The phosphorylated trimeric SOSS1 complex and RNA polymerase II trigger liquid-liquid phase separation at double-strand breaks

Double-strand breaks (DSBs) are the most severe type of DNA damage. Previously, we demonstrated that RNA polymerase II (RNAPII) phosphorylated at the tyrosine 1 (Y1P) residue of its C-terminal domain (CTD) generates RNAs at DSBs. However, the regulation of transcription at DSBs remains enigmatic. Here, we show that the damage-activated tyrosine kinase c-Abl phosphorylates hSSB1, enabling its interaction with Y1P RNAPII at DSBs. Furthermore, the trimeric SOSS1 complex, consisting of hSSB1, INTS3, and c9orf80, binds to Y1P RNAPII in response to DNA damage in an R-loop-dependent manner. Specifically, hSSB1, as a part of the trimeric SOSS1 complex, exhibits a strong affinity for R-loops, even in the presence of replication protein A (RPA). Our in vitro and in vivo data reveal that the SOSS1 complex and RNAPII form dynamic liquid-like repair compartments at DSBs. Depletion of the SOSS1 complex impairs DNA repair, underscoring its biological role in the R-loop-dependent DNA damage response.

Dicer structure and function: conserved and evolving features

RNase III Dicer produces small RNAs guiding sequence-specific regulations, with important biological roles in eukaryotes. Major Dicer-dependent mechanisms are RNA interference (RNAi) and microRNA (miRNA) pathways, which employ distinct types of small RNAs. Small interfering RNAs (siRNAs) for RNAi are produced by Dicer from long double-stranded RNA (dsRNA) as a pool of different small RNAs. In contrast, miRNAs have specific sequences because they are precisely cleaved out from small hairpin precursors. Some Dicer homologs efficiently generate both, siRNAs and miRNAs, while others are adapted for biogenesis of one small RNA type. Here, we review the wealth of recent structural analyses of animal and plant Dicers, which have revealed how different domains and their adaptations contribute to substrate recognition and cleavage in different organisms and pathways. These data imply that siRNA generation was Dicer's ancestral role and that miRNA biogenesis relies on derived features. While the key element of functional divergence is a RIG-I-like helicase domain, Dicer-mediated small RNA biogenesis also documents the impressive functional versatility of the dsRNA-binding domain.

Human senataxin is a bona fide R-loop resolving enzyme and transcription termination factor

Prolonged pausing of the transcription machinery may lead to the formation of three-stranded nucleic acid structures, called R-loops, typically resulting from the annealing of the nascent RNA with the template DNA. Unscheduled persistence of R-loops and RNA polymerases may interfere with transcription itself and other essential processes such as DNA replication and repair. Senataxin (SETX) is a putative helicase, mutated in two neurodegenerative disorders, which has been implicated in the control of R-loop accumulation and in transcription termination. However, understanding the precise role of SETX in these processes has been precluded by the absence of a direct characterisation of SETX biochemical activities. Here, we purify and characterise the helicase domain of SETX in parallel with its yeast orthologue, Sen1. Importantly, we show that SETX is a bona fide helicase with the ability to resolve R-loops. Furthermore, SETX has retained the transcription termination activity of Sen1 but functions in a species-specific manner. Finally, subsequent characterisation of two SETX variants harbouring disease-associated mutations shed light into the effect of such mutations on SETX folding and biochemical properties. Altogether, these results broaden our understanding of SETX function in gene expression and the maintenance of genome integrity and provide clues to elucidate the molecular basis of SETX-associated neurodegenerative diseases.

Cooperation between intrinsically disordered and ordered regions of Spt6 regulates nucleosome and Pol II CTD binding, and nucleosome assembly

Transcription elongation factor Spt6 associates with RNA polymerase II (Pol II) and acts as a histone chaperone, which promotes the reassembly of nucleosomes following the passage of Pol II. The precise mechanism of nucleosome reassembly mediated by Spt6 remains unclear. In this study, we used a hybrid approach combining cryo-electron microscopy and small-angle X-ray scattering to visualize the architecture of Spt6 from Saccharomyces cerevisiae. The reconstructed overall architecture of Spt6 reveals not only the core of Spt6, but also its flexible N- and C-termini, which are critical for Spt6's function. We found that the acidic N-terminal region of Spt6 prevents the binding of Spt6 not only to the Pol II CTD and Pol II CTD-linker, but also to pre-formed intact nucleosomes and nucleosomal DNA. The N-terminal region of Spt6 self-associates with the tSH2 domain and the core of Spt6 and thus controls binding to Pol II and nucleosomes. Furthermore, we found that Spt6 promotes the assembly of nucleosomes in vitro. These data indicate that the cooperation between the intrinsically disordered and structured regions of Spt6 regulates nucleosome and Pol II CTD binding, and also nucleosome assembly.

