Atomic detail investigation of the structure and dynamics of DNA.RNA hybrids: a molecular dynamics study

In-Cell Stability Prediction of RNA/DNA Hybrid Duplexes for Designing Oligonucleotides Aimed at Therapeutics

In cells, the formation of RNA/DNA hybrid duplexes regulates gene expression and modification. The environment inside cellular organelles is heterogeneously crowded with high concentrations of biomolecules that affect the structure and stability of RNA/DNA hybrid duplexes. However, the detailed environmental effects remain unclear. Therefore, the mechanistic details of the effect of such molecular crowding were investigated at the molecular level by using thermodynamic and nuclear magnetic resonance analyses, revealing structure-dependent destabilization of the duplexes under crowded conditions. The transition from B- to A-like hybrid duplexes due to a change in conformation of the DNA strand guided by purine-pyrimidine asymmetry significantly increased the hydration number, which resulted in greater destabilization by the addition of cosolutes. By quantifying the individual contributions of environmental factors and the bulk structure of the duplex, we developed a set of parameters that predict the stability of hybrid duplexes with conformational dissimilarities under diverse crowding conditions. A comparison of the effects of environmental conditions in living cells and in vitro crowded solutions on hybrid duplex formation using the Förster resonance energy transfer technique established the applicability of our parameters to living cells. Moreover, our derived parameters can be used to estimate the efficiency of transcriptional inhibition, genome editing, and silencing techniques in cells. This supports the usefulness of our parameters for the visualization of cellular mechanisms of gene expression and the development of nucleic acid-based therapeutics targeting different cells.

Structure-mechanics statistical learning unravels the linkage between local rigidity and global flexibility in nucleic acids

The mechanical properties of nucleic acids underlie biological processes ranging from genome packaging to gene expression, but tracing their molecular origin has been difficult due to the structural and chemical complexity. We posit that concepts from machine learning can help to tackle this long-standing challenge. Here, we demonstrate the feasibility and advantage of this strategy through developing a structure-mechanics statistical learning scheme to elucidate how local rigidity in double-stranded (ds)DNA and dsRNA may lead to their global flexibility in bend, stretch, and twist. Specifically, the mechanical parameters in a heavy-atom elastic network model are computed from the trajectory data of all-atom molecular dynamics simulation. The results show that the inter-atomic springs for backbone and ribose puckering in dsRNA are stronger than those in dsDNA, but are similar in strengths for base-stacking and base-pairing. Our analysis shows that the experimental observation of dsDNA being easier to bend but harder to stretch than dsRNA comes mostly from the respective B- and A-form topologies. The computationally resolved composition of local rigidity indicates that the flexibility of both nucleic acids is mostly due to base-stacking. But for properties like twist-stretch coupling, backbone springs are shown to play a major role instead. The quantitative connection between local rigidity and global flexibility sets foundation for understanding how local binding and chemical modification of genetic materials effectuate longer-ranged regulatory signals.

The mechanical properties of nucleic acids underlie biological processes ranging from genome packaging to gene expression. We devise structural mechanics statistical learning method to reveal their molecular origin in terms of chemical interactions.

Structural Flexibility of DNA-RNA Hybrid Duplex: Stretching and Twist-Stretch Coupling

DNA-RNA hybrid (DRH) duplexes play essential roles during the replication of DNA and the reverse transcription of RNA viruses, and their flexibility is important for their biological functions. Recent experiments indicated that A-form RNA and B-form DNA have a strikingly different flexibility in stretching and twist-stretch coupling, and the structural flexibility of DRH duplex is of great interest, especially in stretching and twist-stretch coupling. In this work, we performed microsecond all-atom molecular dynamics simulations with new AMBER force fields to characterize the structural flexibility of DRH duplex in stretching and twist-stretch coupling. We have calculated all the helical parameters, stretch modulus, and twist-stretch coupling parameters for the DRH duplex. First, our analyses on structure suggest that the DRH duplex exhibits an intermediate conformation between A- and B-forms and closer to A-form, which can be attributed to the stronger rigidity of the RNA strand than the DNA strand. Second, our calculations show that the DRH duplex has the stretch modulus of 834 ± 34 pN and a very weak twist-stretch coupling. Our quantitative analyses indicate that, compared with DNA and RNA duplexes, the different flexibility of the DRH duplex in stretching and twist-stretch coupling is mainly attributed to its apparently different basepair inclination in the helical structure.

