Cooperative Self-Assembly Process Involving Giant Toroidal Polyoxometalate as a Membrane Building Block in Nanoscale Vesicles

The self-assembly of organic amphiphilic species into various aggregates such as spherical or elongated micelles and cylinders up to the formation of lyotropic hexagonal or lamellar phases results from cooperative processes orchestrated by the hydrophobic effect, while those involving ionic inorganic polynuclear entities and nonionic organic components are still intriguing. Herein, we report on the supramolecular behavior of giant toroidal molybdenum blue-type polyoxometalate, namely, the {Mo154} species in the presence of n-octyl-β-glucoside (C8G1), widely used as a surfactant in biochemistry. Structural investigations were carried out using a set of complementary multiscale methods including single-crystal X-ray diffraction analysis supported by molecular modeling, small-angle X-ray scattering and cryo-TEM observations. In addition, liquid NMR, viscosimetry, surface tension measurement, and isothermal titration calorimetry provided further information to decipher the complex aggregation pathway. Elucidation of the assembly process reveals a rich scenario where the presence of the large {Mo154} anion disrupts the self-assembly of the C8G1, well-known to produce micelles, and induces striking successive phase transitions from fluid-to-gel and from gel-to-fluid. Herein, intimate organic-inorganic primary interactions arising from the superchaotropic nature of the {Mo154} lead to versatile nanoscopic hybrid C8G1-{Mo154} aggregates including crystalline discrete assemblies, smectic lamellar liquid crystals, and large uni- or multilamellar vesicles where the large torus {Mo154} acts a trans-membrane component.

Multi-feature clustering of CTCF binding creates robustness for loop extrusion blocking and Topologically Associating Domain boundaries

Topologically Associating Domains (TADs) separate vertebrate genomes into insulated regulatory neighborhoods that focus genome-associated processes. TADs are formed by Cohesin-mediated loop extrusion, with many TAD boundaries consisting of clustered binding sites of the CTCF insulator protein. Here we determine how this clustering of CTCF binding contributes to the blocking of loop extrusion and the insulation between TADs. We identify enrichment of three features of CTCF binding at strong TAD boundaries, consisting of strongly bound and closely spaced CTCF binding peaks, with a further enrichment of DNA-binding motifs within these peaks. Using multi-contact Nano-C analysis in cells with normal and perturbed CTCF binding, we establish that individual CTCF binding sites contribute to the blocking of loop extrusion, but in an incomplete manner. When clustered, individual CTCF binding sites thus create a stepwise insulation between neighboring TADs. Based on these results, we propose a model whereby multiple instances of temporal loop extrusion blocking create strong insulation between TADs.

Role of surfactants in electron cryo-microscopy film preparation

Single-particle electron cryo-microscopy (cryo-EM) has become an effective and straightforward approach to determine the structure of membrane proteins (MPs). However, obtaining cryo-EM grids of sufficient quality for high-resolution structural analysis remains a major bottleneck. One of the difficulties arises from the presence of detergents, which often leads to a lack of control of the ice thickness. Amphipathic polymers such as amphipols (APols) are detergent substitutes, which have proven to be valuable tools for cryo-EM studies. In this work, we investigate the physico-chemical behavior of APol- and detergent-containing solutions and show a correlation with the properties of vitreous thin films in cryo-EM grids. This study provides new insight on the potential of APols, allowing a better control of ice thickness while limiting protein adsorption at the air-water interface, as shown with the full-length mouse serotonin 5-HT_3A receptor whose structure has been solved in APol. These findings may speed up the process of grid optimization to obtain high-resolution structures of MPs.

TomoFlow: Analysis of Continuous Conformational Variability of Macromolecules in Cryogenic Subtomograms based on 3D Dense Optical Flow

Cryogenic Electron Tomography (cryo-ET) allows structural and dynamics studies of macromolecules in situ. Averaging different copies of imaged macromolecules is commonly used to obtain their structure at higher resolution and discrete classification to analyze their dynamics. Instrumental and data processing developments are progressively equipping cryo-ET studies with the ability to escape the trap of classification into a complete continuous conformational variability analysis. In this work, we propose TomoFlow, a method for analyzing macromolecular continuous conformational variability in cryo-ET subtomograms based on a three-dimensional dense optical flow (OF) approach. The resultant lower-dimensional conformational space allows generating movies of macromolecular motion and obtaining subtomogram averages by grouping conformationally similar subtomograms. The animations and the subtomogram group averages reveal accurate trajectories of macromolecular motion based on a novel mathematical model that makes use of OF properties. This paper describes TomoFlow with tests on simulated datasets generated using different techniques, namely Normal Mode Analysis and Molecular Dynamics Simulation. It also shows an application of TomoFlow on a dataset of nucleosomes in situ, which provided promising results coherent with previous findings using the same dataset but without imposing any prior knowledge on the analysis of the conformational variability. The method is discussed with its potential uses and limitations.