PHF3 regulates neuronal gene expression through the Pol II CTD reader domain SPOC

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) is a regulatory hub for transcription and RNA processing. Here, we identify PHD-finger protein 3 (PHF3) as a regulator of transcription and mRNA stability that docks onto Pol II CTD through its SPOC domain. We characterize SPOC as a CTD reader domain that preferentially binds two phosphorylated Serine-2 marks in adjacent CTD repeats. PHF3 drives liquid-liquid phase separation of phosphorylated Pol II, colocalizes with Pol II clusters and tracks with Pol II across the length of genes. PHF3 knock-out or SPOC deletion in human cells results in increased Pol II stalling, reduced elongation rate and an increase in mRNA stability, with marked derepression of neuronal genes. Key neuronal genes are aberrantly expressed in Phf3 knock-out mouse embryonic stem cells, resulting in impaired neuronal differentiation. Our data suggest that PHF3 acts as a prominent effector of neuronal gene regulation by bridging transcription with mRNA decay.

Termination of non-coding transcription in yeast relies on both an RNA Pol II CTD interaction domain and a CTD-mimicking region in Sen1

Pervasive transcription is a widespread phenomenon leading to the production of a plethora of non‐coding RNAs (ncRNAs) without apparent function. Pervasive transcription poses a threat to proper gene expression that needs to be controlled. In yeast, the highly conserved helicase Sen1 restricts pervasive transcription by inducing termination of non‐coding transcription. However, the mechanisms underlying the specific function of Sen1 at ncRNAs are poorly understood. Here, we identify a motif in an intrinsically disordered region of Sen1 that mimics the phosphorylated carboxy‐terminal domain (CTD) of RNA polymerase II, and structurally characterize its recognition by the CTD‐interacting domain of Nrd1, an RNA‐binding protein that binds specific sequences in ncRNAs. In addition, we show that Sen1‐dependent termination strictly requires CTD recognition by the N‐terminal domain of Sen1. We provide evidence that the Sen1‐CTD interaction does not promote initial Sen1 recruitment, but rather enhances Sen1 capacity to induce the release of paused RNAPII from the DNA. Our results shed light on the network of protein–protein interactions that control termination of non‐coding transcription by Sen1.

Structural insight into recognition of phosphorylated threonine-4 of RNA polymerase II C-terminal domain by Rtt103p

Phosphorylation patterns of the C‐terminal domain (CTD) of largest subunit of RNA polymerase II (called the CTD code) orchestrate the recruitment of RNA processing and transcription factors. Recent studies showed that not only serines and tyrosines but also threonines of the CTD can be phosphorylated with a number of functional consequences, including the interaction with yeast transcription termination factor, Rtt103p. Here, we report the solution structure of the Rtt103p CTD‐interacting domain (CID) bound to Thr4 phosphorylated CTD, a poorly understood letter of the CTD code. The structure reveals a direct recognition of the phospho‐Thr4 mark by Rtt103p CID and extensive interactions involving residues from three repeats of the CTD heptad. Intriguingly, Rtt103p's CID binds equally well Thr4 and Ser2 phosphorylated CTD. A doubly phosphorylated CTD at Ser2 and Thr4 diminishes its binding affinity due to electrostatic repulsion. Our structural data suggest that the recruitment of a CID‐containing CTD‐binding factor may be coded by more than one letter of the CTD code.

RBM7 subunit of the NEXT complex binds U-rich sequences and targets 3'-end extended forms of snRNAs

The Nuclear Exosome Targeting (NEXT) complex is a key cofactor of the mammalian nuclear exosome in the removal of Promoter Upstream Transcripts (PROMPTs) and potentially aberrant forms of other noncoding RNAs, such as snRNAs. NEXT is composed of three subunits SKIV2L2, ZCCHC8 and RBM7. We have recently identified the NEXT complex in our screen for oligo(U) RNA-binding factors. Here, we demonstrate that NEXT displays preference for U-rich pyrimidine sequences and this RNA binding is mediated by the RNA recognition motif (RRM) of the RBM7 subunit. We solved the structure of RBM7 RRM and identified two phenylalanine residues that are critical for interaction with RNA. Furthermore, we showed that these residues are required for the NEXT interaction with snRNAs in vivo. Finally, we show that depletion of components of the NEXT complex alone or together with exosome nucleases resulted in the accumulation of mature as well as extended forms of snRNAs. Thus, our data suggest a new scenario in which the NEXT complex is involved in the surveillance of snRNAs and/or biogenesis of snRNPs.

Can We Execute Stable Microsecond-Scale Atomistic Simulations of Protein-RNA Complexes?

