalytic Contributions of Small Self-Cleaving Ribozymes

Nucleolytic ribozymes utilize general acid-base catalysis to perform phosphodiester cleavage. In most ribozyme classes, a conserved active site guanosine is positioned to act as general base, thereby activating the 2'-OH group to attack the scissile phosphate (γ-catalysis). Here, we present an atomic mutagenesis study for the pistol ribozyme class. Strikingly, "general base knockout" by replacement of the guanine N1 atom by carbon results in only 2.7-fold decreased rate. Therefore, the common view that γ-catalysis critically depends on the N1 moiety becomes challenged. For pistol ribozymes we found that γ-catalysis is subordinate in overall catalysis, made up by two other catalytic factors (α and δ). Our approach allows scaling of the different catalytic contributions (α, β, γ, δ) with unprecedented precision and paves the way for a thorough mechanistic understanding of nucleolytic ribozymes with active site guanines.

Who stole the proton? Suspect general base guanine found with a smoking gun in the pistol ribozyme

The pistol ribozyme (Psr) is one among the most recently discovered classes of small nucleolytic ribozymes that catalyze site-specific RNA self-cleavage through 2'-O-transphosphorylation. The Psr contains a conserved guanine (G40) that in crystal structures is in a position suggesting it plays the role of the general base to abstract a proton from the nucleophile to activate the reaction. Although some functional data is consistent with this mechanistic role, a notable exception is 2-aminopurine (2AP) substitution which has no effect on the rate, unlike similar substitutions across other so-called "G + M" and "G + A" ribozyme classes. Herein we postulate that an alternate conserved guanine, G42, is the primary general base, and provide evidence from molecular simulations that the active site of Psr can undergo local refolding into a structure that is consistent with the common "L-platform/L-scaffold" architecture identified in G + M and G + A ribozyme classes with Psr currently the notable exception. We summarize the key currently available experimental data and present new classical and combined quantum mechanical/molecular mechanical simulation results that collectively suggest a new hypothesis. We hypothesize that there are two available catalytic pathways supported by different conformational states connected by a local refolding of the active site: (1) a primary pathway with an active site architecture aligned with the L-platform/L-scaffold framework where G42 acts as a general base, and (2) a secondary pathway with the crystallographic active site architecture where G40 acts as a general base. We go on to make several experimentally testable predictions, and suggest specific experiments that would ultimately bring closure to the mystery as to "who stole the proton in the pistol ribozyme?".

The role of Na+ in catalysis by the 8-17 DNAzyme

The 8-17 DNAzyme is the most studied deoxyribozyme in terms of its molecular mechanism; hence it has become a model system to understand the basis behind DNA catalysis. New functional studies and the recent attainment of high-resolution X-ray structures, in addition to theoretical calculations have offered a great opportunity to gain a broader comprehension of its mechanism; however many aspects are unclear yet, especially regarding the precise role of metal ions in catalysis. Recently, molecular dynamics simulations have suggested for the first time a specific and dynamical participation of Na⁺ in the mechanism through the reaction pathway, besides the roles proposed for divalent metal cofactors. Herein, we present experimental evidence of a cooperative role of the monovalent cation Na⁺ in catalysis that is in line with these theoretical suggestions. Our findings show a clear influence of the concentration of Na⁺ on the activity of the 8-17 DNAzyme when Pb²⁺ is used as the cofactor. Interestingly, this effect is not noticed with Mg²⁺, indicating a particular contribution of the monovalent ion to catalysis that would operate preferentially with Pb²⁺. We have also found that Na⁺ affects the pK_a of the general base and the general acid, indicating its influence on general acid-base catalysis, already identified as part of the mechanism of the 8-17 DNAzyme. Finally, our results emphasize the need to consider Na⁺ carefully in the design and analysis of functional studies of catalytic DNAs and its possible specific role in their mechanisms.

Structural modifications as tools in mechanistic studies of the cleavage of RNA phosphodiester linkages

The cleavage of RNA phosphodiester bonds by RNase A and hammerhead ribozyme at neutral pH fundamentally differs from the spontaneous reactions of these bonds under the same conditions. While the predominant spontaneous reaction is isomerization of the 3',5'-phosphodiester linkages to their 2',5'-counterparts, this reaction has never been reported to compete with the enzymatic cleavage reaction, not even as a minor side reaction. Comparative kinetic measurements with structurally modified di-nucleoside monophosphates and oligomeric phosphodiesters have played an important role in clarification of mechanistic details of the buffer-independent and buffer-catalyzed reactions. More recently, heavy atom isotope effects and theoretical calculations have refined the picture. The primary aim of all these studies has been to form a solid basis for mechanistic analyses of the action of more complicated catalytic machineries. In other words, to contribute to conception of a plausible unified picture of RNA cleavage by biocatalysts, such as RNAse A, hammerhead ribozyme and DNAzymes. In addition, structurally modified trinucleoside monophosphates as transition state models for Group I and II introns have clarified some features of the action of large ribozymes.

Cryo neutron crystallography demonstrates influence of RNA 2'-OH orientation on conformation, sugar pucker and water structure

The ribose 2′-hydroxyl is the key chemical difference between RNA and DNA and primary source of their divergent structural and functional characteristics. Macromolecular X-ray diffraction experiments typically do not reveal the positions of hydrogen atoms. Thus, standard crystallography cannot determine 2′-OH orientation (H2′-C2′-O2′-HO2′ torsion angle) and its potential roles in sculpting the RNA backbone and the expansive fold space. Here, we report the first neutron crystal structure of an RNA, the Escherichia coli rRNA Sarcin-Ricin Loop (SRL). 2′-OD orientations were established for all 27 residues and revealed O-D bonds pointing toward backbone (O3′, 13 observations), nucleobase (11) or sugar (3). Most riboses in the SRL stem region show a 2′-OD backbone-orientation. GAGA-tetraloop riboses display a 2′-OD base-orientation. An atypical C2′-endo sugar pucker is strictly correlated with a 2′-OD sugar-orientation. Neutrons reveal the strong preference of the 2′-OH to donate in H-bonds and that 2′-OH orientation affects both backbone geometry and ribose pucker. We discuss 2′-OH and water molecule orientations in the SRL neutron structure and compare with results from a solution phase 10 μs MD simulation. We demonstrate that joint cryo-neutron/X-ray crystallography offers an all-in-one approach to determine the complete structural properties of RNA, i.e. geometry, conformation, protonation state and hydration structure.

