Phosphate-Group Recognition by the Aptamer Domain of the Thiamine Pyrophosphate Sensing Riboswitch

Regulation of the thiamine pyrophosphate (TPP)-sensing riboswitch in NMT1 mRNA from Neurospora crassa

The expression of Neurospora crassa NMT1 involved in thiamine pyrophosphate (TPP) metabolism is regulated at the level of mRNA splicing by a TPP-sensing riboswitch within the precursor NMT1 mRNA. Here, using the systematic helix-based computational method, we investigated the regulation of this riboswitch. We find that the function of the riboswitch does not depend on the transcription process. Whether TPP is present or not, the riboswitch predominately folds into the ON state, while the OFF state aptamer structure does not appear during transcription. Since the transition from the ON state to the aptamer structure is extremely slow, TPP may interact with the RNA before full formation of the aptamer structure, promoting the switch flipping. The potential to fully form helix P0 of the ON state is necessary to restore ligand-dependent gene control by the riboswitch.

Role of riboswitches in gene regulation and their potential for algal biotechnology

Riboswitches are regulatory elements in messenger RNA to which specific ligands can bind directly in the absence of proteins. Ligand binding alters the mRNA secondary structure, thereby affecting expression of the encoded protein. Riboswitches are widespread in prokaryotes, with over 20 different effector ligands known, including amino acids, cofactors, and Mg(2+) ions, and gene expression is generally regulated by affecting translation or termination of transcription. In plants, fungi, and microalgae, riboswitches have been found, but only those that bind thiamine pyrophosphate. These eukaryotic riboswitches operate by causing alternative splicing of the transcript. Here, we review the current status of riboswitch research with specific emphasis on microalgae. We discuss new riboswitch discoveries and insights into the underlying mechanism of action, and how next generation sequencing technology provides the motivation and opportunity to improve our understanding of these rare but important regulatory elements. We also highlight the potential of microalgal riboswitches as a tool for synthetic biology and industrial biotechnology.

Observation of long-range tertiary interactions during ligand binding by the TPP riboswitch aptamer

The thiamine pyrophosphate (TPP) riboswitch is a cis-regulatory element in mRNA that modifies gene expression in response to TPP concentration. Its specificity is dependent upon conformational changes that take place within its aptamer domain. Here, the role of tertiary interactions in ligand binding was studied at the single-molecule level by combined force spectroscopy and Förster resonance energy transfer (smFRET), using an optical trap equipped for simultaneous smFRET. The ‘Force-FRET’ approach directly probes secondary and tertiary structural changes during folding, including events associated with binding. Concurrent transitions observed in smFRET signals and RNA extension revealed differences in helix-arm orientation between two previously-identified ligand-binding states that had been undetectable by spectroscopy alone. Our results show that the weaker binding state is able to bind to TPP, but is unable to form a tertiary docking interaction that completes the binding process. Long-range tertiary interactions stabilize global riboswitch structure and confer increased ligand specificity.

DOI:http://dx.doi.org/10.7554/eLife.12362.001

When a gene is switched on, its DNA is first copied to make a molecule of messenger ribonucleic acid (mRNA). The genetic code in the mRNA is then translated into a protein. There are also untranslated regions within mRNAs that do not code for protein themselves, but serve to regulate whether or not a protein is produced from the rest of the mRNA. For example, many mRNAs contain a motif in their untranslated region called a 'riboswitch'. These motifs selectively bind to molecules that are the products of metabolic processes. One riboswitch found in bacteria, animals and plants binds to a molecule known as thiamine pyrophosphate (TPP) and regulates genes that control the uptake of a vitamin called thiamine into cells.

Newly made mRNA molecules are linear strands that then fold into three-dimensional structures. The TPP riboswitch can adopt distinct shapes depending on whether it is bound to TPP or not. Knowledge of these structures is crucial for understanding how riboswitches regulate protein production. Previous research reported the folding of a TPP riboswitch from bacteria.

Here, Duesterberg et al. used a combination of two techniques known as force spectroscopy and Förster resonance energy transfer (FRET) to study the folding of the TPP riboswitch from a plant called Arabidopsis thaliana. The experiments show that in the presence of TPP, structural changes occur in two arm-like appendages – known as helix arms – that extend out of the core of the riboswitch. The riboswitch adopts a particular shape when TPP is strongly bound to it, and in this shape the riboswitch can regulate the activity of certain genes. However, if the riboswitch is only weakly associated with TPP, it takes on a shape in which the two helix arms are further apart and the riboswitch is unable to form the interactions required to complete the process of binding to TPP.

