Human telomere quadruplex: refolding and selection of individual conformers via RNA/DNA chimeric editing

Assembly and Characterization of RNA/DNA Hetero-G-Quadruplexes

Transient association of guanine-rich RNA and DNA in the form of hetero-G-quadruplexes (RDQs) has emerged as an important mechanism for regulating genome transcription and replication but relatively little is known about the structure and biophysical properties of RDQs compared with DNA and RNA homo-G-quadruplexes. Herein, we report the assembly and characterization of three RDQs based on sequence motifs found in human telomeres and mitochondrial nucleic acids. Stable RDQs were assembled using a duplex scaffold, which prevented segregation of the DNA and RNA strands into separate homo-GQs. Each of the RDQs exhibited UV melting temperatures above 50 °C in 100 mM KCl and predominantly parallel morphologies, evidently driven by the RNA component. The fluorogenic dye thioflavin T binds to each RDQ with low micromolar K_D values, similar to its binding to RNA and DNA homo-GQs. These results establish a method for assembling RDQs that should be amenable to screening compounds and libraries to identify selective RDQ-binding small molecules, oligonucleotides, and proteins.

Manipulating DNA G-Quadruplex Structures by Using Guanosine Analogues

The ability to control the folding topology of DNA G-quadruplexes allows for rational design of quadruplex-based scaffolds for potential use in various therapeutic and technological applications. By exploiting the distinct conformational properties of some base- and sugar-modified guanosine surrogates, conformational transitions can be induced through their judicious incorporation at specific sites in the quadruplex core. Changes may involve tetrad polarity inversions with conservation of the global fold or complete refolding to new topologies. Reliable predictions relating to low-energy conformers formed upon specific chemical perturbations of the system and the rational design of modified sequences suffer from our still limited understanding of the subtle interplay of various favorable and unfavorable interactions within a particular quadruplex scaffold. However, aided by an increasing number of systematic substitution experiments and high-resolution structures of modified quadruplex variants, critical interactions, in addition to glycosidic bond angle propensities, are starting to emerge as important contributors to modification-driven quadruplex refolding.

Unexpected Position-Dependent Effects of Ribose G-Quartets in G-Quadruplexes

To understand the role of ribose G-quartets and how they affect the properties of G-quadruplex structures, we studied three systems in which one, two, three, or four deoxyribose G-quartets were substituted with ribose G-quartets. These systems were a parallel DNA intramolecular G-quadruplex, d(TTGGGTGGGTTGGGTGGGTT), and two tetramolecular G-quadruplexes, d(TGGGT) and d(TGGGGT). Thermal denaturation experiments revealed that ribose G-quartets have position-dependent and cumulative effects on G-quadruplex stability. An unexpected destabilization was observed when rG quartets were presented at the 5'-end of the G stack. This observation challenges the general belief that RNA residues stabilize G-quadruplexes. Furthermore, in contrast to past proposals, hydration is not the main factor determining the stability of our RNA/DNA chimeric G-quadruplexes. Interestingly, the presence of rG residues in a central G-quartet facilitated the formation of additional tetramolecular G-quadruplex topologies showing positive circular dichroism signals at 295 nm. 2D NMR analysis of the tetramolecular TGgGGT (lowercase letter indicates ribose) indicates that Gs in the 5'-most G-quartet adopt the syn conformation. These analyses highlight several new aspects of the role of ribose G-quartets on G-quadruplex structure and stability, and demonstrate that the positions of ribose residues are critical for tuning G-quadruplex properties.

Sugar-Edge Interactions in a DNA-RNA G-Quadruplex: Evidence of Sequential C-H⋅⋅⋅O Hydrogen Bonds Contributing to RNA Quadruplex Folding

DNA G-quadruplexes were systematically modified by single riboguanosine (rG) substitutions at anti-dG positions. Circular dichroism and NMR experiments confirmed the conservation of the native quadruplex topology for most of the DNA-RNA hybrid structures. Changes in the C8 NMR chemical shift of guanosines following rG substitution at their 3'-side within the quadruplex core strongly suggest the presence of C8-H⋅⋅⋅O hydrogen-bonding interactions with the O2' position of the C2'-endo ribonucleotide. A geometric analysis of reported high-resolution structures indicates that such interactions are a more general feature in RNA quadruplexes and may contribute to the observed preference for parallel topologies.

