A riboswitch selective for the queuosine precursor preQ1 contains an unusually small aptamer domain

Chemotranscriptomic profiling with a thiamine monophosphate photoaffinity probe

RNA is a multifaceted biomolecule with numerous biological functions and can interact with small molecule metabolites as exemplified by riboswitches. Here, we profile the Escherichia coli transcriptome on interactions with the metabolite Thiamine Monophosphate (TMP). We designed and synthesized a photoaffinity probe based on the scaffold of TMP and applied it to chemotranscriptomic profiling. Using next-generation RNA sequencing, several potential interactions between bacterial transcripts and the probe were identified. A remarkable interaction between the TMP probe and the well-characterized Flavin Mononucleotide (FMN) riboswitch was validated by RT-qPCR, and further verified with competition assays. Localization of the photocrosslinked nucleotides using reverse transcription and docking predictions of the probe suggested binding to the riboswitch aptamer. After examining binding of unmodified TMP to the riboswitch using SHAPE, we found selective yet moderate binding interactions, potentially mediated by the phosphate group of TMP. Lastly, TMP appeared to enhance gene expression of a reporter gene that is under riboswitch control, while the natural ligand FMN displayed an inhibitory effect, hinting at a potential biological role of TMP. This work showcases the possibility of chemotranscriptomic profiling to identify new RNA-small molecule interactions.

Environmental Control of Queuosine Levels in Streptococcus mutans tRNAs

Queuosine (Q) is a modification of the wobble base in tRNAs that decode NA(C/U) codons. It is ubiquitous in bacteria, including many pathogens. Streptococcus mutans is an early colonizer of dental plaque biofilm and a key player in dental caries. Using a combination of genetic and physiological approaches, the predicted Q synthesis and salvage pathways were validated in this organism. These experiments confirmed that S. mutans can synthesize Q de novo through similar pathways found in Bacillus subtilis and Escherichia coli. However, S. mutans has a distinct salvage pathway compared to these model organisms, as it uses a transporter belonging to the energy coupling factor (ECF) family controlled by a preQ1-dependent riboswitch. Furthermore, Q levels in this oral pathogen depended heavily on the media composition, suggesting that micronutrients can affect Q-mediated translation efficiency.

Two riboswitch classes that share a common ligand-binding fold show major differences in the ability to accommodate mutations

Riboswitches are structured RNAs that sense small molecules to control expression. Prequeuosine1 (preQ1)-sensing riboswitches comprise three classes (I, II and III) that adopt distinct folds. Despite this difference, class II and III riboswitches each use 10 identical nucleotides to bind the preQ1 metabolite. Previous class II studies showed high sensitivity to binding-pocket mutations, which reduced preQ1 affinity and impaired function. Here, we introduced four equivalent mutations into a class III riboswitch, which maintained remarkably tight preQ1 binding. Co-crystal structures of each class III mutant showed compensatory interactions that preserve the fold. Chemical modification analysis revealed localized RNA flexibility changes for each mutant, but molecular dynamics (MD) simulations suggested that each mutation was not overtly destabilizing. Although impaired, class III mutants retained tangible gene-regulatory activity in bacteria compared to equivalent preQ1-II variants; mutations in the preQ1-pocket floor were tolerated better than wall mutations. Principal component analysis of MD trajectories suggested that the most functionally deleterious wall mutation samples different motions compared to wildtype. Overall, the results reveal that formation of compensatory interactions depends on the context of mutations within the overall fold and that functionally deleterious mutations can alter long-range correlated motions that link the riboswitch binding pocket with distal gene-regulatory sequences.

Knotty is nice: Metabolite binding and RNA-mediated gene regulation by the preQ1 riboswitch family

Riboswitches sense specific cellular metabolites, leading to messenger RNA conformational changes that regulate downstream genes. Here, we review the three known prequeosine1 (preQ1) riboswitch classes, which encompass five gene-regulatory motifs derived from distinct consensus models of folded RNA pseudoknots. Structural and functional analyses reveal multiple gene-regulation strategies ranging from partial occlusion of the ribosome-binding Shine-Dalgarno sequence (SDS), SDS sequestration driven by kinetic or thermodynamic folding pathways, direct preQ1 recognition by the SDS, and complete SDS burial with in the riboswitch architecture. Family members can also induce elemental transcriptional pausing, which depends on ligand-mediated pseudoknot formation. Accordingly, preQ1 family members provide insight into a wide range of gene-regulatory tactics as well as a diverse repertoire of chemical approaches used to recognize the preQ1 metabolite. From a broader perspective, future challenges for the field will include the identification of new riboswitches in mRNAs that do not possess an SDS or those that induce ligand-dependent transcriptional pausing. When choosing an antibacterial target, the field must also consider how well a riboswitch accommodates mutations. Investigation of riboswitches in their natural context will also be critical to elucidate how RNA-mediated gene regulation influences organism fitness, thus providing a firm foundation for antibiotic development.

The current riboswitch landscape in Clostridioides difficile

Riboswitches are 5' RNA regulatory elements that are capable of binding to various ligands, such as small metabolites, ions and tRNAs, leading to conformational changes and affecting gene transcription or translation. They are widespread in bacteria and frequently control genes that are essential for the survival or virulence of major pathogens. As a result, they represent promising targets for the development of new antimicrobial treatments. Clostridioides difficile, a leading cause of antibiotic-associated nosocomial diarrhoea in adults, possesses numerous riboswitches in its genome. Accumulating knowledge of riboswitch-based regulatory mechanisms provides insights into the potential therapeutic targets for treating C. difficile infections. This review offers an in-depth examination of the current state of knowledge regarding riboswitch-mediated regulation in C. difficile, highlighting their importance in bacterial adaptability and pathogenicity. Particular attention is given to the ligand specificity and function of known riboswitches in this bacterium. The review also discusses the recent progress that has been made in the development of riboswitch-targeting compounds as potential treatments for C. difficile infections. Future research directions are proposed, emphasizing the need for detailed structural and functional analyses of riboswitches to fully harness their regulatory capabilities for developing new antimicrobial strategies.

Mechanistic analysis of Riboswitch Ligand interactions provides insights into pharmacological control over gene expression

Riboswitches are structured RNA elements that regulate gene expression upon binding to small molecule ligands. Understanding the mechanisms by which small molecules impact riboswitch activity is key to developing potent, selective ligands for these and other RNA targets. We report the structure-informed design of chemically diverse synthetic ligands for PreQ1 riboswitches. Multiple X-ray co-crystal structures of synthetic ligands with the Thermoanaerobacter tengcongensis (Tte)-PreQ1 riboswitch confirm a common binding site with the cognate ligand, despite considerable chemical differences among the ligands. Structure probing assays demonstrate that one ligand causes conformational changes similar to PreQ1 in six structurally and mechanistically diverse PreQ1 riboswitch aptamers. Single-molecule force spectroscopy is used to demonstrate differential modes of riboswitch stabilization by the ligands. Binding of the natural ligand brings about the formation of a persistent, folded pseudoknot structure, whereas a synthetic ligand decreases the rate of unfolding through a kinetic mechanism. Single round transcription termination assays show the biochemical activity of the ligands, while a GFP reporter system reveals compound activity in regulating gene expression in live cells without toxicity. Taken together, this study reveals that diverse small molecules can impact gene expression in live cells by altering conformational changes in RNA structures through distinct mechanisms.

