The structure of branched DNA species

Construction of branched DNA-based nanostructures for diagnosis, therapeutics and protein engineering

Branched DNA with multibranch-like anisotropic topology serves as a promising and powerful building block in constructing multifunctional-integrated nanomaterials in a programmable and controllable manner. Recently, a series of branched DNA-based functional nanomaterials were developed by elaborate molecular design. In this review, we focused on the construction of branched DNA-based nanostructures for biological and biomedical applications. First, the molecular design and synthesis method of branched DNA monomer were briefly described. Then, the construction strategies of branched DNA-based nanostructures were categorially discussed, including target-triggered polymerization, enzymatic extension and hybrid assembly. Finally, the biological and biomedical applications including diagnosis, therapeutics and protein engineering were summarized. We envision that the review will contribute to the further development of branched DNA-based nanomaterials with great application potential in the field of biomedicine, thus building a new bridge between material chemistry and biomedicine.

Flexibility defines structure in crystals of amphiphilic DNA nanostars

DNA nanostructures with programmable shape and interactions can be used as building blocks for the self-assembly of crystalline materials with prescribed nanoscale features, holding a vast technological potential. Structural rigidity and bond directionality have been recognised as key design features for DNA motifs to sustain long-range order in 3D, but the practical challenges associated with prescribing building-block geometry with sufficient accuracy have limited the variety of available designs. We have recently introduced a novel platform for the one-pot preparation of crystalline DNA frameworks supported by a combination of Watson-Crick base pairing and hydrophobic forces (Brady et al 2017 Nano Lett. 17 3276-81). Here we use small angle x-ray scattering and coarse-grained molecular simulations to demonstrate that, as opposed to available all-DNA approaches, amphiphilic motifs do not rely on structural rigidity to support long-range order. Instead, the flexibility of amphiphilic DNA building-blocks is a crucial feature for successful crystallisation.

The importance of the N-terminus of T7 endonuclease I in the interaction with DNA junctions

T7 endonuclease I is a dimeric nuclease that is selective for four-way DNA junctions. Previous crystallographic studies have found that the N-terminal 16 amino acids are not visible, neither in the presence nor in the absence of DNA. We have now investigated the effect of deleting the N-terminus completely or partially. N-terminal deleted enzyme binds more tightly to DNA junctions but cleaves them more slowly. While deletion of the N-terminus does not measurably affect the global structure of the complex, the presence of the peptide is required to generate a local opening at the center of the DNA junction that is observed by 2-aminopurine fluorescence. Complete deletion of the peptide leads to a cleavage rate that is 3 orders of magnitude slower and an activation enthalpy that is 3-fold higher, suggesting that the most important interaction of the peptide is with the reaction transition state. Taken together, these data point to an important role of the N-terminus in generating a central opening of the junction that is required for the cleavage reaction to proceed properly. In the absence of this, we find that a cruciform junction is no longer subject to bilateral cleavage, but instead, just one strand is cleaved. Thus, the N-terminus is required for a productive resolution of the junction.

Crystallographic and spectroscopic studies of d(CCGGTACCGG)

The decanucleotide sequence d(CCGGTACCGG) crystallizes as a four-way junction at low cobalt ion concentrations (i.e., 1 mM). When the cobalt concentration in the crystallization solution is increased to 5 mM, the sequence crystallizes as resolved B-DNA duplexes. Gel retardation studies of the decamer show both a faint slow-moving band and a much thicker fast-moving band at low cobalt ion concentrations, and only the intense fast-moving band at higher ion concentration. Circular dichroism (CD) spectroscopy of the decamer indicates a structural transition as the cobalt ion concentration in the solution is increased, probably from B-type to A-type DNA. These studies revealed that the oligomer sequence has several conformations and structures accessible to it, in a manner dependent on sequence, ion concentration, and DNA concentration. [Supplementary materials are available for this article. Go to the publisher's online edition of Nucleosides, Nucleotides & Nucleic Acids for the following free supplemental resources(s): Supplementary Figures 1, 2, and 3.].

DNA energy landscapes via calorimetric detection of microstate ensembles of metastable macrostates and triplet repeat diseases

Biopolymers exhibit rough energy landscapes, thereby allowing biological processes to access a broad range of kinetic and thermodynamic states. In contrast to proteins, the energy landscapes of nucleic acids have been the subject of relatively few experimental investigations. In this study, we use calorimetric and spectroscopic observables to detect, resolve, and selectively enrich energetically discrete ensembles of microstates within metastable DNA structures. Our results are consistent with metastable, "native" DNA states being composed of an ensemble of discrete and kinetically stable microstates of differential stabilities, rather than exclusively being a single, discrete thermodynamic species. This conceptual construct is important for understanding the linkage between biopolymer conformational/configurational space and biological function, such as in protein folding, allosteric control of enzyme activity, RNA and DNA folding and function, DNA structure and biological regulation, etc. For the specific DNA sequences and structures studied here, the demonstration of discrete, kinetically stable microstates potentially has biological consequences for understanding the development and onset of DNA expansion and triplet repeat diseases.

