Francoise Livolant - Academia.edu (original) (raw)

Papers by Francoise Livolant

APS March Meeting Abstracts, 2019

Tissue and Cell, 1984

The fine structure of chromatin in sperm heads was investigated by different microscopic techniqu... more The fine structure of chromatin in sperm heads was investigated by different microscopic techniques: in viva examinations in the polarizing microscope, thin sections and freeze-fracture replicas observed by transmission electron microscopy. The freeze-fractured chromatin appears to be formed of superimposed lamellae, each one 330 A thick. These lamellae are parallel to tbe flattening plane of the sperm head. This situation was already described in other mammal spermatozoa and in particular in the bull and the rabbit. This work presents a new interpretation of this lamellated aspect. The chromatin structure of these spermatozoa is that of a cholesteric liquid crystal, This structure resembles that of a plywood, made of superimposed layers of parallel filaments, but instead of having a right angle between two successive layers, there is a progressive rotation and similar orientation occurs at each 180" rotation. The apparent lamellae result from cleavages due to freeze-fracture between levels of parallel filament orientation. The thickness of lamellae corresponds therefore to the half helicoidal pitch of the cholesteric liquid crystal. This model is consistent with our observations by polarizing microscopy. The lamellation is not visible in thin sections of stallion spermatozoa. There are however biochemical methods to deeondense chromatin and we are able to observe this lamellation in sections normal to the flattening plane of sperm heads. The methods used classically to decondense the sperm chromatin lead to extremely varied aspects which are discussed, some of them being closely related to the structure of cholesteric liquid crystals.

Physica A: Statistical Mechanics and its Applications, 1991

In vitro, pure DNA forms multiple liquid crystalline phases when the polymer concentration is inc... more In vitro, pure DNA forms multiple liquid crystalline phases when the polymer concentration is increased: precholesteric organization, cholesteric phase and columnar hexagonal phase. Similar organizations of ehromatin can be found in vivo: hexagonal packing in bacteriophages and certain sperm heads, cholesteric organization in dinoflagellate chromosomes, bacterial nucleoids and mitochondrial DNA, helical-shaped chromosomes in many species. The different forms of condensed chromatin seem to be related to different local concentrations of DNA. In the highly condensed forms, chromatin is inactive and the double stranded DNA molecule is linear with small amounts of associated proteins. A more detailed analysis is presented in the case of cholesteric structures (helical pitch, defects) in polarizing microscopy and in electron microscopy. Ditfe.rences observed in vitro and in vivo are probably related to the length of the DNA molecule and to the presence of proteins associated to DNA in the chromosomes.

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1992

ABSTRACT

Liquid Crystals, 1994

ABSTRACT

Journal of Molecular Biology, 1999

We investigate the effect of the addition of a trivalent cation, spermidine, to dilute solutions ... more 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 cryoelectron 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 identi®ed 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.

Journal of Molecular Biology, 2008

The transfer of the bacteriophage genome from the capsid into the host cell is a key step of the ... more The transfer of the bacteriophage genome from the capsid into the host cell is a key step of the infectious process. In bacteriophage T5, DNA ejection can be triggered in vitro by simple binding of the phage to its purified Escherichia coli receptor FhuA. Using electrophoresis and cryo-electron microscopy, we measure the extent of DNA ejection as a function of the external osmotic pressure. In the high pressure range (7-16 atm), the amount of DNA ejected decreases with increasing pressure, as theoretically predicted and observed for λ and SPP1 bacteriophages. In the low and moderate pressure range (2-7 atm), T5 exhibits an unexpected behavior. Instead of a unique ejected length, multiple populations coexist. Some phages eject their complete genome, whereas others stop at some nonrandom states that do not depend on the applied pressure. We show that contrarily to what is observed for the phages SPP1 and λ, T5 ejection cannot be explained as resulting from a simple pressure equilibrium between the inside and outside of the capsid. Kinetics parameters and/or structural characteristics of the ejection machinery could play a determinant role in T5 DNA ejection.