We report over 30 μs of unrestrained molecular dynamics simulations of six protein-RNA complexes in explicit solvent. We utilize the AMBER ff99bsc0χ(OL3) RNA force field combined with the ff99SB protein force field and its more recent ff12SB version with reparametrized side-chain dihedrals. The simulations show variable behavior, ranging from systems that are essentially stable to systems with progressive deviations from the experimental structure, which we could not stabilize anywhere close to the starting experimental structure. For some systems, microsecond-scale simulations are necessary to achieve stabilization after initial sizable structural perturbations. The results show that simulations of protein-RNA complexes are challenging and every system should be treated individually. The simulations are affected by numerous factors, including properties of the starting structures (the initially high force field potential energy, resolution limits, conformational averaging, crystal packing, etc.), force field imbalances, and real flexibility of the studied systems. These factors, and thus the simulation behavior, differ from system to system. The structural stability of simulated systems does not correlate with the size of buried interaction surface or experimentally determined binding affinities but reflects the type of protein-RNA recognition. Protein-RNA interfaces involving shape-specific recognition of RNA are more stable than those relying on sequence-specific RNA recognition. The differences between the protein force fields are considerably smaller than the uncertainties caused by sampling and starting structures. The ff12SB improves description of the tyrosine side-chain group, which eliminates some problems associated with tyrosine dynamics.

Structure and semi-sequence-specific RNA binding of Nrd1

In Saccharomyces cerevisiae, the Nrd1-dependent termination and processing pathways play an important role in surveillance and processing of non-coding ribonucleic acids (RNAs). The termination and subsequent processing is dependent on the Nrd1 complex consisting of two RNA-binding proteins Nrd1 and Nab3 and Sen1 helicase. It is established that Nrd1 and Nab3 cooperatively recognize specific termination elements within nascent RNA, GUA[A/G] and UCUU[G], respectively. Interestingly, some transcripts do not require GUA[A/G] motif for transcription termination in vivo and binding in vitro, suggesting the existence of alternative Nrd1-binding motifs. Here we studied the structure and RNA-binding properties of Nrd1 using nuclear magnetic resonance (NMR), fluorescence anisotropy and phenotypic analyses in vivo. We determined the solution structure of a two-domain RNA-binding fragment of Nrd1, formed by an RNA-recognition motif and helix–loop bundle. NMR and fluorescence data show that not only GUA[A/G] but also several other G-rich and AU-rich motifs are able to bind Nrd1 with affinity in a low micromolar range. The broad substrate specificity is achieved by adaptable interaction surfaces of the RNA-recognition motif and helix–loop bundle domains that sandwich the RNA substrates. Our findings have implication for the role of Nrd1 in termination and processing of many non-coding RNAs arising from bidirectional pervasive transcription.

Recognition of asymmetrically dimethylated arginine by TDRD3

Asymmetric dimethylarginine (aDMA) marks are placed on histones and the C-terminal domain (CTD) of RNA Polymerase II (RNAP II) and serve as a signal for recruitment of appropriate transcription and processing factors in coordination with transcription cycle. In contrast to other Tudor domain-containing proteins, Tudor domain-containing protein 3 (TDRD3) associates selectively with the aDMA marks but not with other methylarginine motifs. Here, we report the solution structure of the Tudor domain of TDRD3 bound to the asymmetrically dimethylated CTD. The structure and mutational analysis provide a molecular basis for how TDRD3 recognizes the aDMA mark. The unique aromatic cavity of the TDRD3 Tudor domain with a tyrosine in position 566 creates a selectivity filter for the aDMA residue. Our work contributes to the understanding of substrate selectivity rules of the Tudor aromatic cavity, which is an important structural motif for reading of methylation marks.

In vivo SELEX reveals novel sequence and structural determinants of Nrd1-Nab3-Sen1-dependent transcription termination

The Nrd1-Nab3-Sen1 (NNS) complex pathway is responsible for transcription termination of cryptic unstable transcripts and sn/snoRNAs. The NNS complex recognizes short motifs on the nascent RNA, but the presence of these sequences alone is not sufficient to define a functional terminator. We generated a homogeneous set of several hundreds of artificial, NNS-dependent terminators with an in vivo selection approach. Analysis of these terminators revealed novel and extended sequence determinants for transcription termination and NNS complex binding as well as supermotifs that are critical for termination. Biochemical and structural data revealed that affinity and specificity of RNA recognition by Nab3p relies on induced fit recognition implicating an α-helical extension of the RNA recognition motif. Interestingly, the same motifs can be recognized by the NNS or the mRNA termination complex depending on their position relative to the start of transcription, suggesting that they function as general transcriptional insulators to prevent interference between the non-coding and the coding yeast transcriptomes.

Serine phosphorylation and proline isomerization in RNAP II CTD control recruitment of Nrd1

Recruitment of appropriate RNA processing factors to the site of transcription is controlled by post-translational modifications of the C-terminal domain (CTD) of RNA polymerase II (RNAP II). Here, we report the solution structure of the Ser5 phosphorylated (pSer5) CTD bound to Nrd1. The structure reveals a direct recognition of pSer5 by Nrd1 that requires the cis conformation of the upstream pSer5-Pro6 peptidyl-prolyl bond of the CTD. Mutations at the complex interface diminish binding affinity and impair processing or degradation of noncoding RNAs. These findings underpin the interplay between covalent and noncovalent changes in the CTD structure that constitute the CTD code.

