Mutational analysis of the purine riboswitch aptamer domain

Mechanism of Ligand Discrimination by the NMT1 Riboswitch

Riboswitches are conserved functional domains in mRNA that almost exclusively exist in bacteria. They regulate the biosynthesis and transport of amino acids and essential metabolites such as coenzymes, nucleobases, and their derivatives by specifically binding small molecules. Due to their ability to precisely discriminate between different cognate molecules as well as their common existence in bacteria, riboswitches have become potential antibacterial drug targets that could deliver urgently needed antibiotics with novel mechanisms of action. In this work, we report the recognition mechanisms of four oxidization products (XAN, AZA, UAC, and HPA) generated during purine degradation by an RNA motif termed the NMT1 riboswitch. Specifically, we investigated the physical interactions between the riboswitch and the oxidized metabolites by computing the changes in the free energy on mutating key nucleobases in the ligand binding pocket of the riboswitch. We discovered that the electrostatic interactions are central to ligand discrimination by this riboswitch. The relative binding free energies of the mutations further indicated that some of the mutations can also strengthen the binding affinities of the ligands (AZA, UAC, and HPA). These mechanistic details are also potentially relevant in the design of novel compounds targeting riboswitches.

Disentangling contact and ensemble epistasis in a riboswitch

Mutations introduced into macromolecules often exhibit epistasis, where the effect of one mutation alters the effect of another. Knowing the mechanisms that lead to epistasis is important for understanding how macromolecules work and evolve, as well as for effective macromolecular engineering. Here, we investigate the interplay between "contact epistasis" (epistasis arising from physical interactions between mutated residues) and "ensemble epistasis" (epistasis that occurs when a mutation redistributes the conformational ensemble of a macromolecule, thus changing the effect of the second mutation). We argue that the two mechanisms can be distinguished in allosteric macromolecules by measuring epistasis at differing allosteric effector concentrations. Contact epistasis manifests as nonadditivity in the microscopic equilibrium constants describing the conformational ensemble. This epistatic effect is independent of allosteric effector concentration. Ensemble epistasis manifests as nonadditivity in thermodynamic observables-such as ligand binding-that are determined by the distribution of ensemble conformations. This epistatic effect strongly depends on allosteric effector concentration. Using this framework, we experimentally investigated the origins of epistasis in three pairwise mutant cycles introduced into the adenine riboswitch aptamer domain by measuring ligand binding as a function of allosteric effector concentration. We found evidence for both contact and ensemble epistasis in all cycles. Furthermore, we found that the two mechanisms of epistasis could interact with each other. For example, in one mutant cycle we observed 6 kcal/mol of contact epistasis in a microscopic equilibrium constant. In that same cycle, the maximum epistasis in ligand binding was only 1.5 kcal/mol: shifts in the ensemble masked the contribution of contact epistasis. Finally, our work yields simple heuristics for identifying contact and ensemble epistasis based on measurements of a biochemical observable as a function of allosteric effector concentration.

Exploiting natural riboswitches for aptamer engineering and validation

Over the past three decades, researchers have found that some engineered aptamers can be made to work well in test tubes but that these same aptamers might fail to function in cells. To help address this problem, we developed the 'Graftamer' approach, an experimental platform that exploits the architecture of a natural riboswitch to enhance in vitro aptamer selection and accelerate in vivo testing. Starting with combinatorial RNA pools that contain structural features of a guanine riboswitch aptamer interspersed with regions of random sequence, we performed multiplexed in vitro selection with a collection of small molecules. This effort yielded aptamers for quinine, guanine, and caffeine that appear to maintain structural features of the natural guanine riboswitch aptamer. Quinine and caffeine aptamers were each grafted onto a natural guanine riboswitch expression platform and reporter gene expression was monitored to determine that these aptamers function in cells. Additionally, we determined the secondary structure features and survival mechanism of a class of RNA sequences that evade the intended selection strategy, providing insight into improving this approach for future efforts. These results demonstrate that the Graftamer strategy described herein represents a convenient and straightforward approach to develop aptamers and validate their in vivo function.

Dynamic docking of small molecules targeting RNA CUG repeats causing myotonic dystrophy type 1

Expansion of RNA CUG repeats causes myotonic dystrophy type 1 (DM1). Once transcribed, the expanded CUG repeats strongly attract muscleblind-like 1 (MBNL1) proteins and disturb their functions in cells. Because of its unique structural form, expanded RNA CUG repeats are prospective drug targets, where small molecules can be utilized to target RNA CUG repeats to inhibit MBNL1 binding and ameliorate DM1-associated defects. In this contribution, we developed two physics-based dynamic docking approaches (DynaD and DynaD/Auto) and applied them to nine small molecules known to specifically target RNA CUG repeats. While DynaD uses a distance-based reaction coordinate to study the binding phenomenon, DynaD/Auto combines results of umbrella sampling calculations performed on 1 × 1 UU internal loops and AutoDock calculations to efficiently sample the energy landscape of binding. Predictions are compared with experimental data, displaying a positive correlation with correlation coefficient (R) values of 0.70 and 0.81 for DynaD and DynaD/Auto, respectively. Furthermore, we found that the best correlation was achieved with MM/3D-RISM calculations, highlighting the importance of solvation in binding calculations. Moreover, we detected that DynaD/Auto performed better than DynaD because of the use of prior knowledge about the binding site arising from umbrella sampling calculations. Finally, we developed dendrograms to present how bound states are connected to each other in a binding process. Results are exciting, as DynaD and DynaD/Auto will allow researchers to utilize two novel physics-based and computer-aided drug-design methodologies to perform in silico calculations on drug-like molecules aiming to target complex RNA loops.

The Metabolome Weakens RNA Thermodynamic Stability and Strengthens RNA Chemical Stability

We examined the complex network of interactions among RNA, the metabolome, and divalent Mg2+ under conditions that mimic the Escherichia coli cytoplasm. We determined Mg2+ binding constants for the top 15 E. coli metabolites, comprising 80% of the metabolome by concentration at physiological pH and monovalent ion concentrations. These data were used to inform the development of an artificial cytoplasm that mimics in vivo E. coli conditions, which we term "Eco80". We empirically determined that the mixture of E. coli metabolites in Eco80 approximated single-site binding behavior toward Mg2+ in the biologically relevant free Mg2+ range of ∼0.5 to 3 mM Mg2+, using a Mg2+-sensitive fluorescent dye. Effects of Eco80 conditions on the thermodynamic stability, chemical stability, structure, and catalysis of RNA were examined. We found that Eco80 conditions lead to opposing effects on the thermodynamic and chemical stabilities of RNA. In particular, the thermodynamic stability of RNA helices was weakened by 0.69 ± 0.12 kcal/mol, while the chemical stability was enhanced ∼2-fold, which can be understood using the speciation of Mg2+ between weak and strong Mg2+-metabolite complexes in Eco80. Overall, the use of Eco80 reflects RNA function in vivo and enhances the biological relevance of mechanistic studies of RNA.

