Synthetic riboswitches - A tool comes of age

Switchable and orthogonal gene expression control inside artificial cells by synthetic riboswitches

Here we report two novel synthetic riboswitches that respond to ASP2905 and theophylline and function in reconstituted cell-free protein synthesis (CFPS) system. We encapsulated the CFPS system as well as DNA-templated encoding reporter genes regulated by these orthogonal riboswitches inside liposomes, and achieved switchable and orthogonal control over gene expression by external stimulation with the cognate ligands.

Förster resonance energy transfer within the neomycin aptamer

Förster resonance energy transfer (FRET) measurements between two dyes is a powerful method to interrogate both structure and dynamics of biopolymers. The intensity of a fluorescence signal in a FRET measurement is dependent on both the distance and the relative orientation of the dyes. The latter can at the same time both complicate the analysis and give more detailed information. Here we present a detailed spectroscopic study of the energy transfer between the rigid FRET labels Çmf (donor) and tCnitro (quencher/acceptor) within the neomycin aptamer N1. The energy transfer originates from multiple emitting states of the donor and occurs on a low picosecond to nanosecond time-scale. To fully characterize the energy transfer, ultrafast transient absorption measurements were performed in conjunction with static fluorescence and time-correlated single photon counting (TCSPC) measurements, showing a clear distance dependence of both signal intensity and lifetime. Using a known NMR structure of the ligand-bound neomycin aptamer, the distance between the two labels was used to estimate κ2 and, therefore, make qualitative statements about the change in orientation after ligand binding with unprecedented temporal and spatial resolution. The advantages and potential applications of absorption-based methods using rigid labels for the characterization of FRET processes are discussed.

T7 phage-assisted evolution of riboswitches using error-prone replication and dual selection

Leveraging riboswitches, non-coding mRNA fragments pivotal to gene regulation, poses a challenge in effectively selecting and enriching these functional genetic sensors, which can toggle between ON and OFF states in response to their cognate inducers. Here, we show our engineered phage T7, enabling the evolution of a theophylline riboswitch. We have replaced T7's DNA polymerase with a transcription factor controlled by a theophylline riboswitch and have created two types of host environments to propagate the engineered phage. Both types host an error-prone T7 DNA polymerase regulated by a T7 promoter along with another critical gene-either cmk or pifA, depending on the host type. The cmk gene is necessary for T7 replication and is used in the first host type for selection in the riboswitch's ON state. Conversely, the second host type incorporates the pifA gene, leading to abortive T7 infections and used for selection in the riboswitch's OFF state. This dual-selection system, termed T7AE, was then applied to a library of 65,536 engineered T7 phages, each carrying randomized riboswitch variants. Through successive passage in both host types with and without theophylline, we observed an enrichment of phages encoding functional riboswitches that conferred a fitness advantage to the phage in both hosts. The T7AE technique thereby opens new pathways for the evolution and advancement of gene switches, including non-coding RNA-based switches, setting the stage for significant strides in synthetic biology.

Elimination of editing plasmid mediated by theophylline riboswitch in Zymomonas mobilis

Zymomonas mobilis is regarded as a potential chassis for the production of platform chemicals. Genome editing using the CRISPR-Cas system could meet the need for gene modification in metabolic engineering. However, the low curing efficiency of CRISPR editing plasmid is a common bottleneck in Z. mobilis. In this study, we utilized a theophylline-dependent riboswitch to regulate the expression of the replicase gene of the editing plasmid, thereby promoting the elimination of exogeneous plasmid. The riboswitch D (RSD) with rigorous regulatory ability was identified as the optimal candidate by comparing the transformation efficiency of four theophylline riboswitch-based backbone editing plasmids, and the optimal theophylline concentration for inducing RSD was determined to be 2 mM. A highly effective method for eliminating the editing plasmid, cells with RSD-based editing plasmid which were cultured in liquid and solid RM media in alternating passages at 37 °C without shaking, was established by testing the curing efficiency of backbone editing plasmids pMini and pMini-RSD in RM medium with or without theophylline at 30 °C or 37 °C. Finally, the RSD-based editing plasmid was applied to genome editing, resulting in an increase of more than 10% in plasmid elimination efficiency compared to that of pMini-based editing plasmid. KEY POINTS: • An effective strategy for curing CRISPR editing plasmid has been established in Z. mobilis. • Elimination efficiency of the CRISPR editing plasmid was enhanced by 10% to 20% under the regulation of theophylline-dependent riboswitch RSD.

Facile Expansion of the Variety of Orthogonal Ligand/Aptamer Pairs for Artificial Riboswitches

An RNA aptamer that induces suitable conformational changes upon binding to a user-defined ligand allows us to artificially construct a riboswitch, a ligand-dependent and cis-acting gene regulatory RNA. Although such an aptamer can be obtained through in vitro selection, it is still challenging to rationally expand the variety of orthogonal ligand/aptamer (ligand/riboswitch) pairs. To achieve this in a facile, selection-free way, we herein focused on a specific type of ligand, 6-nt nanosized DNA (nDNA) and its aptamer that was previously selected to construct a eukaryotic artificial riboswitch. Specifically, we merely mutated one or more possible Watson-Crick base pairs in the nDNA/aptamer (nDNA/riboswitch) interactions into another base pair or pairs. Using two sets that each had 16 comprehensive mutations, we obtained three groups of several orthogonal nDNA/riboswitch pairs. These pairs could be used to create complex gene circuits, including multiple simultaneous and/or multistep cascading regulations in synthetic biology.

