HOTAIR forms an intricate and modular secondary structure.

Long noncoding RNAs (lncRNAs) have recently emerged as key players in fundamental cellular processes and diseases, but their functions are poorly understood. HOTAIR is a 2,148-nt-long lncRNA molecule involved in physiological epidermal development and in pathogenic cancer progression, where it has been demonstrated to repress tumor and metastasis suppressor genes. To gain insights into the molecular mechanisms of HOTAIR, we purified it in a stable and homogenous form in vitro, and we determined its functional secondary structure through chemical probing and phylogenetic analysis. The HOTAIR structure reveals a degree of structural organization comparable to well-folded RNAs, like the group II intron, rRNA, or lncRNA steroid receptor activator. It is composed of four independently folding modules, two of which correspond to predicted protein-binding domains. Secondary structure elements that surround protein-binding motifs are evolutionarily conserved. Our work serves as a guide for "navigating" through the lncRNA HOTAIR and ultimately for understanding its function.

Visualizing the ai5γ group IIB intron.

It has become apparent that much of cellular metabolism is controlled by large well-folded noncoding RNA molecules. In addition to crystallographic approaches, computational methods are needed for visualizing the 3D structure of large RNAs. Here, we modeled the molecular structure of the ai5γ group IIB intron from yeast using the crystal structure of a bacterial group IIC homolog. This was accomplished by adapting strategies for homology and de novo modeling, and creating a new computational tool for RNA refinement. The resulting model was validated experimentally using a combination of structure-guided mutagenesis and RNA structure probing. The model provides major insights into the mechanism and regulation of splicing, such as the position of the branch-site before and after the second step of splicing, and the location of subdomains that control target specificity, underscoring the feasibility of modeling large functional RNA molecules.

Exploring Ty1 retrotransposon RNA structure within virus-like particles.

Ty1, a long terminal repeat retrotransposon of Saccharomyces, is structurally and functionally related to retroviruses. However, a differentiating aspect between these retroelements is the diversity of the replication strategies used by long terminal repeat retrotransposons. To understand the structural organization of cis-acting elements present on Ty1 genomic RNA from the GAG region that control reverse transcription, we applied chemoenzymatic probing to RNA/tRNA complexes assembled in vitro and to the RNA in virus-like particles. By comparing different RNA states, our analyses provide a comprehensive structure of the primer-binding site, a novel pseudoknot adjacent to the primer-binding sites, three regions containing palindromic sequences that may be involved in RNA dimerization or packaging and candidate protein interaction sites. In addition, we determined the impact of a novel form of transposon control based on Ty1 antisense transcripts that associate with virus-like particles. Our results support the idea that antisense RNAs inhibit retrotransposition by targeting Ty1 protein function rather than annealing with the RNA genome.

Correlating SHAPE signatures with three-dimensional RNA structures.

Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) is a facile technique for quantitative analysis of RNA secondary structure. In general, low SHAPE signal values indicate Watson-Crick base-pairing, and high values indicate positions that are single-stranded within the RNA structure. However, the relationship of SHAPE signals to structural properties such as non-Watson-Crick base-pairing or stacking has thus far not been thoroughly investigated. Here, we present results of SHAPE experiments performed on several RNAs with published three-dimensional structures. This strategy allows us to analyze the results in terms of correlations between chemical reactivities and structural properties of the respective nucleotide, such as different types of base-pairing, stacking, and phosphate-backbone interactions. We find that the RNA SHAPE signal is strongly correlated with cis-Watson-Crick/Watson-Crick base-pairing and is to a remarkable degree not dependent on other structural properties with the exception of stacking. We subsequently generated probabilistic models that estimate the likelihood that a residue with a given SHAPE score participates in base-pairing. We show that several models that take SHAPE scores of adjacent residues into account perform better in predicting base-pairing compared with individual SHAPE scores. This underscores the context sensitivity of SHAPE and provides a framework for an improved interpretation of the response of RNA to chemical modification.

SHAPE analysis of the FIV Leader RNA reveals a structural switch potentially controlling viral packaging and genome dimerization.

Feline immunodeficiency virus (FIV) infects many species of cat, and is related to HIV, causing a similar pathology. High-throughput selective 2' hydroxyl acylation analysed by primer extension (SHAPE), a technique that allows structural interrogation at each nucleotide, was used to map the secondary structure of the FIV packaging signal RNA. Previous studies of this RNA showed four conserved stem-loops, extensive long-range interactions (LRIs) and a small, palindromic stem-loop (SL5) within the gag open reading frame (ORF) that may act as a dimerization initiation site (DIS), enabling the virus to package two copies of its genome. Our analyses of wild-type (wt) and mutant RNAs suggest that although the four conserved stem-loops are static structures, the 5' and 3' regions previously shown to form LRI also adopt an alternative, yet similarly conserved conformation, in which the putative DIS is occluded, and which may thus favour translational and splicing functions over encapsidation. SHAPE and in vitro dimerization assays were used to examine SL5 mutants. Dimerization contacts appear to be made between palindromic loop sequences in SL5. As this stem-loop is located within the gag ORF, recognition of a dimeric RNA provides a possible mechanism for the specific packaging of genomic over spliced viral RNAs.