Combined QM/MM, Machine Learning Path Integral Approach to Compute Free Energy Profiles and Kinetic Isotope Effects in RNA Cleavage Reactions

We present a fast, accurate, and robust approach for determination of free energy profiles and kinetic isotope effects for RNA 2'-O-transphosphorylation reactions with inclusion of nuclear quantum effects. We apply a deep potential range correction (DPRc) for combined quantum mechanical/molecular mechanical (QM/MM) simulations of reactions in the condensed phase. The method uses the second-order density-functional tight-binding method (DFTB2) as a fast, approximate base QM model. The DPRc model modifies the DFTB2 QM interactions and applies short-range corrections to the QM/MM interactions to reproduce ab initio DFT (PBE0/6-31G*) QM/MM energies and forces. The DPRc thus enables both QM and QM/MM interactions to be tuned to high accuracy, and the QM/MM corrections are designed to smoothly vanish at a specified cutoff boundary (6 Å in the present work). The computational speed-up afforded by the QM/MM+DPRc model enables free energy profiles to be calculated that include rigorous long-range QM/MM interactions under periodic boundary conditions and nuclear quantum effects through a path integral approach using a new interface between the AMBER and i-PI software. The approach is demonstrated through the calculation of free energy profiles of a native RNA cleavage model reaction and reactions involving thio-substitutions, which are important experimental probes of the mechanism. The DFTB2+DPRc QM/MM free energy surfaces agree very closely with the PBE0/6-31G* QM/MM results, and it is vastly superior to the DFTB2 QM/MM surfaces with and without weighted thermodynamic perturbation corrections. 18O and 34S primary kinetic isotope effects are compared, and the influence of nuclear quantum effects on the free energy profiles is examined.

Studying the Key Intermediate of RNA Autohydrolysis by Cryogenic Gas-Phase Infrared Spectroscopy

Over the course of the COVID-19 pandemic, mRNA-based vaccines have gained tremendous importance. The development and analysis of modified RNA molecules benefit from advanced mass spectrometry and require sufficient understanding of fragmentation processes. Analogous to the degradation of RNA in solution by autohydrolysis, backbone cleavage of RNA strands was equally observed in the gas phase; however, the fragmentation mechanism remained elusive. In this work, autohydrolysis-like intermediates were generated from isolated RNA dinucleotides in the gas phase and investigated using cryogenic infrared spectroscopy in helium nanodroplets. Data from both experiment and density functional theory provide evidence for the formation of a five-membered cyclic phosphate intermediate and rule out linear or six-membered structures. Furthermore, the experiments show that another prominent condensed-phase reaction of RNA nucleotides can be induced in the gas phase: the tautomerization of cytosine. Both observed reactions are therefore highly universal and intrinsic properties of the investigated molecules.

Degradation of SARS-CoV-2 specific ribonucleic acid in samples for nucleic acid amplification detection

The degradation of SARS-CoV-2 specific ribonucleic acid (RNA) was investigated by a numerical modeling approach based on nucleic acid amplification test (NAAT) results utilizing the SmartAmp technique. The precision of the measurement was verified by the relative standard deviation (RSD) of repeated measurements at each calibration point. The precision and detection limits were found to be 6% RSD (seven repeated measurements) and 94 copies/tube, respectively, at the lowest calibration point. RNA degradation curves obtained from NAAT data on four different temperatures were in good agreement with the first-order reaction model. By referring to rate constants derived from the results, the Arrhenius model was applied to predict RNA degradation behavior. If the initial RNA concentration was high enough, such as in samples taken from infected bodies, the NAAT results were expected to be positive during testing. On the other hand, if initial RNA concentrations were relatively low, such as RNA in residual viruses on environmental surfaces, special attention should be paid to avoid false-negative results. The results obtained in this study provide a practical guide for RNA sample management in the NAAT of non-human samples.

cyPhyRNA-seq: a genome-scale RNA-seq method to detect active self-cleaving ribozymes by capturing RNAs with 2',3' cyclic phosphates and 5' hydroxyl ends

Self-cleaving ribozymes are catalytically active RNAs that cleave themselves into a 5′-fragment with a 2′,3′-cyclic phosphate and a 3′-fragment with a 5′-hydroxyl. They are widely applied for the construction of synthetic RNA devices and RNA-based therapeutics. However, the targeted discovery of self-cleaving ribozymes remains a major challenge. We developed a transcriptome-wide method, called cyPhyRNA-seq, to screen for ribozyme cleavage fragments in total RNA extract. This approach employs the specific ligation-based capture of ribozyme 5′-fragments using a variant of the Arabidopsis thaliana tRNA ligase we engineered. To capture ribozyme 3′-fragments, they are enriched from total RNA by enzymatic treatments. We optimized and enhanced the individual steps of cyPhyRNA-seq in vitro and in spike-in experiments. Then, we applied cyPhyRNA-seq to total RNA isolated from the bacterium Desulfovibrio vulgaris and detected self-cleavage of the three predicted type II hammerhead ribozymes, whose activity had not been examined to date. cyPhyRNA-seq can be used for the global analysis of active self-cleaving ribozymes with the advantage to capture both ribozyme cleavage fragments from total RNA. Especially in organisms harbouring many self-cleaving RNAs, cyPhyRNA-seq facilitates the investigation of cleavage activity. Moreover, this method has the potential to be used to discover novel self-cleaving ribozymes in different organisms.

