Chun Kit Kwok

In vivo genome-wide profiling of RNA secondary structure reveals novel regulatory features

RNA structure has critical roles in processes ranging from ligand sensing to the regulation of translation, polyadenylation and splicing. However, a lack of genome-wide in vivo RNA structural data has limited our understanding of how RNA structure regulates gene expression in living cells. Here we present a high-throughput, genome-wide in vivo RNA structure probing method, structure-seq, in which dimethyl sulphate methylation of unprotected adenines and cytosines is identified by next-generation sequencing. Application of this method to Arabidopsis thaliana seedlings yielded the first in vivo genome-wide RNA structure map at nucleotide resolution for any organism, with quantitative structural information across more than 10,000 transcripts. Our analysis reveals a three-nucleotide periodic repeat pattern in the structure of coding regions, as well as a less-structured region immediately upstream of the start codon, and shows that these features are strongly correlated with translation efficiency. We also find patterns of strong and weak secondary structure at sites of alternative polyadenylation, as well as strong secondary structure at 5′ splice sites that correlates with unspliced events. Notably, in vivo structures of messenger RNAs annotated for stress responses are poorly predicted in silico, whereas mRNA structures of genes related to cell function maintenance are well predicted. Global comparison of several structural features between these two categories shows that the mRNAs associated with stress responses tend to have more single-strandedness, longer maximal loop length and higher free energy per nucleotide, features that may allow these RNAs to undergo conformational changes in response to environmental conditions. Structure-seq allows the RNA structurome and its biological roles to be interrogated on a genome-wide scale and should be applicable to any organism.

Determination of in vivo RNA structure in low-abundance transcripts

RNA structure plays important roles in diverse biological processes. However, the structures of all but the few most abundant RNAs are presently unknown in vivo. Here we introduce DMS/SHAPE-LMPCR to query the in vivo structures of low-abundance transcripts. DMS/SHAPE-LMPCR achieves attomole sensitivity, a 100,000-fold improvement over conventional methods. We probe the structure of low-abundance U12 small nuclear RNA (snRNA) in Arabidopsis thaliana and provide in vivo evidence supporting our derived phylogenetic structure. Interestingly, in contrast to mammalian U12 snRNAs, the loop of the SLIIb in U12 snRNA is variable among plant species, and DMS/SHAPE-LMPCR determines it to be unstructured. We reveal the effects of proteins on 25S rRNA, 5.8S rRNA and U12 snRNA structure, illustrating the critical importance of mapping RNA structure in vivo. Our universally applicable method opens the door to identifying and exploring the specific structure-function relationships of the multitude of low-abundance RNAs that prevail in living cells.

rG4-seq reveals widespread formation of G-quadruplex structures in the human transcriptome

We introduce RNA G-quadruplex sequencing (rG4-seq), a transcriptome-wide RNA G-quadruplex (rG4) profiling method that couples rG4-mediated reverse transcriptase stalling with next-generation sequencing. Using rG4-seq on polyadenylated-enriched HeLa RNA, we generated a global in vitro map of thousands of canonical and noncanonical rG4 structures. We characterize rG4 formation relative to cytosine content and alternative RNA structure stability, uncover rG4-dependent differences in RNA folding and show evolutionarily conserved enrichment in transcripts mediating RNA processing and stability.

Genome-wide profiling of in vivo RNA structure at single-nucleotide resolution using structure-seq

Structure-seq is a high-throughput and quantitative method that provides genome-wide information on RNA structure at single-nucleotide resolution. Structure-seq can be performed both in vivo and in vitro to study RNA structure-function relationships, RNA regulation of gene expression and RNA processing. Structure-seq can be carried out by an experienced molecular biologist with a basic understanding of bioinformatics. Structure-seq begins with chemical RNA structure probing under single-hit kinetics conditions. Certain chemical modifications, e.g., methylation of the Watson-Crick face of unpaired adenine and cytosine residues by dimethyl sulfate, result in a stop in reverse transcription. Modified RNA is then subjected to reverse transcription using random hexamer primers, which minimizes 3′ end bias; reverse transcription proceeds until it is blocked by a chemically modified residue. Resultant cDNAs are amplified by adapter-based PCR and subjected to high-throughput sequencing, subsequently allowing retrieval of the structural information on a genome-wide scale. In contrast to classical methods that provide information only on individual transcripts, a single structure-seq experiment provides information on tens of thousands of RNA structures in ∼1 month. Although the procedure described here is for Arabidopsis thaliana seedlings in vivo, structure-seq is widely applicable, thereby opening new avenues to explore RNA structure–function relationships in living organisms.