The RNA transport element of the murine musD retrotransposon requires long-range intramolecular interactions for function.

Retrovirus replication requires specialized transport mechanisms to export genomic mRNA from the nucleus to the cytoplasm of the infected cell. This regulation is mediated by a combination of viral and/or cellular factors that interact with cis-acting RNA export elements linking the viral RNA to the cellular CRM1 or NXF1 nuclear export pathways. Endogenous type D murine LTR retrotransposons (musD) were reported to contain an RNA export element located upstream of the 3'-LTR. Although functionally equivalent, the musD export element, termed the musD transport element, is distinct from the other retroviral RNA export elements, such as the constitutive transport element of simian/Mason-Pfizer monkey retroviruses and the RNA transport element found in rodent intracisternal A-particle LTR retrotransposons. We demonstrate here that the minimal RNA transport element (musD transport element) of musD comprises multiple secondary structure elements that presumably serve as recognition signals for the cellular export machinery. We identified two classes of tertiary interactions, namely kissing loops and a pseudoknot. This work constitutes the first example of an RNA transport element requiring such structural motifs to mediate nuclear export.

Resistance to RevM10 inhibition reflects a conformational switch in the HIV-1 Rev response element.

Nuclear export of certain HIV-1 mRNAs requires an interaction between the viral Rev protein and the Rev response element (RRE), a structured element located in the Env region of its RNA genome. This interaction is an attractive target for both drug design and gene therapy, exemplified by RevM10, a transdominant negative protein that, when introduced into host cells, disrupts viral mRNA export. However, two silent G->A mutations in the RRE (RRE61) confer RevM10 resistance, which prompted us to examine RRE structure using a novel chemical probing strategy. Variations in region III/IV/V of mutant RNAs suggest a stepwise rearrangement to RevM10 resistance. Mass spectrometry was used to directly assess Rev "loading" onto RRE and its variants, indicating that this is unaffected by RNA structural changes. Similarity in chemical footprints with mutant protein implicates additional host factors in RevM10 resistance.

CAR T Cell Generation by piggyBac Transposition from Linear Doggybone DNA Vectors Requires Transposon DNA-Flanking Regions.

CD19-specific chimeric antigen receptor (CAR19) T cells, generated using viral vectors, are an efficacious but costly treatment for B cell malignancies. The nonviral piggyBac transposon system provides a simple and inexpensive alternative for CAR19 T cell production. Until now, piggyBac has been plasmid based, facilitating economical vector amplification in bacteria. However, amplified plasmids have several undesirable qualities for clinical translation, including bacterial genetic elements, antibiotic-resistance genes, and the requirement for purification to remove endotoxin. Doggybones (dbDNA) are linear, covalently closed, minimal DNA vectors that can be inexpensively produced enzymatically in vitro at large scale. Importantly, they lack the undesirable features of plasmids. We used dbDNA incorporating piggyBac to generate CAR19 T cells. Initially, expression of functional transposase was evident, but stable CAR expression did not occur. After excluding other causes, additional random DNA flanking the transposon within the dbDNA was introduced, promoting stable CAR expression comparable to that of using plasmid components. Our findings demonstrate that dbDNA incorporating piggyBac can be used to generate CAR T cells and indicate that there is a requirement for DNA flanking the piggyBac transposon to enable effective transposition. dbDNA may further reduce the cost and improve the safety of CAR T cell production with transposon systems.

Antigenomic delta ribozyme variants with mutations in the catalytic core obtained by the in vitro selection method.

We have used the in vitro selection method to search for catalytically active variants of the antigenomic delta ribozyme with mutations in the regions that constitute the ribozyme active site: L3, J1/4 and J4/2. In the initial combinatorial library 16 nt positions were randomized and the library contained a full representation of all possible sequences. Following ten cycles of selection-amplification several catalytically active ribozyme variants were identified. It turned out that one-third of the variants contained only single mutation G80U and their activity was similar to that of the wild-type ribozyme. Unexpectedly, in the next one-third of the variants the C76 residue, which was proposed to play a crucial role in the ribozyme cleavage mechanism, was mutated. In these variants, however, a cytosine residue was present in a neighboring position to the polynucleotide chain. It shows that the ribozyme catalytic core possesses substantial 'structural plasticity' and the capacity of functional adaptation. Four selected ribozyme variants were subjected to more detailed analysis. It turned out that the variants differed in their relative preferences towards Mg2+, Ca2+ and Mn2+ ions. Thus, the functional properties of the variants were dependent on both the structure of their catalytic sites and divalent metal ions performing catalysis.