Air2p is critical for the assembly and RNA-binding of the TRAMP complex and the KOW domain of Mtr4p is crucial for exosome activation

Trf4/5p-Air1/2p-Mtr4p polyadenylation complex (TRAMP) is an essential component of nuclear RNA surveillance in yeast. It recognizes a variety of nuclear transcripts produced by all three RNA polymerases, adds short poly(A) tails to aberrant or unstable RNAs and activates the exosome for their degradation. Despite the advances in understanding the structural features of the isolated complex subunits or their fragments, the details of complex assembly, RNA recognition and exosome activation remain poorly understood. Here we provide the first understanding of the RNA binding mode of the complex. We show that Air2p is an RNA-binding subunit of TRAMP. We identify the zinc knuckles (ZnK) 2, 3 and 4 as the RNA-binding domains, and reveal the essentiality of ZnK4 for TRAMP4 polyadenylation activity. Furthermore, we identify Air2p as the key component of TRAMP4 assembly providing bridging between Mtr4p and Trf4p. The former is bound via the N-terminus of Air2p, while the latter is bound via ZnK5, the linker between ZnK4 and 5 and the C-terminus of the protein. Finally, we uncover the RNA binding part of the Mtr4p arch, the KOW domain, as the essential component for TRAMP-mediated exosome activation.

1H, 13C, and 15N chemical shift assignments of ZCCHC9

ZCCHC9 is a human nuclear protein with sequence homology to yeast Air1p/Air2p proteins which are RNA-binding subunits of the Trf4/Air2/Mtr4 polyadenylation (TRAMP) complex involved in nuclear RNA quality control and degradation in yeast. The ZCCHC9 protein contains four retroviral-type zinc knuckle motifs. Here, we report the NMR spectral assignment of the zinc knuckle region of ZCCHC9. These data will allow performing NMR structural and RNA-binding studies of ZCCHC9 with the aim to investigate its role in the RNA quality control in human.

Recognition of transcription termination signal by the nuclear polyadenylated RNA-binding (NAB) 3 protein

Non-coding RNA polymerase II transcripts are processed by the poly(A)-independent termination pathway that requires the Nrd1 complex. The Nrd1 complex includes two RNA-binding proteins, the nuclear polyadenylated RNA-binding (Nab) 3 and the nuclear pre-mRNA down-regulation (Nrd) 1 that bind their specific termination elements. Here we report the solution structure of the RNA-recognition motif (RRM) of Nab3 in complex with a UCUU oligonucleotide, representing the Nab3 termination element. The structure shows that the first three nucleotides of UCUU are accommodated on the β-sheet surface of Nab3 RRM, but reveals a sequence-specific recognition only for the central cytidine and uridine. The specific contacts we identified are important for binding affinity in vitro as well as for yeast viability. Furthermore, we show that both RNA-binding motifs of Nab3 and Nrd1 alone bind their termination elements with a weak affinity. Interestingly, when Nab3 and Nrd1 form a heterodimer, the affinity to RNA is significantly increased due to the cooperative binding. These findings are in accordance with the model of their function in the poly(A) independent termination, in which binding to the combined and/or repetitive termination elements elicits efficient termination.

The solution structure of the ADAR2 dsRBM-RNA complex reveals a sequence-specific readout of the minor groove

Sequence-dependent recognition of dsDNA-binding proteins is well understood, yet sequence-specific recognition of dsRNA by proteins remains largely unknown, despite their importance in RNA maturation pathways. Adenosine deaminases that act on RNA (ADARs) recode genomic information by the site-selective deamination of adenosine. Here, we report the solution structure of the ADAR2 double-stranded RNA-binding motifs (dsRBMs) bound to a stem-loop pre-mRNA encoding the R/G editing site of GluR-2. The structure provides a molecular basis for how dsRBMs recognize the shape, and also more surprisingly, the sequence of the dsRNA. The unexpected direct readout of the RNA primary sequence by dsRBMs is achieved via the minor groove of the dsRNA and this recognition is critical for both editing and binding affinity at the R/G site of GluR-2. More generally, our findings suggest a solution to the sequence-specific paradox faced by many dsRBM-containing proteins that are involved in post-transcriptional regulation of gene expression.

1H, 13C, and 15N chemical shift assignments for the RNA recognition motif of Nab3

Nuclear polyadenylated RNA-binding (Nab)3 protein is an RNA-binding protein that is involved in the poly(A) independent termination pathway. Here, we report the NMR spectral assignments of RNA-recognition motif (RRM) of Nab3. The assignment will allow performing NMR structural and RNA-binding studies of Nab3 with the aim to investigate its role in the poly(A) independent termination pathway.

The testis-specific human protein RBMY recognizes RNA through a novel mode of interaction

The RBMY (RNA-binding motif gene on Y chromosome) protein encoded by the human Y chromosome is important for normal sperm development. Although its precise molecular RNA targets are unknown at present, it is suggested that human RBMY (hRBMY) participates in splicing in the testis. Using systematic evolution of ligands by exponential enrichment, we found that RNA stem-loops capped by a C(A)/(U)CAA pentaloop are high-affinity binding targets for hRBMY. Subsequent nuclear magnetic resonance structural determination of the hRBMY RNA recognition motif (RRM) in complex with a high-affinity target showed two distinct modes of RNA recognition. First, the RRM beta-sheet surface binds to the RNA loop in a sequence-specific fashion. Second, the beta2-beta3 loop of the hRBMY inserts into the major groove of the RNA stem. The first binding mode might be conserved in the paralogous protein heterogeneous nuclear RNP G, whereas the second mode of binding is found only in hRBMY. This structural difference could be at the origin of the function of RBMY in spermatogenesis.