Lysine-Dependent Entropy Effects in the B. subtilis Lysine Riboswitch: Insights from Single-Molecule Thermodynamic Studies

Riboswitches play an important role in RNA-based sensing/gene regulation control for many bacteria. In particular, the accessibility of multiple conformational states at physiological temperatures allows riboswitches to selectively bind a cognate ligand in the aptamer domain, which triggers secondary structural changes in the expression platform, and thereby "switching" between on or off transcriptional or translational states for the downstream RNA. The present work exploits temperature-controlled, single-molecule total internal reflection fluorescence (TIRF) microscopy to study the thermodynamic landscape of such ligand binding/folding processes, specifically for the Bacillus subtilis lysine riboswitch. The results confirm that the riboswitch folds via an induced-fit (IF) mechanism, in which cognate lysine ligand first binds to the riboswitch before structural rearrangement takes place. The transition state to folding is found to be enthalpically favored (ΔH_fold^‡ < 0), yet with a free-energy barrier that is predominantly entropic (-TΔS_fold^‡ > 0), which results in folding (unfolding) rate constants strongly dependent (independent) of lysine concentration. Analysis of the single-molecule kinetic "trajectories" reveals this rate constant dependence of k_fold on lysine to be predominantly entropic in nature, with the additional lysine conferring preferential advantage to the folding process by the presence of ligands correctly oriented with respect to the riboswitch platform. By way of contrast, van't Hoff analysis reveals enthalpic contributions to the overall folding thermodynamics (ΔH⁰) to be surprisingly constant and robustly independent of lysine concentration. The results demonstrate the crucial role of hydrogen bonding between the ligand and riboswitch platform but with only a relatively modest fraction (45%) of the overall enthalpy change needed to access the transition state and initiate transcriptional switching.

Signal amplification and optimization of riboswitch-based hybrid inputs by modular and titratable toehold switches

BACKGROUND

Synthetic biological circuits are widely utilized to control microbial cell functions. Natural and synthetic riboswitches are attractive sensor modules for use in synthetic biology applications. However, tuning the fold-change of riboswitch circuits is challenging because a deep understanding of the riboswitch mechanism and screening of mutant libraries is generally required. Therefore, novel molecular parts and strategies for straightforward tuning of the fold-change of riboswitch circuits are needed.

RESULTS

In this study, we devised a toehold switch-based modulator approach that combines a hybrid input construct consisting of a riboswitch and transcriptional repressor and de-novo-designed riboregulators named toehold switches. First, the introduction of a pair of toehold switches and triggers as a downstream signal-processing module to the hybrid input for coenzyme B12 resulted in a functional riboswitch circuit. Next, several optimization strategies that focused on balancing the expression levels of the RNA components greatly improved the fold-change from 260- to 887-fold depending on the promoter and host strain. Further characterizations confirmed low leakiness and high orthogonality of five toehold switch pairs, indicating the broad applicability of this strategy to riboswitch tuning.

CONCLUSIONS

The toehold switch-based modulator substantially improved the fold-change compared to the previous sensors with only the hybrid input construct. The programmable RNA-RNA interactions amenable to in silico design and optimization can facilitate further development of RNA-based genetic modulators for flexible tuning of riboswitch circuitry and synthetic biosensors.

Requirements for efficient ligand-gated co-transcriptional switching in designed variants of the B. subtilis pbuE adenine-responsive riboswitch in E. coli

Riboswitches, generally located in the 5'-leader of bacterial mRNAs, direct expression via a small molecule-dependent structural switch that informs the transcriptional or translational machinery. While the structure and function of riboswitch effector-binding (aptamer) domains have been intensely studied, only recently have the requirements for efficient linkage between small molecule binding and the structural switch in the cellular and co-transcriptional context begun to be actively explored. To address this aspect of riboswitch function, we have performed a structure-guided mutagenic analysis of the B. subtilis pbuE adenine-responsive riboswitch, one of the simplest riboswitches that employs a strand displacement switching mechanism to regulate transcription. Using a cell-based fluorescent protein reporter assay to assess ligand-dependent regulatory activity in E. coli, these studies revealed previously unrecognized features of the riboswitch. Within the aptamer domain, local and long-range conformational dynamics influenced by sequences within helices have a significant effect upon efficient regulatory switching. Sequence features of the expression platform including the pre-aptamer leader sequence, a toehold helix and an RNA polymerase pause site all serve to promote strong ligand-dependent regulation. By optimizing these features, we were able to improve the performance of the B. subtilis pbuE riboswitch in E. coli from 5.6-fold induction of reporter gene expression by the wild type riboswitch to over 120-fold in the top performing designed variant. Together, these data point to sequence and structural features distributed throughout the riboswitch required to strike a balance between rates of ligand binding, transcription and secondary structural switching via a strand exchange mechanism and yield new insights into the design of artificial riboswitches.

Structural basis for 2'-deoxyguanosine recognition by the 2'-dG-II class of riboswitches

A recent bioinformatic analysis of well-characterized classes of riboswitches uncovered subgroups unable to bind to the regulatory molecule of the parental class. Within the guanine/adenine class, seven groups of RNAs were identified that deviate from the consensus sequence at one or more of three positions directly involved purine nucleobase recognition, one of which was validated as a second class of 2'-deoxyguanosine riboswitch (called 2'-dG-II). To understand how 2'-dG-II riboswitches recognize their cognate ligand and how they differ from a previously identified class of 2'-deoxyguanosine binding riboswitches, we have solved the crystal structure of a 2'-dG-II aptamer domain bound to 2'-deoxyguanosine. This structure reveals a global architecture similar to other members of the purine riboswitch family, but contains key differences within the ligand binding core. Defining the 2'-dG-II riboswitches is a two-nucleotide insertion in the three-way junction that promotes novel base-base interactions. Unlike 2'-dG-I riboswitches, the 2'-dG-II class only requires local changes to the ligand binding pocket of the guanine/adenine class to achieve a change in ligand preference. Notably, members of the 2'-dG-II family have variable ability to discriminate between 2'-deoxyguanosine and riboguanosine, suggesting that a subset of 2'-dG-II riboswitches may bind either molecule to regulate gene expression.

Effect of mutations on binding of ligands to guanine riboswitch probed by free energy perturbation and molecular dynamics simulations

Riboswitches can regulate gene expression by direct and specific interactions with ligands and have recently attracted interest as potential drug targets for antibacterial. In this work, molecular dynamics (MD) simulations, free energy perturbation (FEP) and molecular mechanics generalized Born surface area (MM-GBSA) methods were integrated to probe the effect of mutations on the binding of ligands to guanine riboswitch (GR). The results not only show that binding free energies predicted by FEP and MM-GBSA obtain an excellent correlation, but also indicate that mutations involved in the current study can strengthen the binding affinity of ligands GR. Residue-based free energy decomposition was applied to compute ligand-nucleotide interactions and the results suggest that mutations highly affect interactions of ligands with key nucleotides U22, U51 and C74. Dynamics analyses based on MD trajectories indicate that mutations not only regulate the structural flexibility but also change the internal motion modes of GR, especially for the structures J12, J23 and J31, which implies that the aptamer domain activity of GR is extremely plastic and thus readily tunable by nucleotide mutations. This study is expected to provide useful molecular basis and dynamics information for the understanding of the function of GR and possibility as potential drug targets for antibacterial.

