High-throughput single-nucleotide structural mapping by capillary automated footprinting analysis

IPANEMAP Suite: a pipeline for probing-informed RNA structure modeling

In addition to their sequence, multiple functions of RNAs are encoded within their structure, which is often difficult to solve using physico-chemical methods. Incorporating low-resolution experimental data such as chemical probing into computational prediction significantly enhances RNA structure modeling accuracy. While medium- and high-throughput RNA structure probing techniques are widely accessible, the subsequent analysis process can be cumbersome, involving multiple software and manual data manipulation. In addition, the relevant interpretation of the data requires proper parameterization of the software and a strict consistency in the analysis pipeline. To streamline such workflows, we introduce IPANEMAP Suite, a comprehensive platform that guides users from chemically probing raw data to visually informative secondary structure models. IPANEMAP Suite seamlessly integrates various experimental datasets and facilitates comparative analysis of RNA structures under different conditions (footprinting), aiding in the study of protein or small molecule interactions with RNA. Here, we show that the unique ability of IPANEMAP Suite to perform integrative modeling using several chemical probing datasets with phylogenetic data can be instrumental in obtaining accurate secondary structure models. The platform's project-based approach ensures full traceability and generates publication-quality outputs, simplifying the entire RNA structure analysis process. IPANEMAP Suite is freely available at https://github.com/Sargueil-CiTCoM/ipasuite under a GPL-3.0 license.

Technological advancements in deciphering RNA-RNA interactions

RNA-RNA interactions (RRIs) can dictate RNA molecules to form intricate higher-order structures and bind their RNA substrates in diverse biological processes. To elucidate the function, binding specificity, and regulatory mechanisms of various RNA molecules, especially the vast repertoire of non-coding RNAs, advanced technologies and methods that globally map RRIs are extremely valuable. In the past decades, many state-of-the-art technologies have been developed for this purpose. This review focuses on those high-throughput technologies for the global mapping of RRIs. We summarize the key concepts and the pros and cons of different technologies. In addition, we highlight the novel biological insights uncovered by these RRI mapping methods and discuss the future challenges for appreciating the crucial roles of RRIs in gene regulation across bacteria, viruses, archaea, and mammals.

Modeling the structure and DAP5-binding site of the FGF-9 5'-UTR RNA utilized in cap-independent translation

Cap-independent or eukaryotic initiation factor (eIF) 4E-independent, translation initiation in eukaryotes requires scaffolding protein eIF4G or its homolog, death-associated protein 5 (DAP5). eIF4G associates with the 40S ribosomal subunit, recruiting the ribosome to the RNA transcript. A subset of RNA transcripts, such as fibroblast growth factor 9 (FGF-9), contain 5' untranslated regions (5' UTRs) that directly bind DAP5 or eIF4GI. For viral mRNA, eIF recruitment usually utilizes RNA structure, such as a pseudoknot or stem-loops, and the RNA-helicase eIF4A is required for DAP5- or 4G-mediated translation, suggesting these 5' UTRs are structured. However, for cellular IRES-like translation, no consensus RNA structures or sequences have yet been identified for eIF binding. However, the DAP5-binding site within the FGF-9 5' UTR is unknown. Moreover, DAP5 binds to other, dissimilar 5' UTRs, some of which require an unpaired, accessible 5' end to stimulate cap-independent translation. Using SHAPE-seq, we modeled the 186 nt FGF-9 5'-UTR RNA's complex secondary structure in vitro. Further, DAP5 footprinting, toeprinting, and UV cross-linking experiments identify DAP5-RNA interactions. Modeling of FGF-9 5'-UTR tertiary structure aligns DAP5-interacting nucleotides on one face of the predicted structure. We propose that RNA structure involving tertiary folding, rather than a conserved sequence or secondary structure, acts as a DAP5-binding site. DAP5 appears to contact nucleotides near the start codon. Our findings offer a new perspective in the hunt for cap-independent translational enhancers. Structural, rather than sequence-specific, eIF-binding sites may act as attractive chemotherapeutic targets or as dosage tools for mRNA-based therapies.

RNA Secondary Structure Modeling Following the IPANEMAP Workflow

The structure of RNA molecules and their complexes are crucial for understanding biology at the molecular level. Resolving these structures holds the key to understanding their manifold structure-mediated functions ranging from regulating gene expression to catalyzing biochemical processes. Predicting RNA secondary structure is a prerequisite and a key step to accurately model their three dimensional structure. Although dedicated modelling software are making fast and significant progresses, predicting an accurate secondary structure from the sequence remains a challenge. Their performance can be significantly improved by the incorporation of experimental RNA structure probing data. Many different chemical and enzymatic probes have been developed; however, only one set of quantitative data can be incorporated as constraints for computer-assisted modelling. IPANEMAP is a recent workflow based on RNAfold that can take into account several quantitative or qualitative data sets to model RNA secondary structure. This chapter details the methods for popular chemical probing (DMS, CMCT, SHAPE-CE, and SHAPE-Map) and the subsequent analysis and structure prediction using IPANEMAP.

Modeling the Structure and DAP5 Binding Site of a Cap-Independent Translational Enhancer mRNA

Cap-independent translation initiation in eukaryotes involves initiation factor (eIF) binding to a transcript's 5' untranslated region (UTR). Internal-ribosome-entry-site (IRES)-like cap-independent translation initiation does not require a free 5' end for eIF binding, as eIFs recruit the ribosome to or near the start codon. For viral mRNA, recruitment usually utilizes RNA structure, such as a pseudoknot. However, for cellular mRNA cap-independent translation, no consensus RNA structures or sequences have yet been identified for eIF binding. Fibroblast-growth factor 9 (FGF-9) is a member of a subset of mRNA that are cap-independently upregulated in breast and colorectal cancer cells using this IRES-like method. Death-associated factor 5 (DAP5), an eIF4GI homolog, binds directly to the FGF-9 5' UTR to initiate translation. However, the DAP5 binding site within the FGF-9 5' UTR is unknown. Moreover, DAP5 binds to other, dissimilar 5' UTRs, some of which need a free 5' end to stimulate cap-independent translation. We propose that a particular RNA structure involving tertiary folding, rather than a conserved sequence or secondary structure, acts as a DAP5 binding site. Using SHAPE-seq, we modeled the FGF-9 5' UTR RNA's complex secondary and tertiary structure in vitro. Further, DAP5 footprinting and toeprinting experiments show DAP5's preference for one face of this structure. DAP5 binding appears to stabilize a higher-energy RNA fold that frees the 5' end to solvent and brings the start codon close to the recruited ribosome. Our findings offer a fresh perspective in the hunt for cap-independent translational enhancers. Structural, rather than sequence-specific, eIF binding sites may act as attractive chemotherapeutic targets or as dosage tools for mRNA-based therapies.

