Parallel-stranded linear homoduplexes of d(A+-G)n > 10 and d(A-G)n > 10 manifesting the contrasting ionic strength sensitivities of poly(A+.A+) and DNA

Switching G-quadruplex to parallel duplex by molecular rotor clustering

Switching of G-quadruplex (G4) structures between variant types of folding has been proved to be a versatile tool for regulation of genomic expression and development of nucleic acid-based constructs. Various specific ligands have been developed to target G4s in K⁺ solution with therapeutic prospects. Although G4 structures have been reported to be converted by sequence modification or a unimolecular ligand binding event in K⁺-deficient conditions, switching G4s towards non-G4 folding continues to be a great challenge due to the stability of G4 in physiological K⁺ conditions. Herein, we first observed the G4 switching towards parallel-stranded duplex (psDNA) by multimolecular ligand binding (namely ligand clustering) to overcome the switching barrier in K⁺. Purine-rich sequences (e.g. those from the KRAS promoter region) can be converted from G4 structures to dimeric psDNAs using molecular rotors (e.g. thioflavin T and thiazole orange) as initiators. The formed psDNAs provided multiple binding sites for molecular rotor clustering to favor subsequent structures with stability higher than the corresponding G4 folding. Our finding provides a clue to designing ligands with the competency of molecular rotor clustering to implement an efficient G4 switching.

Sequence-specific recognition of a coding segment of human DACH1 gene via short pyrimidine/purine oligonucleotides

With growing in vivo evidence of the roles of triplexes in biological processes, oligonucleotide-directed targeting of double-helical DNA for selective modulation of gene functions has become imperative in their therapeutic aspects. This study comprises a comparative investigation of 17-mer Py- and Pu-TFO for the formation of an intermolecular triplex with a 27-bp genomic homopurine-homopyrimidine track present in the transcriptional element of the human DACH1 gene. The biochemical and biophysical studies have revealed that triplex formation takes place only with Py-TFO and not with its Pu-counterpart. Non-denaturating gel electrophoresis indicated the formation of an intermolecular triplex in Py-motif with an increasing amount of Py-TFO, whereas no such interaction was observed for the Pu-counterpart. UV-thermal melting (T m), circular dichroism (CD) and thermal difference spectra (TDS) studies confirmed the pyrimidine motif triplex formation, which was observed to be significantly pH-dependent and stable at acidic pH (5.2) in the presence of 100 mM Na+ ions. Contrarily, Pu-TFO was not found to bind to the target predominantly, owing to its self-association properties. Further studies have revealed that the GA-rich Pu-TFO adopts a homoduplex structure leading to a limit in its availability for triplex formation. These results may add to our understanding of sequence-specific gene targeting and give insight into designing more specific TFOs depending on genomic targets.

Revealing structural peculiarities of homopurine GA repetition stuck by i-motif clip

Non-canonical forms of nucleic acids represent challenging objects for both structure-determination and investigation of their potential role in living systems. In this work, we uncover a structure adopted by GA repetition locked in a parallel homoduplex by an i-motif. A series of DNA oligonucleotides comprising GAGA segment and C3 clip is analyzed by NMR and CD spectroscopies to understand the sequence-structure-stability relationships. We demonstrate how the relative position of the homopurine GAGA segment and the C3 clip as well as single-base mutations (guanine deamination and cytosine methylation) affect base pairing arrangement of purines, i-motif topology and overall stability. We focus on oligonucleotides C3GAGA and methylated GAGAC3 exhibiting the highest stability and structural uniformity which allowed determination of high-resolution structures further analyzed by unbiased molecular dynamics simulation. We describe sequence-specific supramolecular interactions on the junction between homoduplex and i-motif blocks that contribute to the overall stability of the structures. The results show that the distinct structural motifs can not only coexist in the tight neighborhood within the same molecule but even mutually support their formation. Our findings are expected to have general validity and could serve as guides in future structure and stability investigations of nucleic acids.