Development of Range-Corrected Deep Learning Potentials for Fast, Accurate Quantum Mechanical/Molecular Mechanical Simulations of Chemical Reactions in Solution

We develop a new deep potential─range correction (DPRc) machine learning potential for combined quantum mechanical/molecular mechanical (QM/MM) simulations of chemical reactions in the condensed phase. The new range correction enables short-ranged QM/MM interactions to be tuned for higher accuracy, and the correction smoothly vanishes within a specified cutoff. We further develop an active learning procedure for robust neural network training. We test the DPRc model and training procedure against a series of six nonenzymatic phosphoryl transfer reactions in solution that are important in mechanistic studies of RNA-cleaving enzymes. Specifically, we apply DPRc corrections to a base QM model and test its ability to reproduce free-energy profiles generated from a target QM model. We perform these comparisons using the MNDO/d and DFTB2 semiempirical models because they differ in the way they treat orbital orthogonalization and electrostatics and produce free-energy profiles which differ significantly from each other, thereby providing us a rigorous stress test for the DPRc model and training procedure. The comparisons show that accurate reproduction of the free-energy profiles requires correction of the QM/MM interactions out to 6 Å. We further find that the model's initial training benefits from generating data from temperature replica exchange simulations and including high-temperature configurations into the fitting procedure, so the resulting models are trained to properly avoid high-energy regions. A single DPRc model was trained to reproduce four different reactions and yielded good agreement with the free-energy profiles made from the target QM/MM simulations. The DPRc model was further demonstrated to be transferable to 2D free-energy surfaces and 1D free-energy profiles that were not explicitly considered in the training. Examination of the computational performance of the DPRc model showed that it was fairly slow when run on CPUs but was sped up almost 100-fold when using NVIDIA V100 GPUs, resulting in almost negligible overhead. The new DPRc model and training procedure provide a potentially powerful new tool for the creation of next-generation QM/MM potentials for a wide spectrum of free-energy applications ranging from drug discovery to enzyme design.

Investigation of the pKa of the Nucleophilic O2' of the Hairpin Ribozyme

Small ribozymes cleave their RNA phosphodiester backbone by catalyzing a transphosphorylation reaction wherein a specific O2' functions as the nucleophile. While deprotonation of this alcohol through its acidification would increase its nucleophilicity, little is known about the pK_a of this O2' in small ribozymes, in part because high pK_a's are not readily accessible experimentally. Herein, we turn to molecular dynamics to calculate the pK_a of the nucleophilic O2' in the hairpin ribozyme and to study interactions within the active site that may impact its value. We estimate the pK_a of the nucleophilic O2' in the wild-type hairpin ribozyme to be 18.5 ± 0.8, which is higher than the reference compound, and identify a correlation between proper positioning of the O2' for nucleophilic attack and elevation of its pK_a. We find that monovalent ions may play a role in depression of the O2' pK_a, while the exocyclic amine appears to be important for organizing the ribozyme active site. Overall, this study suggests that the pK_a of the O2' is raised in the ground state and lowers during the course of the reaction owing to positioning and metal ion interactions.

Beyond the Plateau: pL Dependence of Proton Inventories as a Tool for Studying Ribozyme and Ribonuclease Catalysis

Acid/base catalysis is an important catalytic strategy used by ribonucleases and ribozymes; however, understanding the number and identity of functional groups involved in proton transfer remains challenging. The proton inventory (PI) technique analyzes the dependence of the enzyme reaction rate on the ratio of D₂O to H₂O and can provide information about the number of exchangeable sites that produce isotope effects and their magnitude. The Gross-Butler (GB) equation is used to evaluate H/D fractionation factors from PI data typically collected under conditions (i.e., a "plateau" in the pH-rate profile) assuming minimal change in active site residue ionization. However, restricting PI analysis to these conditions is problematic for many ribonucleases, ribozymes, and their variants due to ambiguity in the roles of active site residues, the lack of a plateau within the accessible pL range, or cooperative interactions between active site functional groups undergoing ionization. Here, we extend the integration of species distributions for alternative enzyme states in noncooperative models of acid/base catalysis into the GB equation, first used by Bevilacqua and colleagues for the HDV ribozyme, to develop a general population-weighted GB equation that allows simulation and global fitting of the three-dimensional relationship of the D₂O ratio (n) versus pL versus k_n/k₀. Simulations using the GPW-GB equation of PI results for RNase A, HDVrz, and VSrz illustrate that data obtained at multiple selected pL values across the pL-rate profile can assist in the planning and interpreting of solvent isotope effect experiments to distinguish alternative mechanistic models.

Mechanisms of catalytic RNA molecules

Ribozymes are folded catalytic RNA molecules that perform important biological functions. Since the discovery of the first RNA with catalytic activity in 1982, a large number of ribozymes have been reported. While most catalytic RNA molecules act alone, some RNA-based catalysts, such as RNase P, the ribosome, and the spliceosome, need protein components to perform their functions in the cell. In the last decades, the structure and mechanism of several ribozymes have been studied in detail. Aside from the ribosome, which catalyzes peptide bond formation during protein synthesis, the majority of known ribozymes carry out mostly phosphoryl transfer reactions, notably trans-esterification or hydrolysis reactions. In this review, we describe the main features of the mechanisms of various types of ribozymes that can function with or without the help of proteins to perform their biological functions.

Theoretical Studies of the Self Cleavage Pistol Ribozyme Mechanism

Ribozymes are huge complex biological catalysts composed of a combination of RNA and proteins. Nevertheless, there is a reduced number of small ribozymes, the self-cleavage ribozymes, that are formed just by RNA and, apparently, they existed in cells of primitive biological systems. Unveiling the details of these “fossils” enzymes can contribute not only to the understanding of the origins of life but also to the development of new simplified artificial enzymes. A computational study of the reactivity of the pistol ribozyme carried out by means of classical MD simulations and QM/MM hybrid calculations is herein presented to clarify its catalytic mechanism. Analysis of the geometries along independent MD simulations with different protonation states of the active site basic species reveals that only the canonical system, with no additional protonation changes, renders reactive conformations. A change in the coordination sphere of the Mg2+ ion has been observed during the simulations, which allows proposing a mechanism to explain the unique mode of action of the pistol ribozyme by comparison with other ribozymes. The present results are at the center of the debate originated from recent experimental and theoretical studies on pistol ribozyme.