Precise Genomic Riboregulator Control of Metabolic Flux in Microbial Systems

Engineered microbes can be used for producing value-added chemicals from renewable feedstocks, relieving the dependency on nonrenewable resources such as petroleum. These microbes often are composed of synthetic metabolic pathways; however, one major problem in establishing a synthetic pathway is the challenge of precisely controlling competing metabolic routes, some of which could be crucial for fitness and survival. While traditional gene deletion and/or coarse overexpression approaches do not provide precise regulation, cis-repressors (CRs) are RNA-based regulatory elements that can control the production levels of a particular protein in a tunable manner. Here, we describe a protocol for a generally applicable fluorescence-activated cell sorting technique used to isolate eight subpopulations of CRs from a semidegenerate library in Escherichia coli, followed by deep sequencing that permitted the identification of 15 individual CRs with a broad range of protein production profiles. Using these new CRs, we demonstrated a change in production levels of a fluorescent reporter by over two orders of magnitude and further showed that these CRs are easily ported from E. coli to Pseudomonas putida. We next used four CRs to tune the production of the enzyme PpsA, involved in pyruvate to phosphoenolpyruvate (PEP) conversion, to alter the pool of PEP that feeds into the shikimate pathway. In an engineered P. putida strain, where carbon flux in the shikimate pathway is diverted to the synthesis of the commodity chemical cis,cis-muconate, we found that tuning PpsA translation levels increased the overall titer of muconate. Therefore, CRs provide an approach to precisely tune protein levels in metabolic pathways and will be an important tool for other metabolic engineering efforts.

A Riboswitch-Driven Era of New Antibacterials

Riboswitches are structured non-coding RNAs found in the 5′ UTR of important genes for bacterial metabolism, virulence and survival. Upon the binding of specific ligands that can vary from simple ions to complex molecules such as nucleotides and tRNAs, riboswitches change their local and global mRNA conformations to affect downstream transcription or translation. Due to their dynamic nature and central regulatory role in bacterial metabolism, riboswitches have been exploited as novel RNA-based targets for the development of new generation antibacterials that can overcome drug-resistance problems. During recent years, several important riboswitch structures from many bacterial representatives, including several prominent human pathogens, have shown that riboswitches are ideal RNA targets for new compounds that can interfere with their structure and function, exhibiting much reduced resistance over time. Most interestingly, mainstream antibiotics that target the ribosome have been shown to effectively modulate the regulatory behavior and capacity of several riboswitches, both in vivo and in vitro, emphasizing the need for more in-depth studies and biological evaluation of new antibiotics. Herein, we summarize the currently known compounds that target several main riboswitches and discuss the role of mainstream antibiotics as modulators of T-box riboswitches, in the dawn of an era of novel inhibitors that target important bacterial regulatory RNAs.

Eukaryotic artificial ON-riboswitches that respond efficiently to mid-sized short peptides

We chose two types of mid-sized Arg-rich peptides (Rev-pep and Tat-pep) as ligands and used their aptamers to construct efficient eukaryotic ON-riboswitches (ligand-dependently upregulating riboswitches). Due to the aptamers' high affinities, the best Rev-pep-responsive and Tat-pep-responsive riboswitches obtained showed much higher switching efficiencies at low ligand concentrations than small ligand-responsive ON-riboswitches in the same mechanism. In addition, despite the high sequence similarity of Rev-pep and Tat-pep, the two best riboswitches were almost insensitive to each other's peptide ligand. Considering the high responsiveness and specificity along with the versatility of the expression platform used and the applicability of Arg-rich peptides, this orthogonal pair of riboswitches would be widely useful eukaryotic gene regulators or biosensors.

Predicting higher-order mutational effects in an RNA enzyme by machine learning of high-throughput experimental data

Ribozymes are RNA molecules that catalyze biochemical reactions. Self-cleaving ribozymes are a common naturally occurring class of ribozymes that catalyze site-specific cleavage of their own phosphodiester backbone. In addition to their natural functions, self-cleaving ribozymes have been used to engineer control of gene expression because they can be designed to alter RNA processing and stability. However, the rational design of ribozyme activity remains challenging, and many ribozyme-based systems are engineered or improved by random mutagenesis and selection (in vitro evolution). Improving a ribozyme-based system often requires several mutations to achieve the desired function, but extensive pairwise and higher-order epistasis prevent a simple prediction of the effect of multiple mutations that is needed for rational design. Recently, high-throughput sequencing-based approaches have produced data sets on the effects of numerous mutations in different ribozymes (RNA fitness landscapes). Here we used such high-throughput experimental data from variants of the CPEB3 self-cleaving ribozyme to train a predictive model through machine learning approaches. We trained models using either a random forest or long short-term memory (LSTM) recurrent neural network approach. We found that models trained on a comprehensive set of pairwise mutant data could predict active sequences at higher mutational distances, but the correlation between predicted and experimentally observed self-cleavage activity decreased with increasing mutational distance. Adding sequences with increasingly higher numbers of mutations to the training data improved the correlation at increasing mutational distances. Systematically reducing the size of the training data set suggests that a wide distribution of ribozyme activity may be the key to accurate predictions. Because the model predictions are based only on sequence and activity data, the results demonstrate that this machine learning approach allows readily obtainable experimental data to be used for RNA design efforts even for RNA molecules with unknown structures. The accurate prediction of RNA functions will enable a more comprehensive understanding of RNA fitness landscapes for studying evolution and for guiding RNA-based engineering efforts.

Structural changes in aptamers are essential for synthetic riboswitch engineering

Synthetic riboswitches are powerful tools in synthetic biology in which sensing and execution are consolidated in a single RNA molecule. By using SELEX to select aptamers in vitro, synthetic riboswitches can in theory be engineered against any ligand of choice. Surprisingly, very few in vitro selected aptamers have been used for the engineering of synthetic riboswitches. In-depth studies of these aptamers suggest that the key characteristics of such regulatory active RNAs are their structural switching abilities and their binding dynamics. Conventional SELEX approaches seem to be inadequate to select for these characteristics, which may explain the lack of in vitro selected aptamers suited for engineering of synthetic riboswitches. In this review, we explore the functional principles of synthetic riboswitches, identify key characteristics of regulatory active in vitro selected aptamers and integrate these findings in context with available in vitro selection methods. Based on these insights, we propose to use a combination of capture-SELEX and subsequent functional screening for a more successful in vitro selection of aptamers that can be applied for the engineering of synthetic riboswitches.

