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

Theophylline controllable RNAi-based genetic switches regulate expression of lncRNA TINCR and malignant phenotypes in bladder cancer cells

TINCR is a well-known lncRNA which acts as a master regulator in somatic differentiation development. However, it is still unclear whether TINCR is also involved in caner occurrence and progression. In this study, we observed that TINCR was up-regulated in bladder cancer tissues and cells and contributed to oncogenesis and cancer progression. Silencing TINCR expression inhibited cell proliferation and promoted apoptosis in vitro, indicating that TINCR may be the potential therapeutic target for treating bladder urothelial carcinoma. Thus we used the synthetic biology approach to create theophylline controllable RNAi-based genetic switches which silenced TINCR in a dosage-dependent manner. Both RNAi-OFF and ON switches can be used to quantitatively control the expression of TINCR in bladder cancer to suppress the progression of bladder cancer. These findings suggest that lncRNA-TINCR could promote bladder cancer development and progression and artificial control of its expression through inducible RNAi may represent a new kind of therapeutic strategy for treating human bladder cancer.

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

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

Mechanistic Insights into Cofactor-Dependent Coupling of RNA Folding and mRNA Transcription/Translation by a Cobalamin Riboswitch

Riboswitches are mRNA elements regulating gene expression in response to direct binding of a metabolite. While these RNAs are increasingly well understood with respect to interactions between receptor domains and their cognate effector molecules, little is known about the specific mechanistic relationship between metabolite binding and gene regulation by the downstream regulatory domain. Using a combination of cell-based, biochemical, and biophysical techniques, we reveal the specific RNA architectural features enabling a cobalamin-dependent hairpin loop docking interaction between receptor and regulatory domains. Furthermore, these data demonstrate that docking kinetics dictate a regulatory response involving the coupling of translation initiation to general mechanisms that control mRNA abundance. These results yield a comprehensive picture of how RNA structure in the riboswitch regulatory domain enables kinetically constrained ligand-dependent regulation of gene expression.

Exploring the modular nature of riboswitches and RNA thermometers

Natural regulatory RNAs like riboswitches and RNA thermometers (RNAT) have considerable potential in synthetic biology. They are located in the 5′ untranslated region (UTR) of bacterial mRNAs and sense small molecules or changes in temperature, respectively. While riboswitches act on the level of transcription, translation or mRNA stability, all currently known RNATs regulate translation initiation. In this study, we explored the modularity of riboswitches and RNATs and obtained regulatory devices with novel functionalities. In a first approach, we established three riboswitch-RNAT systems conferring dual regulation of transcription and translation depending on the two triggers ligand binding and temperature sensing. These consecutive fusions control gene expression in vivo and can even orchestrate complex cellular behavior. In another approach, we designed two temperature-controlled riboswitches by the integration of an RNAT into a riboswitch aptamer domain. These ‘thermoswitches’ respond to the cognate ligand at low temperatures and are turned into a continuous on-state by a temperature upshift. They represent the first RNATs taking control of transcription. Overall, this study demonstrates that riboswitches and RNATs are ideal for engineering synthetic RNA regulators due to their modular behavior.

Rewiring Riboswitches to Create New Genetic Circuits in Bacteria

Riboswitches are RNA elements that control the expression of genes through a variety of mechanisms in response to the specific binding of small-molecule ligands. Since their discovery, riboswitches have shown promise for the artificial control of transcription or translation of target genes, be it for industrial biotechnology, protein expression, metabolic engineering, antimicrobial target validation, or gene function discovery. However, natural riboswitches are often unsuitable for these purposes due to their regulation by small molecules which are already present within the cell. For this reason, research has focused on creating riboswitches that respond to alternative biologically inert ligands or to molecules which are of interest for biosensing. Here we present methods for the development of artificial riboswitches in Gram-negative and Gram-positive bacteria. These methods are based on reengineering natural aptamers to change their ligand specificity toward molecules which do not bind the original aptamer (ie, that are orthogonal to the original). The first approach involves targeted mutagenesis of native riboswitches to change their specificity toward rationally designed synthetic ligand analogs. The second approach involves the fusion of previously validated orthogonal aptamers with native expression platforms to create novel chimeric riboswitches for the microbial target. We establish the applicability of these methods both for the control of exogenous genes as well as for the control of native genes.

