Influence of DNA sequence on the structure of minicircles under torsional stress

Spectral analysis of DNA superhelical dynamics from molecular minicircle simulations

Torsional and bending deformations of DNA molecules often occur in vivo and are important for biological functions. DNA "under stress" is a conformational state, which is by far the most frequent state during DNA-protein and gene regulation. In DNA minicircles of length <100 base pairs (bp), the combined effect of torsional and bending stresses can cause local unusual conformations, with certain base pair steps often absorbing most of the stress, leaving other steps close to their relaxed conformation. To better understand the superhelical dynamics of DNA under stress, molecular simulations of 94 bp minicircles with different torsional linking numbers were interpreted using Fourier analyses and principal component analyses. Sharp localized bends of nearly 90° in the helical axis were observed, which in turn decreased fluctuations of the rotational register and helped redistribute the torsional stress into writhe, i.e., superhelical turn up to 360°. In these kinked minicircles, only two-thirds of the DNA molecule bends and writhes and the remaining segment stays close to straight and preserves a conformational flexibility typical of canonical B-DNA (bending of 39° ± 17° distributed parsimoniously across 36 bp), which was confirmed and visualized by principal component analysis. These results confirm that stressed DNA molecules are highly heterogeneous along their sequence, with segments designed to locally store and release stress so that nearby segments can stay relaxed.

Two Different Ways of Stress Release in Supercoiled DNA Minicircles under DNA Nick

It is generally believed that DNA nick is an effective way to release stress in supercoiled DNA, resulting from the twisting motion that individual strands rotate around the axis of the DNA helix. Here, we use MD simulations based on the oxDNA model to investigate the relaxation of 336 bp supercoiled minicircular DNA under DNA nick. Our simulations show that stress release, characterized by the abrupt decrease in linking number, may be induced by two types of DNA motion depending on the nick position. Except for the twisting motion, there is a writhing motion, that is, double strands collectively rotating with one plectoneme removal, which may occur in the process of DNA relaxation with the nick position in the loop region. Moreover, the writhing motion is more likely to occur in the DNA with relatively high hardness, such as C-G pairs. Our simulation results uncover the relationship between structural transformation, stress release, and DNA motion during the dynamic process under DNA nick, indicating the influence of nick position on the relaxation of the supercoiled DNA.

DNA supercoiling-induced shapes alter minicircle hydrodynamic properties

DNA in cells is organized in negatively supercoiled loops. The resulting torsional and bending strain allows DNA to adopt a surprisingly wide variety of 3-D shapes. This interplay between negative supercoiling, looping, and shape influences how DNA is stored, replicated, transcribed, repaired, and likely every other aspect of DNA activity. To understand the consequences of negative supercoiling and curvature on the hydrodynamic properties of DNA, we submitted 336 bp and 672 bp DNA minicircles to analytical ultracentrifugation (AUC). We found that the diffusion coefficient, sedimentation coefficient, and the DNA hydrodynamic radius strongly depended on circularity, loop length, and degree of negative supercoiling. Because AUC cannot ascertain shape beyond degree of non-globularity, we applied linear elasticity theory to predict DNA shapes, and combined these with hydrodynamic calculations to interpret the AUC data, with reasonable agreement between theory and experiment. These complementary approaches, together with earlier electron cryotomography data, provide a framework for understanding and predicting the effects of supercoiling on the shape and hydrodynamic properties of DNA.

Revisiting DNA Sequence-Dependent Deformability in High-Resolution Structures: Effects of Flanking Base Pairs on Dinucleotide Morphology and Global Chain Configuration

DNA carries more than the list of biochemical ingredients that drive the basic functions of living systems. The sequence of base pairs includes a multitude of structural and energetic signals, which determine the degree to which the long, threadlike molecule moves and how it responds to proteins and other molecules that control its processing and govern its packaging. The chemical composition of base pairs directs the spatial disposition and fluctuations of successive residues. The observed arrangements of these moieties in high-resolution protein–DNA crystal structures provide one of the best available estimates of the natural, sequence-dependent structure and deformability of the double-helical molecule. Here, we update the set of knowledge-based elastic potentials designed to describe the observed equilibrium structures and configurational fluctuations of the ten unique base-pair steps. The large number of currently available structures makes it possible to characterize the configurational preferences of the DNA base-pair steps within the context of their immediate neighbors, i.e., tetrameric context. Use of these knowledge-based potentials shows promise in accounting for known effects of sequence in long chain molecules, e.g., the degree of curvature reported in classic gel mobility studies and the recently reported sequence-dependent responses of supercoiled minicircles to nuclease cleavage.

