No abstract

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.

RNA nanotechnology takes advantage of structural modularity to build self‐assembling nano‐architectures with applications in medicine and synthetic biology. The use of paranemic motifs, that form without unfolding existing secondary structure, allows for the creation of RNA nanostructures that are compatible with cotranscriptional folding in vitro and in vivo. In previous work, kissing‐loop (KL) motifs have been widely used to design RNA nanostructures that fold cotranscriptionally. However, the paranemic crossover (PX) motif has not yet been explored for cotranscriptional RNA origami architectures and information about the structural geometry of the motif is unknown. Here, a six base pair‐wide paranemic RNA interaction that arranges double helices in a perpendicular manner is introduced, allowing for the generation of a new and versatile building block: the paranemic‐crossover triangle (PXT). The PXT is self‐assembled by cotranscriptional folding and characterized by cryogenic electron microscopy, revealing for the first time an RNA PX interaction in high structural detail. The PXT is used as a building block for the construction of multimers that form filaments and rings and a duplicated PXT motif is used as a building block to self‐assemble cubic structures, demonstrating the PXT as a rigid self‐folding domain for the development of wireframe RNA origami architectures.

RNA nanotechnology uses motifs from nature as well as aptamers from in vitro selection to construct nanostructures and devices for applications in RNA medicine and synthetic biology. The RNA origami method allows cotranscriptional folding of large RNA scaffolds that can position functional motifs in a precise manner, which has been verified by Förster Resonance Energy Transfer (FRET) between fluorescent aptamers. Cryogenic electron microscopy (cryo-EM) is a promising method for characterizing the structure of larger RNA nanostructures. However, the structure of individual aptamers is difficult to solve by cryo-EM due to their low molecular weight. Here, we place aptamers on the RNA origami scaffolds to increase the contrast for cryo-EM and solve the structure of a new Broccoli-Pepper FRET pair. We identify different modes of ligand binding of the two aptamers and verify by selective 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 Angstrom. Our results demonstrate the use of RNA origami scaffolds for characterizing small RNA motifs by cryo-EM and for positioning functional RNA motifs with high spatial precision. The Broccoli-Pepper apta-FRET pair has potential use for developing advanced sensors that are sensitive to small conformational changes.

Although gene synthesis efficiency has improved through the years, current methods offered by DNA synthesis vendors are limited when it comes to repetitive sequences. Here, we describe a method for the enzymatic assembly of repetitive small synthetic genes. The method involves an initial step where the gene of interest is split in silico into small synthons of up to 80 base pairs flanked by Golden Gate-compatible four-base pair overhangs. Synthons are enzymatically synthesized by oligo extension and then assembled into the synthetic gene by Golden Gate Assembly. We construct eight different synthetic genes ranging from 133 to 456 base pairs encoding RNA structures with repetitive elements that are challenging to synthesize. We report assembly fidelities of up to 87.5 % that decrease for increasing number of synthons. This method is envisioned as an important addition to the molecular cloning toolbox and especially useful for construction of challenging and repetitive genes.