RNA Chemical Labeling with Site-Specific, Relative Quantification by Mass Spectrometry for the Structural Study of a Neomycin-Sensing Riboswitch Aptamer Domain

High-resolution mass spectrometry was used for the label-free, direct localization and relative quantification of CMC+ -modifications of a neomycin-sensing riboswitch aptamer domain in the absence and presence of the aminoglycoside ligands neomycin B, ribostamycin, and paromomycin. The chemical probing and MS data for the free riboswitch show high exposure to solvent of the uridine nucleobases U7, U8, U13, U14, U18 as part of the proposed internal and apical loops, but those of U10 and U21 as part of the proposed internal loop were found to be far less exposed than expected. Thus, our data are in better agreement with the proposed secondary structure of the riboswitch in complexes with aminoglycosides than with that of free RNA. For the riboswitch in complexes with neomycin B, ribostamycin, and paromomycin, we found highly similar CMC+ -modification patterns and excellent agreement with previous NMR studies. Differences between the chemical probing and MS data in the absence and presence of the aminoglycoside ligands were quantitative rather than qualitative (i. e., the same nucleobases were labeled, but to different extents) and can be rationalized by stabilization of both the proposed bulge and the apical loop by aminoglycoside binding. Our study shows that chemical probing and mass spectrometry can provide important structural information and complement other techniques such as NMR spectroscopy.

Eukaryotic initiation factor 4F promotes a reorientation of eukaryotic initiation factor 3 binding on the 5' and the 3' UTRs of barley yellow dwarf virus mRNA

Viral mRNAs that lack a 5′ m⁷GTP cap and a 3′ poly-A tail rely on structural elements in their untranslated regions (UTRs) to form unique RNA-protein complexes that regulate viral translation. Recent studies of the barley yellow dwarf virus (BYDV) have revealed eukaryotic initiation factor 3 (eIF3) plays a significant role in facilitating communication between its 5′ and 3′ UTRs by binding both UTRs simultaneously. This report uses in vitro translation assays, fluorescence anisotropy binding assays, and selective 2′-hydroxyl acylation analyzed by primer extension (SHAPE) footprinting to identify secondary structures that are selectively interacting with eIF3. SHAPE data also show that eIF3 alters its interaction with BYDV structures when another factor crucial for BYDV translation, eIF4F, is introduced by the 3′ BYDV translational enhancer (BTE). The observed BTE and eIF4F-induced shift of eIF3 position on the 5’ UTR and the translational effects of altering eIF3-binding structures (SLC and SLII) support a new model for BYDV translation initiation that requires the reorientation of eIF3 on BYDV UTRs. This eIF3 function in BYDV translation initiation is both reminiscent of and distinct from eIF3–RNA interactions found in other non-canonically translating mRNAs (e.g. HCV). This characterization of a new role in translation initiation expands the known functionality of eIF3 and may be broadly applicable to other non-canonically translating mRNAs.

Chemical and Enzymatic Probing of Viral RNAs: From Infancy to Maturity and Beyond

RNA molecules are key players in a variety of biological events, and this is particularly true for viral RNAs. To better understand the replication of those pathogens and try to block them, special attention has been paid to the structure of their RNAs. Methods to probe RNA structures have been developed since the 1960s; even if they have evolved over the years, they are still in use today and provide useful information on the folding of RNA molecules, including viral RNAs. The aim of this review is to offer a historical perspective on the structural probing methods used to decipher RNA structures before the development of the selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) methodology and to show how they have influenced the current probing techniques. Actually, these technological breakthroughs, which involved advanced detection methods, were made possible thanks to the development of next-generation sequencing (NGS) but also to the previous works accumulated in the field of structural RNA biology. Finally, we will also discuss how high-throughput SHAPE (hSHAPE) paved the way for the development of sophisticated RNA structural techniques.

RNA Footprinting Using Small Chemical Reagents

RNA is a pivotal element of the cell which is most of the time found in complex with protein(s) in a cellular environment. RNA can adopt three-dimensional structures that may form specific binding sites not only for proteins but for all sorts of molecules. Since the early days of molecular biology, strategies to probe RNA structure have been developed. Such probes are small molecules or RNases that most of the time specifically react with single strand nucleotides. The precise reaction or cleavage site can be mapped by reverse transcription. It appears that nucleotides in close contact or in proximity of a ligand are no longer reactive to these probes. Carrying the RNA probing experiment in parallel in presence and absence of a ligand yield differences that are known as the ligand "footprint." Such footprints allow for the identification of the precise site of the ligand interaction, but also reveals RNA structural rearrangement upon ligand binding. Here we provide an experimental and analytical workflow to carry RNA footprinting experiments.

In Vitro Synthesis and RNA Structure Probing of CUG Triplet Repeat RNA

Aberrant RNA structure plays a central role in the molecular mechanisms guided by repeat RNAs in diseases like myotonic dystrophy (DM), C9orf72-linked amyotrophic lateral sclerosis (ALS) and fragile X tremor/ataxia syndrome (FXTAS). Much knowledge remains to be gained about how these repeat-expanded transcripts are actually folded, especially regarding the properties specific to very long and interrupted repeats. RNA structure can be interrogated by chemical as well as enzymatic probes. These probes usually bind or cleave single-stranded nucleotides, which can subsequently be detected using reverse transcriptase primer extension. In this chapter, we describe methodology for in vitro synthesis and structure probing of expanded CUG repeat RNAs associated with DM type 1 and approaches for the associated data analysis.

The molecular structure of long non-coding RNAs: emerging patterns and functional implications

Long non-coding RNAs (lncRNAs) are recently-discovered transcripts that regulate vital cellular processes and are crucially connected to diseases. Despite their unprecedented molecular complexity, it is emerging that lncRNAs possess distinct structural motifs. Remarkably, the 3D shape and topology of full-length, native lncRNAs have been visualized for the first time in the last year. These studies reveal that lncRNA structures dictate lncRNA functions. Here, we review experimentally determined lncRNA structures and emphasize that lncRNA structural characterization requires synergistic integration of computational, biochemical and biophysical approaches. Based on these emerging paradigms, we discuss how to overcome the challenges posed by the complex molecular architecture of lncRNAs, with the goal of obtaining a detailed understanding of lncRNA functions and molecular mechanisms in the future.

High-throughput determination of RNA structures

RNA performs and regulates a diverse range of cellular processes, with new functional roles being uncovered at a rapid pace. Interest is growing in how these functions are linked to RNA structures that form in the complex cellular environment. A growing suite of technologies that use advances in RNA structural probes, high-throughput sequencing and new computational approaches to interrogate RNA structure at unprecedented throughput are beginning to provide insights into RNA structures at new spatial, temporal and cellular scales.

The evolution of RNA structural probing methods: From gels to next-generation sequencing

RNA molecules are important players in all domains of life and the study of the relationship between their multiple flexible states and the associated biological roles has increased in recent years. For several decades, chemical and enzymatic structural probing experiments have been used to determine RNA structure. During this time, there has been a steady improvement in probing reagents and experimental methods, and today the structural biologist community has a large range of tools at its disposal to probe the secondary structure of RNAs in vitro and in cells. Early experiments used radioactive labeling and polyacrylamide gel electrophoresis as read-out methods. This was superseded by capillary electrophoresis, and more recently by next-generation sequencing. Today, powerful structural probing methods can characterize RNA structure on a genome-wide scale. In this review, we will provide an overview of RNA structural probing methodologies from a historical and technical perspective. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry RNA Methods > RNA Analyses in vitro and In Silico RNA Methods > RNA Analyses in Cells.