Separation of Quadruplex Polymorphism in DNA Sequences by Reversed-Phase Chromatography

This unit describes a method for the separation of a mixture of quadruplex conformations formed from the same parent sequence via reversed-phase chromatography (RPC). Polymorphism is inherent to quadruplex formation and even relatively simple quadruplex-forming sequences can fold into a cornucopia of possible conformations and topologies. Isolation of a specific conformation for study can be problematic. This is especially true for conformations of the human telomere sequence d(GGG(TTAGGG)3). High performance liquid chromatography (HPLC), especially reversed-phase chromatography, has been a mainstay of nucleic acid research and purification for many decades. We have successfully applied this method to the problem of separating individual quadruplex species in the ensemble from the same parent sequence.

Finding a human telomere DNA-RNA hybrid G-quadruplex formed by human telomeric 6-mer RNA and 16-mer DNA using click chemistry: a protective structure for telomere end

Telomeric repeat-containing RNA is a non-coding RNA molecule newly found in mammalian cells. The telomere RNA has been found to localize to the telomere DNA, but how the newly discovered RNA molecule interacts with telomere DNA is less known. In this study, using the click chemistry we successfully found that a 6-mer human telomere RNA and 16-mer human telomere DNA sequence can form a DNA-RNA hybrid type G-quadruplex structure. Detection of the click-reaction products directly probes DNA-RNA G-quadruplex structures in a complicated solution, whereas traditional methods such as NMR and crystallography may not be suitable. Importantly, we found that formation of DNA-RNA G-quadruplex induced an exonuclease resistance for telomere DNA, indicating that such structures might be important for protecting telomeric DNA from enzyme digestion to avoid telomere DNA shortening. These results provide the direct evidence for formation of DNA-RNA hybrid G-quadruplex structure by human telomere DNA and RNA sequence, suggesting DNA-RNA hybrid G-quadruplex structure associated between telomere DNA and RNA may respond to chromosome end protection and/or present a valuable target for drug design.

Calculation of hydrodynamic properties for G-quadruplex nucleic acid structures from in silico bead models

Nucleic acids enriched in guanine bases can adopt unique quadruple helical tertiary structures known as G-quadruplexes. G-quadruplexes have emerged as attractive drug targets as many G-quadruplex-forming sequences have been discovered in functionally critical sites within the human genome, including the telomere, oncogene promoters, and mRNA processing sites. A single G-quadruplex-forming sequence can adopt one of many folding topologies, often resulting in a lack of a single definitive atomic-level resolution structure for many of these sequences and a major challenge to the discovery of G-quadruplex-selective small molecule drugs. Low-resolution techniques employed to study G-quadruplex structures (e.g., CD spectroscopy) are often unable to discern between G-quadruplex structural ensembles, while high-resolution techniques (e.g., NMR spectroscopy) can be overwhelmed by a highly polymorphic system. Hydrodynamic bead modeling is an approach to studying G-quadruplex structures that could bridge the gap between low-resolution techniques and high-resolution molecular models. Here, we present a discussion of hydrodynamic bead modeling in the context of studying G-quadruplex structures, highlighting recent successes and limitations to this approach, as well as an example featuring a G-quadruplex structure formed from the human telomere. This example can easily be adapted to the investigation of any other G-quadruplex-forming sequences.

Not all G-quadruplexes are created equally: an investigation of the structural polymorphism of the c-Myc G-quadruplex-forming sequence and its interaction with the porphyrin TMPyP4

G-quadruplexes, DNA tertiary structures highly localized to functionally important sites within the human genome, have emerged as important new drug targets. The putative G-quadruplex-forming sequence (Pu27) in the NHE-III(1) promoter region of the c-Myc gene is of particular interest as stabilization of this G-quadruplex with TMPyP4 has been shown to repress c-Myc transcription. In this study, we examine the Pu27 G-quadruplex-forming sequence and its interaction with TMPyP4. We report that the Pu27 sequence exists as a heterogeneous mixture of monomeric and higher-order G-quadruplex species in vitro and that this mixture can be partially resolved by size exclusion chromatography (SEC) separation. Within this ensemble of configurations, the equilibrium can be altered by modifying the buffer composition, annealing procedure, and dialysis protocol thereby affecting the distribution of G-quadruplex species formed. TMPyP4 was found to bind preferentially to higher-order G-quadruplex species suggesting the possibility of stabilization of the junctions of the c-Myc G-quadruplex multimers by porphyrin end-stacking. We also examined four modified c-Myc sequences that have been previously reported and found a narrower distribution of G-quadruplex configurations compared to the parent Pu27 sequence. We could not definitively conclude whether these G-quadruplex structures were selected from the original ensemble or if they are new G-quadruplex structures. Since these sequences differ considerably from the wild-type promoter sequence, it is unclear whether their structures have any actual biological relevance. Additional studies are needed to examine how the polymorphic nature of G-quadruplexes affects the interpretation of in vitro data for c-Myc and other G-quadruplexes. The findings reported here demonstrate that experimental conditions contribute significantly to G-quadruplex formation and should be carefully considered, controlled, and reported in detail.

