Effect of transcription on folding of the Tetrahymena ribozyme

Therapeutic applications of RNA nanostructures

RNA-based therapeutics have gained wide public interest in recent years. RNA is a versatile molecule that exists in many forms including mRNA, siRNA, miRNA, ribozymes, and other non-coding RNAs and is primarily applied for gene therapy. RNA is also used as a modular building block to construct RNA nanostructures. The programmable nature of RNA nanostructures enables the generation of simple, modulable, and multi-functional RNA-based therapeutics. Although the therapeutic application of RNA may be limited due to its structural instability, advances in RNA nanotechnology have improved the stability of RNA nanostructures for greater application. Various strategies have been developed to enhance the stability of RNA nanostructures enabling their application in vivo. In this review, we examine the therapeutic applications of RNA nanostructures. Non-immunogenic RNA nanostructures can be rationally designed with functional RNA molecules to modulate gene expression for gene therapy. On the other hand, nucleic acids can be sensed by cellular receptors to elicit an innate immune response, for which certain DNA and RNA motifs can function as adjuvants. Taking advantage of this adjuvant potential, RNA nanostructures can be used for immunotherapy and be designed for cancer vaccines. Thus, we examine the therapeutic application of immunogenic RNA nanostructures for cancer immunotherapy. RNA nanostructures represent promising platforms to design new nanodrugs, gene therapeutics, immunotherapeutic adjuvants, and cancer vaccines. Ongoing research in the field of RNA nanotechnology will continue to empower the development of RNA nanostructure-based therapeutics with high efficacy and limited toxicity.

RNA structure in alternative splicing regulation: from mechanism to therapy

Alternative splicing is a highly intricate process that plays a crucial role in post-transcriptional regulation and significantly expands the functional proteome of a limited number of coding genes in eukaryotes. Its regulation is multifactorial, with RNA structure exerting a significant impact. Aberrant RNA conformations lead to dysregulation of splicing patterns, which directly affects the manifestation of disease symptoms. In this review, the molecular mechanisms of RNA secondary structure-mediated splicing regulation are summarized, with a focus on the complex interplay between aberrant RNA conformations and disease phenotypes resulted from splicing defects. This study also explores additional factors that reshape structural conformations, enriching our understanding of the mechanistic network underlying structure-mediated splicing regulation. In addition, an emphasis has been placed on the clinical role of targeting aberrant splicing corrections in human diseases. The principal mechanisms of action behind this phenomenon are described, followed by a discussion of prospective development strategies and pertinent challenges.

RNA folding kinetics control riboswitch sensitivity in vivo

Riboswitches are ligand-responsive gene-regulatory RNA elements that perform key roles in maintaining cellular homeostasis. Understanding how riboswitch sensitivity is controlled is critical to understanding how highly conserved aptamer domains are deployed in a variety of contexts with different sensitivity demands. Here we uncover new roles by which RNA folding dynamics control riboswitch sensitivity in cells. By investigating the Clostridium beijerinckii pfl ZTP riboswitch, we identify multiple mechanistic routes of altering expression platform sequence and structure to slow RNA folding, all of which enhance riboswitch sensitivity. Applying these methods to riboswitches with diverse aptamer architectures that regulate transcription and translation with ON and OFF logic demonstrates the generality of our findings, indicating that any riboswitch that operates in a kinetic regime can be sensitized by slowing expression platform folding. Comparison of the most sensitized versions of these switches to equilibrium aptamer:ligand dissociation constants suggests a limit to the sensitivities achievable by kinetic RNA switches. Our results add to the growing suite of knowledge and approaches that can be used to rationally program cotranscriptional RNA folding for biotechnology applications, and suggest general RNA folding principles for understanding dynamic RNA systems in other areas of biology.

Flanking Sequence Cotranscriptionally Regulates Twister Ribozyme Activity

Small nucleolytic ribozymes are RNAs that cleave their own phosphodiester backbone. While proteinaceous enzymes are regulated by a variety of known mechanisms, methods of regulation for ribozymes remain unclear. Twister is one ribozyme class for which many structural and catalytic properties have been elucidated. However, few studies have analyzed the activity of twister ribozymes in the context of a native flanking sequence, even though ribozymes as transcribed in nature do not exist in isolation. Interactions between the ribozyme and its neighboring sequences can induce conformational changes that inhibit self-cleavage, providing a regulatory mechanism that could naturally determine ribozyme activity in vivo and in synthetic applications. To date, eight twister ribozymes have been identified within the staple crop rice (Oryza sativa). Herein, we select several twister ribozymes from rice and show that they are differentially regulated by their flanking sequence using published RNA-seq data sets, structure probing, and cotranscriptional cleavage assays. We found that the Osa 1-2 ribozyme does not interact with its flanking sequences. However, sequences flanking the Osa 1-3 and Osa 1-8 ribozymes form inactive conformations, referred to here as "ribozymogens", that attenuate ribozyme self-cleavage activity. For the Osa 1-3 ribozyme, we show that activity can be rescued upon addition of a complementary antisense oligonucleotide, suggesting ribozymogens can be controlled via external signals. In all, our data provide a plausible mechanism wherein flanking sequence differentially regulates ribozyme activity in vivo. More broadly, the ability to regulate ribozyme behavior locally has potential applications in control of gene expression and synthetic biology.

Observation of coordinated RNA folding events by systematic cotranscriptional RNA structure probing

RNA begins to fold as it is transcribed by an RNA polymerase. Consequently, RNA folding is constrained by the direction and rate of transcription. Understanding how RNA folds into secondary and tertiary structures therefore requires methods for determining the structure of cotranscriptional folding intermediates. Cotranscriptional RNA chemical probing methods accomplish this by systematically probing the structure of nascent RNA that is displayed from an RNA polymerase. Here, we describe a concise, high-resolution cotranscriptional RNA chemical probing procedure called variable length Transcription Elongation Complex RNA structure probing (TECprobe-VL). We demonstrate the accuracy and resolution of TECprobe-VL by replicating and extending previous analyses of ZTP and fluoride riboswitch folding and mapping the folding pathway of a ppGpp-sensing riboswitch. In each system, we show that TECprobe-VL identifies coordinated cotranscriptional folding events that mediate transcription antitermination. Our findings establish TECprobe-VL as an accessible method for mapping cotranscriptional RNA folding pathways.