Classification of RNA structure change by 'gazing' at experimental data

Motivation

Mutations (or Single Nucleotide Variants) in folded RiboNucleic Acid structures that cause local or global conformational change are riboSNitches. Predicting riboSNitches is challenging, as it requires making two, albeit related, structure predictions. The data most often used to experimentally validate riboSNitch predictions is Selective 2' Hydroxyl Acylation by Primer Extension, or SHAPE. Experimentally establishing a riboSNitch requires the quantitative comparison of two SHAPE traces: wild-type (WT) and mutant. Historically, SHAPE data was collected on electropherograms and change in structure was evaluated by 'gel gazing.' SHAPE data is now routinely collected with next generation sequencing and/or capillary sequencers. We aim to establish a classifier capable of simulating human 'gazing' by identifying features of the SHAPE profile that human experts agree 'looks' like a riboSNitch.

Results

We find strong quantitative agreement between experts when RNA scientists 'gaze' at SHAPE data and identify riboSNitches. We identify dynamic time warping and seven other features predictive of the human consensus. The classSNitch classifier reported here accurately reproduces human consensus for 167 mutant/WT comparisons with an Area Under the Curve (AUC) above 0.8. When we analyze 2019 mutant traces for 17 different RNAs, we find that features of the WT SHAPE reactivity allow us to improve thermodynamic structure predictions of riboSNitches. This is significant, as accurate RNA structural analysis and prediction is likely to become an important aspect of precision medicine.

Availability and Implementation

The classSNitch R package is freely available at http://classsnitch.r-forge.r-project.org .

Contact

alain@email.unc.edu.

Supplementary information

Supplementary data are available at Bioinformatics online.

Analyzing the structure of macromolecules in their native cellular environment using hydroxyl radical footprinting

Hydroxyl radical footprinting (HRF) has been successfully used to study the structure of both nucleic acids and proteins in live cells.

Optimization of a novel biophysical model using large scale in vivo antisense hybridization data displays improved prediction capabilities of structurally accessible RNA regions

Current approaches to design efficient antisense RNAs (asRNAs) rely primarily on a thermodynamic understanding of RNA–RNA interactions. However, these approaches depend on structure predictions and have limited accuracy, arguably due to overlooking important cellular environment factors. In this work, we develop a biophysical model to describe asRNA–RNA hybridization that incorporates in vivo factors using large-scale experimental hybridization data for three model RNAs: a group I intron, CsrB and a tRNA. A unique element of our model is the estimation of the availability of the target region to interact with a given asRNA using a differential entropic consideration of suboptimal structures. We showcase the utility of this model by evaluating its prediction capabilities in four additional RNAs: a group II intron, Spinach II, 2-MS2 binding domain and glgC 5΄ UTR. Additionally, we demonstrate the applicability of this approach to other bacterial species by predicting sRNA–mRNA binding regions in two newly discovered, though uncharacterized, regulatory RNAs.

RNA structure through multidimensional chemical mapping

The discoveries of myriad non-coding RNA molecules, each transiting through multiple flexible states in cells or virions, present major challenges for structure determination. Advances in high-throughput chemical mapping give new routes for characterizing entire transcriptomes in vivo, but the resulting one-dimensional data generally remain too information-poor to allow accurate de novo structure determination. Multidimensional chemical mapping (MCM) methods seek to address this challenge. Mutate-and-map (M2), RNA interaction groups by mutational profiling (RING-MaP and MaP-2D analysis) and multiplexed •OH cleavage analysis (MOHCA) measure how the chemical reactivities of every nucleotide in an RNA molecule change in response to modifications at every other nucleotide. A growing body of in vitro blind tests and compensatory mutation/rescue experiments indicate that MCM methods give consistently accurate secondary structures and global tertiary structures for ribozymes, ribosomal domains and ligand-bound riboswitch aptamers up to 200 nucleotides in length. Importantly, MCM analyses provide detailed information on structurally heterogeneous RNA states, such as ligand-free riboswitches that are functionally important but difficult to resolve with other approaches. The sequencing requirements of currently available MCM protocols scale at least quadratically with RNA length, precluding general application to transcriptomes or viral genomes at present. We propose a modify-cross-link-map (MXM) expansion to overcome this and other current limitations to resolving the in vivo 'RNA structurome'.

RiboCAT: a new capillary electrophoresis data analysis tool for nucleic acid probing

Chemical and enzymatic probing of RNA secondary structure and RNA/protein interactions provides the basis for understanding the functions of structured RNAs. However, the ability to rapidly perform such experiments using capillary electrophoresis has been hampered by relatively labor-intensive data analysis software. While these computationally robust programs have been shown to calculate residue-specific reactivities to a high degree of accuracy, they often require time-consuming manual intervention and lack the ability to be easily modified by users. To alleviate these issues, RiboCAT (Ribonucleic acid capillary-electrophoresis analysis tool) was developed as a user-friendly, Microsoft Excel-based tool that reduces the need for manual intervention, thereby significantly shortening the time required for data analysis. Features of this tool include (i) the use of an Excel platform, (ii) a method of intercapillary signal alignment using internal size standards, (iii) a peak-sharpening algorithm to more accurately identify peaks, and (iv) an open architecture allowing for simple user intervention. Furthermore, a complementary tool, RiboDOG (RiboCAT data output generator) was designed to facilitate the comparison of multiple data sets, highlighting potential inconsistencies and inaccuracies that may have occurred during analysis. Using these new tools, the secondary structure of the HIV-1 5' untranslated region (5'UTR) was determined using selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE), matching the results of previous work.

Development of Small Molecules with a Noncanonical Binding Mode to HIV-1 Trans Activation Response (TAR) RNA

Small molecules that bind to RNA potently and specifically are relatively rare. The study of molecules that bind to the HIV-1 transactivation response (TAR) hairpin, a cis-acting HIV genomic element, has long been an important model system for the chemistry of targeting RNA. Here we report the synthesis, biochemical, and structural evaluation of a series of molecules that bind to HIV-1 TAR RNA. A promising analogue, 15, retained the TAR binding affinity of the initial hit and displaced a Tat-derived peptide with an IC₅₀ of 40 μM. NMR characterization of a soluble analogue, 2, revealed a noncanonical binding mode for this class of compounds. Finally, evaluation of 2 and 15 by selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) indicates specificity in binding to TAR within the context of an in vitro-synthesized 365-nt HIV-1 5'-untranslated region (UTR). Thus, these compounds exhibit a novel and specific mode of interaction with TAR, providing important suggestions for RNA ligand design.