Small-Molecule-Binding Riboswitches

RNA is a versatile biomolecule capable of transferring information, taking on distinct three-dimensional shapes, and reacting to ambient conditions. RNA molecules utilize a wide range of mechanisms to control gene expression. An example of such regulation is riboswitches. Consisting exclusively of RNA, they are able to control important metabolic processes, thus providing an elegant and efficient RNA-only regulation system. Existing across all domains of life, riboswitches appear to represent one of the most highly conserved mechanisms for the regulation of a broad range of biochemical pathways. Through binding of a wide range of small-molecule ligands to their so-called aptamer domain, riboswitches undergo a conformational change in their downstream "expression platform." In consequence, the pattern of gene expression changes, which in turn results in increased or decreased protein production. Riboswitches unite the sensing and transduction of a signal that can directly be coupled to the metabolism of the cell; thus they constitute a very potent regulatory mechanism for many organisms. Highly specific RNA-binding domains not only occur in vivo but can also be evolved by means of the SELEX (systematic evolution of ligands by exponential enrichment) method, which allows in vitro selection of aptamers against almost any ligand. Coupling of these aptamers with an expression platform has led to the development of synthetic riboswitches, a highly active research field of great relevance and immense potential. The aim of this review is to summarize developments in the riboswitch field over the last decade and address key questions of recent research.

Gene regulation by a glycine riboswitch singlet uses a finely tuned energetic landscape for helical switching

Riboswitches contain structured aptamer domains that, upon ligand binding, facilitate helical switching in their downstream expression platforms to alter gene expression. To fully dissect how riboswitches function requires a better understanding of the energetic landscape for helical switching. Here, we report a sequencing-based high-throughput assay for monitoring in vitro transcription termination and use it to simultaneously characterize the functional effects of all 522 single point mutants of a glycine riboswitch type-1 singlet. Mutations throughout the riboswitch cause ligand-dependent defects, but only mutations within the terminator hairpin alter readthrough efficiencies in the absence of ligand. A comprehensive analysis of the expression platform reveals that ligand binding stabilizes the antiterminator by just 2-3 kcal/mol, indicating that the competing expression platform helices must be extremely close in energy to elicit a significant ligand-dependent response. These results demonstrate that gene regulation by this riboswitch is highly constrained by the energetics of ligand binding and conformational switching. These findings exemplify the energetic parameters of RNA conformational rearrangements driven by binding events.

Bacterial riboswitches and RNA thermometers: Nature and contributions to pathogenesis

Bacterial pathogens are always challenged by fluctuations of chemical and physical parameters that pose serious threats to cellular integrity and metabolic status. Sudden deprivation of nutrients or key metabolites, changes in surrounding pH, and temperature shifts are the most important examples of such parameters. To elicit a proper response to such fluctuations, bacterial cells coordinate the expression of parameter-relevant genes. Although protein-mediated control of gene expression is well appreciated since many decades, RNA-based regulation has been discovered in early 2000s as a parallel level of regulation. Small regulatory RNAs have emerged as one of the most widespread and important gene regulatory systems in bacteria with rare representatives found in Archaea and Eukarya. Riboswitches and thermosensors are cis-encoded RNA regulatory elements that employ different mechanisms to regulate the expression of related genes controlling key metabolic pathways and genes of temperature relevant proteins including virulence factors. The extent of RNA contributions to gene regulation is not completely known even in well-studied models such E. coli and B. subtilis. In depth understanding of riboswitches is promising for opportunity to discover a narrow spectrum antibacterial drugs that target riboswitches of essential metabolic pathways.

RNA Structural Dynamics As Captured by Molecular Simulations: A Comprehensive Overview

With both catalytic and genetic functions, ribonucleic acid (RNA) is perhaps the most pluripotent chemical species in molecular biology, and its functions are intimately linked to its structure and dynamics. Computer simulations, and in particular atomistic molecular dynamics (MD), allow structural dynamics of biomolecular systems to be investigated with unprecedented temporal and spatial resolution. We here provide a comprehensive overview of the fast-developing field of MD simulations of RNA molecules. We begin with an in-depth, evaluatory coverage of the most fundamental methodological challenges that set the basis for the future development of the field, in particular, the current developments and inherent physical limitations of the atomistic force fields and the recent advances in a broad spectrum of enhanced sampling methods. We also survey the closely related field of coarse-grained modeling of RNA systems. After dealing with the methodological aspects, we provide an exhaustive overview of the available RNA simulation literature, ranging from studies of the smallest RNA oligonucleotides to investigations of the entire ribosome. Our review encompasses tetranucleotides, tetraloops, a number of small RNA motifs, A-helix RNA, kissing-loop complexes, the TAR RNA element, the decoding center and other important regions of the ribosome, as well as assorted others systems. Extended sections are devoted to RNA-ion interactions, ribozymes, riboswitches, and protein/RNA complexes. Our overview is written for as broad of an audience as possible, aiming to provide a much-needed interdisciplinary bridge between computation and experiment, together with a perspective on the future of the field.

Allosteric mechanism of the V. vulnificus adenine riboswitch resolved by four-dimensional chemical mapping

The structural interconversions that mediate the gene regulatory functions of RNA molecules may be different from classic models of allostery, but the relevant structural correlations have remained elusive in even intensively studied systems. Here, we present a four-dimensional expansion of chemical mapping called lock-mutate-map-rescue (LM²R), which integrates multiple layers of mutation with nucleotide-resolution chemical mapping. This technique resolves the core mechanism of the adenine-responsive V. vulnificus add riboswitch, a paradigmatic system for which both Monod-Wyman-Changeux (MWC) conformational selection models and non-MWC alternatives have been proposed. To discriminate amongst these models, we locked each functionally important helix through designed mutations and assessed formation or depletion of other helices via compensatory rescue evaluated by chemical mapping. These LM²R measurements give strong support to the pre-existing correlations predicted by MWC models, disfavor alternative models, and suggest additional structural heterogeneities that may be general across ligand-free riboswitches.

Biophysical properties, thermal stability and functional impact of 8-oxo-7,8-dihydroguanine on oligonucleotides of RNA-a study of duplex, hairpins and the aptamer for preQ1 as models

A better understanding of the effects that oxidative lesions have on RNA is of importance to understand their role in the development/progression of disease. 8-oxo-7,8-dihydroguanine was incorporated into RNA to understand its structural and functional impact on RNA:RNA and RNA:DNA duplexes, hairpins and pseudoknots. One to three modifications were incorporated into dodecamers of RNA [AAGAGGGAUGAC] resulting in thermal destabilization (ΔT_m – 10°C per lesion). Hairpins with tetraloops c-UUCG*-g* (8-10), a-ACCG-g* (11-12), c-UUG*G*-g* (13-16) and c-ACG*G*-g* (17-20) were modified and used to determine thermal stabilities, concluding that: (i) modifying the stem leads to destabilization unless adenosine is the opposing basepair of 8-oxoGua; (ii) modification at the loop is position- and sequence-dependent and varies from slight stabilization to large destabilization, in some cases leading to formation of other secondary structures (hairpin→duplex). Functional effects were established using the aptamer for preQ₁ as model. Modification at G5 disrupted the stem P1 and inhibited recognition of the target molecule 7-methylamino-7-deazaguanine (preQ₁). Modifying G11 results in increased thermal stability, albeit with a K_d 4-fold larger than its canonical analog. These studies show the capability of 8-oxoG to affect structure and function of RNA, resulting in distinct outcomes as a function of number and position of the lesion.