HEMNMA-3D: Cryo Electron Tomography Method Based on Normal Mode Analysis to Study Continuous Conformational Variability of Macromolecular Complexes

Cryogenic electron tomography (cryo-ET) allows structural determination of biomolecules in their native environment (in situ). Its potential of providing information on the dynamics of macromolecular complexes in cells is still largely unexploited, due to the challenges of the data analysis. The crowded cell environment and continuous conformational changes of complexes make difficult disentangling the data heterogeneity. We present HEMNMA-3D, which is, to the best of our knowledge, the first method for analyzing cryo electron subtomograms in terms of continuous conformational changes of complexes. HEMNMA-3D uses a combination of elastic and rigid-body 3D-to-3D iterative alignments of a flexible 3D reference (atomic structure or electron microscopy density map) to match the conformation, orientation, and position of the complex in each subtomogram. The elastic matching combines molecular mechanics simulation (Normal Mode Analysis of the 3D reference) and experimental, subtomogram data analysis. The rigid-body alignment includes compensation for the missing wedge, due to the limited tilt angle of cryo-ET. The conformational parameters (amplitudes of normal modes) of the complexes in subtomograms obtained through the alignment are processed to visualize the distribution of conformations in a space of lower dimension (typically, 2D or 3D) referred to as space of conformations. This allows a visually interpretable insight into the dynamics of the complexes, by calculating 3D averages of subtomograms with similar conformations from selected (densest) regions and by recording movies of the 3D reference's displacement along selected trajectories through the densest regions. We describe HEMNMA-3D and show its validation using synthetic datasets. We apply HEMNMA-3D to an experimental dataset describing in situ nucleosome conformational variability. HEMNMA-3D software is available freely (open-source) as part of ContinuousFlex plugin of Scipion V3.0 (http://scipion.i2pc.es).

Local structure of DNA toroids reveals curvature-dependent intermolecular forces

In viruses and cells, DNA is closely packed and tightly curved thanks to polyvalent cations inducing an effective attraction between its negatively charged filaments. Our understanding of this effective attraction remains very incomplete, partly because experimental data is limited to bulk measurements on large samples of mostly uncurved DNA helices. Here we use cryo electron microscopy to shed light on the interaction between highly curved helices. We find that the spacing between DNA helices in spermine-induced DNA toroidal condensates depends on their location within the torus, consistent with a mathematical model based on the competition between electrostatic interactions and the bending rigidity of DNA. We use our model to infer the characteristics of the interaction potential, and find that its equilibrium spacing strongly depends on the curvature of the filaments. In addition, the interaction is much softer than previously reported in bulk samples using different salt conditions. Beyond viruses and cells, our characterization of the interactions governing DNA-based dense structures could help develop robust designs in DNA nanotechnologies.

Nucleosome conformational variability in solution and in interphase nuclei evidenced by cryo-electron microscopy of vitreous sections

In Eukaryotes, DNA is wound around the histone octamer forming the basic chromatin unit, the nucleosome. Atomic structures have been obtained from crystallography and single particle cryo-electron microscopy (cryoEM) of identical engineered particles. But native nucleosomes are dynamical entities with diverse DNA sequence and histone content, and little is known about their conformational variability, especially in the cellular context. Using cryoEM and tomography of vitreous sections we analyse native nucleosomes, both in vitro, using purified particles solubilized at physiologically relevant concentrations (25–50%), and in situ, within interphase nuclei. We visualize individual nucleosomes at a level of detail that allows us to measure the distance between the DNA gyres wrapped around. In concentrated solutions, we demonstrate a salt-dependent transition, with a high salt compact conformation resembling the canonical nucleosome and an open low salt one, closer to nuclear nucleosomes. Although further particle characterization and cartography are needed to understand the relationship between this conformational variability and chromatin functional states, this work opens a route to chromatin exploration in situ.

Can Changes in Temperature or Ionic Conditions Modify the DNA Organization in the Full Bacteriophage Capsid?