Biosensing with DNAzymes

This article provides a comprehensive review of biosensing with DNAzymes, providing an overview of different sensing applications while highlighting major progress and seminal contributions to the field of portable biosensor devices and point-of-care diagnostics. Specifically, the field of functional nucleic acids is introduced, with a specific focus on DNAzymes. The incorporation of DNAzymes into bioassays is then described, followed by a detailed overview of recent advances in the development of in vivo sensing platforms and portable sensors incorporating DNAzymes for molecular recognition. Finally, a critical perspective on the field, and a summary of where DNAzyme-based devices may make the biggest impact are provided.

Hydrated metal ion as a general acid in the catalytic mechanism of the 8-17 DNAzyme

The RNA-cleaving 8-17 DNAzyme, which is a metalloenzyme that depends on divalent metal ions for its function, is the most studied catalytic DNA in terms of its mechanism. By the end of 2017, a report of the crystal structure of the enzyme-substrate complex in the presence of Pb²⁺ probed some of the previous findings and opened new questions, especially around the participation of the metal ion in the catalytic mechanism and the promiscuity exhibited by the enzyme in terms of the metal cofactor required for catalysis. In this article we explore the role of the divalent metal ion in the mechanism of the 8-17 DNAzyme as a general acid, by measuring the influence of pH over the activity of a slower variant of the enzyme in the presence of Pb²⁺. We replaced G14, which has been identified as a general base in the mechanism of the enzyme, by the unnatural analog 2-aminopurine, with a lower pK_a value of the N1 group. With this approach, we obtained a bell-shaped pH-rate profile with experimental pK_a values of 5.4 and 7.0. Comparing these results with previous pH-rate profiles in the presence of Mg²⁺, our findings suggest the stabilization of the 5'-O leaving group by the hydrated metal ion acting as a general acid, in addition to the activation of the 2'-OH nucleophile by the general base G14.

Probing Metal-Dependent Phosphate Binding for the Catalysis of the 17E DNAzyme

The RNA-cleaving 17E DNAzyme exhibits different levels of cleavage activity in the presence of various divalent metal ions, with Pb²⁺ giving the fastest cleavage. In this study, the metal-phosphate interaction is probed to understand the trend of activity with different metal ions. For the first-row transition metals, the lowest activity shown by Ni²⁺ correlates with the inhibition by the inorganic phosphate and its water ligand exchange rate, suggesting inner-sphere metal coordination. Cleavage activity with the two stereoisomers of the phosphorothioate-modified substrates, R_p and S_p, indicated that Mg²⁺, Mn²⁺, Fe²⁺, and Co²⁺ had the highest S_p:R_p activity ratio of >900. Comparatively, the activity was much less affected using the thiophilic metals, including Pb²⁺, suggesting inner-sphere coordination. The pH-rate profiles showed that Pb²⁺ was different than the rest of the metal ions in having a smaller slope and a similar fitted apparent pK_a and the pK_a of metal-bound water. Combining previous reports and our current results, we propose that Pb²⁺ most likely plays the role of a general acid while the other metal ions are Lewis acid catalysts interacting with the scissile phosphate.

Opportunities for innovation: Building on the success of lipid nanoparticle vaccines

Lipid nanoparticle (LNP) formulations of messenger RNA (mRNA) have demonstrated high efficacy as vaccines against SARS-CoV-2. The success of these nanoformulations underscores the potential of LNPs as a delivery system for next-generation biological therapies. In this article, we highlight the key considerations necessary for engineering LNPs as a vaccine delivery system and explore areas for further optimisation. There remain opportunities to improve the protection of mRNA, optimise cytosolic delivery, target specific cells, minimise adverse side-effects and control the release of RNA from the particle. The modular nature of LNP formulations and the flexibility of mRNA as a payload provide many pathways to implement these strategies. Innovation in LNP vaccines is likely to accelerate with increased enthusiasm following recent successes; however, any advances will have implications for a broad range of therapeutic applications beyond vaccination such as gene therapy.

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.

Site-Selective Artificial Ribonucleases: Renaissance of Oligonucleotide Conjugates for Irreversible Cleavage of RNA Sequences

RNA-targeting therapeutics require highly efficient sequence-specific devices capable of RNA irreversible degradation in vivo. The most developed methods of sequence-specific RNA cleavage, such as siRNA or antisense oligonucleotides (ASO), are currently based on recruitment of either intracellular multi-protein complexes or enzymes, leaving alternative approaches (e.g., ribozymes and DNAzymes) far behind. Recently, site-selective artificial ribonucleases combining the oligonucleotide recognition motifs (or their structural analogues) and catalytically active groups in a single molecular scaffold have been proven to be a great competitor to siRNA and ASO. Using the most efficient catalytic groups, utilising both metal ion-dependent (Cu(II)-2,9-dimethylphenanthroline) and metal ion-free (Tris(2-aminobenzimidazole)) on the one hand and PNA as an RNA recognising oligonucleotide on the other, allowed site-selective artificial RNases to be created with half-lives of 0.5-1 h. Artificial RNases based on the catalytic peptide [(ArgLeu)₂Gly]₂ were able to take progress a step further by demonstrating an ability to cleave miRNA-21 in tumour cells and provide a significant reduction of tumour growth in mice.

Biochemical analysis of cleavage and ligation activities of the pistol ribozyme from Paenibacillus polymyxa

Nine distinct classes of self-cleaving ribozymes are known to date, of which the pistol ribozyme class was discovered only 5 years ago. Self-cleaving ribozymes are able to cleave their own phosphodiester backbone at a specific site with rates much higher than those of spontaneous RNA degradation. Our study focuses on a bioinformatically predicted pistol ribozyme from the bacterium Paenibacillus polymyxa. We provide a biochemical characterization of this ribozyme, which includes an investigation of the effect of various metal ions on ribozyme cleavage and a kinetic analysis of ribozyme activity under increasing Mg²⁺ concentrations and pH. Based on the obtained results, we discuss a possible catalytic role of divalent metal ions. Moreover, we investigated the ligation activity of the P. polymyxa pistol ribozyme - an aspect that has not been previously analysed for this ribozyme class. We determined that the P. polymyxa pistol ribozyme is almost fully cleaved at equilibrium with the ligation rate constant being nearly 30-fold lower than the cleavage rate constant. In summary, we have characterized an additional representative of this recently discovered ribozyme class isolated from P. polymyxa. We expect that our biochemical characterization of a pistol representative in a cultivatable, genetically tractable organism will support our future investigation of the biological roles of this ribozyme class in bacteria.