Structure and specific RNA binding of ADAR2 double-stranded RNA binding motifs

Adenosine deaminases that act on RNA (ADARs) site-selectively modify adenosines to inosines within RNA transcripts, thereby recoding genomic information. How ADARs select specific adenosine moieties for deamination is poorly understood. Here, we report NMR structures of the two double-stranded RNA binding motifs (dsRBMs) of rat ADAR2 and an NMR chemical shift perturbation study of the interaction of the two dsRBMs with a 71 nucleotide RNA encoding the R/G site of the GluR-B. We have identified the protein and the RNA surfaces involved in complex formation, allowing us to present an NMR-based model of the complex. We have found that dsRBM1 recognizes a conserved pentaloop, whereas dsRBM2 recognizes two bulged bases adjacent to the editing site, demonstrating RNA structure-dependent recognition by the ADAR2 dsRBMs. In vitro mutagenesis studies with both the protein and the RNA further support our structural findings.

Shape-specific recognition in the structure of the Vts1p SAM domain with RNA

Although the abundant sterile alpha motif (SAM) domain was originally classified as a protein-protein interaction domain, it has recently been shown that certain SAM domains have the ability to bind RNA, defining a new type of post-transcriptional gene regulator. To further understand the function of SAM-RNA recognition, we determined the solution structures of the SAM domain of the Saccharomyces cerevisiae Vts1p (Vts1p-SAM) and the Smaug response element (SRE) stem-loop RNA as a complex and in isolation. The structures show that Vts1p-SAM recognizes predominantly the shape of the SRE rather than its sequence, with the exception of a G located at the tip of the pentaloop. Using microarray gene profiling, we identified several genes in S. cerevisiae that seem to be regulated by Vts1p and contain one or more copies of the SRE.

RNA sequence- and shape-dependent recognition by proteins in the ribonucleoprotein particle

At all stages of its life (from transcription to translation), an RNA transcript interacts with many different RNA-binding proteins. The composition of this supramolecular assembly, known as a ribonucleoprotein particle, is diverse and highly dynamic. RNA-binding proteins control the generation, maturation and lifespan of the RNA transcript and thus regulate and influence the cellular function of the encoded gene. Here, we review our current understanding of protein-RNA recognition mediated by the two most abundant RNA-binding domains (the RNA-recognition motif and the double-stranded RNA-binding motif) plus the zinc-finger motif, the most abundant nucleic-acid-binding domain. In addition, we discuss how not only the sequence but also the shape of the RNA are recognized by these three classes of RNA-binding protein.

A novel RNA pentaloop fold involved in targeting ADAR2

Adenosine deaminases that act on RNA (ADARs) catalyze the site-specific conversion of adenosine to inosine in primary mRNA transcripts, thereby affecting coding potential of mature mRNAs. Structural determinants that define the adenosine moieties for specific ADARs-mediated deaminations are currently unknown. We report the solution structure of the central region of the human R/G stem-loop pre-mRNA, a natural ADAR2 substrate encoding the subunit B of the glutamate receptor (R/G site). The structure reveals that the GCU(A/C)A pentaloop that is conserved in mammals and birds adopts a novel fold. The fold is stabilized by a complex interplay of hydrogen bonds and stacking interactions. We propose that this new pentaloop structure is an important determinant of the R/G site recognition by ADAR2.

Molecular dynamics simulations of Guanine quadruplex loops: advances and force field limitations

A computational analysis of d(GGGGTTTTGGGG)(2) guanine quadruplexes containing either lateral or diagonal four-thymidine loops was carried out using molecular dynamics (MD) simulations in explicit solvent, locally enhanced sampling (LES) simulations, systematic conformational search, and free energy molecular-mechanics, Poisson Boltzmann, surface area (MM-PBSA) calculations with explicit inclusion of structural monovalent cations. The study provides, within the approximations of the applied all-atom additive force field, a qualitatively complete analysis of the available loop conformational space. The results are independent of the starting structures. Major conformational transitions not seen in conventional MD simulations are observed when LES is applied. The favored LES structures consistently provide lower free energies (as estimated by molecular-mechanics, Poisson Boltzmann, surface area) than other structures. Unfortunately, the predicted optimal structure for the diagonal loop arrangement differs substantially from the atomic resolution experiments. This result is attributed to force field deficiencies, such as the potential misbalance between solute-cation and solvent-cation terms. The MD simulations are unable to maintain the stable coordination of the monovalent cations inside the diagonal loops as reported in a recent x-ray study. The optimal diagonal and lateral loop arrangements appear to be close in energy although a proper inclusion of the loop monovalent cations could stabilize the diagonal architecture.