Structure-based virtual screening of unbiased and RNA-focused libraries to identify new ligands for the HCV IRES model system

Targeting RNA including viral RNAs with small molecules is an emerging field. The hepatitis C virus internal ribosome entry site (HCV IRES) is a potential target for translation inhibitor development to raise drug resistance mutation preparedness. Using RNA-focused and unbiased molecule libraries, a structure-based virtual screening (VS) by molecular docking and pharmacophore analysis was performed against the HCV IRES subdomain IIa. VS hits were validated by a microscale thermophoresis (MST) binding assay and a Förster resonance energy transfer (FRET) assay elucidating ligand-induced conformational changes. Ten hit molecules were identified with potencies in the high to medium micromolar range proving the suitability of structure-based virtual screenings against RNA-targets. Hit compounds from a 2-guanidino-quinazoline series, like the strongest binder, compound 8b with an EC50 of 61 μM, show low molecular weight, moderate lipophilicity and reduced basicity compared to previously reported IRES ligands. Therefore, it can be considered as a potential starting point for further optimization by chemical derivatization.

RNA-Small-Molecule Interaction: Challenging the "Undruggable" Tag

RNA targeting, specifically with small molecules, is a relatively new and rapidly emerging avenue with the promise to expand the target space in the drug discovery field. From being "disregarded" as an "undruggable" messenger molecule to FDA approval of an RNA-targeting small-molecule drug Risdiplam, a radical change in perspective toward RNA has been observed in the past decade. RNAs serve important regulatory functions beyond canonical protein synthesis, and their dysregulation has been reported in many diseases. A deeper understanding of RNA biology reveals that RNA molecules can adopt a variety of structures, carrying defined binding pockets that can accommodate small-molecule drugs. Due to its functional diversity and structural complexity, RNA can be perceived as a prospective target for therapeutic intervention. This perspective highlights the proof of concept of RNA-small-molecule interactions, exemplified by targeting of various transcripts with functional modulators. The advent of RNA-oriented knowledge would help expedite drug discovery.

Biosynthesis and function of 7-deazaguanine derivatives in bacteria and phages

SUMMARYDeazaguanine modifications play multifaceted roles in the molecular biology of DNA and tRNA, shaping diverse yet essential biological processes, including the nuanced fine-tuning of translation efficiency and the intricate modulation of codon-anticodon interactions. Beyond their roles in translation, deazaguanine modifications contribute to cellular stress resistance, self-nonself discrimination mechanisms, and host evasion defenses, directly modulating the adaptability of living organisms. Deazaguanine moieties extend beyond nucleic acid modifications, manifesting in the structural diversity of biologically active natural products. Their roles in fundamental cellular processes and their presence in biologically active natural products underscore their versatility and pivotal contributions to the intricate web of molecular interactions within living organisms. Here, we discuss the current understanding of the biosynthesis and multifaceted functions of deazaguanines, shedding light on their diverse and dynamic roles in the molecular landscape of life.

Flipping the script: Understanding riboswitches from an alternative perspective

Riboswitches are broadly distributed regulatory elements most frequently found in the 5'-leader sequence of bacterial mRNAs that regulate gene expression in response to the binding of a small molecule effector. The occupancy status of the ligand-binding aptamer domain manipulates downstream information in the message that instructs the expression machinery. Currently, there are over 55 validated riboswitch classes, where each class is defined based on the identity of the ligand it binds and/or sequence and structure conservation patterns within the aptamer domain. This classification reflects an "aptamer-centric" perspective that dominates our understanding of riboswitches. In this review, we propose a conceptual framework that groups riboswitches based on the mechanism by which RNA manipulates information directly instructing the expression machinery. This scheme does not replace the established aptamer domain-based classification of riboswitches but rather serves to facilitate hypothesis-driven investigation of riboswitch regulatory mechanisms. Based on current bioinformatic, structural, and biochemical studies of a broad spectrum of riboswitches, we propose three major mechanistic groups: (1) "direct occlusion", (2) "interdomain docking", and (3) "strand exchange". We discuss the defining features of each group, present representative examples of riboswitches from each group, and illustrate how these RNAs couple small molecule binding to gene regulation. While mechanistic studies of the occlusion and docking groups have yielded compelling models for how these riboswitches function, much less is known about strand exchange processes. To conclude, we outline the limitations of our mechanism-based conceptual framework and discuss how critical information within riboswitch expression platforms can inform gene regulation.

Probing and perturbing riboswitch folding using a fluorescent base analogue

Riboswitches are mRNA segments that regulate gene expression in response to ligand binding. The Class I preQ1 riboswitch consists of a stem-loop and an adenine-rich single-stranded tail ("L3"), which adopt a pseudoknot structure upon binding of the ligand preQ1 . We inserted 2-aminopurine (2-AP), a fluorescent analogue of adenine (A), into the riboswitch at six different positions within L3. Here, 2-AP functions both as a spectroscopic probe and as a "mutation" that reveals how alteration of specific A residues impacts the riboswitch. Using fluorescence and circular dichroism spectroscopy, we found that 2-AP decreases the affinity of the riboswitch for preQ1 at all labeling positions tested, although modified and unmodified variants undergo the same global conformational changes at sufficiently high preQ1 concentration. 2-AP substitution is most detrimental to ligand binding at sites proximal to the ligand-binding pocket, while distal labeling sites exhibit the largest impacts on the stability of the L3 domain in the absence of ligand. Insertion of multiple 2-AP residues does not induce significant additional disruptions. Our results show that interactions involving the A residues in L3 play a critical role in ligand recognition by the preQ1 riboswitch and that 2-AP substitution exerts complex and varied impacts on this riboswitch.

Mechanistic Analysis of Riboswitch Ligand Interactions Provides Insights into Pharmacological Control over Gene Expression

Riboswitches are structured RNA elements that regulate gene expression upon binding to small molecule ligands. Understanding the mechanisms by which small molecules impact riboswitch activity is key to developing potent, selective ligands for these and other RNA targets. We report the structure-informed design of chemically diverse synthetic ligands for PreQ1 riboswitches. Multiple X-ray co-crystal structures of synthetic ligands with the Thermoanaerobacter tengcongensis (Tte)-PreQ1 riboswitch confirm a common binding site with the cognate ligand, despite considerable chemical differences among the ligands. Structure probing assays demonstrate that one ligand causes conformational changes similar to PreQ1 in six structurally and mechanistically diverse PreQ1 riboswitch aptamers. Single-molecule force spectroscopy is used to demonstrate differential modes of riboswitch stabilization by the ligands. Binding of the natural ligand brings about the formation of a persistent, folded pseudoknot structure, whereas a synthetic ligand decreases the rate of unfolding through a kinetic mechanism. Single round transcription termination assays show the biochemical activity of the ligands, while a GFP reporter system reveals compound activity in regulating gene expression in live cells without toxicity. Taken together, this study reveals that diverse small molecules can impact gene expression in live cells by altering conformational changes in RNA structures through distinct mechanisms.

Excited States of apo-Guanidine-III Riboswitch Contribute to Guanidinium Binding through Both Conformational and Induced-Fit Mechanisms

Riboswitches are mRNA segments that regulate gene expression through conformational changes driven by their cognate ligand binding. The ykkC motif forms a riboswitch class that selectively senses a guanidinium ion (Gdm+) and regulates the downstream expression of proteins which aid in the efflux of excess Gdm+ from the cells. The aptamer domain (AD) of the guanidine-III riboswitch forms an H-type pseudoknot with a triple helical domain that binds a Gdm+. We studied the binding of Gdm+ to the AD of the guanidine (ykkC)-III riboswitch using computer simulations to probe the specificity of the riboswitch to Gdm+ binding. We show that Gdm+ binding is a fast process occurring on the nanosecond time scale, with minimal conformational changes to the AD. Using machine learning and Markov-state models, we identified the excited conformational states of the AD, which have a high Gdm+ binding propensity, making the Gdm+ binding landscape complex exhibiting both conformational selection and induced-fit mechanisms. The proposed apo-AD excited states and their role in the ligand-sensing mechanism are amenable to experimental verification. Further, targeting these excited-state conformations in discovering new antibiotics can be explored.