Replication restart: a pathway for (CTG).(CAG) repeat deletion in Escherichia coli

(CTG)n.(CAG)n repeats undergo deletion at a high rate in plasmids in Escherichia coli in a process that involves RecA and RecB. In addition, DNA replication fork progression can be blocked during synthesis of (CTG)n.(CAG)n repeats. Replication forks stalled at (CTG)n.(CAG)n repeats may be rescued by replication restart that involves recombination as well as enzymes involved in replication and DNA repair, and this process may be responsible for the high rate of repeat deletion in E. coli. To test this hypothesis (CAG)n.(CTG)n deletion rates were measured in several E. coli strains carrying mutations involved in replication restart. (CAG)n.(CTG)n deletion rates were decreased, relative to the rates in wild type cells, in strains containing mutations in priA, recG, ruvAB, and recO. Mutations in priB and priC resulted in small reductions in deletion rates. In a recF strain, rates were decreased when (CAG)n comprised the leading template strand, but rates were increased when (CTG)n comprised the leading template. Deletion rates were increased slightly in a recJ strain. The mutational spectra for most mutant strains were altered relative to those in parental strains. In addition, purified PriA and RecG proteins showed unexpected binding to single-stranded, duplex, and forked DNAs containing (CAG)n and/or (CTG)n loop-outs in various positions. The results presented are consistent with an interpretation that the high rates of trinucleotide repeat instability observed in E. coli result from the attempted restart of replication forks stalled at (CAG)n.(CTG)n repeats.

Low-resolution reconstruction of a synthetic DNA holliday junction

We have studied the low-resolution solution conformation of a Holliday (or four-way) DNA junction by using small-angle x-ray scattering, sedimentation velocity, and computational modeling techniques. The scattering data were analyzed in two independent ways: firstly, by rigid-body modeling of the scattering data using previously suggested models for the Holliday junction (HJ), and secondly, by ab initio reconstruction methods. The models found by both methods agree with experimentally determined sedimentation coefficients and are compatible with the results of previous studies using different techniques, but provide a more direct and accurate determination of the solution conformation of the HJ. Our results show that addition of Mg(2+) alters the conformation of the HJ from an extended to a stacked arrangement.

Topological effects on the electrophoretic mobility of rigid rodlike DNA in polyacrylamide gels

The electrophoretic migration of rigid rodlike DNA structures with well defined topologies has been investigated in polyacrylamide (PA) hydrogels prepared by copolymerization of acrylamide and N, N'-methylenebisacrylamide. Previous studies have reported structural and dynamic characteristics of linear and branched DNA during electrophoresis in PA gels using a variety of experimental parameters. However, a thorough investigation aimed at establishing specific relationships between topological features of rigid rodlike DNA structures and their electrophoretic behavior is still needed. In order to study these topological effects on mobility, an intensive examination of the electrophoretic mobility of small linear and starlike DNA was performed. A series of model DNA structures with well-defined branched topologies were synthesized with varying molecular parameters, such as number of arms surrounding the branch point and arm length. The electrophoretic mobility of these structures was then contrasted with a series of data obtained using linear DNA of comparable molecular size. When large DNA stars (M >/= 60 bp) were compared with linear DNA of identical molecular weight, the Ferguson plots were quite different. However, small DNA stars (24-32 bp) and linear analogues had identical Ferguson plots. This indicates that a different motional mode or greater interaction with the gel exists for the larger DNA stars. When the total molecular weight of the DNA stars was held constant and the number of arms varied, the Ferguson plots for all the stars were identical. Additionally, a critical pore size was reached when the ratio of linear DNA mobility to star DNA mobility increased dramatically. Thus, while the incorporation of a single branch point can produce a large reduction in mobility, above a critical molecular size, the incorporation of additional branch points does not appear to provide further reduction in mobility. This finding is consistent with the transport properties of large synthetic star polymers, where a large reduction in their diffusion coefficient is observed when a single branch is added. When additional arms are incorporated, large synthetic stars do not display an appreciable further reduction in diffusion coefficient. The effect of arm length on mobility for rigid rod DNA stars was also studied. For four-arm DNA stars, the mobility was found to scale as an exponential function of the arm length. Finally, a recently proposed phenomenological model was used to successfully fit the mobility data for linear rigid rod DNA at various concentrations of PA.

Sequence-specific and nonspecific mobilities of single-stranded oligonucleotides observed by changing the borate buffer concentration

The suitability of gel electrophoresis to structural analysis of nucleic acids has been examined, using from borate buffer concentrations (in the range from 4.5 to 450 mM. The gel electrophoretic mobility of single-stranded oligonucleotides was shown to be sequence-dependent at higher concentrations than 27 mM of borate buffer and nondependent at lower one (less than 9 mM). As a result, each dodecamer had a sequence-specific critical concentration of Tris-borate-EDTA (TBE) buffer at which each seems to change its structural state. At the lower concentration than the critical one, all the dodecamers turned to the state of a finite mobility and migrated in a sharp band. This finding is discussed and rationalized by the assumption that the difference in conformational dynamics of oligonucleotides, due to the difference in their sequence, is mainly responsible for the observed difference in their mobility.

