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RNA molecules with novel functions have revived interest in the accurate prediction of RNA three-dimensional (3D) structure and folding dynamics. However, existing methods are inefficient in automated 3D structure prediction. Here, we report a robust computational approach for rapid folding of RNA molecules. We develop a simplified RNA model for discrete molecular dynamics (DMD) simulations, incorporating base-pairing and base-stacking interactions. We demonstrate correct folding of 150 structurally diverse RNA sequences. The majority of DMD-predicted 3D structures have <4 Å deviations from experimental structures. The secondary structures corresponding to the predicted 3D structures consist of 94% native base-pair interactions. Folding thermodynamics and kinetics of tRNA Phe , pseudoknots, and mRNA fragments in DMD simulations are in agreement with previous experimental findings. Folding of RNA molecules features transient, non-native conformations, suggesting nonhierarchical RNA folding. Our method allows rapid conformational sampling of RNA folding, with computational time increasing linearly with RNA length. We envision this approach as a promising tool for RNA structural and functional analyses.

Molecular beacons are sensitive fluorescent probes hybridizing selectively to designated DNA and RNA targets. They have recently become practical tools for quantitative real-time monitoring of single-stranded nucleic acids. Here, we comparatively study the performance of a variety of such probes, stemless and stem-containing DNA and PNA (peptide nucleic acid) beacons, in Tris-buffer solutions containing various concentrations of NaCl and MgCl(2). We demonstrate that different molecular beacons respond differently to the change of salt concentration, which could be attributed to the differences in their backbones and constructions. We have found that the stemless PNA beacon hybridizes rapidly to the complementary oligodeoxynucleotide and is less sensitive than the DNA beacons to the change of salt thus allowing effective detection of nucleic acid targets under various conditions. Though we found stemless DNA beacons improper for diagnostic purposes due to high background fluorescence, we believe that use of these DNA and similar RNA constructs in molecular-biophysical studies may be helpful for analysis of conformational flexibility of single-stranded nucleic acids. With the aid of PNA "openers", molecular beacons were employed for the detection of a chosen target sequence directly in double-stranded DNA (dsDNA). Conditions are found where the stemless PNA beacon strongly discriminates the complementary versus mismatched dsDNA targets. Together with the insensitivity of PNA beacons to the presence of salt and DNA-binding/processing proteins, the latter results demonstrate the potential of these probes as robust tools for recognition of specific sequences within dsDNA without denaturation and deproteinization of duplex DNA.

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To elucidate the mechanism of recognition of double-stranded DNA (dsDNA) by homopyrimidine polyamide ("peptide") nucleic acid (PNA) leading to the stranddisplacement, the kinetics of the sequence-specific PNA/DNA binding have been studied. The binding was monitored with time by the gel retardation and nuclease S1 cleavage assays. The experimental kinetic curves obey pseudo-first-order kinetics and the dependence of the pseudo-first-order rate constant, kps, on PNA concentration, P, obeys a power law kps the proposed kinetic scheme is performed. The interpretation of our experimental data in the framework of the proposed kinetic scheme leads to the following conclusions. The sequence specificity of the recognition is essentially provided at the "search" step of the process, which consists in the highly reversible transient formation of duplex between one PNA molecule and the complementary strand of duplex DNA while the other DNA strand is displaced. This search step is followed by virtually irreversible "locking" step via PNA2/DNA triplex formation. The proposed mechanism explains how the binding of homopyrimidine PNA to dsDNA meets two apparently mutually contradictory features: high sequence specificity of binding and remarkable stability of both correct and mismatched PNA/DNA complexes.A new type of DNA analogue, polyamide ("peptide") nucleic acid (PNA), was described in 1991 (1). This sequence-specific DNA binding reagent is believed to be a very promising drug (2, 3) with numerous potential applications (4). For homopyrimidine PNAs a unique type of duplex DNA/drug interaction is observed. It consists of PNA binding to one of the DNA strands through formation of stable PNA2/DNA triplex while the noncomplementary DNA strand is left in single-stranded state (1, 5, 6) thus forming a structure that we call the P loop. P-loop formation leads to selective inhibition of protein binding to DNA (7,8), results in transcription elongation arrest (2,7,9,10), creates an artificial transcription promoter (11), makes it possible to convert single-strand-specific nucleases into sequence-selective cutters (12), and, if PNA is biotinylated, to place electron-microscopy markers on doublestranded DNA (dsDNA) (13).For biomedical and molecular biological applications of PNA it is essential to understand the factors controlling PNA/ DNA binding and its sequence specificity. The data indicate that under conditions in which the PNA/DNA complexes are normally studied, the binding is virtually irreversible (5), thus implying a crucial role of kinetic factors in the stranddisplacement reaction. Here we present a kinetic study of homopyrimidine PNAs binding to dsDNA and propose a kinetic model for PNA/DNA sequence-specific recognition.