Opportunities in the design and application of RNA for gene expression control

The past decade of synthetic biology research has witnessed numerous advances in the development of tools and frameworks for the design and characterization of biological systems. Researchers have focused on the use of RNA for gene expression control due to its versatility in sensing molecular ligands and the relative ease by which RNA can be modeled and designed compared to proteins. We review the recent progress in the field with respect to RNA-based genetic devices that are controlled through small molecule and protein interactions. We discuss new approaches for generating and characterizing these devices and their underlying components. We also highlight immediate challenges, future directions and recent applications of synthetic RNA devices in engineered biological systems.

Novel technologies provide more engineering strategies for amino acid-producing microorganisms

Traditionally, amino acid-producing strains were obtained by random mutagenesis and subsequent selection. With the development of genetic and metabolic engineering techniques, various microorganisms with high amino acid production yields are now constructed by rational design of targeted biosynthetic pathways. Recently, novel technologies derived from systems and synthetic biology have emerged and open a new promising avenue towards the engineering of amino acid production microorganisms. In this review, these approaches, including rational engineering of rate-limiting enzymes, real-time sensing of end-products, pathway optimization on the chromosome, transcription factor-mediated strain improvement, and metabolic modeling and flux analysis, were summarized with regard to their application in microbial amino acid production.

Design criteria for synthetic riboswitches acting on transcription

Riboswitches are RNA-based regulators of gene expression composed of a ligand-sensing aptamer domain followed by an overlapping expression platform. The regulation occurs at either the level of transcription (by formation of terminator or antiterminator structures) or translation (by presentation or sequestering of the ribosomal binding site). Due to a modular composition, these elements can be manipulated by combining different aptamers and expression platforms and therefore represent useful tools to regulate gene expression in synthetic biology. Using computationally designed theophylline-dependent riboswitches we show that 2 parameters, terminator hairpin stability and folding traps, have a major impact on the functionality of the designed constructs. These have to be considered very carefully during design phase. Furthermore, a combination of several copies of individual riboswitches leads to a much improved activation ratio between induced and uninduced gene activity and to a linear dose-dependent increase in reporter gene expression. Such serial arrangements of synthetic riboswitches closely resemble their natural counterparts and may form the basis for simple quantitative read out systems for the detection of specific target molecules in the cell.

RNA aptamers as genetic control devices: the potential of riboswitches as synthetic elements for regulating gene expression

RNA utilizes many different mechanisms to control gene expression. Among the regulatory elements that respond to external stimuli, riboswitches are a prominent and elegant example. They consist solely of RNA and couple binding of a small molecule ligand to the so-called "aptamer domain" with a conformational change in the downstream "expression platform" which then determines system output. The modular organization of riboswitches and the relative ease with which ligand-binding RNA aptamers can be selected in vitro against almost any molecule have led to the rapid and widespread adoption of engineered riboswitches as artificial genetic control devices in biotechnology and synthetic biology over the past decade. This review highlights proof-of-principle applications to demonstrate the versatility and robustness of engineered riboswitches in regulating gene expression in pro- and eukaryotes. It then focuses on strategies and parameters to identify aptamers that can be integrated into synthetic riboswitches that are functional in vivo, before finishing with a reflection on how to improve the regulatory properties of engineered riboswitches, so that we can not only further expand riboswitch applicability, but also finally fully exploit their potential as control elements in regulating gene expression.