Novel Biochemical Tools for Probing HIV RNA Structure

Functional analysis of viral RNA requires knowledge of secondary structure arrangements and tertiary base interactions. Thus, high-throughput and comprehensive methods for assessing RNA structure are highly desirable. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) has proven highly useful for modeling the secondary structures of HIV and other retroviral RNAs in recent years. This technology is not without its limitations however, as SHAPE data can be severely compromised when the RNA under study is structurally heterogeneous. In addition, the method reveals little information regarding the three-dimensional (3D) organization of an RNA. This chapter outlines four detailed SHAPE-related methodologies that circumvent these limitations. "Ensemble" and "in-gel" variations of SHAPE permit structural analysis of individual conformers within structurally heterogeneous mixtures of RNA, while probing strategies that utilize "through-space" cleavage reagents such as methidiumpropyl-EDTA (MPE) and peptides appended with an ATCUN (amino terminal copper/nickel binding motif) can provide insight into 3D organization. Combinational application of these techniques provides a formidable arsenal for exploring the structures of HIV RNAs and associated nucleoprotein complexes.

A small stem-loop structure of the Ebola virus trailer is essential for replication and interacts with heat-shock protein A8

Ebola virus (EBOV) is a single-stranded negative-sense RNA virus belonging to the Filoviridae family. The leader and trailer non-coding regions of the EBOV genome likely regulate its transcription, replication, and progeny genome packaging. We investigated the cis-acting RNA signals involved in RNA–RNA and RNA–protein interactions that regulate replication of eGFP-encoding EBOV minigenomic RNA and identified heat shock cognate protein family A (HSC70) member 8 (HSPA8) as an EBOV trailer-interacting host protein. Mutational analysis of the trailer HSPA8 binding motif revealed that this interaction is essential for EBOV minigenome replication. Selective 2′-hydroxyl acylation analyzed by primer extension analysis of the secondary structure of the EBOV minigenomic RNA indicates formation of a small stem-loop composed of the HSPA8 motif, a 3′ stem-loop (nucleotides 1868–1890) that is similar to a previously identified structure in the replicative intermediate (RI) RNA and a panhandle domain involving a trailer-to-leader interaction. Results of minigenome assays and an EBOV reverse genetic system rescue support a role for both the panhandle domain and HSPA8 motif 1 in virus replication.

Fluorescence Methods for Probing G-Quadruplex Structure in Single- and Double-Stranded DNA

Interest in exploring G-quadruplex (G4) structures in nucleic acids is growing as it becomes more widely recognized that these structures have many interesting biological roles and chemical properties. Probing the G4-forming potential of DNA with dimethyl sulfate, polymerase stop assays, or nuclease digestion are three commonly used techniques that usually employ radio-isotopic labels for visualization. However, as fluorescent labeling methods have grown in popularity and versatility, many laboratories have moved away from the routine use of radio-isotopic methods. We have adapted traditional procedures for structural analysis of G4-forming DNA sequences by using fluorescent labels and capillary electrophoresis and demonstrate their application to well-studied G4 structures, including c-MYC PU27 G4. The three fluorescent assays described here allow interrogation of G4 structures in double- and single-stranded DNA substrates, using either chemical or enzymatic cleavage. When combined, these techniques can provide valuable information for the investigation of G4 topology and structure, as well as visualizing any structural effects caused by interaction of quadruplexes with the complementary C-rich DNA strand.

Characterizing 3D RNA structure by single molecule FRET

The importance of elucidating the three dimensional structures of RNA molecules is becoming increasingly clear. However, traditional protein structural techniques such as NMR and X-ray crystallography have several important drawbacks when probing long RNA molecules. Single molecule Förster resonance energy transfer (smFRET) has emerged as a useful alternative as it allows native sequences to be probed in physiological conditions and allows multiple conformations to be probed simultaneously. This review serves to describe the method of generating a three dimensional RNA structure from smFRET data from the biochemical probing of the secondary structure to the computational refinement of the final model.

Genome-Wide Probing of RNA Structures In Vitro Using Nucleases and Deep Sequencing

RNA structure probing is an important technique that studies the secondary and tertiary conformations of an RNA. While it was traditionally performed on one RNA at a time, recent advances in deep sequencing has enabled the secondary structure mapping of thousands of RNAs simultaneously. Here, we describe the method Parallel Analysis for RNA Structures (PARS), which couples double and single strand specific nuclease probing to high throughput sequencing. Upon cloning of the cleavage sites into a cDNA library, deep sequencing and mapping of reads to the transcriptome, the position of paired and unpaired bases along cellular RNAs can be identified. PARS can be performed under diverse solution conditions and on different organismal RNAs to provide genome-wide RNA structural information. This information can also be further used to constrain computational predictions to provide better RNA structure models under different conditions.

Probing Xist RNA Structure in Cells Using Targeted Structure-Seq

The long non-coding RNA (lncRNA) Xist is a master regulator of X-chromosome inactivation in mammalian cells. Models for how Xist and other lncRNAs function depend on thermodynamically stable secondary and higher-order structures that RNAs can form in the context of a cell. Probing accessible RNA bases can provide data to build models of RNA conformation that provide insight into RNA function, molecular evolution, and modularity. To study the structure of Xist in cells, we built upon recent advances in RNA secondary structure mapping and modeling to develop Targeted Structure-Seq, which combines chemical probing of RNA structure in cells with target-specific massively parallel sequencing. By enriching for signals from the RNA of interest, Targeted Structure-Seq achieves high coverage of the target RNA with relatively few sequencing reads, thus providing a targeted and scalable approach to analyze RNA conformation in cells. We use this approach to probe the full-length Xist lncRNA to develop new models for functional elements within Xist, including the repeat A element in the 5’-end of Xist. This analysis also identified new structural elements in Xist that are evolutionarily conserved, including a new element proximal to the C repeats that is important for Xist function.

To do their jobs, many RNAs need to fold into structures (through base-paring). We were interested in the conformation of a specific mammalian RNA, Xist, when it is inside a cell. Xist is a very large non-coding RNA (lncRNA), that is >17,000 nt long. Xist is particularly important because it is one of the first lncRNAs to be discovered, and turns genes off across an entire chromosome. To figure out how Xist RNA is folded in mouse cells, we developed a new approach, Targeted Structure-Seq, to examine the conformation of large RNAs like Xist. Using computer modeling, we identified parts of Xist that are base paired into RNA duplexes. We also determined which parts of the Xist RNA are likely to be structured. This work provides a new tool for studying the secondary structure of any large RNA, and helps us understand what the important pieces of Xist look like while Xist does its work in the cell.

Expression of Alternatively Spliced Human T-Cell Leukemia Virus Type 1 mRNAs Is Influenced by Mitosis and by a Novel cis-Acting Regulatory Sequence

Human T-cell leukemia virus type 1 (HTLV-1) expression depends on the concerted action of Tax, which drives transcription of the viral genome, and Rex, which favors expression of incompletely spliced mRNAs and determines a 2-phase temporal pattern of viral expression. In the present study, we investigated the Rex dependence of the complete set of alternatively spliced HTLV-1 mRNAs. Analyses of cells transfected with Rex-wild-type and Rex-knockout HTLV-1 molecular clones using splice site-specific quantitative reverse transcription (qRT)-PCR revealed that mRNAs encoding the p30Tof, p13, and p12/8 proteins were Rex dependent, while the p21rex mRNA was Rex independent. These findings provide a rational explanation for the intermediate-late temporal pattern of expression of the p30tof, p13, and p12/8 mRNAs described in previous studies. All the Rex-dependent mRNAs contained a 75-nucleotide intronic region that increased the nuclear retention and degradation of a reporter mRNA in the absence of other viral sequences. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis revealed that this sequence formed a stable hairpin structure. Cell cycle synchronization experiments indicated that mitosis partially bypasses the requirement for Rex to export Rex-dependent HTLV-1 transcripts. These findings indicate a link between the cycling properties of the host cell and the temporal pattern of viral expression/latency that might influence the ability of the virus to spread and evade the immune system.