Activity and fidelity of human DNA polymerase α depend on primer structure

DNA polymerase α (Polα) plays an important role in genome replication. In a complex with primase, Polα synthesizes chimeric RNA-DNA primers necessary for replication of both chromosomal DNA strands. During RNA primer extension with deoxyribonucleotides, Polα needs to use double-stranded helical substrates having different structures. Here, we provide a detailed structure-function analysis of human Polα's interaction with dNTPs and DNA templates primed with RNA, chimeric RNA-DNA, or DNA. We report the crystal structures of two ternary complexes of the Polα catalytic domain containing dCTP, a DNA template, and either a DNA or an RNA primer. Unexpectedly, in the ternary complex with a DNA:DNA duplex and dCTP, the "fingers" subdomain of Polα is in the open conformation. Polα induces conformational changes in the DNA and hybrid duplexes to produce the universal double helix form. Pre-steady-state kinetic studies indicated for both duplex types that chemical catalysis rather than product release is the rate-limiting step. Moreover, human Polα extended DNA primers with higher efficiency but lower processivity than it did with RNA and chimeric primers. Polα has a substantial propensity to make errors during DNA synthesis, and we observed that its fidelity depends on the type of sugar at the primer 3'-end. A detailed structural comparison of Polα with other replicative DNA polymerases disclosed common features and some differences, which may reflect the specialization of each polymerase in genome replication.

Dynamic and Energetic Signatures of Adenine Tracts in a rA-dT RNA-DNA Hybrid and in Homologous RNA-DNA, RNA-RNA, and DNA-DNA Double Helices

Nuclear magnetic resonance spectroscopy and proton exchange are being used to characterize the opening reactions of individual base pairs in the RNA-DNA hybrid 5'-rGCGAUAAAAAGGCC-3'/5'-dGGCCTTTTTATCGC-3'. The hybrid contains a central tract of five rA-dT base pairs. The rates and the equilibrium constant of the opening reaction for each base pair are determined from the dependence of the exchange rates of imino protons on ammonia concentration, at 10 °C. The results are compared to those previously obtained by our laboratory for three homologous duplexes of the same base sequence (except for the appropriate T/U substitution), containing tracts of dA-rU, rA-rU, or dA-dT base pairs. The rA-dT tract is distinguished by an enhanced propensity of the base pairs to exist in the extrahelical state. The opening rates of rA-dT base pairs also exhibit a strong dependence on the location of the base pair in the structure; namely, as one advances into the tract, the opening rates of rA-dT base pairs gradually decrease. The local stability of each rA-dT base pair within the tract is the same as that of the corresponding rA-rU base pair in the homologous RNA-only duplex but differs from the stabilities of dA-dT and dA-rU base pairs in the other two duplexes (namely, dA-dT > rA-dT > dA-rU). These results demonstrate that, in nucleic acid double helices with the same base sequence, the opening dynamics and the energetics of individual base pairs are strongly influenced by the nature of the strand and by the structural context of the base pair.

Inclusion of methoxy groups inverts the thermodynamic stabilities of DNA-RNA hybrid duplexes: A molecular dynamics simulation study

Modified nucleic acids have found profound applications in nucleic acid based technologies such as antisense and antiviral therapies. Previous studies on chemically modified nucleic acids have suggested that modifications incorporated in furanose sugar especially at 2'-position attribute special properties to nucleic acids when compared to other modifications. 2'-O-methyl modification to deoxyribose sugars of DNA-RNA hybrids is one such modification that increases nucleic acid stability and has become an attractive class of compounds for potential antisense applications. It has been reported that modification of DNA strands with 2'-O-methyl group reverses the thermodynamic stability of DNA-RNA hybrid duplexes. Molecular dynamics simulations have been performed on two hybrid duplexes (DR and RD) which differ from each other and 2'-O-methyl modified counterparts to investigate the effect of 2'-O-methyl modification on their duplex stability. The results obtained suggest that the modification drives the conformations of both the hybrid duplexes towards A-RNA like conformation. The modified hybrid duplexes exhibit significantly contrasting dynamics and hydration patterns compared to respective parent duplexes. In line with the experimental results, the relative binding free energies suggest that the introduced modifications stabilize the less stable DR hybrid, but destabilize the more stable RD duplex. Binding free energy calculations suggest that the increased hydrophobicity is primarily responsible for the reversal of thermodynamic stability of hybrid duplexes. Free energy component analysis further provides insights into the stability of modified duplexes.