Linking aptamer-ligand binding and expression platform folding in riboswitches: prospects for mechanistic modeling and design

The power of riboswitches in regulation of bacterial metabolism derives from coupling of two characteristics: recognition and folding. Riboswitches contain aptamers, which function as biosensors. Upon detection of the signaling molecule, the riboswitch transduces the signal into a genetic decision. The genetic decision is coupled to refolding of the expression platform, which is distinct from, although overlapping with, the aptamer. Early biophysical studies of riboswitches focused on recognition of the ligand by the aptamer-an important consideration for drug design. A mechanistic understanding of ligand-induced riboswitch RNA folding can further enhance riboswitch ligand design, and inform efforts to tune and engineer riboswitches with novel properties. X-ray structures of aptamer/ligand complexes point to mechanisms through which the ligand brings together distal strand segments to form a P1 helix. Transcriptional riboswitches must detect the ligand and form this P1 helix within the timescale of transcription. Depending on the cell's metabolic state and cellular environmental conditions, the folding and genetic outcome may therefore be affected by kinetics of ligand binding, RNA folding, and transcriptional pausing, among other factors. Although some studies of isolated riboswitch aptamers found homogeneous, prefolded conformations, experimental, and theoretical studies point to functional and structural heterogeneity for nascent transcripts. Recently it has been shown that some riboswitch segments, containing the aptamer and partial expression platforms, can form binding-competent conformers that incorporate an incomplete aptamer secondary structure. Consideration of the free energy landscape for riboswitch RNA folding suggests models for how these conformers may act as transition states-facilitating rapid, ligand-mediated aptamer folding.

Improving fold activation of small transcription activating RNAs (STARs) with rational RNA engineering strategies

Regulatory RNAs have become integral components of the synthetic biology and bioengineering toolbox for controlling gene expression. We recently expanded this toolbox by creating small transcription activating RNAs (STARs) that act by disrupting the formation of a target transcriptional terminator hairpin placed upstream of a gene. While STARs are a promising addition to the repertoire of RNA regulators, much work remains to be done to optimize the fold activation of these systems. Here we apply rational RNA engineering strategies to improve the fold activation of two STAR regulators. We demonstrate that a combination of promoter strength tuning and multiple RNA engineering strategies can improve fold activation from 5.4-fold to 13.4-fold for a STAR regulator derived from the pbuE riboswitch terminator. We then validate the generality of our approach and show that these same strategies improve fold activation from 2.1-fold to 14.6-fold for an unrelated STAR regulator, opening the door to creating a range of additional STARs to use in a broad array of biotechnologies. We also establish that the optimizations preserve the orthogonality of these STARs between themselves and a set of RNA transcriptional repressors, enabling these optimized STARs to be used in sophisticated circuits.

A Highly Coupled Network of Tertiary Interactions in the SAM-I Riboswitch and Their Role in Regulatory Tuning

RNA folding in vivo is significantly influenced by transcription, which is not necessarily recapitulated by Mg(2+)-induced folding of the corresponding full-length RNA in vitro. Riboswitches that regulate gene expression at the transcriptional level are an ideal system for investigating this aspect of RNA folding as ligand-dependent termination is obligatorily co-transcriptional, providing a clear readout of the folding outcome. The folding of representative members of the SAM-I family of riboswitches has been extensively analyzed using approaches focusing almost exclusively upon Mg(2+) and/or S-adenosylmethionine (SAM)-induced folding of full-length transcripts of the ligand binding domain. To relate these findings to co-transcriptional regulatory activity, we have investigated a set of structure-guided mutations of conserved tertiary architectural elements of the ligand binding domain using an in vitro single-turnover transcriptional termination assay, complemented with phylogenetic analysis and isothermal titration calorimetry data. This analysis revealed a conserved internal loop adjacent to the SAM binding site that significantly affects ligand binding and regulatory activity. Conversely, most single point mutations throughout key conserved features in peripheral tertiary architecture supporting the SAM binding pocket have relatively little impact on riboswitch activity. Instead, a secondary structural element in the peripheral subdomain appears to be the key determinant in observed differences in regulatory properties across the SAM-I family. These data reveal a highly coupled network of tertiary interactions that promote high-fidelity co-transcriptional folding of the riboswitch but are only indirectly linked to regulatory tuning.