Thioflavin T binds dimeric parallel-stranded GA-containing non-G-quadruplex DNAs: a general approach to lighting up double-stranded scaffolds

A molecular rotor thioflavin T (ThT) is usually used as a fluorescent ligand specific for G-quadruplexes. Here, we demonstrate that ThT can tightly bind non-G-quadruplex DNAs with several GA motifs and dimerize them in a parallel double-stranded mode, accompanied by over 100-fold enhancement in the fluorescence emission of ThT. The introduction of reverse Watson–Crick T-A base pairs into these dimeric parallel-stranded DNA systems remarkably favors the binding of ThT into the pocket between G•G and A•A base pairs, where ThT is encapsulated thereby restricting its two rotary aromatic rings in the excited state. A similar mechanism is also demonstrated in antiparallel DNA duplexes where several motifs of two consecutive G•G wobble base pairs are incorporated and serve as the active pockets for ThT binding. The insight into the interactions of ThT with non-G-quadruplex DNAs allows us to introduce a new concept for constructing DNA-based sensors and devices. As proof-of-concept experiments, we design a DNA triplex containing GA motifs in its Hoogsteen hydrogen-bonded two parallel strands as a pH-driven nanoswitch and two GA-containing parallel duplexes as novel metal sensing platforms where C–C and T–T mismatches are included. This work may find further applications in biological systems (e.g. disease gene detection) where parallel duplex or triplex stretches are involved.

A triple stranded G-quadruplex formation in the promoter region of human myosin β(Myh7) gene

Regulatory regions in human genome, enriched in guanine-rich DNA sequences have the propensity to fold into G-quadruplex structures. On exploring the genome for search of G-tracts, it was interesting to find that promoter of Human Myosin Gene (MYH7) contains a conserved 23-mer G-rich sequence (HM-23). Mutations in this gene are associated with familial cardiomyopathy. Enrichment of MYH7 gene in G-rich sequences could possibly play a critical role in its regulation. We used polyacrylamide gel electrophoresis (PAGE), UV-Thermal denaturation (UV-Tm) and Circular Dichroism (CD), to demonstrate the formation of a G-quadruplex by 23-mer G-rich sequence HM23 in promoter location of MYH7 gene. We observed that the wild G-rich sequence HM23 containing consecutive G₅ stretch in two stacks adopt G-quadruplexes of diverse molecularity by involvement of four-strand, three-strand and two-strands with same parallel topology. Interestingly, the mutated sequence in the absence of continuous G₅ stretch obstructs the formation of three-stranded G-quadruplex. We demonstrated that continuous G₅ stretch is mandatory for the formation of a unique three-stranded G-quadruplex. Presence of various transcription factors (TF) in vicinity of the sequence HM23 leave fair possibility of recognition by TF binding sites, and so modulate gene expression. These findings may add on our understanding about the effect of base change in the formation of varied structural species in similar solution condition. This study may give insight about structural polymorphism arising due to recognition of non-Watson-Crick G-quadruplex structures by cellular proteins and designing structure specific molecules.

A bouquet of DNA structures: Emerging diversity

Structural polymorphism of DNA has constantly been evolving from the time of illustration of the double helical model of DNA by Watson and Crick. A variety of non-canonical DNA structures have constantly been documented across the globe. DNA attracted worldwide attention as a carrier of genetic information. In addition to the classical Watson–Crick duplex, DNA can actually adopt diverse structures during its active participation in cellular processes like replication, transcription, recombination and repair. Structures like hairpin, cruciform, triplex, G-triplex, quadruplex, i-motif and other alternative non-canonical DNA structures have been studied at length and have also shown their in vivo occurrence. This review mainly focuses on non-canonical structures adopted by DNA oligonucleotides which have certain prerequisites for their formation in terms of sequence, its length, number and orientation of strands along with varied solution conditions. This conformational polymorphism of DNA might be the basis of different functional properties of a specific set of DNA sequences, further giving some insights for various extremely complicated biological phenomena. Many of these structures have already shown their linkages with diseases like cancer and genetic disorders, hence making them an extremely striking target for structure-specific drug designing and therapeutic applications.