Time-resolved fluorescence resonance energy transfer: a versatile tool for the analysis of nucleic acids

The biological functions of nucleic acids in processes of DNA replication, transcription, homologous recombination, mRNA translation, and ribozyme catalysis are intimately linked to their three-dimensional structures and to conformational changes induced by proteins, metal ions and other ligands. Fluorescence spectroscopy is a powerful technique for probing the structure and conformational dynamics of biological macromolecules under a wide range of solution conditions. Fluorescence resonance energy transfer (FRET) provides long-range distance information from 10 to 100 A, a range that is useful for probing the global structure of nucleic acids. While steady-state measurements of FRET provide the average distance between donor and acceptor, much more information is available from the analysis of the nanosecond emission decay of the donor in time-resolved FRET (trFRET) experiments. Analysis of the decay in terms of donor-acceptor distance distributions can resolve different conformers in a heterogeneous mixture, providing information on the global structure and flexibility of each species as well as their equilibrium populations. In this review, we outline the principles of trFRET and the methods used to incorporate fluorescent probes into DNA and RNA. Examples of specific applications are presented to illustrate the versatility of trFRET as a tool to define global structures, to identify conformational heterogeneity and flexibility, to investigate the energetics of tertiary structure formation and to probe structural rearrangements of nucleic acids.

Drug recognition of a DNA single strand break: nogalamycin intercalation between coaxially stacked hairpins

Two DNA hairpin motifs (5'-GCGAAGC-3' and 5'-ACGA AGT-3'), both stabilized by a 5'-GAA loop, have been used to design novel intramolecular double hairpin structures (5'-GCGAAGCACGAAGT-3' and 5'-ACGAAGTGCG AAGC-3') in which coaxial stacking of the two hairpin components generates a double-stranded stem region effectively with a single-strand break in the middle of the sequence at either the TG or CA step between unconnected 3' and 5' terminal bases. We have investigated by NMR the conformation and dynamics of the DNA at the strand break site. We show that mutual stacking significantly enhances the stability of each hairpin. Further, the anthracycline antibiotic nogalamycin binds cleanly to the 5'-TG (5'-CA) site formed by the mutually stacked hairpins despite the break in the sugar-phosphate backbone on one strand. The complex resembles the structure of nogalamycin-DNA complexes with the drug bound at 5'-TG sites in intact duplex sequences, with pi-stacking interactions probably the single dominant stabilizing interaction.

Triplet repeat DNA structures and human genetic disease: dynamic mutations from dynamic DNA

Fourteen genetic neurodegenerative diseases and three fragile sites have been associated with the expansion of (CTG)n (CAG)n, (CGG)n (CCG)n, or (GAA)n (TTC)n repeat tracts. Different models have been proposed for the expansion of triplet repeats, most of which presume the formation of alternative DNA structures in repeat tracts. One of the most likely structures, slipped strand DNA, may stably and reproducibly form within triplet repeat sequences. The propensity to form slipped strand DNA is proportional to the length and homogeneity of the repeat tract. The remarkable stability of slipped strand DNA may, in part, be due to loop-loop interactions facilitated by the sequence complementarity of the loops and the dynamic structure of three-way junctions formed at the loop-outs.

Liquid crystals of polyelectrolyte networks

The Onsager theory of nematic liquid crystals is extended to rigid polyelectrolytes cross-linked by polyvalent ions. Recent synchrotron x-ray diffraction experiments showed that dilute, birefringent networks are formed under these conditions. The application of Onsager theory to this system leads to the prediction of the existence of a range of exotic mesophases such as the "cubatic," the "tetratic," and the "trigatic." The exotic network phases appear on the border of regions of phase coexistence of network phase with isotropic material (at low polyvalent ion concentration) and with dense bundles (at high polyvalent ion concentration).

Charge transport through DNA four-way junctions

Long range oxidative damage as a result of charge transport is shown to occur through single crossover junctions assembled from four semi-complementary strands of DNA. When a rhodium complex is tethered to one of the arms of the four-way junction assembly, thereby restricting its intercalation into the pi-stack, photo-induced oxidative damage occurs to varying degrees at all guanine doublets in the assembly, though direct strand scission only occurs at the predicted site of intercalation. In studies where the Mg(2+) concentration was varied, so as to perturb base stacking at the junction, charge transport was found to be enhanced but not to be strongly localized to the arms that preferentially stack on each other. These data suggest that the conformations of four-way junctions can be relatively mobile. Certainly, in four-way junctions charge transport is less discriminate than in the more rigidly stacked DNA double crossover assemblies.

Folding of branched RNA species

The global structures of branched RNA species are important to their function. Branched RNA species are defined as molecules in which double-helical segments are interrupted by abrupt discontinuities. These include helical junctions of different orders, and base bulges and loops. Common helical junctions are three- and four-way junctions, often interrupted by mispairs or additional nucleotides. There are many interesting examples of functional RNA junctions, including the hammerhead and hairpin ribozymes, and junctions that serve as binding sites for proteins. The junctions display some common structural properties. These include a tendency to undergo pairwise helical stacking and ion-induced conformational transitions. Helical branchpoints can act as key architectural components and as important sites for interactions with proteins. Copyright 1999 John Wiley & Sons, Inc.

DNA recognition by structure-selective nucleases

The nucleases discussed in this review show little sequence specificity but instead recognize certain structural features of their respective DNA substrates. The level of their structural selectivity ranges from simple discrimination between single- and double-stranded DNA (nucleases P1 and S1), the recognition of helical parameters like groove width and flexibility (DNase I), the recognition of helical distortions caused by abasic sites (exonuclease III, HAP1), to the recognition of specialized structures like flap DNA (5'-nucleases of eukaryotes, phages, and eubacterial DNA polymerases) and four-way junctions (T4 endonuclease VII, RuvC). The discussion is focused on the structural basis of the recognition process. In most cases the available x-ray structures of the nucleases and/or their DNA complexes have revealed the presence of structural motifs explaining the observed structural selectivity.

