Long RNA dangling end has large energetic contribution to duplex stability

Enhancement and inactivation effect of CRISPR/Cas12a via extending hairpin activators for detection of transcription factors

An enhancement effect for the activation of CRISPR/Cas12a (CRISPR = clustered regularly interspaced short palindromic repeats; Cas = CRISPR-associated) was discovered. That was, a hairpin model with dangling 5' end complementary to crRNA (CRISPR RNA) greatly improved the activity of CRISPR/Cas12a after extention of two random sequences. But, the corresponding intact hairpin without PAM (protospacer adjacent motif) or suboptimal PAM sequences was completely inactive to CRISPR/Cas12a because of the superhigh stability of intact hairpin. According to the finding, a CRISPR/Cas12a-based strategy coupled with a signal reported system was designed for transcription factors detection. By using mono-labeled ssDNA (single-stranded DNA) as reporter and two newly synthesized N-C (nitrogen-doped carbon) nanosheets as scavenger to eliminate the fluorescent background, the strategy realized the detection of NF-ĸB p50 (p50 subunit of nuclear factor kappa-B) with a linear detection range of 0.8 - 2000.0 pM and a LOD of 0.5 pM. The discovery of "enhancement and inactivation effect" not only deepened insight into CRISPR/Cas12a but also broadened the practical application of CRISPR/Cas systems for the molecular detection and disease diagnostics.

Split-tracrRNA as an efficient tracrRNA system with an improved potential of scalability

Due to the relatively long sequence, tracrRNAs are chemically less synthesizable than crRNAs, leading to limited scalability of RNA guides for CRISPR-Cas9 systems. To develop shortened versions of RNA guides with improved cost-effectiveness, we have developed a split-tracrRNA system by nicking the 67-mer tracrRNA (tracrRNA(67)). Cellular gene editing assays and in vitro DNA cleavage assays revealed that the position of the nick is critical for maintaining the activity of tracrRNA(67). TracrRNA(41 + 23), produced by nicking in stem loop 2, showed gene editing efficiency and specificity comparable to those of tracrRNA(67). Removal of the loop of stem loop 2 was further possible without compromising the efficiency and specificity when the stem duplex was stabilized via a high GC content. Binding assays and single-molecule experiments suggested that efficient split-tracrRNAs could be engineered as long as their binding affinity to Cas9 and their reaction kinetics are similar to those of tracrRNA(67).

"RESET" Effect: Random Extending Sequences Enhance the Trans-Cleavage Activity of CRISPR/Cas12a

The trans-cleavage activity of CRISPR/Cas12a has been widely used in biosensing applications. However, the lack of exploration on the fundamental properties of CRISPR/Cas12a not only discourages further in-depth studies of the CRISPR/Cas12a system but also limits the design space of CRISPR/Cas12a-based applications. Herein, a "RESET" effect (random extending sequences enhance trans-cleavage activity) is discovered for the activation of CRISPR/Cas12a trans-cleavage activity. That is, a single-stranded DNA, which is too short to work as the activator, can efficiently activate CRISPR/Cas12a after being extended a random sequence from its 3'-end, even when the random sequence folds into secondary structures. The finding of the "RESET" effect enriches the CRISPR/Cas12a-based sensing strategies. Based on this effect, two CRISPR/Cas12a-based biosensors are designed for the sensitive and specific detection of two biologically important enzymes.

Best practice for improved accuracy: a critical reassessment of van't Hoff analysis of melt curves

Biomolecular thermodynamics, particularly for DNA, are frequently determined via van't Hoff analysis of optically-measured melt curves. Accurate and precise values of thermodynamic parameters are essential for the modelling of complex systems involving cooperative effects, such as RNA tertiary structure and DNA origami because the uncertainties associated with each motif in a folding energy landscape can compound, significantly reducing the power of predictive models. We follow the sources of uncertainty as they propagate through a typical van't Hoff analysis to derive best practices for melt experiments and subsequent data analysis, assuming perfect signal baseline correction. With appropriately designed experiments and analysis, a van't Hoff approach can provide surprisingly high precision, e.g., enthalpies may be determined with a precision as low as a 10^-2 kJ∙mol^-1 for an 8 base DNA oligomer.

Plausible Minimal Substrate for Erm Protein

Erm proteins methylate a specific adenine residue (A2058, Escherichia coli coordinates) conferring macrolide-lincosamide-streptogramin B (MLSB) antibiotic resistance on a variety of microorganisms, ranging from antibiotic producers to pathogens. To identify the minimal motif required to be recognized and methylated by the Erm protein, various RNA substrates from 23S rRNA were constructed, and the substrate activity of these constructs was studied using three Erm proteins, namely, ErmB from Firmicutes and ErmE and ErmS from Actinobacteria The shortest motif of 15 nucleotides (nt) could be recognized and methylated by ErmS, consisting of A2051 to the methylatable adenine (A2058) and its base-pairing counterpart strand, presumably assuming a quite similar structure to that in 23S rRNA, an unpaired target adenine immediately followed by an irregular double-stranded RNA region. This observation confirms the ultimate end of each side in helix 73 for methylation, determined by the approaches described above, and could reveal the mechanism behind the binding, recognition, induced fit, methylation, and conformational change for product release in the minimal context of substrate, presumably with the help of structural determination of the protein-RNA complex. In the course of determining the minimal portion of substrate from domain V, protein-specific features could be observed among the Erm proteins in terms of the methylation of RNA substrate and cooperativity and/or allostery between the region in helix 73 furthest away from the target adenine and the large portion of domain V above the methylatable adenine.