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DNA can adopt diverse range of structures other than classical Watson–Crick duplex.

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Discussion of alternate structures like hairpin, cruciform, triplex, quadruplex etc.

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This review gives some insights for the biological relevance of DNA structures.

Effect of the alkaline cations on the stability of the model polynucleotide poly(dG-dC)·poly(dG-dC)

When the model polynucleotide poly(dG-dC)∙poly(dG-dC) [polyGC] is titrated with a strong acid (HCl) in unbuffered aqueous solutions containing the chlorides of the alkali metals in the concentration range 0.010 M-0.600 M, two transitions in the absorbance vs. pH plots are evidenced, characterized by the constants pK(a(₁)) and pK(a(₂)). The limiting values at infinite saline concentrations of these two constants, namely pK(∞)(a(₁)) and pK(∞)(a(₂)) obtained making use of the "one site saturation constant" equation or, in turn, of the double logarithmic plot: pK(a) vs. log([salt]⁻¹), exhibit a clear dependence on the nature of the cations. The effects of the different alkali cations on the pK(∞)(a) values follow the Hofmeister series. In fact, the pK(∞)(a(₁)) and the pK(∞)(a(₂)) values are smaller for Li+ and Na+ than for Rb+ and Cs+, with K+ at the border between the two, showing that the transitions require higher concentrations of protons to occur in the presence of high concentrations of the cosmotropic ions.

Probing DNA's interstrand orientation with gold nanoparticles

The interstrand orientation of a DNA duplex plays a pivotal role in its biological and chemical functions. Therefore, developing an efficient way to determine (control and monitor) the parallel or antiparallel conformation of a DNA duplex is of great significance, which, however, remains a big challenge under some circumstances. In this work, we demonstrate that gold nanoparticles tagged on DNA are especially useful in trapping and detecting a special interstrand orientation of a DNA double helix, based on inherent electrostatic and steric repulsions between nanoparticles which will affect their self-assembly into a large structure. More importantly, some of the conformations revealed by the gold nanoparticle assay may even not be thermodynamically preferred and thus will be hard to detect using currently available methods. This simple, straightforward, and efficient methodology capable of dictating and probing a special DNA duplex structure provides a useful tool for conformational analyses and functional explorations of biomolecules as well as biophysical and nanobiomedical research.

Interaction of isoquinoline alkaloids with polymorphic DNA structures

The interaction of berberine, palmatine, and coralyne with the B, Z, and H(L) form of poly[d(G-C)] was studied. Berberine and palmatine showed moderate binding to the B form, while coralyne showed higher binding, as revealed from spectroscopic and thermodynamic data. Berberine and coralyne binding to the B form was exothermic and enthalpy-driven, while palmatine showed exothermic binding which was favored by both negative enthalpy and negative entropy changes. Berberine and palmatine neither bind nor converted the Z-form structure to B form. Coralyne, on the other hand, exhibited a strong binding affinity to Z DNA structure that was enthalpy-driven. Berberine binding to the H(L) form was cooperative, exothermic, and favored by both negative enthalpy and negative entropy changes with the formation of an induced CD band. Palmatine showed weak binding, while coralyne showed a strong binding with the H(L) form. The structural differences in the isoquinoline alkaloids appear to influence the affinity and mode of interactions with these polymorphic DNA structures.

Circular dichroism and conformational polymorphism of DNA

Here we review studies that provided important information about conformational properties of DNA using circular dichroic (CD) spectroscopy. The conformational properties include the B-family of structures, A-form, Z-form, guanine quadruplexes, cytosine quadruplexes, triplexes and other less characterized structures. CD spectroscopy is extremely sensitive and relatively inexpensive. This fast and simple method can be used at low- as well as high-DNA concentrations and with short- as well as long-DNA molecules. The samples can easily be titrated with various agents to cause conformational isomerizations of DNA. The course of detected CD spectral changes makes possible to distinguish between gradual changes within a single DNA conformation and cooperative isomerizations between discrete structural states. It enables measuring kinetics of the appearance of particular conformers and determination of their thermodynamic parameters. In careful hands, CD spectroscopy is a valuable tool for mapping conformational properties of particular DNA molecules. Due to its numerous advantages, CD spectroscopy significantly participated in all basic conformational findings on DNA.