Generating in vitro transcripts with homogenous 3' ends using trans-acting antigenomic delta ribozyme.

In most in vitro run-off transcription reactions with T7 RNA polymerase, transcripts with heterogeneous ends are commonly obtained. Towards the goal of finding a simple and effective procedure for correct processing of their 3' ends we propose the use of trans-acting antigenomic delta ribozyme. We demonstrate that the extension of nascent transcripts with only seven nucleotides complementary to the ribozyme's recognition site, and subsequently, the removal of those nucleotides with the ribozyme acting in trans, is an efficient procedure for generating transcripts with homogenous 3' ends. This approach was tested on two model RNA molecules: an in vitro transcript of yeast tRNA(Phe) and a delta ribozyme, which processed itself during transcription. The proposed procedure is a simple alternative to the use of ribozymes as cis-cleaving autocatalytic cassettes attached to transcript 3' ends. As there is little possibility that the required additional stretch, only seven nucleotides long, enters into stable interactions with other parts of the transcripts, it can be cleaved off with high efficacy.

Native Purification and Analysis of Long RNAs.

The purification and analysis of long noncoding RNAs (lncRNAs) in vitro is a challenge, particularly if one wants to preserve elements of functional structure. Here, we describe a method for purifying lncRNAs that preserves the cotranscriptionally derived structure. The protocol avoids the misfolding that can occur during denaturation-renaturation protocols, thus facilitating the folding of long RNAs to a native-like state. This method is simple and does not require addition of tags to the RNA or the use of affinity columns. LncRNAs purified using this type of native purification protocol are amenable to biochemical and biophysical analysis. Here, we describe how to study lncRNA global compaction in the presence of divalent ions at equilibrium using sedimentation velocity analytical ultracentrifugation and analytical size-exclusion chromatography as well as how to use these uniform RNA species to determine robust lncRNA secondary structure maps by chemical probing techniques like selective 2'-hydroxyl acylation analyzed by primer extension and dimethyl sulfate probing.

A more complex isoleucine aptamer with a cognate triplet.

The simplest RNA that can meet a column affinity selection for isoleucine was previously defined using selection amplification with decreasing numbers of randomized nucleotides. This simplest UAUU motif was a small asymmetric internal loop. Conserved positions of the loop include isoleucine codon and anticodon triplets (Lozupone C., Changayil, S., Majerfeld, I., and Yarus, M. (2003) RNA (N. Y.) 9, 1315-1322). Using new primer sequences, we now select a somewhat more complex isoleucine binding RNA, requiring 4.7 more bits of information to describe. The newly selected structure is a terminal or hairpin loop of 20 nucleotides, 15 being invariant. An information profile shows that the new binding site contains five short functional loop regions joined by less significant single nucleotide positions. Among the important nucleotides is a conserved isoleucine anticodon, supporting the escaped triplet theory, which posits a stereochemical genetic code originating in RNA amino acid binding sites.

Size, constant sequences, and optimal selection.

Because the abundance of functional molecules in RNA sequence space has many unexplored aspects, we compared the outcome of 11 independent selections, performed using the same affinity selection protocol and contiguous randomized regions of 16, 22, 26, 50, 70, and 90 nucleotides. All affinity selections targeted the simplest isoleucine aptamer, an asymmetric internal loop. This loop should be abundant in all selections, so that it can be compared across all experiments. In some cases, two primer sets intended to favor selection of different structures have also been compared. The simplest isoleucine aptamer dominates all selections except with the shortest tract, 16 contiguous randomized nucleotides. Here the isoleucine aptamer cannot be accommodated and no other motif can be selected. Our results suggest an optimum length for selection; surprisingly, both the shortest and the longest randomized tracts make it more difficult to recover the motif. Estimated apparent initial abundances suggest that the simplest isoleucine motif was 20- to 40-fold more frequent in selection with 50- or 70-nucleotide randomized regions than with any other length. Considering primer sets, a pre-formed stable stem within fixed flanking sequences had a five-to 10-fold negative effect on apparent motif abundance at all lengths. Differing random tract lengths also determined the probable motif permutation and the most abundant helix lengths. These data support a significant but lesser role for primer sequences in the outcome of selections.