IMPORTANCE

HTLV-1 is a complex retrovirus that causes two distinct pathologies termed adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-1-associated myelopathy in about 5% of infected individuals. Expression of the virus depends on the concerted action of Tax, which drives transcription of the viral genome, and Rex, which favors expression of incompletely spliced mRNAs and determines a 2-phase temporal pattern of virus expression. The findings reported in this study revealed a novel cis-acting regulatory element and indicated that mitosis partially bypasses the requirement for Rex to export Rex-dependent HTLV-1 transcripts. Our results add a layer of complexity to the mechanisms controlling the expression of alternatively spliced HTLV-1 mRNAs and suggest a link between the cycling properties of the host cell and the temporal pattern of viral expression/latency that might influence the ability of the virus to spread and evade the immune system.

Multiple conformations are a conserved and regulatory feature of the RB1 5' UTR

Folding to a well-defined conformation is essential for the function of structured ribonucleic acids (RNAs) like the ribosome and tRNA. Structured elements in the untranslated regions (UTRs) of specific messenger RNAs (mRNAs) are known to control expression. The importance of unstructured regions adopting multiple conformations, however, is still poorly understood. High-resolution SHAPE-directed Boltzmann suboptimal sampling of the Homo sapiens Retinoblastoma 1 (RB1) 5' UTR yields three distinct conformations compatible with the experimental data. Private single nucleotide variants (SNVs) identified in two patients with retinoblastoma each collapse the structural ensemble to a single but distinct well-defined conformation. The RB1 5' UTRs from Bos taurus (cow) and Trichechus manatus latirostris (manatee) are divergent in sequence from H. sapiens (human) yet maintain structural compatibility with high-probability base pairs. SHAPE chemical probing of the cow and manatee RB1 5' UTRs reveals that they also adopt multiple conformations. Luciferase reporter assays reveal that 5' UTR mutations alter RB1 expression. In a traditional model of disease, causative SNVs disrupt a key structural element in the RNA. For the subset of patients with heritable retinoblastoma-associated SNVs in the RB1 5' UTR, the absence of multiple structures is likely causative of the cancer. Our data therefore suggest that selective pressure will favor multiple conformations in eukaryotic UTRs to regulate expression.

Consistent global structures of complex RNA states through multidimensional chemical mapping

Accelerating discoveries of non-coding RNA (ncRNA) in myriad biological processes pose major challenges to structural and functional analysis. Despite progress in secondary structure modeling, high-throughput methods have generally failed to determine ncRNA tertiary structures, even at the 1-nm resolution that enables visualization of how helices and functional motifs are positioned in three dimensions. We report that integrating a new method called MOHCA-seq (Multiplexed •OHCleavage Analysis with paired-end sequencing) with mutate-and-map secondary structure inference guides Rosetta 3D modeling to consistent 1-nm accuracy for intricately folded ncRNAs with lengths up to 188 nucleotides, including a blind RNA-puzzle challenge, the lariat-capping ribozyme. This multidimensional chemical mapping (MCM) pipeline resolves unexpected tertiary proximities for cyclic-di-GMP, glycine, and adenosylcobalamin riboswitch aptamers without their ligands and a loose structure for the recently discovered human HoxA9D internal ribosome entry site regulon. MCM offers a sequencing-based route to uncovering ncRNA 3D structure, applicable to functionally important but potentially heterogeneous states.

DOI:http://dx.doi.org/10.7554/eLife.07600.001

Our genetic material, in the form of molecules of DNA, provides instructions for many different processes in our cells. To issue these instructions, particular sections of DNA are copied to make a type of molecule called ribonucleic acid (RNA). Some of these RNA molecules contain instructions to make proteins, but others—known as non-coding RNAs—regulate the activity of genes in cells.

The genetic information within RNA is encoded by the sequence of four different chemical parts called ‘nucleotides’. RNA can exist as a single strand of nucleotides, but the nucleotides can also pair up in specific combinations to form sections of double-stranded RNA. Therefore, a single strand of non-coding RNA can fold into a complex three-dimensional shape that contains loops, twists, and bulges.

The three-dimensional structures of non-coding RNAs are crucial for their roles in cells, but the variety and complexity of shapes that they can form makes it technically difficult to study them. In 2008, researchers developed a new method called MOHCA that can map the positions of nucleotides that are close together in the three-dimensional structure. Highly reactive chemicals are attached to the nucleotides and these can react with, and damage, other nearby nucleotides. By detecting which nucleotides have been damaged, it is possible to map the positions of these nucleotides and decipher the structure of the RNA molecule using computer algorithms.

MOHCA is a promising approach, but the initial methods to find the damaged nucleotides were tedious and required specialized equipment. Now, Cheng, Das et al.—including some of the researchers involved in the 2008 work—have developed an improved version of MOHCA that uses readily available RNA sequencing techniques to find the damaged nucleotides. The RNA sequencing data are then analyzed by a new algorithm in the Rosetta computer modeling software.

Cheng, Das et al. used this newly developed ‘MOHCA-seq’ and Rosetta to reveal the structures of a human non-coding RNA and several other non-coding RNA molecules to a much higher level of detail than before. Together, MOHCA-seq and Rosetta provide a rapid method for researchers to decipher the three-dimensional structure of non-coding RNAs. This method is likely to speed up the analysis of the complex structures of non-coding RNAs. It will be useful in future efforts to work out what roles these RNAs play in cells, including their activity in cancer, neurodegeneration, and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.07600.002

Current perspectives on RNA secondary structure probing

The range of roles played by structured RNAs in biological systems is vast. At the same time as we are learning more about the importance of RNA structure, recent advances in reagents, methods and technology mean that RNA secondary structural probing has become faster and more accurate. As a result, the capabilities of laboratories that already perform this type of structural analysis have increased greatly, and it has also become more widely accessible. The present review summarizes established and recently developed techniques. The information we can derive from secondary structural analysis is assessed, together with the areas in which we are likely to see exciting developments in the near future.

Automated band annotation for RNA structure probing experiments with numerous capillary electrophoresis profiles

MOTIVATION

Capillary electrophoresis (CE) is a powerful approach for structural analysis of nucleic acids, with recent high-throughput variants enabling three-dimensional RNA modeling and the discovery of new rules for RNA structure design. Among the steps composing CE analysis, the process of finding each band in an electrophoretic trace and mapping it to a position in the nucleic acid sequence has required significant manual inspection and remains the most time-consuming and error-prone step. The few available tools seeking to automate this band annotation have achieved limited accuracy and have not taken advantage of information across dozens of profiles routinely acquired in high-throughput measurements.