Biophysical studies with parallel stranded oligoduplex

The thermal denaturation profiles of parallel stranded oligoduplexes during UV-spectroscopy and fluorescence spectroscopy, for the first time, were shown to follow the similar patterns as were described for antiparallel stranded oligoduplexes. The B-form structure of these center-bubble-forming oligoduplexes was determined with circular dichroism. Thermodynamic properties were also discussed.

Conformations of DNA strands containing GAGT, GACA, or GAGC tetranucleotide repeats

The (GA)(n) microsatellite has been known from previous studies to adopt unusual, ordered, cooperatively melting secondary structures in neutral aqueous solutions containing physiological concentrations of salts, at acid pH values or in aqueous ethanol solutions. To find more about the primary structure specificity of these structures, we performed parallel comparative studies of related tetranucleotide repeats (GAGC)(5), (GAGT)(5), and (GACA)(5). The general conclusion following from these comparative studies is that the primary structure specificity is fairly high, indicating that not only guanines but also adenines play a significant role in the stabilization of these unusual structures. (GAGC)(5) is a hairpin or a duplex depending on DNA concentration. Neither acid pH nor ionic strength or the presence of ethanol changed the secondary structure of (GAGC)(5) in a significant way. (GACA)(5) forms a weakly stable hairpin in neutral aqueous solutions but forms a duplex at acid pH where cytosine is protonated. (GAGT)(5) behaves most similar to (GAGA)(5). Salt induces its hairpin to duplex transition at neutral pH and an isomerization into another, probably parallel stranded, duplex takes place at acid pH. (GAGT)(5) is the only of the three present 20-mers that responds to ethanol like (GAGA)(5).

Alkaline titrations of poly(dG-dC).poly(dG-dC): microemulsion versus solution behavior

PolyGC was titrated with a strong base in the presence of increasing concentrations of NaCl (from 0.00 to 0.60M) either in water solution or with the polynucleotide solubilized in the aqueous core of reverse micelles, i.e., the cationic quaternary water-in-oil microemulsion CTAB/n-hexane/n-pentanol/water. The results for matched samples in the two media were compared. CD and UV spectroscopies and, for the solution experiments, pH measurements were used to follow the course of deprotonation. In both media the primary effect of the addition of base was denaturation of the polynucleotide, reversible by back-titration with a strong acid. In solution, the apparent pK(a) of the transition decreases with increasing the salt concentration and a roughly linear dependence of pK(a) on p[NaCl] has been found. A parallel monotonic decay with ionic strength has been found in solution for R(OH), defined as the number of hydroxyl ions required per monomeric unit of polyGC to reach half-transition. By contrast, in microemulsion, R(OH) has been found to be independent of the NaCl concentration (and 10 to 50 times lower than in solution). This result is proposed as an indirect evidence of the independence of pK(a) on the salt concentration in microemulsion, where the pH cannot be measured. A sort of buffering effect of the positive charges on the micellar wall and of their counter-ions on the ionic strength could well explain this discrepancy of behavior in the two media.