Rational Re-engineering of a Transcriptional Silencing PreQ1 Riboswitch

Re-engineered riboswitches that no longer respond to cellular metabolites, but that instead can be controlled by synthetic molecules, are potentially useful gene regulatory tools for use in synthetic biology and biotechnology fields. Previously, extensive genetic selection and screening approaches were employed to re-engineer a natural adenine riboswitch to create orthogonal ON-switches, enabling translational control of target gene expression in response to synthetic ligands. Here, we describe how a rational targeted approach was used to re-engineer the PreQ1 riboswitch from Bacillus subtilis into an orthogonal OFF-switch. In this case, the evaluation of just six synthetic compounds with seven riboswitch mutants led to the identification of an orthogonal riboswitch-ligand pairing that effectively repressed the transcription of selected genes in B. subtilis. The streamlining of the re-engineering approach, and its extension to a second class of riboswitches, provides a methodological platform for the creation of new orthogonal regulatory components for biotechnological applications including gene functional analysis and antimicrobial target validation and screening.

RiboTALE: A modular, inducible system for accurate gene expression control

A limiting factor in synthetic gene circuit design is the number of independent control elements that can be combined together in a single system. Here, we present RiboTALEs, a new class of inducible repressors that combine the specificity of TALEs with the ability of riboswitches to recognize exogenous signals and differentially control protein abundance. We demonstrate the capacity of RiboTALEs, constructed through different combinations of TALE proteins and riboswitches, to rapidly and reproducibly control the expression of downstream targets with a dynamic range of 243.7 ± 17.6-fold, which is adequate for many biotechnological applications.

Ribozyme-based aminoglycoside switches of gene expression engineered by genetic selection in S. cerevisiae

Systems for conditional gene expression are powerful tools in basic research as well as in biotechnology. For future applications, it is of great importance to engineer orthogonal genetic switches that function reliably in diverse contexts. RNA-based switches have the advantage that effector molecules interact immediately with regulatory modules inserted into the target RNAs, getting rid of the need of transcription factors usually mediating genetic control. Artificial riboswitches are characterized by their simplicity and small size accompanied by a high degree of modularity. We have recently reported a series of hammerhead ribozyme-based artificial riboswitches that allow for post-transcriptional regulation of gene expression via switching mRNA, tRNA, or rRNA functions. A more widespread application was so far hampered by moderate switching performances and a limited set of effector molecules available. Here, we report the re-engineering of hammerhead ribozymes in order to respond efficiently to aminoglycoside antibiotics. We first established an in vivo selection protocol in Saccharomyces cerevisiae that enabled us to search large sequence spaces for optimized switches. We then envisioned and characterized a novel strategy of attaching the aptamer to the ribozyme catalytic core, increasing the design options for rendering the ribozyme ligand-dependent. These innovations enabled the development of neomycin-dependent RNA modules that switch gene expression up to 25-fold. The presented aminoglycoside-responsive riboswitches belong to the best-performing RNA-based genetic regulators reported so far. The developed in vivo selection protocol should allow for sampling of large sequence spaces for engineering of further optimized riboswitches.