DNA A-tract bending in three dimensions: solving the dA4T4 vs. dT4A4 conundrum

DNA A-tracts have been defined as four or more consecutive A.T base pairs without a TpA step. When inserted in phase with the DNA helical repeat, bending is manifested macroscopically as anomalous migration on polyacrylamide gels, first observed >20 years ago. An unsolved conundrum is why DNA containing in-phase A-tract repeats of A(4)T(4) are bent, whereas T(4)A(4) is straight. We have determined the solution structures of the DNA duplexes formed by d(GCAAAATTTTGC) [A4T4] and d(CGTTTTAAAACG) [T4A4] with NH(4)(+) counterions by using NMR spectroscopy, including refinement with residual dipolar couplings. Analysis of the structures shows that the ApT step has a large negative roll, resulting in a local bend toward the minor groove, whereas the TpA step has a positive roll and locally bends toward the major groove. For A4T4, this bend is nearly in phase with bends at the two A-tract junctions, resulting in an overall bend toward the minor groove of the A-tract, whereas for T4A4, the bends oppose each other, resulting in a relatively straight helix. NMR-based structural modeling of d(CAAAATTTTG)(15) and d(GTTTTAAAAC)(15) reveals that the former forms a left-handed superhelix with a diameter of approximately 110 A and pitch of 80 A, similar to DNA in the nucleosome, whereas the latter has a gentle writhe with a pitch of >250 A and diameter of approximately 50 A. Results of gel electrophoretic mobility studies are consistent with the higher-order structure of the DNA and furthermore depend on the nature of the monovalent cation present in the running buffer.

Formation pathways of a guanine-quadruplex DNA revealed by molecular dynamics and thermodynamic analysis of the substates

The formation of a cation-stabilized guanine quadruplex (G-DNA) stem is an exceptionally slow process involving complex kinetics that has not yet been characterized at atomic resolution. Here, we investigate the formation of a parallel stranded G-DNA stem consisting of four strands of d(GGGG) using molecular dynamics simulations with explicit inclusion of counterions and solvent. Due to the limitations imposed by the nanosecond timescale of the simulations, rather than watching for the spontaneous formation of G-DNA, our approach probes the stability of possible supramolecular intermediates (including two-, three-, and four-stranded assemblies with out-of-register base pairing between guanines) on the formation pathway. The simulations suggest that "cross-like" two-stranded assemblies may serve as nucleation centers in the initial formation of parallel stranded G-DNA quadruplexes, proceeding through a series of rearrangements involving trapping of cations, association of additional strands, and progressive slippage of strands toward the full stem. To supplement the analysis, approximate free energies of the models are obtained with explicit consideration of the integral cations. The approach applied here serves as a prototype for qualitatively investigating other G-DNA molecules using molecular dynamics simulation and free-energy analysis.

Non-Watson-Crick basepairing and hydration in RNA motifs: molecular dynamics of 5S rRNA loop E

Explicit solvent and counterion molecular dynamics simulations have been carried out for a total of >80 ns on the bacterial and spinach chloroplast 5S rRNA Loop E motifs. The Loop E sequences form unique duplex architectures composed of seven consecutive non-Watson-Crick basepairs. The starting structure of spinach chloroplast Loop E was modeled using isostericity principles, and the simulations refined the geometries of the three non-Watson-Crick basepairs that differ from the consensus bacterial sequence. The deep groove of Loop E motifs provides unique sites for cation binding. Binding of Mg(2+) rigidifies Loop E and stabilizes its major groove at an intermediate width. In the absence of Mg(2+), the Loop E motifs show an unprecedented degree of inner-shell binding of monovalent cations that, in contrast to Mg(2+), penetrate into the most negative regions inside the deep groove. The spinach chloroplast Loop E shows a marked tendency to compress its deep groove compared with the bacterial consensus. Structures with a narrow deep groove essentially collapse around a string of Na(+) cations with long coordination times. The Loop E non-Watson-Crick basepairing is complemented by highly specific hydration sites ranging from water bridges to hydration pockets hosting 2 to 3 long-residing waters. The ordered hydration is intimately connected with RNA local conformational variations.

Refinement of d(GCGAAGC) hairpin structure using one- and two-bond residual dipolar couplings

The structure of the 13C,15N-labeled d(GCGAAGC) hairpin, as determined by NMR spectroscopy and refined using molecular dynamics with NOE-derived distances, torsion angles, and residual dipolar couplings (RDCs), is presented. Although the studied molecule is of small size, it is demonstrated that the incorporation of diminutive RDCs can significantly improve local structure determination of regions undefined by the conventional restraints. Very good correlation between the experimental and back-calculated small one- and two-bond 1H-13C, 1H-15N, 13C-13C and 13C-15N coupling constants has been attained. The final structures clearly show typical features of the miniloop architecture. The structure is discussed in context of the extraordinary stability of the d(GCGAAGC) hairpin, which originates from a complex interplay between the aromatic base stacking and hydrogen bonding interactions.