Supercoiling and looping promote DNA base accessibility and coordination among distant sites

DNA in cells is supercoiled and constrained into loops and this supercoiling and looping influence every aspect of DNA activity. We show here that negative supercoiling transmits mechanical stress along the DNA backbone to disrupt base pairing at specific distant sites. Cooperativity among distant sites localizes certain sequences to superhelical apices. Base pair disruption allows sharp bending at superhelical apices, which facilitates DNA writhing to relieve torsional strain. The coupling of these processes may help prevent extensive denaturation associated with genomic instability. Our results provide a model for how DNA can form short loops, which are required for many essential processes, and how cells may use DNA loops to position nicks to facilitate repair. Furthermore, our results reveal a complex interplay between site-specific disruptions to base pairing and the 3-D conformation of DNA, which influences how genomes are stored, replicated, transcribed, repaired, and many other aspects of DNA activity.

A Primer on the oxDNA Model of DNA: When to Use it, How to Simulate it and How to Interpret the Results

The oxDNA model of Deoxyribonucleic acid has been applied widely to systems in biology, biophysics and nanotechnology. It is currently available via two independent open source packages. Here we present a set of clearly documented exemplar simulations that simultaneously provide both an introduction to simulating the model, and a review of the model's fundamental properties. We outline how simulation results can be interpreted in terms of-and feed into our understanding of-less detailed models that operate at larger length scales, and provide guidance on whether simulating a system with oxDNA is worthwhile.

Structural Landscape of the Transition from an ssDNA Dumbbell Plus Its Complementary Hairpin to a dsDNA Microcircle Via a Kissing Loop Intermediate

The experimental construction of a double-stranded DNA microcircle of only 42 base pairs entailed a great deal of ingenuity and hard work. However, figuring out the three-dimensional structures of intermediates and the final product can be particularly baffling. Using a combination of model building and unrestrained molecular dynamics simulations in explicit solvent we have characterized the different DNA structures involved along the process. Our 3D models of the single-stranded DNA molecules provide atomic insight into the recognition event that must take place for the DNA bases in the cohesive tail of the hairpin to pair with their complementary bases in the single-stranded loops of the dumbbell. We propose that a kissing loop involving six base pairs makes up the core of the nascent dsDNA microcircle. We also suggest a feasible pathway for the hybridization of the remaining complementary bases and characterize the final covalently closed dsDNA microcircle as possessing two well-defined U-turns. Additional models of the pre-ligation complex of T4 DNA ligase with the DNA dumbbell and the post-ligation pre-release complex involving the same enzyme and the covalently closed DNA microcircle are shown to be compatible with enzyme recognition and gap ligation.

Biomolecular Modeling and Simulation: A Prospering Multidisciplinary Field

We reassess progress in the field of biomolecular modeling and simulation, following up on our perspective published in 2011. By reviewing metrics for the field's productivity and providing examples of success, we underscore the productive phase of the field, whose short-term expectations were overestimated and long-term effects underestimated. Such successes include prediction of structures and mechanisms; generation of new insights into biomolecular activity; and thriving collaborations between modeling and experimentation, including experiments driven by modeling. We also discuss the impact of field exercises and web games on the field's progress. Overall, we note tremendous success by the biomolecular modeling community in utilization of computer power; improvement in force fields; and development and application of new algorithms, notably machine learning and artificial intelligence. The combined advances are enhancing the accuracy andscope of modeling and simulation, establishing an exemplary discipline where experiment and theory or simulations are full partners.