Imaging metabolite dynamics in living cells using a Spinach-based riboswitch

Riboswitches are natural ligand-sensing RNAs typically that are found in the 5' UTRs of mRNA. Numerous classes of riboswitches have been discovered, enabling mRNA to be regulated by diverse and physiologically important cellular metabolites and small molecules. Here we describe Spinach riboswitches, a new class of genetically encoded metabolite sensor derived from naturally occurring riboswitches. Drawing upon the structural switching mechanism of natural riboswitches, we show that Spinach can be swapped for the expression platform of various riboswitches, allowing metabolite binding to induce Spinach fluorescence directly. In the case of the thiamine 5'-pyrophosphate (TPP) riboswitch from the Escherichia coli thiM gene encoding hydroxyethylthiazole kinase, we show that insertion of Spinach results in an RNA sensor that exhibits fluorescence upon binding TPP. This TPP Spinach riboswitch binds TPP with affinity and selectivity similar to that of the endogenous riboswitch and enables the discovery of agonists and antagonists of the TPP riboswitch using simple fluorescence readouts. Furthermore, expression of the TPP Spinach riboswitch in Escherichia coli enables live imaging of dynamic changes in intracellular TPP concentrations in individual cells. Additionally, we show that other riboswitches that use a structural mechanism similar to that of the TPP riboswitch, including the guanine and adenine riboswitches from the Bacillus subtilis xpt gene encoding xanthine phosphoribosyltransferase, and the S-adenosyl-methionine-I riboswitch from the B. subtilis yitJ gene encoding methionine synthase, can be converted into Spinach riboswitches. Thus, Spinach riboswitches constitute a novel class of RNA-based fluorescent metabolite sensors that exploit the diversity of naturally occurring ligand-binding riboswitches.

Programmed evolution for optimization of orthogonal metabolic output in bacteria

Current use of microbes for metabolic engineering suffers from loss of metabolic output due to natural selection. Rather than combat the evolution of bacterial populations, we chose to embrace what makes biological engineering unique among engineering fields - evolving materials. We harnessed bacteria to compute solutions to the biological problem of metabolic pathway optimization. Our approach is called Programmed Evolution to capture two concepts. First, a population of cells is programmed with DNA code to enable it to compute solutions to a chosen optimization problem. As analog computers, bacteria process known and unknown inputs and direct the output of their biochemical hardware. Second, the system employs the evolution of bacteria toward an optimal metabolic solution by imposing fitness defined by metabolic output. The current study is a proof-of-concept for Programmed Evolution applied to the optimization of a metabolic pathway for the conversion of caffeine to theophylline in E. coli. Introduced genotype variations included strength of the promoter and ribosome binding site, plasmid copy number, and chaperone proteins. We constructed 24 strains using all combinations of the genetic variables. We used a theophylline riboswitch and a tetracycline resistance gene to link theophylline production to fitness. After subjecting the mixed population to selection, we measured a change in the distribution of genotypes in the population and an increased conversion of caffeine to theophylline among the most fit strains, demonstrating Programmed Evolution. Programmed Evolution inverts the standard paradigm in metabolic engineering by harnessing evolution instead of fighting it. Our modular system enables researchers to program bacteria and use evolution to determine the combination of genetic control elements that optimizes catabolic or anabolic output and to maintain it in a population of cells. Programmed Evolution could be used for applications in energy, pharmaceuticals, chemical commodities, biomining, and bioremediation.

Unexpected versatility in bacterial riboswitches

Bacterial riboswitches are elements present in the 5'-untranslated regions (UTRs) of mRNA molecules that bind to ligands and regulate the expression of downstream genes. Riboswitches typically regulate the expression of protein-coding genes. However, mechanisms of riboswitch-mediated regulation have recently been shown to be more diverse than originally thought, with reports showing that riboswitches can regulate the expression of noncoding RNAs and control the access of proteins, such as transcription termination factor Rho and RNase E, to a nascent RNA. Riboswitches are also increasingly used in biotechnology, with advances in the engineering of synthetic riboswitches and the development of riboswitch-based sensors. In this review we address the emerging roles and mechanisms of riboswitch-mediated regulation in natura and recent progress in the development of riboswitch-based technology.