We compared four bacteriophage species, T5, λ, T7, and Φ29, to explore the possibilities of DNA reorganization in the capsid where the chain is highly concentrated and confined. First, we did not detect any change in DNA organization as a function of temperature between 20 to 40 °C. Second, the presence of spermine (4+) induces a significant enlargement of the typical size of the hexagonal domains in all phages. We interpret these changes as a reorganization of DNA by slight movements of defects in the structure, triggered by a partial screening of repulsive interactions. We did not detect any signal characteristic of a long-range chiral organization of the encapsidated DNA in the presence and in the absence of spermine.

Coexistence of coil and globule domains within a single confined DNA chain

The highly charged DNA chain may be either in an extended conformation, the coil, or condensed into a highly dense and ordered structure, the toroid. The transition, also called collapse of the chain, can be triggered in different ways, for example by changing the ionic conditions of the solution. We observe individual DNA molecules one by one, kept separated and confined inside a protein shell (the envelope of a bacterial virus, 80 nm in diameter). For subcritical concentrations of spermine (4+), part of the DNA is condensed and organized in a toroid and the other part of the chain remains uncondensed around. Two states coexist along the same DNA chain. These ‘hairy’ globules are imaged by cryo-electron microscopy. We describe the global conformation of the chain and the local ordering of DNA segments inside the toroid.

Polymorphism of DNA conformation inside the bacteriophage capsid

Double-stranded DNA bacteriophage genomes are packaged into their icosahedral capsids at the highest densities known so far (about 50 % w:v). How the molecule is folded at such density and how its conformation changes upon ejection or packaging are fascinating questions still largely open. We review cryo-TEM analyses of DNA conformation inside partially filled capsids as a function of the physico-chemical environment (ions, osmotic pressure, temperature). We show that there exists a wide variety of DNA conformations. Strikingly, the different observed structures can be described by some of the different models proposed over the years for DNA organisation inside bacteriophage capsids: either spool-like structures with axial or concentric symmetries, or liquid crystalline structures characterised by a DNA homogeneous density. The relevance of these conformations for the understanding of DNA folding and unfolding upon ejection and packaging in vivo is discussed.

Protein-DNA interactions determine the shapes of DNA toroids condensed in virus capsids

DNA toroids that form inside the bacteriophage capsid present different shapes according to whether they are formed by the addition of spermine or polyethylene glycol to the bathing solution. Spermine-DNA toroids present a convex, faceted section with no or minor distortions of the DNA interstrand spacing with respect to those observed in the bulk, whereas polyethylene glycol-induced toroids are flattened to the capsid inner surface and show a crescent-like, nonconvex shape. By modeling the energetics of the DNA toroid using a free-energy functional composed of energy contributions related to the elasticity of the wound DNA, exposed surface DNA energy, and adhesion between the DNA and the capsid, we established that the crescent shape of the toroidal DNA section comes from attractive interactions between DNA and the capsid. Such attractive interactions seem to be specific to the PEG condensation process and are not observed in the case of spermine-induced DNA condensation.

Are liquid crystalline properties of nucleosomes involved in chromosome structure and dynamics?

Nucleosome core particles correspond to the structural units of eukaryotic chromatin. They are charged colloids, 101 Angstrom in diameter and 55 Angstrom in length, formed by the coiling of a 146/147 bp DNA fragment (50 nm) around the histone protein octamer. Solutions of these particles can be concentrated, under osmotic pressure, up to the concentrations found in the nuclei of living cells. In the presence of monovalent cations (Na(+)), nucleosomes self-assemble into crystalline or liquid crystalline phases. A lamello-columnar phase is observed at 'low salt' concentrations, while a two-dimensional hexagonal phase and a three-dimensional quasi-hexagonal phase form at 'high salt' concentrations. We followed the formation of these phases from the dilute isotropic solutions to the ordered phases by combining cryoelectron microscopy and X-ray diffraction analyses. The phase diagram is presented as a function of the monovalent salt concentration and applied osmotic pressure. An alternative method to condense nucleosomes is to induce their aggregation upon addition of divalent or multivalent cations (Mg(2+), spermidine(3+) and spermine(4+)). Ordered phases are also found in the aggregates. We also discuss whether these condensed phases of nucleosomes may be relevant from a biological point of view.