RNA target highlights in CASP15: Evaluation of predicted models by structure providers

The first RNA category of the Critical Assessment of Techniques for Structure Prediction competition was only made possible because of the scientists who provided experimental structures to challenge the predictors. In this article, these scientists offer a unique and valuable analysis of both the successes and areas for improvement in the predicted models. All 10 RNA-only targets yielded predictions topologically similar to experimentally determined structures. For one target, experimentalists were able to phase their x-ray diffraction data by molecular replacement, showing a potential application of structure predictions for RNA structural biologists. Recommended areas for improvement include: enhancing the accuracy in local interaction predictions and increased consideration of the experimental conditions such as multimerization, structure determination method, and time along folding pathways. The prediction of RNA-protein complexes remains the most significant challenge. Finally, given the intrinsic flexibility of many RNAs, we propose the consideration of ensemble models.

Single-molecule FRET observes opposing effects of urea and TMAO on structurally similar meso- and thermophilic riboswitch RNAs

Bacteria live in a broad range of environmental temperatures that require adaptations of their RNA sequences to maintain function. Riboswitches are regulatory RNAs that change conformation upon typically binding metabolite ligands to control bacterial gene expression. The paradigmatic small class-I preQ1 riboswitches from the mesophile Bacillus subtilis (Bsu) and the thermophile Thermoanaerobacter tengcongensis (Tte) adopt similar pseudoknot structures when bound to preQ1. Here, we use UV-melting analysis combined with single-molecule detected chemical denaturation by urea to compare the thermodynamic and kinetic folding properties of the two riboswitches, and the urea-countering effects of trimethylamine N-oxide (TMAO). Our results show that, first, the Tte riboswitch is more thermotolerant than the Bsu riboswitch, despite only subtle sequence differences. Second, using single-molecule FRET, we find that urea destabilizes the folded pseudoknot structure of both riboswitches, yet has a lower impact on the unfolding kinetics of the thermodynamically less stable Bsu riboswitch. Third, our analysis shows that TMAO counteracts urea denaturation and promotes folding of both the riboswitches, albeit with a smaller effect on the more stable Tte riboswitch. Together, these findings elucidate how subtle sequence adaptations in a thermophilic bacterium can stabilize a common RNA structure when a new ecological niche is conquered.

Advances in RNA Labeling with Trifluoromethyl Groups

Fluorine labeling of ribonucleic acids (RNA) in conjunction with 19 F NMR spectroscopy has emerged as a powerful strategy for spectroscopic analysis of RNA structure and dynamics, and RNA-ligand interactions. This study presents the first syntheses of 2'-OCF3 guanosine and uridine phosphoramidites, their incorporation into oligoribonucleotides by solid-phase synthesis and a comprehensive study of their properties. NMR spectroscopic analysis showed that the 2'-OCF3 modification is associated with preferential C2'-endo conformation of the U and G ribose in single-stranded RNA. When paired to the complementary strand, slight destabilization of the duplex caused by the modification was revealed by UV melting curve analysis. Moreover, the power of the 2'-OCF3 label for NMR spectroscopy is demonstrated by dissecting RNA pseudoknot folding and its binding to a small molecule. Furthermore, the 2'-OCF3 modification has potential for applications in therapeutic oligonucleotides. To this end, three 2'-OCF3 modified siRNAs were tested in silencing of the BASP1 gene which indicated enhanced performance for one of them. Importantly, together with earlier work, the present study completes the set of 2'-OCF3 nucleoside phosphoramidites to all four standard nucleobases (A, U, C, G) and hence enables applications that utilize the favorable properties of the 2'-OCF3 group without any restrictions in placing the modification into the RNA target sequence.

8-oxoguanine riboswitches in bacteria detect and respond to oxidative DNA damage

Riboswitches rely on structured aptamer domains to selectively sense their target ligands and regulate gene expression. However, some riboswitch aptamers in bacteria carry mutations in their otherwise strictly conserved binding pockets that change ligand specificities. The aptamer domain of a riboswitch class originally found to selectively sense guanine forms a three-stem junction that has since been observed to exploit numerous alterations in its ligand-binding pocket. These rare variants have modified their ligand specificities to sense other purines or purine derivatives, including adenine, 2'-deoxyguanosine (three classes), and xanthine. Herein, we report the characteristics of a rare variant that is narrowly distributed in the Paenibacillaceae family of bacteria. Known representatives are always associated with genes encoding 8-oxoguanine deaminase. As predicted from this gene association, these variant riboswitches tightly bind 8-oxoguanine (8-oxoG), strongly discriminate against other purine derivatives, and function as genetic "ON" switches. Following exposure of cells to certain oxidative stresses, a representative 8-oxoG riboswitch activates gene expression, likely caused by the accumulation of 8-oxoG due to oxidative damage to G nucleobases in DNA, RNA, and the nucleotide pool. Furthermore, an engineered version of the variant aptamer was prepared that exhibits specificity for 8-oxoadenine, further demonstrating that RNA aptamers can acquire mutations that expand their ability to detect and respond to oxidative damage.

Structure and function analysis of a type III preQ1-I riboswitch from Escherichia coli reveals direct metabolite sensing by the Shine-Dalgarno sequence

Riboswitches are small noncoding RNAs found primarily in the 5' leader regions of bacterial messenger RNAs where they regulate expression of downstream genes in response to binding one or more cellular metabolites. Such noncoding RNAs are often regulated at the translation level, which is thought to be mediated by the accessibility of the Shine-Dalgarno sequence (SDS) ribosome-binding site. Three classes (I-III) of prequeuosine1 (preQ1)-sensing riboswitches are known that control translation. Class I is divided into three subtypes (types I-III) that have diverse mechanisms of sensing preQ1, which is involved in queuosine biosynthesis. To provide insight into translation control, we determined a 2.30 Å-resolution cocrystal structure of a class I type III preQ1-sensing riboswitch identified in Escherichia coli (Eco) by bioinformatic searches. The Eco riboswitch structure differs from previous preQ1 riboswitch structures because it has the smallest naturally occurring aptamer and the SDS directly contacts the preQ1 metabolite. We validated structural observations using surface plasmon resonance and in vivo gene-expression assays, which showed strong switching in live E. coli. Our results demonstrate that the Eco riboswitch is relatively sensitive to mutations that disrupt noncanonical interactions that form the pseudoknot. In contrast to type II preQ1 riboswitches, a kinetic analysis showed that the type III Eco riboswitch strongly prefers preQ1 over the chemically similar metabolic precursor preQ0. Our results reveal the importance of noncanonical interactions in riboswitch-driven gene regulation and the versatility of the class I preQ1 riboswitch pseudoknot as a metabolite-sensing platform that supports SDS sequestration.