High-throughput screening of cell-free riboswitches by fluorescence-activated droplet sorting

Cell-free systems that display complex functions without using living cells are emerging as new platforms to test our understanding of biological systems as well as for practical applications such as biosensors and biomanufacturing. Those that use cell-free protein synthesis (CFPS) systems to enable genetically programmed protein synthesis have relied on genetic regulatory components found or engineered in living cells. However, biological constraints such as cell permeability, metabolic stability, and toxicity of signaling molecules prevent development of cell-free devices using living cells even if cell-free systems are not subject to such constraints. Efforts to engineer regulatory components directly in CFPS systems thus far have been based on low-throughput experimental approaches, limiting the availability of basic components to build cell-free systems with diverse functions. Here, we report a high-throughput screening method to engineer cell-free riboswitches that respond to small molecules. Droplet-sorting of riboswitch variants in a CFPS system rapidly identified cell-free riboswitches that respond to compounds that are not amenable to bacterial screening methods. Finally, we used a histamine riboswitch to demonstrate chemical communication between cell-sized droplets.

RNA Design Principles for Riboswitches that Regulate RNase P-Mediated tRNA Processing

Riboswitches are an attractive target for the directed design of RNA-based regulators by in silico prediction. These noncoding RNA elements consist of an aptamer platform for the highly selective ligand recognition and an expression platform which controls gene activity typically at the level of transcription or translation. In previous work, we could successfully apply RNA folding prediction to implement a new riboswitch mechanism regulating processing of a tRNA by RNase P. In this contribution, we present detailed information about our pipeline consisting of in silico design combined with the biochemical analysis for the verification of the implemented mechanism. Furthermore, we discuss the applicability of the presented biochemical in vivo and in vitro methods for the characterization of other artificial riboswitches.

Streamlined CRISPR genome engineering in wild-type bacteria using SIBR-Cas

CRISPR-Cas is a powerful tool for genome editing in bacteria. However, its efficacy is dependent on host factors (such as DNA repair pathways) and/or exogenous expression of recombinases. In this study, we mitigated these constraints by developing a simple and widely applicable genome engineering tool for bacteria which we termed SIBR-Cas (Self-splicing Intron-Based Riboswitch-Cas). SIBR-Cas was generated from a mutant library of the theophylline-dependent self-splicing T4 td intron that allows for tight and inducible control over CRISPR-Cas counter-selection. This control delays CRISPR-Cas counter-selection, granting more time for the editing event (e.g. by homologous recombination) to occur. Without the use of exogenous recombinases, SIBR-Cas was successfully applied to knock-out several genes in three wild-type bacteria species (Escherichia coli MG1655, Pseudomonas putida KT2440 and Flavobacterium IR1) with poor homologous recombination systems. Compared to other genome engineering tools, SIBR-Cas is simple, tightly regulated and widely applicable for most (non-model) bacteria. Furthermore, we propose that SIBR can have a wider application as a simple gene expression and gene regulation control mechanism for any gene or RNA of interest in bacteria.

Riboswitches for Controlled Expression of Therapeutic Transgenes Delivered by Adeno-Associated Viral Vectors

Vectors developed from adeno-associated virus (AAV) are powerful tools for in vivo transgene delivery in both humans and animal models, and several AAV-delivered gene therapies are currently approved for clinical use. However, AAV-mediated gene therapy still faces several challenges, including limited vector packaging capacity and the need for a safe, effective method for controlling transgene expression during and after delivery. Riboswitches, RNA elements which control gene expression in response to ligand binding, are attractive candidates for regulating expression of AAV-delivered transgene therapeutics because of their small genomic footprints and non-immunogenicity compared to protein-based expression control systems. In addition, the ligand-sensing aptamer domains of many riboswitches can be exchanged in a modular fashion to allow regulation by a variety of small molecules, proteins, and oligonucleotides. Riboswitches have been used to regulate AAV-delivered transgene therapeutics in animal models, and recently developed screening and selection methods allow rapid isolation of riboswitches with novel ligands and improved performance in mammalian cells. This review discusses the advantages of riboswitches in the context of AAV-delivered gene therapy, the subsets of riboswitch mechanisms which have been shown to function in human cells and animal models, recent progress in riboswitch isolation and optimization, and several examples of AAV-delivered therapeutic systems which might be improved by riboswitch regulation.

In Vitro Selection of RNA Aptamers Binding to Nanosized DNA for Constructing Artificial Riboswitches

We here designed an in vitro selection scheme for obtaining an aptamer with which to rationally construct an artificial riboswitch as its component part. In fact, a nanosized DNA-binding aptamer obtained through this scheme allowed us to easily and successfully create eukaryotic riboswitches that upregulate internal ribosome entry site-mediated translation in response to the ligand (nanosized DNA) in wheat germ extract, a eukaryotic cell-free expression system. The induction ratio of the best riboswitch ligand-dose-dependently increased to 21 at 300 μM ligand. This switching efficiency is much higher than that of the same type of riboswitch with a widely used theophylline-binding aptamer, which was in vitro selected without considering its utility for constructing riboswitches. The selection scheme described here would facilitate obtaining various ligand/aptamer pairs suitable for constructing artificial riboswitches, which could serve as elements of synthetic gene circuits in synthetic biology.