Molecular aspects on the interaction of protoberberine, benzophenanthridine, and aristolochia group of alkaloids with nucleic acid structures and biological perspectives

Alkaloids occupy an important position in chemistry and pharmacology. Among the various alkaloids, berberine and coralyne of the protoberberine group, sanguinarine of the benzophenanthridine group, and aristololactam-beta-d-glucoside of the aristolochia group have potential to form molecular complexes with nucleic acid structures and have attracted recent attention for their prospective clinical and pharmacological utility. This review highlights (i) the physicochemical properties of these alkaloids under various environmental conditions, (ii) the structure and functional aspects of various forms of deoxyribonucleic acid (DNA) (B-form, Z-form, H(L)-form, protonated form, and triple helical form) and ribonucleic acid (RNA) (A-form, protonated form, and triple helical form), and (iii) the interaction of these alkaloids with various polymorphic DNA and RNA structures reported by several research groups employing various analytical techniques like absorbance, fluorescence, circular dichroism, and NMR spectroscopy; electrospray ionization mass spectrometry, thermal melting, viscosity, and DNase footprinting as well as molecular modeling and thermodynamic studies to provide detailed binding mechanism at the molecular level for structure-activity relationship. Nucleic acids binding properties of these alkaloids are interpreted in relation to their biological activity.

Acid titrations of poly(dG-dC).poly(dG-dC) in aqueous solution and in a w/o microemulsion

The model polynucleotide poly(dG-dC).poly(dG-dC) (polyGC) was titrated with a strong acid (HCl) in aqueous unbuffered solutions and in the quaternary w/o microemulsion CTAB/n-pentanol/n-hexane/water. The titrations, performed at several concentrations of NaCl in the range 0.005 to 0.600 M, were followed by recording the modifications of the electronic absorption and of the CD spectra (210< or = lambda < or =350 nm) upon addition of the acid. In solution, the polynucleotide undergoes two acid-induced transitions, neither of which corresponds to denaturation of the duplex to single coil. The first transition leads to the Hoogsteen type synG.C+ duplex, while the second leads to the C+.C duplex. The initial B-form of polyGC was recovered by back-titration with NaOH. The apparent pKa values were obtained for both steps of the titration, at all salt concentrations. A reasonably linear dependence of pKa1 and pKa2 from p[NaCl] was obtained, with both pKa values decreasing with increasing ionic strength. In microemulsion, at salt concentrations < or = 0.300 M, an acid-induced transition was observed, matching the first conformational transition recorded also in solution. However, further addition of acid led to denaturation of the protonated duplex. Renaturation of polyGC was obtained by back-titration with NaOH. At salt concentrations > 0.300 M, polyGC is present as a mixture of B-form and psi- aggregates, that slowly separate from the microemulsion. The acid titration induces at first a conformational transition similar to the one observed at low salt or in solution, then denaturation occurs, which is however preceded by the appearance of a transient conformation, that has been tentatively classified as a left-handed Z double helix.

Towards a better understanding of the unusual conformations of the alternating guanine–adenine repeat strands of DNA

Alternating guanine-adenine strands of DNA are known to self-associate into a parallel-stranded homoduplex at neutral pH, fold into an ordered single-stranded structure at acid pH, and adopt yet another ordered single-stranded conformer in aqueous ethanol. The unusual conformers melt cooperatively and exhibit distinct circular dichroism spectra suggestive of a substantial conformational order, but their molecular structures are not known yet. Here, we have probed the molecular structures using guanine and adenine analogs lacking the N7 atom, and thus unable of Hoogsteen pairing, or those restrained in the less-frequent syn glycosidic orientation. The studies showed that the syn glycosidic orientation of dA residues promoted the neutral homoduplex, whereas the syn orientation of dG was incompatible with the homoduplex. In addition, Hoogsteen pairing of dA seemed to be a crucial property of the homoduplex whereas dG did not pair in this way. The situation was the same in both single-stranded conformers with the dG residues. On the other hand, the presence of N7 was important with dA but its syn geometry was not favorable. The present data can be used as restraints to model the unusual molecular structures of the alternating guanine-adenine strands of DNA.

Are stop codons recognized by base triplets in the large ribosomal RNA subunit?

