Recombination R-triplex: H-bonds contribution to stability as revealed with minor base substitutions for adenine

Simple Repeats as Building Blocks for Genetic Computers

Processing of RNA involves heterogeneous nuclear ribonucleoproteins. The simple sequence repeats (SSRs) they bind can also adopt alternative DNA structures, like Z DNA, triplexes, G quadruplexes, and I motifs. Those SSRs capable of switching conformation under physiological conditions (called flipons) are genetic elements that can encode alternative RNA processing by their effects on RNA processivity, most likely as DNA:RNA hybrids. Flipons are elements of a binary, instructive genetic code directing how genomic sequences are compiled into transcripts. The combinatorial nature of this code provides a rich set of options for creating genetic computers able to reproduce themselves and use a heritable and evolvable code to optimize survival. The underlying computational logic potentiates a diverse set of genetic programs that modify cis-mediated heritability and disease risk.

Conformational variability of recombination R-triplex formed by the mammalian telomeric sequence

Alignment of three nucleic acids strands, in which the third strand is identical to one of the DNA duplex strands, occurs in various cellular systems. In the case of telomeric t-loops, recognition between the DNA duplex and the homologous single strand is likely to be mediated by proteins through formation of the transient recombination-type R-triplex. Earlier, using 2-aminopurine as a fluorescent reporting base, we evaluated the thermodynamic characteristics of intramolecular R-triplex formed by a mixed nucleotide sequence. Here, we used this approach to explore a propensity of the telomeric TTAGGG repeat to form the R-triplex. The circular dichroism spectral changes detected upon formation of the R-triplex suggest that this process is accompanied by specific conformational changes in DNA, including a local destabilization of the target duplex next to a GGG run revealed by the fluorescence of the reporting 2-aminopurine base. Surprisingly, stability of the R-triplex formed by telomeric sequence depends strikingly on the counter ion, being higher for Na⁺ than for Li⁺. Taken together these findings indicate a significant conformational variability of telomeric DNA in the context of recombination-type R-triplex, a phenomenon of possible biological relevance.

Ionic microenvironmental effects on triplex DNA stabilization: cationic counterion effects on poly(dT)·poly(dA)·poly(dT)

The structure and conformation of nucleic acids are influenced by metal ions, polyamines, and the microenvironment. In poly(purine) · poly(pyrimidine) sequences, triplex DNA formation is facilitated by metal ions, polyamines and other ligands. We studied the effects of mono- and di-valent metal ions, and ammonium salts on the stability of triple- and double-stranded structures formed from poly(dA) and poly(dT) by measuring their respective melting temperatures. In the presence of metal ions, the absorbance versus temperature profile showed two transitions: Tm1 for triplex to duplex and single stranded DNA, and Tm2 for duplex DNA melting to single stranded DNA. Monovalent cations (Li(+), Na(+), K(+), Rb(+), Cs(+) and [Formula: see text] ) promoted triplex DNA at concentrations ≥150 mM. Tm1 varied from 49.8 °C in the presence of 150 mM Li(+) to 30.6 °C in the presence of 150 mM K(+). [Formula: see text] was very effective in stabilizing triplex DNA and its efficacy decreased with increasing substitution of the hydrogen atoms with methyl, ethyl, propyl and butyl groups. As in the case of monovalent cations, a concentration-dependent increase in Tm1 was observed with divalent ions and triplex DNA stabilization decreased in the order: Mg(2+) > Ca(2+) > Sr(2+) > Ba(2+). All positively charged cations increased the melting temperature of duplex DNA. Values of Δn (number of ions released) on triplex DNA melting were 0.46 ± 0.06 and 0.18 ± 0.02, respectively, for mono- and di-valent cations, as calculated from 1/Tm1 versus ln[M(+,2+)] plots. The corresponding values for duplex DNA were 0.25 ± 0.02 and 0.12 ± 0.02, respectively, for mono- and di-valent cations. Circular dichroism spectroscopic studies showed distinct conformational changes in triplex DNA stabilized by alkali metal and ammonium ions. Our results might be useful in developing triplex forming oligonucleotide based gene silencing techniques.