DNA mechanics and its biological impact

Almost all nucleoprotein interactions and DNA manipulation events involve mechanical deformations of DNA. Extraordinary progresses in single-molecule, structural, and computational methods have characterized the average mechanical properties of DNA, such as bendability and torsional rigidity, in high resolution. Further, the advent of sequencing technology has permitted measuring, in high-throughput, how such mechanical properties vary with sequence and epigenetic modifications along genomes. We review these recent technological advancements, and discuss how they have contributed to the emerging idea that variations in the mechanical properties of DNA play a fundamental role in regulating, genome-wide, diverse processes involved in chromatin organization.

Sequence-Dependent Kink Formation in Short DNA Loops: Theory and Molecular Dynamics Simulations

Kink formation is essential in highly bent DNA complexed with gene regulatory proteins such as histones to release the bending stress stored within the DNA duplex. Local opening of the double-stranded DNA creates a sharp turn along the specific sequence, which leads to the global bending of the DNA strand. Despite the critical role of kink formation, it is still challenging to predict the position of kink formation for a given DNA sequence. In this study, we propose a theoretical model and perform molecular dynamics simulations to quantify the sequence-dependent kink probability of a strongly bent DNA. By incorporating the elastic bending energy and the sequence-specific thermodynamic parameters, we investigate the importance of the DNA sequence on kink formation. We find that the sequence with TA dinucleotide repeats flanked by GC steps increases the kink propensity by more than an order of magnitude under the same bending stress. The number of base pairs involved in the local opening is found to be coupled with the sequence-specific bubble formation free energy. Our study elucidates the molecular origin of the sequence heterogeneity on kink formation, which is fundamental to understanding protein-DNA recognition.

WASP: a software package for correctly characterizing the topological development of ribbon structures

We introduce the Writhe Application Software Package (WASP) which can be used to characterisze the topology of ribbon structures, the underlying mathematical model of DNA, Biopolymers, superfluid vorticies, elastic ropes and magnetic flux ropes. This characterization is achieved by the general twist–writhe decomposition of both open and closed ribbons, in particular through a quantity termed the polar writhe. We demonstrate how this decomposition is far more natural and straightforward than artificial closure methods commonly utilized in DNA modelling. In particular, we demonstrate how the decomposition of the polar writhe into local and non-local components distinctly characterizes the local helical structure and knotting/linking of the ribbon. This decomposition provides additional information not given by alternative approaches. As example applications, the WASP routines are used to characterise the evolving topology (writhe) of DNA minicircle and open ended plectoneme formation magnetic/optical tweezer simulations, and it is shown that the decomponsition into local and non-local components is particularly important for the detection of plectonemes. Finally it is demonstrated that a number of well known alternative writhe expressions are actually simplifications of the polar writhe measure.

Effects of Model Shape, Volume, and Softness of the Capsid for DNA Packaging of phi29

Double-stranded DNA is under extreme confinement when packed in phage phi29 with osmotic pressures approaching 60 atm and densities near liquid crystalline. The shape of the capsid determined from experiment is elongated. We consider the effects of the capsid shape and volume on the DNA distribution. We propose simple models for the capsid of phage phi29 to capture volume, shape, and wall flexibility, leading to an accurate DNA density profile. The effect of the packaging motor twisting the DNA on the resulting density distribution has been explored. We find packing motor induced twisting leads to a greater numbers of defects formed. The emergence of defects such as bubbles or large roll angles along the DNA shows a sequence dependence, and the resulting flexibility leads to an inhomogeneous distribution of defects occurring more often at TpA steps and AT-rich regions. In conjunction with capsid elongation, this has effects on the global DNA packing structures.

