Identification and Characterization of New RNA Tetraloop Sequence Families

General Strategies for RNA X-ray Crystallography

An extremely small proportion of the X-ray crystal structures deposited in the Protein Data Bank are of RNA or RNA-protein complexes. This is due to three main obstacles to the successful determination of RNA structure: (1) low yields of pure, properly folded RNA; (2) difficulty creating crystal contacts due to low sequence diversity; and (3) limited methods for phasing. Various approaches have been developed to address these obstacles, such as native RNA purification, engineered crystallization modules, and incorporation of proteins to assist in phasing. In this review, we will discuss these strategies and provide examples of how they are used in practice.

Identification and characterization of RNA pentaloop sequence families

One of the current methods for predicting RNA tertiary structure is fragment-based homology, which predicts tertiary structure from secondary structure. For a successful prediction, this method requires a library of the tertiary structures of small motifs clipped from previously solved RNA 3D structures. Because of the limited number of available tertiary structures, it is not practical to find structures for all sequences of all motifs. Identifying sequence families for motifs can fill the gaps because all sequences within a family are expected to have similar structural features. Currently, a collection of well-characterized sequence families has been identified for tetraloops. Because of their prevalence and biological functions, pentaloop structures should also be well-characterized. In this study, 10 pentaloop sequence families are identified. For each family, the common and distinguishing structural features are highlighted. These sequence families can be used to predict the tertiary structure of pentaloop sequences for which a solved structure is not available.

Directed Evolution of 2'-Fluoro-Modified, RNA-Supported Carbohydrate Clusters That Bind Tightly to HIV Antibody 2G12

Carbohydrate binding proteins (CBPs) are attractive targets in medicine and biology. Multivalency, with several glycans binding to several binding pockets in the CBP, is important for high-affinity interactions. Herein, we describe a novel platform for design of multivalent carbohydrate cluster ligands by directed evolution, in which serum-stable 2'-fluoro modified RNA (F-RNA) backbones evolve to present the glycan in optimal clusters. We have validated this method by the selection of oligomannose (Man₉) glycan clusters from a sequence pool of ∼10¹³ that bind to broadly neutralizing HIV antibody 2G12 with 13 to 36 nM affinities.

A NusG Specialized Paralog That Exhibits Specific, High-Affinity RNA-Binding Activity

Bacterial NusG associates with RNA polymerase (RNAP) through its N-terminal domain, while the C-terminal domain (CTD) forms dynamic interactions with Rho, S10, NusB and NusA to affect transcription elongation. While virtually all bacteria encode for a core NusG, many also synthesize paralogs that transiently bind RNAP to alter expression of targeted genes. Yet, despite the importance of the genes they regulate, most of the subfamilies of NusG paralogs (e.g., UpxY, TaA, ActX and LoaP) have not been investigated in depth. Herein, we discover that LoaP requires a small RNA hairpin located within the 5' leader region of its targeted operons. LoaP binds the RNA element with nanomolar affinity and high specificity, in contrast to other NusG proteins, which have not been shown to exhibit RNA-binding activity. These data reveal a sequence feature that can be used to identify LoaP-regulated operons. This discovery also expands the repertoire of macromolecular interactions exhibited by the NusG CTD during transcription elongation to include an RNA ligand.

Stable Hairpin Structures Formed by Xylose-Based Nucleic Acids

Xenobiology explores synthetic nucleic acid polymers as alternative carriers of genetic information to expand the central dogma. The xylo- and deoxyxylo-nucleic acids (XyNA and dXyNA), containing 3' epimers of riboses and deoxyriboses, are considered to be potential candidates for an orthogonal system. In this study, thermal and spectroscopic analyses show that XyNA and dXyNA form stable hairpins. The dXyNA hairpin structure determined by NMR spectroscopy contains a flexible loop that locks the stem into a stable ladder-like duplex with marginal right-handed helicity. The reduced flexibility of the dXyNA duplex observed in the stem of the hairpin demonstrates that folding of dXyNA yields more stable structures described so far.

Incorporating a Thiophosphate Modification into a Common RNA Tetraloop Motif Causes an Unanticipated Stability Boost

GNRA (N = A, C, G, or U; R = A or G) tetraloops are common RNA secondary structural motifs and feature a phosphate stacked atop a nucleobase. The rRNA sarcin/ricin loop (SRL) is capped by GApGA, and the phosphate p stacks on G. We recently found that regiospecific incorporation of a single dithiophosphate (PS2) but not a monothiophosphate (PSO) instead of phosphate in the backbone of RNA aptamers dramatically increases the binding affinity for their targets. In the RNA:thrombin complex, the key contribution to the 1000-fold tighter binding stems from an edge-on contact between PS2 and a phenylalanine ring. Here we investigated the consequences of replacing the SRL phosphate engaged in a face-on interaction with guanine with either PS2 or PSO for stability. We found that PS2···G and Rp-PSO···G contacts stabilize modified SRLs compared to the parent loop to unexpected levels: up to 6.3 °C in melting temperature T_m and -4.7 kcal/mol in ΔΔG°. Crystal structures demonstrate that the vertical distance to guanine for the closest sulfur is just 0.05 Å longer on average compared to that of oxygen despite the larger van der Waals radius of the former (1.80 Å for S vs 1.52 Å for O). The higher stability is enthalpy-based, and the negative charge as assessed by a neutral methylphosphonate modification plays only a minor role. Quantum mechanical/molecular mechanical calculations are supportive of favorable dispersion attraction interactions by sulfur making the dominant contribution. A stacking interaction between phosphate and guanine (SRL) or uracil (U-turn) is also found in newly classified RNA tetraloop families besides GNRA.

RNA CoSSMos 2.0: an improved searchable database of secondary structure motifs in RNA three-dimensional structures

The RNA Characterization of Secondary Structure Motifs, RNA CoSSMos, database is a freely accessible online database that allows users to identify secondary structure motifs among RNA 3D structures and explore their structural features. RNA CoSSMos 2.0 now requires two closing base pairs for all RNA loop motifs to create a less redundant database of secondary structures. Furthermore, RNA CoSSMos 2.0 represents an upgraded database with new features that summarize search findings and aid in the search for 3D structural patterns among RNA secondary structure motifs. Previously, users were limited to viewing search results individually, with no built-in tools to compare search results. RNA CoSSMos 2.0 provides two new features, allowing users to summarize, analyze and compare their search result findings. A function has been added to the website that calculates the average and representative structures of the search results. Additionally, users can now view a summary page of their search results that reports percentages of each structural feature found, including sugar pucker, glycosidic linkage, hydrogen bonding patterns and stacking interactions. Other upgrades include a newly embedded NGL structural viewer, the option to download the clipped structure coordinates in *.pdb format and improved NMR structure results. RNA CoSSMos 2.0 is no longer simply a search engine for a structure database; it now has the capability of analyzing, comparing and summarizing search results.

Database URL: http://rnacossmos.com