Small RNAs and Hfq capture unfolded RNA target sites during transcription

Small ribonucleoproteins (sRNPs) target nascent precursor RNAs to guide folding, modification, and splicing during transcription. Yet, rapid co-transcriptional folding of the RNA can mask sRNP sites, impeding target recognition and regulation. To examine how sRNPs target nascent RNAs, we monitored binding of bacterial Hfq⋅DsrA sRNPs to rpoS transcripts using single-molecule co-localization co-transcriptional assembly (smCoCoA). We show that Hfq⋅DsrA recursively samples the mRNA before transcription of the target site to poise it for base pairing with DsrA. We adapted smCoCoA to precisely measure when the target site is synthesized and revealed that Hfq⋅DsrA often binds the mRNA during target site synthesis close to RNA polymerase (RNAP). We suggest that targeting transcripts near RNAP allows an sRNP to capture a site before the transcript folds, providing a kinetic advantage over post-transcriptional targeting. We propose that other sRNPs may also use RNAP-proximal targeting to hasten recognition and regulation.

A trailing ribosome speeds up RNA polymerase at the expense of transcript fidelity via force and allostery

In prokaryotes, translation can occur on mRNA that is being transcribed in a process called coupling. How the ribosome affects the RNA polymerase (RNAP) during coupling is not well understood. Here, we reconstituted the E. coli coupling system and demonstrated that the ribosome can prevent pausing and termination of RNAP and double the overall transcription rate at the expense of fidelity. Moreover, we monitored single RNAPs coupled to ribosomes and show that coupling increases the pause-free velocity of the polymerase and that a mechanical assisting force is sufficient to explain the majority of the effects of coupling. Also, by cryo-EM, we observed that RNAPs with a terminal mismatch adopt a backtracked conformation, while a coupled ribosome allosterically induces these polymerases toward a catalytically active anti-swiveled state. Finally, we demonstrate that prolonged RNAP pausing is detrimental to cell viability, which could be prevented by polymerase reactivation through a coupled ribosome.

Observation of coordinated cotranscriptional RNA folding events

RNA begins to fold as it is transcribed by an RNA polymerase. Consequently, RNA folding is constrained by the direction and rate of transcription. Understanding how RNA folds into secondary and tertiary structures therefore requires methods for determining the structure of cotranscriptional folding intermediates. Cotranscriptional RNA chemical probing methods accomplish this by systematically probing the structure of nascent RNA that is displayed from RNA polymerase. Here, we have developed a concise, high-resolution cotranscriptional RNA chemical probing procedure called Transcription Elongation Complex RNA structure probing-Multilength (TECprobe-ML). We validated TECprobe-ML by replicating and extending previous analyses of ZTP and fluoride riboswitch folding, and mapped the folding pathway of a ppGpp-sensing riboswitch. In each system, TECprobe-ML identified coordinated cotranscriptional folding events that mediate transcription antitermination. Our findings establish TECprobe-ML as an accessible method for mapping cotranscriptional RNA folding pathways.

DrTransformer: heuristic cotranscriptional RNA folding using the nearest neighbor energy model

MOTIVATION

Folding during transcription can have an important influence on the structure and function of RNA molecules, as regions closer to the 5' end can fold into metastable structures before potentially stronger interactions with the 3' end become available. Thermodynamic RNA folding models are not suitable to predict structures that result from cotranscriptional folding, as they can only calculate properties of the equilibrium distribution. Other software packages that simulate the kinetic process of RNA folding during transcription exist, but they are mostly applicable for short sequences.

RESULTS

We present a new algorithm that tracks changes to the RNA secondary structure ensemble during transcription. At every transcription step, new representative local minima are identified, a neighborhood relation is defined and transition rates are estimated for kinetic simulations. After every simulation, a part of the ensemble is removed and the remainder is used to search for new representative structures. The presented algorithm is deterministic (up to numeric instabilities of simulations), fast (in comparison with existing methods), and it is capable of folding RNAs much longer than 200 nucleotides.

AVAILABILITY AND IMPLEMENTATION

This software is open-source and available at https://github.com/ViennaRNA/drtransformer.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

ADAR RNA editing on antisense RNAs results in apparent U-to-C base changes on overlapping sense transcripts

Despite hundreds of RNA modifications described to date, only RNA editing results in a change in the nucleotide sequence of RNA molecules compared to the genome. In mammals, two kinds of RNA editing have been described so far, adenosine to inosine (A-to-I) and cytidine to uridine (C-to-U) editing. Recent improvements in RNA sequencing technologies have led to the discovery of a continuously growing number of editing sites. These methods are powerful but not error-free, making routine validation of newly-described editing sites necessary. During one of these validations on DDX58 mRNA, along with A-to-I RNA editing sites, we encountered putative U-to-C editing. These U-to-C edits were present in several cell lines and appeared regulated in response to specific environmental stimuli. The same findings were also observed for the human long intergenic non-coding RNA p21 (hLincRNA-p21). A more in-depth analysis revealed that putative U-to-C edits result from A-to-I editing on overlapping antisense RNAs that are transcribed from the same loci. Such editing events, occurring on overlapping genes transcribed in opposite directions, have recently been demonstrated to be immunogenic and have been linked with autoimmune and immune-related diseases. Our findings, also confirmed by deep transcriptome data, demonstrate that such loci can be recognized simply through the presence of A-to-I and U-to-C mismatches within the same locus, reflective A-to-I editing both in the sense-oriented transcript and in the cis-natural antisense transcript (cis-NAT), implying that such clusters could be a mark of functionally relevant ADAR1 editing events.

Dynamic RNA synthetic biology: new principles, practices and potential

An increased appreciation of the role of RNA dynamics in governing RNA function is ushering in a new wave of dynamic RNA synthetic biology. Here, we review recent advances in engineering dynamic RNA systems across the molecular, circuit and cellular scales for important societal-scale applications in environmental and human health, and bioproduction. For each scale, we introduce the core concepts of dynamic RNA folding and function at that scale, and then discuss technologies incorporating these concepts, covering new approaches to engineering riboswitches, ribozymes, RNA origami, RNA strand displacement circuits, biomaterials, biomolecular condensates, extracellular vesicles and synthetic cells. Considering the dynamic nature of RNA within the engineering design process promises to spark the next wave of innovation that will expand the scope and impact of RNA biotechnologies.