New Modified Deoxythymine with Dibranched Tetraethylene Glycol Stabilizes G-Quadruplex Structures

Methods for stabilizing G-quadruplex formation is a promising therapeutic approach for cancer treatment and other biomedical applications because stable G-quadruplexes efficiently inhibit biological reactions. Oligo and polyethylene glycols are promising biocompatible compounds, and we have shown that linear oligoethylene glycols can stabilize G-quadruplexes. Here, we developed a new modified deoxythymine with dibranched or tribranched tetraethylene glycol (TEG) and incorporated these TEG-modified deoxythymines into a loop region that forms an antiparallel G-quadruplex. We analyzed the stability of the modified G-quadruplexes, and the results showed that the tribranched TEG destabilized G-quadruplexes through entropic contributions, likely through steric hindrance. Interestingly, the dibranched TEG modification increased G-quadruplex stability relative to the unmodified DNA structures due to favorable enthalpic contributions. Molecular dynamics calculations suggested that dibranched TEG interacts with the G-quadruplex through hydrogen bonding and CH-π interactions. Moreover, these branched TEG-modified deoxythymine protected the DNA oligonucleotides from degradation by various nucleases in human serum. By taking advantage of the unique interactions between DNA and branched TEG, advanced DNA materials can be developed that affect the regulation of DNA structure.

Stability prediction of canonical and non-canonical structures of nucleic acids in various molecular environments and cells

This review provides the biophysicochemical background and recent advances in stability prediction of canonical and non-canonical structures of nucleic acids in various molecular environments and cells.

A Temporal Order in 5'- and 3'- Processing of Eukaryotic tRNAHis

For flawless translation of mRNA sequence into protein, tRNAs must undergo a series of essential maturation steps to be properly recognized and aminoacylated by aminoacyl-tRNA synthetase, and subsequently utilized by the ribosome. While all tRNAs carry a 3'-terminal CCA sequence that includes the site of aminoacylation, the additional 5'-G-1 position is a unique feature of most histidine tRNA species, serving as an identity element for the corresponding synthetase. In eukaryotes including yeast, both 3'-CCA and 5'-G-1 are added post-transcriptionally by tRNA nucleotidyltransferase and tRNA^His guanylyltransferase, respectively. Hence, it is possible that these two cytosolic enzymes compete for the same tRNA. Here, we investigate substrate preferences associated with CCA and G-1-addition to yeast cytosolic tRNA^His, which might result in a temporal order to these important processing events. We show that tRNA nucleotidyltransferase accepts tRNA^His transcripts independent of the presence of G-1; however, tRNA^His guanylyltransferase clearly prefers a substrate carrying a CCA terminus. Although many tRNA maturation steps can occur in a rather random order, our data demonstrate a likely pathway where CCA-addition precedes G-1 incorporation in S. cerevisiae. Evidently, the 3'-CCA triplet and a discriminator position A73 act as positive elements for G-1 incorporation, ensuring the fidelity of G-1 addition.

Contribution of 3'T and 3'TT overhangs to the thermodynamic stability of model siRNA duplexes

Herein, we report comprehensive thermodynamic studies on 36 RNA/DNA duplexes designed as siRNA mimics to determine the energetic contribution of 3'T and 3'TT dangling ends. The thermodynamic effect induced by the presence of 3'T overhangs on the stability of RNA duplexes ranges from -0.28 to -0.92 kcal/mol and strongly depends on the type and orientation of the adjacent base pair. Further extension of the 3'-dangling end length, by a second T residue, results in additional stabilization of 0.14 to 0.21 kcal/mol. The results revealed that the thermodynamic contribution of 3'-dangling T and TT on RNA duplexes differs from the influence of 3'-dangling U and UU on RNA duplexes and 3'-dangling T and TT on DNA duplexes. This data suggests that using the contribution of 3'-dangling T values for RNA duplexes, instead of 3'-dangling T values for DNA duplexes or 3'-dangling U values for RNA duplexes, would improve the prediction of the stability of siRNA duplexes.

Effects of trimethylamine N-oxide and urea on DNA duplex and G-quadruplex

We systematically investigated effects of molecular crowding with trimethylamine N-oxide (TMAO) as a zwitterionic and protective osmolyte and urea as a nonionic denaturing osmolyte on conformation and thermodynamics of the canonical DNA duplex and the non-canonical DNA G-quadruplex. It was found that TMAO and urea stabilized and destabilized, respectively, the G-quadruplex. On the other hand, these osmolytes generally destabilize the duplex; however, it was observed that osmolytes having the trimethylamine group stabilized the duplex at the lower concentrations because of a direct binding to a groove of the duplex. These results are useful not only to predict DNA structures and their thermodynamics under physiological environments in living cells, but also design of polymers and materials to regulate structure and stability of DNA sequences.

Stabilization of duplex DNA and RNA by dangling ends studied by free energy simulations

Single unpaired nucleotides at the end of double-stranded nucleic acids, termed dangling ends, can contribute to duplex stability. Umbrella sampling free energy simulations of dangling cytosine and guanine nucleotides at the end of duplex and single stranded RNA and DNA molecules have been used to investigate the molecular origin of dangling end effects. In unrestraint simulations, the dangling end nucleotides stayed close to placements observed in experimental structures. Calculated free energy contributions associated with the presence of dangling nucleotides were in reasonable agreement with experiment predicting the general trend of a more stabilizing effect of purine vs. pyrimidine dangling ends. In addition, the calculations indicate a more significant stabilizing effect of dangling ends at the 5'-end vs. 3'-end in case of DNA and the opposite trend in case of RNA. Both electrostatic and van der Waals interactions contribute to the duplex stabilizing effect of dangling end nucleotides. The free energy simulation scheme could also be used to design dangling end nucleotides that result in enhanced duplex stabilization.