Determination of the glycosidic torsion angles in uniformly 13C-labeled nucleic acids from vicinal coupling constants 3J(C2)/4-H1' and 3J(C6)/8-H1'

A two-dimensional, quantitative J-correlation NMR experiment for precise measurements of the proton-carbon vicinal coupling constants 3J(C2)/4-H1' and 3J(C6)/8-H1' in uniformly 13C-labeled nucleic acids is presented. To reduce loss of signal due to 1H-13C dipole-dipole relaxation, a multiple-quantum constant time experiment with appropriately incorporated band selective 1H and 13C pulses was applied. The experiment is used to obtain the 3J(C2)/4-H1' and 3J(C6)/8-H1' coupling constants in a uniformly 13C, 15N-labeled [d(G4T4G4)]2 quadruplex. The measured values and glycosidic torsion angles in the G-quadruplex, obtained by restrained molecular dynamics with explicit solvent using the previously published restraints, along with selected data from the literature are used to check and modify existing parameters of the Karplus equations. The parameterizations obtained using glycosidic torsion angles derived from the original solution and recently determined X-ray structures are also compared.

Molecular dynamics of the frame-shifting pseudoknot from beet western yellows virus: the role of non-Watson-Crick base-pairing, ordered hydration, cation binding and base mutations on stability and unfolding

Molecular dynamics simulations of the frame-shifting pseudoknot from beet western yellows virus (BWYV, NDB file UR0004) were performed with explicit inclusion of solvent and counterions. In all, 33 ns of simulation were carried out, including 10 ns of the native structure with protonation of the crucial cytosine residue, C8(N3+). The native structure exhibited stable trajectories retaining all Watson-Crick and tertiary base-pairs, except for fluctuations or transient disruptions at specific sites. The most significant fluctuations involved the change or disruption of hydrogen-bonding between C8(N3+) and bases G12, A25, and C26, as well as disruption of the water bridges linking C8(N3+) with A25 and C26. To increase sampling of rare events, the native simulation was continued at 400 K. A partial, irreversible unfolding of the molecule was initiated by slippage of C8(N3+) relative to G12 and continued by sudden concerted changes in hydrogen-bonding involving A23, A24, and A25. These events were followed by a gradual loss of stacking interactions in loop 2. Of the Watson-Crick base-pairs, only the 5'-terminal pair of stem 1 dissociated at 400 K, while the trans sugar-edge/sugar-edge A20.G4 interaction remained surprisingly stable. Four additional room-temperature simulations were carried out to obtain insights into the structural and dynamic effects of selected mutations. In two of these, C8 was left unprotonated. Considerable local rearrangements occurred that were not observed in the crystal structure, thus confirming N3-protonation of C8 in the native molecule. We also investigated the effect of mutating C8(N3+) to U8, to correlate with experimental and phylogenetic studies, and of changing the G4 x C17 base-pair to A4 x U17 to weaken the trans sugar-edge interaction between positions 4 and 20 and to test models of unfolding. The simulations indicate that the C8 x G12 x C26 base-triple at the junction is the most labile region of the frame-shifting pseudoknot. They provide insights into the roles of the other non-Watson-Crick base-pairs in the early stages of unfolding of the pseudoknot, which must occur to allow readthrough of the message by the ribosome. The simulations revealed several critical, highly ordered hydration sites with close to 100 % occupancies and residency times of individual water molecules of up to 5 ns. Sodium cation coordination sites with occupancies above 50 % were also observed.

NMR methodology for the study of nucleic acids

During the past few years, NMR methodology for the study of nucleic acids has benefited from new developments that greatly improved state-of-the-art technology for the precise determination of three-dimensional structures. Substantial progress has been made in designing experimental protocols for the measurement of residual dipolar couplings, in sensitivity optimization of triple-resonance experiments and in detection of hydrogen bonds and in developing computational methods for structure refinement using NMR restraints.

A-like guanine-guanine stacking in the aqueous DNA duplex of d(GGGGCCCC)

We have used CD spectroscopy, NMR spectroscopy and unrestrained molecular dynamics to study conformational properties of a DNA duplex formed by the self-complementary octamer d(GGGGCCCC). Its unusual CD spectrum contains features indicating A-like stacking of half of the bases, whereas the other half stack in a B-like fashion. Unrestrained molecular dynamics simulations converged to a stable B-like double-helix of d(GGGGCCCC). However, the double-helix contained a central hole whose size was half of that occurring in structure A. In the canonical structure B, the hole does not exist at all because the base-pairs cross the double-helix centre. The cytosine bases were stacked in the duplex of d(GGGGCCCC) as in structure B, while stacking of the guanine bases displayed features characteristic for structure A. NMR spectroscopy revealed that the A-like guanine-guanine stacking was accompanied by an increased tendency of the deoxyribose rings attached to the guanine bases to be puckered in an A-like fashion. Otherwise, the duplex of d(GGGGCCCC) showed no clash, no bend and no other significant deviation from structure B. The present analysis demonstrates a remarkable propensity of the guanine runs to stack in an A-like fashion even within the B-DNA framework. This property explains why the oligo(dG). oligo(dC) tracts switch into structure A so easily. Secondly, this property may influence replication, because structure A is replicated more faithfully than structure B. Thirdly, the oligo(dG) runs might have played an important role in early evolution, when DNA took on functions that originally evolved on RNA. Fourthly, the present study extends the vocabulary of DNA secondary structures by the heteronomous duplex of d(GGGGCCCC) in which the B-like strand of oligo(dC) is bound to the A-like strand of oligo(dG).