Ligand-mediated and tertiary interactions cooperatively stabilize the P1 region in the guanine-sensing riboswitch

Riboswitches are genetic regulatory elements that control gene expression depending on ligand binding. The guanine-sensing riboswitch (Gsw) binds ligands at a three-way junction formed by paired regions P1, P2, and P3. Loops L2 and L3 cap the P2 and P3 helices and form tertiary interactions. Part of P1 belongs to the switching sequence dictating the fate of the mRNA. Previous studies revealed an intricate relationship between ligand binding and presence of the tertiary interactions, and between ligand binding and influence on the P1 region. However, no information is available on the interplay among these three main regions in Gsw. Here we show that stabilization of the L2-L3 region by tertiary interactions, and the ligand binding site by ligand binding, cooperatively influences the structural stability of terminal base pairs in the P1 region in the presence of Mg2+ ions. The results are based on molecular dynamics simulations with an aggregate simulation time of ~10 μs across multiple systems of the unbound state of the Gsw aptamer and a G37A/C61U mutant, and rigidity analyses. The results could explain why the three-way junction is a central structural element also in other riboswitches and how the cooperative effect could become contextual with respect to intracellular Mg2+ concentration. The results suggest that the transmission of allosteric information to P1 can be entropy-dominated.

Engineering and In Vivo Applications of Riboswitches

Riboswitches are common gene regulatory units mostly found in bacteria that are capable of altering gene expression in response to a small molecule. These structured RNA elements consist of two modular subunits: an aptamer domain that binds with high specificity and affinity to a target ligand and an expression platform that transduces ligand binding to a gene expression output. Significant progress has been made in engineering novel aptamer domains for new small molecule inducers of gene expression. Modified expression platforms have also been optimized to function when fused with both natural and synthetic aptamer domains. As this field expands, the use of these privileged scaffolds has permitted the development of tools such as RNA-based fluorescent biosensors. In this review, we summarize the methods that have been developed to engineer new riboswitches and highlight applications of natural and synthetic riboswitches in enzyme and strain engineering, in controlling gene expression and cellular physiology, and in real-time imaging of cellular metabolites and signals.

Bioinformatic analysis of riboswitch structures uncovers variant classes with altered ligand specificity

Riboswitches are RNAs that form complex, folded structures that selectively bind small molecules or ions. As with certain groups of protein enzymes and receptors, some riboswitch classes have evolved to change their ligand specificity. We developed a procedure to systematically analyze known riboswitch classes to find additional variants that have altered their ligand specificity. This approach uses multiple-sequence alignments, atomic-resolution structural information, and riboswitch gene associations. Among the discoveries are unique variants of the guanine riboswitch class that most tightly bind the nucleoside 2'-deoxyguanosine. In addition, we identified variants of the glycine riboswitch class that no longer recognize this amino acid, additional members of a rare flavin mononucleotide (FMN) variant class, and also variants of c-di-GMP-I and -II riboswitches that might recognize different bacterial signaling molecules. These findings further reveal the diverse molecular sensing capabilities of RNA, which highlights the potential for discovering a large number of additional natural riboswitch classes.

Ultrasensitive fluorescence detection of transcription factors based on kisscomplex formation and the T7 RNA polymerase amplification method

Herein, we report a kisscomplex based protein fluorescence assay (KPFA) method, which employed the formation of a kisscomplex and the T7 RNA polymerase amplification method, for the assay of transcription factors with high sensitivity. The detection limits of MITF and NF-κB p65 are 0.23 pM and 0.496 pM, respectively.

Role of lysine binding residues in the global folding of the lysC riboswitch

Riboswitches regulate gene expression by rearranging their structure upon metabolite binding. The lysine-sensing lysC riboswitch is a rare example of an RNA aptamer organized around a 5-way helical junction in which ligand binding is performed exclusively through nucleotides located at the junction core. We have probed whether the nucleotides involved in ligand binding play any role in the global folding of the riboswitch. As predicted, our findings indicate that ligand-binding residues are critical for the lysine-dependent gene regulation mechanism. We also find that these residues are not important for the establishment of key magnesium-dependent tertiary interactions, suggesting that folding and ligand recognition are uncoupled in this riboswitch for the formation of specific interactions. However, FRET assays show that lysine binding results in an additional conformational change, indicating that lysine binding may also participate in a specific folding transition. Thus, in contrast to helical junctions being primary determinants in ribozymes and rRNA folding, we speculate that the helical junction of the lysine-sensing lysC riboswitch is not employed as structural a scaffold to direct global folding, but rather has a different role in establishing RNA-ligand interactions required for riboswitch regulation. Our work suggests that helical junctions may adopt different functions such as the coordination of global architecture or the formation of specific ligand binding site.

'Z-DNA like' fragments in RNA: a recurring structural motif with implications for folding, RNA/protein recognition and immune response

Since the work of Alexander Rich, who solved the first Z-DNA crystal structure, we have known that d(CpG) steps can adopt a particular structure that leads to forming left-handed helices. However, it is still largely unrecognized that other sequences can adopt ‘left-handed’ conformations in DNA and RNA, in double as well as single stranded contexts. These ‘Z-like’ steps involve the coexistence of several rare structural features: a C2’-endo puckering, a syn nucleotide and a lone pair–π stacking between a ribose O4’ atom and a nucleobase. This particular arrangement induces a conformational stress in the RNA backbone, which limits the occurrence of Z-like steps to ≈0.1% of all dinucleotide steps in the PDB. Here, we report over 600 instances of Z-like steps, which are located within r(UNCG) tetraloops but also in small and large RNAs including riboswitches, ribozymes and ribosomes. Given their complexity, Z-like steps are probably associated with slow folding kinetics and once formed could lock a fold through the formation of unique long-range contacts. Proteins involved in immunologic response also specifically recognize/induce these peculiar folds. Thus, characterizing the conformational features of these motifs could be a key to understanding the immune response at a structural level.

Multilevel Regulation and Translational Switches in Synthetic Biology

In contrast to the versatility of regulatory mechanisms in natural systems, synthetic genetic circuits have been so far predominantly composed of transcriptionally regulated modules. This is about to change as the repertoire of foundational tools for post-transcriptional regulation is quickly expanding. We provide an overview of the different types of translational regulators: protein, small molecule and ribonucleic acid (RNA) responsive and we describe the new emerging circuit designs utilizing these tools. There are several advantages of achieving multilevel regulation via translational switches and it is likely that such designs will have the greatest and earliest impact in mammalian synthetic biology for regenerative medicine and gene therapy applications.