Duesterberg et al.’s findings reveal that the way in which the A. thaliana riboswitch changes shape when it is bound to TPP is different to that of its bacterial counterpart. The next challenge will be to observe these shape changes in even more detail, and to use these new techniques to study other riboswitches in various organisms.

DOI:http://dx.doi.org/10.7554/eLife.12362.002

Dynamics of riboswitches: Molecular simulations

Riboswitch RNAs play key roles in bacterial metabolism and represent a promising new class of antibiotic targets for treatment of infectious disease. While many studies of riboswitches have been performed, the exact mechanism of riboswitch operation is still not fully understood at the atomistic level of detail. Molecular dynamics simulations are useful for interpreting existing experimental data and producing predictions for new experiments. Here, a wide range of computational studies on riboswitches is reviewed. By elucidating the key principles of riboswitch operation, computation may aid in the effort to design more specific antibiotics with affinities greater than those of the native ligand. Such a detailed understanding may be required to improve efficacy and reduce side effects. These studies are laying the groundwork for understanding the action mechanism of new compounds that inhibit riboswitch activity. Future directions such as magnesium effects, large-scale conformational changes, expression platforms and co-transcriptional folding are also discussed. This article is part of a Special Issue entitled: Riboswitches.

Folding energy landscape of the thiamine pyrophosphate riboswitch aptamer

Riboswitches are motifs in the untranslated regions (UTRs) of RNA transcripts that sense metabolite levels and modulate the expression of the corresponding genes for metabolite import, export, synthesis, or degradation. All riboswitches contain an aptamer: an RNA structure that, upon binding ligand, folds to expose or sequester regulatory elements in the adjacent sequence through alternative nucleotide pairing. The coupling between ligand binding and aptamer folding is central to the regulatory mechanisms of thiamine pyrophosphate (TPP) riboswitches and has not been fully characterized. Here, we show that TPP aptamer folding can be decomposed into ligand-independent and -dependent steps that correspond to the formation of secondary and tertiary structures, respectively. We reconstructed the full energy landscape for folding of the wild-type (WT) aptamer and measured perturbations of this landscape arising from mutations or ligand binding. We show that TPP binding proceeds in two steps, from a weakly to a strongly bound state. Our data imply a hierarchical folding sequence, and provide a framework for understanding molecular mechanism throughout the TPP riboswitch family.

Riboswitches: discovery of drugs that target bacterial gene-regulatory RNAs

Riboswitches are messenger RNA (mRNA) domains that regulate gene function in response to the intracellular concentration of a variety of metabolites and second messengers. They control essential genes in many pathogenic bacteria, thus representing an inviting new class of biomolecular target for the development of antibiotics and chemical-biological tools. In this Account, we briefly review the discovery of riboswitches in the first years of the 21st century and their ensuing characterization over the past decade. We then discuss the progress achieved so far in using riboswitches as a focus for drug discovery, considering both the value of past serendipity and the particular challenges that confront current researchers. Five mechanisms of gene regulation have been determined for riboswitches. Most bacterial riboswitches modulate either transcription termination or translation initiation in response to ligand binding. All known examples of eukaryotic riboswitches, and some bacterial riboswitches, control gene expression by alternative splicing. The glmS riboswitch, which is widespread in Gram-positive bacteria, is a catalytic RNA activated by ligand binding: its self-cleavage destabilizes the mRNA of which it is part. Finally, one example of a trans-acting riboswitch is known. Three-dimensional structures have been determined for representatives of 13 structurally distinct riboswitch classes, providing atomic-level insight into their mechanisms of ligand recognition. While cellular and viral RNAs have attracted widespread interest as potential drug targets, riboswitches show special promise due to the diversity of small-molecule recognition strategies that are on display in their ligand-binding pockets. Moreover, riboswitches have evolved to recognize small-molecule ligands, which is unique among known structured RNA domains. Structural and biochemical advances in the study of riboswitches provide an impetus for the development of methods for the discovery of novel riboswitch activators and inhibitors. Recent rational drug design efforts focused on select riboswitch classes have yielded a small number of candidate antibiotic compounds, including one active in a mouse model of Staphylococcus aureus infection. The development of high-throughput methods suitable for riboswitch-specific drug discovery is ongoing. A fragment-based screening approach employing equilibrium dialysis that may be generically useful has demonstrated early success. Riboswitch-mediated gene regulation is widely employed by bacteria; however, only the thiamine pyrophosphate-responsive riboswitch has thus far been found in eukaryotes. Thus, riboswitches are particularly attractive as targets for antibacterials. Indeed, antimicrobials with previously unknown mechanisms have been found to function by binding riboswitches and causing aberrant gene expression.