Overview of the creative genome: effects of genome structure and sequence on the generation of variation and evolution

This overview of a special issue of Annals of the New York Academy of Sciences discusses uneven distribution of distinct types of variation across the genome, the dependence of specific types of variation upon distinct classes of DNA sequences and/or the induction of specific proteins, the circumstances in which distinct variation-generating systems are activated, and the implications of this work for our understanding of evolution and of cancer. Also discussed is the value of non text-based computational methods for analyzing information carried by DNA, early insights into organizational frameworks that affect genome behavior, and implications of this work for comparative genomics.

Targeting duplex DNA with chimeric α,β-triplex-forming oligonucleotides

Triplex-directed DNA recognition is strictly limited by polypurine sequences. In an attempt to address this problem with synthetic biology tools, we designed a panel of short chimeric α,β-triplex-forming oligonucleotides (TFOs) and studied their interaction with fluorescently labelled duplex hairpins using various techniques. The hybridization of hairpin with an array of chimeric probes suggests that recognition of double-stranded DNA follows complicated rules combining reversed Hoogsteen and non-canonical homologous hydrogen bonding. In the presence of magnesium ions, chimeric TFOs are able to form highly stable α,β-triplexes, as indicated by native gel-electrophoresis, on-array thermal denaturation and fluorescence-quenching experiments. CD spectra of chimeric triplexes exhibited features typically observed for anti-parallel purine triplexes with a GA or GT third strand. The high potential of chimeric α,β-TFOs in targeting double-stranded DNA was demonstrated in the EcoRI endonuclease protection assay. In this paper, we report, for the first time, the recognition of base pair inversions in a duplex by chimeric TFOs containing α-thymidine and α-deoxyguanosine.

Altering the electrostatic potential in the major groove: thermodynamic and structural characterization of 7-deaza-2'-deoxyadenosine:dT base pairing in DNA

As part of an ongoing effort to explore the effect of major groove electrostatics on the thermodynamic stability and structure of DNA, a 7-deaza-2′-deoxyadenosine:dT (7-deaza-dA:dT) base pair in the Dickerson–Drew dodecamer (DDD) was studied. The removal of the electronegative N7 atom on dA and the replacement with an electropositive C–H in the major groove was expected to have a significant effect on major groove electrostatics. The structure of the 7-deaza-dA:dT base pair was determined at 1.1 Å resolution in the presence of Mg²⁺. The 7-deaza-dA, which is isosteric for dA, had minimal effect on the base pairing geometry and the conformation of the DDD in the crystalline state. There was no major groove cation association with the 7-deaza-dA heterocycle. In solution, circular dichroism showed a positive Cotton effect centered at 280 nm and a negative Cotton effect centered at 250 nm that were characteristic of a right-handed helix in the B-conformation. However, temperature-dependent NMR studies showed increased exchange between the thymine N3 imino proton of the 7-deaza-dA:dT base pair and water, suggesting reduced stacking interactions and an increased rate of base pair opening. This correlated with the observed thermodynamic destabilization of the 7-deaza-dA modified duplex relative to the DDD. A combination of UV melting and differential scanning calorimetry experiments were conducted to evaluate the relative contributions of enthalpy and entropy in the thermodynamic destabilization of the DDD. The most significant contribution arose from an unfavorable enthalpy term, which probably results from less favorable stacking interactions in the modified duplex, which was accompanied by a significant reduction in the release of water and cations from the 7-deaza-dA modified DNA.