Selection of High-Affinity RNA Aptamers That Distinguish between Doxycycline and Tetracycline

Oligonucleotide aptamers are found in prokaryotes and eukaryotes, and they can be selected from large synthetic libraries to bind protein or small-molecule ligands with high affinities and specificities. Aptamers can function as biosensors, as protein recognition elements, and as components of riboswitches allowing ligand-dependent control of gene expression. One of the best studied laboratory-selected aptamers binds the antibiotic tetracycline, but it binds with a much lower affinity to the closely related but more bioavailable antibiotic doxycycline. Here we report enrichment of doxycycline binding aptamers from a selectively randomized library of tetracycline aptamer variants over four selection rounds. Selected aptamers distinguish between doxycycline, which they bind with dissociation constants of approximately 7 nM, and tetracycline, which they bind undetectably. They thus function as orthogonal complements to the original tetracycline aptamer. Unexpectedly, doxycycline aptamers adopt a conformation distinct from that of the tetracycline aptamer and depend on constant regions originally installed as primer binding sites. We show that the fluorescence emission intensity of doxycycline increases upon aptamer binding, permitting their use as biosensors. This new class of aptamers can be used in multiple contexts where doxycycline detection, or doxycycline-mediated regulation, is necessary.

The complex formed between a synthetic RNA aptamer and the transcription repressor TetR is a structural and functional twin of the operator DNA-TetR regulator complex

RNAs play major roles in the regulation of gene expression. Hence, designer RNA molecules are increasingly explored as regulatory switches in synthetic biology. Among these, the TetR-binding RNA aptamer was selected by its ability to compete with operator DNA for binding to the bacterial repressor TetR. A fortuitous finding was that induction of TetR by tetracycline abolishes both RNA aptamer and operator DNA binding in TetR. This enabled numerous applications exploiting both the specificity of the RNA aptamer and the efficient gene repressor properties of TetR. Here, we present the crystal structure of the TetR-RNA aptamer complex at 2.7 Å resolution together with a comprehensive characterization of the TetR–RNA aptamer versus TetR–operator DNA interaction using site-directed mutagenesis, size exclusion chromatography, electrophoretic mobility shift assays and isothermal titration calorimetry. The fold of the RNA aptamer bears no resemblance to regular B-DNA, and neither does the thermodynamic characterization of the complex formation reaction. Nevertheless, the functional aptamer-binding epitope of TetR is fully contained within its DNA-binding epitope. In the RNA aptamer complex, TetR adopts the well-characterized DNA-binding-competent conformation of TetR, thus revealing how the synthetic TetR-binding aptamer strikes the chords of the bimodal allosteric behaviour of TetR to function as a synthetic regulator.

Robust gene expression control in human cells with a novel universal TetR aptamer splicing module

Fine-tuning of gene expression is desirable for a wide range of applications in synthetic biology. In this context, RNA regulatory devices provide a powerful and highly functional tool. We developed a versatile, robust and reversible device to control gene expression by splicing regulation in human cells using an aptamer that is recognized by the Tet repressor TetR. Upon insertion in proximity to the 5′ splice site, intron retention can be controlled via the binding of TetR to the aptamer. Although we were able to demonstrate regulation for different introns, the genomic context had a major impact on regulation. In consequence, we advanced the aptamer to develop a splice device. Our novel device contains the aptamer integrated into a context of exonic and intronic sequences that create and maintain an environment allowing a reliable and robust splicing event. The exon-born, additional amino acids will then be cleaved off by a self-cleaving peptide. This design allows portability of the splicing device, which we confirmed by demonstrating its functionality in different gene contexts. Intriguingly, our splicing device shows a high dynamic range and low basal activity, i.e. desirable features that often prove a major challenge when implementing synthetic biology in mammalian cell lines.

High-throughput identification of synthetic riboswitches by barcode-free amplicon-sequencing in human cells

Synthetic riboswitches mediating ligand-dependent RNA cleavage or splicing-modulation represent elegant tools to control gene expression in various applications, including next-generation gene therapy. However, due to the limited understanding of context-dependent structure-function relationships, the identification of functional riboswitches requires large-scale-screening of aptamer-effector-domain designs, which is hampered by the lack of suitable cellular high-throughput methods. Here we describe a fast and broadly applicable method to functionally screen complex riboswitch libraries (~1.8 × 104 constructs) by cDNA-amplicon-sequencing in transiently transfected and stimulated human cells. The self-barcoding nature of each construct enables quantification of differential mRNA levels without additional pre-selection or cDNA-manipulation steps. We apply this method to engineer tetracycline- and guanine-responsive ON- and OFF-switches based on hammerhead, hepatitis-delta-virus and Twister ribozymes as well as U1-snRNP polyadenylation-dependent RNA devices. In summary, our method enables fast and efficient high-throughput riboswitch identification, thereby overcoming a major hurdle in the development cascade for therapeutically applicable gene switches.

Heterologous production of small molecules in the optimized Streptomyces hosts

Time span of literature covered: 2010-2018The genome mining of streptomycetes has revealed their great biosynthetic potential to produce novel natural products. One of the most promising exploitation routes of this biosynthetic potential is the refactoring and heterologous expression of corresponding biosynthetic gene clusters in a panel of specifically selected and optimized chassis strains. This article will review selected recent reports on heterologous production of natural products in streptomycetes. In the first part, the importance of heterologous production for drug discovery will be discussed. In the second part, the review will discuss recently developed genetic control elements (such as promoters, ribosome binding sites, terminators) and their application to achieve successful heterologous expression of biosynthetic gene clusters. Finally, the most widely used Streptomyces hosts for heterologous expression of biosynthetic gene clusters will be compared in detail. The article will be of interest to natural product chemists, molecular biologists, pharmacists and all individuals working in the natural products drug discovery field.