Dynamical characterization and multiple unbinding paths of two PreQ1 ligands in one pocket

Riboswitches naturally regulate gene expression in bacteria by binding to specific small molecules. Class 1 preQ1 riboswitch aptamer is an important model not only for RNA folding but also as a target for designing small molecule antibiotics due to its well-known minimal aptamer domain. Here, we ran a total of 62.4 μs conventional and enhanced-sampling molecular dynamics (MD) simulations to characterize the determinants underlying the binding of the preQ1-II riboswitch aptamer to two preQ1 ligands in one binding pocket. Decomposition of binding free energy suggested that preQ1 ligands at α and β sites interact with four nucleotides (G5, C17, C18, and A30) and two nucleotides (A12 and C31), respectively. Mg2+ ions play a crucial role in both stabilizing the binding pocket and facilitating ligand binding. The flexible preQ1 ligand at the β site leads to the top of the binding pocket loosening and thus pre-organizes the riboswitch for ligand entry. Enhanced sampling simulations further revealed that the preQ1 ligand at the α site unbinds through two orthogonal pathways, which are dependent on whether or not a β site preQ1 ligand is present. One of the two preQ1 ligands has been identified in the binding pocket, which will aid to identify the second preQ1 Ligand. Our work provides new information for designing robust ligands.

Riboswitches, from cognition to transformation

Riboswitches are functional RNA elements that regulate gene expression by directly detecting metabolites. Twenty years have passed since it was first discovered, researches on riboswitches are becoming increasingly standardized and refined, which could significantly promote people's cognition of RNA function as well. Here, we focus on some representative orphan riboswitches, enumerate the structural and functional transformation and artificial design of riboswitches including the coupling with ribozymes, hoping to attain a comprehensive understanding of riboswitch research.

Structural basis for control of bacterial RNA polymerase pausing by a riboswitch and its ligand

Folding of nascent transcripts can be modulated by the RNA polymerase (RNAP) that carries out their transcription, and vice versa. A pause of RNAP during transcription of a preQ1 riboswitch (termed que-PEC) is stabilized by a previously characterized template consensus sequence and the ligand-free conformation of the nascent RNA. Ligand binding to the riboswitch induces RNAP pause release and downstream transcription termination; however, the mechanism by which riboswitch folding modulates pausing is unclear. Here, we report single-particle cryo-electron microscopy reconstructions of que-PEC in ligand-free and ligand-bound states. In the absence of preQ1, the RNA transcript is in an unexpected hyper-translocated state, preventing downstream nucleotide incorporation. Strikingly, on ligand binding, the riboswitch rotates around its helical axis, expanding the surrounding RNAP exit channel and repositioning the transcript for elongation. Our study reveals the tight coupling by which nascent RNA structures and their ligands can functionally regulate the macromolecular transcription machinery.

A riboswitch separated from its ribosome-binding site still regulates translation

Riboswitches regulate downstream gene expression by binding cellular metabolites. Regulation of translation initiation by riboswitches is posited to occur by metabolite-mediated sequestration of the Shine-Dalgarno sequence (SDS), causing bypass by the ribosome. Recently, we solved a co-crystal structure of a prequeuosine1-sensing riboswitch from Carnobacterium antarcticum that binds two metabolites in a single pocket. The structure revealed that the second nucleotide within the gene-regulatory SDS, G34, engages in a crystal contact, obscuring the molecular basis of gene regulation. Here, we report a co-crystal structure wherein C10 pairs with G34. However, molecular dynamics simulations reveal quick dissolution of the pair, which fails to reform. Functional and chemical probing assays inside live bacterial cells corroborate the dispensability of the C10-G34 pair in gene regulation, leading to the hypothesis that the compact pseudoknot fold is sufficient for translation attenuation. Remarkably, the C. antarcticum aptamer retained significant gene-regulatory activity when uncoupled from the SDS using unstructured spacers up to 10 nucleotides away from the riboswitch-akin to steric-blocking employed by sRNAs. Accordingly, our work reveals that the RNA fold regulates translation without SDS sequestration, expanding known riboswitch-mediated gene-regulatory mechanisms. The results infer that riboswitches exist wherein the SDS is not embedded inside a stable fold.

Discovering riboswitches: the past and the future

Riboswitches are structured noncoding RNA domains used by many bacteria to monitor the concentrations of target ligands and regulate gene expression accordingly. In the past 20 years over 55 distinct classes of natural riboswitches have been discovered that selectively sense small molecules or elemental ions, and thousands more are predicted to exist. Evidence suggests that some riboswitches might be direct descendants of the RNA-based sensors and switches that were likely present in ancient organisms before the evolutionary emergence of proteins. We provide an overview of the current state of riboswitch research, focusing primarily on the discovery of riboswitches, and speculate on the major challenges facing researchers in the field.

Isothermal Titration Calorimetry Analysis of a Cooperative Riboswitch Using an Interdependent-Sites Binding Model

Isothermal titration calorimetry (ITC) is a powerful biophysical tool to characterize energetic profiles of biomacromolecular interactions without any alteration of the underlying chemical structures. In this protocol, we describe procedures for performing, analyzing, and interpreting ITC data obtained from a cooperative riboswitch-ligand interaction.

Decoding the Identification Mechanism of an SAM-III Riboswitch on Ligands through Multiple Independent Gaussian-Accelerated Molecular Dynamics Simulations

S-Adenosyl-l-methionine (SAM)-responsive riboswitches play a central role in the regulation of bacterial gene expression at the level of transcription attenuation or translation inhibition. In this study, multiple independent Gaussian-accelerated molecular dynamics simulations were performed to decipher the identification mechanisms of SAM-III (S_MK) on ligands SAM, SAH, and EEM. The results reveal that ligand binding highly affects the structural flexibility, internal dynamics, and conformational changes of SAM-III. The dynamic analysis shows that helices P3 and P4 as well as two junctions J23 and J24 of SAM-III are highly susceptible to ligand binding. Analyses of free energy landscapes suggest that ligand binding induces different free energy profiles of SAM-III, which leads to the difference in identification sites of SAM-III on ligands. The information on ligand-nucleotide interactions not only uncovers that the π-π, cation-π, and hydrogen bonding interactions drive identification of SAM-III on the three ligands but also reveals that different electrostatic properties of SAM, SAH, and EEM alter the active sites of SAM-III. Meanwhile, the results also verify that the adenine group of SAM, SAH, and EEM is well recognized by conserved nucleotides G7, A29, U37, A38, and G48. We expect that this study can provide useful information for understanding the applications of SAM-III in chemical, synthetic RNA biology, and biomedical fields.

Bioinformatics and Genomic Analyses of the Suitability of Eight Riboswitches for Antibacterial Drug Targets

Antibiotic resistance (AR) is an acute problem that results in prolonged and debilitating illnesses. AR mortality worldwide is growing and causes a pressing need to research novel mechanisms of action and untested target molecules. This article presents in silico analyses of eight bacterial riboswitches for their suitability for antibacterial drug targets. Most bacterial riboswitches are located in the 5′-untranslated region of messenger RNAs, act as allosteric cis-acting gene control elements, and have not been found in humans before. Sensing metabolites, the riboswitches regulate the synthesis of vital cellular metabolites in various pathogenic bacteria. The analyses performed in this article represent a complete and informative genome-wide bioinformatics analysis of the adequacy of eight riboswitches as antibacterial drug targets in different pathogenic bacteria based on four criteria. Due to the ability of the riboswitch to control biosynthetic pathways and transport proteins of essential metabolites and the presence/absence of alternative biosynthetic pathways, we classified them into four groups based on their suitability for use as antibacterial drug targets guided by our in silico analyses. We concluded that some of them are promising targets for antibacterial drug discovery, such as the PreQ1, MoCo RNA, cyclic-di-GMP I, and cyclic-di-GMP II riboswitches.