Force field dependence of riboswitch dynamics

Riboswitches are noncoding regulatory elements that control gene expression in response to the presence of metabolites, which bind to the aptamer domain. Metabolite binding appears to occur through a combination of conformational selection and induced fit mechanism. This demands to characterize the structural dynamics of the apo state of aptamer domains. In principle, molecular dynamics (MD) simulations can give insights at the atomistic level into the dynamics of the aptamer domain. However, it is unclear to what extent contemporary force fields can bias such insights. Here, we show that the Amber force field ff99 yields the best agreement with detailed experimental observations on differences in the structural dynamics of wild type and mutant aptamer domains of the guanine-sensing riboswitch (Gsw), including a pronounced influence of Mg2+. In contrast, applying ff99 with parmbsc0 and parmχOL modifications (denoted ff10) results in strongly damped motions and overly stable tertiary loop-loop interactions. These results are based on 58 MD simulations with an aggregate simulation time>11 μs, careful modeling of Mg2+ ions, and thorough statistical testing. Our results suggest that the moderate stabilization of the χ-anti region in ff10 can have an unwanted damping effect on functionally relevant structural dynamics of marginally stable RNA systems. This suggestion is supported by crystal structure analyses of Gsw aptamer domains that reveal χ torsions with high-anti values in the most mobile regions. We expect that future RNA force field development will benefit from considering marginally stable RNA systems and optimization toward good representations of dynamics in addition to structural characteristics.

Structure-guided mutational analysis of gene regulation by the Bacillus subtilis pbuE adenine-responsive riboswitch in a cellular context

Riboswitches are a broadly distributed form of RNA-based gene regulation in Bacteria and, more rarely, Archaea and Eukarya. Most often found in the 5'-leader sequence of bacterial mRNAs, they are generally composed of two functional domains: a receptor (aptamer) domain that binds an effector molecule and a regulatory domain (or expression platform) that instructs the expression machinery. One of the most studied riboswitches is the Bacillus subtilis adenine-responsive pbuE riboswitch, which regulates gene expression at the transcriptional level, up-regulating expression in response to increased intracellular effector concentrations. In this work, we analyzed sequence and structural elements that contribute to efficient ligand-dependent regulatory activity in a co-transcriptional and cellular context. Unexpectedly, we found that the P1 helix, which acts as the antitermination element of the switch in this RNA, supported ligand-dependent activation of a reporter gene over a broad spectrum of lengths from 3 to 10 bp. This same trend was also observed using a minimal in vitro single-turnover transcription assay, revealing that this behavior is intrinsic to the RNA sequence. We also found that the sequences at the distal tip of the terminator not directly involved in alternative secondary structure formation are highly important for efficient regulation. These data strongly support a model in which the switch is highly localized to the P1 helix adjacent to the ligand-binding pocket that likely presents a local kinetic block to invasion of the aptamer by the terminator.

High-throughput mutate-map-rescue evaluates SHAPE-directed RNA structure and uncovers excited states

The three-dimensional conformations of noncoding RNAs underpin their biochemical functions but have largely eluded experimental characterization. Here, we report that integrating a classic mutation/rescue strategy with high-throughput chemical mapping enables rapid RNA structure inference with unusually strong validation. We revisit a 16S rRNA domain for which SHAPE (selective 2'-hydroxyl acylation with primer extension) and limited mutational analysis suggested a conformational change between apo- and holo-ribosome conformations. Computational support estimates, data from alternative chemical probes, and mutate-and-map (M(2)) experiments highlight issues of prior methodology and instead give a near-crystallographic secondary structure. Systematic interrogation of single base pairs via a high-throughput mutation/rescue approach then permits incisive validation and refinement of the M(2)-based secondary structure. The data further uncover the functional conformation as an excited state (20 ± 10% population) accessible via a single-nucleotide register shift. These results correct an erroneous SHAPE inference of a ribosomal conformational change, expose critical limitations of conventional structure mapping methods, and illustrate practical steps for more incisively dissecting RNA dynamic structure landscapes.

The purine riboswitch as a model system for exploring RNA biology and chemistry

Over the past decade the purine riboswitch, and in particular its nucleobase-binding aptamer domain, has emerged as an important model system for exploring various aspects of RNA structure and function. Its relatively small size, structural simplicity and readily observable activity enable application of a wide variety of experimental approaches towards the study of this RNA. These analyses have yielded important insights into small molecule recognition, co-transcriptional folding and secondary structural switching, and conformational dynamics that serve as a paradigm for other RNAs. In this article, the current state of understanding of the purine riboswitch family and how this growing knowledge base is starting to be exploited in the creation of novel RNA devices are examined. This article is part of a Special Issue entitled: Riboswitches.

ITC analysis of ligand binding to preQ₁ riboswitches

Riboswitches regulate genes by binding to small-molecule effectors. Isothermal titration calorimetry (ITC) provides a label-free method to quantify the equilibrium association constant, K(A), of a riboswitch interaction with its cognate ligand. In addition to probing affinity and specific chemical contributions that contribute to binding, ITC can be used to measure the thermodynamic parameters of an interaction (ΔG, ΔH, and ΔS), in addition to the binding stoichiometry (N). Here, we describe methods developed to measure the binding affinity of various preQ1 riboswitch classes for the pyrrolopyrimidine effector, preQ1. Example isotherms are provided along with a review of various preQ1-II (class 2) riboswitch mutants that were interrogated by ITC to quantify the energetic contributions of specific interactions visualized in the crystal structure. Protocols for ITC are provided in sufficient detail that the reader can reproduce experiments independently, or develop derivative methods suitable for analyzing novel riboswitch-ligand binding interactions.

Comparative sequence and structure analysis reveals the conservation and diversity of nucleotide positions and their associated tertiary interactions in the riboswitches

The tertiary motifs in complex RNA molecules play vital roles to either stabilize the formation of RNA 3D structure or to provide important biological functionality to the molecule. In order to better understand the roles of these tertiary motifs in riboswitches, we examined 11 representative riboswitch PDB structures for potential agreement of both motif occurrences and conservations. A total of 61 unique tertiary interactions were found in the reference structures. In addition to the expected common A-minor motifs and base-triples mainly involved in linking distant regions the riboswitch structures three highly conserved variants of A-minor interactions called G-minors were found in the SAM-I and FMN riboswitches where they appear to be involved in the recognition of the respective ligand’s functional groups. From our structural survey as well as corresponding structure and sequence alignments, the agreement between motif occurrences and conservations are very prominent across the representative riboswitches. Our analysis provide evidence that some of these tertiary interactions are essential components to form the structure where their sequence positions are conserved despite a high degree of diversity in other parts of the respective riboswitches sequences. This is indicative of a vital role for these tertiary interactions in determining the specific biological function of riboswitch.