StructureFold: genome-wide RNA secondary structure mapping and reconstruction in vivo

MOTIVATION RNAs fold into complex structures that are integral to the diverse mechanisms underlying RNA regulation of gene expression. Recent development of transcriptome-wide RNA structure profiling through the application of structure-probing enzymes or chemicals combined with high-throughput sequencing has opened a new field that greatly expands the amount of in vitro and in vivo RNA structural information available. The resultant datasets provide the opportunity to investigate RNA structural information on a global scale. However, the analysis of high-throughput RNA structure profiling data requires considerable computational effort and expertise. RESULTS We present a new platform, StructureFold, that provides an integrated computational solution designed specifically for large-scale RNA structure mapping and reconstruction across any transcriptome. StructureFold automates the processing and analysis of raw high-throughput RNA structure profiling data, allowing the seamless incorporation of wet-bench structural information from chemical probes and/or ribonucleases to restrain RNA secondary structure prediction via the RNAstructure and ViennaRNA package algorithms. StructureFold performs reads mapping and alignment, normalization and reactivity derivation, and RNA structure prediction in a single user-friendly web interface or via local installation. The variation in transcript abundance and length that prevails in living cells and consequently causes variation in the counts of structure-probing events between transcripts is accounted for. Accordingly, StructureFold is applicable to RNA structural profiling data obtained in vivo as well as to in vitro or in silico datasets. StructureFold is deployed via the Galaxy platform. AVAILABILITY AND IMPLEMENTATION StructureFold is freely available as a component of Galaxy available at: https://usegalaxy.org/. CONTACT yxt148@psu.edu or sma3@psu.edu SUPPLEMENTARY INFORMATION Supplementary data are available at Bioinformatics online.

Targeted Detection of G‐Quadruplexes in Cellular RNAs

The G-quadruplex (G4) is a non-canonical nucleic acid structure which regulates important cellular processes. RNA G4s have recently been shown to exist in human cells and be biologically significant. Described herein is a new approach to detect and map RNA G4s in cellular transcripts. This method exploits the specific control of RNA G4–cation and RNA G4–ligand interactions during reverse transcription, by using a selective reverse transcriptase to monitor RNA G4-mediated reverse transcriptase stalling (RTS) events. Importantly, a ligation-amplification strategy is coupled with RTS, and enables detection and mapping of G4s in important, low-abundance cellular RNAs. Strong evidence is provided for G4 formation in full-length cellular human telomerase RNA, offering important insights into its cellular function.

Effect of loop sequence and loop length on the intrinsic fluorescence of G-quadruplexes.

Guanine quadruplex structures (GQSs) exhibit unique spectroscopic features, including an inverse melting profile at 295 nm, distinctive circular dichroism features, and intrinsic fluorescence. Herein, we investigate effects of loop sequence and loop length on the intrinsic fluorescence of 13 DNA GQSs. We report label-free fluorescence enhancements upon intramolecular GQS formation of up to 16-fold and a shift in the emission maximum to the visible portion of the spectrum. Effects can be understood in the context of available nuclear magnetic resonance GQS structures. The intrinsic fluorescence of GQSs may be useful for nucleic acid studies and for the development of label-free detection methods.

Decrease in RNA folding cooperativity by deliberate population of intermediates in RNA G-quadruplexes

Keeping a broad (RNA) perspective: conventional biochemical detection systems only have a 100-fold linear response range. The range of potassium concentrations detected by an RNA G-quadruplex sequence can be broadened by intentionally populating multiple intermediate folding states. The folding of the RNA G-quadruplexes was monitored by both circular dichroism and intrinsic fluorescence spectroscopy.

G-Quadruplexes: Prediction, Characterization, and Biological Application

Guanine (G)-rich sequences in nucleic acids can assemble into G-quadruplex structures that involve G-quartets linked by loop nucleotides. The structural and topological diversity of G-quadruplexes have attracted great attention for decades. Recent methodological advances have advanced the identification and characterization of G-quadruplexes in vivo as well as in vitro, and at a much higher resolution and throughput, which has greatly expanded our current understanding of G-quadruplex structure and function. Accumulating knowledge about the structural properties of G-quadruplexes has helped to design and develop a repertoire of molecular and chemical tools for biological applications. This review highlights how these exciting methods and findings have opened new doors to investigate the potential functions and applications of G-quadruplexes in basic and applied biosciences.

Structural Analysis using SHALiPE to Reveal RNA G-Quadruplex Formation in Human Precursor MicroRNA

Abstract RNA G‐quadruplex (rG4) structures are of fundamental importance to biology. A novel approach is introduced to detect and structurally map rG4s at single‐nucleotide resolution in RNAs. The approach, denoted SHALiPE, couples selective 2′‐hydroxyl acylation with lithium ion‐based primer extension, and identifies characteristic structural fingerprints for rG4 mapping. We apply SHALiPE to interrogate the human precursor microRNA 149, and reveal the formation of an rG4 structure in this non‐coding RNA. Additional analyses support the SHALiPE results and uncover that this rG4 has a parallel topology, is thermally stable, and is conserved in mammals. An in vitro Dicer assay shows that this rG4 inhibits Dicer processing, supporting the potential role of rG4 structures in microRNA maturation and post‐transcriptional regulation of mRNAs.