RESULTS

We present a dynamic-programming-based approach to automate band annotation for high-throughput capillary electrophoresis. The approach is uniquely able to define and optimize a robust target function that takes into account multiple CE profiles (sequencing ladders, different chemical probes, different mutants) collected for the RNA. Over a large benchmark of multi-profile datasets for biological RNAs and designed RNAs from the EteRNA project, the method outperforms prior tools (QuSHAPE and FAST) significantly in terms of accuracy compared with gold-standard manual annotations. The amount of computation required is reasonable at a few seconds per dataset. We also introduce an 'E-score' metric to automatically assess the reliability of the band annotation and show it to be practically useful in flagging uncertainties in band annotation for further inspection.

AVAILABILITY AND IMPLEMENTATION

The implementation of the proposed algorithm is included in the HiTRACE software, freely available as an online server and for download at http://hitrace.stanford.edu.

CONTACT

sryoon@snu.ac.kr or rhiju@stanford.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Heterogeneous dynamics in DNA site discrimination by the structurally homologous DNA-binding domains of ETS-family transcription factors

The ETS family of transcription factors exemplifies current uncertainty in how eukaryotic genetic regulators with overlapping DNA sequence preferences achieve target site specificity. PU.1 and Ets-1 represent archetypes for studying site discrimination by ETS proteins because their DNA-binding domains are the most divergent in sequence, yet they share remarkably superimposable DNA-bound structures. To gain insight into the contrasting thermodynamics and kinetics of DNA recognition by these two proteins, we investigated the structure and dynamics of site discrimination by their DNA-binding domains. Electrophoretic mobilities of complexes formed by the two homologs with circularly permuted binding sites showed significant dynamic differences only for DNA complexes of PU.1. Free solution measurements by dynamic light scattering showed PU.1 to be more dynamic than Ets-1; moreover, dynamic changes are strongly coupled to site discrimination by PU.1, but not Ets-1. Interrogation of the protein/DNA interface by DNA footprinting showed similar accessibility to dimethyl sulfate for PU.1/DNA and Ets-1/DNA complexes, indicating that the dynamics of PU.1/DNA complexes reside primarily outside that interface. An information-based analysis of the two homologs’ binding motifs suggests a role for dynamic coupling in PU.1's ability to enforce a more stringent sequence preference than Ets-1 and its proximal sequence homologs.

Computational analysis of RNA structures with chemical probing data

RNAs play various roles, not only as the genetic codes to synthesize proteins, but also as the direct participants of biological functions determined by their underlying high-order structures. Although many computational methods have been proposed for analyzing RNA structures, their accuracy and efficiency are limited, especially when applied to the large RNAs and the genome-wide data sets. Recently, advances in parallel sequencing and high-throughput chemical probing technologies have prompted the development of numerous new algorithms, which can incorporate the auxiliary structural information obtained from those experiments. Their potential has been revealed by the secondary structure prediction of ribosomal RNAs and the genome-wide ncRNA function annotation. In this review, the existing probing-directed computational methods for RNA secondary and tertiary structure analysis are discussed.

Probing small non-coding RNAs structures

The diverse roles of RNAs depend on their ability to fold so as to form biologically functional structures. Thus, understanding the function of a given RNA molecule often requires experimental analysis of its secondary structure by in vitro RNA probing, which is more accurate than using prediction programs only. This chapter presents in vitro RNA probing protocols that we routinely use, from RNA transcript production and purification to RNA structure determination using enzymatic (RNases T1, T2, and V1) and chemical (DMS, CMCT, kethoxal, and Pb(2+)) probing performed on both unlabeled and end-labeled RNAs.

Large scale purification of RNA nanoparticles by preparative ultracentrifugation

Purification of large quantities of supramolecular RNA complexes is of paramount importance due to the large quantities of RNA needed and the purity requirements for in vitro and in vivo assays. Purification is generally carried out by liquid chromatography (HPLC), polyacrylamide gel electrophoresis (PAGE), or agarose gel electrophoresis (AGE). Here, we describe an efficient method for the large-scale purification of RNA prepared by in vitro transcription using T7 RNA polymerase by cesium chloride (CsCl) equilibrium density gradient ultracentrifugation and the large-scale purification of RNA nanoparticles by sucrose gradient rate-zonal ultracentrifugation or cushioned sucrose gradient rate-zonal ultracentrifugation.

Rational experiment design for sequencing-based RNA structure mapping

Structure mapping is a classic experimental approach for determining nucleic acid structure that has gained renewed interest in recent years following advances in chemistry, genomics, and informatics. The approach encompasses numerous techniques that use different means to introduce nucleotide-level modifications in a structure-dependent manner. Modifications are assayed via cDNA fragment analysis, using electrophoresis or next-generation sequencing (NGS). The recent advent of NGS has dramatically increased the throughput, multiplexing capacity, and scope of RNA structure mapping assays, thereby opening new possibilities for genome-scale, de novo, and in vivo studies. From an informatics standpoint, NGS is more informative than prior technologies by virtue of delivering direct molecular measurements in the form of digital sequence counts. Motivated by these new capabilities, we introduce a novel model-based in silico approach for quantitative design of large-scale multiplexed NGS structure mapping assays, which takes advantage of the direct and digital nature of NGS readouts. We use it to characterize the relationship between controllable experimental parameters and the precision of mapping measurements. Our results highlight the complexity of these dependencies and shed light on relevant tradeoffs and pitfalls, which can be difficult to discern by intuition alone. We demonstrate our approach by quantitatively assessing the robustness of SHAPE-Seq measurements, obtained by multiplexing SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) chemistry in conjunction with NGS. We then utilize it to elucidate design considerations in advanced genome-wide approaches for probing the transcriptome, which recently obtained in vivo information using dimethyl sulfate (DMS) chemistry.

Exploiting post-transcriptional regulation to probe RNA structures in vivo via fluorescence

While RNA structures have been extensively characterized in vitro, very few techniques exist to probe RNA structures inside cells. Here, we have exploited mechanisms of post-transcriptional regulation to synthesize fluorescence-based probes that assay RNA structures in vivo. Our probing system involves the co-expression of two constructs: (i) a target RNA and (ii) a reporter containing a probe complementary to a region in the target RNA attached to an RBS-sequestering hairpin and fused to a sequence encoding the green fluorescent protein (GFP). When a region of the target RNA is accessible, the area can interact with its complementary probe, resulting in fluorescence. By using this system, we observed varied patterns of structural accessibility along the length of the Tetrahymena group I intron. We performed in vivo DMS footprinting which, along with previous footprinting studies, helped to explain our probing results. Additionally, this novel approach represents a valuable tool to differentiate between RNA variants and to detect structural changes caused by subtle mutations. Our results capture some differences from traditional footprinting assays that could suggest that probing in vivo via oligonucleotide hybridization facilitates the detection of folding intermediates. Importantly, our data indicate that intracellular oligonucleotide probing can be a powerful complement to existing RNA structural probing methods.