RecA force generation by hydrolysis waves

We present a simple theory of the dynamics of force generation by RecA during homologous strand exchange and a continuous, deterministic mathematical model of the proposed process. Calculations show that force generation is possible in this model for certain reasonable values of the parameters. We predict the shape of the force-velocity curve for the Holliday junction, which exhibits a distinctive kink at large retarding force, and suggest experiments which should distinguish between the proposed model and other models in the literature.

Structure of branched DNA molecules: gel retardation and atomic force microscopy studies

DNA heteroduplexes as models for slipped strand DNA have been analyzed by polyacrylamide gel migration and atomic force microscopy (AFM). All heteroduplexes containing one hairpin or loop have reduced electrophoretic mobilities compared with that expected for their molecular weights. The retarded gel mobility correlates with the formation of a sharp kink detected by AFM. Increasing the hairpin length from 7 bp to 50 bp results in a monotonous decrease in gel mobility of heteroduplexes. This secondary retardation effect appears to depend only on the hairpin size since the AFM data show no dependence of the kink angle on the hairpin length. Heteroduplex isomers with a loop or hairpin in opposite strands migrate with distinct mobilities. Analysis of gel migration of heteroduplexes with altered hairpin orientations as well as of truncated heteroduplexes indicates that the difference in mobility is due to an inherent curvature in one of the long arms. This is confirmed by the end-to-end distance measurements from AFM images. In addition, significant variation of the end-to-end distances is consistent with a dynamic structure of heteroduplexes at the three-way junction. Double heteroduplexes containing one hairpin in each of the complementary strands also separate in a gel as two isomers. Their appearance in AFM showed a complicated pattern of flat representations of the three-dimensional structure and may indicate a certain degree of interaction between complementary parts of the hairpins that are several helical turns apart.

DNA-sequence asymmetry directs the alignment of recombination sites in the FLP synaptic complex

The FLP recombinase promotes site-specific recombination in the 2 micrometer circle of Saccharomyces cerevisiae. FLP recognizes a 48 bp target site (FLP recombination target, or FRT) consisting of three 13 bp protein binding sites, or symmetry elements, flanking an 8 bp spacer region. Efficient recombination also occurs with DNA substrates that have minimal FRT sites, consisting only of the spacer and two surrounding 13 bp symmetry elements arranged in inverse orientation; thus, the wild-type spacer sequence is the main asymmetric feature of the minimal recombination site. FLP carries out recombination with many minimal target sites bearing symmetric or asymmetric mutant spacer sequences; however, the overall directionality of recombination defined in terms of inversion or excision of a DNA domain is determined by spacer-sequence asymmetry. In order to evaluate the potential influence of spacer-sequence asymmetry on structures formed during early steps in recombination, we used electron microscopy to investigate the structure of the FLP synaptic complex, which is the intermediate protein-DNA complex involved in site pairing and strand exchange. Using linear substrate DNAs that have minimal FRTs with wild-type spacer sequences, we find that 85 to 90% of the FLP synaptic complexes examined contain the two FRTs aligned in parallel. This strong preference for parallel site alignment stands in contrast with prevailing models for lambda integrase-class recombination systems, which postulate antiparallel site alignment, and results from biophysical studies on synthetic, immobile four-way DNA junctions. Our results show that the strong preference for parallel alignment can be attributed to conformational preferences of Holliday junctions present in the synaptosome.

Left-handed DNA crossovers. Implications for DNA-DNA recognition and structural alterations

The close approach of DNA segments participates in many biological functions including DNA condensation and DNA processing. Previous crystallographic studies have shown that B-DNA self-fitting by mutual groove-backbone interaction produces right-handed DNA crossovers. These structures have opened new perspectives on the role of close DNA-DNA interactions in the architecture and activity the DNA molecule. In the present study, the analysis of the crystal packing of two B-DNA decamer duplexes d(CCIIICCCGG) and d(CCGCCGGCGG) reveals the existence of new modes of DNA crossing. Symmetric left-handed crossovers are produced by mutual fitting of DNA grooves at the crossing point. New sequence patterns contribute to stabilize longitudinal fitting of the sugar-phosphate backbone into the major groove. In addition, the close approach of DNA segments greatly influences the DNA conformation in a sequence dependent manner. This study provides new insights into the role of DNA sequence and structure in DNA-DNA recognition. In providing detailed molecular views of DNA crossovers of opposite chirality, this study can also help to elucidate the role of symmetry and chirality in the recognition of complex DNA structures by protein dimers or tetramers, such as topoisomerase II and recombinase enzymes. These results are discussed in the context of the possible relationships between DNA condensation and DNA processing.

