Optical spectroscopic study of the effects of a single deoxyribose substitution in a ribose backbone: implications in RNA-RNA interaction

Selective Metal Ion Utilization Contributes to the Transformation of the Activity of Yeast Polymerase η from DNA Polymerization toward RNA Polymerization

Polymerase eta (Polη) is a translesion synthesis DNA polymerase directly linked to cancer development. It can bypass several DNA lesions thereby rescuing DNA damage-stalled replication complexes. We previously presented evidence implicating Saccharomyces cerevisiae Polη in transcription elongation, and identified its specific RNA extension and translesion RNA synthetic activities. However, RNA synthesis by Polη proved rather inefficient under conditions optimal for DNA synthesis. Searching for factors that could enhance its RNA synthetic activity, we have identified the divalent cation of manganese. Here, we show that manganese triggers drastic changes in the activity of Polη. Kinetics experiments indicate that manganese increases the efficiency of ribonucleoside incorporation into RNA by ~400–2000-fold opposite undamaged DNA, and ~3000 and ~6000-fold opposite TT dimer and 8oxoG, respectively. Importantly, preference for the correct base is maintained with manganese during RNA synthesis. In contrast, activity is strongly impaired, and base discrimination is almost lost during DNA synthesis by Polη with manganese. Moreover, Polη shows strong preference for manganese during RNA synthesis even at a 25-fold excess magnesium concentration. Based on this, we suggest that a new regulatory mechanism, selective metal cofactor utilization, modulates the specificity of Polη helping it to perform distinct activities needed for its separate functions during replication and transcription.

Stacking geometry between two sheared Watson-Crick basepairs: Computational chemistry and bioinformatics based prediction

BACKGROUND

Molecular modeling of RNA double helices is possible using most probable values of basepair parameters obtained from crystal structure database. The A:A w:wC non-canonical basepair, involving Watson-Crick edges of two Adenines in cis orientation, appears quite frequently in database. Bimodal distribution of its Shear, due to two different H-bonding schemes, introduces the confusion in assigning most the probable value. Its effect is pronounced when the A:A w:wC basepair stacks on Sheared wobble G:U W:WC basepairs.

METHODS

We employed molecular dynamics simulations of three possible double helices with GAG, UAG and GAU sequence motifs at their centers and quantum chemical calculation for non-canonical A:A w:wC basepair stacked on G:U W:WC basepair.

RESULTS

We noticed stable structures of GAG motif with specifically negative Shear of the A:A basepair but stabilities of the other motifs were not found with A:A w:wC basepairing. Hybrid DFT-D and MP2 stacking energy analyses on dinucleotide step sequences, A:A w:wC::G:U W:WC and A:A w:wC::U:G W:WC reveal that viable orientation of A:A::G:U prefers one of the H-bonding modes with negative Shear, supported by crystal structure database. The A:A::U:G dinucleotide, however, prefers structure with only positive Shear.

CONCLUSIONS

The quantum chemical calculations explain why MD simulations of GAG sequence motif only appear stable. In the cases of the GAU and UAG motifs "tug of war" situation between positive and negative Shears of A:A w:wC basepair induces conformational plasticity.

GENERAL SIGNIFICANCE

We have projected comprehensive reason behind the promiscuous nature of A:A w:wC basepair which brings occasional structural plasticity.

Interactions of p53 with poly(ADP-ribose) and DNA induce distinct changes in protein structure as revealed by ATR-FTIR spectroscopy

Due to multiple domains and in part intrinsically disordered regions, structural analyses of p53 remain a challenging task, particularly in complex with DNA and other macromolecules. Here, we applied a novel attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic approach to investigate changes in secondary structure of full-length p53 induced by non-covalent interactions with DNA and poly(ADP-ribose) (PAR). To validate our approach, we confirmed a positive regulatory function of p53’s C-terminal domain (CTD) with regard to sequence-specific DNA binding and verified that the CTD mediates p53–PAR interaction. Further, we demonstrate that DNA and PAR interactions result in distinct structural changes of p53, indicating specific binding mechanisms via different domains. A time-dependent analysis of the interplay of DNA and PAR binding to p53 revealed that PAR represents p53’s preferred binding partner, which efficiently controls p53–DNA interaction. Moreover, we provide infrared spectroscopic data on PAR pointing to the absence of regular secondary structural elements. Finally, temperature-induced melting experiments via CD spectroscopy show that DNA binding stabilizes the structure of p53, while PAR binding can shift the irreversible formation of insoluble p53 aggregates to higher temperatures. In conclusion, this study provides detailed insights into the dynamic interplay of p53 binding to DNA and PAR at a formerly inaccessible molecular level.