Diverse modes of 5'-[4-(aminoiminomethyl)phenyl]-[2,2'-bifuran]-5-carboximidamide (DB832) interaction with multi-stranded DNA structures

The modes of binding of 5'-[4-(aminoiminomethyl)phenyl]-[2,2'-Bifuran]-5-carboximidamide (DB832) to multi-stranded DNAs: human telomere quadruplex, monomolecular R-triplex, pyr/pur/pyr triplex consisting of 12 T*(T x A) triplets, and DNA double helical hairpin were studied. The optical adsorption of the ligand was used for monitoring the binding and for determination of the association constants and the numbers of binding sites. CD spectra of DB832 complexes with the oligonucleotides and the data on the energy transfer from DNA bases to the bound DB832 assisted in elucidating the binding modes. The affinity of DB832 to the studied multi-stranded DNAs was found to be greater (K(ass) approximately 10(7)M(-1)) than to the duplex DNA (K(ass) approximately 2 x 10(5)M(-1)). A considerable stabilizing effect of DB832 binding on R-triplex conformation was detected. The nature of the ligand tight binding differed for the studied multi-stranded DNA depending on their specific conformational features: recombination-type R-triplex demonstrated the highest affinity for DB832 groove binding, while pyr/pur/pyr TTA triplex favored DB832 intercalation at the end stacking contacts and the human telomere quadruplex d[AG(3)(T(2)AG(3))(3)] accommodated the ligand in a capping mode. Additionally, the pyr/pur/pyr TTA triplex and d[AG(3)(T(2)AG(3))(3)] quadruplex bound DB832 into their grooves, though with a markedly lesser affinity. DB832 may be useful for discrimination of the multi-sranded DNA conformations and for R-triplex stabilization.

Intramolecular recombination R-triplex in solution: stabilization by bis-intercalator YOYO

Recognition of double-stranded DNA with a mixed nucleotide sequence by oligonucleotide is a long-term challenge. This aim can be achieved via formation of the recombination R-triplex, accommodating two identical DNA strands in parallel orientation, and antiparallel complementary strand. In the absence of proteins the R-triplex stability is low, however, so that intermolecular R-triplex is not formed by three DNA strands in a ligand-free system. Recently, recognition of DNA with mixed base sequence by single-stranded oligonucleotide in the presence of bis-intercalator YOYO was reported. Here, we describe thermodynamic characteristics of YOYO complexes with the model oligonucleotides 5'-GT-2AP-GACTGAG TTTT CTCAGTCTACGC GAA GCGTAGACTGAG-3' (R(2AP)CW) bearing a single reporting 2-aminopurine (2AP) in place of adenine and 5'-CTCAGTCTACGC GAA GCGTAGACTGAG-3' (CW). We found that each oligonucleotide is able to bind two YOYO molecules via intercalation mode in 0.5 M LiCl. Fluorescence intensity of YOYO intercalated in triplex R(2AP)CW and in CW hairpin increased 40-fold compared to the free YOYO. Remarkably, the melting temperature of the triplex (determined using temperature dependence of the 2AP fluorescence) increased from 19 degrees C to 33 degrees C upon binding two YOYO molecules. Further increase in the YOYO concentration resulted in binding of up to five YOYO molecules to R(2AP)CW triplex and up to six YOYO molecules to CW hairpin.

Pseudoknot structures with conserved base triples in telomerase RNAs of ciliates

Telomerase maintains the integrity of telomeres, the ends of linear chromosomes, by adding G-rich repeats to their 3′-ends. Telomerase RNA is an integral component of telomerase. It contains a template for the synthesis of the telomeric repeats by the telomerase reverse transcriptase. Although telomerase RNAs of different organisms are very diverse in their sequences, a functional non-template element, a pseudoknot, was predicted in all of them. Pseudoknot elements in human and the budding yeast Kluyveromyces lactis telomerase RNAs contain unusual triple-helical segments with AUU base triples, which are critical for telomerase function. Such base triples in ciliates have not been previously reported. We analyzed the pseudoknot sequences in 28 ciliate species and classified them in six different groups based on the lengths of the stems and loops composing the pseudoknot. Using miniCarlo, a helical parameter-based modeling program, we calculated 3D models for a representative of each morphological group. In all cases, the predicted structure contains at least one AUU base triple in stem 2, except for that of Colpidium colpoda, which contains unconventional GCG and AUA triples. These results suggest that base triples in a pseudoknot element are a conserved feature of all telomerases.