Prime Editing: An All-Rounder for Genome Editing

Prime editing (PE), as a "search-and-replace" genome editing technology, has shown the attractive potential of versatile genome editing ability, which is, in principle, currently superior to other well-established genome-editing technologies in the all-in-one operation scope. However, essential technological solutions of PE technology, such as the improvement of genome editing efficiency, the inhibition of potential off-targets and intended edits accounting for unexpected side-effects, and the development of effective delivery systems, are necessary to broaden its application. Since the advent of PE, many optimizations have been performed on PE systems to improve their performance, resulting in bright prospects for application in many fields. This review briefly discusses the development of PE technology, including its functional principle, noteworthy barriers restraining its application, current efforts in technical optimization, and its application directions and potential risks. This review may provide a concise and informative insight into the burgeoning field of PE, highlight the exciting prospects for this powerful tool, and provide clues for questions that may propel the field forward.

Protein-Based Virtual Screening Tools Applied for RNA-Ligand Docking Identify New Binders of the preQ1-Riboswitch

Targeting RNA with small molecules is an emerging field. While several ligands for different RNA targets are reported, structure-based virtual screenings (VSs) against RNAs are still rare. Here, we elucidated the general capabilities of protein-based docking programs to reproduce native binding modes of small-molecule RNA ligands and to discriminate known binders from decoys by the scoring function. The programs were found to perform similar compared to the RNA-based docking tool rDOCK, and the challenges faced during docking, namely, protomer and tautomer selection, target dynamics, and explicit solvent, do not largely differ from challenges in conventional protein-ligand docking. A prospective VS with the Bacillus subtilis preQ₁-riboswitch aptamer domain performed with FRED, HYBRID, and FlexX followed by microscale thermophoresis assays identified six active compounds out of 23 tested VS hits with potencies between 29.5 nM and 11.0 μM. The hits were selected not solely based on their docking score but for resembling key interactions of the native ligand. Therefore, this study demonstrates the general feasibility to perform structure-based VSs against RNA targets, while at the same time it highlights pitfalls and their potential solutions when executing RNA-ligand docking.

Variants of the guanine riboswitch class exhibit altered ligand specificities for xanthine, guanine, or 2'-deoxyguanosine

The aptamer portions of previously reported riboswitch classes that sense guanine, adenine, or 2′-deoxyguanosine are formed by a highly similar three-stem junction with distinct nucleotide sequences in the regions joining the stems. The nucleotides in these joining regions form the major features of the selective ligand-binding pocket for each aptamer. Previously, we reported the existence of additional, rare variants of the predominant guanine-sensing riboswitch class that carry nucleotide differences in the ligand-binding pocket, suggesting that these RNAs have further diversified their structures and functions. Herein, we report the discovery and analysis of three naturally occurring variants of guanine riboswitches that are narrowly distributed across Firmicutes. These RNAs were identified using comparative sequence analysis methods, which also revealed that some of the gene associations for these variants are atypical for guanine riboswitches or their previously known natural variants. Binding assays demonstrate that the newfound variant riboswitch representatives recognize xanthine, guanine, or 2′-deoxyguanosine, with the guanine class exhibiting greater discrimination against related purines than the more common guanine riboswitch class reported previously. These three additional variant classes, together with the four previously discovered riboswitch classes that employ the same three-stem junction architecture, reveal how a simple structural framework can be diversified to expand the range of purine-based ligands sensed by RNA.

Engineered pegRNAs improve prime editing efficiency

Prime editing enables the installation of virtually any combination of point mutations, small insertions or small deletions in the DNA of living cells. A prime editing guide RNA (pegRNA) directs the prime editor protein to the targeted locus and also encodes the desired edit. Here we show that degradation of the 3' region of the pegRNA that contains the reverse transcriptase template and the primer binding site can poison the activity of prime editing systems, impeding editing efficiency. We incorporated structured RNA motifs to the 3' terminus of pegRNAs that enhance their stability and prevent degradation of the 3' extension. The resulting engineered pegRNAs (epegRNAs) improve prime editing efficiency 3-4-fold in HeLa, U2OS and K562 cells and in primary human fibroblasts without increasing off-target editing activity. We optimized the choice of 3' structural motif and developed pegLIT, a computational tool to identify non-interfering nucleotide linkers between pegRNAs and 3' motifs. Finally, we showed that epegRNAs enhance the efficiency of the installation or correction of disease-relevant mutations.

The Biochemical Landscape of Riboswitch Ligands

More than 55 distinct classes of riboswitches that respond to small metabolites or elemental ions have been experimentally validated to date. The ligands sensed by these riboswitches are biased in favor of fundamental compounds or ions that are likely to have been relevant to ancient forms of life, including those that might have populated the "RNA World", which is a proposed biochemical era that predates the evolutionary emergence of DNA and proteins. In the following text, I discuss the various types of ligands sensed by some of the most common riboswitches present in modern bacterial cells and consider implications for ancient biological processes centered on the proven capabilities of these RNA-based sensors. Although most major biochemical aspects of metabolism are represented by known riboswitch classes, there are striking sensory gaps in some key areas. These gaps could reveal weaknesses in the performance capabilities of RNA that might have hampered RNA World evolution, or these could highlight opportunities to discover additional riboswitch classes that sense essential metabolites.

A small RNA that cooperatively senses two stacked metabolites in one pocket for gene control

Riboswitches are structured non-coding RNAs often located upstream of essential genes in bacterial messenger RNAs. Such RNAs regulate expression of downstream genes by recognizing a specific cellular effector. Although nearly 50 riboswitch classes are known, only a handful recognize multiple effectors. Here, we report the 2.60-Å resolution co-crystal structure of a class I type I preQ₁-sensing riboswitch that reveals two effectors stacked atop one another in a single binding pocket. These effectors bind with positive cooperativity in vitro and both molecules are necessary for gene regulation in bacterial cells. Stacked effector recognition appears to be a hallmark of the largest subgroup of preQ₁ riboswitches, including those from pathogens such as Neisseria gonorrhoeae. We postulate that binding to stacked effectors arose in the RNA World to closely position two substrates for RNA-mediated catalysis. These findings expand known effector recognition capabilities of riboswitches and have implications for antimicrobial development.

Riboswitches contain an aptamer domain that recognizes a metabolite and an expression platform that regulates gene expression. Here the authors report the crystal structure of a preQ₁-sensing riboswitch from Carnobacterium antarcticus that shows two metabolites in a single binding pocket.