The promise of riboswitches as potential antibacterial drug targets

Riboswitches represent promising novel RNA structures for developing compounds that artificially regulate gene expression and, thus, bacterial growth. The past years have seen increasing efforts to identify metabolite-analogues which act on riboswitches and which reveal antibacterial activity. Here, we summarize the current inventory of riboswitch-targeting compounds, their characteristics and antibacterial potential.

Modularity of select riboswitch expression platforms enables facile engineering of novel genetic regulatory devices

RNA-based biosensors and regulatory devices have received significant attention for their potential in a broad array of synthetic biology applications. One of the primary difficulties in engineering these molecules is the lack of facile methods to link sensory modules, or aptamers, to readout domains. Such efforts typically require extensive screening or selection of sequences that facilitate interdomain communication. Bacteria have evolved a widespread form of gene regulation known as riboswitches that perform this task with sufficient fidelity to control expression of biosynthetic and transport proteins essential for normal cellular homeostasis. In this work, we demonstrate that select riboswitch readout domains, called expression platforms, are modular in that they can host a variety of natural and synthetic aptamers to create novel chimeric RNAs that regulate transcription both in vitro and in vivo. Importantly, this technique does not require selection of device-specific "communication modules" required to transmit ligand binding to the regulatory domain, enabling rapid engineering of novel functional RNAs.

Nucleotides adjacent to the ligand-binding pocket are linked to activity tuning in the purine riboswitch

Direct sensing of intracellular metabolite concentrations by riboswitch RNAs provides an economical and rapid means to maintain metabolic homeostasis. Since many organisms employ the same class of riboswitch to control different genes or transcription units, it is likely that functional variation exists in riboswitches such that activity is tuned to meet cellular needs. Using a bioinformatic approach, we have identified a region of the purine riboswitch aptamer domain that displays conservation patterns linked to riboswitch activity. Aptamer domain compositions within this region can be divided into nine classes that display a spectrum of activities. Naturally occurring compositions in this region favor rapid association rate constants and slow dissociation rate constants for ligand binding. Using X-ray crystallography and chemical probing, we demonstrate that both the free and bound states are influenced by the composition of this region and that modest sequence alterations have a dramatic impact on activity. The introduction of non-natural compositions result in the inability to regulate gene expression in vivo, suggesting that aptamer domain activity is highly plastic and thus readily tunable to meet cellular needs.

Information-theoretic uncertainty of SCFG-modeled folding space of the non-coding RNA

RNA secondary structure ensembles define probability distributions for alternative equilibrium secondary structures of an RNA sequence. Shannon's entropy is a measure for the amount of diversity present in any ensemble. In this work, Shannon's entropy of the SCFG ensemble on an RNA sequence is derived and implemented in polynomial time for both structurally ambiguous and unambiguous grammars. Micro RNA sequences generally have low folding entropy, as previously discovered. Surprisingly, signs of significantly high folding entropy were observed in certain ncRNA families. More effective models coupled with targeted randomization tests can lead to a better insight into folding features of these families.

AVAILABILITY

URL http://www.plantbio.uga.edu/~russell/index.php?s=1&n=5&r=0.

Denaturation of RNA secondary and tertiary structure by urea: simple unfolded state models and free energy parameters account for measured m-values

To investigate the mechanism by which urea destabilizes RNA structure, urea-induced unfolding of four different RNA secondary and tertiary structures was quantified in terms of an m-value, the rate at which the free energy of unfolding changes with urea molality. From literature data and our osmometric study of a backbone analogue, we derived average interaction potentials (per square angstrom of solvent accessible surface) between urea and three kinds of RNA surfaces: phosphate, ribose, and base. Estimates of the increases in solvent accessible surface areas upon RNA denaturation were based on a simple model of unfolded RNA as a combination of helical and single-strand segments. These estimates, combined with the three interaction potentials and a term to account for interactions of urea with released ions, yield calculated m-values that are in good agreement with experimental values (200 mm monovalent salt). Agreement was obtained only if single-stranded RNAs were modeled in a highly stacked, A-form conformation. The primary driving force for urea-induced denaturation is the strong interaction of urea with the large surface areas of bases that become exposed upon denaturation of either RNA secondary or tertiary structure, though interactions of urea with backbone and released ions may account for up to a third of the m-value. Urea m-values for all four RNAs are salt-dependent, which we attribute to an increased extension (or decreased charge density) of unfolded RNAs with an increased urea concentration. The sensitivity of the urea m-value to base surface exposure makes it a potentially useful probe of the conformations of RNA unfolded states.

Structure and mechanism of purine-binding riboswitches

A riboswitch is a non-protein coding sequence capable of directly binding a small molecule effector without the assistance of accessory proteins to regulate expression of the mRNA in which it is embedded. Currently, over 20 different classes of riboswitches have been validated in bacteria with the promise of many more to come, making them an important means of regulating the genome in the bacterial kingdom. Strikingly, half of the known riboswitches recognize effector compounds that contain a purine or related moiety. In the last decade, significant progress has been made to determine how riboswitches specifically recognize these compounds against the background of many other similar cellular metabolites and transduce this signal into a regulatory response. Of the known riboswitches, the purine family containing guanine, adenine and 2'-deoxyguanosine-binding classes are the most extensively studied, serving as a simple and useful paradigm for understanding how these regulatory RNAs function. This review provides a comprehensive summary of the current state of knowledge regarding the structure and mechanism of these riboswitches, as well as insights into how they might be exploited as therapeutic targets and novel biosensors.

Mechanisms for differentiation between cognate and near-cognate ligands by purine riboswitches

Riboswitches are elements in the 5'-untranslated region of mRNAs that regulate gene expression by directly interacting with metabolites related to their own gene products. A remarkable feature of this gene regulation mechanism is the high specificity of riboswitches for their cognate ligands. In this study, we used a combination of static and time-resolved NMR-spectroscopic methods to investigate the mechanisms for ligand specificity in purine riboswitches. We investigate the xpt-aptamer domain from a guanine-responsive riboswitch and the mfl-aptamer domain from a 2'-deoxyguanosine-responsive riboswitch. The xpt-aptamer binds the purine nucleobases guanine/hypoxanthine with high affinity, but, unexpectedly, also the nucleoside 2'-deoxyguanosine. On the other hand, the mfl-aptamer is highly specific for its cognate ligand 2'-deoxyguanosine, and does not bind purine ligands. We addressed the question of aptamer`s ligand specificity by real-time NMR spectroscopy. Our studies of ligand binding and subsequently induced aptamer folding revealed that the xpt-aptamer discriminates against non-cognate ligands by enhanced life-times of the cognate complex compared with non-cognate complexes, whereas the mfl-aptamer rejects non-cognate ligands at the level of ligand association, employing a kinetic proofreading mechanism.

