Monomolecular tetrahelix of polyguanine with a strictly defined folding pattern

Defined folding pattern of poly(rG) supports inherent ability to encode biological information

The RNA World hypothesis posits that RNA can represent a primitive life form by reproducing itself and demonstrating catalytic activity. However, this hypothesis is incapable of addressing several major origin-of-life (OoL) questions. A recently described paradox-free alternative OoL hypothesis, the Quadruplex (G4) World, is based on the ability of poly(dG) to fold into a stable architecture with an unambiguous folding pattern using G-tetrads as building elements. Because of the folding pattern of three G-tetrads and single-G loops, dG15 is programmable and has the capability to encode biological information. Here, we address two open questions of the G4 World hypothesis: (1) Does RNA follow the same folding pattern as DNA? (2) How do stable quadruplexes evolve into the present-day system of information transfer, which is based on Watson-Crick base pair complementarity? To address these questions, we systematically studied the thermodynamic and optical properties of both DNA and RNA G15- and G3T (GGGTGGGTGGGTGGG)-derived sequences. Our study revealed that similar to DNA sequences, RNAs adopt quadruplexes with only three G-tetrads. Thus, both poly(dG) and poly(rG) possess inherent ability to fold into 3D quadruplex architecture with strictly defined folding pattern. The study also revealed that despite high stability of both DNA and RNA quadruplexes, they are vulnerable to single-nucleotide substitutions, which drop the thermal stability by ~40°C and can facilitate introduction of the complementarity principle into the G4 World.

In vitro higher-order oligomeric assembly of the respiratory syncytial virus M2-1 protein with longer RNAs

The respiratory syncytial virus (RSV) M2-1 protein is a transcriptional antitermination factor crucial for efficiently synthesizing multiple full-length viral mRNAs. During RSV infection, M2-1 exists in a complex with mRNA within cytoplasmic compartments called inclusion body-associated granules (IBAGs). Prior studies showed that M2-1 can bind along the entire length of viral mRNAs instead of just gene-end (GE) sequences, suggesting that M2-1 has more sophisticated RNA recognition and binding characteristics. Here, we analyzed the higher oligomeric complexes formed by M2-1 and RNAs in vitro using size exclusion chromatography (SEC), electrophoretic mobility shift assays (EMSA), negative stain electron microscopy (EM), and mutagenesis. We observed that the minimal RNA length for such higher oligomeric assembly is about 14 nucleotides for polyadenine sequences, and longer RNAs exhibit distinct RNA-induced binding modality to M2-1, leading to enhanced particle formation frequency and particle homogeneity as the local RNA concentration increases. We showed that particular cysteine residues of the M2-1 cysteine-cysteine-cystine-histidine (CCCH) zinc-binding motif are essential for higher oligomeric assembly. Furthermore, complexes assembled with long polyadenine sequences remain unaffected when co-incubated with ribonucleases or a zinc chelation agent. Our study provided new insights into the higher oligomeric assembly of M2-1 with longer RNA.IMPORTANCERespiratory syncytial virus (RSV) causes significant respiratory infections in infants, the elderly, and immunocompromised individuals. The virus forms specialized compartments to produce genetic material, with the M2-1 protein playing a pivotal role. M2-1 acts as an anti-terminator in viral transcription, ensuring the creation of complete viral mRNA and associating with both viral and cellular mRNA. Our research focuses on understanding M2-1's function in viral mRNA synthesis by modeling interactions in a controlled environment. This approach is crucial due to the challenges of studying these compartments in vivo. Reconstructing the system in vitro uncovers structural and biochemical aspects and reveals the potential functions of M2-1 and its homologs in related viruses. Our work may contribute to identifying targets for antiviral inhibitors and advancing RSV infection treatment.

Which came first: the chicken, the egg, or guanine?

The main goal of the origin of life (OoL) hypothesis is to reconstruct the missing link between the primordial soup and the extant biology. However, the OoL itself is just the initial part of the link representing the bootstrapping operation of Darwinian evolution. The rest of the link is the emergence of the evolution to the present day primary biological system-the ribosome-based translation apparatus. A valid hypothesis must (i) not invoke Darwinian evolution in the bootstrapping and (ii) transform the ab initio life form into the translation apparatus without violating the principle of continuity (i.e., only incremental steps without foresight). Currently, no such hypothesis exists. Here, I discuss the Quadruplex World hypothesis, which fully complies with these requirements and suggests a spontaneous emergence of the ab initio life form. The spontaneity of OoL arises from the physicochemical properties of guanine monomers in a manner of causal determinism: each step of the process (i.e., scaffolding, polymerization, and folding) is caused by the most recent past step such that in the end only the specific 3D architecture forms. The architecture (i) has a length-independent folding pattern; (ii) can play the role of the predecessor of tRNA and single-handedly conduct a primitive form of translation; and (iii) can evolve into the extant translation apparatus without any paradoxes.