Molecular Dynamics Simulations of the Aptamer Domain of Guanidinium Ion Binding Riboswitch ykkC-III: Structural Insights into the Discrimination of Cognate and Alternate Ligands

Guanidinium ion is a toxic cellular metabolite. The ykkC-III riboswitch, an mRNA stretch, regulates the gene expression by undergoing a conformational change in response to the binding of a free guanidinium ion and thereby plays a potentially important role in alleviating guanidinium toxicity in cells. An experimental crystal structure of the guanidinium-bound aptamer domain of the riboswitch from Thermobifida Fusca revealed the overall RNA architecture and mapped the specific noncovalent interactions that stabilize the ligand within the binding pocket aptamer. However, details of how the aptamer domain discriminates the cognate ligand from its closest structurally analogous physiological metabolites (arginine and urea), and how the binding of cognate ligand arrays information from the aptamer domain to the expression platform for regulating the gene expression, are not well understood. To fill this void, we perform a cumulative of 2 μs all-atom explicit-solvent molecular dynamics (MD) simulations on the full aptamer domain, augmented with quantum-chemical calculations on the ligand-binding pocket, to compare the structural and dynamical details of the guanidinium-bound state with the arginine or urea bound states, as well as the unbound (open) state. Analysis of the ligand-binding pocket reveals that due to unfavorable interactions with the binding-pocket residues, urea cannot bind the aptamer domain and thereby cannot alter the gene expression. Although interaction of the guanidyl moiety of arginine within the binding pocket is either comparable or stronger than the guanidinium ion, additional non-native hydrogen-bonding networks, as well as differences in the dynamical details of the arginine-bound state, explain why arginine cannot transmit the information from the aptamer domain to the expression platform. Based on our simulations, we propose a mechanism of how the aptamer domain communicates with the expression platform. Overall, our work provides interesting insights into the ligand recognition by a specific class of riboswitches and may hopefully inspire future studies to further understand the gene regulation by riboswitches.

A chemical probe based on the PreQ1 metabolite enables transcriptome-wide mapping of binding sites

The role of metabolite-responsive riboswitches in regulating gene expression in bacteria is well known and makes them useful systems for the study of RNA-small molecule interactions. Here, we study the PreQ₁ riboswitch system, assessing sixteen diverse PreQ₁-derived probes for their ability to selectively modify the class-I PreQ₁ riboswitch aptamer covalently. For the most active probe (11), a diazirine-based photocrosslinking analog of PreQ₁, X-ray crystallography and gel-based competition assays demonstrated the mode of binding of the ligand to the aptamer, and functional assays demonstrated that the probe retains activity against the full riboswitch. Transcriptome-wide mapping using Chem-CLIP revealed a highly selective interaction between the bacterial aptamer and the probe. In addition, a small number of RNA targets in endogenous human transcripts were found to bind specifically to 11, providing evidence for candidate PreQ₁ aptamers in human RNA. This work demonstrates a stark influence of linker chemistry and structure on the ability of molecules to crosslink RNA, reveals that the PreQ₁ aptamer/ligand pair are broadly useful for chemical biology applications, and provides insights into how PreQ₁, which is similar in structure to guanine, interacts with human RNAs.

Effect of Sequence on the Interactions of Divalent Cations with M-Box Riboswitches from Mycobacterium tuberculosis and Bacillus subtilis

RNA is highly negatively charged and often acquires complex structures that require the presence of divalent cations. Subtle changes in conformation resulting from changes in sequence can affect the way ions associate with RNA. Riboswitches are RNA molecules that are involved in the control of gene expression in bacteria and are excellent systems for testing the effects of sequence variations on the conformation of RNA because they contain a highly conserved binding pocket but present sequence variability among different organisms. In this work, we have compared the aptamer domain of a proposed M-box riboswitch from Mycobacterium tuberculosis with the aptamer domain of a validated M-box riboswitch from Bacillus subtilis. We have in vitro transcribed and purified wild-type (WT) M-box riboswitches from M. tuberculosis and B. subtilis as well as a variety of mutated aptamers in which regions from one riboswitch have been replaced with regions from the other riboswitch. We have used ultraviolet unfolding experiments and circular dichroism to characterize the interactions of WT and related M-box riboswitches with divalent cations. Our results show that M-box from M. tuberculosis associates with Mg²⁺ and Sr²⁺ in a similar fashion while M-box from B. subtilis discriminates between these two ions and appears to associate better with Mg²⁺. Our overall results show that M-box from M. tuberculosis interacts differently with cations than M-box from B. subtilis and suggest conformational differences between these two riboswitches.

Synthesis of O 6-alkylated preQ1 derivatives

A naturally occurring riboswitch can utilize 7-aminomethyl-O 6-methyl-7-deazaguanine (m6preQ1) as cofactor for methyl group transfer resulting in cytosine methylation. This recently discovered riboswitch-ribozyme activity opens new avenues for the development of RNA labeling tools based on tailored O 6-alkylated preQ1 derivatives. Here, we report a robust synthesis for this class of pyrrolo[2,3-d]pyrimidines starting from readily accessible N 2-pivaloyl-protected 6-chloro-7-cyano-7-deazaguanine. Substitution of the 6-chloro atom with the alcoholate of interest proceeds straightforward. The transformation of the 7-cyano substituent into the required aminomethyl group turned out to be challenging and was solved by a hydration reaction sequence on a well-soluble dimethoxytritylated precursor via in situ oxime formation. The synthetic path now provides a solid foundation to access O 6-alkylated 7-aminomethyl-7-deazaguanines for the development of RNA labeling tools based on the preQ1 class-I riboswitch scaffold.

A structural intermediate pre-organizes the add adenine riboswitch for ligand recognition

Riboswitches are RNA sequences that regulate gene expression by undergoing structural changes upon the specific binding of cellular metabolites. Crystal structures of purine-sensing riboswitches have revealed an intricate network of interactions surrounding the ligand in the bound complex. The mechanistic details about how the aptamer folding pathway is involved in the formation of the metabolite binding site have been previously shown to be highly important for the riboswitch regulatory activity. Here, a combination of single-molecule FRET and SHAPE assays have been used to characterize the folding pathway of the adenine riboswitch from Vibrio vulnificus. Experimental evidences suggest a folding process characterized by the presence of a structural intermediate involved in ligand recognition. This intermediate state acts as an open conformation to ensure ligand accessibility to the aptamer and folds into a structure nearly identical to the ligand-bound complex through a series of structural changes. This study demonstrates that the add riboswitch relies on the folding of a structural intermediate that pre-organizes the aptamer global structure and the ligand binding site to allow efficient metabolite sensing and riboswitch genetic regulation.

Microsecond Folding of preQ1 Riboswitch and Its Biological Significance Revealed by Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy

Riboswitches are regulatory elements of bacterial mRNA which function with conformational switching upon binding of specific cellular metabolites. In particular, transcriptional riboswitches regulate gene expression kinetically through the conformational change of the aptamer domain. In this study, we investigate the conformational dynamics and ligand binding mechanisms of the aptamer domain of a transcriptional prequeuosine (preQ₁) riboswitch from Bacillus subtilis using two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS) with microsecond time resolution. The obtained time-resolved single-molecule data indicate that the aptamer domain undergoes folding/unfolding including three forms, which are attributed to hairpin (O), pseudoknot-like (pF), and H-type pseudoknot (fF) structures. It is found that a cofactor, Mg²⁺, binds only to the fF form with the conformational selection mechanism. In contrast, it is indicated that the ligand, preQ₁, binds to the O form with the induced-fit mechanism and significantly accelerates the microsecond O → pF folding process. It is also shown that the binding with preQ₁ substantially stabilizes the fF form that is generated from the pF form with a long time constant (>10 ms). Combining these results with the results of a former smFRET study on the slower time scale, we obtain an overall picture of the folding/unfolding dynamics of the aptamer domain as well as its energy landscape. On the basis of the picture obtained, we discuss the significance of the microsecond folding/unfolding of the aptamer domain for biological function of the riboswitch and propose the molecular mechanism of the gene expression controlled by the structural dynamics of the aptamer domain.