Theory and simulation of DNA-coated colloids: a guide for rational design

By exploiting the exquisite selectivity of DNA hybridization, DNA-coated colloids (DNACCs) can be made to self-assemble in a wide variety of structures. The beauty of this system stems largely from its exceptional versatility and from the fact that a proper choice of the grafted DNA sequences yields fine control over the colloidal interactions. Theory and simulations have an important role to play in the optimal design of self assembling DNACCs. At present, the powerful model-based design tools are not widely used, because the theoretical literature is fragmented and the connection between different theories is often not evident. In this Perspective, we aim to discuss the similarities and differences between the different models that have been described in the literature, their underlying assumptions, their strengths and their weaknesses. Using the tools described in the present Review, it should be possible to move towards a more rational design of novel self-assembling structures of DNACCs and, more generally, of systems where ligand-receptor are used to control interactions.

Native characterization of nucleic acid motif thermodynamics via non-covalent catalysis

DNA hybridization thermodynamics is critical for accurate design of oligonucleotides for biotechnology and nanotechnology applications, but parameters currently in use are inaccurately extrapolated based on limited quantitative understanding of thermal behaviours. Here, we present a method to measure the ΔG° of DNA motifs at temperatures and buffer conditions of interest, with significantly better accuracy (6- to 14-fold lower s.e.) than prior methods. The equilibrium constant of a reaction with thermodynamics closely approximating that of a desired motif is numerically calculated from directly observed reactant and product equilibrium concentrations; a DNA catalyst is designed to accelerate equilibration. We measured the ΔG° of terminal fluorophores, single-nucleotide dangles and multinucleotide dangles, in temperatures ranging from 10 to 45 °C.

DNA hybridisation thermodynamics parameters underlie rational design of oligonucleotides for diagnostics and nanotechnology. Here, the authors present an accurate method to measure the free energy of a given DNA structure at specific temperature and buffer conditions.

Microarrays for identifying binding sites and probing structure of RNAs

Oligonucleotide microarrays are widely used in various biological studies. In this review, application of oligonucleotide microarrays for identifying binding sites and probing structure of RNAs is described. Deep sequencing allows fast determination of DNA and RNA sequence. High-throughput methods for determination of secondary structures of RNAs have also been developed. Those methods, however, do not reveal binding sites for oligonucleotides. In contrast, microarrays directly determine binding sites while also providing structural insights. Microarray mapping can be used over a wide range of experimental conditions, including temperature, pH, various cations at different concentrations and the presence of other molecules. Moreover, it is possible to make universal microarrays suitable for investigations of many different RNAs, and readout of results is rapid. Thus, microarrays are used to provide insight into oligonucleotide sequences potentially able to interfere with biological function. Better understanding of structure-function relationships of RNA can be facilitated by using microarrays to find RNA regions capable to bind oligonucleotides. That information is extremely important to design optimal sequences for antisense oligonucleotides and siRNA because both bind to single-stranded regions of target RNAs.

Dangling ends perturb the stability of RNA duplexes responsive to surrounding conditions

Unpaired terminal nucleotides (dangling ends) occur in various biologically important RNA structures. We studied the thermal stability of RNA duplexes with dangling ends under conditions that mimic those in cells. Dangling ends of one or two nucleotides stabilized a duplex up to approximately 2.7 kcal mol(-1) in the absence of cosolutes. RNA duplexes with dangling purine nucleotides were more stable than those with pyrimidine nucleotides. Interestingly, in the presence of various cosolutes, RNA duplexes with purine dangling ends were significantly destabilized, although those with pyrimidine dangling ends were destabilized slightly. For example, in 30 wt % poly(ethylene glycol), stabilization resulting from adenine dangling ends was reduced by 1.4 kcal mol(-1) . Our quantitative analyses also showed that the number of water molecules bound to the dangling ends in an aqueous solution was independent of the nucleotide type but dependent on the stability of the dangling-end region. It has been considered that dangling ends stabilize helices; however, our results suggest that the stabilization is responsive to the surrounding conditions.

Effects of sugar functional groups, hydrophobicity, and fluorination on carbohydrate-DNA stacking interactions in water

Carbohydrate-aromatic interactions are highly relevant for many biological processes. Nevertheless, experimental data in aqueous solution relating structure and energetics for sugar-arene stacking interactions are very scarce. Here, we evaluate how structural variations in a monosaccharide including carboxyl, N-acetyl, fluorine, and methyl groups affect stacking interactions with aromatic DNA bases. We find small differences on stacking interaction among the natural carbohydrates examined. The presence of fluorine atoms within the pyranose ring slightly increases the interaction with the C-G DNA base pair. Carbohydrate hydrophobicity is the most determinant factor. However, gradual increase in hydrophobicity of the carbohydrate does not translate directly into a steady growth in stacking interaction. The energetics correlates better with the amount of apolar surface buried upon sugar stacking on top of the aromatic DNA base pair.