A small, portable RNA device for the control of exon skipping in mammalian cells

Splicing is an essential and highly regulated process in mammalian cells. We developed a synthetic riboswitch that efficiently controls alternative splicing of a cassette exon in response to the small molecule ligand tetracycline. The riboswitch was designed to control the accessibility of the 3' splice site by placing the latter inside the closing stem of a conformationally controlled tetracycline aptamer. In the presence of tetracycline, the cassette exon is skipped, whereas it is included in the ligand's absence. The design allows for an easy, context-independent integration of the regulatory device into any gene of interest. Portability of the device was shown through its functionality in four different systems: a synthetic minigene, a reporter gene and two endogenous genes. Furthermore, riboswitch functionality to control cellular signaling cascades was demonstrated by using it to specifically induce cell death through the conditionally controlled expression of CD20, which is a target in cancer therapy.

Small synthetic molecule-stabilized RNA pseudoknot as an activator for -1 ribosomal frameshifting

Programmed –1 ribosomal frameshifting (−1PRF) is a recoding mechanism to make alternative proteins from a single mRNA transcript. −1PRF is stimulated by cis-acting signals in mRNA, a seven-nucleotide slippery sequence and a downstream secondary structure element, which is often a pseudoknot. In this study we engineered the frameshifting pseudoknot from the mouse mammary tumor virus to respond to a rationally designed small molecule naphthyridine carbamate tetramer (NCTn). We demonstrate that NCTn can stabilize the pseudoknot structure in mRNA and activate –1PRF both in vitro and in human cells. The results illustrate how NCTn-inducible –1PRF may serve as an important component of the synthetic biology toolbox for the precise control of gene expression using small synthetic molecules.

RiboCas: A Universal CRISPR-Based Editing Tool for Clostridium

Members of the genus Clostridium represent a diverse assemblage of species exhibiting both medical and industrial importance. Deriving both a greater understanding of their biology, while at the same time enhancing their exploitable properties, requires effective genome editing tools. Here, we demonstrate the first implementation in the genus of theophylline-dependent, synthetic riboswitches exhibiting a full set of dynamic ranges, also suitable for applications where tight control of gene expression is required. Their utility was highlighted by generating a novel riboswitch-based editing tool-RiboCas-that overcomes the main obstacles associated with CRISPR/Cas9 systems, including low transformation efficiencies and excessive Cas9 toxicity. The universal nature of the tool was established by obtaining chromosomal modifications in C. pasteurianum, C. difficile, and C. sporogenes, as well as by carrying out the first reported example of CRISPR-targeted gene disruption in C. botulinum. The high efficiency (100% mutant generation) and ease of application of RiboCas make it suitable for use in a diverse range of microorganisms.

Next-level riboswitch development-implementation of Capture-SELEX facilitates identification of a new synthetic riboswitch

The development of synthetic riboswitches has always been a challenge. Although a number of interesting proof-of-concept studies have been published, almost all of these were performed with the theophylline aptamer. There is no shortage of small molecule-binding aptamers; however, only a small fraction of them are suitable for RNA engineering since a classical SELEX protocol selects only for high-affinity binding but not for conformational switching. We now implemented RNA Capture-SELEX in our riboswitch developmental pipeline to integrate the required selection for high-affinity binding with the equally necessary RNA conformational switching. Thus, we successfully developed a new paromomycin-binding synthetic riboswitch. It binds paromomycin with a KD of 20 nM and can discriminate between closely related molecules both in vitro and in vivo. A detailed structure-function analysis confirmed the predicted secondary structure and identified nucleotides involved in ligand binding. The riboswitch was further engineered in combination with the neomycin riboswitch for the assembly of an orthogonal Boolean NOR logic gate. In sum, our work not only broadens the spectrum of existing RNA regulators, but also signifies a breakthrough in riboswitch development, as the effort required for the design of sensor domains for RNA-based devices will in many cases be much reduced.

Synthetic antibody: Prospects in aquaculture biosecurity

The emerging technology of aptamers that is also known as synthetic antibodies is rivalling antibodies research in the recent years. The unique yet important features of aptamers are advancing antibodies in diverse applications, which include disease diagnosis, prophylactic and therapeutic. The versatility of aptamer has further extended its application to function as gene expression modulator, known as synthetic riboswitches. This report reviewed and discussed the applications of aptamers technology in the biosecurity of aquaculture, the promising developments in biosensor detection for disease diagnosis as well as prophylactic and therapeutic measurements. The application of aptamers technology in immunophenotyping study of aquatic animal is highlighted. Lastly, the future perspective of aptamers in the management of aquatic animal health is discussed, special emphasis on the potential application of aptamers as synthetic riboswitches to enhance host immunity, as well as the growth performance.

Riboswitch Signal Amplification by Controlling Plasmid Copy Number

Riboswitches are cis-acting RNA devices in mRNAs that control gene expression in response to chemical inputs. As RNA aptamers that recognize diverse classes of molecules can be isolated by in vitro selection, synthetic riboswitches hold promise for various applications in synthetic biology. One of the major drawbacks of riboswitches, however, is their limited dynamic range. A high level of gene expression in the OFF state (leakage) is also a common problem. To address these challenges, we designed and constructed a dual-riboswitch plasmid in which two genes are controlled by theophylline-activated riboswitches. One riboswitch controls the gene of interest, and another riboswitch controls RepL, a phage-derived replication protein that regulates the plasmid copy number. This single-plasmid system afforded an ON/OFF ratio as high as 3900. Furthermore, we used the system to control CRISPR interference (CRISPRi) targeting endogenous genes, and successfully observed expected phenotypic changes in Escherichia coli.

Tuning the Performance of Synthetic Riboswitches using Machine Learning

Riboswitch development for clinical, technological, and synthetic biology applications constantly seeks to optimize regulatory behavior. Here, we present a machine learning approach to improve the regulation of a tetracycline (tc)-dependent riboswitch device composed of two individual tc aptamers. We developed a bioinformatics model that combines random forest analysis with a convolutional neural network to predict the switching behavior of such tandem riboswitches. We found that both biophysical parameters and the hydrogen bond pattern influence regulation. Our new design pipeline led to significant improvement of the tc riboswitch device with a dynamic range extension from 8.5 to 40-fold. We are confident that our novel method not only results in an excellent tc-dependent riboswitch device but further holds great promise and potential for the optimization of other riboswitches.