The precise mechanism of stop codon recognition in translation termination is still unclear. A previously published study by Ivanov and colleagues proposed a new model for stop codon recognition in which 3-nucleotide Ter-anticodons within the loops of hairpin helices 69 (domain IV) and 89 (domain V) in large ribosomal subunit (LSU) rRNA recognize stop codons to terminate protein translation in eubacteria and certain organelles. We evaluated this model by extensive bioinformatic analysis of stop codons and their putative corresponding Ter-anticodons across a much wider range of species, and found many cases for which it cannot explain the stop codon usage without requiring the involvement of one or more of the eight possible noncomplementary base pairs. Involvement of such base pairs may not be structurally or thermodynamically damaging to the model. However, if, according to the model, Ter-anticodon interaction with stop codons occurs within the ribosomal A-site, the structural stringency which that site imposes on sense codon.tRNA anticodon interaction should also extend to stop codon.Ter-anticodon interactions. Moreover, with Ter-tRNA in place of an aminoacyl-tRNA, for each of the various Ter-anticodons there is a sense codon that can interact with it preferentially by complementary and wobble base-pairing. Both these considerations considerably weaken the arguments put forth previously.

Protonated structures of naturally occurring deoxyribonucleic acids and their interaction with berberine

Protonation-induced conformational changes in natural DNAs of diverse base composition under the influence of low pH, low temperature, and low ionic strength have been studied using various spectroscopic techniques. At pH3.40, 10mM [Na+], and at 5 degrees C, all natural DNAs irrespective of base composition adopted an unusual and stable conformation remarkably different from the canonical B-form conformation. This protonated conformation has been characterized to have unique absorption and circular dichroic spectral characteristics and exhibited cooperative thermal melting profiles with decreased thermal melting temperatures compared to their respective B-form counterparts. The nature of this protonated structure was further investigated by monitoring the interaction of the plant alkaloid, berberine that was previously shown from our laboratory to differentially bind to B-form and H(L)-form of poly[d(G-C)] [Bioorg. Med. Chem.2003, 11, 4861]. Binding of berberine to protonated conformation of natural DNAs resulted in intrinsic circular dichroic changes as well as generation of induced circular dichroic bands for the bound berberine molecule with opposite signs and magnitude compared with B-form structures. Nevertheless, the binding of the alkaloid to both the B and protonated forms was non-linear and non-cooperative as revealed from Scatchard plots derived from spectrophotometric titration data. Steady state fluorescence studies on the other hand showed remarkable increase of the rather weak intrinsic fluorescence of berberine on binding to the protonated structure compared to the B-form structure. Taken together, these results suggest that berberine can detect the formation of significant population of H(L)-form structures under the influence of protonation irrespective of heterogeneous base compositions in natural DNAs.

Titration of poly(dA-dT) . poly(dA-dT) in solution at variable NaCl concentration

CD and uv absorption data showed that high molecular weight poly(dA-dT) . poly(dA-dT), at 298 K, undergoes an acid-induced transition from B-double helix to random coil in NaCl solutions of different concentrations, ranging from 0.005 to 0.600M. Similarly, titration of the polynucleotide with a strong base causes duplex-to-single strands transition. The base- and acid-induced transitions were both reversible by back-titration (with an acid or, respectively, with a base): the apparent pKa were the same in both directions. However, the number of protons per titratable site (adenine N1) required to reach half-denaturation was in great excess over the stoichiometric value; to a much larger extent, the same effect was observed also for the deprotonation of the N3H sites of thymine. Moreover, in the basic denaturation experiments, at low salt concentrations ([NaCl]< or =0.300M) less acid than calculated was needed to back-titrate the base excess to half-denaturation. Both effects could be qualitatively justified on the basis of the counterion condensation theory of polyelectrolytes and considering the energy barrier created by the negatively charged phosphodiester groups to the penetration of the OH- ions inside the double helix and the screening effect of the Na+ ions on such charges, in the deprotonation experiments.