Convergent donor and acceptor substrate utilization among kinase ribozymes

Accommodation of donor and acceptor substrates is critical to the catalysis of (thio)phosphoryl group transfer, but there has been no systematic study of donor nucleotide recognition by kinase ribozymes, and there is relatively little known about the structural requirements for phosphorylating internal 2′OH. To address these questions, new self-phosphorylating ribozymes were selected that utilize ATP(gammaS) or GTP(gammaS) for 2′OH (thio)phosphorylation. Eight independent sequence families were identified among 57 sequenced isolates. Kinetics, donor nucleotide recognition and secondary structures were analyzed for representatives from each family. Each ribozyme was highly specific for its cognate donor. Competition assays with nucleotide analogs showed a remarkable convergence of donor recognition requirements, with critical contributions to recognition provided by the Watson–Crick face of the nucleobase, lesser contributions from donor nucleotide ribose hydroxyls, and little or no contribution from the Hoogsteen face. Importantly, most ribozymes showed evidence of significant interaction with one or more donor phosphates, suggesting that—unlike most aptamers—these ribozymes use phosphate interactions to orient the gamma phosphate within the active site for in-line displacement. All but one of the mapped (thio)phosphorylation sites are on unpaired guanosines within internal bulges. Comparative structural analysis identified three loosely-defined consensus structural motifs for kinase ribozyme active sites.

Investigations on the specificity of DNA aptamers binding to ethanolamine

In our previous work, we selected aptamers binding to ethanolamine, one of the smallest molecular aptamer targets so far (Mann, D., Reinemann, C., Stoltenburg, R. and Strehlitz, B. Biochem. Biophys. Res. Commun. 2005, 338, 1928-1934). Two representatives of these aptamers (EA#14.3 and EA#9.4) were analyzed regarding their specificity. Ethanolamine is a very small organic molecule (M(w) = 61.08) with biological, medical, and industrial relevance. Its small size represented a challenge for aptamer development, as ethanolamine only consists of a short carbon chain (2C) and two functional groups (amino and hydroxyl group). Related organic molecules, ethanolamine derivatives, and some amino acids were tested to act as potential binding partners for these aptamers. In this way we were able to determine the exact binding domain within the target. The results revealed that both aptamers bind to various molecules, which contain a freely accessible ethyl- or methylamine group. In contrast to the amino group (in a primary, secondary, or tertiary amine) the hydroxyl group was not necessary for the aptamer binding. The aptamers were not able to bind to negatively charged organic molecules, despite containing an ethyl- or methylamine group, nor did they bind to molecules with quaternary amines. The selected ethanolamine binding aptamers are useful for the detection of molecules containing accessible ethyl- or methylamine groups; they can be used as linker elements to immobilize a target molecule of interest on a surface or to purify targets from complex samples.

In vitro selection of conformational probes for riboswitches

Riboswitches are non-coding RNA elements mainly located in the 5' untranslated regions (UTR) of bacterial genes. They bind to small metabolites and upon binding conformational changes occur that trigger the expression of a certain gene. Riboswitches have been identified that bind to amino acids, purines, and other small metabolites such as thiamine pyrophosphate. Riboswitches contain an aptamer domain which is necessary for interaction with the metabolite and a related expression domain which harbours structural and sequence information required for interference with gene expression. The binding of a metabolite to the aptamer domain induces structural rearrangements that are relayed to the expression domain, thereby interfering with gene expression. To investigate and determine domains of the riboswitches which undergo conformational changes upon metabolite binding we used a dynamic SELEX process and identified RNA aptamers that bind to the metabolite-free variant of the riboswitch but are released upon metabolite addition. By this means, and after determination of the binding region, domains which are necessary for proper function of a full-length riboswitch can be identified.