Engineered assembly of intertwined oligomers of an immunoglobulin chain

Domain 1 of CD2 (CD2.D1) forms a conventional Ig fold stabilised by non-covalent antiparallel contacts between beta-strands. Removing two residues from the middle of the protein sequence, where the polypeptide chain normally folds back upon itself, stabilises an open conformation. In this modified molecule, the optimum evolved contacts between side-chains can only be satisfied through the antiparallel association of two chains to create a symmetrical pair of pseudo-domains. Here, we describe the dynamics of the switch between monomeric and dimeric states and demonstrate the extension of this novel underlying principle to trimer and tetramer formation. The ability of a protein molecule to form higher-order antiparallel structures is reminiscent of the behaviour of hairpins, duplexes, three-way and Holliday junctions in DNA.

Nucleotide excision repair affects the stability of long transcribed (CTG*CAG) tracts in an orientation-dependent manner in Escherichia coli

The influence of nucleotide excision repair (NER), the principal in vivo repair system for DNA damages, was investigated in Escherichia coli with uvrA, uvrB and uvrAuvrB mutants with the triplet repeat sequences (TRS) involved in myotonic dystrophy, the fragile X syndrome and Friedreich's ataxia. (CTG*CAG)175was more stable when the (CTG) strand was transcribed than when the (CAG) strand was transcribed in the alternate orientation. A lack of the UvrA protein dramatically increases the instability of this TRS in vivo as compared with the stability of the same sequence in uvrB mutant, which produces an intact UvrA protein. We propose that transcription transiently dissociates the triplet repeat complementary strands enabling the non-transcribed strand to fold into a hairpin conformation which is then sufficiently stable that replication bypasses the hairpin to give large deletions. If the TRS was not transcribed, fewer deletions were observed. Alternatively, in the uvrA-mutant, the hairpins existing on the lagging strand will suffer bypass DNA synthesis to generate deleted molecules. Hence, NER, functionally similar in both prokaryotes and eukaryotes, is an important factor in the genetic instabilities of long transcribed TRS implicated in human hereditary neuro-logical diseases.

Symmetry and chirality in topoisomerase II-DNA crossover recognition

Several experimental data support the notion that the recognition of DNA crossovers play an important role in the multiple functions of topoisomerase II. Here, a theoretical analysis of the possible modes of assembly of yeast topoisomerase II with right and left-handed tight DNA crossovers is performed, using the crystal coordinates of the docking partners. The DNA crossovers are assumed to be clamped into the central hole of the enzyme. Taking into account the rules for building symmetric ternary complexes and the structural constraints imposed by DNA-DNA and protein-DNA interactions, this analysis shows that two geometric solutions could exist, depending on the chirality of the DNA crossovers. In the first one, the two DNA segments are symmetrically recognized by the enzyme while each single double helix binds asymmetrically the protein dimer. In the second one, each double helix is symmetrically recognized by the protein around its dyad axis, while the two DNA segments have their own binding modes. The finding of potential DNA-binding domains which could interact with the crossovers provides structural supports for each model. The structural similarity of a loop containing a cluster of conserved basic residues pointing into the central hole of topoisomerase II and the second DNA-binding site of histone H5 which binds DNA crossover is of particular interest. Each solution, which is consistent with different sets of experimental data found in the literature, could either correspond to different functions of the enzyme or different steps of the reaction. This work provides structural insights for better understanding the role of chirality and symmetry in topoisomerase II-DNA crossover recognition, suggests testable experiments to further elucidate the structure of ternary complexes, and raises new questions about the relationships between the mechanism of strand-passage and strand-exchange catalyzed by the enzyme.

Excess counterion binding and ionic stability of kinked and branched DNA

We compute the excess number of counterions associated with kinked and branched DNA, and the ionic stabilities of these structures as a function of chain length and both sodium and magnesium salt concentration, using numerical counterion condensation theory. The DNA structures are modeled as two or more finite lines of phosphate charges radiating from the kink or junction center. The number of excess counterions around the (40-90 degrees) kinked duplex is very small (at most four). The geometries of large three- and four-way DNA junctions (with > 50 base pairs per arm) in solutions containing low to moderate NaCl concentrations, by contrast, accumulate a substantial number of excess sodium ions (> 20) but no more than 15 magnesium counterions. The excess number of counterions surrounding the kinked linear chain and the branched DNA structures either remains invariant or increases with chain length, tending to reach a plateau value. Open configurations, such as the planar Y-shaped three-way junction (with three 120 degrees inter-arm angles) and the 90 degrees cross-shaped four-way junction, are ionically more stable than compact geometries, such as pyramidal three-way junctions and X-shaped four-way junctions, over the entire range of salt concentration considered (10(-5)-10(-1) M NaCl or MgCl2). The ionic stabilities of the compact forms increase with increasing salt concentration and become comparable to those of the extended geometries at high salt (especially when magnesium is the supporting salt).

