NMR solution structures of bistranded abasic site lesions in DNA

Ionizing radiation produces clustered lesions in DNA. Since the orientation of bistranded lesions affects their recognition by DNA repair enzymes, clustered damages are more difficult to process and thus more toxic than single oxidative lesions. In order to understand the structural determinants that lead to differential recognition, we used NMR spectroscopy and restrained molecular dynamics to solve the structure of two DNA duplexes, each containing two stable abasic site analogues positioned on opposite strands of the duplex and staggered in the 3' (-1 duplex, (AP) 2-1 duplex) or 5' (+1 duplex, (AP) 2+1 duplex) direction. Cross-peak connectivities observed in the nonexchangeable NOESY spectra indicate compression of the helix at the lesion site of the duplexes, resulting in the formation of two abasic bulges. The exchangeable proton spectra show the AP site partner nucleotides forming interstrand hydrogen bonds that are characteristic of a Watson-Crick G.C base pairs, confirming the extra helical nature of the AP residues. Restrained molecular dynamics simulations generate a set of converging structures in full agreement with the spectroscopic data. In the (AP) 2-1 duplex, the extra helical abasic site residues reside in the minor groove of the helix, while they appear in the major groove in the (AP) 2+1 duplex. These structural differences are consistent with the differential recognition of bistranded abasic site lesions by human AP endonuclease.     

Conformational Dynamics of DNA Repair by Escherichia coli Endonuclease III

Escherichia coli endonuclease III (Endo III or Nth) is a DNA glycosylase with a broad substrate specificity for oxidized or reduced pyrimidine bases. Endo III possesses two types of activities: N-glycosylase (hydrolysis of the N-glycosidic bond) and AP lyase (elimination of the 3′-phosphate of the AP-site). We report a pre-steady-state kinetic analysis of structural rearrangements of the DNA substrates and uncleavable ligands during their interaction with Endo III. Oligonucleotide duplexes containing 5,6-dihydrouracil, a natural abasic site, its tetrahydrofuran analog, and undamaged duplexes carried fluorescent DNA base analogs 2-aminopurine and 1,3-diaza-2-oxophenoxazine as environment-sensitive reporter groups. The results suggest that Endo III induces several fast sequential conformational changes in DNA during binding, lesion recognition, and adjustment to a catalytically competent conformation. A comparison of two fluorophores allowed us to distinguish between the events occurring in the damaged and undamaged DNA strand. Combining our data with the available structures of Endo III, we conclude that this glycosylase uses a multistep mechanism of damage recognition, which likely involves Gln⁴¹ and Leu⁸¹ as DNA lesion sensors.     

Structural, dynamic, and enzymatic properties of mixed alpha/beta-oligonucleotides containing polarity reversals

We employ NMR structure determination, thermodynamics, and enzymatics to uncover the structural, thermodynamic and enzymatic properties of alpha/beta-ODNs containing 3'-3' and 5'-5' linkages. RNase H studies show that alpha/beta-gapmers that are designed to target erbB-2 efficiently elicit RNase H activity. NMR structures of DNA.DNA and DNA.RNA duplexes reveal that single alpha-anomeric residues fit well into either duplex, but alter the dynamic properties of the backbone and deoxyriboses as well as the topology of the minor groove in the DNA.RNA hybrid.     

NF-kappa B binding mechanism: a nuclear magnetic resonance and modeling study of a GGG --> CTC mutation

We present the solution structure of the nonpalindromic 16 bp DNA 5'd(CTGCTCACTTTCCAGG)3'. 5'd(CCTGGAAAGTGAGCAG)3' containing a mutated kappaB site for which the mutation of a highly conserved GGG tract of the native kappaB HIV-1 site to CTC abolishes NF-kappaB binding. 1H and 31P NMR spectroscopies have been used together with molecular modeling to determine the fine structure of the duplex. NMR data show evidence for a BI-BII equilibrium of the CpA.TpG steps at the 3'-end of the oligomer. Models for the extreme conformations reached by the mutated duplex (denoted 16M) are proposed in agreement with the NMR data. Since the distribution of BII sites is changed in the mutated duplex compared to that of the native duplex (denoted 16N), large differences are induced in the intrinsic structural properties of both duplexes. In particular, in BII structures, 16M shows a kink located at the 3'-end of the duplex, and in contrast, 16N exhibits an intrinsic global curvature toward the major groove. Whereas 16N can reach a conformation very favorable for the interaction with NF-kappaB, 16M cannot mimic such a conformation and, moreover, its deeper and narrower major groove could hinder the DNA-protein interactions.     