Insights into secondary and tertiary interactions of dengue virus RNA by SHAPE

Dengue virus (DENV) is a single-stranded positive-sense RNA virus belonging to the Flaviviridae family. The DENV RNA genome contains multiple cis-acting elements that continue to unravel their essential role in managing viral molecular processes. Attempts have been made to predict the secondary structure of DENV RNA using a variety of computational tools. Nevertheless, a greater degree of accuracy is achieved when these methods are complemented with structure probing experimentation. This chapter outlines detailed methodology for the structural study of DENV subgenomic minigenome RNA by applying high-throughput selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE). High-throughput SHAPE combines a novel chemical probing technology with reverse transcription, capillary electrophoresis, and secondary structure prediction software to rapidly and reproducibly determine the structure of RNAs from several hundred to several thousand nucleotides at single-nucleotide resolution. This methodology investigates local structure for all positions in a sequence-independent manner and as such it is particularly useful in predicting RNA secondary and tertiary interactions.

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.

Identification of biologically active, HIV TAR RNA-binding small molecules using small molecule microarrays

Identifying small molecules that selectively bind to structured RNA motifs remains an important challenge in developing potent and specific therapeutics. Most strategies to find RNA-binding molecules have identified highly charged compounds or aminoglycosides that commonly have modest selectivity. Here we demonstrate a strategy to screen a large unbiased library of druglike small molecules in a microarray format against an RNA target. This approach has enabled the identification of a novel chemotype that selectively targets the HIV transactivation response (TAR) RNA hairpin in a manner not dependent on cationic charge. Thienopyridine 4 binds to and stabilizes the TAR hairpin with a Kd of 2.4 μM. Structure-activity relationships demonstrate that this compound achieves activity through hydrophobic and aromatic substituents on a heterocyclic core, rather than cationic groups typically required. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis was performed on a 365-nucleotide sequence derived from the 5' untranslated region (UTR) of the HIV-1 genome to determine global structural changes in the presence of the molecule. Importantly, the interaction of compound 4 can be mapped to the TAR hairpin without broadly disrupting any other structured elements of the 5' UTR. Cell-based anti-HIV assays indicated that 4 inhibits HIV-induced cytopathicity in T lymphocytes with an EC50 of 28 μM, while cytotoxicity was not observed at concentrations approaching 1 mM.

Positional effects of AAN motifs in rpoS regulation by sRNAs and Hfq

The Escherichia coli stationary phase transcription factor RpoS is translated in response to small noncoding RNAs (sRNAs), which base pair with the rpoS mRNA leader. The bacterial Sm-like protein Hfq anneals sRNAs with their mRNA targets by simultaneously binding the mRNA and sRNA. Intriguingly, Hfq is recruited to the rpoS leader via AAN motifs far upstream of the sRNA. SHAPE (selective 2'-hydroxyl acylation and primer extension) chemical footprinting showed that the rpoS leader is divided into a far upstream domain, an Hfq binding domain, and a downstream inhibitory stem-loop containing the sRNA and ribosome binding sites. To investigate how Hfq promotes sRNA-mRNA base pairing from a distance, we deleted the natural AAN Hfq binding site, and we inserted artificial AAN binding sites at various positions in the rpoS leader. All the relocated AAN motifs restored tight Hfq binding in vitro, but only insertion at the natural position restored Hfq-dependent sRNA annealing in vitro and sRNA regulation of rpoS translation in vivo. Furthermore, U-rich motifs in the downstream inhibitory domain stabilized the rpoS mRNA-Hfq complex and contributed to regulation of rpoS expression. We propose that the natural Hfq binding domain is optimal for positive regulation because it recruits Hfq to the mRNA and allows it to act on incoming sRNAs without opening the inhibitory stem-loop when sRNA is absent.

DNA Binding Polyamides and the Importance of DNA Recognition in their use as Gene-Specific and Antiviral Agents

There is a long history for the bioorganic and biomedical use of N-methyl-pyrrole-derived polyamides (PAs) that are higher homologs of natural products such as distamycin A and netropsin. This work has been pursued by many groups, with the Dervan and Sugiyama groups responsible for many breakthroughs. We have studied PAs since about 1999, partly in industry and partly in academia. Early in this program, we reported methods to control cellular uptake of polyamides in cancer cell lines and other cells likely to have multidrug resistance efflux pumps induced. We went on to discover antiviral polyamides active against HPV31, where SAR showed that a minimum binding size of about 10 bp of DNA was necessary for activity. Subsequently we discovered polyamides active against two additional high-risk HPVs, HPV16 and 18, a subset of which showed broad spectrum activity against HPV16, 18 and 31. Aspects of our results presented here are incompatible with reported DNA recognition rules. For example, molecules with the same cognate DNA recognition properties varied from active to inactive against HPVs. We have since pursued the mechanism of action of antiviral polyamides, and polyamides in general, with collaborators at NanoVir, the University of Missouri-St. Louis, and Georgia State University. We describe dramatic consequences of β-alanine positioning even in relatively small, 8-ring polyamides; these results contrast sharply with prior reports. This paper was originally presented by JKB as a Keynote Lecture in the 2^nd International Conference on Medicinal Chemistry and Computer Aided Drug Design Conference in Las Vegas, NV, October 2013.

The mutate-and-map protocol for inferring base pairs in structured RNA

Chemical mapping is a widespread technique for structural analysis of nucleic acids in which a molecule's reactivity to different probes is quantified at single nucleotide resolution and used to constrain structural modeling. This experimental framework has been extensively revisited in the past decade with new strategies for high-throughput readouts, chemical modification, and rapid data analysis. Recently, we have coupled the technique to high-throughput mutagenesis. Point mutations of a base paired nucleotide can lead to exposure of not only that nucleotide but also its interaction partner. Systematically carrying out the mutation and mapping for the entire system gives an experimental approximation of the molecule's "contact map." Here, we give our in-house protocol for this "mutate-and-map" (M2) strategy, based on 96-well capillary electrophoresis, and we provide practical tips on interpreting the data to infer nucleic acid structure.

Computational analysis of conserved RNA secondary structure in transcriptomes and genomes

Transcriptomics experiments and computational predictions both enable systematic discovery of new functional RNAs. However, many putative noncoding transcripts arise instead from artifacts and biological noise, and current computational prediction methods have high false positive rates. I discuss prospects for improving computational methods for analyzing and identifying functional RNAs, with a focus on detecting signatures of conserved RNA secondary structure. An interesting new front is the application of chemical and enzymatic experiments that probe RNA structure on a transcriptome-wide scale. I review several proposed approaches for incorporating structure probing data into the computational prediction of RNA secondary structure. Using probabilistic inference formalisms, I show how all these approaches can be unified in a well-principled framework, which in turn allows RNA probing data to be easily integrated into a wide range of analyses that depend on RNA secondary structure inference. Such analyses include homology search and genome-wide detection of new structural RNAs.