Molecular dynamics of DNA quadruplex molecules containing inosine, 6-thioguanine and 6-thiopurine

The ability of the four-stranded guanine (G)-DNA motif to incorporate nonstandard guanine analogue bases 6-oxopurine (inosine, I), 6-thioguanine (tG), and 6-thiopurine (tI) has been investigated using large-scale molecular dynamics simulations. The simulations suggest that a G-DNA stem can incorporate inosines without any marked effect on its structure and dynamics. The all-inosine quadruplex stem d(IIII)(4) shows identical dynamical properties as d(GGGG)(4) on the nanosecond time scale, with both molecular assemblies being stabilized by monovalent cations residing in the channel of the stem. However, simulations carried out in the absence of these cations show dramatic differences in the behavior of d(GGGG)(4) and d(IIII)(4). Whereas vacant d(GGGG)(4) shows large fluctuations but does not disintegrate, vacant d(IIII)(4) is completely disrupted within the first nanosecond. This is a consequence of the lack of the H-bonds involving the N2 amino group that is not present in inosine. This indicates that formation of the inosine quadruplex could involve entirely different intermediate structures than formation of the guanosine quadruplex, and early association of cations in this process appears to be inevitable. In the simulations, the incorporation of 6-thioguanine and 6-thiopurine sharply destabilizes four-stranded G-DNA structures, in close agreement with experimental data. The main reason is the size of the thiogroup leading to considerable steric conflicts and expelling the cations out of the channel of the quadruplex stem. The G-DNA stem can accommodate a single thioguanine base with minor perturbations. Incorporation of a thioguanine quartet layer is associated with a large destabilization of the G-DNA stem whereas the all-thioguanine quadruplex immediately collapses.

An A-type double helix of DNA having B-type puckering of the deoxyribose rings

DNA usually adopts structure B in aqueous solution, while structure A is preferred in mixtures of trifluoroethanol (TFE) with water. However, the octamer d(CCCCGGGG) and other d(C(n)G(n)) fragments of DNA provide CD spectra that suggest that the base-pairs are stacked in an A-like fashion even in aqueous solution. Yet, d(CCCCGGGG) undergoes a cooperative TFE-induced transition into structure A, indicating that an important part of the aqueous duplex retains structure B. NMR spectroscopy shows that puckering of the deoxyribose rings is of the B-type. Hence, combination of the information provided by CD spectroscopy and NMR spectroscopy suggests an unprecedented double helix of DNA in which A-like base stacking is combined with B-type puckering of the deoxyribose rings. In order to determine whether this combination is possible, we used molecular dynamics to simulate the duplex of d(CCCCGGGG). Remarkably, the simulations, completely unrestrained by the experimental data, provided a very stable double helix of DNA, exhibiting just the intermediate B/A features described above. The double helix contained well-stacked guanine bases but almost unstacked cytosine bases. This generated a hole in the double helix center, which is a property characteristic for A-DNA, but absent from B-DNA. The minor groove was narrow at the double helix ends but wide at the central CG step where the Watson-Crick base-pairs were buckled in opposite directions. The base-pairs stacked tightly at the ends but stacking was loose in the duplex center. The present double helix, in which A-like base stacking is combined with B-type sugar puckering, is relevant to replication and transcription because both of these phenomena involve a local B-to-A transition.

Differences in conformational behavior of ATA and TAT sequences in single strand DNA trimer

The conformational behavior of single strand (ss) TAT and ATA trimers of DNA have been studied by computational chemistry tools including CICADA software interfaced with AMBER molecular mechanics and dynamics. The Single-Coordinate-Driving (SCD) method has been used in conjunction with molecular dynamics simulated annealing. It has been revealed that the conformational flexibility of each sequence differs substantially from the other one. Four common conformational families have been found for both trimers. These are: helical, reverse-stacked (base 3), half-stacked (base 3), reverse-stacked (base 1). However, the energies of conformers representing the families are different for both the studied systems. An additional conformational family, bulged, has been found for ss(ATA), while ss(TAT) has been found also in half-stacked (base 1) conformation. In general, ss(TAT) exhibits a higher number of low energy conformations while ss(ATA) shows one interesting low energy conformational interconversion between reverse-stacked (A3) family and half-stacked (A3) family. The high conformational variability of the trimers has been confirmed by flexibility analysis and by molecular dynamics simulations, which have also shown the conformational stability of single conformational families. It has been concluded that the methodology used is able to provide a very detailed picture of the conformational space of these molecules.