Creating small transcription activating RNAs

We expanded the mechanistic capability of small RNAs by creating an entirely synthetic mode of regulation: small transcription activating RNAs (STARs). Using two strategies, we engineered synthetic STAR regulators to disrupt the formation of an intrinsic transcription terminator placed upstream of a gene in Escherichia coli. This resulted in a group of four highly orthogonal STARs that had up to 94-fold activation. By systematically modifying sequence features of this group, we derived design principles for STAR function, which we then used to forward engineer a STAR that targets a terminator found in the Escherichia coli genome. Finally, we showed that STARs could be combined in tandem to create previously unattainable RNA-only transcriptional logic gates. STARs provide a new mechanism of regulation that will expand our ability to use small RNAs to construct synthetic gene networks that precisely control gene expression.

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

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

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

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

Riboswitch engineering - making the all-important second and third steps

Synthetic biology uses our understanding of biological systems to develop innovative solutions for challenges in fields as diverse as genetic control and logic devices, bioremediation, materials production or diagnostics and therapy in medicine by designing new biological components. RNA-based elements are key components of these engineered systems. Their structural and functional diversity is ideal for generating regulatory riboswitches that react with many different types of output to molecular and environmental signals. Recent advances have added new sensor and output domains to the existing toolbox, and demonstrated the portability of riboswitches to many different organisms. Improvements in riboswitch design and screens for selecting in vivo active switches provide the means to isolate riboswitches with regulatory properties more like their natural counterparts.

Modular riboswitch toolsets for synthetic genetic control in diverse bacterial species

Ligand-dependent control of gene expression is essential for gene functional analysis, target validation, protein production, and metabolic engineering. However, the expression tools currently available are difficult to transfer between species and exhibit limited mechanistic diversity. Here we demonstrate how the modular architecture of purine riboswitches can be exploited to develop orthogonal and chimeric switches that are transferable across diverse bacterial species, modulating either transcription or translation, to provide tunable activation or repression of target gene expression, in response to synthetic non-natural effector molecules. Our novel riboswitch-ligand pairings are shown to regulate physiologically important genes required for bacterial motility in Escherichia coli and cell morphology in Bacillus subtilis. These findings are relevant for future gene function studies and antimicrobial target validation, while providing new modular and orthogonal regulatory components for deployment in synthetic biology regimes.

RNA self-assembly and RNA nanotechnology

CONSPECTUS: Nanotechnology's central goal involves the direct control of matter at the molecular nanometer scale to build nanofactories, nanomachines, and other devices for potential applications including electronics, alternative fuels, and medicine. In this regard, the nascent use of nucleic acids as a material to coordinate the precise arrangements of specific molecules marked an important milestone in the relatively recent history of nanotechnology. While DNA served as the pioneer building material in nucleic acid nanotechnology, RNA continues to emerge as viable alternative material with its own distinct advantages for nanoconstruction. Several complementary assembly strategies have been used to build a diverse set of RNA nanostructures having unique structural attributes and the ability to self-assemble in a highly programmable and controlled manner. Of the different strategies, the architectonics approach uniquely endeavors to understand integrated structural RNA architectures through the arrangement of their characteristic structural building blocks. Viewed through this lens, it becomes apparent that nature routinely uses thermodynamically stable, recurrent modular motifs from natural RNA molecules to generate unique and more complex programmable structures. With the design principles found in natural structures, a number of synthetic RNAs have been constructed. The synthetic nanostructures constructed to date have provided, in addition to affording essential insights into RNA design, important platforms to characterize and validate the structural self-folding and assembly properties of RNA modules or building blocks. Furthermore, RNA nanoparticles have shown great promise for applications in nanomedicine and RNA-based therapeutics. Nevertheless, the synthetic RNA architectures achieved thus far consist largely of static, rigid particles that are still far from matching the structural and functional complexity of natural responsive structural elements such as the ribosome, large ribozymes, and riboswitches. Thus, the next step in synthetic RNA design will involve new ways to implement these same types of dynamic and responsive architectures into nanostructures functioning as real nanomachines in and outside the cell. RNA nanotechnology will likely garner broader utility and influence with a greater focus on the interplay between thermodynamic and kinetic influences on RNA self-assembly and using natural RNAs as guiding principles.