Circular dichroism spectroscopy of conformers of (guanine + adenine) repeat strands of DNA

(Guanine+adenine) strands of DNA are known to associate into guanine tetraplexes, homodimerize into parallel or antiparallel duplexes, and fold into a cooperatively melting single strand resembling the protein alpha helix. Using CD spectroscopy and other methods, we studied how this conformational polymorphism depended on the primary structure of DNA. The study showed that d(GGGA)(5) and d(GGA)(7) associated into homoduplexes at low salt or in the presence of LiCl but were prone to guanine tetraplex formation, especially in the presence of KCl. In addition, they yielded essentially the same CD spectrum in the presence of ethanol as observed with the ordered single strand of d(GA)(10). Strands of d(GA)(10), d(GGAA)(5), d(GAA)(7), and d(GAAA)(5) associated into homoduplexes in both LiCl and KCl solutions, but not into guanine tetraplexes. d(GAAA)(5) and d(GAA)(7) further failed to form the single-stranded conformer in aqueous ethanol. Adenine protonation, however, stabilized the single-stranded conformer even in these adenine-rich fragments. The ordered single strands, homoduplexes as well as the guanine tetraplexes, all provided strikingly similar CD spectra, indicating that all of the conformers shared similar base stacking geometries. The increasing adenine content only decreased the conformer thermostability.

NMR structure of a parallel-stranded DNA duplex at atomic resolution

DNA dodecamers have been designed with two cytosines on each end and intervening A and T stretches, such that the oligomers have fully complementary A:T base pairs when aligned in the parallel orientation. Spectroscopic (UV, CD and IR), NMR and molecular dynamics studies have shown that oligomers having the sequences d(CCATAATTTACC) and d(CCTATTAAATCC) form a parallel-stranded duplex when dissolved at 1:1 stoichiometry in aqueous solution. This is due to the C:C+ clamps on either end and extensive mismatches in the antiparallel orientation. The structure is stable at neutral and acidic pH. At higher temperatures, the duplex melts into single strands in a highly cooperative fashion. All adenine, cytosine and thymine nucleotides adopt the anti conformation with respect to the glycosidic bond. The A:T base pairs form reverse Watson-Crick base pairs. The duplex shows base stacking and NOEs between the base protons T(H6)/A(H8) and the sugar protons (H1'/H2'/H2") of the preceding nucleotide, as has been observed in antiparallel duplexes. However, no NOEs are observed between base protons H2/H6/H8 of sequential nucleotides, though such NOEs are observed between T(CH3) and A(H8). A three-dimensional structure of the parallel-stranded duplex at atomic resolution has been obtained using molecular dynamics simulations under NMR constraints. The simulated structures have torsional angles very similar to those found in B-DNA duplexes, but the base stacking and helicoid parameters are significantly different.

Unexpected formation of parallel duplex in GAA and TTC trinucleotide repeats of Friedreich's ataxia

The onset and progress of Friedreich's ataxia (FRDA) is associated with the genetic instability of the (GAA).(TTC) trinucleotide repeats located within the frataxin gene. The instability of these repeats may involve the formation of an alternative DNA structure. Poly-purine (R)/poly-pyrimidine (Y) sequences typically form triplex DNA structures which may contribute to genetic instability. Conventional wisdom suggested that triplex structures formed by these poly-purine (R)/poly-pyrimidine (Y) sequences may contribute to their genetic instability. Here, we report the characterization of the single-stranded GAA and TTC sequences and their mixtures using NMR, UV-melting, and gel electrophoresis, as well as chemical and enzymatic probing methods. We show that the FRDA GAA/TTC, repeats are capable of forming various alternative structures. The most intriguing is the observation of a parallel (GAA).(TTC) duplex in equilibrium with the antiparallel Watson-Crick (GAA).(TTC) duplex. We also show that the GAA strands form self-assembled structures, whereas the TTC strands are essentially unstructured. Finally, we demonstrate that the FRDA repeats form only the YRY triplex (but not the RRY triplex) at neutral pH and the complete formation of the YRY triplex requires the ratio of GAA to TTC strand larger than 1:2. The structural features presented here and in other studies distinguish the FRDA (GAA)¿(TTC) repeats from the fragile X (CGG).CCG), myotonic dystrophy (CTG).(CAG) and the Huntington (CAG).(CTG) repeats.