Effects of salt, polyethylene glycol, and locked nucleic acids on the thermodynamic stabilities of consecutive terminal adenosine mismatches in RNA duplexes

Consecutive terminal mismatches add thermodynamic stability to RNA duplexes and occur frequently in microRNA-mRNA interactions. Accurate thermodynamic stabilities of consecutive terminal mismatches contribute to the development of specific, high-affinity siRNA therapeutics. Consecutive terminal adenosine mismatches (TAMS) are studied at different salt concentrations, with polyethylene glycol cosolutes, and with locked nucleic acid (LNA) substitutions. These measurements provide benchmarks for the application of thermodynamic predictions to different physiological or therapeutic conditions. The salt dependence for RNA duplex stability is similar for TAMS, internal loops, and Watson-Crick duplexes. A unified model for predicting the free energy of an RNA duplex with or without loops and mismatches at lower sodium concentrations is presented. The destabilizing effects of PEG 200 are larger for TAMS than internal loops or Watson-Crick duplexes, which may result from different base stacking conformations, dynamics, and water hydration. In contrast, LNA substitutions stabilize internal loops much more than TAMS. Surprisingly, the average per adenosine increase in stability for LNA substitutions in internal loops is -1.82 kcal/mol and only -0.20 kcal/mol for TAMS. The stabilities of TAMS and internal loops with LNA substitutions have similar favorable free energies. Thus, the unfavorable free energy of adenosine internal loops is largely an entropic effect. The favorable stabilities of TAMS result mainly from base stacking. The ability of RNA duplexes to form extended terminal mismatches in the absence of proteins such as argonaute and identifying the enthalpic contributions to terminal mismatch stabilities provide insight into the physical basis of microRNA-mRNA molecular recognition and specificity.

Lost in folding space? Comparing four variants of the thermodynamic model for RNA secondary structure prediction

BACKGROUND

Many bioinformatics tools for RNA secondary structure analysis are based on a thermodynamic model of RNA folding. They predict a single, "optimal" structure by free energy minimization, they enumerate near-optimal structures, they compute base pair probabilities and dot plots, representative structures of different abstract shapes, or Boltzmann probabilities of structures and shapes. Although all programs refer to the same physical model, they implement it with considerable variation for different tasks, and little is known about the effects of heuristic assumptions and model simplifications used by the programs on the outcome of the analysis.

RESULTS

We extract four different models of the thermodynamic folding space which underlie the programs RNAFOLD, RNASHAPES, and RNASUBOPT. Their differences lie within the details of the energy model and the granularity of the folding space. We implement probabilistic shape analysis for all models, and introduce the shape probability shift as a robust measure of model similarity. Using four data sets derived from experimentally solved structures, we provide a quantitative evaluation of the model differences.

CONCLUSIONS

We find that search space granularity affects the computed shape probabilities less than the over- or underapproximation of free energy by a simplified energy model. Still, the approximations perform similar enough to implementations of the full model to justify their continued use in settings where computational constraints call for simpler algorithms. On the side, we observe that the rarely used level 2 shapes, which predict the complete arrangement of helices, multiloops, internal loops and bulges, include the "true" shape in a rather small number of predicted high probability shapes. This calls for an investigation of new strategies to extract high probability members from the (very large) level 2 shape space of an RNA sequence. We provide implementations of all four models, written in a declarative style that makes them easy to be modified. Based on our study, future work on thermodynamic RNA folding may make a choice of model based on our empirical data. It can take our implementations as a starting point for further program development.

A 5'-cap for DNA probes binding RNA target strands

Detecting short RNA strands with high fidelity at any of the bases of their sequence, including the termini, can be challenging, since fraying, wobbling, and refolding all compete with canonical base pairing. We performed a search for 5'-substituents of oligodeoxynucleotides that increase base pairing fidelity at the terminus of duplexes with RNA target strands. From a total of over 70 caps, differing in stacking moiety and linker, a phosphodiester-linked sequence of the residues of L-prolinol, glycine, and oxolinic acid, dubbed ogOA, was identified as a 5'-cap that stabilizes any of the four canonical base pairs, with ΔT(m) values of up to +13.1 °C for an octamer. At the same time, the cap increases discrimination against any of the 12 possible terminal mismatches, including mismatches that are more stable than their perfectly matched counterparts in the control duplex, such as A:A. A probe with the cap also showed increased selectivity in the detection of two closely related microRNAs, let7c and let7a, with a ΔT(m) value of 9.2 °C. Melting curves also yielded thermodynamic data that shed light on the uniformity of molecular recognition in the sequence space of DNA:DNA and DNA:RNA duplexes. Hybridization probes with fidelity-enhancing caps should find applications in the individual and parallel detection of biologically active RNA species.

Use of nucleic Acid analogs for the study of nucleic Acid interactions

Unnatural nucleosides have been explored to expand the properties and the applications of oligonucleotides. This paper briefly summarizes nucleic acid analogs in which the base is modified or replaced by an unnatural stacking group for the study of nucleic acid interactions. We also describe the nucleoside analogs of a base pair-mimic structure that we have examined. Although the base pair-mimic nucleosides possess a simplified stacking moiety of a phenyl or naphthyl group, they can be used as a structural analog of Watson-Crick base pairs. Remarkably, they can adopt two different conformations responding to their interaction energies, and one of them is the stacking conformation of the nonpolar aromatic group causing the site-selective flipping of the opposite base in a DNA double helix. The base pair-mimic nucleosides can be used to study the mechanism responsible for the base stacking and the flipping of bases out of a nucleic acid duplex.