A translational riboswitch coordinates nascent transcription-translation coupling

Bacterial messenger RNA (mRNA) synthesis by RNA polymerase (RNAP) and first-round translation by the ribosome are often coupled to regulate gene expression, yet how coupling is established and maintained is ill understood. Here, we develop biochemical and single-molecule fluorescence approaches to probe the dynamics of RNAP-ribosome interactions on an mRNA with a translational preQ₁-sensing riboswitch in its 5' untranslated region. Binding of preQ₁ leads to the occlusion of the ribosome binding site (RBS), inhibiting translation initiation. We demonstrate that RNAP poised within the mRNA leader region promotes ribosomal 30S subunit binding, antagonizing preQ₁-induced RBS occlusion, and that the RNAP-30S bridging transcription factors NusG and RfaH distinctly enhance 30S recruitment and retention, respectively. We further find that, while 30S-mRNA interaction significantly impedes RNAP in the absence of translation, an actively translating ribosome promotes productive transcription. A model emerges wherein mRNA structure and transcription factors coordinate to dynamically modulate the efficiency of transcription-translation coupling.

Beyond Plug and Pray: Context Sensitivity and in silico Design of Artificial Neomycin Riboswitches

Gene regulation in prokaryotes often depends on RNA elements such as riboswitches or RNA thermometers located in the 5' untranslated region of mRNA. Rearrangements of the RNA structure in response, e.g., to the binding of small molecules or ions control translational initiation or premature termination of transcription and thus mRNA expression. Such structural responses are amenable to computational modelling, making it possible to rationally design synthetic riboswitches for a given aptamer. Starting from an artificial aptamer, we construct the first synthetic transcriptional riboswitches that respond to the antibiotic neomycin. We show that the switching behaviour in vivo critically depends not only on the sequence of the riboswitch itself, but also on its sequence context. We therefore developed in silico methods to predict the impact of the context, making it possible to adapt the design and to rescue non-functional riboswitches. We furthermore analyse the influence of 5' hairpins with varying stability on neomycin riboswitch activity. Our data highlight the limitations of a simple plug-and-play approach in the design of complex genetic circuits and demonstrate that detailed computational models significantly simplify, improve, and automate the design of transcriptional circuits. Our design software is available under a free licence on GitHub (https://github.com/xileF1337/riboswitch_design).

Small molecule-RNA targeting: starting with the fundamentals

The structural and regulatory elements in therapeutically relevant RNAs offer many opportunities for targeting by small molecules, yet fundamental understanding of what drives selectivity in small molecule:RNA recognition has been a recurrent challenge. In particular, RNAs tend to be more dynamic and offer less chemical functionality than proteins, and biologically active ligands must compete with the highly abundant and highly structured RNA of the ribosome. Indeed, the only small molecule drug targeting RNA other than the ribosome was just approved in August 2020, and our recent survey of the literature revealed fewer than 150 reported chemical probes that target non-ribosomal RNA in biological systems. This Feature outlines our efforts to improve small molecule targeting strategies and gain fundamental insights into small molecule:RNA recognition by analyzing patterns in both RNA-biased small molecule chemical space and RNA topological space privileged for differentiation. First, we synthesized libraries based on RNA binding scaffolds that allowed us to reveal general principles in small molecule:recognition and to ask precise chemical questions about drivers of affinity and selectivity. Elaboration of these scaffolds has led to recognition of medicinally relevant RNA targets, including viral and long noncoding RNA structures. More globally, we identified physicochemical, structural, and spatial properties of biologically active RNA ligands that are distinct from those of protein-targeted ligands, and we have provided the dataset and associated analytical tools as part of a publicly available online platform to facilitate RNA ligand discovery. At the same time, we used pattern recognition protocols to identify RNA topologies that can be differentially recognized by small molecules and have elaborated this technique to visualize conformational changes in RNA secondary structure. These fundamental insights into the drivers of RNA recognition in vitro have led to functional targeting of RNA structures in biological systems. We hope that these initial guiding principles, as well as the approaches and assays developed in their pursuit, will enable rapid progress toward the development of RNA-targeted chemical probes and ultimately new therapeutic approaches to a wide range of deadly human diseases.

2'-O-Trifluoromethylated RNA - a powerful modification for RNA chemistry and NMR spectroscopy

New RNA modifications are needed to advance our toolbox for targeted manipulation of RNA. In particular, the development of high-performance reporter groups facilitating spectroscopic analysis of RNA structure and dynamics, and of RNA-ligand interactions has attracted considerable interest. To this end, fluorine labeling in conjunction with 19F-NMR spectroscopy has emerged as a powerful strategy. Appropriate probes for RNA previously focused on single fluorine atoms attached to the 5-position of pyrimidine nucleobases or at the ribose 2'-position. To increase NMR sensitivity, trifluoromethyl labeling approaches have been developed, with the ribose 2'-SCF3 modification being the most prominent one. A major drawback of the 2'-SCF3 group, however, is its strong impact on RNA base pairing stability. Interestingly, RNA containing the structurally related 2'-OCF3 modification has not yet been reported. Therefore, we set out to overcome the synthetic challenges toward 2'-OCF3 labeled RNA and to investigate the impact of this modification. We present the syntheses of 2'-OCF3 adenosine and cytidine phosphoramidites and their incorporation into oligoribonucleotides by solid-phase synthesis. Importantly, it turns out that the 2'-OCF3 group has only a slight destabilizing effect when located in double helical regions which is consistent with the preferential C3'-endo conformation of the 2'-OCF3 ribose as reflected in the 3 J (H1'-H2') coupling constants. Furthermore, we demonstrate the exceptionally high sensitivity of the new label in 19F-NMR analysis of RNA structure equilibria and of RNA-small molecule interactions. The study is complemented by a crystal structure at 0.9 Å resolution of a 27 nt hairpin RNA containing a single 2'-OCF3 group that well integrates into the minor groove. The new label carries high potential to outcompete currently applied fluorine labels for nucleic acid NMR spectroscopy because of its significantly advanced performance.

Former orphan riboswitches reveal unexplored areas of bacterial metabolism, signaling, and gene control processes

Comparative sequence analyses have been used to discover numerous classes of structured noncoding RNAs, some of which are riboswitches that specifically recognize small-molecule or elemental ion ligands and influence expression of adjacent downstream genes. Determining the correct identity of the ligand for a riboswitch candidate typically is aided by an understanding of the genes under its regulatory control. Riboswitches whose ligands were straightforward to identify have largely been associated with well-characterized metabolic pathways, such as coenzyme or amino acid biosynthesis. Riboswitch candidates whose ligands resist identification, collectively known as orphan riboswitches, are often associated with genes coding for proteins of unknown function, or genes for various proteins with no established link to one another. The cognate ligands for 16 former orphan riboswitch motifs have been identified to date. The successful pursuit of the ligands for these classes has provided insight into areas of biology that are not yet fully explored, such as ion homeostasis, signaling networks, and other previously underappreciated biochemical or physiological processes. Herein we discuss the strategies and methods used to match ligands with orphan riboswitch classes, and overview the lessons learned to inform and motivate ongoing efforts to identify ligands for the many remaining candidates.