Influence of an exocyclic guanine adduct on the thermal stability, conformation, and melting thermodynamics of a DNA duplex.

As part of an overall program to characterize the impact of mutagenic lesions on the physiochemical properties of DNA, we report here the results of a comparative spectroscopic study on pairs of DNA duplexes both with and without an exocyclic guanine lesion. Specifically, we have studied a family of four 13-mer duplexes of the form d(CGCATGYGTACGC).d(GCGTACZCATGCG) in which Y is either the normal deoxyguanosine residue (G) or the exocyclic guanine adduct 1,N2-propanodeoxyguanosine (X), while Z is either deoxycytosine (C) or deoxyadenosine (A). Thus, the four duplexes studied, which can be designated by the identity of their central Y.Z base pair, are a Watson-Crick duplex (GC), a duplex with a central mismatch (GA), and two duplexes with exocyclic guanine lesions (X), that differ only by the base opposite the lesion (XC and XA). The data derived from our spectroscopic measurements on these four duplexes have allowed us to evaluate the influence of the exocyclic guanine lesion, as well as the base opposite the lesion, on the conformation, thermal stability, and melting energetics of the host DNA duplex. To be specific, our circular dichroism (CD) spectra show that the exocyclic guanine lesion induces alterations in the duplex structure, while our temperature-dependent optical measurements reveal that these lesion-induced structural alterations reduce the thermal stability, the transition enthalpy, and the transition free energy of the duplex.(ABSTRACT TRUNCATED AT 250 WORDS)     

DNase I-induced DNA conformation. 2 A structure of a DNase I-octamer complex.

The structure of a complex between DNase I and d(GCGATCGC)2 has been solved by molecular replacement and refined to an R-factor of 0.174 for all data between 6 and 2 A resolution. The nicked octamer duplexes have lost a dinucleotide from the 3' ends of one strand and are hydrogen-bonded across a 2-fold axis to form a quasi-continuous double helix of 14 base-pairs. DNase I is bound in the minor groove of the B-type DNA duplex forming contacts in and along both sides of the minor groove extending over a total of six base-pairs. As a consequence of binding of DNase I to the DNA-substrate the minor groove opens by about 3 A and the duplex bends towards the major groove by about 20 degrees. Apart from these more global distortions the bound duplex also shows significant deviations in local geometry. A major cause for the observed perturbations in the DNA conformation seems to be the stacking type interaction of a tyrosine ring (Y76) with a deoxyribose. In contrast, the enzyme structure is nearly unchanged compared to free DNase I (0.49 A root-mean-square deviations for main-chain atoms) thus providing a rigid framework to which the DNA substrate has to adapt on binding. These results confirm the hypothesis that groove width and stiffness are major factors determining the global sequence dependence of the enzyme's cutting rates. The nicked octamer present in the crystals did not allow us to draw detailed conclusions about the catalytic mechanism but confirmed the location of the active site near H134 on top of the central beta-sheets. A second cut of the DNA induced by diffusion of Mn2+ into the crystals may suggest the presence of a secondary active site in DNase I.     

Structure of NaeI-DNA complex reveals dual-mode DNA recognition and complete dimer rearrangement

NaeI, a novel DNA endonuclease, shows topoisomerase and recombinase activities when a Lys residue is substituted for Leu 43. The NaeI-DNA structure demonstrates that each of the two domains of NaeI recognizes one molecule of DNA duplex. DNA recognition induces dramatic rearrangements: narrowing the binding site of the Topo domain 16 A to grip DNA, widening that of the Endo domain 8 A to encircle and bend DNA 45 degrees for cleavage, and completely rebuilding the homodimer interface. The NaeI-DNA structure presents the first example of novel recognition of two copies of one DNA sequence by two different amino acid sequences and two different structural motifs in one polypeptide.     

Solution structure of duplex DNA containing the mutagenic lesion 1,N(2)-etheno-2'-deoxyguanine

We have used high-resolution NMR spectroscopy and molecular dynamics simulations to determine the solution structure of DNA containing the genotoxic lesion 1, N (2)-etheno-2'-deoxyguanosine (epsilonG), paired to dC. The NMR data suggest the presence of a major, minimally perturbed structure at neutral pH. NOESY spectra indicate the presence of a right-handed helix with all nucleotides in anti, 2'-deoxyribose conformations within the C2'-endo/C1'-exo range and proper Watson-Crick base pair alignments outside the lesion site. The epsilonG residue remains deeply embedded inside the helix and stacks between the flanking base pairs. The lesion partner dC is extrahelical and is located in the minor groove of the duplex, where it is highly exposed to solvent. Upon acidification of the sample, a second conformation at the lesion site of the duplex emerges, with protonation of the lesion partner dC and possible formation of a Hoogsteen base pair. Restrained molecular dynamics simulations of the neutral-pH structure generated a set of three-dimensional models that show epsilonG inside the helix, where the lesion is stabilized by stacking interactions with flanking bases but without participating in hydrogen bonding. The lesion counterbase dC is displaced in the minor groove of the duplex where it can form a hydrogen bond with the sugar O4' atom of a residue 2 bp away.     