Evidence for recycling of external guide sequences during cleavage of bipartite substrates in vitro by reconstituted archaeal RNase P

RNA-mediated RNA cleavage events are being increasingly exploited to disrupt RNA function, an important objective in post-genomic biology. RNase P, a ribonucleoprotein enzyme that catalyzes the removal of 5'-leaders from precursor tRNAs, has previously been utilized for sequence-specific cleavage of cellular RNAs. In one of these strategies, borne out in bacterial and mammalian cell culture, an external guide sequence (EGS) RNA base-paired to a target RNA makes the latter a substrate for endogenous RNase P by rendering the bipartite target RNA-EGS complex a precursor tRNA structural mimic. In this study, we first obtained evidence that four different mesophilic and thermophilic archaeal RNase P holoenzymes, reconstituted in vitro using their respective constituent RNA and protein subunits, recognize and cleave such substrate-EGS complexes. We further demonstrate that these EGSs engage in multiple rounds of substrate recognition while assisting archaeal RNase P-mediated cleavage of a target RNA in vitro. Taken together, the EGS-based approach merits consideration as a gene knockdown tool in archaea.

Highly polar carbohydrates stack onto DNA duplexes via CH/π interactions

Carbohydrate-nucleic acid contacts are known to be a fundamental part of some drug-DNA recognition processes. Most of these interactions occur through the minor groove of DNA, such as in the calicheamicin or anthracycline families, or through both minor and major groove binders such as in the pluramycins. Here, we demonstrate that carbohydrate-DNA interactions are also possible through sugar capping of a DNA double helix. Highly polar mono- and disaccharides are capable of CH/π stacking onto the terminal DNA base pair of a duplex as shown by NMR spectroscopy. The energetics of the carbohydrate-DNA interactions vary depending on the stereochemistry, polarity, and contact surface of the sugar involved and also on the terminal base pair. These results reveal carbohydrate-DNA base stacking as a potential recognition motif to be used in drug design, supramolecular chemistry, or biobased nanomaterials.

Fluorescence competition and optical melting measurements of RNA three-way multibranch loops provide a revised model for thermodynamic parameters

Three-way multibranch loops (junctions) are common in RNA secondary structures. Computer algorithms such as RNAstructure and MFOLD do not consider the identity of unpaired nucleotides in multibranch loops when predicting secondary structure. There is limited experimental data, however, to parametrize this aspect of these algorithms. In this study, UV optical melting and a fluorescence competition assay are used to measure stabilities of multibranch loops containing up to five unpaired adenosines or uridines or a loop E motif. These results provide a test of our understanding of the factors affecting multibranch loop stability and provide revised parameters for predicting stability. The results should help to improve predictions of RNA secondary structure.

Direct measurement of the rates and barriers on forward and reverse diffusions of intramolecular collision in overhang oligonucleotides

The dynamics of end-to-interior (Type I) and end-to-end (Type II) collisions in a dangling overhang anchored on a double-stranded DNA (dsDNA) was studied by monitoring the fluorescence quenching of tetramethylrhodamine (TMR) by guanosine residues through combining photoinduced electron transfer (PET) with fluorescence correlation spectroscopy (FCS) at different temperatures. TMR and guanosine residues are separated by a double helix with dangling bases ranging from 2 to 16. By analyzing the FCS data, we obtained the forward and reverse intrachain diffusion rate constants and respective barriers. For both Type I and Type II collisions, the intrachain diffusion rates followed the scaling law of the Gaussian chain model. Especially, the reverse intrachain diffusion rate was insensitive to the base length of separation. Both the activation enthalpy and activation entropy of the forward and reverse diffusions were length independent. The comparison between Type I and Type II collisions shows that the collision rate of end-to-interior is slower than that of end-to-end. The phenomenon is further checked in detail by a series of dangling DNA with the same separation length but different tail lengths (Type III).

Stacking interaction in the middle and at the end of a DNA helix studied with non-natural nucleotides

Base stacking is important for the base pair interaction of a DNA duplex, DNA replication by polymerases, and single-stranded nucleotide overhangs. To study the mechanisms responsible for DNA stacking interactions, we measured the thermal stability of DNA duplexes containing a non-natural nucleotide tethered to a simple aromatic hydrocarbon group devoid of dipole moments and hydrogen bonding sites. The duplexes containing tetrahydrofuran were paired with a deoxyadenosine derivative (A/T base pair analog) or a deoxycytidine derivative (C/G base pair analog) and showed a lower stability than Watson-Crick base pairing, partly due to the loss of interbase hydrogen bonds. Conversely, non-natural nucleotides present at a dangling end yielded an interaction energy as high as that observed with base pairing. Importantly, the non-natural nucleotides yielded an interaction energy with a linear correlation similar to that of the analogous Watson-Crick base pairs both in the middle and at the end of a DNA duplex, although a different stacking mechanism between the middle and the end was suggested. Moreover, a positive cooperativity was observed in dangling end stacking of the nucleotide base moiety and aromatic hydrocarbon group. These observations are useful to understand nucleic acid interactions and to design new non-natural nucleotides.

Prediction of RNA multiloop and pseudoknot conformations from a lattice-based, coarse-grain tertiary structure model

We present a semiquantitative lattice model of RNA folding, which is able to reproduce complex folded structures such as multiloops and pseudoknots without relying on the frequently employed ad hoc generalization of the Jacobson-Stockmayer loop entropy. We derive the model parameters from the Turner description of simple secondary structural elements and pay particular attention to the unification of mismatch and coaxial stacking parameters as well as of border and nonlocal loop parameters, resulting in a reduced, unified parameter set for simple loops of arbitrary type and size. For elementary structures, the predictive power of the model is comparable to the standard secondary structure approaches, from which its parameters are derived. For complex structures, our approach offers a systematic treatment of generic effects of chain connectivity as well as of excluded volume or attractive interactions between and within all elements of the secondary structure. We reproduce the native structures of tRNA multiloops and of viral frameshift signal pseudoknots.