Upstream Flanking Sequence Assists Folding of an RNA Thermometer

Many heat shock genes in bacteria are regulated through a class of temperature-sensitive stem-loop (SL) RNAs called RNA thermometers (RNATs). One of the most widely studied RNATs is the Repression Of heat Shock Expression (ROSE) element associated with expression of heat shock proteins. Located in the 5'UTR, the RNAT contains one to three auxiliary hairpins upstream of it. Herein, we address roles of these upstream SLs in the folding and function of an RNAT. Bradyrhizobium japonicum is a nitrogen-fixing bacterium that experiences a wide range of temperatures in the soil and contains ROSE elements, each having multiple upstream SLs. The 5'UTR of the messenger (mRNA) for heat shock protein A (hspA) in B. japonicum has an intricate secondary structure containing three SLs upstream of the RNAT SL. While structure-function studies of the hspA RNAT itself have been reported, it has been unclear if these auxiliary SLs contribute to the temperature-sensing function of the ROSE elements. Herein, we show that the full length (FL) sequence has several melting transitions indicating that the ROSE element unfolds in a non-two-state manner. The upstream SLs are more stable than the RNAT itself, and a variant with disrupted base pairing in the SL immediately upstream of the RNAT has little influence on the melting of the RNAT. On the basis of these results and modeling of the co-transcriptional folding of the ROSE element, we propose that the upstream SLs function to stabilize the transcript and aid proper folding and dynamics of the RNAT.

Topological crossing in the misfolded Tetrahymena ribozyme resolved by cryo-EM

Taking advantage of single-particle cryogenic electron microscopy (cryo-EM) to analyze highly heterogeneous or flexible samples, we obtained long-awaited three-dimensional (3D) structures of the misfolded Tetrahymena ribozyme. These structures provide clear evidence for a previously proposed topological isomer model, in which the stereochemically impossible crossing of two core RNA strands prevents rapid rearrangement of the misfolded state to the native state. Topological isomers may be widespread in misfolding of complex RNA, and these cryo-EM structures set a foundation for dissecting their detailed kinetic mechanisms and functional consequences in a paradigmatic model system.

The Tetrahymena group I intron has been a key system in the understanding of RNA folding and misfolding. The molecule folds into a long-lived misfolded intermediate (M) in vitro, which has been known to form extensive native-like secondary and tertiary structures but is separated by an unknown kinetic barrier from the native state (N). Here, we used cryogenic electron microscopy (cryo-EM) to resolve misfolded structures of the Tetrahymena L-21 ScaI ribozyme. Maps of three M substates (M1, M2, M3) and one N state were achieved from a single specimen with overall resolutions of 3.5 Å, 3.8 Å, 4.0 Å, and 3.0 Å, respectively. Comparisons of the structures reveal that all the M substates are highly similar to N, except for rotation of a core helix P7 that harbors the ribozyme’s guanosine binding site and the crossing of the strands J7/3 and J8/7 that connect P7 to the other elements in the ribozyme core. This topological difference between the M substates and N state explains the failure of 5′-splice site substrate docking in M, supports a topological isomer model for the slow refolding of M to N due to a trapped strand crossing, and suggests pathways for M-to-N refolding.

Alkaline pH has an unexpected effect on transcriptional pausing during synthesis of the E. coli pH-responsive riboswitch

Riboswitches are 5′-untranslated regions of mRNA that change their conformation in response to ligand binding, allowing post-transcriptional gene regulation. This ligand-based model of riboswitch function has been expanded with the discovery of a “pH-responsive element” (PRE) riboswitch in Escherichia coli. At neutral pH, the PRE folds into a translationally inactive structure with an occluded ribosome-binding sequence, whereas at alkaline pH, the PRE adopts a translationally active structure. This unique riboswitch does not rely on ligand binding in a traditional sense to modulate its alternative folding outcomes. Rather, pH controls riboswitch folding by two possible modes that are yet to be distinguished; pH either regulates the transcription rate of RNA polymerase (RNAP) or acts on the RNA itself. Previous work suggested that RNAP pausing is prolonged by alkaline pH at two sites, stimulating PRE folding into the active structure. To date, there has been no rigorous exploration into how pH influences RNAP pausing kinetics during PRE synthesis. To provide that understanding and distinguish between pH acting on RNAP versus RNA, we investigated RNAP pausing kinetics at key sites for PRE folding under different pH conditions. We find that pH influences RNAP pausing but not in the manner proposed previously. Rather, alkaline pH either decreases or has no effect on RNAP pause longevity, suggesting that the modulation of RNAP pausing is not the sole mechanism by which pH affects PRE folding. These findings invite the possibility that the RNA itself actively participates in the sensing of pH.

Cotranscriptional RNA strand exchange underlies the gene regulation mechanism in a purine-sensing transcriptional riboswitch

RNA folds cotranscriptionally to traverse out-of-equilibrium intermediate structures that are important for RNA function in the context of gene regulation. To investigate this process, here we study the structure and function of the Bacillus subtilis yxjA purine riboswitch, a transcriptional riboswitch that downregulates a nucleoside transporter in response to binding guanine. Although the aptamer and expression platform domain sequences of the yxjA riboswitch do not completely overlap, we hypothesized that a strand exchange process triggers its structural switching in response to ligand binding. In vivo fluorescence assays, structural chemical probing data and experimentally informed secondary structure modeling suggest the presence of a nascent intermediate central helix. The formation of this central helix in the absence of ligand appears to compete with both the aptamer's P1 helix and the expression platform's transcriptional terminator. All-atom molecular dynamics simulations support the hypothesis that ligand binding stabilizes the aptamer P1 helix against central helix strand invasion, thus allowing the terminator to form. These results present a potential model mechanism to explain how ligand binding can induce downstream conformational changes by influencing local strand displacement processes of intermediate folds that could be at play in multiple riboswitch classes.