Riboswitches: emerging themes in RNA structure and function

Riboswitches are RNAs capable of binding cellular metabolites using a diverse array of secondary and tertiary structures to modulate gene expression. The recent determination of the three-dimensional structures of parts of six different riboswitches illuminates common features that allow riboswitches to be grouped into one of two types. Type I riboswitches, as exemplified by the purine riboswitch, are characterized by a single, localized binding pocket supported by a largely pre-established global fold. This arrangement limits ligand-induced conformational changes in the RNA to a small region. In contrast, Type II riboswitches, such as the thiamine pyrophosphate riboswitch, contain binding pockets split into at least two spatially distinct sites. As a result, binding induces both local changes to the binding pocket and global architecture. Similar organizational themes are found in other noncoding RNAs, making it possible to begin to build a hierarchical classification of RNA structure based on the spatial organization of their active sites and associated secondary structural elements.

Secondary structures and functional requirements for thiM riboswitches from Desulfovibrio vulgaris, Erwinia carotovora and Rhodobacter spheroides

Abstract Bacterial thiM riboswitches contain aptamer domains that bind the metabolite thiamine pyrophosphate (TPP). Binding of TPP to the aptamer domain induces structural rearrangements that are relayed to the expression domain, thereby interfering with gene expression. Here, we report identification of three putative thiM riboswitches from different bacteria and analysis of their secondary structures. Chemical probing revealed that the riboswitches share similar secondary structures in their aptamer domains that can communicate with the highly variant expression domains in a mechanism likely involving sequestration of the Shine-Dalgarno sequence. Remarkably, the aptamer domain of the thiM gene of Desulfovibrio vulgaris binds TPP with similar affinity and selectivity as that of Escherichia coli, although nucleotides previously shown to form direct contacts to the metabolite are mutated. We also designed small RNA hairpins for each riboswitch that bind the RNA only in the absence of the metabolite. Our study shows that aptamer domains in riboswitches with high similarity in their secondary structures can communicate with a broad variety of non-related expression domains by similar mechanisms.

Prior interaction of ATP with yeast mRNAs enhances protein synthesis at the initiation step

ATP hydrolysis is important for different stages of the protein synthesis process. A novel effect of this nucleotide was detected using mRNAs isolated from S. cerevisiae after phenol extraction of polysomes. When polysomal mRNA (pmRNA) or poly(A)(+) RNA were preincubated with ATP (approximately 3 mM, near physiological concentration), their translational activity in a cell-free system from yeast was stimulated 2-3 fold. This increased translational activity is specific for the poly(A)(+) RNA fraction, correlates with facilitated assembly of 80S initiation complexes, and is associated to increased synthesis of high molecular weight polypeptides. TCA precipitation assays of RNA incubated with [(14)C]ATP suggested an association of the nucleotide with the nucleic acid. The amount of [(14)C]ATP co-precipitated was dependent on magnesium (optimum at 5-6 mM), was partially inhibited by monovalent ions, and was maximal with poli(A)(+) RNA. Existence of RNA-associated kinases or ATPases appear unlikely since neither phosphorylation nor nucleotide hydrolysis were observed during preincubation of pmRNA with ATP. Another evidence of ATP-RNA interaction was an increased absorbance at 260 nm after incubation suggesting unwinding of the RNA secondary structure. Therefore, preincubation with ATP may affect the conformation of mRNAs and thereby facilitate the initiation of protein synthesis. This event could be part of an in vivo energy-dependent mechanism for translational control.