Intracellular light-activation of riboswitch activity

By combining a riboswitch with a cell-permeable photocaged small-molecule ligand, an optochemical gene control element was constructed that enabled spatial and temporal control of gene expression in bacterial cells. The simplicity of this strategy, coupled with the ability to create synthetic riboswitches with tailored ligand specificities and output in a variety of microorganisms, plants, and fungi might afford a general strategy to photocontrol gene expression in vivo. The ability to activate riboswitches by using light enables the interrogation and manipulation of a wide range of biological processes with high precision, and will have broad utility in the regulation of artificial genetic circuits.

Synthetic riboswitches - A tool comes of age

Within the last decade, it has become obvious that RNA plays an important role in regulating gene expression. This has led to a plethora of approaches aiming at exploiting the outstanding chemical properties of RNA to develop synthetic RNA regulators for conditional gene expression systems. Consequently, many different regulators have been developed to act on various stages of gene expression. They can be engineered to respond to almost any ligand of choice and are, therefore, of great interest for applications in synthetic biology. This review presents an overview of such engineered riboswitches, discusses their applicability and points out recent trends in their development. This article is part of a Special Issue entitled: Riboswitches.

Structure-guided design of fluorescent S-adenosylmethionine analogs for a high-throughput screen to target SAM-I riboswitch RNAs

Many classes of S-adenosylmethionine (SAM)-binding RNAs and proteins are of interest as potential drug targets in diverse therapeutic areas, from infectious diseases to cancer. In the former case, the SAM-I riboswitch is an attractive target because this structured RNA element is found only in bacterial mRNAs and regulates multiple genes in several human pathogens. Here, we describe the synthesis of stable and fluorescent analogs of SAM in which the fluorophore is introduced through a functionalizable linker to the ribose. A Cy5-labeled SAM analog was shown to bind several SAM-I riboswitches via in-line probing and fluorescence polarization assays, including one from Staphylococcus aureus that controls the expression of SAM synthetase in this organism. A fluorescent ligand displacement assay was developed and validated for high-throughput screening of compounds to target the SAM-I riboswitch class.

Themes and variations in riboswitch structure and function

The complexity of gene expression control by non-coding RNA has been highlighted by the recent progress in the field of riboswitches. Discovered a decade ago, riboswitches represent a diverse group of non-coding mRNA regions that possess a unique ability to directly sense cellular metabolites and modulate gene expression through formation of alternative metabolite-free and metabolite-bound conformations. Such protein-free metabolite sensing domains utilize sophisticated three-dimensional folding of RNA molecules to discriminate between a cognate ligand from related compounds so that only the right ligand would trigger a genetic response. Given the variety of riboswitch ligands ranging from small cations to large coenzymes, riboswitches adopt a great diversity of structures. Although many riboswitches share structural principles to build metabolite-competent folds, form precise ligand-binding pockets, and communicate a ligand-binding event to downstream regulatory regions, virtually all riboswitch classes possess unique features for ligand recognition, even those tuned to recognize the same metabolites. Here we present an overview of the biochemical and structural research on riboswitches with a major focus on common principles and individual characteristics adopted by these regulatory RNA elements during evolution to specifically target small molecules and exert genetic responses. This article is part of a Special Issue entitled: Riboswitches.

A disconnect between high-affinity binding and efficient regulation by antifolates and purines in the tetrahydrofolate riboswitch

The tetrahydrofolate (THF) riboswitch regulates folate transport and metabolism in a number of Firmicutes by cooperatively binding two molecules of THF. To further understand this riboswitch's specificity for THF, binding and regulatory activity of a series of THF analogs and antifolates were examined. Our data reveal that although binding is dominated by the RNA's interactions with the pterin moiety, the para-aminobenzoic acid (pABA) moiety plays a significant role in transcriptional regulation. Further, we find that adenine and several other analogs bind with high affinity by an alternative binding mechanism. Despite a similar affinity to THF, adenine is a poor regulator of transcriptional attenuation. These results demonstrate that binding alone does not determine a compound's effectiveness in regulating the activity of the riboswitch-a complication in current efforts to develop antimicrobials that target these RNAs.