Cryo-electron microscopy of vitreous sections

Since the beginning of the 1980s, cryo-electron microscopy of a thin film of vitrified aqueous suspension has made it possible to observe biological particles in their native state, in the absence of the usual artefacts of dehydration and staining. Combined with 3-d reconstruction, it has become an important tool for structural molecular biology. Larger objects such as cells and tissues cannot generally be squeezed in a thin enough film. Cryo-electron microscopy of vitreous sections (CEMOVIS) provides then a solution. It requires vitrification of a sizable piece of biological material and cutting it into ultrathin sections, which are observed in the vitrified state. Each of these operations raises serious difficulties that have now been overcome. In general, the native state seen with CEMOVIS is very different from what has been seen before and it is seen in more detail. CEMOVIS will give its full potential when combined with computerized electron tomography for 3-d reconstruction.

Role of histone tails in the conformation and interactions of nucleosome core particles

The goal of this work was to test the role of the histone tails in the emergence of attractive interactions between nucleosomes above a critical salt concentration that corresponds to the complete tail extension outside the nucleosome [Mangenot, S., et al (2002) Biophys. J. 82, 345-356; Mangenot, S., et al (2002) Eur. Phys. J. E 7, 221-231]. Small angle X-ray scattering experiments were performed in parallel with intact and trypsin tail-deleted nucleosomes with 146 +/- 3 bp DNA. We varied the monovalent salt concentration from 10 to 300 monovalent salt concentration and followed the evolution of (i) the second virial coefficient that characterizes the interactions between particles and (ii) the conformation of the particle. The attractive interactions do not emerge in the absence of the tails, which validates the proposed hypothesis.

Phase diagram of nucleosome core particles

We present a phase diagram of the nucleosome core particle (NCP) as a function of the monovalent salt concentration and applied osmotic pressure. Above a critical pressure, NCPs stack on top of each other to form columns that further organize into multiple columnar phases. An isotropic (and in some cases a nematic) phase of columns is observed in the moderate pressure range. Under higher pressure conditions, a lamello-columnar phase and an inverse hexagonal phase form under low salt conditions, whereas a 2D hexagonal phase or a 3D orthorhombic phase is found at higher salt concentration. For intermediate salt concentrations, microphase separation occurs. The richness of the phase diagram originates from the heterogeneous distribution of charges at the surface of the NCP, which makes the particles extremely sensitive to small ionic variations of their environment, with consequences on their interactions and supramolecular organization. We discuss how the polymorphism of NCP supramolecular organization may be involved in chromatin changes in the cellular context.

X-ray diffraction characterization of the dense phases formed by nucleosome core particles

Multiple dense phases of nucleosome core particles (NCPs) were formed in controlled ionic conditions (15-160 mM monovalent salt, no divalent ions), under osmotic pressures ranging from 4.7 x 10(5) to 2.35 x 10(6) Pa. We present here the x-ray diffraction analysis of these phases. In the lamello-columnar phase obtained at low salt concentration (<25 mM), NCPs stack into columns that align to form bilayers, kept separated from one another by a layer of solvent. NCPs form a monoclinic lattice in the plane of the bilayer. For high salt concentration (>50 mM), NCPs order into either a two-dimensional columnar hexagonal phase or into three-dimensional orthorhombic (quasi-hexagonal) crystals. The lamellar and hexagonal (or quasi-hexagonal) organizations coexist in the intermediate salt range; their demixing requires a long time. For an applied pressure P = 4.7 10(5) Pa, the calculated NCPs concentration ranges from approximately 280 to 320 mg/ml in the lamello-columnar phase to 495 to 585 mg/ml in the three-dimensional orthorhombic phase. These concentrations cover the concentration of the living cell.

Salt-induced conformation and interaction changes of nucleosome core particles

Small angle x-ray scattering was used to follow changes in the conformation and interactions of nucleosome core particles (NCP) as a function of the monovalent salt concentration C(s). The maximal extension (D(max)) of the NCP (145 +/- 3-bp DNA) increases from 137 +/- 5 A to 165 +/- 5 A when C(s) rises from 10 to 50 mM and remains constant with further increases of C(s) up to 200 mM. In view of the very weak increase of the R(g) value in the same C(s) range, we attribute this D(max) variation to tail extension, a proposal confirmed by simulations of the entire I(q) curves, considering an ideal solution of particles with tails either condensed or extended. This tail extension is observed at higher salt values when particles contain longer DNA fragments (165 +/- 10 bp). The maximal extension of the tails always coincides with the screening of repulsive interactions between particles. The second virial coefficient becomes smaller than the hard sphere virial coefficient and eventually becomes negative (net attractive interactions) for NCP(145). Addition of salt simultaneously screens Coulombic repulsive interactions between NCP and Coulombic attractive interactions between tails and DNA inside the NCP. We discuss how the coupling of these two phenomena may be of biological relevance.

