Dynamics of intramolecular recognition: base-pairing in DNA/RNA near and far from equilibrium

Tailor made: the art of therapeutic mRNA design

mRNA medicine is a new and rapidly developing field in which the delivery of genetic information in the form of mRNA is used to direct therapeutic protein production in humans. This approach, which allows for the quick and efficient identification and optimization of drug candidates for both large populations and individual patients, has the potential to revolutionize the way we prevent and treat disease. A key feature of mRNA medicines is their high degree of designability, although the design choices involved are complex. Maximizing the production of therapeutic proteins from mRNA medicines requires a thorough understanding of how nucleotide sequence, nucleotide modification and RNA structure interplay to affect translational efficiency and mRNA stability. In this Review, we describe the principles that underlie the physical stability and biological activity of mRNA and emphasize their relevance to the myriad considerations that factor into therapeutic mRNA design.

Unified approach to partition functions of RNA secondary structures

RNA secondary structure formation is a field of considerable biological interest as well as a model system for understanding generic properties of heteropolymer folding. This system is particularly attractive because the partition function and thus all thermodynamic properties of RNA secondary structure ensembles can be calculated numerically in polynomial time for arbitrary sequences and homopolymer models admit analytical solutions. Such solutions for many different aspects of the combinatorics of RNA secondary structure formation share the property that the final solution depends on differences of statistical weights rather than on the weights alone. Here, we present a unified approach to a large class of problems in the field of RNA secondary structure formation. We prove a generic theorem for the calculation of RNA folding partition functions. Then, we show that this approach can be applied to the study of the molten-native transition, denaturation of RNA molecules, as well as to studies of the glass phase of random RNA sequences.

Energetic coupling between clustered lesions modulated by intervening triplet repeat bulge loops: allosteric implications for DNA repair and triplet repeat expansion

Clusters of closely spaced oxidative DNA lesions present challenges to the cellular repair machinery. When located in opposing strands, base excision repair (BER) of such lesions can lead to double strand DNA breaks (DSB). Activation of BER and DSB repair pathways has been implicated in inducing enhanced expansion of triplet repeat sequences. We show here that energy coupling between distal lesions (8oxodG and/or abasic sites) in opposing DNA strands can be modulated by a triplet repeat bulge loop located between the lesion sites. We find this modulation to be dependent on the identity of the lesions (8oxodG vs. abasic site) and the positions of the lesions (upstream vs. downstream) relative to the intervening bulge loop domain. We discuss how such bulge loop-mediated lesion crosstalk might influence repair processes, while favoring DNA expansion, the genotype of triplet repeat diseases.

Mechanism of RNA double helix-propagation at atomic resolution

The conversion of a nucleic acid from single strands to double strands is thought to involve slow nucleation followed by fast double-strand propagation. Here, for RNA double-strand propagation, we propose an atomic resolution reaction mechanism. This mechanism, called the stack-ratchet, is based on data-mining of three-dimensional structures and on available thermodynamic information. The stack-ratchet mechanism extends and adds detail to the classic zipper model proposed by Porschke (Porschke, D. Biophysical Chemistry 1974, 2, pp. 97-101). Porschke's zipper model describes the addition of a base pair to a nucleated helix in terms of a single type of elementary reaction; a concerted process in which the two bases, one from each strand, participate in the transition state. In the stack-ratchet mechanism proposed here a net base-pairing step consists of two elementary reactions. Motions of only one strand are required to achieve a given transition state. One elementary reaction preorganizes and stacks the 3' single-strand, driven by base--base stacking interactions. A second elementary reaction stacks the 5' strand and pairs it with the preorganized 3' strand. In the stack-ratchet mechanism, a variable length 3' stack leads the single-strand/double-strand junction. The stack-ratchet mechanism is not a two-state process. A base can be (i) unstacked and unpaired, (ii) stacked and paired, or (ii) stacked and unpaired (only on the 3' strand). The data suggests that helices of DNA and of RNA do not propagate by similar mechanisms.

DNA repair and DNA triplet repeat expansion: the impact of abasic lesions on triplet repeat DNA energetics

Enhanced levels of DNA triplet expansion are observed when base excision repair (BER) of oxidative DNA base damage (e.g., 8-oxo-dG) occurs at or near CAG repeat sequences. This observation suggests an interplay between processing mechanisms required for DNA repair and expansion pathways that yield genotypes associated with many neurological/developmental disorders. It has been proposed that DNA expansion involves the transient formation within the triplet repeat domains of non-native slipped DNA structures that are incorrectly processed by the BER machinery of repair during DNA synthesis. We show here that replacement within a triplet repeat bulge loop domain of a guanosine residue by an abasic site, the universal BER intermediate, increases the population of slipped/looped DNA structures relative to the corresponding lesion-free construct. Such abasic lesion-induced energetic enhancement of slipped/looped structures provides a linkage between BER and DNA expansion. We discuss how the BER machinery of repair may be influenced by abasic-induced energetic alterations in the properties of regions proximal to and/or within triplet repeat domains, thereby potentially modulating levels of DNA expansion.

Inferring DNA sequences from mechanical unzipping data: the large-bandwidth case

The complementary strands of DNA molecules can be separated when stretched apart by a force; the unzipping signal is correlated to the base content of the sequence but is affected by thermal and instrumental noise. We consider here the ideal case where opening events are known to a very good time resolution (very large bandwidth), and study how the sequence can be reconstructed from the unzipping data. Our approach relies on the use of statistical Bayesian inference and of Viterbi decoding algorithm. Performances are studied numerically on Monte Carlo generated data, and analytically. We show how multiple unzippings of the same molecule may be exploited to improve the quality of the prediction, and calculate analytically the number of required unzippings as a function of the bandwidth, the sequence content, and the elasticity parameters of the unzipped strands.

Chromatin physics: Replacing multiple, representation-centered descriptions at discrete scales by a continuous, function-dependent self-scaled model

This commentary on the inspiring works and ideas by Langowski, Mangeol et al., Lee et al., Bundschuh and Gerland, Schiessel, Vaillant et al., Lesne and Victor, Claudet and Bednar, Fuks, Allemand et al., and Blossey, all appearing in this issue (Eur. Phys. J. E 19 (2006)), expresses our felt need of novel approaches to chromatin modeling.