Application of constrained real-space refinement of flexible molecular fragments to automatic model building of RNA structures

New methods have been developed for locating phosphate groups and nucleic acid bases in the electron density of RNA structures. These methods utilize a constrained real-space refinement of molecular fragments and a phased rotation–conformation–translation function. Real-space refinement has also contributed to the improvement of the bone/base method of RNA model building and to redesigning the method of building double helices in nucleic acid structures. This improvement is reflected in the increased accuracy of the model building and the ability to better distinguish between correct and false solutions. A program,RSR, was created, and the programsNUT,HELandDHLwere upgraded and organized into a program system, which is CCP4 oriented. Source codes will also be released.

Role of ligand binding in structural organization of add A-riboswitch aptamer: a molecular dynamics simulation

The specific binding of ligands is the first step of gene expression or translation regulation by riboswitches. However, understanding the mechanism of the specific binding is still difficult because the tertiary structures of the riboswitch aptamers are available almost only for ligand-bound state at present. In this paper we hope to give some insights into this problem through the studies of the role of ligand-aptamer interaction in the structural organization of add A-riboswitch aptamer, based on the crystal structure of the ligand-bound aptamer. We use all-atom molecular dynamics to simulate the behaviors of the aptamer in ligand-bound, free and mutated states by Amber force field. The results show that the correct paring of the ligand adenine with the nucleotide U74 in the binding pocket is crucial to stabilizing the conformations of the ligand-bound aptamer, especially the helix P1 connecting the expression platform. Our results also suggest that both the nucleotide U74 and U51 may be the key sites of the ligand recognition but the former has much higher probability as the initial docking site. This is in agreement with previous experimental results.

The structure of a tetrahydrofolate-sensing riboswitch reveals two ligand binding sites in a single aptamer

Transport and biosynthesis of folate and its derivatives are frequently controlled by the tetrahydrofolate (THF) riboswitch in Firmicutes. We have solved the crystal structure of the THF riboswitch aptamer in complex with folinic acid, a THF analog. Uniquely, this structure reveals two molecules of folinic acid binding to a single structured domain. These two sites interact with ligand in a similar fashion, primarily through recognition of the reduced pterin moiety. 7-deazaguanine, a soluble analog of guanine, binds the riboswitch with nearly the same affinity as its natural effector. However, 7-deazaguanine effects transcriptional termination to a substantially lesser degree than folinic acid, suggesting that the cellular guanine pool does not act upon the THF riboswitch. Under physiological conditions the ligands display strong cooperative binding, with one of the two sites playing a greater role in eliciting the regulatory response, which suggests that the second site may play another functional role.

Structural principles of nucleoside selectivity in a 2'-deoxyguanosine riboswitch

Purine riboswitches play an essential role in genetic regulation of bacterial metabolism. This family includes the 2′-deoxyguanosine (dG) riboswitch, involved in feedback control of deoxyguanosine biosynthesis. To understand the principles that define dG selectivity, we determined crystal structures of natural Mesoplasma florum riboswitch bound to cognate dG, as well as non-cognate guanosine, deoxyguanosine monophosphate and guanosine monophosphate. Comparison with related purine riboswitch structures reveals that the dG riboswitch achieves its specificity by modifying key interactions involving the nucleobase and through rearrangement of the ligand-binding pocket, so as to accommodate the additional sugar moiety. In addition, we observe novel conformational changes beyond the junctional binding pocket, extending as far as peripheral loop-loop interactions. It appears that re-engineering riboswitch scaffolds will require consideration of selectivity features dispersed throughout the riboswitch tertiary fold, and that structure-guided drug design efforts targeted to junctional RNA scaffolds need to be addressed within such an expanded framework.

Classification of pseudo pairs between nucleotide bases and amino acids by analysis of nucleotide-protein complexes

Nucleotide bases are recognized by amino acid residues in a variety of DNA/RNA binding and nucleotide binding proteins. In this study, a total of 446 crystal structures of nucleotide–protein complexes are analyzed manually and pseudo pairs together with single and bifurcated hydrogen bonds observed between bases and amino acids are classified and annotated. Only 5 of the 20 usual amino acid residues, Asn, Gln, Asp, Glu and Arg, are able to orient in a coplanar fashion in order to form pseudo pairs with nucleotide bases through two hydrogen bonds. The peptide backbone can also form pseudo pairs with nucleotide bases and presents a strong bias for binding to the adenine base. The Watson–Crick side of the nucleotide bases is the major interaction edge participating in such pseudo pairs. Pseudo pairs between the Watson–Crick edge of guanine and Asp are frequently observed. The Hoogsteen edge of the purine bases is a good discriminatory element in recognition of nucleotide bases by protein side chains through the pseudo pairing: the Hoogsteen edge of adenine is recognized by various amino acids while the Hoogsteen edge of guanine is only recognized by Arg. The sugar edge is rarely recognized by either the side-chain or peptide backbone of amino acid residues.

Constitutive regulatory activity of an evolutionarily excluded riboswitch variant

The exquisite specificity of the adenine-responsive riboswitch toward its cognate metabolite has been shown to arise from the formation of a Watson-Crick interaction between the adenine ligand and residue U65. A recent crystal structure of a U65C adenine aptamer variant has provided a rationale for the phylogenetic conservation observed at position 39 for purine aptamers. The G39-C65 variant adopts a compact ligand-free structure in which G39 is accommodated by the ligand binding site and is base-paired to the cytosine at position 65. Here, we demonstrate using a combination of biochemical and biophysical techniques that the G39-C65 base pair not only severely impairs ligand binding but also disrupts the functioning of the riboswitch in vivo by constitutively activating gene expression. Folding studies using single-molecule FRET revealed that the G39-C65 variant displays a low level of dynamic heterogeneity, a feature reminiscent of ligand-bound wild-type complexes. A restricted conformational freedom together with an ability to significantly fold in monovalent ions are exclusive to the G39-C65 variant. This work provides a mechanistic framework to rationalize the evolutionary exclusion of certain nucleotide combinations in favor of sequences that preserve ligand binding and gene regulation functionalities.

Riboswitches: structures and mechanisms

A critical feature of the hypothesized RNA world would have been the ability to control chemical processes in response to environmental cues. Riboswitches present themselves as viable candidates for a sophisticated mechanism of regulatory control in RNA-based life. These regulatory elements in the modern world are most commonly found in the 5'-untranslated regions of bacterial mRNAs, directly interacting with metabolites as a means of regulating expression of the coding region via a secondary structural switch. In this review, we focus on recent insights into how these RNAs fold into complex architectures capable of both recognizing a specific small molecule compound and exerting regulatory control over downstream sequences, with an emphasis on transcriptional regulation.

A new way to see RNA

Unlike proteins, the RNA backbone has numerous degrees of freedom (eight, if one counts the sugar pucker), making RNA modeling, structure building and prediction a multidimensional problem of exceptionally high complexity. And yet RNA tertiary structures are not infinite in their structural morphology; rather, they are built from a limited set of discrete units. In order to reduce the dimensionality of the RNA backbone in a physically reasonable way, a shorthand notation was created that reduced the RNA backbone torsion angles to two (η and θ, analogous to φ and ψ in proteins). When these torsion angles are calculated for nucleotides in a crystallographic database and plotted against one another, one obtains a plot analogous to a Ramachandran plot (the η/θ plot), with highly populated and unpopulated regions. Nucleotides that occupy proximal positions on the plot have identical structures and are found in the same units of tertiary structure. In this review, we describe the statistical validation of the η/θ formalism and the exploration of features within the η/θ plot. We also describe the application of the η/θ formalism in RNA motif discovery, structural comparison, RNA structure building and tertiary structure prediction. More than a tool, however, the η/θ formalism has provided new insights into RNA structure itself, revealing its fundamental components and the factors underlying RNA architectural form.