Atomistic investigation of the effect of incremental modification of deoxyribose sugars by locked nucleic acid (β-D-LNA and α-L-LNA) moieties on the structures and thermodynamics of DNA-RNA hybrid duplexes

Chemically modified oligonucleotides offer many possibilities in utilizing their special features for a vast number of applications in nucleic acid based therapies and synthetic molecular biology. Locked nucleic acid analogues (α-/β-LNA) are modifications having an extra ring of 2'-O,4'-C-methylene group in the furanose sugar. LNA strands have been shown to exhibit high binding affinity toward RNA and DNA strands, and the resultant duplexes show significantly high melting temperatures. In the present study, molecular dynamics (MD) simulations were performed on DNA-RNA hybrid duplexes by systematically modifying their deoxyribose sugars with locked nucleic acid analogues. Several geometrical and energetic analyses were performed using principal component (PCA) analysis and binding free energy methods to understand the consequence of incorporated isomeric LNA modifications on the structure, dynamics, and stability of DNA-RNA hybrid duplex. The β-modification systematically changes the conformation of the DNA-RNA hybrid duplex whereas drastic changes are observed for α-modification. The fully modified duplexes have distinct properties compared to partial and unmodified duplexes, and the partly modified duplexes have properties intermediate to full strand and unmodified duplexes. The distribution of BI versus BII populations suggests that backbone rearrangement is minimal for β-LNA modification in order to accommodate it in duplexes whereas extensive backbone rearrangement is necessary in order to incorporate α-LNA modification which subsequently alters the energetic and structural properties of the duplexes. The simulation results also suggest that the alteration of DNA-RNA hybrid properties depends on the position of modification and the gap between the modifications.

The free energy landscape of pseudorotation in 3'-5' and 2'-5' linked nucleic acids

The five-membered furanose ring is a central component of the chemical structure of biological nucleic acids. The conformations of the furanose ring can be analytically described using the concept of pseudorotation, and for RNA and DNA they are dominated by the C2'-endo and C3'-endo conformers. While the free energy difference between these two conformers can be inferred from NMR measurements, a free energy landscape of the complete pseudorotation cycle of nucleic acids in solution has remained elusive. Here, we describe a new free energy calculation method for molecular dynamics (MD) simulations using the two pseudorotation parameters directly as the collective variables. To validate our approach, we calculated the free energy surface of ribose pseudorotation in guanosine and 2'-deoxyguanosine. The calculated free energy landscape reveals not only the relative stability of the different pseudorotation conformers, but also the main transition path between the stable conformations. Applying this method to a standard A-form RNA duplex uncovered the expected minimum at the C3'-endo state. However, at a 2'-5' linkage, the minimum shifts to the C2'-endo conformation. The free energy of the C3'-endo conformation is 3 kcal/mol higher due to a weaker hydrogen bond and a reduced base stacking interaction. Unrestrained MD simulations suggest that the conversion from C3'-endo to C2'-endo and vice versa is on the nanosecond and microsecond time scale, respectively. These calculations suggest that 2'-5' linkages may enable folded RNAs to sample a wider spectrum of their pseudorotation conformations.

DNA–RNA hybrid duplexes with decreasing pyrimidine content in the DNA strand provide structural snapshots for the A- to B-form conformational transition of nucleic acids

A gradual increase in the deoxypyrimidine content in DNA–RNA hybrids leads to B- to A-form nucleic acid transition. Possible factors that govern nuclease activity on hybrid duplexes are presented.

Structures, dynamics, and stabilities of fully modified locked nucleic acid (β-D-LNA and α-L-LNA) duplexes in comparison to pure DNA and RNA duplexes

Locked nucleic acid (LNA) is a chemical modification which introduces a -O-CH2- linkage in the furanose sugar of nucleic acids and blocks its conformation in a particular state. Two types of modifications, namely, 2'-O,4'-C-methylene-β-D-ribofuranose (β-D-LNA) and 2'-O,4'-C-methylene-α-L-ribofuranose (α-L-LNA), have been shown to yield RNA and DNA duplex-like structures, respectively. LNA modifications lead to increased melting temperatures of DNA and RNA duplexes, and have been suggested as potential therapeutic agents in antisense therapy. In this study, molecular dynamics (MD) simulations were performed on fully modified LNA duplexes and pure DNA and RNA duplexes sharing a similar sequence to investigate their structure, stabilities, and solvation properties. Both LNA duplexes undergo unwinding of the helical structure compared to the pure DNA and RNA duplexes. Though the α-LNA substituent has been proposed to mimic deoxyribose sugar in its conformational properties, the fully modified duplex was found to exhibit unique structural and dynamic properties with respect to the other three nucleic acid structures. Free energy calculations accurately capture the enhanced stabilization of the LNA duplex structures compared to DNA and RNA molecules as observed in experiments. π-stacking interaction between bases from complementary strands is shown to be one of the contributors to enhanced stabilization upon LNA substitution. A combination of two factors, namely, nature of the -O-CH2- linkage in the LNAs vs their absence in the pure duplexes and similar conformations of the sugar rings in DNA and α-LNA vs the other two, is suggested to contribute to the stark differences among the four duplexes studied here in terms of their structural, dynamic, and energetic properties.