Fundamental studies of functional nucleic acids: aptamers, riboswitches, ribozymes and DNAzymes

This review aims at juxtaposing common versus distinct structural and functional strategies that are applied by aptamers, riboswitches, and ribozymes/DNAzymes. Focusing on recently discovered systems, we begin our analysis with small-molecule binding aptamers, with emphasis on in vitro-selected fluorogenic RNA aptamers and their different modes of ligand binding and fluorescence activation. Fundamental insights are much needed to advance RNA imaging probes for detection of exo- and endogenous RNA and for RNA process tracking. Secondly, we discuss the latest gene expression-regulating mRNA riboswitches that respond to the alarmone ppGpp, to PRPP, to NAD⁺, to adenosine and cytidine diphosphates, and to precursors of thiamine biosynthesis (HMP-PP), and we outline new subclasses of SAM and tetrahydrofolate-binding RNA regulators. Many riboswitches bind protein enzyme cofactors that, in principle, can catalyse a chemical reaction. For RNA, however, only one system (glmS ribozyme) has been identified in Nature thus far that utilizes a small molecule - glucosamine-6-phosphate - to participate directly in reaction catalysis (phosphodiester cleavage). We wonder why that is the case and what is to be done to reveal such likely existing cellular activities that could be more diverse than currently imagined. Thirdly, this brings us to the four latest small nucleolytic ribozymes termed twister, twister-sister, pistol, and hatchet as well as to in vitro selected DNA and RNA enzymes that promote new chemistry, mainly by exploiting their ability for RNA labelling and nucleoside modification recognition. Enormous progress in understanding the strategies of nucleic acids catalysts has been made by providing thorough structural fundaments (e.g. first structure of a DNAzyme, structures of ribozyme transition state mimics) in combination with functional assays and atomic mutagenesis.

Imaginary Ribozymes

Biocatalysis is dominated by protein enzymes, and only a few classes of ribozymes are known to contribute to the task of promoting biochemical transformations. The RNA World theory encompasses the notion that earlier forms of life made use of a much greater diversity of ribozymes and other functional RNAs to guide complex metabolic states long before proteins had emerged in evolution. In recent years, the discoveries of various classes of ribozymes, riboswitches, and other noncoding RNAs in bacteria have provided additional support for the hypothesis that RNA molecules indeed have the catalytic competence to promote diverse chemical reactions without the aid of protein enzymes. Herein, some of the most striking observations made from examinations of natural riboswitches that bind small ligands are highlighted and used as a basis to imagine the characteristics and functions of long-extinct ribozymes from the RNA World.

Molecular Features and Metal Ions That Influence 10-23 DNAzyme Activity

Deoxyribozymes (DNAzymes) with RNA hydrolysis activity have a tremendous potential as gene suppression agents for therapeutic applications. The most extensively studied representative is the 10-23 DNAzyme consisting of a catalytic loop and two substrate binding arms that can be designed to bind and cleave the RNA sequence of interest. The RNA substrate is cleaved between central purine and pyrimidine nucleotides. The activity of this DNAzyme in vitro is considerably higher than in vivo, which was suggested to be related to its divalent cation dependency. Understanding the mechanism of DNAzyme catalysis is hindered by the absence of structural information. Numerous biological studies, however, provide comprehensive insights into the role of particular deoxynucleotides and functional groups in DNAzymes. Here we provide an overview of the thermodynamic properties, the impact of nucleobase modifications within the catalytic loop, and the role of different metal ions in catalysis. We point out features that will be helpful in developing novel strategies for structure determination and to understand the mechanism of the 10-23 DNAzyme. Consideration of these features will enable to develop improved strategies for structure determination and to understand the mechanism of the 10-23 DNAzyme. These insights provide the basis for improving activity in cells and pave the way for developing DNAzyme applications.

Molecular dynamics analysis of Mg2+ -dependent cleavage of a pistol ribozyme reveals a fail-safe secondary ion for catalysis

Pistol ribozymes comprise a class of small, self-cleaving RNAs discovered via comparative genomic analysis. Prior work in the field has probed the kinetics of the cleavage reaction, as well as the influence of various metal ion cofactors that accelerate the process. In the current study, we performed unbiased and unconstrained molecular dynamics simulations from two current high-resolution pistol crystal structures, and we analyzed trajectory data within the context of the currently accepted ribozyme mechanistic framework. Root-mean-squared deviations, radial distribution functions, and distributions of nucleophilic angle-of-attack reveal insights into the potential roles of three magnesium ions with respect to catalysis and overall conformational stability of the molecule. A series of simulation trajectories containing in silico mutations reveal the relatively flexible and partially interchangeable roles of two particular magnesium ions within solvated hydrogen-bonding distances from the catalytic center.

The L-platform/L-scaffold framework: a blueprint for RNA-cleaving nucleic acid enzyme design

We develop an L-platform/L-scaffold framework we hypothesize may serve as a blueprint to facilitate site-specific RNA-cleaving nucleic acid enzyme design. Building on the L-platform motif originally described by Suslov and coworkers, we identify new critical scaffolding elements required to anchor a conserved general base guanine ("L-anchor") and bind functionally important metal ions at the active site ("L-pocket"). Molecular simulations, together with a broad range of experimental structural and functional data, connect the L-platform/L-scaffold elements to necessary and sufficient conditions for catalytic activity. We demonstrate that the L-platform/L-scaffold framework is common to five of the nine currently known naturally occurring ribozyme classes (Twr, HPr, VSr, HHr, Psr), and intriguingly from a design perspective, the framework also appears in an artificially engineered DNAzyme (8-17dz). The flexibility of the L-platform/L-scaffold framework is illustrated on these systems, highlighting modularity and trends in the variety of known general acid moieties that are supported. These trends give rise to two distinct catalytic paradigms, building on the classifications proposed by Wilson and coworkers and named for the implicated general base and acid. The "G + A" paradigm (Twr, HPr, VSr) exclusively utilizes nucleobase residues for chemistry, and the "G + M + " paradigm (HHr, 8-17dz, Psr) involves structuring of the "L-pocket" metal ion binding site for recruitment of a divalent metal ion that plays an active role in the chemical steps of the reaction. Finally, the modularity of the L-platform/L-scaffold framework is illustrated in the VS ribozyme where the "L-pocket" assumes the functional role of the "L-anchor" element, highlighting a distinct mechanism, but one that is functionally linked with the hammerhead ribozyme.