Structure and dynamics of the deoxyguanosine-sensing riboswitch studied by NMR-spectroscopy

The mfl-riboswitch regulates expression of ribonucleotide reductase subunit in Mesoplasma florum by binding to 2′-deoxyguanosine and thereby promoting transcription termination. We characterized the structure of the ligand-bound aptamer domain by NMR spectroscopy and compared the mfl-aptamer to the aptamer domain of the closely related purine-sensing riboswitches. We show that the mfl-aptamer accommodates the extra 2′-deoxyribose unit of the ligand by forming a more relaxed binding pocket than these found in the purine-sensing riboswitches. Tertiary structures of the xpt-aptamer bound to guanine and of the mfl-aptamer bound to 2′-deoxyguanosine exhibit very similar features, although the sequence of the mfl-aptamer contains several alterations compared to the purine-aptamer consensus sequence. These alterations include the truncation of a hairpin loop which is crucial for complex formation in all purine-sensing riboswitches characterized to date. We further defined structural features and ligand binding requirements of the free mfl-aptamer and found that the presence of Mg²⁺ is not essential for complex formation, but facilitates ligand binding by promoting pre-organization of key structural motifs in the free aptamer.

Nanoporous silica colloidal films with molecular transport gated by aptamers responsive to small molecules

We report the preparation of colloidal nanoporous silica films whose function mimics that of protein channels in gating the transport of small molecules across a cell membrane. Specifically, we report a means of controlling the molecular flux through colloidal nanopores that employ aptamer oligonucleotides binding to a specific organic small molecule (cocaine). These biomacromolecules have been introduced onto the nanopore surface by attaching pre-made oligonucleotides to the activated nanopore surface. The aptamers change their conformation in response to the binding events, and thus alter the free volume of the colloidal nanopores available for molecular transport.

Molecular insights into the ligand-controlled organization of the SAM-I riboswitch

S-adenosylmethionine (SAM) riboswitches are widespread in bacteria, and up to five different SAM riboswitch families have been reported, highlighting the relevance of SAM regulation. On the basis of crystallographic and biochemical data, it has been postulated, but never demonstrated, that ligand recognition by SAM riboswitches involves key conformational changes in the RNA architecture. We show here that the aptamer follows a two-step hierarchical folding selectively induced by metal ions and ligand binding, each of them leading to the formation of one of the two helical stacks observed in the crystal structure. Moreover, we find that the anti-antiterminator P1 stem is rotated along its helical axis upon ligand binding, a mechanistic feature that could be common to other riboswitches. We also show that the nonconserved P4 helical domain is used as an auxiliary element to enhance the ligand-binding affinity. This work provides the first comprehensive characterization, to our knowledge, of a ligand-controlled riboswitch folding pathway.

Effects of Mg2+ on the free energy landscape for folding a purine riboswitch RNA

There are potentially several ways Mg2+ might promote formation of an RNA tertiary structure: by causing a general "collapse" of the unfolded ensemble to more compact conformations, by favoring a reorganization of structure within a domain to a form with specific tertiary contacts, and by enhancing cooperative linkages between different sets of tertiary contacts. To distinguish these different modes of action, we have studied Mg2+ interactions with the adenine riboswitch, in which a set of tertiary interactions that forms around a purine-binding pocket is thermodynamically linked to the tertiary "docking" of two hairpin loops in another part of the molecule. Each of four RNA forms with different extents of tertiary structure were characterized by small-angle X-ray scattering. The free energy of interconversion between different conformations in the absence of Mg2+ and the free energy of Mg2+ interaction with each form have been estimated, yielding a complete picture of the folding energy landscape as a function of Mg2+ concentration. At 1 mM Mg2+ (50 mM K+), the overall free energy of stabilization by Mg2+ is large, -9.8 kcal/mol, and about equally divided between its effect on RNA collapse to a partially folded structure and on organization of the binding pocket. A strong cooperative linkage between the two sets of tertiary contacts is intrinsic to the RNA. This quantitation of the effects of Mg2+ on an RNA with two distinct sets of tertiary interactions suggests ways that Mg2+ may work to stabilize larger and more complex RNA structures.

Riboswitches as hormone receptors: hypothetical cytokinin-binding riboswitches in Arabidopsis thaliana

Background

Riboswitches are mRNA elements that change conformation when bound to small molecules. They are known to be key regulators of biosynthetic pathways in both prokaryotes and eukaryotes.

Presentation of the Hypothesis

The hypothesis presented here is that riboswitches function as receptors in hormone perception. We propose that riboswitches initiate or integrate signaling cascades upon binding to classic signaling molecules. The molecular interactions for ligand binding and gene expression control would be the same as for biosynthetic pathways, but the context and the cadre of ligands to consider is dramatically different. The hypothesis arose from the observation that a compound used to identify adenine binding RNA sequences is chemically similar to the classic plant hormone, or growth regulator, cytokinin. A general tenet of the hypothesis is that riboswitch-binding metabolites can be used to make predictions about chemically related signaling molecules. In fact, all cell permeable signaling compounds can be considered as potential riboswitch ligands. The hypothesis is plausible, as demonstrated by a cursory review of the transcriptome and genome of the model plant Arabidopsis thaliana for transcripts that i) contain an adenine aptamer motif, and ii) are also predicted to be cytokinin-regulated. Here, one gene, CRK10 (for Cysteine-rich Receptor-like Kinase 10, At4g23180), contains an adenine aptamer-related sequence and is down-regulated by cytokinin approximately three-fold in public gene expression data. To illustrate the hypothesis, implications of cytokinin-binding to the CRK10 mRNA are discussed.

Testing the hypothesis

At the broadest level, screening various cell permeable signaling molecules against random RNA libraries and comparing hits to sequence and gene expression data bases could determine how broadly the hypothesis applies. Specific cases, such as CRK10 presented here, will require experimental validation of direct ligand binding, altered RNA conformation, and effect on gene expression. Each case will be different depending on the signaling pathway and the physiology involved.

Implications of the hypothesis

This would be a very direct signal perception mechanism for regulating gene expression; rivaling animal steroid hormone receptors, which are frequently ligand dependent transcription initiation factors. Riboswitch-regulated responses could occur by modulating target RNA stability, translatability, and alternative splicing - all known expression platforms used in riboswitches. The specific illustration presented, CRK10, implies a new mechanism for the perception of cytokinin, a classic plant hormone. Experimental support for the hypothesis would add breadth to the growing list of important functions attributed to riboswitches.

Reviewers

This article was reviewed by Anthony Poole, Rob Knight, Mikhail Gelfand.