Influence of Mg2+ on the conformational flexibility of a tetracycline aptamer

The tetracycline-binding RNA aptamer (TC-aptamer) is a synthetic riboswitch that binds the antibiotic tetracycline (TC) with exceptionally high affinity. Although a crystal structure exists of the TC-bound state, little is known about the conformational dynamics and changes upon ligand binding. In this study, pulsed electron paramagnetic resonance techniques for measuring distances (PELDOR) in combination with rigid nitroxide spin labels (Çm spin label) were used to investigate the conformational flexibility of the TC-aptamer in the presence and absence of TC at different Mg²⁺ concentrations. TC was found to be the essential factor for stabilizing the tertiary structure at intermediate Mg²⁺ concentrations. At higher Mg²⁺ concentrations, Mg²⁺ alone is sufficient to stabilize the tertiary structure. In addition, the orientation of the two spin-labeled RNA helices with respect to each other was analyzed with orientation-selective PELDOR and compared to the crystal structure. These results demonstrate for the first time the unique value of the Çm spin label in combination with PELDOR to provide information about conformational flexibilities and orientations of secondary structure elements of biologically relevant RNAs.

Development of an inducible anti-VEGF rAAV gene therapy strategy for the treatment of wet AMD

Vascular endothelial growth factor (VEGF) is a key mediator in the development and progression of choroidal neovascularization (CNV) in patients with wet age-related macular degeneration (AMD). As a consequence, current treatment strategies typically focus on the administration of anti-VEGF agents, such as Aflibercept (Eylea), that inhibit VEGF function. While this approach is largely successful at counteracting CNV progression, the treatment can require repetitive (i.e. monthly) intravitreal injections of the anti-VEGF agent throughout the patient’s lifetime, imposing a substantial financial and medical burden on the patient. Moreover, repetitive injection of anti-VEGF agents over a period of years may encourage progression of retinal and choroidal atrophy in patients with AMD, leading to a decrease in visual acuity. Herein, we have developed a single-injection recombinant adeno-associated virus (rAAV)-based gene therapy treatment for wet AMD that prevents CNV formation through inducible over-expression of Eylea. First, we demonstrate that by incorporating riboswitch elements into the rAAV expression cassette allows protein expression levels to be modulated in vivo through oral supplementation on an activating ligand (e.g. tetracycline). We subsequently utilized this technology to modulate the intraocular concentration of Eylea following rAAV delivery, leading to nearly complete (p = 0.0008) inhibition of clinically significant CNV lesions in an established mouse model of wet AMD. The results shown in this study pave the way for the development of a personalized gene therapy strategy for the treatment of wet AMD that is substantially less invasive and more clinically adaptable than the current treatment paradigm of repetitive bolus injections of anti-VEGF agents.

Recognition of cyclic-di-GMP by a riboswitch conducts translational repression through masking the ribosome-binding site distant from the aptamer domain

The riboswitch is a class of RNA-based gene regulatory machinery that is dependent on recognition of its target ligand by RNA tertiary structures. Ligand recognition is achieved by the aptamer domain, and ligand-dependent structural changes of the expression platform then usually mediate termination of transcription or translational initiation. Ligand-dependent structural changes of the aptamer domain and expression platform have been reported for several riboswitches with short (<40 nucleotides) expression platforms. In this study, we characterized structural changes of the Vc2 c-di-GMP riboswitch that represses translation of downstream open reading frames in a ligand-dependent manner. The Vc2 riboswitch has a long (97 nucleotides) expression platform, but its structure and function are largely unknown. Through mutational analysis and chemical probing, we identified its secondary structures that are possibly responsible for switch-OFF and switch-ON states of translational initiation.

Sensitive and specific detection of ligands using engineered riboswitches

Riboswitches are RNA elements found in non-coding regions of messenger RNAs that regulate gene expression through a ligand-triggered conformational change. Riboswitches typically bind tightly and specifically to their ligands, so they have the potential to serve as highly effective sensors in vitro. In B. subtilis and other gram-positive bacteria, purine nucleotide synthesis is regulated by riboswitches that bind to guanine. We modified the xpt-pbuX guanine riboswitch for use in a fluorescence quenching assay that allowed us to specifically detect and quantify guanine in vitro. Using this assay, we reproducibly detected as little as 5 nM guanine. We then produced sensors for 2'-deoxyguanosine and cyclic diguanylate (c-diGMP) by appending the P1 stem of the guanine riboswitch to the ligand-binding domains of a 2'-deoxyguanosine riboswitch and a c-diGMP riboswitch. These hybrid sensors could detect 15 nM 2'-deoxyguanosine and 3 nM c-diGMP, respectively. Each sensor retained the ligand specificity of its corresponding natural riboswitch. In order to extend the utility of our approach, we developed a strategy for the in vitro selection of sensors with novel ligand specificity. Here we report a proof-of-principle experiment that demonstrated the feasibility of our selection strategy.

Analyzing and Tuning Ribozyme Activity by Deep Sequencing To Modulate Gene Expression Level in Mammalian Cells

Self-cleaving ribozymes, in combination with aptamers and various classes of RNAs, have been heavily engineered to create RNA devices to control gene expression. Although understanding of sequence-function relationships of ribozymes is critical for such efforts, our current knowledge of self-cleaving ribozymes is mostly limited to the results from small scale mutational studies performed under different conditions, or qualitative results of mutate-and-select experiments that may contain experimental biases. Here, we applied our strategy based on deep sequencing to comprehensively assay a large number of mutants to systematically examine the effect of the P4 stem sequence on the activity of an HDV-like ribozyme. We discovered that the ribozyme activity is highly sensitive to the sequence and the apparent stability of the varied positions. Furthermore, we demonstrated that the collection of the ribozyme variants with different activities can be used as a convenient device to fine-tune the level of gene expression in mammalian cells.