Confluence of theory and experiment reveals the catalytic mechanism of the Varkud satellite ribozyme

The Varkud satellite ribozyme catalyses site-specific RNA cleavage and ligation, and serves as an important model system to understand RNA catalysis. Here, we combine stereospecific phosphorothioate substitution, precision nucleobase mutation and linear free-energy relationship measurements with molecular dynamics, molecular solvation theory and ab initio quantum mechanical/molecular mechanical free-energy simulations to gain insight into the catalysis. Through this confluence of theory and experiment, we unify the existing body of structural and functional data to unveil the catalytic mechanism in unprecedented detail, including the degree of proton transfer in the transition state. Further, we provide evidence for a critical Mg²⁺ in the active site that interacts with the scissile phosphate and anchors the general base guanine in position for nucleophile activation. This novel role for Mg²⁺ adds to the diversity of known catalytic RNA strategies and unifies functional features observed in the Varkud satellite, hairpin and hammerhead ribozyme classes.

Insights into DNA catalysis from structural and functional studies of the 8-17 DNAzyme

The review examines functional knowledge gathered over two decades of research on the 8-17 DNAzyme, focusing on three aspects: the structural requirements for catalysis, the role of metal ions and the participation of general acid-base catalysis.

Systematic Screening, Rational Development, and Initial Optimization of Efficacious RNA Silencing Agents for Human Rod Opsin Therapeutics

Purpose

To systematically evaluate human rod opsin (hRHO) mRNA for potential target sites sensitive to posttranscriptional gene silencing (PTGS) by hammerhead ribozyme (hhRz) or RNA interference (RNAi) in human cells. To develop a comprehensive strategy to identify and optimize lead candidate agents for PTGS gene therapeutics.

Methods

In multidisciplinary RNA drug discovery, computational mRNA accessibility and in vitro experimental methods using reverse transcription–polymerase chain reaction (RT-PCR) were used to map accessibility in full-length hRHO transcripts. HhRzs targeted predicted accessible and inaccessible sites and were screened for cellular knockdown using a bicistronic reporter construct. Lead hhRz and RNAi PTGS agents were rationally optimized for target knockdown in human cells.

Results

Systematic screening of hRHO mRNA targeting agents resulted in lead candidate identification of a novel hhRz embedded in an RNA scaffold. Rational optimization strategies identified a minimal 725 hhRz as the most active agent. Recently identified tertiary accessory elements did not enhance activity. A 725-short-hairpin RNA (shRNA) agent exerts log-order knockdown. Silent modulation of the 725-hhRz target site in hRHO mRNA resulted in resistance to knockdown.

Conclusions

Combining rational RNA drug design with cell-based screening allowed rapid identification of lead agents targeting hRHO. Optimization strategies identified the agent with highest intracellular activity. These agents have therapeutic potential in a mutation-independent strategy for adRP, or other degenerations where hRHO is a target. This approach can be broadly applied to any validated target mRNA, regardless of the disease.

Translational Relevance

This work establishes a platform approach to develop RNA biologicals for the treatment of human disease.

Evidence for a Catalytic Strategy to Promote Nucleophile Activation in Metal-Dependent RNA-Cleaving Ribozymes and 8-17 DNAzyme

An unique catalytic strategy was recently reported for the glmS ribozyme [Bingaman et al., Nat. Chem. Biol.2017, 13, 439-445] that involves promotion of productive hydrogen bonding of the O2' nucleophile to facilitate its activation. We provide broad evidence of this strategy in the hammerhead, pistol, and VS ribozymes and 8-17 DNAzyme, enabled by a functionally important divalent metal ion that interacts with the scissile phosphate and disrupts nonproductive competitive hydrogen bonding with the O2' nucleophile. This strategy, designated tertiary gamma (3°γ) catalysis, illustrates an additional role for divalent ions in ribozyme catalysis.

Small Molecule Rescue and Glycosidic Conformational Analysis of the Twister Ribozyme

The number of self-cleaving ribozymes has increased sharply in recent years, giving rise to elaborations of the four known ribozyme catalytic strategies, α, β, γ, and δ. One such extension is utilized by the twister ribozyme, which is hypothesized to conduct δ, or general acid catalysis, via N3 of the syn adenine +1 nucleobase indirectly via buffer catalysis at biological pH and directly at lower pH. Herein, we test the δ catalysis role of A1 via chemical rescue and the catalytic relevance of the syn orientation of the nucleobase by conformational analysis. Using inhibited twister ribozyme variants with A1(N3) deaza or A1 abasic modifications, we observe >100-fold chemical rescue effects in the presence of protonatable biological small molecules such as imidazole and histidine, similar to observed rescue values previously reported for C75U/C76Δ in the HDV ribozyme. Brønsted plots for the twister variants support a model in which small molecules rescue catalytic activity via a proton transfer mechanism, suggesting that A1 in the wild type is involved in proton transfer, most likely general acid catalysis. Additionally, through glycosidic conformational analysis in an appropriate background that accommodates the bromine atom, we observe that an 8BrA1-modified twister ribozyme is up to 10-fold faster than a nonmodified A1 ribozyme, supporting crystallographic data that show that A1 is syn when conducting proton transfer. Overall, this study provides functional evidence that the nucleotide immediately downstream of the cleavage site participates directly or indirectly in general acid-base catalysis in the twister ribozyme while occupying the syn conformation.