Improving therapeutic potential of non-viral minimized DNA vectors

The tragic deaths of three patients in a recent AAV-based X-linked myotubular myopathy clinical trial highlight once again the pressing need for safe and reliable gene delivery vectors. Non-viral minimized DNA vectors offer one possible way to meet this need. Recent pre-clinical results with minimized DNA vectors have yielded promising outcomes in cancer therapy, stem cell therapy, stem cell reprograming, and other uses. Broad clinical use of these vectors, however, remains to be realized. Further advances in vector design and production are ongoing. An intriguing and promising potential development results from manipulation of the specific shape of non-viral minimized DNA vectors. By improving cellular uptake and biodistribution specificity, this approach could impact gene therapy, DNA nanotechnology, and personalized medicine.

Chromosomal Organization and Regulation of Genetic Function in Escherichia coli Integrates the DNA Analog and Digital Information

In this article, we summarize our current understanding of the bacterial genetic regulation brought about by decades of studies using the Escherichia coli model. It became increasingly evident that the cellular genetic regulation system is organizationally closed, and a major challenge is to describe its circular operation in quantitative terms. We argue that integration of the DNA analog information (i.e., the probability distribution of the thermodynamic stability of base steps) and digital information (i.e., the probability distribution of unique triplets) in the genome provides a key to understanding the organizational logic of genetic control. During bacterial growth and adaptation, this integration is mediated by changes of DNA supercoiling contingent on environmentally induced shifts in intracellular ionic strength and energy charge. More specifically, coupling of dynamic alterations of the local intrinsic helical repeat in the structurally heterogeneous DNA polymer with structural-compositional changes of RNA polymerase holoenzyme emerges as a fundamental organizational principle of the genetic regulation system. We present a model of genetic regulation integrating the genomic pattern of DNA thermodynamic stability with the gene order and function along the chromosomal OriC-Ter axis, which acts as a principal coordinate system organizing the regulatory interactions in the genome.

Strongly Bent Double-Stranded DNA: Reconciling Theory and Experiment

The strong bending of polymers is poorly understood. We propose a general quantitative framework of polymer bending that includes both the weak and strong bending regimes on the same footing, based on a single general physical principle. As the bending deformation increases beyond a certain (polymer-specific) point, the change in the convexity properties of the effective bending energy of the polymer makes the harmonic deformation energetically unfavorable: in this strong bending regime the energy of the polymer varies linearly with the average bending angle as the system follows the convex hull of the deformation energy function. For double-stranded DNA, the effective bending deformation energy becomes non-convex for bends greater than ~ 2° per base-pair, equivalent to the curvature of a closed circular loop of ~ 160 base pairs. A simple equation is derived for the polymer loop energy that covers both the weak and strong bending regimes. The theory shows quantitative agreement with recent DNA cyclization experiments on short DNA fragments, while maintaining the expected agreement with experiment in the weak bending regime. Counter-intuitively, cyclization probability (j-factor) of very short DNA loops is predicted to increase with decreasing loop length; the j-factor reaches its minimum for loops of ≃ 45 base pairs. Atomistic simulations reveal that the attractive component of the short-range Lennard-Jones interaction between the backbone atoms can explain the underlying non-convexity of the DNA effective bending energy, leading to the linear bending regime. Applicability of the theory to protein-DNA complexes, including the nucleosome, is discussed.

Stokesian dynamics of sedimenting elastic rings

We consider elastic microfilaments which form closed loops. We investigate how the loops change shape and orientation while settling under gravity in a viscous fluid. Loops are circular at the equilibrium. Their dynamics are investigated numerically based on the Stokes equations for the fluid motion and the bead-spring model of the microfilament. The Rotne-Prager approximation for the bead mobility is used. We demonstrate that the relevant dimensionless parameter is the ratio of the bending resistance of the filament to the gravitation force corrected for buoyancy. The inverse of this ratio, called the elasto-gravitation number B, is widely used in the literature for sedimenting elastic linear filaments. We assume that B is of the order of 104-106, which corresponds to easily deformable loops. We find out that initially tilted circles evolve towards different sedimentation modes, depending on B. Very stiff or stiff rings attain almost planar, oval shapes, which are vertical or tilted, respectively. More flexible loops deform significantly and converge towards one of several characteristic periodic motions. These sedimentation modes are also detected when starting from various shapes, and for different loop lengths. In general, multi-stability is observed: an elastic ring converges to one of several sedimentation modes, depending on the initial conditions. This effect is pronounced for very elastic loops. The surprising diversity of long-lasting periodic motions and shapes of elastic rings found in this work gives a new perspective for the dynamics of more complex deformable objects at micrometer and nanometer scales, sedimenting under gravity or rotating in a centrifuge, such as red blood cells, ring polymers or circular DNA.