Cotranscriptional RNA Chemical Probing

Cotranscriptional folding is a fundamental step in RNA biogenesis and the basis for many RNA-mediated gene regulation systems. Understanding how RNA folds as it is synthesized requires experimental methods that can systematically identify intermediate RNA structures that form during transcription. Cotranscriptional RNA chemical probing experiments achieve this by applying high-throughput RNA structure probing to an in vitro transcribed array of cotranscriptionally folded intermediate transcripts. In this chapter, we present guidelines and procedures for integrating single-round in vitro transcription using E. coli RNA polymerase with high-throughput RNA chemical probing workflows. We provide an overview of key concepts including DNA template design, transcription roadblocking strategies, single-round in vitro transcription with E. coli RNA polymerase, and RNA chemical probing and describe procedures for DNA template preparation, cotranscriptional RNA chemical probing, RNA purification, and 3' adapter ligation. The end result of these procedures is a purified RNA library that can be prepared for Illumina sequencing using established high-throughput RNA structure probing library construction strategies.

Transcription complexes as RNA chaperones

To exert their functions, RNAs adopt diverse structures, ranging from simple secondary to complex tertiary and quaternary folds. In vivo, RNA folding starts with RNA transcription, and a wide variety of processes are coupled to co-transcriptional RNA folding events, including the regulation of fundamental transcription dynamics, gene regulation by mechanisms like attenuation, RNA processing or ribonucleoprotein particle formation. While co-transcriptional RNA folding and associated co-transcriptional processes are by now well accepted as pervasive regulatory principles in all organisms, investigations into the role of the transcription machinery in co-transcriptional folding processes have so far largely focused on effects of the order in which RNA regions are produced and of transcription kinetics. Recent structural and structure-guided functional analyses of bacterial transcription complexes increasingly point to an additional role of RNA polymerase and associated transcription factors in supporting co-transcriptional RNA folding by fostering or preventing strategic contacts to the nascent transcripts. In general, the results support the view that transcription complexes can act as RNA chaperones, a function that has been suggested over 30 years ago. Here, we discuss transcription complexes as RNA chaperones based on recent examples from bacterial transcription.

A roadmap for rRNA folding and assembly during transcription

Ribonucleoprotein (RNP) assembly typically begins during transcription when folding of the newly synthesized RNA is coupled with the recruitment of RNA-binding proteins (RBPs). Upon binding, the proteins induce structural rearrangements in the RNA that are crucial for the next steps of assembly. Focusing primarily on bacterial ribosome assembly, we discuss recent work showing that early RNA-protein interactions are more dynamic than previously supposed, and remain so, until sufficient proteins are recruited to each transcript to consolidate an entire domain of the RNP. We also review studies showing that stable assembly of an RNP competes against modification and processing of the RNA. Finally, we discuss how transcription sets the timeline for competing and cooperative RNA-RBP interactions that determine the fate of the nascent RNA. How this dance is coordinated is the focus of this review.

Regulation of polyphosphate glucokinase gene expression through co-transcriptional processing in Mycobacterium tuberculosis H37Rv

Transcription is a molecular process that involves the synthesis of RNA chain into the 5'-3' direction, and simultaneously nascent RNA chain tends to form geometric structures, known as co-transcriptional folding. This folding determines the functional properties of RNA molecules and possibly has a critical role during the synthesis. This functioning includes the characterized properties of riboswitches and ribozymes, which are significant when the transcription rate is comparable to the cellular environment. This study reports a novel non-coding region important in the genetic expression of polyphosphate glucokinase (ppgk) in Mycobacterium tuberculosis. This non-coding element of ppgk gene undergoes cleavage activity during the transcriptional process in Mycobacterium tuberculosis. We revealed that cleavage occurs within the nascent RNA, and the resultant cleaved 3'RNA fragment carries the Shine- Dalgarno (SD) sequence and expression platform. We concluded co-transcriptional processing at the non-coding region as the required mechanism for ppgk expression that remains constitutive within the bacterial environment. This study defines the molecular mechanism dependent on the transient but highly active structural features of the nascent RNA.

Transcription Regulation Through Nascent RNA Folding

Folding of RNA into secondary structures through intramolecular base pairing determines an RNA's three-dimensional architecture and associated function. Simple RNA structures like stem loops can provide specialized functions independent of coding capacity, such as protein binding, regulation of RNA processing and stability, stimulation or inhibition of translation. RNA catalysis is dependent on tertiary structures found in the ribosome, tRNAs and group I and II introns. While the extent to which non-coding RNAs contribute to cellular maintenance is generally appreciated, the fact that both non-coding and coding RNA can assume relevant structural states has only recently gained attention. In particular, the co-transcriptional folding of nascent RNA of all classes has the potential to regulate co-transcriptional processing, RNP (ribonucleoprotein particle) formation, and transcription itself. Riboswitches are established examples of co-transcriptionally folded coding RNAs that directly regulate transcription, mainly in prokaryotes. Here we discuss recent studies in both prokaryotes and eukaryotes showing that structure formation may carry a more widespread regulatory logic during RNA synthesis. Local structures forming close to the catalytic center of RNA polymerases have the potential to regulate transcription by reducing backtracking. In addition, stem loops or more complex structures may alter co-transcriptional RNA processing or its efficiency. Several examples of functional structures have been identified to date, and this review provides an overview of physiologically distinct processes where co-transcriptionally folded RNA plays a role. Experimental approaches such as single-molecule FRET and in vivo structural probing to further advance our insight into the significance of co-transcriptional structure formation are discussed.

Computationally reconstructing cotranscriptional RNA folding from experimental data reveals rearrangement of non-native folding intermediates

The series of RNA folding events that occur during transcription can critically influence cellular RNA function. Here, we present reconstructing RNA dynamics from data (R2D2), a method to uncover details of cotranscriptional RNA folding. We model the folding of the Escherichia coli signal recognition particle (SRP) RNA and show that it requires specific local structural fluctuations within a key hairpin to engender efficient cotranscriptional conformational rearrangement into the functional structure. All-atom molecular dynamics simulations suggest that this rearrangement proceeds through an internal toehold-mediated strand-displacement mechanism, which can be disrupted with a point mutation that limits local structural fluctuations and rescued with compensating mutations that restore these fluctuations. Moreover, a cotranscriptional folding intermediate could be cleaved in vitro by recombinant E. coli RNase P, suggesting potential cotranscriptional processing. These results from experiment-guided multi-scale modeling demonstrate that even an RNA with a simple functional structure can undergo complex folding and processing during synthesis.