RNA modularity for synthetic biology

RNA molecules are highly modular components that can be used in a variety of contexts for building new metabolic, regulatory and genetic circuits in cells. The majority of synthetic RNA systems to date predominately rely on two-dimensional modularity. However, a better understanding and integration of three-dimensional RNA modularity at structural and functional levels is critical to the development of more complex, functional bio-systems and molecular machines for synthetic biology applications.

The centrality of RNA for engineering gene expression

Synthetic biology holds promise as both a framework for rationally engineering biological systems and a way to revolutionize how we fundamentally understand them. Essential to realizing this promise is the development of strategies and tools to reliably and predictably control and characterize sophisticated patterns of gene expression. Here we review the role that RNA can play towards this goal and make a case for why this versatile, designable, and increasingly characterizable molecule is one of the most powerful substrates for engineering gene expression at our disposal. We discuss current natural and synthetic RNA regulators of gene expression acting at key points of control – transcription, mRNA degradation, and translation. We also consider RNA structural probing and computational RNA structure predication tools as a way to study RNA structure and ultimately function. Finally, we discuss how next-generation sequencing methods are being applied to the study of RNA and to the characterization of RNA's many properties throughout the cell.

Engineering modular 'ON' RNA switches using biological components

Riboswitches are cis-acting regulatory elements broadly distributed in bacterial mRNAs that control a wide range of critical metabolic activities. Expression is governed by two distinct domains within the mRNA leader: a sensory 'aptamer domain' and a regulatory 'expression platform'. Riboswitches have also received considerable attention as important tools in synthetic biology because of their conceptually simple structure and the ability to obtain aptamers that bind almost any conceivable small molecule using in vitro selection (referred to as SELEX). In the design of artificial riboswitches, a significant hurdle has been to couple the two domains enabling their efficient communication. We previously demonstrated that biological transcriptional 'OFF' expression platforms are easily coupled to diverse aptamers, both biological and SELEX-derived, using simple design rules. Here, we present two modular transcriptional 'ON' riboswitch expression platforms that are also capable of hosting foreign aptamers. We demonstrate that these biological parts can be used to facilely generate artificial chimeric riboswitches capable of robustly regulating transcription both in vitro and in vivo. We expect that these modular expression platforms will be of great utility for various synthetic biological applications that use RNA-based biosensors.

A modular strategy for engineering orthogonal chimeric RNA transcription regulators

Antisense RNA transcription attenuators are a key component of the synthetic biology toolbox, with their ability to serve as building blocks for both signal integration logic circuits and transcriptional cascades. However, a central challenge to building more sophisticated RNA genetic circuitry is creating larger families of orthogonal attenuators that function independently of each other. Here, we overcome this challenge by developing a modular strategy to create chimeric fusions between the engineered transcriptional attenuator from plasmid pT181 and natural antisense RNA translational regulators. Using in vivo gene expression assays in Escherichia coli, we demonstrate our ability to create chimeric attenuators by fusing sequences from five different translational regulators. Mutagenesis of these functional attenuators allowed us to create a total of 11 new chimeric attenutaors. A comprehensive orthogonality test of these culminated in a 7 × 7 matrix of mutually orthogonal regulators. A comparison between all chimeras tested led to design principles that will facilitate further engineering of orthogonal RNA transcription regulators, and may help elucidate general principles of non-coding RNA regulation. We anticipate that our strategy will accelerate the development of even larger families of orthogonal RNA transcription regulators, and thus create breakthroughs in our ability to construct increasingly sophisticated RNA genetic circuitry.