Effect of Loops and G‑Quartets on the Stability of RNA G‑Quadruplexes

The loop length, loop composition, salt concentration, and number of G-quartets are major determinants of G-quadruplex stability. We examined the effect of each of these factors on the thermal stability and folding topology of a library of RNA quadruplexes. The thermal stability of G2 and G3 RNA quadruplexes was investigated upon varying the loop length (from 1-1-1 to 15-15-15) and salt concentration (from 1 to 100 mM KCl), while the effect of loop composition was explored using 18 naturally occurring potential RNA quadruplexes predicted in untranslated regions (UTRs). We found loop length and quadruplex stability to be inversely related for G2 RNA quadruplexes and G3 RNA quadruplexes with shorter loops. However, melting temperature saturates for G3 RNA quadruplexes with longer loops. RNA G-quadruplexes with longer loops (G3 15-15-15) displayed <i>T</i>_m values significantly higher than the physiological temperature. This study thus highlights the need to modify the consensus motif presently used by quadruplex prediction tools. An increase in the loop size from 7 bases to 15 bases in the consensus motif will add to its predictive value for the discovery of potential RNA quadruplexes across transcriptomes

Human Telomeric RNA G‑Quadruplex Response to Point Mutation in the G‑Quartets

Many putative G-quadruplex forming sequences have been predicted to exist in the human genome and transcriptome. As these sequences are subject to point mutations or SNPs (single nucleotide polymorphisms) during the course of evolution, we attempt to understand impact of these mutations in context of RNA G-quadruplex formation using human telomeric RNA (TERRA) as a model sequence. Our studies suggest that G-quadruplex stability is sensitive to substitution of the guanines comprising G-quartets. While central G-quartet plays a crucial role in maintaining the DNA G-quadruplex stability as evident in literature, there is equal importance of three G-quartets in the stability of RNA quadruplex structure. The work here highlights the alterations in the G-quartet are detrimental to the integrity of overall RNA G-quadruplex structure. Furthermore, TmPyP4 molecules are shown to exhibit similar binding behavior toward telomeric RNA G-quadruplex harboring base substitutions employing CD titrations and isothermal titration calorimetry; well indicating that mutation does not influence TmPyP4 recognition ability as it affects the stability of RNA G-quadruplex. Thus, our study implicates that mutation in G-quartets causes destabilization of RNA G-quadruplex without affecting its trans factor binding ability

The G‑Quadruplex Augments Translation in the 5′ Untranslated Region of Transforming Growth Factor β2

Transforming growth factor β2 (TGFβ2) is a versatile cytokine with a prominent role in cell migration, invasion, cellular development, and immunomodulation. TGFβ2 promotes the malignancy of tumors by inducing epithelial–mesenchymal transition, angiogenesis, and immunosuppression. As it is well-documented that nucleic acid secondary structure can regulate gene expression, we assessed whether any secondary motif regulates its expression at the post-transcriptional level. Bioinformatics analysis predicts an existence of a 23-nucleotide putative G-quadruplex sequence (PG4) in the 5′ untranslated region (UTR) of TGFβ2 mRNA. The ability of this stretch of sequence to form a highly stable, intramolecular parallel quadruplex was demonstrated using ultraviolet and circular dichroism spectroscopy. Footprinting studies further validated its existence in the presence of a neighboring nucleotide sequence. Following structural characterization, we evaluated the biological relevance of this secondary motif using a dual luciferase assay. Although PG4 inhibits the expression of the reporter gene, its presence in the context of the entire 5′ UTR sequence interestingly enhances gene expression. Mutation or removal of the G-quadruplex sequence from the 5′ UTR of the gene diminished the level of expression of this gene at the translational level. Thus, here we highlight an activating role of the G-quadruplex in modulating gene expression of TGFβ2 at the translational level and its potential to be used as a target for the development of therapeutics against cancer

The RNA Stem–Loop to G‑Quadruplex Equilibrium Controls Mature MicroRNA Production inside the Cell

The biological role of the existence of overlapping structures in RNA is possible yet remains very unexplored. G-Rich tracts of RNA form G-quadruplexes, while GC-rich sequences prefer stem–loop structures. The equilibrium between alternate structures within RNA may occur and influence its functionality. We tested the equilibrium between G-quadruplex and stem–loop structure in RNA and its effect on biological processes using pre-miRNA as a model system. Dicer enzyme recognizes canonical stem–loop structures in pre-miRNA to produce mature miRNAs. Deviation from stem–loop leads to deregulated mature miRNA levels, providing readout of the existence of an alternate structure per se G-quadruplex-mediated structural interference in miRNA maturation. <i>In vitro</i> analysis using beacon and Dicer cleavage assays indicated that mature miRNA levels depend on relative amounts of K⁺ and Mg²⁺ ions, suggesting an ion-dependent structural shift. Further <i>in cellulo</i> studies with and without TmPyP₄ (RNA G-quadruplex destabilizer) demonstrated that miRNA biogenesis is modulated by G-quadruplex to stem–loop equilibrium in a subset of pre-miRNAs. Our combined analysis thus provides evidence of the formation of noncanonical G-quadruplexes in competition with canonical stem–loop structure inside the cell and its effect on miRNA maturation in a comprehensive manner

Optical bias with optical wedges in point sampling

Supplementary Tables and Figures. (PDF 454 kb

Additional file 3: of RNA secondary structure profiling in zebrafish reveals unique regulatory features

Multi-conformation position counts in transcripts with overlapping peaks. (TXT 712 kb

Additional file 4: of RNA secondary structure profiling in zebrafish reveals unique regulatory features

Number of read starts for every position covered in RNase V1 sample. (TXT 32051 kb

Additional file 9: of RNA secondary structure profiling in zebrafish reveals unique regulatory features

PARS scores of 54,083 transcripts and non-coding RNAs. (TXT 35418 kb

Additional file 5: of RNA secondary structure profiling in zebrafish reveals unique regulatory features

Number of read starts for every position covered in S1 nuclease sample. (TXT 45980 kb

Additional file 2: of RNA secondary structure profiling in zebrafish reveals unique regulatory features

Load score and percentage coverage of 54,083 transcripts. (TXT 3635 kb