Cooperativity and Allostery in RNA Systems

Allostery is among the most basic biological principles employed by biological macromolecules to achieve a biologically active state in response to chemical cues. Although initially used to describe the impact of small molecules on the conformation and activity of protein enzymes, the definition of this term has been significantly broadened to describe long-range conformational change of macromolecules in response to small or large effectors. Such a broad definition could be applied to RNA molecules, which do not typically serve as protein-free cellular enzymes but fold and form macromolecular assemblies with the help of various ligand molecules, including ions and proteins. Ligand-induced allosteric changes in RNA molecules are often accompanied by cooperative interactions between RNA and its ligand, thus streamlining the folding and assembly pathways. This chapter provides an overview of the interplay between cooperativity and allostery in RNA systems and outlines methods to study these two biological principles.

Repositioning the Sm-Binding Site in Saccharomyces cerevisiae Telomerase RNA Reveals RNP Organizational Flexibility and Sm-Directed 3'-End Formation

Telomerase RNA contains a template for synthesizing telomeric DNA and has been proposed to act as a flexible scaffold for holoenzyme protein subunits in the RNP. In Saccharomyces cerevisiae, the telomerase RNA, TLC1, is bound by the Sm₇ protein complex, which is required for stabilization of the predominant, non-polyadenylated (poly(A)–) TLC1 isoform. However, it remains unclear (1) whether Sm₇ retains this function when its binding site is repositioned within TLC1, as has been shown for other TLC1-binding telomerase subunits, and (2) how Sm₇ stabilizes poly(A)– TLC1. Here, we first show that Sm₇ can stabilize poly(A)– TLC1 even when its binding site is repositioned via circular permutation to several different positions within TLC1, further supporting the conclusion that the telomerase holoenzyme is organizationally flexible. Next, we show that when an Sm site is inserted 5′ of its native position and the native site is mutated, Sm₇ stabilizes shorter forms of poly(A)– TLC1 in a manner corresponding to how far upstream the new site was inserted, providing strong evidence that Sm₇ binding to TLC1 controls where the mature poly(A)– 3′ is formed by directing a 3′-to-5′ processing mechanism. In summary, our results show that Sm₇ and the 3′ end of yeast telomerase RNA comprise an organizationally flexible module within the telomerase RNP and provide insights into the mechanistic role of Sm₇ in telomerase RNA biogenesis.

Branched kissing loops for the construction of diverse RNA homooligomeric nanostructures

In biological systems, large and complex structures are often assembled from multiple simpler identical subunits. This strategy-homooligomerization-allows efficient genetic encoding of structures and avoids the need to control the stoichiometry of multiple distinct units. It also allows the minimal number of distinct subunits when designing artificial nucleic acid structures. Here, we present a robust self-assembly system in which homooligomerizable tiles are formed from intramolecularly folded RNA single strands. Tiles are linked through an artificially designed branched kissing-loop motif, involving Watson-Crick base pairing between the single-stranded regions of a bulged helix and a hairpin loop. By adjusting the tile geometry to gain control over the curvature, torsion and the number of helices, we have constructed 16 different linear and circular structures, including a finite-sized three-dimensional cage. We further demonstrate cotranscriptional self-assembly of tiles based on branched kissing loops, and show that tiles inserted into a transfer RNA scaffold can be overexpressed in bacterial cells.

NMR Chemical Exchange Measurements Reveal That N6-Methyladenosine Slows RNA Annealing

N⁶-Methyladenosine (m⁶A) is an abundant epitranscriptomic modification that plays important roles in many aspects of RNA metabolism. While m⁶A is thought to mainly function by recruiting reader proteins to specific RNA sites, the modification can also reshape RNA-protein and RNA-RNA interactions by altering RNA structure mainly by destabilizing base pairing. Little is known about how m⁶A and other epitranscriptomic modifications might affect the kinetic rates of RNA folding and other conformational transitions that are also important for cellular activity. Here, we used NMR R_1ρ relaxation dispersion and chemical exchange saturation transfer to noninvasively and site-specifically measure nucleic acid hybridization kinetics. The methodology was validated on two DNA duplexes and then applied to examine how a single m⁶A alters the hybridization kinetics in two RNA duplexes. The results show that m⁶A minimally impacts the rate constant for duplex dissociation, changing k_off by ∼1-fold but significantly slows the rate of duplex annealing, decreasing k_on by ∼7-fold. A reduction in the annealing rate was observed robustly for two different sequence contexts at different temperatures, both in the presence and absence of Mg²⁺. We propose that rotation of the N⁶-methyl group from the preferred syn conformation in the unpaired nucleotide to the energetically disfavored anti conformation required for Watson-Crick pairing is responsible for the reduced annealing rate. The results help explain why in mRNA m⁶A slows down tRNA selection and more generally suggest that m⁶A may exert cellular functions by reshaping the kinetics of RNA conformational transitions.

Self-powered RNA nanomachine driven by metastable structure

Many non-coding and regulatory RNA elements have evolved to exploit transient or metastable structures that emerge during transcription to control complex folding pathways or to encode dynamic functions. However, efforts to engineer synthetic RNA devices have mostly focused on the thermodynamically stable structures. Consequently, significant challenges and opportunities exist in engineering functional RNAs that explicitly take advantage of cotranscriptionally generated transient or metastable structures. In this work, we designed a short RNA sequence that adopts a robust metastable structure when transcribed by an RNA polymerase. Although the metastable structure persists for hours at low temperature, it refolds almost completely into the thermodynamically stable structure upon heat denaturation followed by cooling. The synthetic RNA was also equipped with the Broccoli aptamer so that it can bind its ligand and become fluorescent only in the thermodynamically stable structure. We further demonstrated that the relaxation to the thermodynamically stable and fluorescent structure can be catalyzed by a short trigger RNA in a sequence-specific manner. Finally, the RNA architecture was redesigned to sense and respond to microRNA sequences. In summary, we designed RNA nanomachines that can detect an RNA sequence, amplify signal and produce an optical output, all encoded in a single RNA transcript, self-powered by a metastable structure.