Large domain motions in Ago protein controlled by the guide DNA-strand seed region determine the Ago-DNA-mRNA complex recognition process

The recognition mechanism and cleavage activity of argonaute (Ago), miRNA, and mRNA complexes are the core processes to the small non-coding RNA world. The 5′ nucleation at the ‘seed’ region (position 2–8) of miRNA was believed to play a significant role in guiding the recognition of target mRNAs to the given miRNA family. In this paper, we have performed all-atom molecular dynamics simulations of the related and recently revealed Ago-DNA:mRNA ternary complexes to study the dynamics of the guide-target recognition and the effect of mutations by introducing “damaging” C·C mismatches at different positions in the seed region of the DNA-RNA duplex. Our simulations show that the A-form-like helix duplex gradually distorts as the number of seed mismatches increases and the complex can survive no more than two such mismatches. Severe distortions of the guide-target heteroduplex are observed in the ruinous 4-sites mismatch mutant, which give rise to a bending motion of the PAZ domain along the L1/L2 “hinge-like” connection segment, resulting in the opening of the nucleic-acid-binding channel. These long-range interactions between the seed region and PAZ domain, moderated by the L1/L2 segments, reveal the central role of the seed region in the guide-target strands recognition: it not only determines the guide-target heteroduplex’s nucleation and propagation, but also regulates the dynamic motions of Ago domains around the nucleic-acid-binding channel.

Recent Developments and Applications of the CHARMM force fields

Empirical force fields commonly used to describe the condensed phase properties of complex systems such as biological macromolecules are continuously being updated. Improvements in quantum mechanical (QM) methods used to generate target data, availability of new experimental target data, incorporation of new classes of compounds and new theoretical developments (eg. polarizable methods) make force-field development a dynamic domain of research. Accordingly, a number of improvements and extensions of the CHARMM force fields have occurred over the years. The objective of the present review is to provide an up-to-date overview of the CHARMM force fields. A limited presentation on the historical aspects of force fields will be given, including underlying methodologies and principles, along with a brief description of the strategies used for parameter development. This is followed by information on the CHARMM additive and polarizable force fields, including examples of recent applications of those force fields.

Conformational flexibility influences degree of hydration of nucleic acid hybrids

Four nucleic acid duplexes-DNA/RNA hybrid, RNA/DNA hybrid, RNA duplex, and DNA duplex-were studied under molecular crowding conditions of osmolytes. Destabilization of duplexes (ΔΔG°(25)) indicated that the ΔΔG°(25) values of hybrids were intermediate between those of DNA and RNA duplexes. In the presence of polyethylene glycol 200, the ΔΔG°(25) values were estimated to be +3.0, +3.5, +3.5, and +4.1 kcal mol(-1) for the DNA duplex, DNA/RNA hybrid, RNA/DNA hybrid, and RNA duplex, respectively. Differences in the number of water molecules taken up (-Δn(w)) upon duplex formations between 0 and 37 °C (Δ(-Δn(w))) were estimated to be 44.8 and 59.7 per duplex structure for the DNA/RNA and RNA/DNA hybrids, respectively. While the Δ(-Δn(w)) value for the DNA/RNA hybrid was intermediate between those of the DNA (26.1) and RNA (59.2) duplexes, the value for RNA/DNA hybrid was close to that of RNA duplex. These differences in the thermodynamic parameters and hydration are probably a consequence of the enhanced global flexibility of the RNA/DNA hybrid structure relative to the DNA/RNA hybrid structure observed in molecular dynamics simulations. This molecular crowding study provides information not only on hydration but also on the flexibility of the conformation of nucleic acid duplexes.