Branched oligonucleotide-intercalator conjugate forming a parallel stranded structure inhibits HIV-1 integrase

Integration of a DNA copy of the HIV-1 genome into chromosomal DNA of infected cells is a key step of viral replication. Integration is carried out by integrase, a viral protein which binds to both ends of viral DNA and catalyses reactions of the 3'-end processing and strand transfer. A 3'-3' branched oligonucleotide functionalised by the intercalator oxazolopyridocarbazole at each 5'-end was found to inhibit integration in vitro. We show that both a specific (G,A) sequence and the OPC intercalating agent contribute to the capability of the branched oligonucleotide to form a parallel stranded structure responsible for the inhibition.

Dimerization of the guanine-adenine repeat strands of DNA

Jovin and co-workers have demonstrated that DNA strands containing guanine-adenine repeats generate a parallel-stranded homoduplex. Here we propose that the homoduplex is a dimer of the ordered single strand discovered by Fresco and co-workers at acid pH. The Fresco single strand is shown here to be stabilized in aqueous ethanol where adenine is not protonated. Furthermore, we demonstrate that the strands dimerize at higher salt concentrations without significantly changing their conformation, so that the dimerization is non-cooperative. Hence, the Jovin homoduplex can form through a non-cooperative dimerization of two cooperatively melting single strands. The available data indicate that the guanines stabilize the Fresco single strand whereas the adenines cause dimerization owing to their known intercalation or clustering tendency. The guanine-adenine repeat dimer seems to be a DNA analog of the leucine zipper causing dimerization of proteins.

Conformational properties of DNA strands containing guanine-adenine and thymine-adenine repeats

CD spectroscopy and PAGE were used to cooperatively analyze melting conformers of DNA strands containing GA and TA dinucleotide repeats. The 20mer (GA)10 formed a homoduplex in neutral solutions containing physiological concentrations of salts and this homoduplex was not destabilized even in the terminal (GA)3 hexamers of (GA)3(TA)4(GA)3, although the central (TA)4 portion of this oligonucleotide preserved the conformation adopted by (TA)10. This observation demonstrates that homoduplexes of alternating GA and TA sequences can co-exist in a single DNA molecule. Another 20mer, (GATA)5, adopted as a whole either the AT duplex, like (TA)10, or the GA duplex, like (GA)10, and switched between them reversibly. The concentration of salt controlled the conformational switching. Hence, guanine and thymine share significant properties regarding complementarity to adenine, while the TA and GA sequences can stack in at least two mutually compatible ways within the DNA duplexes analyzed here. These properties extend our knowledge of non-canonical structures of DNA.

Effect of the 1-(2'-deoxy-beta-D-ribofuranosyl)-3-nitropyrrole residue on the stability of DNA duplexes and triplexes

3-Nitropyrrole (M) was introduced as a non-discriminating 'universal' base in nucleic acid duplexes by virtue of small size and a presumed tendency to stack but not hydrogen bond with canonical bases. However, the absence of thermally-induced hyperchromic changes by single-stranded deoxyoligomers in which M alternates with A or C residues shows that M does not stack strongly with A or C nearest neighbors. Yet, the insertion of a centrally located M opposite any canonical base in a duplex is sometimes even less destabilizing than that of some mismatches, and the variation in duplex stability is small. In triplexes, on the other hand, an M residue centrally located in the third strand reduces triplex stability drastically even when the X.Y target base pair is A.T or G. C in a homopurine. homopyrimidine segment. But, when the target duplex opposition is M-T and the third strand residue is T, the presence of M in the test triplet has little effect on triplex stability. Therefore, a lack of hydrogen bonding in an otherwise helix-compatible test triplet cannot be responsible for triplex destabilization when M is the third strand residue. Thus, M is non-discriminating and none-too-destabilizing in a duplex, but in a triplex it is extremely destabilizing when in the third strand.