Why are Hoogsteen base pairs energetically disfavored in A-RNA compared to B-DNA?

A(syn)-U/T and G(syn)-C⁺ Hoogsteen (HG) base pairs (bps) are energetically more disfavored relative to Watson–Crick (WC) bps in A-RNA as compared to B-DNA by >1 kcal/mol for reasons that are not fully understood. Here, we used NMR spectroscopy, optical melting experiments, molecular dynamics simulations and modified nucleotides to identify factors that contribute to this destabilization of HG bps in A-RNA. Removing the 2′-hydroxyl at single purine nucleotides in A-RNA duplexes did not stabilize HG bps relative to WC. In contrast, loosening the A-form geometry using a bulge in A-RNA reduced the energy cost of forming HG bps at the flanking sites to B-DNA levels. A structural and thermodynamic analysis of purine-purine HG mismatches reveals that compared to B-DNA, the A-form geometry disfavors syn purines by 1.5–4 kcal/mol due to sugar-backbone rearrangements needed to sterically accommodate the syn base. Based on MD simulations, an additional penalty of 3–4 kcal/mol applies for purine-pyrimidine HG bps due to the higher energetic cost associated with moving the bases to form hydrogen bonds in A-RNA versus B-DNA. These results provide insights into a fundamental difference between A-RNA and B-DNA duplexes with important implications for how they respond to damage and post-transcriptional modifications.

Vibrational Dynamics and Couplings of the Hydrated RNA Backbone: A Two-Dimensional Infrared Study

The equilibrium structure of the RNA sugar-phosphate backbone and its hydration shell is distinctly different from hydrated DNA. Applying femtosecond two-dimensional infrared (2D-IR) spectroscopy in a range from 950 to 1300 cm^-1, we elucidate the character, dynamics, and couplings of backbone modes of a double-stranded RNA A-helix geometry in its aqueous environment. The 2D-IR spectra display a greater number of backbone modes than for DNA, with distinctly different lineshapes of diagonal peaks. Phosphate-ribose interactions and local hydration structures are reflected in the complex coupling pattern of RNA modes. Interactions with the fluctuating water shell give rise to spectral diffusion on a 300 fs time scale, leading to a quasi-homogeneous line shape of the symmetric (PO₂)^- stretching mode of the strongly hydrated phosphate groups. The RNA results are benchmarked by 2D-IR spectra of DNA oligomers in water and analyzed by molecular dynamics and quantum mechanical molecular mechanics simulations.

Translesion synthesis DNA polymerase η exhibits a specific RNA extension activity and a transcription-associated function

Polymerase eta (Polη) is a low fidelity translesion synthesis DNA polymerase that rescues damage-stalled replication by inserting deoxy-ribonucleotides opposite DNA damage sites resulting in error-free or mutagenic damage bypass. In this study we identify a new specific RNA extension activity of Polη of Saccharomyces cerevisiae. We show that Polη is able to extend RNA primers in the presence of ribonucleotides (rNTPs), and that these reactions are an order of magnitude more efficient than the misinsertion of rNTPs into DNA. Moreover, during RNA extension Polη performs error-free bypass of the 8-oxoguanine and thymine dimer DNA lesions, though with a 10³ and 10²-fold lower efficiency, respectively, than it synthesizes opposite undamaged nucleotides. Furthermore, in vivo experiments demonstrate that the transcription of several genes is affected by the lack of Polη, and that Polη is enriched over actively transcribed regions. Moreover, inactivation of its polymerase activity causes similar transcription inhibition as the absence of Polη. In summary, these results suggest that the new RNA synthetic activity of Polη can have in vivo relevance.