Circular dichroism and guanine quadruplexes

Circular dichroism (CD) is remarkably sensitive to the conformational states of nucleic acids; therefore, CD spectroscopy has been used to study most features of DNA and RNA structures. Quadruplexes are among the significant noncanonical nucleic acids architectures that have received special attentions recently. This article presents examples on the contribution of CD spectroscopy to our knowledge of quadruplex structures and their polymorphism. The examples were selected to demonstrate the potential of this simple method in the quadruplex field. As CD spectroscopy detects only the global feature of a macromolecule, it should preferably be used in combination with other techniques. On the other hand, CD spectroscopy, often as a pioneering approach, can reveal the formation of particular structural arrangements, to search for the conditions stabilizing the structures, to follow the transitions between various structural states, to explore kinetics of their appearance, to determine thermodynamic parameters and also detect formation of higher order structures. This article aims to show that CD spectroscopy is an important complementary technique to NMR spectroscopy and X-ray diffraction in quadruplex studies.

Resolution of quadruplex polymorphism by size-exclusion chromatography

This unit describes a method for separation of quadruplex species formed from the same sequence via size-exclusion chromatography (SEC). Polymorphism is inherent to quadruplex formation, and even relatively simple quadruplex-forming sequences, such as the human telomere sequence d(GGG(TTAGGG)(3)), can form a myriad of possible configurations. HPLC, especially using reversed-phase and anion-exchange methods, has been a mainstay of nucleic acids research and purification for many decades. These methods have been applied for separation of individual quadruplex species formed in a mixture from the same parent sequence.

Polymorphism and resolution of oncogene promoter quadruplex-forming sequences

We report the separation of several quadruplex species formed by ten promoter sequences by Size Exclusion Chromatography (SEC). Modification at the 5' or 3' ends or in loop regions of quadruplex forming sequences has become the standard technique for dealing with quadruplex polymorphism. However, conformations produced employing this method or by other means of artificially shifting the equilibrium may not represent the species that are present in vivo. This method enables an unperturbed view of the structural polymorphism inherent to quadruplex formation. Separation via SEC facilitates studies on quadruplex structure and biophysical properties without the need for sequence modification.

The application of DNA and RNA G-quadruplexes to therapeutic medicines

The intriguing structural diversity in folded topologies available to guanine-rich nucleic acid repeat sequences have made four-stranded G-quadruplex structures the focus of both basic and applied research, from cancer biology and novel therapeutics through to nanoelectronics. Distributed widely in the human genome as targets for regulating gene expression and chromosomal maintenance, they offer unique avenues for future cancer drug development. In particular, the recent advances in chemical and structural biology have enabled the construction of bespoke selective DNA based aptamers to be used as novel therapeutic agents and access to detailed structural models for structure based drug discovery. In this critical review, we will explore the important underlying characteristics of G-quadruplexes that make them functional, stable, and predictable nanoscaffolds. We will review the current structural database of folding topologies, molecular interfaces and novel interaction surfaces, with a consideration to their future exploitation in drug discovery, molecular biology, supermolecular assembly and aptamer design. In recent years the number of potential applications for G-quadruplex motifs has rapidly grown, so in this review we aim to explore the many future challenges and highlight where possible successes may lie. We will highlight the similarities and differences between DNA and RNA folded G-quadruplexes in terms of stability, distribution, and exploitability as small molecule targets. Finally, we will provide a detailed review of basic G-quadruplex geometry, experimental tools used, and a critical evaluation of the application of high-resolution structural biology and its ability to provide meaningful and valid models for future applications (255 references).