BIOPHYSICS MEETS GENE THERAPY: HOW EXPLORING SUPERCOILING-DEPENDENT STRUCTURAL CHANGES IN DNA LED TO THE DEVELOPMENT OF MINIVECTOR DNA

Supercoiling affects every aspect of DNA function (replication, transcription, repair, recombination, etc.), yet the vast majority of studies on DNA and crystal structures of the molecule utilize short linear duplex DNA, which cannot be supercoiled. To study how supercoiling drives DNA biology, we developed and patented methods to make milligram quantities of tiny supercoiled circles of DNA called minicircles. We used a collaborative and multidisciplinary approach, including computational simulations (both atomistic and coarse-grained), biochemical experimentation, and biophysical methods to study these minicircles. By determining the three-dimensional conformations of individual supercoiled DNA minicircles, we revealed the structural diversity of supercoiled DNA and its highly dynamic nature. We uncovered profound structural changes, including sequence-specific base-flipping (where the DNA base flips out into the solvent), bending, and denaturing in negatively supercoiled minicircles. Counterintuitively, exposed DNA bases emerged in the positively supercoiled minicircles, which may result from inside-out DNA (Pauling-like, or "P-DNA"). These structural changes strongly influence how enzymes interact with or act on DNA. We hypothesized that, because of their small size and lack of bacterial sequences, these small supercoiled DNA circles may be efficient at delivering DNA into cells for gene therapy applications. "Minivectors," as we named them for this application, have proven to have therapeutic potential. We discovered that minivectors efficiently transfect a wide range of cell types, including many clinically important cell lines that are refractory to transfection with conventional plasmid vectors. Minivectors can be aerosolized for delivery to lungs and transfect human cells in culture to express RNA or genes. Importantly, minivectors demonstrate no obvious vector-associated toxicity. Minivectors can be repeatedly delivered and are long-lasting without integrating into the genome. Requests from colleagues around the world for minicircle and minivector DNA revealed a demand for our invention. We successfully obtained start-up funding for Twister Biotech, Inc. to help fulfill this demand, providing DNA for those who needed it, with a long-term goal of developing human therapeutics. In summary, what started as a tool for studying DNA structure has taken us in new and unanticipated directions.

Identifying Physical Causes of Apparent Enhanced Cyclization of Short DNA Molecules with a Coarse-Grained Model

DNA cyclization is a powerful technique to gain insight into the nature of DNA bending. While the wormlike chain model provides a good description of small to moderate bending fluctuations, it is expected to break down for large bending. Recent cyclization experiments on strongly bent shorter molecules indeed suggest enhanced flexibility over and above that expected from the wormlike chain. Here, we use a coarse-grained model of DNA to investigate the subtle thermodynamics of DNA cyclization for molecules ranging from 30 to 210 base pairs. As the molecules get shorter, we find increasing deviations between our computed equilibrium j-factor and the classic wormlike chain predictions of Shimada and Yamakawa for a torsionally aligned looped molecule. These deviations are due to sharp kinking, first at nicks, and only subsequently in the body of the duplex. At the shortest lengths, substantial fraying at the ends of duplex domains is the dominant method of relaxation. We also estimate the dynamic j-factor measured in recent FRET experiments. We find that the dynamic j-factor is systematically larger than its equilibrium counterpart—with the deviation larger for shorter molecules—because not all the stress present in the fully cyclized state is present in the transition state. These observations are important for the interpretation of recent cyclization experiments, suggesting that measured anomalously high j-factors may not necessarily indicate non-WLC behavior in the body of duplexes.