Driving forces for nucleic acid pK(a) shifting in an A(+).C wobble: effects of helix position, temperature, and ionic strength

Secondary structure plays critical roles in nucleic acid function. Mismatches in DNA can lead to mutation and disease, and some mismatches involve a protonated base. Among protonated mismatches, A(+).C wobble pairs form near physiological pH and have relatively minor effects on helix geometry, making them especially important in biology. Herein, we investigate effects of helix position, temperature, and ionic strength on pK(a) shifting in A(+).C wobble pairs in DNA. We observe that pK(a) shifting is favored by internal A(+).C wobbles, which have low cooperativities of folding and make large contributions to stability, and disfavored by external A(+).C wobbles, which have high folding cooperativities but make small contributions to stability. An inverse relationship between pK(a) shifting and temperature is also found, which supports a model in which protonation is enthalpically favored overall and entropically correlated with cooperativity of folding. We also observe greater pK(a) shifts as the ionic strength decreases, consistent with anticooperativity between proton binding and counterion-condensed monovalent cation. Under the most favorable temperature and ionic strength conditions tested, a pK(a) of 8.0 is observed for the A(+).C wobble pair, which represents an especially large shift ( approximately 4.5 pK(a) units) from the unperturbed pK(a) value of adenosine. This study suggests that protonated A(+).C wobble pairs exist in DNA under biologically relevant conditions, where they can drive conformational changes and affect replication and transcription fidelity.

Binding of short oligonucleotides to RNA: studies of the binding of common RNA structural motifs to isoenergetic microarrays

Binding of short oligonucleotides to RNA is important for many biological processes. On the basis of RNAi phenomena, antisense, and ribozyme approaches, it is useful in the inhibition of biological functions. To be considered as potential therapeutics, oligonucleotides must bind strongly and selectively to a complementary fragment of target RNA. Microarray technologies also involve the binding of oligonucleotide probes to DNA or RNA. Herein, the hybridization of common structural motifs of RNA, i.e., hairpins, internal loops, bulges, 3'- and 5'-dangling ends, and pseudoknots to isoenergetic microarray probes is presented. The analysis demonstrates that microarray probes bind to bulges, internal loops, and dangling ends as expected. Probes may also bind to terminal helixes, however, possibly due to the rearrangement of base pairs. These results suggest that isoenergetic microarray mapping can provide data to facilitate and improve RNA secondary structure prediction. However, optimal results require combination with chemical and/or enzymatic mapping.

Mechanism of RNA double helix-propagation at atomic resolution

The conversion of a nucleic acid from single strands to double strands is thought to involve slow nucleation followed by fast double-strand propagation. Here, for RNA double-strand propagation, we propose an atomic resolution reaction mechanism. This mechanism, called the stack-ratchet, is based on data-mining of three-dimensional structures and on available thermodynamic information. The stack-ratchet mechanism extends and adds detail to the classic zipper model proposed by Porschke (Porschke, D. Biophysical Chemistry 1974, 2, pp. 97-101). Porschke's zipper model describes the addition of a base pair to a nucleated helix in terms of a single type of elementary reaction; a concerted process in which the two bases, one from each strand, participate in the transition state. In the stack-ratchet mechanism proposed here a net base-pairing step consists of two elementary reactions. Motions of only one strand are required to achieve a given transition state. One elementary reaction preorganizes and stacks the 3' single-strand, driven by base--base stacking interactions. A second elementary reaction stacks the 5' strand and pairs it with the preorganized 3' strand. In the stack-ratchet mechanism, a variable length 3' stack leads the single-strand/double-strand junction. The stack-ratchet mechanism is not a two-state process. A base can be (i) unstacked and unpaired, (ii) stacked and paired, or (ii) stacked and unpaired (only on the 3' strand). The data suggests that helices of DNA and of RNA do not propagate by similar mechanisms.

A CA(+) pair adjacent to a sheared GA or AA pair stabilizes size-symmetric RNA internal loops

RNA internal loops are often important sites for folding and function. Residues in internal loops can have pK_a values shifted close to neutral pH because of the local structural environment. A series of RNA internal loops were studied at different pH by UV absorbance versus temperature melting experiments and imino proton nuclear magnetic resonance (NMR). A stabilizing CA pair forms at pH 7 in the and nearest neighbors when the CA pair is the first noncanonical pair (loop-terminal pair) in 3 × 3 nucleotide and larger size-symmetric internal loops. These and nearest neighbors, with CA adjacent to a closing Watson−Crick pair, are further stabilized when the pH is lowered from 7 to 5.5. The results are consistent with a significantly larger fraction (from ∼20% at pH 7 to ∼90% at pH 5.5) of adenines being protonated at the N1 position to form stabilizing wobble CA⁺ pairs adjacent to a sheared GA or AA pair. The noncanonical pair adjacent to the GA pair in can either stabilize or destabilize the loop, consistent with the sequence-dependent thermodynamics of GA pairs. No significant pH-dependent stabilization is found for most of the other nearest neighbor combinations involving CA pairs (e.g., and ), which is consistent with the formation of various nonwobble pairs observed in different local sequence contexts in crystal and NMR structures. A revised free-energy model, including stabilization by wobble CA⁺ pairs, is derived for predicting stabilities of medium-size RNA internal loops.