A novel method for sequence specific double strand DNA cleavage using PNA (peptide nucleic acid) targeting is described. Nuclease S1 digestion of double stranded DNA gives rise to double strand cleavage at an occupied PNA strand displacement binding site, and under optimized conditions complete cleavage can be obtained. The efficiency of this cleavage is more than 10 fold enhanced when a tandem PNA site is targeted, and additionally enhanced if this site is in trans rather than in cis orientation. Thus in effect, the PNA targeting makes the single strand specific nuclease S1 behave like a pseudo restriction endonuclease.

If adenines and thymines in two mutually complementary mixedbase peptide nucleic acid (PNA) oligomers are substituted with diaminopurines and thiouracils, respectively, so-called pseudocomplementary PNAs (pcPNAs) are created. Pairs of pcPNAs have recently demonstrated an ability to highly selectively target essentially any designated site on double-stranded DNA (dsDNA) by forming very stable PNA-DNA strand-displacement complexes via double duplex invasion (helix invasion). These properties of pcPNAs make them unique and very promising ligands capable of denying the access of DNA-binding proteins to dsDNA. To elucidate the sequence-unrestricted mechanism of sequence-specific dsDNA recognition by pcPNAs, we have studied the kinetics of formation of corresponding PNA-DNA complexes at various temperatures by the gel-shift assay. In parallel, the conditions for possible selfhybridization of pcPNA oligomers have been assayed by mixing curve (Job plot) and thermal melting experiments. The data indicate that, at physiological temperatures (Ϸ37°C), the equilibrium is shifted toward the pairing of corresponding pcPNAs with each other. This finding explains a linear concentration dependence, within the submicromolar range, of the pcPNA invasion rate into dsDNA at 37°C. At elevated temperatures (>50°C), the rather unstable pcPNA duplexes dissociate, yielding the expected quadratic dependence for the rate of pcPNA invasion on the PNA concentration. The polycationic character of pcPNA pairs, carrying the duplicated number of protonated terminal PNA residues commonly used to increase the PNA solubility and binding affinity, also explains the self-inhibition of pcPNA invasion observed at higher PNA concentrations. Melting of pcPNA duplexes occurs with the integral transition enthalpies ranged from ؊235 to ؊280 kJ⅐mol ؊1 , contributing to an anomalously high activation energy of Ϸ150 kJ⅐mol ؊1 found for the helix invasion of pcPNAs carrying four different nucleobases. A simplified kinetic model for pcPNAs helix invasion is proposed that interprets all unusual features of pcPNAs binding to dsDNA. Our findings have important implications for rational use of pcPNAs.double-stranded DNA ͉ pseudocomplementary PNA ͉ sequence-selective recognition P eptide nucleic acids (PNAs) and their derivatives are of significant biomedical and biotechnological interest as prospective biomolecular tools for highly selective manipulation of nucleic acids (1-8). Recently, a new modification of PNAs has been introduced for sequence-unrestricted targeting of doublestranded DNA (dsDNA). Along with ordinary guanines and cytosines, these PNAs, dubbed pseudocomplementary PNAs (pcPNAs; refs. 9 and 10), carry 2,6-diaminopurines (D) and 2-thiouracils ( s U) instead of adenines and thymines, respectively.Model building revealed a steric clash between the bulky thio group of s U (or similarly modified thymine) and one of the two amino groups of D within the s U-D pair (9, 11). This clashing effect must severely destabilize the pcPNA-pcPNA duplexes whereas bindin...

Due to its robustness and simplicity, the rolling replication of circular DNA probes holds a distinct position in DNA diagnostics among other isothermal methods of target, probe or signal amplification. Major rolling-circle amplification approaches to DNA detection via posthybridization probe/signal turn-by-turn enhancement are briefly overviewed here with an emphasis on the new concepts and latest progress in the field, including the single-molecule and single-mutation detection assays as exemplary applications. Underlying mechanisms, current controversies and principal advantages of rolling-circle amplification are also considered. Possible future directions for the further advancement of this diagnostic methodology are outlined.