Solution structure of DNA containing α-OH-PdG: the mutagenic adduct produced by acrolein

Acrolein is a cell metabolic product and a main component of cigarette smoke. Its reaction with DNA produces two guanine lesions γ-OH-PdG, a major adduct that is nonmutagenic in mammalian cells, and the positional isomer α-OH-PdG. We describe here the solution structure of a short DNA duplex containing a single α-OH-PdG lesion, as determined by solution NMR spectroscopy and restrained molecular dynamics simulations. The spectroscopic data show a mostly regular right-handed helix, locally perturbed at its center by the presence of the lesion. All undamaged residues of the duplex are in anti orientation, forming standard Watson–Crick base-pair alignments. Duplication of proton signals near the damaged site differentiates two enantiomeric duplexes, thus establishing the exocyclic nature of the lesion. At the lesion site, α-OH-PdG rotates to a syn conformation, pairing to its counter cytosine residue that is protonated at pH 5.9. Three-dimensional models produced by restrained molecular dynamics simulations show different hydrogen-bonding patterns between the lesion and its cytosine partner and identify further stabilization of α-OH-PdG in a syn conformation by intra-residue hydrogen bonds. We compare the α-OH-PdG•dC duplex structure with that of duplexes containing the analogous lesion propano-dG and discuss the implications of our findings for the mutagenic bypass of acrolein lesions.     

DNA structure and polymerase fidelity.

The accuracy of DNA replication results from both the intrinsic DNA polymerase fidelity and the DNA sequence. Although the recent structural studies on polymerases have brought new insights on polymerase fidelity, the role of DNA sequence and structure is less well understood. Here, the analysis of the crystal structures of hotspots for polymerase slippage including (CA)n and (A)n tracts in different intermolecular contexts reveals that, in the B-form, these sequences share common structural alterations which may explain the high rate of replication errors. In particular, a two-faced "Janus-like" structure with shifted base-pairs in the major groove but an apparent normal geometry in the minor groove constitutes a molecular decoy specifically suitable to mislead the polymerases. A model of the rat polymerase beta bound to this structure suggests that an altered conformation of the nascent template-primer duplex can interfere with correct nucleotide incorporation by affecting the geometry of the active site and breaking the rules of base-pairing, while at the same time escaping enzymatic mechanisms of error discrimination which scan for the correct geometry of the minor groove.In contrast, by showing that the A-form greatly attenuates the sequence-dependent structural alterations in hotspots, this study suggests that the A-conformation of the nascent template-primer duplex at the vicinity of the polymerase active site will contribute to fidelity. The A-form may play the role of a structural buffer which preserves the correct geometry of the active site for all sequences. The detailed comparison of the conformation of the nascent template-primer duplex in the available crystal structures of DNA polymerase-DNA complexes shows that polymerase beta, the least accurate enzyme, is unique in binding to a B-DNA duplex even close to its active site. This model leads to several predictions which are discussed in the light of published experimental data.     

Impact of the oxidized guanine lesion spiroiminodihydantoin on the conformation and thermodynamic stability of a 15-mer DNA duplex

Spiroiminodihydantoin (Sp) is a hyperoxidized guanine base produced from oxidation of the mutagenic DNA lesion 7,8-dihydro-8-oxo-2'-deoxguanosine (8-oxoG) by a variety of species including peroxynitrite, singlet oxygen, and the high-valent metals Ir(IV) and Cr(V). In this study, the conformation and thermodynamic stability of a 15-mer DNA duplex containing an Sp lesion are examined using spectroscopic techniques and differential scanning calorimetry (DSC). The Sp lesion does not alter the global B-form conformation of the DNA duplex as determined by circular dichroism spectroscopy. Thermal denaturation experiments find that Sp significantly lowers the thermal stability of the duplex by approximately 20 degrees C. The enthalpies, entropies, and free energies of duplex formation for 15-mers containing guanine, 8-oxoG, and Sp were determined by performing DSC experiments as well as van't Hoff analysis of UV melting spectroscopic data. The thermodynamic stability of the Sp duplex is significantly reduced compared to that of both the 8-oxoG and parent G duplexes, with the thermodynamic destabilization being enthalpic in origin. The thermodynamic impact of the Sp lesion is compared to what is found for other types of DNA base damage and discussed in relation to how the presence of this lesion could affect cellular processes, in particular the recognition and repair of these adducts by the base excision repair enzymes.     