Protein unties the pseudoknot: S1-mediated unfolding of RNA higher order structure

Ribosomal protein S1 plays important roles in the translation initiation step of many Escherichia coli mRNAs, particularly those with weak Shine-Dalgarno sequences or structured 5′ UTRs, in addition to a variety of cellular processes beyond the ribosome. In all cases, the RNA-binding activity of S1 is a central feature of its function. While sequence determinants of S1 affinity and many elements of the interactions of S1 with simple secondary structures are known, mechanistic details of the protein's interactions with RNAs of more complex secondary and tertiary structure are less understood. Here, we investigate the interaction of S1 with the well-characterized H-type pseudoknot of a class-I translational preQ₁ riboswitch as a highly structured RNA model whose conformation and structural dynamics can be tuned by the addition of ligands of varying binding affinity, particularly preQ₁, guanine, and 2,6-diaminopurine. Combining biochemical and single molecule fluorescence approaches, we show that S1 preferentially interacts with the less folded form of the pseudoknot and promotes a dynamic, partially unfolded conformation. The ability of S1 to unfold the RNA is inversely correlated with the structural stability of the pseudoknot. These mechanistic insights delineate the scope and limitations of S1-chaperoned unfolding of structured RNAs.

Nucleobase mutants of a bacterial preQ1-II riboswitch that uncouple metabolite sensing from gene regulation

Riboswitches are a class of nonprotein-coding RNAs that directly sense cellular metabolites to regulate gene expression. They are model systems for analyzing RNA-ligand interactions and are established targets for antibacterial agents. Many studies have analyzed the ligand-binding properties of riboswitches, but this work has outpaced our understanding of the underlying chemical pathways that govern riboswitch-controlled gene expression. To address this knowledge gap, we prepared 15 mutants of the preQ1-II riboswitch-a structurally and biochemically well-characterized HLout pseudoknot that recognizes the metabolite prequeuosine1 (preQ1). The mutants span the preQ1-binding pocket through the adjoining Shine-Dalgarno sequence (SDS) and include A-minor motifs, pseudoknot-insertion helix P4, U·A-U base triples, and canonical G-C pairs in the anti-SDS. As predicted-and confirmed by in vitro isothermal titration calorimetry measurements-specific mutations ablated preQ1 binding, but most aberrant binding effects were corrected by compensatory mutations. In contrast, functional analysis in live bacteria using a riboswitch-controlled GFPuv-reporter assay revealed that each mutant had a deleterious effect on gene regulation, even when compensatory changes were included. Our results indicate that effector binding can be uncoupled from gene regulation. We attribute loss of function to defects in a chemical interaction network that links effector binding to distal regions of the fold that support the gene-off RNA conformation. Our findings differentiate effector binding from biological function, which has ramifications for riboswitch characterization. Our results are considered in the context of synthetic ligands and drugs that bind tightly to riboswitches without eliciting a biological response.

Exploring the RNA-Recognition Mechanism Using Supervised Molecular Dynamics (SuMD) Simulations: Toward a Rational Design for Ribonucleic-Targeting Molecules?

Although proteins have represented the molecular target of choice in the development of new drug candidates, the pharmaceutical importance of ribonucleic acids has gradually been growing. The increasing availability of structural information has brought to light the existence of peculiar three-dimensional RNA arrangements, which can, contrary to initial expectations, be recognized and selectively modulated through small chemical entities or peptides. The application of classical computational methodologies, such as molecular docking, for the rational development of RNA-binding candidates is, however, complicated by the peculiarities characterizing these macromolecules, such as the marked conformational flexibility, the singular charges distribution, and the relevant role of solvent molecules. In this work, we have thus validated and extended the applicability domain of SuMD, an all-atoms molecular dynamics protocol that allows to accelerate the sampling of molecular recognition events on a nanosecond timescale, to ribonucleotide targets of pharmaceutical interest. In particular, we have proven the methodological ability by reproducing the binding mode of viral or prokaryotic ribonucleic complexes, as well as that of artificially engineered aptamers, with an impressive degree of accuracy.

Computational study of switching mechanism in add A-riboswitch

Riboswitch can bind small molecules to regulate gene expression. Unlike other RNAs, riboswitch relies on its conformational switching for regulation. However, the understanding of the switching mechanism is still limited. Here, we focussed on the add A-riboswitch to illustrate the dynamical switching mechanism as an example. We performed molecular dynamics simulation, conservation and co-evolution calculations to infer the dynamical motions and evolutionary base pairings. The results suggest that the binding domain is stable for molecule recognition and binding, whereas the switching base pairings are co-evolutionary for translation. The understanding of the add A-riboswitch switching mechanism provides a potential solution for riboswitch drug design.

Development of a new oligonucleotide block location-based feature extraction (BLBFE) method for the classification of riboswitches

As knowledge of genetics and genome elements increases, the demand for the development of bioinformatics tools for analyzing these data is raised. Riboswitches are genetic components, usually located in the untranslated regions of mRNAs, that regulate gene expression. Additionally, their interaction with antibiotics has been recently suggested, implying a role in antibiotic effects and resistance. Following a previously published sequential block finding algorithm, herein, we report the development of a new block location-based feature extraction strategy (BLBFE). This procedure utilizes the locations of family-specific sequential blocks on riboswitch sequences as features. Furthermore, the performance of other feature extraction strategies, including mono- and dinucleotide frequencies, k-mer, DAC, DCC, DACC, PC-PseDNC-General and SC-PseDNC-General methods, was investigated. KNN, LDA, naïve Bayes, PNN and decision tree classifiers accompanied by V-fold cross-validation were applied for all methods of feature extraction, and their performances based on the defined feature extraction strategies were compared. Performance measures of accuracy, sensitivity, specificity and F-score for each method of feature extraction were studied. The proposed feature extraction strategy resulted in classification of riboswitches with an average correct classification rate (CCR) of 90.8%. Furthermore, the obtained data confirmed the performance of the developed feature extraction method with an average accuracy of 96.1%, an average sensitivity of 90.8%, an average specificity of 97.52% and an average F-score of 90.69%. Our results implied that the proposed feature extraction (BLBFE) method can classify and discriminate riboswitch families with high CCR, accuracy, sensitivity, specificity and F-score values.

Classification of Riboswitch Families Using Block Location-Based Feature Extraction (BLBFE) Method

Purpose: Riboswitches are special non-coding sequences usually located in mRNAs' un-translated regions and regulate gene expression and consequently cellular function. Furthermore, their interaction with antibiotics has been recently implicated. This raises more interest in development of bioinformatics tools for riboswitch studies. Herein, we describe the development and employment of novel block location-based feature extraction (BLBFE) method for classification of riboswitches. Methods: We have already developed and reported a sequential block finding (SBF) algorithm which, without operating alignment methods, identifies family specific sequential blocks for riboswitch families. Herein, we employed this algorithm for 7 riboswitch families including lysine, cobalamin, glycine, SAM-alpha, SAM-IV, cyclic-di-GMP-I and SAH. Then the study was extended toward implementation of BLBFE method for feature extraction. The outcome features were applied in various classifiers including linear discriminant analysis (LDA), probabilistic neural network (PNN), decision tree and k-nearest neighbors (KNN) classifiers for classification of the riboswitch families. The performance of the classifiers was investigated according to performance measures such as correct classification rate (CCR), accuracy, sensitivity, specificity and f-score. Results: As a result, average CCR for classification of riboswitches was 87.87%. Furthermore, application of BLBFE method in 4 classifiers displayed average accuracies of 93.98% to 96.1%, average sensitivities of 76.76% to 83.61%, average specificities of 96.53% to 97.69% and average f-scores of 74.9% to 81.91%. Conclusion: Our results approved that the proposed method of feature extraction; i.e. BLBFE method; can be successfully used for classification and discrimination of the riboswitch families with high CCR, accuracy, sensitivity, specificity and f-score values.

Quantum chemical and well-tempered metadynamics study to design adenine analogs for orthogonal Preq1 riboswitch

Communicated by Ramaswamy H. Sarma.