Transient Protein-RNA Interactions Guide Nascent Ribosomal RNA Folding

Ribosome assembly is an efficient but complex and heterogeneous process during which ribosomal proteins assemble on the nascent rRNA during transcription. Understanding how the interplay between nascent RNA folding and protein binding determines the fate of transcripts remains a major challenge. Here, using single-molecule fluorescence microscopy, we follow assembly of the entire 3' domain of the bacterial small ribosomal subunit in real time. We find that co-transcriptional rRNA folding is complicated by the formation of long-range RNA interactions and that r-proteins self-chaperone the rRNA folding process prior to stable incorporation into a ribonucleoprotein (RNP) complex. Assembly is initiated by transient rather than stable protein binding, and the protein-RNA binding dynamics gradually decrease during assembly. This work questions the paradigm of strictly sequential and cooperative ribosome assembly and suggests that transient binding of RNA binding proteins to cellular RNAs could provide a general mechanism to shape nascent RNA folding during RNP assembly.

The Biogenesis of SRP RNA Is Modulated by an RNA Folding Intermediate Attained during Transcription

The signal recognition particle (SRP), responsible for co-translational protein targeting and delivery to cellular membranes, depends on the native long-hairpin fold of its RNA to confer functionality. Since RNA initiates folding during its synthesis, we used high-resolution optical tweezers to follow in real time the co-transcriptional folding of SRP RNA. Surprisingly, SRP RNA folding is robust to transcription rate changes and the presence or absence of its 5'-precursor sequence. The folding pathway also reveals the obligatory attainment of a non-native hairpin intermediate (H1) that eventually rearranges into the native fold. Furthermore, H1 provides a structural platform alternative to the native fold for RNase P to bind and mature SRP RNA co-transcriptionally. Delays in attaining the final native fold are detrimental to the cell, altogether showing that a co-transcriptional folding pathway underpins the proper biogenesis of function-essential SRP RNA.

A ligand-gated strand displacement mechanism for ZTP riboswitch transcription control

Cotranscriptional folding is an obligate step of RNA biogenesis that can guide RNA structure formation and function through transient intermediate folds. This process is particularly important for transcriptional riboswitches in which the formation of ligand-dependent structures during transcription regulates downstream gene expression. However, the intermediate structures that comprise cotranscriptional RNA folding pathways, and the mechanisms that enable transit between them, remain largely unknown. Here, we determine the series of cotranscriptional folds and rearrangements that mediate antitermination by the Clostridium beijerinckii pfl ZTP riboswitch in response to the purine biosynthetic intermediate ZMP. We uncover sequence and structural determinants that modulate an internal RNA strand displacement process and identify biases within natural ZTP riboswitch sequences that promote on-pathway folding. Our findings establish a mechanism for pfl riboswitch antitermination and suggest general strategies by which nascent RNA molecules navigate cotranscriptional folding pathways.

Real-time assembly of ribonucleoprotein complexes on nascent RNA transcripts

Cellular protein-RNA complexes assemble on nascent transcripts, but methods to observe transcription and protein binding in real time and at physiological concentrations are not available. Here, we report a single-molecule approach based on zero-mode waveguides that simultaneously tracks transcription progress and the binding of ribosomal protein S15 to nascent RNA transcripts during early ribosome biogenesis. We observe stable binding of S15 to single RNAs immediately after transcription for the majority of the transcripts at 35 °C but for less than half at 20 °C. The remaining transcripts exhibit either rapid and transient binding or are unable to bind S15, likely due to RNA misfolding. Our work establishes the foundation for studying transcription and its coupled co-transcriptional processes, including RNA folding, ligand binding, and enzymatic activity such as in coupling of transcription to splicing, ribosome assembly or translation.

The early steps of ribosome assembly occur co-transcriptionally on the nascent ribosomal RNA. Here the authors demonstrate an approach that allows simultaneous monitoring of transcription and ribosomal protein assembly at the single-molecule level in real time.

Effects of flanking regions on HDV cotranscriptional folding kinetics

Hepatitis delta virus (HDV) ribozyme performs the self-cleavage activity through folding to a double pseudoknot structure. The folding of functional RNA structures is often coupled with the transcription process. In this work, we developed a new approach for predicting the cotranscriptional folding kinetics of RNA secondary structures with pseudoknots. We theoretically studied the cotranscriptional folding behavior of the 99-nucleotide (nt) HDV sequence, two upstream flanking sequences, and one downstream flanking sequence. During transcription, the 99-nt HDV can effectively avoid the trap intermediates and quickly fold to the cleavage-active state. It is different from its refolding kinetics, which folds into an intermediate trap state. For all the sequences, the ribozyme regions (from 1 to 73) all fold to the same structure during transcription. However, the existence of the 30-nt upstream flanking sequence can inhibit the ribozyme region folding into the active native state through forming an alternative helix Alt1 with the segments 70-90. The longer upstream flanking sequence of 54 nt itself forms a stable hairpin structure, which sequesters the formation of the Alt1 helix and leads to rapid formation of the cleavage-active structure. Although the 55-nt downstream flanking sequence could invade the already folded active structure during transcription by forming a more stable helix with the ribozyme region, the slow transition rate could keep the structure in the cleavage-active structure to perform the activity.

RNA Regulations and Functions Decoded by Transcriptome-wide RNA Structure Probing

RNA folds into intricate structures that are crucial for its functions and regulations. To date, a multitude of approaches for probing structures of the whole transcriptome, i.e., RNA structuromes, have been developed. Applications of these approaches to different cell lines and tissues have generated a rich resource for the study of RNA structure–function relationships at a systems biology level. In this review, we first introduce the designs of these methods and their applications to study different RNA structuromes. We emphasize their technological differences especially their unique advantages and caveats. We then summarize the structural insights in RNA functions and regulations obtained from the studies of RNA structuromes. And finally, we propose potential directions for future improvements and studies.

Co-Transcriptional Folding and Regulation Mechanisms of Riboswitches

Riboswitches are genetic control elements within non-coding regions of mRNA. These self-regulatory elements have been found to sense a range of small metabolites, ions, and other physical signals to exert regulatory control of transcription, translation, and splicing. To date, more than a dozen riboswitch classes have been characterized that vary widely in size and secondary structure. Extensive experiments and theoretical studies have made great strides in understanding the general structures, genetic mechanisms, and regulatory activities of individual riboswitches. As the ligand-dependent co-transcriptional folding and unfolding dynamics of riboswitches are the key determinant of gene expression, it is important to investigate the thermodynamics and kinetics of riboswitches both in the presence and absence of metabolites under the transcription. This review will provide a brief summary of the studies about the regulation mechanisms of the pbuE, S_MK, yitJ, and metF riboswitches based on the ligand-dependent co-transcriptional folding of the riboswitches.