Impact of the C1' configuration of abasic sites on DNA duplex structure

Naturally occurring abasic sites in DNA exist as an equilibrium mixture of the aldehyde, the hydrated aldehyde, and the hemiacetal forms (dominant). The influence of the configuration of the C1' hydroxyl group of the hemiacetal form on duplex structure and abasic site repair has been examined using novel carbocyclic analogues. Both the alpha- and beta-forms of this novel abasic site were introduced into oligomeric DNA using the standard DMT-phosphoramidite approach in an automated solid-phase synthesizer. Solution structures of the d(CGTACXCATGC).d(GCATGAGTACG) duplex (where X is the alpha- or beta-anomer of the carbocyclic abasic site analogue) were determined by NMR spectroscopy and restrained molecular dynamics simulations. The structures were only minimally perturbed by the presence of either anomer of the abasic site. All residues adopted an anti conformation, and Watson-Crick alignments were observed on all base pairs of the duplexes. At the lesion site, the abasic residues and their partner adenines showed increased dynamic behavior but adopted intrahelical positions in the final refined structures. Incision of duplexes having the alpha- or beta-anomer of the carbocyclic abasic site by human AP endonuclease showed that the enzyme recognizes both configurations of the lesion and nicks the DNA backbone with similar efficiency. Our results challenge the suggestion that Ape1 is stereoselective and imply a plasticity at the active site of the enzyme for accommodating either anomer of the lesion.     

Computational analysis of the mode of binding of 8-oxoguanine to formamidopyrimidine-DNA glycosylase

8-Oxoguanine (8OG) is the most prevalent form of oxidative DNA damage. In bacteria, 8OG is excised by formamidopyrimidine glycosylase (Fpg) as the initial step in base excision repair. To efficiently excise this lesion, Fpg must discriminate between 8OG and an excess of guanine in duplex DNA. In this study, we explore the structural basis underlying this high degree of selectivity. Two structures have been reported in which Fpg is bound to DNA, differing with respect to the position of the lesion in the active site, one structure showing 8OG bound in the syn conformation and the other in the anti conformation. Remarkably, the results of our all-atom simulations are consistent with both structures. The syn conformation observed in the crystallographic structure of Fpg obtained from Bacillus stearothermophilus is stabilized through interaction with E77, a nonconserved residue. Replacement of E77 with Ser, creating the Fpg sequence found in Escherichia coli and other bacteria, results in preferred binding of 8OG in the anti conformation. Our calculations provide novel insights into the roles of active site residues in binding and recognition of 8OG by Fpg.     

Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. IX. Cleavage of substrates with point modifications in the recognition site and flanking sequences

Ability of the EcoRII restriction endonuclease to cleave 14-base-pair DNA duplexes with nucleotide substitutions in the recognition site CCA/TGG and in the adjacent base pair has been studied. Modifications leading to a local change in the substrate conformation (rU residue in and outside the recognition site, A.A- or A.C-pairs in the flanking sequence) reduce the rate of hydrolysis, the effect being maximal when the modified base pair is outside the recognition site. No digestion occurs when the internal dC-residue of the recognition site is 5-methylated in one or both strands. Replacement of dT residue in the EcoRII recognition site by dfl5U residue results in a dramatic inhibition of hydrolysis. Km and kcat for the cleavage of 14-base-pair DNA duplex have been determined. The cleavage rate of the dT-containing strand of the recognition site in 1.5 fold higher comparing with the dA-containing strand. The cleavage of both strands of the substrate by EcoRII endonuclease is confirmed to proceed in one enzyme-substrate complex.     