Kinetic discrimination in recognition of DNA quadruplex targets by guanine-rich heteroquadruplex-forming PNA probes

Guanine-rich peptide nucleic acid probes hybridize to DNA G quadruplex targets with high affinity, forming PNA-DNA heteroquadruplexes. We report a surprising degree of kinetic discrimination for PNA heteroquadruplex formation with a series of DNA targets. The fastest hybridization is observed for targets folded into parallel morphologies.

Selection of G-quadruplex folding topology with LNA-modified human telomeric sequences in K+ solution

G-rich nucleic acid oligomers can form G-quadruplexes built by G-tetrads stacked upon each other. Depending on the nucleotide sequence, G-quadruplexes fold mainly with two topologies: parallel, in which all G-tracts are oriented parallel to each other, or antiparallel, in which one or more G-tracts are oriented antiparallel to the other G-tracts. In the former topology, all glycosidic bond angles conform to anti conformations, while in the latter topology they adopt both syn and anti conformations. It is of interest to understand the molecular forces that govern G-quadruplex folding. Here, we approach this problem by examining the impact of LNA (locked nucleic acid) modifications on the folding topology of the dimeric model system of the human telomere sequence. In solution, this DNA G-quadruplex forms a mixture of G-quadruplexes with antiparallel and parallel topologies. Using CD and NMR spectroscopies, we show that LNA incorporations can modulate this equilibrium in a rational manner and we establish a relationship between incorporation of LNA nucleotides in syn and/or anti positions and the shift of the equilibrium to obtain exclusively the parallel G-quadruplex. The change in topology is driven by a combination of the C3'-endo puckering of LNA nucleotides and their preference for the anti glycosidic conformation. In addition, the parallel LNA-modified G-quadruplexes are thermally stabilised by about 11 °C relative to their DNA counterparts.

Sole and stable RNA duplexes of G-rich sequences located in the 5'-untranslated region of protooncogenes

Guanine- (G-) rich nucleic acid sequences can form four-stranded structures called G-quadruplexes. It is widely held that the formation of a G-quadruplex in RNA is more feasible than in DNA because of the lack of a complementary strand in mRNA. Here, we analyzed sequences of 5'-untranslated regions of protooncogenes and surprisingly found that these regions showed an enrichment of not only guanine (G) but also cytosine (C) nucleotides. Since neighboring cytosine- (C-) rich regions can affect the formation and stability of a G-quadruplex structure, we further investigated the properties of DNA and RNA structures of G-rich and GC-rich regions. We selected typical GC-rich RNA sequences from protooncogenes and corresponding DNA sequences and investigated their structures. It was found that the GC-rich RNA sequences formed stable A-form duplexes as their major structure independent of the surrounding conditions, including the presence of different cations (Na(+), K(+), or Li(+)) or molecular crowding with 40 wt % poly(ethylene glycol) with an average molecular mass of 200 Da although there are a few exceptions in which only a combination of K(+) and molecular crowding induced a G-quadruplex structure of an extremely G-rich RNA sequence. In contrast, structural polymorphisms involving duplexes, G-quadruplexes, and i-motifs were observed for GC-rich DNA sequences depending on the surrounding factors. These results demonstrate the considerable structural and functional differences in GC-rich sequences of the genome (DNA) and transcriptosome (mRNA) with respect to the nucleic acid backbone. Moreover, it was suggested that structural study for a G-rich RNA sequence should be carried out under cell-mimicking condition where K(+) and crowding cosolutes exist.

Monomorphic RNA G-quadruplex and polymorphic DNA G-quadruplex structures responding to cellular environmental factors

We systematically and quantitatively investigated the structure and thermodynamics of G-quadruplexes of RNAs and corresponding DNAs of the same sequences under molecular crowding conditions that mimic the high osmotic stress induced by the numerous molecules inside of living cells. Structural analyses demonstrated that various telomere RNA sequences folded into parallel-stranded G-quadruplexes in a manner independent of the surrounding conditions with different cations under both dilute and molecular crowding conditions. In contrast, DNA G-quadruplexes showed structural polymorphism. Moreover, we demonstrated that the G-quadruplexes of the RNA sequences were more stable than those of the same DNA sequences. These results show that a single and robust RNA G-quadruplex structure can exist in a manner independent of the sequence and surrounding conditions. To confirm this, we studied a guanine-rich sequence located in the 5'-untranslated region of human bcl-2 mRNA that is thought to play a role in translation. The results revealed a stable parallel G-quadruplex that formed under all conditions tested. For example, a bcl-RNA G-quadruplex in the presence of 5 mM KCl [free energy change at 25 degrees C (DeltaG degrees (25)) of -5.42 kcal/mol] was more stable than its corresponding DNA G-quadruplex (DeltaG degrees (25) = -2.31 kcal/mol). Our results further indicated that water molecules binding to the 2'-OH group of RNA G-quadruplexes play a critical role in their formation and stability.