In silico methods for co-transcriptional RNA secondary structure prediction and for investigating alternative RNA structure expression

RNA transcripts are the primary products of active genes in any living organism, including many viruses. Their cellular destiny not only depends on primary sequence signals, but can also be determined by RNA structure. Recent experimental evidence shows that many transcripts can be assigned more than a single functional RNA structure throughout their cellular life and that structure formation happens co-transcriptionally, i.e. as the transcript is synthesised in the cell. Moreover, functional RNA structures are not limited to non-coding transcripts, but can also feature in coding transcripts. The picture that now emerges is that RNA structures constitute an additional layer of information that can be encoded in any RNA transcript (and on top of other layers of information such as protein-context) in order to exert a wide range of functional roles. Moreover, different encoded RNA structures can be expressed at different stages of a transcript's life in order to alter the transcript's behaviour depending on its actual cellular context. Similar to the concept of alternative splicing for protein-coding genes, where a single transcript can yield different proteins depending on cellular context, it is thus appropriate to propose the notion of alternative RNA structure expression for any given transcript. This review introduces several computational strategies that my group developed to detect different aspects of RNA structure expression in vivo. Two aspects are of particular interest to us: (1) RNA secondary structure features that emerge during co-transcriptional folding and (2) functional RNA structure features that are expressed at different times of a transcript's life and potentially mutually exclusive.

NMR Structural Profiling of Transcriptional Intermediates Reveals Riboswitch Regulation by Metastable RNA Conformations

Gene repression induced by the formation of transcriptional terminators represents a prime example for the coupling of RNA synthesis, folding, and regulation. In this context, mapping the changes in available conformational space of transcription intermediates during RNA synthesis is important to understand riboswitch function. A majority of riboswitches, an important class of small metabolite-sensing regulatory RNAs, act as transcriptional regulators, but the dependence of ligand binding and the subsequent allosteric conformational switch on mRNA transcript length has not yet been investigated. We show a strict fine-tuning of binding and sequence-dependent alterations of conformational space by structural analysis of all relevant transcription intermediates at single-nucleotide resolution for the I-A type 2'dG-sensing riboswitch from Mesoplasma florum by NMR spectroscopy. Our results provide a general framework to dissect the coupling of synthesis and folding essential for riboswitch function, revealing the importance of metastable states for RNA-based gene regulation.

Inverted repeat Alu elements in the human lincRNA-p21 adopt a conserved secondary structure that regulates RNA function

LincRNA-p21 is a long intergenic non-coding RNA (lincRNA) involved in the p53-mediated stress response. We sequenced the human lincRNA-p21 (hLincRNA-p21) and found that it has a single exon that includes inverted repeat Alu elements (IRAlus). Sense and antisense Alu elements fold independently of one another into a secondary structure that is conserved in lincRNA-p21 among primates. Moreover, the structures formed by IRAlus are involved in the localization of hLincRNA-p21 in the nucleus, where hLincRNA-p21 colocalizes with paraspeckles. Our results underscore the importance of IRAlus structures for the function of hLincRNA-p21 during the stress response.

Cotranscriptional folding of a riboswitch at nucleotide resolution

RNAs can begin to fold immediately after emerging from RNA polymerase during transcription. Interactions between nascent RNAs and ligands during cotranscriptional folding can direct the formation of alternative RNA structures, a feature exploited by non-coding RNAs called riboswitches to make gene regulatory decisions. Despite their importance, cotranscriptional folding pathways have yet to be uncovered with sufficient resolution to reveal how cotranscriptional folding governs RNA structure and function. To access cotranscriptional folding at nucleotide resolution, we extend selective 2’-hydroxyl acylation analyzed by primer extension sequencing (SHAPE-Seq) to measure structural information of nascent RNAs during transcription. With cotranscriptional SHAPE-Seq, we determine how the B. cereus crcB fluoride riboswitch cotranscriptional folding pathway undergoes a ligand-dependent bifurcation that delays or promotes terminator formation via a series of coordinated structural transitions. Our results directly link cotranscriptional RNA folding to a genetic decision and establish a framework for cotranscriptional analysis of RNA structure at nucleotide resolution.

Using RNA as Molecular Code for Programming Cellular Function

RNA is involved in a wide-range of important molecular processes in the cell, serving diverse functions: regulatory, enzymatic, and structural. Together with its ease and predictability of design, these properties can lead RNA to become a useful handle for biological engineers with which to control the cellular machinery. By modifying the many RNA links in cellular processes, it is possible to reprogram cells toward specific design goals. We propose that RNA can be viewed as a molecular programming language that, together with protein-based execution platforms, can be used to rewrite wide ranging aspects of cellular function. In this review, we catalogue developments in the use of RNA parts, methods, and associated computational models that have contributed to the programmability of biology. We discuss how RNA part repertoires have been combined to build complex genetic circuits, and review recent applications of RNA-based parts and circuitry. We explore the future potential of RNA engineering and posit that RNA programmability is an important resource for firmly establishing an era of rationally designed synthetic biology.

Coupling of RNA Polymerase II Transcription Elongation with Pre-mRNA Splicing

Pre-mRNA maturation frequently occurs at the same time and place as transcription by RNA polymerase II. The co-transcriptionality of mRNA processing has permitted the evolution of mechanisms that functionally couple transcription elongation with diverse events that occur on the nascent RNA. This review summarizes the current understanding of the relationship between transcriptional elongation through a chromatin template and co-transcriptional splicing including alternative splicing decisions that affect the expression of most human genes.