Trinity of G-tetrads and origin of translation

BACKGROUND

The RNA world hypothesis cannot address most of the questions of the origin of life without violating the continuity principle (small Darwinian steps without foresight and miracles). Moreover, the RNA world is an isolated system incapable of accommodating the genetic code and evolving into extant biochemistry. All these problems are rooted in the central assumption of the hypothesis: de novo appearance of the ribozymes, production of which represents a multistep reaction requiring the complementarity principle. Thus, even the basis of the RNA world is at odds with the continuity principle-it uses foresight (multistep reaction) and a miracle (complementarity principle). Can a three-dimensional (3D) architecture, capable of molecular recognition and catalysis, be formed in a single-step reaction without the complementarity or any other preexisting rules?

HYPOTHESIS

At first glance, the above question sounds rhetoric since the complementarity principle is the essential feature of the RNA world; it turns an RNA polymer into a genetic material. Without it, the RNA world becomes as shapeless and unconvincing as other hypotheses based on the non-hereditary molecules (i.e., protein world). However, it was suggested recently that the quadruplexes could initiate life and take necessary evolutionary steps before the arrival of the complementarity rules. The hypothesis relies on the unique properties of guanines (Gs) to self-assemble into G-tetrads and efficiently polymerize without any external help or preexisting rules. Interestingly, polyG folds into an unusually stable and well-structured monomolecular architecture that uses the quadruplex domain (QD) assembly. The QD has a strictly defined zigzag-like building pattern to accommodate only three G-tetrads. Since both QD architecture and codon length are based on triplets, the inevitable question arises: are they related? Or could QD play the role of the early adapter and determine the codon length? The current paper is an attempt to answer this question.

CONCLUSION

While without translation apparatus most of the steps of the extant translation are physically impossible, the QD-mediated translation is sterically feasible and can be explained by physicochemical properties of the QD and the amino acids without violating the continuity principle. Astonishingly, the quadruplex world hypothesis can address all the shortcomings of the RNA world, including its most significant challenge-step-by-step evolution from the polymerization of the first polynucleotide to the extant biochemistry.

Structure of Tetrahelical DNA Homopolymers Supports Quadruplex World Hypothesis

We previously reported a tetrahelical monomolecular architecture of DNA, tmDNA, which employs G-quartets and an all-parallel GGGTGGGTGGGTGGG (G3T) quadruplex as the repeating unit. Based on thermodynamic and kinetic studies, we proposed that covalently joined (G3T) _n units formed an uninterrupted programmable homopolymer; however, structural evidence for the tmDNA architecture was lacking. Here, we used NMR spectroscopy of wild-type and single-inosine-substituted constructs to characterize both monomolecular (G3T)₂ and bimolecular quadruplex-Mg-coupled versions of tmDNA. The NMR results support an architecture consisting of uninterrupted stacked G-tetrads in both the monomolecular constructs and bimolecular assemblies. Taken together, these data support the formation of a stable programmable homopolymeric tmDNA architecture, which may have been a precursor to the modern-day Watson-Crick DNA duplex.

A Chimeric DNA/RNA Antiparallel Quadruplex with Improved Stability

Nucleic acid quadruplexes are proposed to play a role in the regulation of gene expression, are often present in aptamers selected for specific binding functions and have potential applications in medicine and biotechnology. Therefore, understanding their structure and thermodynamic properties and designing highly stable quadruplexes is desirable for a variety of applications. Here, we evaluate DNA→RNA substitutions in the context of a monomolecular, antiparallel quadruplex, the thrombin-binding aptamer (TBA, GGTTGGTGTGGTTGG) in the presence of either K+ or Sr2+ . TBA predominantly folds into a chair-type configuration containing two G-tetrads, with G residues in both syn and anti conformation. All chimeras with DNA→RNA substitutions (G→g) at G residues requiring the syn conformation demonstrated strong destabilization. In contrast, G→g substitutions at Gs with anti conformation increased stability without affecting the monomolecular chair-type topology. None of the DNA→RNA substitutions in loop positions affected the quadruplex topology; however, these substitutions varied widely in their stabilizing or destabilizing effects in an unpredictable manner. This analysis allowed us to design a chimeric DNA/RNA TBA construct that demonstrated substantially improved stability relative to the all-DNA construct. These results have implications for a variety of quadruplex-based applications including for the design of dynamic nanomachines.