Overview of formation of G-quadruplex structures

There are many structures that can be adopted by nucleic acids other than the Watson-Crick duplex. In particular, a noncanonical four-stranded topology, called a G-quadruplex, is of great interest because of its roles in key biological processes such as the maintenance of telomeres and regulation of gene transcription. This review describes the condition for forming the G-quadruplex structure, G-quadruplex-forming sequences, and methods for studying the structures.

Supramolecular structure and polymorphism of alkali metal salts of guanosine 5'-monophosphate: SEM and NMR study

Scanning electron microscopy images of the Na(+), K(+), and Rb(+) salts of guanosine 5'-monophosphate (5'-GMP) in the presence of the corresponding metal chloride have shown the formation of exceptionally large molecular aggregates. These are much larger than those previously reported in solution. Each cation system produced a solid with a different morphology. The SEM samples were prepared from concentrated aqueous (D(2)O) solutions containing various amounts of 5'-GMP and metal chloride, and they approached the limit of solubility for the 5'-GMP under these conditions. Straight or slightly curved "free standing" rods composed of bundles of parallel stacks of G-quartets were formed from solutions of 0.85-1.0 M Na(2)(5'-GMP) containing 0.25-0.50 M NaCl. The rods had varying lengths of 6000-40 000 nm and an average diameter of 2000 nm. Calculations estimate this diameter to correspond to approximately 650 parallel stacks of G-quartets. Alignment of the individual G-quartet stacks into bundles and rods occurred as a result of phosphate charge neutralization by the high concentration of Na(+) ions. The SEM image of the K(+) system showed the presence of two types of morphologies, a rodlike lattice formation interpreted to be formed of stacked G-quartets, and irregular twisting fibers of varying diameter. In conjunction with the (1)H NMR data, the latter are proposed to be composed of continuous helices of doubly hydrogen-bonded guanines having the same H-bonding motif as the planar G-quartets. The Rb(+) system had some similarities to both the Na(+) system and the K(+) system. (1)H NMR spectra were different for each cation system, corresponding to the differences observed by SEM imaging of the solids. Polymorphism has been observed in telomeric sequences but has not been extensively explored in 5'-GMP.

Structure of propeller-type parallel-stranded RNA G-quadruplexes, formed by human telomeric RNA sequences in K+ solution

Very recent studies showed that mammalian telomeres were transcribed into telomeric-repeat-containing RNAs and suggested that these RNA molecules were biologically important. Here we report on a structural study of RNA G-quadruplexes formed by human telomeric RNA sequences in K(+) solution. Our data indicated that these sequences formed propeller-type parallel-stranded RNA G-quadruplexes. We have determined the NMR-based solution structure of a dimeric propeller-type RNA G-quadruplex formed by the 12-nt human telomeric RNA sequence r(UAGGGUUAGGGU). We also observed the stacking of two such propeller-type G-quadruplex blocks for the 10-nt human telomeric RNA sequence r(GGGUUAGGGU) and a higher-order G-quadruplex structure for the 9-nt human telomeric RNA sequence r(GGGUUAGGG). Based on these findings we proposed how higher-order structures might be formed by long telomeric RNA.

Structure of long telomeric RNA transcripts: the G-rich RNA forms a compact repeating structure containing G-quartets

Recent studies have identified RNA transcripts arising from mammalian telomeres with the transcript of the C-rich strand (r(5'-UUAGGG-3')(n)) being much more abundant than transcripts of the G-rich strand. Here we used transmission electron microscopy, CD, and nuclease digestion to investigate the structure of approximately 640-nucleotide (nt) RNA transcripts of the C-rich and G-rich strands of mammalian telomeric DNA. The CD spectrum of the C-rich RNA in low salt (10 mm KCl) or high salt (100 mm KCl) was typical of mixed sequence RNA, whereas the CD spectrum for the G-rich RNA differed with changes characteristic of parallel G-quadruplexes at the higher salt concentration. Electron microscopy visualization of the C-rich RNA revealed relatively extended unstructured molecules 59.7 +/- 17.8 nm in length and with a width consistent with single-stranded RNA following metal coating. In contrast, the G-rich RNA was observed as round particles and short, thick rods with the rods being most prevalent in high salt conditions and absent in low salt. The rods were 22.7 +/- 4.8 nm in length and 7.6 nm in width. Digestion of the G-rich RNA with T1 RNA nuclease revealed a ladder of bands whose sizes were integral multiples of 24 nt plus a 4-nt overhang. These observations suggest a model in which G-rich telomeric RNA folds into chains of particles each consisting of four (UUAGGG) repeats stabilized by parallel G-quartets and joined by UUA linkers. These chains further condense to form short rods and round particles.