The role of sequence context, nucleotide pool balance and stress in 2'-deoxynucleotide misincorporation in viral, bacterial and mammalian RNA

The misincorporation of 2′-deoxyribonucleotides (dNs) into RNA has important implications for the function of non-coding RNAs, the translational fidelity of coding RNAs and the mutagenic evolution of viral RNA genomes. However, quantitative appreciation for the degree to which dN misincorporation occurs is limited by the lack of analytical tools. Here, we report a method to hydrolyze RNA to release 2′-deoxyribonucleotide-ribonucleotide pairs (dNrN) that are then quantified by chromatography-coupled mass spectrometry (LC-MS). Using this platform, we found misincorporated dNs occurring at 1 per 10³ to 10⁵ ribonucleotide (nt) in mRNA, rRNAs and tRNA in human cells, Escherichia coli, Saccharomyces cerevisiae and, most abundantly, in the RNA genome of dengue virus. The frequency of dNs varied widely among organisms and sequence contexts, and partly reflected the in vitro discrimination efficiencies of different RNA polymerases against 2′-deoxyribonucleoside 5′-triphosphates (dNTPs). Further, we demonstrate a strong link between dN frequencies in RNA and the balance of dNTPs and ribonucleoside 5′-triphosphates (rNTPs) in the cellular pool, with significant stress-induced variation of dN incorporation. Potential implications of dNs in RNA are discussed, including the possibilities of dN incorporation in RNA as a contributing factor in viral evolution and human disease, and as a host immune defense mechanism against viral infections.

DNA damage induced by carbon ions (C³⁺) beam accessed by independent component analysis of infrared spectra

PURPOSE

To understand the degradation processes and role of secondary species on deoxyribonucleic acid (DNA)-damaged fragments as a result of irradiation with energetic ions.

MATERIALS AND METHODS

Damage caused to DNA thin films as a result of exposure to 4 keV carbon ions beam was accessed by analyzing the infrared spectra, obtained in situ for different irradiation times, with both bi-dimensional (2D) correlation spectroscopy and independent component analysis (ICA).

RESULTS

Results indicated that deoxyribose, phosphate and base groups of the DNA molecule were being damaged and new reaction products as oxime and furfural groups are being formed.

CONCLUSIONS

Damage on DNA bases is consistent with the formation of oxime products which react with DNA deoxyribose products forming furfural groups and confirming that DNA damage is caused by direct and indirect processes.

Anharmonic vibrational modes of nucleic acid bases revealed by 2D IR spectroscopy

Polarization-dependent two-dimensional infrared (2D IR) spectra of the purine and pyrimadine base vibrations of five nucleotide monophosphates (NMPs) were acquired in D(2)O at neutral pH in the frequency range 1500-1700 cm(-1). The distinctive cross-peaks between the ring deformations and carbonyl stretches of NMPs indicate that these vibrational modes are highly coupled, in contrast with the traditional peak assignment, which is based on a simple local mode picture such as C═O, C═N, and C═C double bond stretches. A model of multiple anharmonically coupled oscillators was employed to characterize the transition energies, vibrational anharmonicities and couplings, and transition dipole strengths and orientations. No simple or intuitive structural correlations are found to readily assign the spectral features, except in the case of guanine and cytosine, which contain a single local CO stretching mode. To help interpret the nature of these vibrational modes, we performed density functional theory (DFT) calculations and found that multiple ring vibrations are coupled and delocalized over the purine and pyrimidine rings. Generally, there is close correspondence between the experimental and computational results, provided that the DFT calculations include explicit waters solvating hydrogen-bonding sites. These results provide direct experimental evidence of the delocalized nature of the nucleotide base vibrations via a nonperturbative fashion and will serve as building blocks for constructing a structure-based model of DNA and RNA vibrational spectroscopy.

