Development of a novel tobramycin dependent riboswitch.

We herein report the selection and characterization of a new riboswitch dependent on the aminoglycoside tobramycin. Its dynamic range rivals even the tetracycline dependent riboswitch to be the current best performing, synthetic riboswitch that controls translation initiation. The riboswitch was selected with RNA Capture-SELEX, a method that not only selects for binding but also for structural changes in aptamers on binding. This study demonstrates how this method can fundamentally reduce the labour required for the de novo identification of synthetic riboswitches. The initially selected riboswitch candidate harbours two distinct tobramycin binding sites with KDs of 1.1 nM and 2.4 µM, respectively, and can distinguish between tobramycin and the closely related compounds kanamycin A and B. Using detailed genetic and biochemical analyses and 1H NMR spectroscopy, the proposed secondary structure of the riboswitch was verified and the tobramycin binding sites were characterized. The two binding sites were found to be essentially non-overlapping, allowing for a separate investigation of their contribution to the activity of the riboswitch. We thereby found that only the high-affinity binding site was responsible for regulatory activity, which allowed us to engineer a riboswitch from only this site with a minimal sequence size of 33 nt and outstanding performance.

RNA origami scaffolds facilitate cryo-EM characterization of a Broccoli-Pepper aptamer FRET pair.

Cryogenic electron microscopy (cryo-EM) is a promising method for characterizing the structure of larger RNA structures and complexes. However, the structure of individual aptamers is difficult to solve by cryo-EM due to their low molecular weight and a high signal-to-noise ratio. By placing RNA aptamers on larger RNA scaffolds, the contrast for cryo-EM can be increased to allow the determination of the tertiary structure of the aptamer. Here we use the RNA origami method to scaffold two fluorescent aptamers (Broccoli and Pepper) in close proximity and show that their cognate fluorophores serve as donor and acceptor for FRET. Next, we use cryo-EM to characterize the structure of the RNA origami with the two aptamers to a resolution of 4.4 Å. By characterizing the aptamers with and without ligand, we identify two distinct modes of ligand binding, which are further supported by selective chemical probing. 3D variability analysis of the cryo-EM data show that the relative position between the two bound fluorophores on the origami fluctuate by only 3.5 Å. Our results demonstrate a general approach for using RNA origami scaffolds for characterizing small RNA motifs by cryo-EM and for positioning functional RNA motifs with high spatial precision.

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.

RNA-based Capture-SELEX for the selection of small molecule-binding aptamers.

Despite their wide applicability, the selection of small molecule-binding RNA aptamers with both high affinity binding and specificity is still challenging. Aptamers that excel at both binding and structure switching are particularly rare and difficult to find. Here, we present the protocol of a Capture-SELEX that specifically allows the in vitro selection of small-molecule binding aptamers, which are essential building blocks for the design process of synthetic riboswitches and biosensors. Moreover, we provide a comparative overview of our proposed methodology versus alternative in vitro selection protocols with a special focus on the design of the pool. Finally, we have included detailed notes to point out useful tips and pitfalls for future application.