Small DNA circles as bacterial topoisomerase I inhibitors

Bacterial topoisomerase I is a potential target during the course of antibacterial drug therapy. In our studies, specifically designed small DNA circles with high bending stress were synthesized. It is demonstrated that small DNA circles showed high inhibitory effect on the activity of bacterial topoisomerase I and the single-stranded regions associated with bending deformation in DNA circles are believed to be the crucial factor for trapping the enzymes and decreasing the effective concentration of the topoisomerases in the reaction solution. In addition, the DNA circles showed high thermal stability and excellent nuclease resistance. In consideration of the low cytotoxicity of DNA-based biopharmaceuticals, our results may provide a new idea for the future design and optimization of DNA-based therapeutic agents for antibacterial therapy.

DNA sequence encodes the position of DNA supercoils

The three-dimensional organization of DNA is increasingly understood to play a decisive role in vital cellular processes. Many studies focus on the role of DNA-packaging proteins, crowding, and confinement in arranging chromatin, but structural information might also be directly encoded in bare DNA itself. Here, we visualize plectonemes (extended intertwined DNA structures formed upon supercoiling) on individual DNA molecules. Remarkably, our experiments show that the DNA sequence directly encodes the structure of supercoiled DNA by pinning plectonemes at specific sequences. We develop a physical model that predicts that sequence-dependent intrinsic curvature is the key determinant of pinning strength and demonstrate this simple model provides very good agreement with the data. Analysis of several prokaryotic genomes indicates that plectonemes localize directly upstream of promoters, which we experimentally confirm for selected promotor sequences. Our findings reveal a hidden code in the genome that helps to spatially organize the chromosomal DNA.

Sequence-dependent response of DNA to torsional stress: a potential biological regulation mechanism

Torsional restraints on DNA change in time and space during the life of the cell and are an integral part of processes such as gene expression, DNA repair and packaging. The mechanical behavior of DNA under torsional stress has been studied on a mesoscopic scale, but little is known concerning its response at the level of individual base pairs and the effects of base pair composition. To answer this question, we have developed a geometrical restraint that can accurately control the total twist of a DNA segment during all-atom molecular dynamics simulations. By applying this restraint to four different DNA oligomers, we are able to show that DNA responds to both under- and overtwisting in a very heterogeneous manner. Certain base pair steps, in specific sequence environments, are able to absorb most of the torsional stress, leaving other steps close to their relaxed conformation. This heterogeneity also affects the local torsional modulus of DNA. These findings suggest that modifying torsional stress on DNA could act as a modulator for protein binding via the heterogeneous changes in local DNA structure.

Chromosomal organization of transcription: in a nutshell

Early studies of transcriptional regulation focused on individual gene promoters defined specific transcription factors as central agents of genetic control. However, recent genome-wide data propelled a different view by linking spatially organized gene expression patterns to chromosomal dynamics. Therefore, the major problem in contemporary molecular genetics concerned with transcriptional gene regulation is to establish a unifying model that reconciles these two views. This problem, situated at the interface of polymer physics and network theory, requires development of an integrative methodology. In this review, we discuss recent achievements in classical model organism E. coli and provide some novel insights gained from studies of a bacterial plant pathogen, D. dadantii. We consider DNA topology and the basal transcription machinery as key actors of regulation, in which activation of functionally relevant genes is coupled to and coordinated with the establishment of extended chromosomal domains of coherent transcription. We argue that the spatial organization of genome plays a fundamental role in its own regulation.

Advances in Non-Viral DNA Vectors for Gene Therapy

Uses of viral vectors have thus far eclipsed uses of non-viral vectors for gene therapy delivery in the clinic. Viral vectors, however, have certain issues involving genome integration, the inability to be delivered repeatedly, and possible host rejection. Fortunately, development of non-viral DNA vectors has progressed steadily, especially in plasmid vector length reduction, now allowing these tools to fill in specifically where viral or other non-viral vectors may not be the best options. In this review, we examine the improvements made to non-viral DNA gene therapy vectors, highlight opportunities for their further development, address therapeutic needs for which their use is the logical choice, and discuss their future expansion into the clinic.