DNA sequence context conceals α-anomeric lesions

DNA sequence context has long been known to modulate detection and repair of DNA damage. Recent studies using experimental and computational approaches have sought to provide a basis for this observation. We have previously shown that an α-anomeric adenosine (αA) flanked by cytosines (5'CαAC-3') resulted in a kinked DNA duplex with an enlarged minor groove. Comparison of different flanking sequences revealed that a DNA duplex containing a 5'CαAG-3' motif exhibits unique substrate properties. However, this substrate was not distinguished by unusual thermodynamic properties. To understand the structural basis of the altered recognition, we have determined the solution structure of a DNA duplex with a 5'CαAG-3' core, using an extensive set of restraints including dipolar couplings and backbone torsion angles. The NMR structure exhibits an excellent agreement with the data (total R(X) <5.3%). The αA base is intrahelical, in a reverse Watson-Crick orientation, and forms a weak base pair with a thymine of the opposite strand. In comparison to the DNA duplex with a 5'CαAC-3' core, we observe a significant reduction of the local perturbation (backbone, stacking, tilt, roll, and twist), resulting in a straighter DNA with narrower minor groove. Overall, these features result in a less perturbed DNA helix and obscure the presence of the lesion compared to the 5'CαAC-3' sequence. The improved stacking of the 5'CαAG-3' core also affects the energetics of the DNA deformation that is required to form a catalytically competent complex. These traits provide a rationale for the modulation of the recognition by endonuclease IV.     

Solution structure of DAPI selectively bound in the minor groove of a DNA T.T mismatch-containing site: NMR and molecular dynamics studies.

The solution structure of the complex between 4', 6-diamidino-2-phenylindole (DAPI) and DNA oligomer [d(GCGATTCGC)]2, containing a central T.T mismatch, has been characterized by combined use of proton one- and two-dimensional NMR spectroscopy, molecular mechanics and molecular dynamics computations including relaxation matrix refinement. The results show that the DAPI molecule binds in the minor groove of the central region 5'-ATT-3' of the DNA oligomer, which predominantly adopts a duplex structure with a global right-handed B-like conformation. In the final models of the complex, the DAPI molecule is located nearly isohelical with its NH indole proton oriented towards the DNA helix axis and forming a bifurcated hydrogen bond with the carbonyl O2 groups of a mismatched T5 and the T6 residue of the opposite strand. Mismatched thymines adopt a wobble base pair conformation and are found stacked between the flanking base pairs, inducing only minor local conformational changes in global duplex structure. In addition, no other binding mechanisms were observed, showing that minor groove binding of DAPI to the mismatch-containing site is favoured in comparison with any other previously reported interaction with G.C sequences.     

Structural effect of the anticancer agent 6-thioguanine on duplex DNA

The incorporation of 6-thioguanine (S6G) into DNA is an essential step in the cytotoxic activity of thiopurines. However, the structural effects of this substitution on duplex DNA have not been fully characterized. Here, we present the solution structures of DNA duplexes containing S6G opposite thymine (S6G·T) and opposite cytosine (S6G·C), solved by high-resolution NMR spectroscopy and restrained molecular dynamics. The data indicate that both duplexes adopt right-handed helical conformations with all Watson–Crick hydrogen bonding in place. The S6G·T structures exhibit a wobble-type base pairing at the lesion site, with thymine shifted toward the major groove and S6G displaced toward the minor groove. Aside from the lesion site, the helices, including the flanking base pairs, are not highly perturbed by the presence of the lesion. Surprisingly, thermal dependence experiments suggest greater stability in the S6G-T mismatch than the S6G-C base pair.     

DNA oligonucleotides with A, T, G or C opposite an abasic site: structure and dynamics

Abasic sites are common DNA lesions resulting from spontaneous depurination and excision of damaged nucleobases by DNA repair enzymes. However, the influence of the local sequence context on the structure of the abasic site and ultimately, its recognition and repair, remains elusive. In the present study, duplex DNAs with three different bases (G, C or T) opposite an abasic site have been synthesized in the same sequence context (5′-CCA AAG₆ XA₈C CGG G-3′, where X denotes the abasic site) and characterized by 2D NMR spectroscopy. Studies on a duplex DNA with an A opposite the abasic site in the same sequence has recently been reported [Chen,J., Dupradeau,F.-Y., Case,D.A., Turner,C.J. and Stubbe,J. (2007) Nuclear magnetic resonance structural studies and molecular modeling of duplex DNA containing normal and 4′-oxidized abasic sites. Biochemistry, 46, 3096–3107]. Molecular modeling based on NMR-derived distance and dihedral angle restraints and molecular dynamics calculations have been applied to determine structural models and conformational flexibility of each duplex. The results indicate that all four duplexes adopt an overall B-form conformation with each unpaired base stacked between adjacent bases intrahelically. The conformation around the abasic site is more perturbed when the base opposite to the lesion is a pyrimidine (C or T) than a purine (G or A). In both the former cases, the neighboring base pairs (G6-C21 and A8-T19) are closer to each other than those in B-form DNA. Molecular dynamics simulations reveal that transient H-bond interactions between the unpaired pyrimidine (C20 or T20) and the base 3′ to the abasic site play an important role in perturbing the local conformation. These results provide structural insight into the dynamics of abasic sites that are intrinsically modulated by the bases opposite the abasic site.