Dynamical ensemble of the active state and transition state mimic for the RNA-cleaving 8-17 DNAzyme in solution

We perform molecular dynamics simulations, based on recent crystallographic data, on the 8-17 DNAzyme at four states along the reaction pathway to determine the dynamical ensemble for the active state and transition state mimic in solution. A striking finding is the diverse roles played by Na+ and Pb2+ ions in the electrostatically strained active site that impact all four fundamental catalytic strategies, and share commonality with some features recently inferred for naturally occurring hammerhead and pistol ribozymes. The active site Pb2+ ion helps to stabilize in-line nucleophilic attack, provides direct electrostatic transition state stabilization, and facilitates leaving group departure. A conserved guanine residue is positioned to act as the general base, and is assisted by a bridging Na+ ion that tunes the pKa and facilitates in-line fitness. The present work provides insight into how DNA molecules are able to solve the RNA-cleavage problem, and establishes functional relationships between the mechanism of these engineered DNA enzymes with their naturally evolved RNA counterparts. This adds valuable information to our growing body of knowledge on general mechanisms of phosphoryl transfer reactions catalyzed by RNA, proteins and DNA.

Molecular simulations of the pistol ribozyme: unifying the interpretation of experimental data and establishing functional links with the hammerhead ribozyme

The pistol ribozyme (Psr) is among the most recently discovered RNA enzymes and has been the subject of experiments aimed at elucidating the mechanism. Recent biochemical studies have revealed exciting clues about catalytic interactions in the active site not apparent from available crystallographic data. The present work unifies the interpretation of the existing body of structural and functional data on Psr by providing a dynamical model for the catalytically active state in solution from molecular simulation. Our results suggest that a catalytic Mg2+ ion makes inner-sphere contact with G33:N7 and outer-sphere coordination to the pro-RP of the scissile phosphate, promoting electrostatic stabilization of the dianionic transition state and neutralization of the developing charge of the leaving group through a metal-coordinated water molecule that is made more acidic by a hydrogen bond donated from the 2'OH of P32. This model is consistent with experimental activity-pH and mutagenesis data, including sensitivity to G33(7cG) and phosphorothioate substitution/metal ion rescue. The model suggests several experimentally testable predictions, including the response of cleavage activity to mutations at G42 and P32 positions in the ribozyme, and thio substitutions of the substrate in the presence of different divalent metal ions. Further, the model identifies striking similarities of Psr to the hammerhead ribozyme (HHr), including similar global fold, organization of secondary structure around an active site three-way junction, catalytic metal ion binding mode, and guanine general base. However, the specific binding mode and role of the Mg2+ ion, as well as a conserved 2'-OH in the active site, are interrelated but subtly different between the ribozymes.

Novel ribozymes: discovery, catalytic mechanisms, and the quest to understand biological function

Small endonucleolytic ribozymes promote the self-cleavage of their own phosphodiester backbone at a specific linkage. The structures of and the reactions catalysed by members of individual families have been studied in great detail in the past decades. In recent years, bioinformatics studies have uncovered a considerable number of new examples of known catalytic RNA motifs. Importantly, entirely novel ribozyme classes were also discovered, for most of which both structural and biochemical information became rapidly available. However, for the majority of the new ribozymes, which are found in the genomes of a variety of species, a biological function remains elusive. Here, we concentrate on the different approaches to find catalytic RNA motifs in sequence databases. We summarize the emerging principles of RNA catalysis as observed for small endonucleolytic ribozymes. Finally, we address the biological functions of those ribozymes, where relevant information is available and common themes on their cellular activities are emerging. We conclude by speculating on the possibility that the identification and characterization of proteins that we hypothesize to be endogenously associated with catalytic RNA might help in answering the ever-present question of the biological function of the growing number of genomically encoded, small endonucleolytic ribozymes.

Reinventing the Wheel: Synthetic Circular RNAs for Mammalian Cell Engineering

The circular RNA renaissance is upon us. Recent reports demonstrate applications of synthetic circular RNA molecules as gene therapies and in the production of biologics from cell-based expression systems. Circular RNAs are covalently closed loop RNA species that are formed naturally through noncolinear splicing of pre-mRNA. Although once thought to be noncoding artefacts from splicing errors, it is now accepted that circular RNAs are abundant and have diverse functions in gene regulation and protein coding in eukaryotes. Numerous reports have investigated circular RNAs in various diseases, but the promise of synthetic circular RNAs in the production of recombinant proteins and as RNA-based therapies is only now coming into focus. This review highlights reported uses of synthetic circular RNAs and describes methods for generating these molecules.

Cleaning Up Mechanistic Debris Generated by Twister Ribozymes Using Computational RNA Enzymology

The catalytic properties of RNA have been a subject of fascination and intense research since their discovery over 30 years ago. Very recently, several classes of nucleolytic ribozymes have emerged and been characterized structurally. Among these, the twister ribozyme has been center-stage, and a topic of debate about its architecture and mechanism owing to conflicting interpretations of different crystal structures, and in some cases conflicting interpretations of the same functional data. In the present work, we attempt to clean up the mechanistic "debris" generated by twister ribozymes using a comprehensive computational RNA enzymology approach aimed to provide a unified interpretation of existing structural and functional data. Simulations in the crystalline environment and in solution provide insight into the origins of observed differences in crystal structures, and coalesce on a common active site architecture, and dynamical ensemble in solution. We use GPU-accelerated free energy methods with enhanced sampling to ascertain microscopic nucleobase pK a values of the implicated general acid and base, from which predicted activity-pH profiles can be compared directly with experiments. Next, ab initio quantum mechanical/molecular mechanical (QM/MM) simulations with full dynamic solvation under periodic boundary conditions are used to determine mechanistic pathways through multi-dimensional free energy landscapes for the reaction. We then characterize the rate-controlling transition state, and make predictions about kinetic isotope effects and linear free energy relations. Computational mutagenesis is performed to explain the origin of rate effects caused by chemical modifications and make experimentally testable predictions. Finally, we provide evidence that helps to resolve conflicting issues related to the role of metal ions in catalysis. Throughout each stage, we highlight how a conserved L-platform structural motif, to- gether with a key L-anchor residue, forms the characteristic active site scaffold enabling each of the catalytic strategies to come together not only for the twister ribozyme, but the majority of the known small nucleolytic ribozyme classes.