Bilayers of nucleosome core particles

Among the multiple effects involved in chromatin condensation and decondensation processes, interactions between nucleosome core particles are suspected to play a crucial role. We analyze them in the absence of linker DNA and added proteins, after the self-assembly of isolated nucleosome core particles under controlled ionic conditions. We describe an original lamellar mesophase forming tubules on the mesoscopic scale. High resolution imaging of cryosections of vitrified samples reveals how nucleosome core particles stack on top of one another into columns which themselves align to form bilayers that repel one another through a solvent layer. We deduce from this structural organization how the particles interact through attractive interactions between top and bottom faces and lateral polar interactions that originate in the heterogeneous charge distribution at the surface of the particle. These interactions, at work under conditions comparable with those found in the living cell, should be of importance in the mechanisms governing chromatin compaction in vivo.

Aggregation of nucleosomes by divalent cations

Conditions of precipitation of nucleosome core particles (NCP) by divalent cations (Ca(2+) and Mg(2+)) have been explored over a large range of nucleosome and cation concentrations. Precipitation of NCP occurs for a threshold of divalent cation concentration, and redissolution is observed for further addition of salt. The phase diagram looks similar to those obtained with DNA and synthetic polyelectrolytes in the presence of multivalent cations, which supports the idea that NCP/NCP interactions are driven by cation condensation. In the phase separation domain the effective charge of the aggregates was determined by measurements of their electrophoretic mobility. Aggregates formed in the presence of divalent cations (Mg(2+)) remain negatively charged over the whole concentration range. They turn positively charged when aggregation is induced by trivalent (spermidine) or tetravalent (spermine) cations. The higher the valency of the counterions, the more significant is the reversal of the effective charge of the aggregates. The sign of the effective charge has no influence on the aspect of the phase diagram. We discuss the possible reasons for this charge reversal in the light of actual theoretical approaches.

DNA in human and stallion spermatozoa forms local hexagonal packing with twist and many defects

In human and other mammal sperm nuclei, DNA is packed in a highly condensed state, the structure of which remains unsolved. Cryoelectron microscopy of vitrified sections provides a first direct view of the local arrangement of the nucleoprotamine filament. DNA aligns in parallel in layers and its orientation rotates along a single-twist direction as in a cholesteric liquid crystal. The structure contains numerous defects, which introduce locally double-twist configurations. Destruction of the SS bonds with dithiotrehitol relaxes the twist and favors the extension of the hexagonal close packing of the filaments, though keeping constant their interfilament distance.

Chiral discotic columnar germs of nucleosome core particles

In concentrated solution and in the presence of high concentrations of monovalent cations, nucleosome core particles order into a discotic columnar mesophase. This phase is limited to finite-sized hexagonal germs that further divide into six coiled branches, following an iterative process. We show how the structure of the germs comes from the competition between hexagonal packing and chirality with a combination of dendritic facetting and double-twist configurations. Geometrical considerations lead us to suspect that the chirality of the eukaryotic chromosomes may originate from the same competition.

Spermine-induced aggregation of DNA, nucleosome, and chromatin

We have analyzed the conditions of aggregation or precipitation of DNA in four different states: double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), mononucleosome core particles (NCP), and H1-depleted chromatin fragments (ChF) in the presence of the multivalent cation spermine (4+). In an intermediate regime of DNA concentration, these conditions are identical for the four states. This result demonstrates that the mechanism involved is general from flexible chains to rigid rods and quasi-colloidal states. It is dominated by local electrostatic attractions that are considered, for instance, by the "ion-bridging" model. The onset of precipitation does not require the electroneutrality of the DNA chains. Above a given spermine concentration dsDNA aggregates remain neutral, whereas NCP aggregates turn positively charged. The difference is thought to originate from the extension of the positively charged proteic tails of the NCP. This suggests that local fluctuations of polyamine concentrations can induce either positively or negatively charged chromatin domains.