Guanine quadruplex formation by RNA/DNA hybrid analogs of Oxytricha telomere G(4)T(4)G(4) fragment

Using circular dichroism spectroscopy, gel electrophoresis, and ultraviolet absorption spectroscopy, we have studied quadruplex folding of RNA/DNA analogs of the Oxytricha telomere fragment, G(4)T(4)G(4), which forms the well-known basket-type, antiparallel quadruplex. We have substituted riboguanines (g) for deoxyriboguanines (G) in the positions G1, G9, G4, and G12; these positions form the terminal tetrads of the G(4)T(4)G(4) quadruplex and adopt syn, syn, anti, and anti glycosidic geometries, respectively. We show that substitution of a single sugar was able to change the quadruplex topology. With the exception of G(4)T(4)G(3)g, which adopted an antiparallel structure, all the RNA/DNA hybrid analogs formed parallel, bimolecular quadruplexes in concentrated solution at low salt. In dilute solutions ( approximately 0.1 mM nucleoside), the RNA/DNA hybrids substituted at positions 4 or 12 adopted antiparallel quadruplexes, which were especially stable in Na(+) solutions. The hybrids substituted at positions 1 and 9 preferably formed parallel quadruplexes, which were more stable than the nonmodified G(4)T(4)G(4) quadruplex in K(+) solutions. Substitutions near the 3'end of the molecule affected folding more than substitutions near the 5'end. The ability to control quadruplex folding will allow further studies of biophysical and biological properties of the various folding topologies.

Role of loops in the guanine quadruplex formation by DNA/RNA hybrid analogs of G4T4G4

CD spectroscopy, gel electrophoresis and absorption-based thermal stability were used to analyze quadruplex formation of RNA and RNA/DNA hybrid analogs of the deoxyoligonucleotide G4T4G4, which forms a well-characterized basket-type quadruplex. All RNA-containing dodecamers, g4u4g4, G4u4G4 and g4T4g4 (RNA lower-case, DNA capital letters), formed parallel, namely tetramolecular quadruplexes in Na+-containing solutions. The u4 loop forced DNA tetrads into the same conformation as adopted by g4u4g4. In contrast, the T4 loop destabilized the RNA tetrads. Potassium ions markedly stabilized parallel quadruplexes of RNA-containing analogs as well as their bimolecular folding. In the presence of K+, g4T4g4 formed exclusively bimolecular quadruplexes of both parallel and antiparallel types as indicated by CD. Thus, the T4 loop permits RNA strands to adopt an antiparallel arrangement. These findings may be useful for engineering particular quadruplex foldings in different quadruplex-forming sequences.

G-quadruplex formation by human telomeric repeats-containing RNA in Na+ solution

For a long time, telomeres have been considered to be transcriptionally silent. Very recently, a breaking finding from two groups demonstrated that telomere DNA is transcribed into telomeric repeat-containing RNA in mammalian cells (Azzalin, C. M.; Reichenbach, P.; Khoriauli, L.; Giulotto, E.; Lingner, J. Science 2007, 318, 798-801. Schoefter, S.; Blasco, M. A. Nat. Cell Biol. 2008, 10, 228-236). The telomeric RNA, a newly appeared player in telomere biology, may be a key component of telomere machinery. In the current study, we used a combination of NMR, circular dichroism (CD), matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOFMS), and gel electrophoresis to investigate the structural features of a human telomere RNA sequence. We demonstrated that human telomere RNA can form a parallel G-quadruplex structure in the presence of Na(+). Importantly, we found for the first time that the G-quadruplex forming telomere RNA protects itself from enzymatic digestion. These results provide valuable information to allow understanding of the structure and function of human telomeric RNA.