Regulatory effects of cotranscriptional RNA structure formation and transitions

RNAs, which play significant roles in many fundamental biological processes of life, fold into sophisticated and precise structures. RNA folding is a dynamic and intricate process, which conformation transition of coding and noncoding RNAs form the primary elements of genetic regulation. The cellular environment contains various intrinsic and extrinsic factors that potentially affect RNA folding in vivo, and experimental and theoretical evidence increasingly indicates that the highly flexible features of the RNA structure are affected by these factors, which include the flanking sequence context, physiochemical conditions, cis RNA-RNA interactions, and RNA interactions with other molecules. Furthermore, distinct RNA structures have been identified that govern almost all steps of biological processes in cells, including transcriptional activation and termination, transcriptional mutagenesis, 5'-capping, splicing, 3'-polyadenylation, mRNA export and localization, and translation. Here, we briefly summarize the dynamic and complex features of RNA folding along with a wide variety of intrinsic and extrinsic factors that affect RNA folding. We then provide several examples to elaborate RNA structure-mediated regulation at the transcriptional and posttranscriptional levels. Finally, we illustrate the regulatory roles of RNA structure and discuss advances pertaining to RNA structure in plants. WIREs RNA 2016, 7:562-574. doi: 10.1002/wrna.1350 For further resources related to this article, please visit the WIREs website.

Secondary Structure Prediction of Single Sequences Using RNAstructure

RNA secondary structure is often predicted using folding thermodynamics. RNAstructure is a software package that includes structure prediction by free energy minimization, prediction of base pairing probabilities, prediction of structures composed of highly probably base pairs, and prediction of structures with pseudoknots. A user-friendly graphical user interface is provided, and this interface works on Windows, Apple OS X, and Linux. This chapter provides protocols for using RNAstructure for structure prediction.

Progress and challenges for chemical probing of RNA structure inside living cells

Proper gene expression is essential for the survival of every cell. Once thought to be a passive transporter of genetic information, RNA has recently emerged as a key player in nearly every pathway in the cell. A full description of its structure is critical to understanding RNA function. Decades of research have focused on utilizing chemical tools to interrogate the structures of RNAs, with recent focus shifting to performing experiments inside living cells. This Review will detail the design and utility of chemical reagents used in RNA structure probing. We also outline how these reagents have been used to gain a deeper understanding of RNA structure in vivo. We review the recent merger of chemical probing with deep sequencing. Finally, we outline some of the hurdles that remain in fully characterizing the structure of RNA inside living cells, and how chemical biology can uniquely tackle such challenges.

Genome-wide profiling of mouse RNA secondary structures reveals key features of the mammalian transcriptome

Background

The understanding of RNA structure is a key feature toward the comprehension of RNA functions and mechanisms of action. In particular, non-coding RNAs are thought to exert their functions by specific secondary structures, but an efficient annotation on a large scale of these structures is still missing.

Results

By using a novel high-throughput method, named chemical inference of RNA structures, CIRS-seq, that uses dimethyl sulfate, and N-cyclohexyl- N'-(2-morpholinoethyl)carbodiimide metho-p-toluenesulfonate to modify RNA residues in single-stranded conformation within native deproteinized RNA secondary structures, we investigate the structural features of mouse embryonic stem cell transcripts. Our analysis reveals an unexpected higher structuring of the 5′ and 3′ untranslated regions compared to the coding regions, a reduced structuring at the Kozak sequence and stop codon, and a three-nucleotide periodicity across the coding region of messenger RNAs. We also observe that ncRNAs exhibit a higher degree of structuring with respect to protein coding transcripts. Moreover, we find that the Lin28a binding protein binds selectively to RNA motifs with a strong preference toward a single stranded conformation.

Conclusions

This work defines for the first time the complete RNA structurome of mouse embryonic stem cells, revealing an extremely distinct RNA structural landscape. These results demonstrate that CIRS-seq constitutes an important tool for the identification of native deproteinized RNA structures.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0491-2) contains supplementary material, which is available to authorized users.

Native Purification and Analysis of Long RNAs

The purification and analysis of long noncoding RNAs (lncRNAs) in vitro is a challenge, particularly if one wants to preserve elements of functional structure. Here, we describe a method for purifying lncRNAs that preserves the cotranscriptionally derived structure. The protocol avoids the misfolding that can occur during denaturation-renaturation protocols, thus facilitating the folding of long RNAs to a native-like state. This method is simple and does not require addition of tags to the RNA or the use of affinity columns. LncRNAs purified using this type of native purification protocol are amenable to biochemical and biophysical analysis. Here, we describe how to study lncRNA global compaction in the presence of divalent ions at equilibrium using sedimentation velocity analytical ultracentrifugation and analytical size-exclusion chromatography as well as how to use these uniform RNA species to determine robust lncRNA secondary structure maps by chemical probing techniques like selective 2'-hydroxyl acylation analyzed by primer extension and dimethyl sulfate probing.

RNA folding: structure prediction, folding kinetics and ion electrostatics

Beyond the "traditional" functions such as gene storage, transport and protein synthesis, recent discoveries reveal that RNAs have important "new" biological functions including the RNA silence and gene regulation of riboswitch. Such functions of noncoding RNAs are strongly coupled to the RNA structures and proper structure change, which naturally leads to the RNA folding problem including structure prediction and folding kinetics. Due to the polyanionic nature of RNAs, RNA folding structure, stability and kinetics are strongly coupled to the ion condition of solution. The main focus of this chapter is to review the recent progress in the three major aspects in RNA folding problem: structure prediction, folding kinetics and ion electrostatics. This chapter will introduce both the recent experimental and theoretical progress, while emphasize the theoretical modelling on the three aspects in RNA folding.

Looking at LncRNAs with the ribozyme toolkit

A diverse population of large RNA molecules controls every aspect of cellular function, and yet we know very little about their molecular structures. However, robust technologies developed for visualizing ribozymes and riboswitches, together with new approaches for mapping RNA inside cells, provide the foundation for visualizing the structures of long noncoding RNAs, mRNAs, and viral RNAs, thereby facilitating new mechanistic insights.

A single-stranded architecture for cotranscriptional folding of RNA nanostructures

Artificial DNA and RNA structures have been used as scaffolds for a variety of nanoscale devices. In comparison to DNA structures, RNA structures have been limited in size, but they also have advantages: RNA can fold during transcription and thus can be genetically encoded and expressed in cells. We introduce an architecture for designing artificial RNA structures that fold from a single strand, in which arrays of antiparallel RNA helices are precisely organized by RNA tertiary motifs and a new type of crossover pattern. We constructed RNA tiles that assemble into hexagonal lattices and demonstrated that lattices can be made by annealing and/or cotranscriptional folding. Tiles can be scaled up to 660 nucleotides in length, reaching a size comparable to that of large natural ribozymes.