3' terminal nucleotides determine thermodynamic stabilities of mismatches at the ends of RNA helices

The thermodynamic stabilities of consecutive mismatches at the ends of RNA helices are determined by the 3' terminal nucleotides. More than 40 RNA duplexes containing terminal motifs of 3 or more nucleotides were studied by optical melting experiments. Up to three noncanonical pairs of nucleotides at the end of RNA helices provide additional thermodynamic stability. 3' nucleotides contribute more stability than 5' nucleotides, and purines contribute more stability than pyrimidines. The additional stability of a second or third 3' nucleotide stacking on a purine is the same for both dangling ends and consecutive terminal mismatches. Current predictions underestimate RNA duplex stabilities with terminal motifs by 1.4 kcal/mol on average, which is an order of magnitude in a binding constant at 37 degrees C. Accurate thermodynamic parameters for these terminal motifs will contribute to improvements in RNA secondary structure predictions, identification of microRNA targets, and design of siRNA therapeutics with fewer off-target effects.

Synthesis of nuclease-resistant siRNAs possessing universal overhangs

RNA interference (RNAi) induced by small interfering RNA (siRNA) has emerged as a powerful technique for the silencing of gene expression at the post-transcriptional level. It has been shown that in the RNAi machinery, the 3'-overhang region of a guide strand (an antisense strand) of siRNA is recognized by the PAZ domain in the Argonaute protein, and the 2-nucleotide (nt) 3'-overhang is accommodated into a binding pocket composed of hydrophobic amino acids in the PAZ domain. Based on this background information, we designed and synthesized siRNAs possessing aromatic compounds at their 3'-overhang regions. It was found that the modified siRNAs possessing aromatic compounds are more potent than the siRNAs without the 3'-overhang regions. Further, the silencing activities of the modified siRNAs are almost equal to those of normal siRNAs with natural nucleosides at their 3'-overhang regions. We also found that the siRNAs possessing the aromatic compounds at their 3'-overhang region could be used to inhibit hepatitis C virus (HCV) replication. Moreover, the RNAs with aromatic groups at their 3'-ends were more resistant to nucleolytic degradation by snake venom phosphodiesterase (SVPD) (a 3'-exonuclease) than natural RNAs. The aromatic compounds described in this report do not have functional groups capable of forming hydrogen bonds with nucleobases. Therefore, we expect that they can serve as the universal overhang units that can improve the nuclease resistance of siRNAs.

A computational study of expanded heterocyclic nucleosides in DNA

The first molecular dynamics study of a series of heterospacer-expanded tricyclic bases in DNA using modified force field parameters in AMBER is detailed. The expanded purine nucleoside monomers have been designed to probe the effects of a heteroaromatic spacer ring on the structure, function, and dynamics of the DNA helix. The heterobase scaffold has been expanded with a furan, pyrrole, or thiophene spacer ring. This structural modification increases the polarizability of the bases and provides an additional hydrogen bond donor with the amine hydrogen of the pyrrole ring or hydrogen bond acceptor with the furan or thiophene ring free electron pairs. The polarizability of the expanded bases were determined by AM1 calculations and the results of the MD simulations of 20-mers predict that the modified curvature of the expanded base leads to a much larger major groove, while the effect on the minor groove is negligible. Overall, the structure resembles A-DNA. MD simulations of 10-mers suggest that the balance between base pairing vs. base stacking and intercalation can be shifted towards the latter due to the increased surface area and polarizability of the expanded bases.

Experimental measurement of carbohydrate-aromatic stacking in water by using a dangling-ended DNA model system

Protein-carbohydrate recognition is of fundamental importance for a large number of biological processes; carbohydrate-aromatic stacking is a widespread, but poorly understood, structural motif in this recognition. We describe, for the first time, the measurement of carbohydrate-aromatic interactions from their contribution to the stability of a dangling-ended DNA model system. We observe clear differences in the energetics of the interactions of several monosaccharides with a benzene moiety depending on the number of hydroxy groups, the stereochemistry, and the presence of a methyl group in the pyranose ring. A fucose-benzene pair is the most stabilizing of the studied series (-0.4 Kcal mol(-1)) and this interaction can be placed in the same range as other more studied interactions with aromatic residues of proteins, such as Phe-Phe, Phe-Met, or Phe-His. The noncovalent forces involved seem to be dispersion forces and nonconventional hydrogen bonds, whereas hydrophobic effects do not seem to drive the interaction.

Thermodynamic analysis of 5' and 3' single- and 3' double-nucleotide overhangs neighboring wobble terminal base pairs

Thermodynamic parameters are reported for duplex formation of 40 self-complementary RNA duplexes containing wobble terminal base pairs with all possible 3′ single and double-nucleotide overhangs, mimicking the structures of short interfering RNAs (siRNA) and microRNAs (miRNA). Based on nearest neighbor analysis, the addition of a single 3′ dangling nucleotide increases the stability of duplex formation up to 1 kcal/mol in a sequence-dependent manner. The addition of a second dangling nucleotide increases the stability of duplexes closed with wobble base pairs in an idiosyncratic manner. The results allow for the development of a nearest neighbor model, which improves the predication of free energy and melting temperature for duplexes closed by wobble base pairs with 3′ single or double-nucleotide overhangs. Phylogenetic analysis of naturally occurring miRNAs was performed. Selection of the effector miR strand of the mature miRNA duplex appears to be dependent on the orientation of the GU closing base pair rather than the identity of the 3′ double-nucleotide overhang. Thermodynamic parameters for the 5′ single terminal overhangs adjacent to wobble closing base pairs are also presented.