Molecular dynamics simulation and binding energy calculation for estimation of oligonucleotide duplex thermostability in RNA-based therapeutics

For oligonucleotide-based therapeutics, a thorough understanding of the thermodynamic properties of duplex formation is critical to developing stable and potent drugs. For unmodified small interfering RNA (siRNA), DNA antisense oligonucleotide (AON) and locked nucleic acid (LNA), DNA/LNA modified oligonucleotides, nearest neighbor (NN) methods can be effectively used to quickly and accurately predict duplex thermodynamic properties such as melting point. Unfortunately, for chemically modified olignonucleotides, there has been no accurate prediction method available. Here we describe the potential of estimating melting temperature (T(m)) for nonstandard oligonucleotides by using the correlation of the experimental T(m) with the calculated duplex binding energy (BE) for oligonucleotides of a given length. This method has been automated into a standardized molecular dynamics (MD) protocol through Pipeline Pilot (PP) using the CHARMm component in Discovery Studio (DS). Results will be presented showing the correlation of the predicted data with experiment for both standard and chemically modified siRNA and AON.

Impact of 2'-hydroxyl sampling on the conformational properties of RNA: update of the CHARMM all-atom additive force field for RNA

Here, we present an update of the CHARMM27 all-atom additive force field for nucleic acids that improves the treatment of RNA molecules. The original CHARMM27 force field parameters exhibit enhanced Watson-Crick base pair opening which is not consistent with experiment, whereas analysis of molecular dynamics (MD) simulations show the 2'-hydroxyl moiety to almost exclusively sample the O3' orientation. Quantum mechanical (QM) studies of RNA related model compounds indicate the energy minimum associated with the O3' orientation to be too favorable, consistent with the MD results. Optimization of the dihedral parameters dictating the energy of the 2'-hydroxyl proton targeting the QM data yielded several parameter sets, which sample both the base and O3' orientations of the 2'-hydroxyl to varying degrees. Selection of the final dihedral parameters was based on reproduction of hydration behavior as related to a survey of crystallographic data and better agreement with experimental NMR J-coupling values. Application of the model, designated CHARMM36, to a collection of canonical and noncanonical RNA molecules reveals overall improved agreement with a range of experimental observables as compared to CHARMM27. The results also indicate the sensitivity of the conformational heterogeneity of RNA to the orientation of the 2'-hydroxyl moiety and support a model whereby the 2'-hydroxyl can enhance the probability of conformational transitions in RNA.

Toward a consensus view of duplex RNA flexibility

The structure and flexibility of the RNA duplex has been studied using extended molecular dynamics simulations on four diverse 18-mer oligonucleotides designed to contain many copies of the 10 unique dinucleotide steps in different sequence environments. Simulations were performed using the two most popular force fields for nucleic acids simulations (AMBER and CHARMM) in their latest versions, trying to arrive to a consensus picture of the RNA flexibility. Contrary to what was found for DNA duplex (DNA(2)), no clear convergence is found for the RNA duplex (RNA(2)), but one of the force field seems to agree better with experimental data. MD simulations performed with this force field were used to fully characterize, for the first time to our knowledge, the sequence-dependent elastic properties of RNA duplexes at different levels of resolutions. The flexibility pattern of RNA(2) shows similarities with DNA(2), but also surprising differences, which help us to understand the different biological functions of both molecules. A full mesoscopic model of RNA duplex at different resolution levels is derived to be used for genome-wide description of the flexibility of double-helical fragments of RNA.

Dynamics and stability of individual base pairs in two homologous RNA-DNA hybrids

Nuclear magnetic resonance spectroscopy and proton exchange have been used to characterize two RNA-DNA hybrids from the tR2 intrinsic transcription terminator site of phage lambda. The hybrids have the same base sequence [5'-GGCGCAGGCC(T/U)(T/U)CC-3'/5'-GGAAGGCC(T/U)GCGCC-3'] but differ from each other by an interchange of DNA and RNA strands. The opening of single base pairs in the two hybrids is characterized by measuring the rates of exchange of imino protons with solvent protons as a function of the concentration of a proton acceptor (ammonia base) at 10 degrees C. The free energy change in the opening reaction provides a measure of the stability of the base pair, while the rates of opening and closing define the base pair dynamics. The results demonstrate that, within the same base sequence context, dA-rU base pairs are less stable than dT-rA base pairs. The differences in stability are enhanced when two dA-rU base pairs are located next to each other in the hybrid structure. For the G-C base pairs, the rates of opening and closing and the stability are affected by the base sequence context and by the nature of the sugar moiety attached to the guanine. The dominant feature of the base sequence is the proximity of the dA-rU base pair, which destabilizes the G-C base pair when the guanine is located on the DNA strand. Two G-C base pairs (namely, those in the fourth and 10th positions) exhibit large differences in their opening and closing rates between the two hybrids, while maintaining the same stability. These results provide the first demonstration that, for RNA-DNA hybrid structures with the same base sequence, the opening dynamics and the stability of individual base pairs are strongly influenced by the chemical nature of each strand.