Application of supervised machine learning algorithms for the classification of regulatory RNA riboswitches

Riboswitches, the small structured RNA elements, were discovered about a decade ago. It has been the subject of intense interest to identify riboswitches, understand their mechanisms of action and use them in genetic engineering. The accumulation of genome and transcriptome sequence data and comparative genomics provide unprecedented opportunities to identify riboswitches in the genome. In the present study, we have evaluated the following six machine learning algorithms for their efficiency to classify riboswitches: J48, BayesNet, Naïve Bayes, Multilayer Perceptron, sequential minimal optimization, hidden Markov model (HMM). For determining effective classifier, the algorithms were compared on the statistical measures of specificity, sensitivity, accuracy, F-measure and receiver operating characteristic (ROC) plot analysis. The classifier Multilayer Perceptron achieved the best performance, with the highest specificity, sensitivity, F-score and accuracy, and with the largest area under the ROC curve, whereas HMM was the poorest performer. At present, the available tools for the prediction and classification of riboswitches are based on covariance model, support vector machine and HMM. The present study determines Multilayer Perceptron as a better classifier for the genome-wide riboswitch searches.

Controlling Bdellovibrio bacteriovorus Gene Expression and Predation Using Synthetic Riboswitches

Bdellovibrio bacteriovorus is a predatory bacterium that feeds on Gram-negative bacteria including a wide range of pathogens and thus has potential applications as a biocontrol agent. Owing to its unique life cycle, however, there are limited tools that enable genetic manipulation of B. bacteriovorus. This work describes our first steps toward engineering the predatory bacterium for practical applications by developing basic genetic parts to control gene expression. Specifically, we evaluated four robust promoters that are active during the attack phase of B. bacteriovorus. Subsequently, we tested several synthetic riboswitches that have been reported to function in Escherichia coli, and identified theophylline-activated riboswitches that function in B. bacteriovorus. Finally, we inserted the riboswitch into the bacterial chromosome to regulate expression of the flagellar sigma factor fliA, which was previously predicted to be essential for predation, and observed that the engineered strain shows a faster predation kinetics in the presence of theophylline.

An Orphan Riboswitch Unveils Guanidine Regulation in Bacteria

In this issue, Nelson and colleagues (2017) determined that guanidine, the prevalent protein denaturant, is the long-lost ligand sensed by the ykkC class of riboswitches, and identified that members of its regulon are involved in guanidine detoxification and export.

Artificial OFF-Riboswitches That Downregulate Internal Ribosome Entry without Hybridization Switches in a Eukaryotic Cell-Free Translation System

We constructed novel artificial riboswitches that function in a eukaryotic translation system (wheat germ extract), by rationally implanting an in vitro-selected aptamer into the intergenic internal ribosome entry site (IRES) of Plautia stali intestine virus. These eukaryotic OFF-riboswitches (OFF-eRSs) ligand-dose-dependently downregulate IRES-mediated translation without hybridization switches, which typical riboswitches utilize for gene regulation. The hybridization-switch-free mechanism not only allows for easy design but also requires less energy for regulation, resulting in a higher switching efficiency than hybridization-switch-based OFF-eRSs provide. In addition, even a small ligand such as theophylline can induce satisfactory repression, in contrast to other types of OFF-eRSs that modulate the 5' cap-dependent canonical translation. Because our proposed hybridization-switch-free OFF-eRSs are based on a versatile IRES that functions well in many types of eukaryotic translation systems, they would be widely usable elements for synthetic gene circuits in both cell-free and cell-based synthetic biology.

Design of Artificial Riboswitches as Biosensors

RNA aptamers readily recognize small organic molecules, polypeptides, as well as other nucleic acids in a highly specific manner. Many such aptamers have evolved as parts of regulatory systems in nature. Experimental selection techniques such as SELEX have been very successful in finding artificial aptamers for a wide variety of natural and synthetic ligands. Changes in structure and/or stability of aptamers upon ligand binding can propagate through larger RNA constructs and cause specific structural changes at distal positions. In turn, these may affect transcription, translation, splicing, or binding events. The RNA secondary structure model realistically describes both thermodynamic and kinetic aspects of RNA structure formation and refolding at a single, consistent level of modelling. Thus, this framework allows studying the function of natural riboswitches in silico. Moreover, it enables rationally designing artificial switches, combining essentially arbitrary sensors with a broad choice of read-out systems. Eventually, this approach sets the stage for constructing versatile biosensors.

Applicability of a computational design approach for synthetic riboswitches

Riboswitches have gained attention as tools for synthetic biology, since they enable researchers to reprogram cells to sense and respond to exogenous molecules. In vitro evolutionary approaches produced numerous RNA aptamers that bind such small ligands, but their conversion into functional riboswitches remains difficult. We previously developed a computational approach for the design of synthetic theophylline riboswitches based on secondary structure prediction. These riboswitches have been constructed to regulate ligand-dependent transcription termination in Escherichia coli. Here, we test the usability of this design strategy by applying the approach to tetracycline and streptomycin aptamers. The resulting tetracycline riboswitches exhibit robust regulatory properties in vivo. Tandem fusions of these riboswitches with theophylline riboswitches represent logic gates responding to two different input signals. In contrast, the conversion of the streptomycin aptamer into functional riboswitches appears to be difficult. Investigations of the underlying aptamer secondary structure revealed differences between in silico prediction and structure probing. We conclude that only aptamers adopting the minimal free energy (MFE) structure are suitable targets for construction of synthetic riboswitches with design approaches based on equilibrium thermodynamics of RNA structures. Further improvements in the design strategy are required to implement aptamer structures not corresponding to the calculated MFE state.