Quadruplex World

The RNA world hypothesis relies on the double-helix complementarity principle for both replication and catalytic activity of RNA. However, the de novo appearance of the complementarity rules, without previous evolution steps, is doubtful. Another major problem of the RNA world is its isolated nature, making it almost impossible to accommodate the genetic code and transform it into modern biochemistry. These and many other unanswered questions of the RNA world led to suggestions that some simpler molecules must have preceded RNA. Most of these alternative hypotheses proposed the double-helical polymers with different backbones but used the same complementarity principle. The current paper describes a fundamentally different idea: the de novo appearance of a nucleic acid polymer without any preexisting rules or requirements. This approach, coined as the quadruplex world hypothesis, is based on (i) the ability of guanines to form stable G-tetrads that facilitate polymerization; and (ii) the unique property of polyguanines to fold into a monomolecular tetrahelix with a strictly defined building pattern and tertiary structure. The tetrahelix is capable of high-affinity intermolecular interactions and catalytic activities. The quadruplex world hypothesis has the potential to address almost all the shortcomings of the RNA world.

HIV-1 genomic RNA U3 region forms a stable quadruplex-hairpin structure

The U3 promoter region of the HIV-1 long terminal repeat (LTR) has previously been shown to fold into a series of dynamic G-quadruplex structures. Among the G-quadruplexes identified in the LTR sequence, LTR-III was shown to be the most stable in vitro. NMR studies of this 28-nucleotide (nt) DNA revealed a unique quadruplex-hairpin structure. Whether the hairpin forms in RNA element is unknown and the role of the hairpin in the structure and stability of quadruplexes has not been characterized. Here, we used optical and thermodynamic studies to address these questions. The wild-type LTR-III RNA formed a monomolecular quadruplex with a parallel topology using only propeller loops, including the hairpin loop element. By comparison to the WT and variant RNAs, LTR-III DNA structures were more heterogeneous and less stable. Increased stability of the RNA suggests that the RNA quadruplex-hairpin structure may be a more attractive therapeutic target than the analogous DNA element.

Quadruplex-Templated and Catalyzed Ligation of Nucleic Acids

Template-guided chemical reactions between nucleic acid strands are an important process in biomedical research. However, almost all of these reactions employ an oligonucleotide-templated approach that is based on the double-helix alignment. The moderate stability of the double helix makes this approach unsuitable for many chemical reactions, so alternative nucleic acid alignment mechanisms, demonstrating higher thermal and chemical stability, are desirable. Earlier, we described a noncovalent coupling mechanism between DNA strands through a quadruplex-and-Mg²⁺ connection (QMC). QMC is based on G-quadruplexes and allows unusually stable and specific interactions. Herein, a novel catalytic nucleic acid reaction, based on QMC, is described. This approach uses G-tetrads as a structural and recognition element without employing Watson-Crick complementarity rules at any stage of substrate/catalyst formation or interaction between them. Quadruplex-templated ligation can be achieved through the self-ligation of two nucleic acid strands, or through a quadruplex catalyst, which forms a G-triplex and specifically connects the strands. The process is extraordinarily robust and efficient. For instance, the ligation of carbodiimide-activated substrates can proceed in boiling solutions, and complete ligation is demonstrated within a minute. The quadruplex-templated and catalyzed reactions will create new opportunities for chemical reactions requiring harsh experimental conditions.

Duplex-tetraplex equilibria in guanine- and cytosine-rich DNA

Noncanonical four-stranded DNA structures, including G-quadruplexes and i-motifs, have been discovered in the cell and are implicated in a variety of genomic regulatory functions. The tendency of a specific guanine- and cytosine-rich region of genomic DNA to adopt a four-stranded conformation depends on its ability to overcome the constraints of duplex base-pairing by undergoing consecutive duplex-to-coil and coil-to-tetraplex transitions. The latter ability is determined by the balance between the free energies of participating ordered and disordered structures. In this review, we present an overview of the literature on the stability of G-quadruplex and i-motif structures and discuss the extent of duplex-tetraplex competition as a function of the sequence context of the DNA and environmental conditions including temperature, pH, salt, molecular crowding, and the presence of G-quadruplex-binding ligands. We outline how the results of in vitro studies can be expanded to understanding duplex-tetraplex equilibria in vivo.