Small RNAs derived from lncRNA RNase MRP have gene-silencing activity relevant to human cartilage-hair hypoplasia

Post-transcriptional processing of some long non-coding RNAs (lncRNAs) reveals that they are a source of miRNAs. We show that the 268-nt non-coding RNA component of mitochondrial RNA processing endoribonuclease, (RNase MRP), is the source of at least two short (∼20 nt) RNAs designated RMRP-S1 and RMRP-S2, which function as miRNAs. Point mutations in RNase MRP cause human cartilage-hair hypoplasia (CHH), and several disease-causing mutations map to RMRP-S1 and -S2. SHAPE chemical probing identified two alternative secondary structures altered by disease mutations. RMRP-S1 and -S2 are significantly reduced in two fibroblast cell lines and a B-cell line derived from CHH patients. Tests of gene regulatory activity of RMRP-S1 and -S2 identified over 900 genes that were significantly regulated, of which over 75% were down-regulated, and 90% contained target sites with seed complements of RMRP-S1 and -S2 predominantly in their 3' UTRs. Pathway analysis identified regulated genes that function in skeletal development, hair development and hematopoietic cell differentiation including PTCH2 and SOX4 among others, linked to major CHH phenotypes. Also, genes associated with alternative RNA splicing, cell proliferation and differentiation were highly targeted. Therefore, alterations RMRP-S1 and -S2, caused by point mutations in RMRP, are strongly implicated in the molecular mechanism of CHH.

Mapping small DNA ligand hydroxyl radical footprinting and affinity cleavage products for capillary electrophoresis

The mapping of DNA footprints and affinity cleavage sites for small DNA ligands is affected by the choice of sequencing chemistry and end label, and the potential for indexing errors can be significant when mapping small ligand-DNA interactions. Described here is a mechanism for avoiding such errors based on a summary of standard labeling, cleavage, and indexing chemistries and a comparison among them for analysis of these interactions by capillary electrophoresis. The length dependence of the difference between Sanger and Maxam-Gilbert indexing is examined for a number of duplexes of mixed sequence.

HiTRACE-Web: an online tool for robust analysis of high-throughput capillary electrophoresis

To facilitate the analysis of large-scale high-throughput capillary electrophoresis data, we previously proposed a suite of efficient analysis software named HiTRACE (High Throughput Robust Analysis of Capillary Electrophoresis). HiTRACE has been used extensively for quantitating data from RNA and DNA structure mapping experiments, including mutate-and-map contact inference, chromatin footprinting, the Eterna RNA design project and other high-throughput applications. However, HiTRACE is based on a suite of command-line MATLAB scripts that requires nontrivial efforts to learn, use and extend. Here, we present HiTRACE-Web, an online version of HiTRACE that includes standard features previously available in the command-line version and additional features such as automated band annotation and flexible adjustment of annotations, all via a user-friendly environment. By making use of parallelization, the on-line workflow is also faster than software implementations available to most users on their local computers. Free access: http://hitrace.org.

RNA secondary structure prediction using high-throughput SHAPE

Understanding the function of RNA involved in biological processes requires a thorough knowledge of RNA structure. Toward this end, the methodology dubbed "high-throughput selective 2' hydroxyl acylation analyzed by primer extension", or SHAPE, allows prediction of RNA secondary structure with single nucleotide resolution. This approach utilizes chemical probing agents that preferentially acylate single stranded or flexible regions of RNA in aqueous solution. Sites of chemical modification are detected by reverse transcription of the modified RNA, and the products of this reaction are fractionated by automated capillary electrophoresis (CE). Since reverse transcriptase pauses at those RNA nucleotides modified by the SHAPE reagents, the resulting cDNA library indirectly maps those ribonucleotides that are single stranded in the context of the folded RNA. Using ShapeFinder software, the electropherograms produced by automated CE are processed and converted into nucleotide reactivity tables that are themselves converted into pseudo-energy constraints used in the RNAStructure (v5.3) prediction algorithm. The two-dimensional RNA structures obtained by combining SHAPE probing with in silico RNA secondary structure prediction have been found to be far more accurate than structures obtained using either method alone.

Identifying and characterizing Hfq-RNA interactions

To regulate stress responses and virulence, bacteria use small regulatory RNAs (sRNAs). These RNAs can up or down regulate target mRNAs through base pairing by influencing ribosomal access and RNA decay. A large class of these sRNAs, called trans-encoded sRNAs, requires the RNA binding protein Hfq to facilitate base pairing between the regulatory RNA and its target mRNA. The resulting network of regulation is best characterized in Escherichia coli and Salmonella typhimurium, but the importance of Hfq dependent sRNA regulation is recognized in a diverse population of bacteria. In this review we present the approaches and methods used to discover Hfq binding RNAs, characterize their interactions and elucidate their functions.

The HIV-2 Rev-response element: determining secondary structure and defining folding intermediates

Interaction between the viral protein Rev and the RNA motifs known as Rev response elements (RREs) is required for transport of unspliced and partially spliced human immunodeficiency virus (HIV)-1 and HIV-2 RNAs from the nucleus to the cytoplasm during the later stages of virus replication. A more detailed understanding of these nucleoprotein complexes and the host factors with which they interact should accelerate the development of new antiviral drugs targeting cis-acting RNA regulatory signals. In this communication, the secondary structures of the HIV-2 RRE and two RNA folding precursors have been identified using the SHAPE (selective 2′-hydroxyl acylation analyzed by primer extension) chemical probing methodology together with a novel mathematical approach for determining the secondary structures of RNA conformers present in a mixture. A complementary chemical probing technique was also used to support these secondary structure models, to confirm that the RRE2 RNA undergoes a folding transition and to obtain information about the relative positioning of RRE2 substructures in three dimensions. Our analysis collectively suggests that the HIV-2 RRE undergoes two conformational transitions before assuming the energetically most favorable conformer. The 3D models for the HIV-2 RRE and folding intermediates are also presented, wherein the Rev-binding stem–loops (IIB and I) are located coaxially in the former, which is in agreement with previous models for HIV-1 Rev-RRE binding.

Structural complexity of Dengue virus untranslated regions: cis-acting RNA motifs and pseudoknot interactions modulating functionality of the viral genome

The Dengue virus (DENV) genome contains multiple cis-acting elements required for translation and replication. Previous studies indicated that a 719-nt subgenomic minigenome (DENV-MINI) is an efficient template for translation and (−) strand RNA synthesis in vitro. We performed a detailed structural analysis of DENV-MINI RNA, combining chemical acylation techniques, Pb²⁺ ion-induced hydrolysis and site-directed mutagenesis. Our results highlight protein-independent 5′–3′ terminal interactions involving hybridization between recognized cis-acting motifs. Probing analyses identified tandem dumbbell structures (DBs) within the 3′ terminus spaced by single-stranded regions, internal loops and hairpins with embedded GNRA-like motifs. Analysis of conserved motifs and top loops (TLs) of these dumbbells, and their proposed interactions with downstream pseudoknot (PK) regions, predicted an H-type pseudoknot involving TL1 of the 5′ DB and the complementary region, PK2. As disrupting the TL1/PK2 interaction, via ‘flipping’ mutations of PK2, previously attenuated DENV replication, this pseudoknot may participate in regulation of RNA synthesis. Computer modeling implied that this motif might function as autonomous structural/regulatory element. In addition, our studies targeting elements of the 3′ DB and its complementary region PK1 indicated that communication between 5′–3′ terminal regions strongly depends on structure and sequence composition of the 5′ cyclization region.