Effects of supercoiling in electrophoretic trapping of circular DNA in polyacrylamide gels

Electrophoretic velocity and orientation have been used to study the electric-field-induced trapping of supercoiled and relaxed circular DNA (2926 and 5386 bp) in polyacrylamide gels (5% T, 3.3% C) at 7.5-22.5 V/cm, using as controls linear molecules of either the same contour length or the same radius of gyration. The circle-specific trapping is reversible. From the duration of the reverse pulse needed to detrap the molecules, the average trap depth is estimated to be 90 A, which is consistent with the molecular charge and the field strengths needed to keep molecules trapped. Trapped circles exhibit a strong field alignment compared to the linear form, and there is a good correlation between the enhanced field alignment for the circles and the onset of trapping in both constant and pulsed fields. The circles do not exhibit the orientation overshoot response to a field pulse seen with linear DNA, and the rate of orientation growth scales as E(-2+/-0.1) with the field, as opposed to E(-1.1+/-0.1) for the linear form. These results show that the linear form migrates by cyclic reptation, whereas the circles most likely are trapped by impalement on gel fibers. This proposal is supported by very similar velocity and orientation behavior of circular DNA in agarose gels, where impalement has been deemed more likely because of stiffer gel fibers. The trapping efficiency is sensitive to DNA topology, as expected for impalement. In polyacrylamide the supercoiled form (superhelical density sigma = -0.05) has a two- to fourfold lower probability of trapping than the corresponding relaxed species, whereas in agarose gels the supercoiled form is not trapped at all. These results are consistent with existing data on the average holes in the plectonemic supercoiled structures and the fiber thicknesses in the two gel types. On the basis of the topology effect, it is argued that impalement during pulsed-field electrophoresis in polyacrylamide gels may be useful for the separation of more intricate DNA structures such as knots. The results also indicate that linear dichroism on field-aligned molecules can be used to measure the supercoiling angle, if relaxed DNA circles are used as controls for the global degree of orientation.

Trinucleotide repeat DNA structures: dynamic mutations from dynamic DNA

Models for the disease-associated expansion of (CTG)n.(CAG)n, (CGG)n.(CCG)n, and (GAA)n.(TTC)n trinucleotide repeats involve alternative DNA structures formed during DNA replication, repair and recombination. These repeat sequences are inherently flexible and can form a variety of hairpins, intramolecular triplexes, quadruplexes, and slipped-strand structures that may be important intermediates and result in their genetic instability.

CTG repeats associated with human genetic disease are inherently flexible

The lengthening of tracts of CTG, CGG and GAA triplet repeats during progression of a pedigree has been associated with more than 12 human genetic diseases, including fragile X syndrome, myotonic dystrophy and Friedreich's ataxia. These repetitive sequence elements have the potential to form alternative DNA secondary structures that may contribute to their instability. The alternative DNA secondary structures may mediate errors during DNA replication, repair or recombination of the triplet repeat, leading to expansion. Here we show that DNA composed of pure CTG or CGG repeats exhibits anomalously fast mobility on polyacrylamide gels, confirming a previous observation for DNA containing CTG and CGG triplet repeats flanked by mixed sequence DNA. Moreover, we show that even short tracts of duplex CTG repeats have an unusual helix structure. CTG repeats reduce overall curvature associated with phased A-tract or GGCC curves, but alone they do not introduce curvature into DNA. The reduction in curvature of phased A-tracts by CTG repeats is similar to that afforded by an interspersed flexible region associated with a (TT).(TT) mispair. CTG-containing DNAs exhibit a rapid rate of cyclization, consistent with a flexible helix. These results suggest that tracts of (CTG).(CAG) repeats are inherently flexible. In addition, our results suggest that the unusual rapid electrophoretic mobility of CTG or CGG-containing DNA may be a consequence of an extended flexible DNA chain.

Minor groove-binding architectural proteins: structure, function, and DNA recognition

To date, high-resolution structures have been solved for five different architectural proteins complexed to their DNA target sites. These include TATA-box-binding protein, integration host factor (IHF), high mobility group I(Y)[HMG I(Y)], and the HMG-box-containing proteins SRY and LEF-1. Each of these proteins interacts with DNA exclusively through minor groove contacts and alters DNA conformation. This paper reviews the structural features of these complexes and the roles they play in facilitating assembly of higher-order protein-DNA complexes and discusses elements that contribute to sequence-specific recognition and conformational changes.

Design and NMR study of an immobile DNA four-way junction containing 38 nucleotides

The DNA Holliday junction is a central intermediate in genetic recombination. We have designed and synthesized a DNA oligomer, J1a, as a model compound for the Holliday junction suitable to be studied by NMR spectroscopy and future molecular modelling. The design was based on a 46-base oligomer, J4, previously studied by Pikkemaat, J. A., van den Elst, H., van Boom, J. H. & Altona, C. [Biochemistry 33, 14896-14907 (1994)], including the propensity to undergo a self-folding process to give a four-way junction in which three of the four arms are capped with a hairpin loop. J1a, however, is considerably shortened by eight bases and thus contains only 38 residues which significantly facilitates the proton resonance assignments. The base sequence at the branch point is identical to that in J4. 1H-NMR data clearly point to the presence of three hairpin loops in J1a and show that the double-helical arms adopt the B-DNA form. Quasicontinuous pairwise stacking between helical arms to give a single preferred stacked X-conformation is evident. The extent of folding into this stacked conformation is strongly dependent upon the magnesium concentration. Full Watson-Crick base pairing at the branch point is completely preserved. The A/D-stacking preference of the small junction is the same as that exhibited by J4.

Crossover isomer bias is the primary sequence-dependent property of immobilized Holliday junctions

Recombination of genes is essential to the evolution of genetic diversity, the segregation of chromosomes during cell division, and certain DNA repair processes. The Holliday junction, a four-arm, four-strand branched DNA crossover structure, is formed as a transient intermediate during genetic recombination and repair processes in the cell. The recognition and subsequent resolution of Holliday junctions into parental or recombined products appear to be critically dependent on their three-dimensional structure. Complementary NMR and time-resolved fluorescence resonance energy transfer experiments on immobilized four-arm DNA junctions reported here indicate that the Holliday junction cannot be viewed as a static structure but rather as an equilibrium mixture of two conformational isomers. Furthermore, the distribution between the two possible crossover isomers was found to depend on the sequence in a manner that was not anticipated on the basis of previous low-resolution experiments.