Spermidine-induced aggregation of nucleosome core particles: evidence for multiple liquid crystalline phases

We investigate the effect of the addition of a trivalent cation, spermidine, to dilute solutions of nucleosome core particles (NCP). In the presence of spermidine, part of the NCP segregates from the initial homogeneous solution, forming dense aggregates. We follow this precipitation process over a wide range of spermidine and NaCl concentrations and determine the conditions of aggregation of the particles. The structure of the dense phases is analyzed by means of polarizing light microscopy and cryo-electron microscopy. We report the existence of multiple supramolecular organizations. According to the relative concentrations of spermidine, monovalent salt and NCP, the particles may aggregate into amorphous phases, stack into randomly oriented columns, or form liquid crystalline phases. Two discotic liquid crystalline phases are identified and analyzed: a columnar nematic corresponding to columns of NCP simply aligned in parallel, and a columnar hexagonal phase in which the columns order into a transversal 2D hexagonal array. We discuss the nature and origin of the interactions possibly involved in the formation and maintenance of these different types of order.

Liquid crystalline ordering of nucleosome core particles under macromolecular crowding conditions: evidence for a discotic columnar hexagonal phase

Macromolecular crowding conditions occurring inside the cell nucleus were reproduced experimentally with solutions of mononucleosome core particles to study their supramolecular organization. We report here that under these conditions, and over a large range of monovalent salt concentrations, mononucleosome core particles self-assemble to form a discotic liquid crystalline phase characterized in polarizing and freeze-fracture electron microscopy. Mononucleosomes are stacked on each other to form columns, which are themselves closely packed into an hexagonal array. The nucleosome concentration, estimated from the network parameters, falls in the range of values measured in cell nuclei. We suggest that these concentrated solutions, although their organization cannot be immediately compared to the organization of chromatin in vivo, may be used to investigate the nucleosome-nucleosome interactions. Furthermore, this approach may be complexified to take into account the complexity of the eucaryotic chromatin.

Comparison of slam-freezing and high-pressure freezing effects on the DNA cholesteric liquid crystalline structure

Using in parallel electron microscopy of ultrathin frozenhydrated sections and freeze-fracture replicas, we compare the ultrastructural consequences of two freezing techniques: slam-freezing at liquid helium temperature and high-pressure freezing, on a model system, the DNA cholesteric liquid crystalline phase. Both freezing techniques are able to vitrify DNA liquid crystalline solutions containing up to 85% water, but induce structural rearrangements of the molecular organization. The cholesteric structure is preserved by the slam-freezing method despite the formation of periodic distortions induced by the mechanical compressive stress. In contrast, high-pressure freezing does not preserve the structure of the liquid crystal: the long-range cholesteric stratification disappears, and the local continuous twist between molecules is modified. These results show that vitrification, though necessary, may not be a sufficient token of preservation of the native state of hydrated materials. We discuss the possible origins of the molecular rearrangements that have time to occur in the specimens as a result of the low freezing rate permitted by the high-pressure freezing process.

DNA-DNA interaction in thin layer analysed by cryo-electron microscopy

Thin vitrified films of 146 base pair double-stranded DNA fragments in semi-dilute solutions are observed by cryo-electron microscopy. Depending on the salt conditions of the buffer and the DNA concentration, the molecules form different characteristic arrangements: they are arranged either locally parallel to each other in random overall orientation or packed perpendicular to the film surface, or else are randomly oriented. These arrangements reflect the weak forces acting between the DNA molecules themselves, as well as between the molecules and the thin film surface.

Supramolecular ordering of DNA in the cholesteric liquid crystalline phase: an ultrastructural study

Aqueous solutions of 146-base pair DNA fragments form a cholesteric liquid crystalline phase in the range of about 160-290 mg/ml. We present a structural analysis of this phase by comparing the data obtained from polarizing and electron microscopy. This phase shows multiple aspects or "textures" which are presented and interpreted. They mainly depend on the orientation of the structure relative to the observation plane and on the nature, distribution, and amount of defects present in the phase. These defects are then analyzed with the two methods, and the molecular orientations can be defined precisely in their core. The biological interest of such structural analyses is discussed in relation with the understanding of chromatin structure and function.

Cholesteric liquid crystalline DNA; a comparative analysis of cryofixation methods

The ultrastructure of liquid crystalline phases of DNA raises numerous problems because of the structure itself which is fluid and which nature depends on the relative amount of DNA, water and ions. Different cryofixation methods were tested and compared after freeze-fracture of the specimen. A good ultrastructural preservation of the samples can be achieved without addition of any cryoprotectant by quick-freezing against a copper block cooled down to liquid helium temperature. Then, molecular orientations can be followed very accurately and the local disorder around a mean direction which exists in the liquid state is kept in the frozen structure.