The dynamic structural basis of differential enhancement of conformational stability by 5'- and 3'-dangling ends in RNA

Unpaired bases at the end of an RNA duplex (dangling ends) can stabilize the core duplex in a sequence-dependent manner and are important determinants of RNA folding, recognition, and functions. Using 2-aminopurine as a dangling end purine base, we have employed femtosecond time-resolved fluorescence spectroscopy, combined with UV optical melting, to quantitatively investigate the physical and structural nature of the stacking interactions between the dangling end bases and the terminal base pairs. A 3'-dangling purine base has a large subpopulation that stacks on the guanine base of the terminal GC or UG pair, either intrastrand or cross-strand depending on the orientation of the pair, thus providing stabilization of different magnitudes. On the contrary, a 5'-dangling purine base only has a marginal subpopulation that stacks on the purine of the same strand (intrastrand) but has little cross-strand stacking. Thus a 5'-dangling purine does not provide significant stabilization. These stacking structures are not static, and a dangling end base samples a range of stacked and unstacked conformations with respect to the terminal base pair. Femtosecond time-resolved anisotropy decay reveals certain hindered base conformational dynamics that occur on the picosecond to nanosecond time scales, which allow the dangling base to sample these substates. When the dangling purine is opposite to a U and is able to form a potential base pair at the end of the duplex, there is an interplay of base stacking and hydrogen-bonding interactions that depends on the orientation of the base pair relative to the adjacent GC pair. By resolving these populations that are dynamically exchanging on fast time scales, we elucidated the correlation between dynamic conformational distributions and thermodynamic stability.

On the structural repertoire of pools of short, random RNA sequences

A detailed knowledge of the mapping between sequence and structure spaces in populations of RNA molecules is essential to better understand their present-day functional properties, to envisage a plausible early evolution of RNA in a prebiotic chemical environment and to improve the design of in vitro evolution experiments, among others. Analysis of natural RNAs, as well as in vitro and computational studies, show that certain RNA structural motifs are much more abundant than others, pointing out a complex relation between sequence and structure. Within this framework, we have investigated computationally the structural properties of a large pool (10(8) molecules) of single-stranded, 35 nt-long, random RNA sequences. The secondary structures obtained are ranked and classified into structure families. The number of structures in main families is analytically calculated and compared with the numerical results. This permits a quantification of the fraction of structure space covered by a large pool of sequences. We further show that the number of structural motifs and their frequency is highly unbalanced with respect to the nucleotide composition: simple structures such as stem-loops and hairpins arise from sequences depleted in G, while more complex structures require an enrichment of G. In general, we observe a strong correlation between subfamilies-characterized by a fixed number of paired nucleotides-and nucleotide composition. Our results are compared to the structural repertoire obtained in a second pool where isolated base pairs are prohibited.

Comprehensive thermodynamic analysis of 3' double-nucleotide overhangs neighboring Watson-Crick terminal base pairs

Thermodynamic parameters are reported for duplex formation of 48 self-complementary RNA duplexes containing Watson-Crick terminal base pairs (GC, AU and UA) with all 16 possible 3' double-nucleotide overhangs; mimicking the structures of short interfering RNAs (siRNA) and microRNAs (miRNA). Based on nearest-neighbor analysis, the addition of a second dangling nucleotide to a single 3' dangling nucleotide increases stability of duplex formation up to 0.8 kcal/mol in a sequence dependent manner. Results from this study in conjunction with data from a previous study [A. S. O'Toole, S. Miller and M. J. Serra (2005) RNA, 11, 512.] allows for the development of a refined nearest-neighbor model to predict the influence of 3' double-nucleotide overhangs on the stability of duplex formation. The model improves the prediction of free energy and melting temperature when tested against five oligomers with various core duplex sequences. Phylogenetic analysis of naturally occurring miRNAs was performed to support our results. Selection of the effector miR strand of the mature miRNA duplex appears to be dependent upon the identity of the 3' double-nucleotide overhang. Thermodynamic parameters for 3' single terminal overhangs adjacent to a UA pair are also presented.

A DNA duplex with extremely enhanced thermal stability based on controlled immobilization on gold nanoparticles

The effect of DNA loadings on the thermal stability of DNA duplex immobilized on gold nanoparticles has been investigated. The modestly loaded duplexes on the gold nanoparticles showed enhanced thermal stability, as compared to that of the free duplex (without gold nanoparticles). However, the highly loaded duplex showed stability similar to that of free duplex. The stability could be controlled over a wide temperature range simply by varying the salt concentration (over 50 degrees C). Additionally, the gold nanoparticles with modestly loaded oligonucleotides could be used as nanoprobes for effective and fast strand exchange reactions, based on the increased thermal stability of the immobilized duplex. These results indicate that the interaction between the duplex and the nanoparticle surface plays an important role in determining the stability of the duplex.

A nano-circular single-stranded DNA as a novel tool for SNPs detection

Analysis of single nucleotides polymorphisms (SNPs) is very important for the elucidation of gene-based physiological differences. For high-throughput applications, detection systems need to be highly selective, highly sensitive, and simple. In this study, we investigated whether synchronous transcription and translation from nano-circular single-stranded DNA (ssDNA) might be useful for the detection of SNPs. A nano-circular probe ssDNA was designed to contain codons for a (His)6 peptide, and the sense mRNA transcribed from this ssDNA contains a Shine-Dalgarno sequence, a start AUG codon, 6 His codons (CAU), and a stop UAG codon. The entire circular ssDNA is 55 nt and contains T20 as a linker sequence and a binding site for the target SNP-containing DNAs. Our results that show SNPs can be detected by the cell-free synthesis of the (His)6 peptide from this ssDNA. Because this method allows sequence distinction, signal amplification, and easy detection in one system, it should improve the efficacy of high-throughput gene analysis.