Recognition and manipulation of branched DNA structure by junction-resolving enzymes

The junction-resolving enzymes are a class of nucleases that introduce paired cleavages into four-way DNA junctions. They are important in DNA recombination and repair, and are found throughout nature, from eubacteria and their bacteriophages through to higher eukaryotes and their viruses. These enzymes exhibit structure-selective binding to DNA junctions; although cleavage may be more or less sequence-dependent, binding affinity is purely related to the branched structure of the DNA. Binding and cleavage events can be separated for a number of the enzymes by mutagenesis, and mutant proteins that are defective in cleavage while retaining normal junction-selective binding have been isolated. Critical acidic residues have been identified in several resolving enzymes, suggesting a role in the coordination of metal ions that probably deliver the hydrolytic water molecule. The resolving enzymes all bind to junctions in dimeric form, and the subunits introduce independent cleavages within the lifetime of the enzyme-junction complex to ensure resolution of the four-way junction. In addition to recognising the structure of the junction, recent data from four different junction-resolving enzymes indicate that they also manipulate the global structure. In some cases this results in severe distortion of the folded structure of the junction. Understanding the recognition and manipulation of DNA structure by these enzymes is a fascinating challenge in molecular recognition.

Action of site-specific recombinases XerC and XerD on tethered Holliday junctions

In Xer site-specific recombination, two related recombinases, XerC and XerD, mediate the formation of recombinant products using Holliday junction-containing DNA molecules as reaction intermediates. Each recombinase catalyses the exchange of one pair of specific strands. By using synthetic Holliday junction-containing recombination substrates in which two of the four arms are tethered in an antiparallel configuration by a nine thymine oligonucleotide, we show that XerD catalyses efficient strand exchange only when its substrate strands are 'crossed'. XerC also catalyses very efficient strand exchange when its substrate strands are 'crossed', though it also appears to be able to mediate strand exchange when its substrate strands are 'continuous'. By using chemical probes of Holliday junction structure in the presence and absence of bound recombinases, we show that recombinase binding induces unstacking of the bases in the centre of the recombination site, indicating that the junction branch point is positioned there and is distorted as a consequence of recombinase binding.

The resolving enzyme CCE1 of yeast opens the structure of the four-way DNA junction

Junction-resolving enzymes exhibit structure-selective binding to DNA, but may also manipulate the DNA structure. CCE1 is a junction-resolving enzyme found in the yeast mitochondrion. To facilitate the analysis of the CCE1-junction interaction, we have exploited the sequence dependence of the cleavage reaction to devise a junction that is refractory to cleavage by this enzyme, even in the presence of magnesium ions. On binding to four-way DNA junctions, pure recombinant CCE1 opens the global structure into a 4-fold symmetrical configuration of arms with an open, chemically reactive centre. The structure of the CCE1-junction complex is independent of the sequence of the junction, and of the presence or absence of magnesium or other ions. This and other functional properties of CCE1 are strikingly similar to those of RuvC resolving enzyme of Escherichia coli.

Cruciform structures and functions

In this paper, a structure-function analysis of B-DNA self-fitting is reviewed in the light of recent oligonucleotide crystal structures. Their crystal packings provided a high-resolution view of B-DNA helices closely and specifically fitted by groove-backbone interaction, a natural and biologically relevant manner to assemble B-DNA helices. In revealing that new properties of the DNA molecule emerge during condensation, these crystallographic studies have pointed to the biological importance of DNA—DNA interactions.

Fluorescence characteristics of 5-carboxytetramethylrhodamine linked covalently to the 5' end of oligonucleotides: multiple conformers of single-stranded and double-stranded dye-DNA complexes

Fluorescence steady-state and lifetime experiments have been carried out on duplex and single-stranded DNA molecules labeled at the 5' ends with 5-carboxytetramethylrhodamine (TMRh). The temperature and ionic strength of the solutions were varied over large ranges. The results reveal at least three well-defined states of the TMRh-DNA molecules for the single-stranded as well as for the double-stranded DNA molecules. Two states are fluorescent, with lifetimes in the range of 0.5-1 ns and 2.5-3 ns. A third state of TMRh-DNA does not fluoresce (a dark species of TMRh-DNA). The distribution of the TMRh-DNA molecules among these three states is strongly temperature and ionic strength dependent. Estimates are made of some reaction parameters of the multistate model. The results are discussed in terms of the photophysics of TMRh, and consequences of the multiple conformers of TMRh-DNA for studies involving fluorescence studies with TMRh-labeled DNA are considered.

Three-way and four-way junctions in DNA: a conformational viewpoint

DNA junctions are potential intermediates in various important genetic processes, including mutagenesis and recombination. The quantity of research carried out in this area is rapidly increasing. Examples of three-way and four-way junctions are now relatively well characterized and a few common properties have been recognized, of which the most important is the tendency of junctions to fold into one or more coaxially stacked helical conformations or cross-over structures.