Structure and Function of Multimeric G-Quadruplexes

G-quadruplexes are noncanonical nucleic acid structures formed from stacked guanine tetrads. They are frequently used as building blocks and functional elements in fields such as synthetic biology and also thought to play widespread biological roles. G-quadruplexes are often studied as monomers, but can also form a variety of higher-order structures. This increases the structural and functional diversity of G-quadruplexes, and recent evidence suggests that it could also be biologically important. In this review, we describe the types of multimeric topologies adopted by G-quadruplexes and highlight what is known about their sequence requirements. We also summarize the limited information available about potential biological roles of multimeric G-quadruplexes and suggest new approaches that could facilitate future studies of these structures.

Label-Free Direct Detection of Saxitoxin Based on a Localized Surface Plasmon Resonance Aptasensor

Seafood is an emerging health food, and interest in improving the quality of seafood is increasing. Saxitoxin (STX) is a neurotoxin produced by marine dinoflagellates that is accumulated in seafood. It can block the neuronal transmission between nerves and muscle cell membranes, resulting in the disturbance of neuromuscular transmission and subsequent voluntary muscle paralysis. Here, we developed a new aptamer for the detection of STX using graphene oxide–systematic evolution of ligands by exponential enrichment (GO-SELEX). Furthermore, we confirmed sensitivity and selectivity of the developed aptamer specific to STX using a localized surface plasmon resonance (LSPR) sensor. The sensing chip was fabricated by fixing the new STX aptamer immobilized on the gold nanorod (GNR) substrate. The STX LSPR aptasensor showed a broad, linear detection range from 5 to 10,000 μg/L, with a limit of detection (LOD) of 2.46 μg/L (3σ). Moreover, it was suitable for the detection of STX (10, 100, and 2000 μg/L) in spiked mussel samples and showed a good recovery rate (96.13–116.05%). The results demonstrated that the new STX aptamer-modified GNR chip was sufficiently sensitive and selective to detect STX and can be applied to real samples as well. This LSPR aptasensor is a simple, label-free, cost-effective sensing system with a wide detectable range.

Stability Factors of the Parallel Quadruplexes: DNA Versus RNA

One of the most stable quadruplexes is formed by the G3T sequence (GGGTGGGTGGGTGGG) that folds into a parallel quadruplex with three G-tetrads and chain-reversal T-loops. For example, in 1 mM K⁺, it unfolds at 75 °C and at physiological conditions, it unfolds above 100 °C. The RNA analogue, ggguggguggguggg (g3u), which employs exactly same folding topology, demonstrates even higher thermal stability. Here, we performed melting experiments of G3T, g3u, and more than 30 chimeric constructs (G3T with RNA nucleotides at certain positions). Although the g3u quadruplex is 13 °C more stable than G3T, majority of G → g (DNA-for-RNA) substitutions destabilize G3T. Only three G → g and loop T → u substitutions stabilize the structure. However, stabilization effects of these six substitutions overcome destabilization of other nine G → g, resulting in higher stability of all-RNA g3u. The present work clearly indicates that the stacking interactions are more favorable in parallel DNA quadruplexes, whereas the chain-reversal loops play an important role in higher stability of RNA quadruplexes. In addition, we have shown that the 5'-end of RNA quadruplexes represents a more favorable target for stacking interactions than the 3'-end. Based on the current study, rational design of the quadruplexes for particular biotechnological applications and drugs, targeting the quadruplexes, may be envisaged.

DNA Quadruple Helices in Nanotechnology

DNA has played an early and powerful role in the development of bottom-up nanotechnologies, not least because of DNA's precise, predictable, and controllable properties of assembly on the nanometer scale. Watson-Crick complementarity has been used to build complex 2D and 3D architectures and design a number of nanometer-scale systems for molecular computing, transport, motors, and biosensing applications. Most of such devices are built with classical B-DNA helices and involve classical A-T/U and G-C base pairs. However, in addition to the above components underlying the iconic double helix, a number of alternative pairing schemes of nucleobases are known. This review focuses on two of these noncanonical classes of DNA helices: G-quadruplexes and the i-motif. The unique properties of these two classes of DNA helix have been utilized toward some remarkable constructions and applications: G-wires; nanostructures such as DNA origami; reconfigurable structures and nanodevices; the formation and utilization of hemin-utilizing DNAzymes, capable of generating varied outputs from biosensing nanostructures; composite nanostructures made up of DNA as well as inorganic materials; and the construction of nanocarriers that show promise for the therapeutics of diseases.