An Ontology for Facilitating Discussion of Catalytic Strategies of RNA-Cleaving Enzymes

A predictive understanding of the mechanisms of RNA cleavage is important for the design of emerging technology built from biological and synthetic molecules that have promise for new biochemical and medicinal applications. Over the past 15 years, RNA cleavage reactions involving 2'-O-transphosphorylation have been discussed using a simplified framework introduced by Breaker that consists of four fundamental catalytic strategies (designated α, β, γ, and δ) that contribute to rate enhancement. As more detailed mechanistic data emerge, there is need for the framework to evolve and keep pace. We develop an ontology for discussion of strategies of enzymes that catalyze RNA cleavage via 2'-O-transphosphorylation that stratifies Breaker's framework into primary (1°), secondary (2°), and tertiary (3°) contributions to enable more precise interpretation of mechanism in the context of structure and bonding. Further, we point out instances where atomic-level changes give rise to changes in more than one catalytic contribution, a phenomenon we refer to as "functional blurring". We hope that this ontology will help clarify our conversations and pave the path forward toward a consensus view of these fundamental and fascinating mechanisms. The insight gained will deepen our understanding of RNA cleavage reactions catalyzed by natural protein and RNA enzymes, as well as aid in the design of new engineered DNA and synthetic enzymes.

Highly efficient expression of circular RNA aptamers in cells using autocatalytic transcripts

RNA aptamers and RNA aptamer-based devices can be genetically encoded and expressed in cells to probe and manipulate cellular function. However, their usefulness in the mammalian cell is limited by low expression and rapid degradation. Here we describe the Tornado (Twister-optimized RNA for durable overexpression) expression system for achieving rapid RNA circularization, resulting in RNA aptamers with high stability and expression levels. Tornado-expressed transcripts contain an RNA of interest flanked by Twister ribozymes. The ribozymes rapidly undergo autocatalytic cleavage, leaving termini that are ligated by the ubiquitous endogenous RNA ligase RtcB. Using this approach, protein-binding aptamers that otherwise have minimal effects in cells become potent inhibitors of cellular signaling. Additionally, an RNA-based fluorescent metabolite biosensor for S-adenosyl methionine (SAM) that is expressed at low levels when expressed as a linear RNA achieves levels sufficient for detection of intracellular SAM dynamics when expressed as a circular RNA. The Tornado expression system thus markedly enhances the utility of RNA-based approaches in the mammalian cell.

Hatchet ribozyme structure and implications for cleavage mechanism

Self-cleaving ribozymes are RNAs that catalyze position-specific cleavage of their phosphodiester backbone. The cleavage site of the newly discovered hatchet ribozyme is located at the very 5′ end of its consensus secondary structure motif. Here we report on the 2.1-Å crystal structure of the hatchet ribozyme in the product state, which defines its intricate tertiary fold and identifies key residues lining the catalytic pocket. This in turn has allowed us to propose a model of the precatalytic state structure and a role in catalysis for a conserved guanine. This study therefore provides a structure-based platform toward an improved understanding of the catalytic mechanism of hatchet ribozymes.

Small self-cleaving ribozymes catalyze site-specific cleavage of their own phosphodiester backbone with implications for viral genome replication, pre-mRNA processing, and alternative splicing. We report on the 2.1-Å crystal structure of the hatchet ribozyme product, which adopts a compact pseudosymmetric dimeric scaffold, with each monomer stabilized by long-range interactions involving highly conserved nucleotides brought into close proximity of the scissile phosphate. Strikingly, the catalytic pocket contains a cavity capable of accommodating both the modeled scissile phosphate and its flanking 5′ nucleoside. The resulting modeled precatalytic conformation incorporates a splayed-apart alignment at the scissile phosphate, thereby providing structure-based insights into the in-line cleavage mechanism. We identify a guanine lining the catalytic pocket positioned to contribute to cleavage chemistry. The functional relevance of structure-based insights into hatchet ribozyme catalysis is strongly supported by cleavage assays monitoring the impact of selected nucleobase and atom-specific mutations on ribozyme activity.

Transesterification Reaction and the Repair of Embedded Ribonucleotides in DNA Are Suppressed upon the Assembly of DNA into Nucleosome Core Particles †

Ribonucleotides can be incorporated into DNA through many different cellular processes, and abundant amounts of ribonucleotides are detected in genomic DNA. Embedded ribonucleotides lead to genomic instability through either spontaneous ribonucleotide cleavage via internal transesterification or by inducing mutagenesis, recombination, and chromosome rearrangements. Ribonucleotides misincorporated in genomic DNA can be removed by the ribonucleotide excision repair (RER) pathway in which RNase HII initiates the repair by cleaving the 5'-phosphate of the ribonucleotide. Herein, based on in vitro reconstituted nucleosome core particles (NCPs) containing a single ribonucleotide at different positions, we studied the kinetics of ribonucleotide cleavage via the internal transesterification reaction and repair of the ribonucleotides by RNase HII in NCPs. Our results show that ribonucleotide cleavage via the internal transesterification in NCPs is suppressed compared to that in free DNA. DNA bending and structural rigidity account for the suppressed ribonucleotide cleavage in NCPs. Ribonucleotide repair by RNase HII in NCPs exhibits a strong correlation between the translational and rotational positions of the ribonucleotides. An embedded ribonucleotide located at the entry site while facing outward in NCP is repaired as efficiently as that in free DNA. However, the repair of those located in the central part of NCPs and facing inward are inhibited by up to 273-fold relative to those in free dsDNA. The difference in repair efficiency appears to arise from their different accessibility to repair enzymes in NCPs. This study reveals that a ribonucleotide misincorporated in DNA assembled into NCPs is protected against cleavage. Hence, the spontaneous cleavage of the misincorporated ribonucleotides under physiological conditions is not an essential threat to the stability of chromatin DNA. Instead, their decreased repair efficiency in NCPs may result in numerous and persistent ribonucleotides in genomic DNA, which could exert other deleterious effects on DNA such as mutagenesis and recombination.