Regulation of MAP4K4 gene expression by RNA interference through an engineered theophylline-dependent hepatitis delta virus ribozyme switch

Riboswitches are functional non-coding RNA regulatory components that play an important role in the regulation of gene expression in diverse organisms. In particular, using riboswitches to modulate RNA interference (RNAi) enables temporal and spatial control of gene expression in mammalian cells. Herein, a ribozyme gene switch to activate RNAi was fabricated for the artificial regulation of versatile gene silencing through the interaction of an RNA aptamer with small molecules. The device comprised an allosteric HDV ribozyme with an embedded theophylline aptamer and a primary miRNA (pri-miRNA) to silence the MAP4K4 gene in hepatic (HepG2) cells, aiming to achieve dose-dependent control of the activation of RNAi, and then the regulation of the MAP4K4 gene by theophylline. Finally, we demonstrated the feasibility and applicability of utilizing HDV ribozyme switches to activate RNAi for regulating an endogenous gene in mammalian cells.

ROC'n'Ribo: Characterizing a Riboswitching Expression System by Modeling Single-Cell Data

RNA-engineered systems offer simple and versatile control over gene expression in many organisms. In particular, the design and implementation of riboswitches presents a unique opportunity to manipulate any reporter device in cis, executing tight temporal and spatial control at low metabolic costs. Assembled to higher order genetic circuits, such riboswitch-regulated devices may efficiently process logical operations. Here, we propose a hierarchical stochastic modeling approach to characterize an in silico repressor gate based on neomycin- and tetracycline-sensitive riboswitches. The model was calibrated on rich, transient in vivo single-cell data to account for cell-to-cell variability. To capture the effect of this variability on gate performance we employed the well-known ROC-analysis and derived a novel performance indicator for logic gates. Introduction of such a performance measure is necessary, since we aimed to assess the correct functionality of the gate at the single-cell level-a prerequisite for its further adaption to a genetic circuitry. Our results may be applied to other genetic devices to analyze their efficiency and ensure their correct performance in the light of cell-to-cell variability.

Deep Sequencing Analysis of Aptazyme Variants Based on a Pistol Ribozyme

Chemically regulated self-cleaving ribozymes, or aptazymes, are emerging as a promising class of genetic devices that allow dynamic control of gene expression in synthetic biology. However, further expansion of the limited repertoire of ribozymes and aptamers, and development of new strategies to couple the RNA elements to engineer functional aptazymes are highly desirable for synthetic biology applications. Here, we report aptazymes based on the recently identified self-cleaving pistol ribozyme class using a guanine aptamer as the molecular sensing element. Two aptazyme architectures were studied by constructing and assaying 17 728 mutants by deep sequencing. Although one of the architectures did not yield functional aptazymes, a novel aptazyme design in which the aptamer and the ribozyme were placed in tandem yielded a number of guanine-inhibited ribozymes. Detailed analysis of the extensive sequence-function data suggests a mechanism that involves a competition between two mutually exclusive RNA structures reminiscent of natural bacterial riboswitches.

Riboswitch-Mediated Gene Regulation: Novel RNA Architectures Dictate Gene Expression Responses

Riboswitches are RNA elements that act on the mRNA with which they are cotranscribed to modulate expression of that mRNA. These elements are widely found in bacteria, where they have a broad impact on gene expression. The defining feature of riboswitches is that they directly recognize a physiological signal, and the resulting shift in RNA structure affects gene regulation. The majority of riboswitches respond to cellular metabolites, often in a feedback loop to repress synthesis of the enzymes used to produce the metabolite. Related elements respond to the aminoacylation status of a specific tRNA or to a physical parameter, such as temperature or pH. Recent studies have identified new classes of riboswitches and have revealed new insights into the molecular mechanisms of signal recognition and gene regulation. Application of structural and biophysical approaches has complemented previous genetic and biochemical studies, yielding new information about how different riboswitches operate.

Inducing circular RNA formation using the CRISPR endoribonuclease Csy4

Circular RNAs (circRNAs) are highly stable, covalently closed RNAs that are regulated in a spatiotemporal manner and whose functions are largely unknown. These molecules have the potential to be incorporated into engineered systems with broad technological implications. Here we describe a switch for inducing back-splicing of an engineered circRNA that relies on the CRISPR endoribonuclease, Csy4, as an activator of circularization. The endoribonuclease activity and 3' end-stabilizing properties of Csy4 are particularly suited for this task. Coexpression of Csy4 and the circRNA switch allows for the removal of downstream competitive splice sites and stabilization of the 5' cleavage product. This subsequently results in back-splicing of the 5' cleavage product into a circRNA that can translate a reporter protein from an internal ribosomal entry site (IRES). Our platform outlines a straightforward approach toward regulating splicing and could find potential applications in synthetic biology as well as in studying the properties of different circRNAs.

Optimization of industrial microorganisms: recent advances in synthetic dynamic regulators

Production of biochemicals by industrial fermentation using microorganisms requires maintaining cellular production capacity, because maximal productivity is economically important. High-productivity microbial strains can be developed using static engineering, but these may not maintain maximal productivity throughout the culture period as culture conditions and cell states change dynamically. Additionally, economic reasons limit heterologous protein expression using inducible promoters to prevent metabolic burden for commodity chemical and biofuel production. Recently, synthetic and systems biology has been used to design genetic circuits, precisely controlling gene expression or influencing genetic behavior toward a desired phenotype. Development of dynamic regulators can maintain cellular phenotype in a maximum production state in response to factors including cell concentration, oxygen, temperature, pH, and metabolites. Herein, we introduce dynamic regulators of industrial microorganism optimization and discuss metabolic flux fine control by dynamic regulators in response to metabolites or extracellular stimuli, robust production systems, and auto-induction systems using quorum sensing.