Pairwise coupling analysis of helical junction hydrogen bonding interactions in luteoviral RNA pseudoknots

A 28-nucleotide mRNA pseudoknot that overlaps the P1 and P2 genes of sugarcane yellow leaf virus (ScYLV) stimulates -1 ribosomal frameshifting. The in vitro frameshifting efficiency is decreased >or=8-fold upon substitution of the 3'-most loop 2 nucleotide (C27) with adenosine, which accepts a hydrogen bond from the 2'-OH group of C14 in stem S1. The solution structures of the wild-type (WT) and C27A ScYLV RNA pseudoknots show that while the RNAs adopt virtually identical overall structures, there are significant structural differences at the helical junctions of the two RNAs. Specifically, C8(+) in loop L1 in the C8(+).(G12.C28) L1-S2 major groove base triple is displaced by approximately 2.3 A relative to the accepting stem 2 base pair (G12.C28) in the C27A RNA. Here, we use a double mutant cycle approach to analyze the pairwise coupling of the C8(+).(G12.C28)...C27.(C14-G7) and ...A27.(C14-G7) hydrogen bonds in the WT and C27A ScYLV RNAs, respectively, and compare these findings with previous results from the beet western yellows virus (BWYV) RNA. We find that the pairwise coupling free energy (delta(AB)(i)) is favorable for the WT RNA (-0.7 +/- 0.1 kcal/mol), thus revealing that formation of these two hydrogen bonds is positively cooperative. In contrast, delta(AB)(i) is 0.9 +/- 0.4 kcal/mol for the poorly functional C27A ScYLV RNA, indicative of nonadditive hydrogen bond formation. These results reveal that cooperative hydrogen bond formation across the helical stem junction in H-type pseudoknots correlates with enhanced frameshift stimulation by luteoviral mRNA pseudoknots.

A nature of conformational changes of yeast tRNAPhe

We analysed conformational changes of yeast tRNA(Phe) induced by high hydrostatic pressure (HHP) measured by Fourier-transform infrared (FTIR) and fluorescence spectroscopies. High pressure influences RNA conformation without other cofactors, such as metal ions and salts. FTIR spectra of yeast tRNA(Phe) recorded at high hydrostatic pressure up to 13 kbar with and without magnesium ions showed a shift of the bands towards higher frequencies. That blue shift is due to an increase a higher energy of bonds as a result of shortening of hydrogen bonds followed by dehydration of tRNA. The fluorescence spectra of Y-base tRNA(Phe) at high pressure up to 3 kbar showed a decrease of the intensity band at 430 nm as a consequence of conformational rearrangement of the anticodon loop leading to exposure of Y-base side chain to the solution. We suggest that structural transition of nucleic acids is driven by the changes of water structure from tetrahedral to a cubic-like geometry induced by high pressure and, in consequence, due to economy of hydration.

Effects of base substitutions in an RNA hairpin from molecular dynamics and free energy simulations

Contributions of individual interactions in the GGCGCAAGCC hairpin containing a GCAA tetraloop were studied by computer simulations using base substitutions. The G in the first tetraloop position was replaced by inosine (I) or adenosine (A), and the G in the C-G basepair closing the tetraloop was replaced by I. These substitutions eliminate particular hydrogen bonds proposed in the nuclear magnetic resonance model of the GCAA tetraloop. Molecular dynamics simulations of the GCAA tetraloop in aqueous solvent displayed a well-defined hydrogen pattern between the first and last loop nucleotides (G and A) stabilized by a bridging water molecule. Substitution of G-->I in the basepair closing the tetraloop did not significantly influence the loop structure and dynamics. The ICAA loop maintained the overall structure, but displayed variation in the hydrogen-bond network within the tetraloop itself. Molecular dynamics simulations of the ACAA loop led to conformational heterogeneity of the resulting structures. Changes of hairpin formation free energy associated with substitutions of individual bases were calculated by the free energy perturbation method. The calculated decrease of the hairpin stability upon G-->I substitution in the C-G basepair closing the tetraloop was in good agreement with experimental thermodynamic data. Our theoretical estimates for G-->I and G-->A mutations located in the tetraloop suggest larger loop destabilization than corresponding experimental results. The extent of conformational sampling of the structures resulting from base substitutions and its impact on the calculated free energy was discussed.