Metal-ion binding and metal-ion induced folding of the adenine-sensing riboswitch aptamer domain

Divalent cations are important in the folding and stabilization of complex RNA structures. The adenine-sensing riboswitch controls the expression of mRNAs for proteins involved in purine metabolism by directly sensing intracellular adenine levels. Adenine binds with high affinity and specificity to the ligand binding or aptamer domain of the adenine-sensing riboswitch. The X-ray structure of this domain in complex with adenine revealed an intricate RNA-fold consisting of a three-helix junction stabilized by long-range base-pairing interactions and identified five binding sites for hexahydrated Mg2+-ions. Furthermore, a role for Mg2+-ions in the ligand-induced folding of this RNA was suggested. Here, we describe the interaction of divalent cations with the RNA-adenine complex in solution as studied by high-resolution NMR spectroscopy. Paramagnetic line broadening, chemical shift mapping and intermolecular nuclear Overhauser effects (NOEs) indicate the presence of at least three binding sites for divalent cations. Two of them are similar to those in the X-ray structure. The third site, which is important for the folding of this RNA, has not been observed previously. The ligand-free state of the RNA is conformationally heterogeneous and contains base-pairing patterns detrimental to ligand binding in the absence of Mg2+, but becomes partially pre-organized for ligand binding in the presence of Mg2+. Compared to the highly similar guanine-sensing riboswitch, the folding pathway for the adenine-sensing riboswitch aptamer domain is more complex and the influence of Mg2+ is more pronounced.

Riboswitches: small-molecule recognition by gene regulatory RNAs

Riboswitches demonstrate the ability of highly structured RNA molecules to recognize small-molecule metabolites with high specificity and subsequently harness the binding energy for the control of gene expression. Crystal structures have now been determined for the metabolite-binding domains of riboswitches that respond to purines, thiamine pyrophosphate and S-adenosylmethionine, as well as for the glmS ribozyme, a catalytic riboswitch that is activated by the metabolite glucosamine-6-phosphate. In addition to these riboswitch structures, a solution NMR structure has been reported for a ribosensor that regulates heat shock genes in response to changes in temperature. These studies reveal the structural basis of the remarkable selectivity of riboswitches and, in conjunction with biochemical and biophysical measurements, provide a framework for detailed mechanistic understanding of riboswitch-mediated modulation of gene expression.

Conformational changes in the expression domain of the Escherichia coli thiM riboswitch

The thiM riboswitch contains an aptamer domain that adaptively binds the coenzyme thiamine pyrophosphate (TPP). The binding of TPP to the aptamer domain induces structural rearrangements that are relayed to a second domain, the so-called expression domain, thereby interfering with gene expression. The recently solved crystal structures of the aptamer domains of the thiM riboswitches in complex with TPP revealed how TPP stabilizes secondary and tertiary structures in the RNA ligand complex. To understand the global modes of reorganization between the two domains upon metabolite binding the structure of the entire riboswitch in presence and absence of TPP needs to be determined. Here we report the secondary structure of the entire thiM riboswitch from Escherichia coli in its TPP-free form and its transition into the TPP-bound variant, thereby depicting domains of the riboswitch that serve as communication links between the aptamer and the expression domain. Furthermore, structural probing provides an explanation for the lack of genetic control exerted by a riboswitch variant with mutations in the expression domain that still binds TPP.

The THI-box Riboswitch, or How RNA Binds Thiamin Pyrophosphate

Riboswitches are genetic control elements present mainly in the 5' untranslated regions of messenger RNAs that, upon binding of a small metabolite (like some vitamins, amino acids, and nucleobases), undergo conformational changes, affecting the expression of downstream genes. Structural studies of riboswitches are important for understanding how they recognize their ligands with high specificity and affinity. The thiamin pyrophosphate binding riboswitch (THI- box) is widely distributed in the three kingdoms of life and is involved in very distinct modes of gene regulation. Three recent THI-box structural analyses revealed how polyanionic RNA is able to bind a molecule with a negatively charged pyrophosphate group like thiamin pyrophosphate (TPP) and how it can discriminate between TPP and monophosphorylated analog molecules. These studies give insight into the genetic regulatory mechanisms in which the THI-box is involved.

Structures of RNA Switches: Insight into Molecular Recognition and Tertiary Structure

RNA switches (riboswitches) have important functions in gene regulation. They comprise an aptamer domain, which is responsible for ligand binding, and an expression platform that transmits the ligand-binding state of the aptamer domain through a conformational change. Riboswitches can regulate gene expression either at the level of transcription or translation, and it has been proposed that riboswitch mechanisms are even used to regulate the processing of mRNA. This Minireview summarizes the current understanding of the structures and mode of action of RNA switches, with particular focus on secondary and tertiary interactions, which stabilize the global RNA structure and thus determine the function of the aptamer domain.