The global folding of four-way helical junctions in RNA, including that in U1 snRNA

Helical junctions are important elements in the architecture of folded RNA molecules. The global geometry of fully base-paired four-way junctions between RNA helices has been analyzed by comparative gel electrophoresis. Junctions appear to fold by pairwise coaxial helical stacking in one of two possible stereochemically equivalent isomers based upon alternative selections of stacking partners. In the presence of 1 mM Mg2+, the two continuous helical axes are approximately at right angles to each other for all junctions studied, but the RNA junctions exhibit significant sequence-dependent differences in their structures as a function of ionic conditions. The four-way junction found in the U1 snRNA folded by coaxial helical stacking. It retained the 90 degrees crossed stacked structure under all ionic conditions tested, despite the presence of a G.A mismatch at the point of strand exchange.

Interaction of normal and mutant SRY proteins with DNA

In mammals, sex determination is caused by the Y-chromosome gene SRY. The DNA-binding domain of human SRY protein is similar to those of the chromatin protein HMG1. Like HMG1, SRY binds to kinked DNA structures, and bends linear DNA sharply upon binding. We analysed the biochemical properties of mutant SRY proteins from five patients with complete gonadal dysgenesis: two bind and bend DNA almost normally, two bind inefficiently but bend DNA normally, and one binds DNA with almost normal affinity but produces a different angle. The mutations with moderate effect on complex formation can be transmitted to progeny, the ones with severe effects on either binding or bending are de novo. The angle induced by SRY depends on the exact DNA sequence, thus discriminating different target sites. We suggest that the exact spatial arrangement of the nucleoprotein complex organized by SRY in chromatin is essential for the expression of genes involved in testis differentiation.

Suicide substrates reveal properties of the homology-dependent steps during integrative recombination of bacteriophage lambda

BACKGROUND

A fundamental feature of bacteriophage lambda site-specific recombination is the strict requirement for a region of sequence identity between recombining DNA duplexes. It has been difficult to understand how the recombination machinery identifies and responds to nonhomologies as subtle as a single base-pair substitution, because the reaction intermediates are transient and there are likely to be several different homology-dependent steps. In order to understand better how the recombination machinery compares parental sequences, we have used the recently developed 'suicide substances'--DNA containing 5'-bridging phosphorothioate linkages--to monitor the timing of homology-sensing relative to the strand cleavage reactions.

RESULTS

The cleavage reactions for the two different strands of attB, the bacterial recombination locus for lambda integration, show very different degrees of dependence on homology with the partner locus, attP. Strand cleavage at the B binding site for Int recombinase is insensitive to homology. In contrast, cleavage at the B' binding site strongly depends on homology in the three base pairs adjacent to the B site. Strand cleavage at the B site is apparently required for the readout of this homology but, surprisingly, joining of the cleaved B site to a partner is not.

CONCLUSIONS

Our finding that cleavage at the B site is insensitive to homology shows that effective synapsis between partners does not depend on sequence matching. Cleavage at the B' site provides the earliest positive signal for a homology-dependent switch in the lambda recombination machinery. Because this switch can occur in the absence of strand joining, the results argue against models that invoke strand ligation as the critical element of homology-sensing. Alternative mechanisms are presented that involve varieties of non-covalent strand swapping. A synthesis of the present results and other recent experiments highlights the importance of the disannealing of complementary strands and their reannealing to new partners, a process traditionally described as branch migration. The reversibility of branch migration and its bias away from mismatched combinations are proposed to be the major mechanisms of homology-sensing during lambda integration.

Conformation of short DNA fragments by modulated fluorescence polarization anisotropy

The technique of fluorescence polarization anisotropy (FPA) decay of intercalated ethidium has been used to study DNA conformation and dynamics, which are being recognized as primary determinants in transcription control and other cellular processes. Frequency modulated FPA when applied to two DNA molecules, a "straight" 50 base-pairs duplex fragment, and a bent fragment of similar length, has yielded different rotational diffusion coefficients for the two fragments. The data have been processed with an analytical model and with Brownian dynamics simulations, obtaining a good fit and a quantitative agreement between the two models. Both analyses have confirmed that one fragment can be described as a straight cylinder, while the other fragment is bent, with an angle estimated to be 45 degrees +/- 3 degrees. FPA has proved to be very powerful in determining simple conformations of short DNA duplexes and also particularly apt to probe the dynamical features of DNA fragments where conventional methods are either too cumbersome or fail to give quantitative results. In addition, the ligand no longer behaves ideally due to its complex structure and charge distribution. Thus for the protein the slope is no longer related simply to the net ligand charge, and the PB model gives a much larger slope than the LL model.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the mechanism of DNA recombination using tangles

The DNA of all organisms has a complex and essential topology. The three topological properties of naturally occurring DNA are supercoiling, catenation, and knotting. Although these properties are denned rigorously only for closed circular DNA, even linear DNAin vivocan have topological properties because it is divided into topologically separate subdomains (Drlica 1987; Roberge & Gasser, 1992). The essentiality of topological properties is demonstrated by the lethal consequence of interfering with topoisomerases, the enzymes that regulate the level of DNA supercoiling and that unlink DNA during its replication (reviewed in Wang, 1991; Bjornsti, 1991; Drlica, 1992; Ullspergeret al. 1995).