Substrate 2'-hydroxyl groups required for ribozyme-catalyzed polymerization

A polymerase ribozyme has been generated that uses nucleoside triphosphates to elongate an RNA primer by the successive addition of nucleotides complementary to an RNA template. Its polymerization is accurate, with an average error rate less than 3%, and it is general in terms of the sequence and the length of the primer and template RNAs. To begin to understand how the substrate contacts contribute to this accurate and general activity, we investigated which primer and template 2'-hydroxyl groups are involved in substrate recognition. We identified eight positions where 2'-deoxy substitutions can influence polymerization kinetics. All eight are within five nucleotides of the primer 3' terminus. Some, but not all, of the 2'-deoxy effects appear to be sequence dependent. These results begin to build a picture of how the polymerase ribozyme recognizes its substrates.

A library of IR bands of nucleic acids in solution

This review presents a compilation and discussion of infrared (IR) bands characteristic of nucleic acids in various conformations. The entire spectral range 1800-800 cm(-1) relevant for DNA/RNA in aqueous solution has been subdivided into four sections. Each section contains descriptions of bands appearing from group specific parts of nucleic acid structure, such as nucleobase, base-sugar, sugar-phosphate and sugar moiety. The approach allows comparisons of information obtained from one spectral region with another. The IR band library should facilitate detailed and unambiguous assignment of structural changes, ligand binding, etc. in nucleic acids from IR spectra. is aimed at highlighting specific features that are useful for following major changes in nucleic acid structures. also concerns some recent results, where IR spectroscopy has been used to obtain semi-quantitative information on coexisting modes of sugar pucker in oligonucleotides.

MD simulations of homomorphous PNA, DNA, and RNA single strands: characterization and comparison of conformations and dynamics

MD simulations of homomorphous single-stranded PNA, DNA, and RNA with the same base sequence have been performed in aqueous solvent. For each strand two separate simulations were performed starting from a (i) helical conformation and (ii) random coiled state. Comparisons of the simulations with the single-stranded helices (case i) show that the differences in the covalent nature of the backbones cause significant differences in the structural and dynamical properties of the strands. It is found that the PNA strand maintains its nice base-stacked initial helical structure throughout the 1.5-ns MD simulation at 300 K, while DNA/RNA show relatively larger fluctuations in the structures with a few local unstacking events during -ns MD simulation each. It seems that the weak physical coupling between the bases and the backbone in PNA causes a loss of correlation between the dynamics of the bases and the backbone compared to the DNA/RNA and helps maintain the base-stacked helical conformation. The global flexibility of a single-stranded PNA helix was also found to be lowest, while RNA appears to be the most flexible single-stranded helix. The sugar pucker of several nucleotides in single-stranded DNA and RNA was found to adopt both C2'-endo and C3'-endo conformations for significant times. This effect is more pronounced for single strands in completely coiled states. The simulations with single-stranded coils as the initial structure also indicate that a PNA can adopt a more compact globular structure, while DNA/RNA of the same size adopts a more extended coil structure. This allows even a short PNA in the coiled state to form a significantly stable nonsequentially base-stacked globular structure in solution. Due to the hydrophobic nature of the PNA backbone, it interacts with surrounding water rather weakly compared to DNA/RNA.

Probing RNA in vivo with methylation guide small nucleolar RNAs

Most box C/D small nucleolar RNAs (snoRNAs) direct the formation of 2'-O-methylated nucleotides in ribosomal RNA and, apparently, other RNAs present in the nucleolar complex. Sites to be modified are selected by a long (>10-nt) antisense guide sequence in the snoRNA and a distance measurement from a box D or D' element that follows the snoRNA guide sequence. Modification of the substrate occurs in the region of complementarity, at a position five nucleotides upstream from box D/D'. Methylation can be targeted to novel sites by expressing a snoRNA with a new guide sequence. In some cases methylation impairs the growth rate of the cell, indicating that a functionally important nucleotide has been altered. With a view to harnessing snoRNA-directed methylation for functional mapping, we have developed a method for constructing libraries of snoRNA genes that, in principle, can introduce methylation point mutations into any rRNA segment of interest. The strategy and procedures are described here, and preliminary results are presented that show the feasibility of using this technology to probe a region of the yeast large subunit rRNA that includes the core of the peptidyltransferase center.