Journal of Molecular Biology, 1991

I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase o... more I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase of DNA. Freeze-fracture methods reveal that this phase is a lamellar structure, each layer (30 to 40 A thick) composed of DNA molecules aligned in parallel. Numerous defects can be seen in the structure, and their nature is determined. I show that they are mainly screw dislocations of both handedness. By this method it is possible to follow individual double-stranded DNA molecules in this highly packed structure. I show, moreover, that there is a local twist between DNA molecules along the screw dislocation lines and that this twist can be either right-handed or left-handed. The interest of such ultrastructural analysis is discussed in relation to the understanding of chromatin structure.

Journal of Molecular Biology, 2010

Double-stranded DNA bacteriophage genomes are densely packaged into capsids until the ejection is... more Double-stranded DNA bacteriophage genomes are densely packaged into capsids until the ejection is triggered upon interaction of the tail with the bacterial receptor. Using cryo-electron microscopy, we describe the organization of the genome in the full capsid of T5 and show how it undergoes a series of phase transitions upon progressive ejection when the encapsidated DNA length decreases. Monodomains of hexagonally crystallized DNA segments initially form a three-dimensional lattice of defects. The structure turns liquid crystalline (twodimensional hexagonal and then cholesteric) and finally isotropic. These structures suggest a mechanism in which defects of the full capsid would initiate the ejection and introduce the necessary fluidity to relax the constrained mosaic crystal to let the genome start flowing out of the capsid.

Chromosoma, 1989

Freeze-fracture-etch replicas of concentrated DNA solutions which appeared, by polarized light mi... more Freeze-fracture-etch replicas of concentrated DNA solutions which appeared, by polarized light microscopy, to be in a cholesteric-like liquid crystalline state were examined by high resolution transmission electron microscopy (TEM). Individual DNA molecules were resolvable, and the microscopic morphologies observed for such replicas confirmed the cholesteric organization of DNA molecules in this liquid crystalline state. Furthermore, replica morphologies were strikingly similar to TEM images of dinoflagellate chromosomes in both thin section and freeze-etch replicas, providing strong support for the cholesteric DNA packing model proposed for the organization of DNA in these chromosomes by Bouligand and Livolant.

Chromosoma, 1980

Dinoflagellate chromosomes observed in thin section show regular patterns which have been shown t... more Dinoflagellate chromosomes observed in thin section show regular patterns which have been shown to correspond to a liquid crystalline helicoidal arrangement of DNA. Peripheral DNA filaments form a system of loops in the surrounding nucleoplasm. When such chromosomes (studied in Prorocentrum micans) are in presence of water, they extend considerably and form a double helical bundle. At the periphery of

Chromosoma, 1978

By using the optical properties of birefringence of DNA, the arrangement of these molecules has b... more By using the optical properties of birefringence of DNA, the arrangement of these molecules has been studied in Dinoflagellate chromosomes and Dipteran polytene chromosomes. These latter are used, here, as a reference material. These observations have been made under a polarizing microscope on intact and stretched chromosomes.--Intact Dinoflagellate chromosomes show a positive birefringence, in contrast with polytene chromosomes bands which are negatively birefringent. From these observations one can deducd the preferential orientation of DNA filaments, in Dinoflagellates, normal to the chromosome axis, and in polytene chromosomes parallel to the same axis.--After stretching, these two kinds of chromosomes are negatively birefringent. In both cases, DNA molecules have been aligned along the stretch axis.--In Dinoflagellate chromosomes the passage from a positive to a negative birefringence is realized without any isotropic stage. The intermediary state presents a biaxial structure.

Chromosoma, 1978

The Dinoflagellate Prorocentrum micans has been studied in classical and high voltage transmissio... more The Dinoflagellate Prorocentrum micans has been studied in classical and high voltage transmission electron microscopy, with the help of a goniometric stage. The general structure of the nucleus is analysed with special reference to the links observed between chromosomes and the nuclear envelope, the nucleoplasm and the nucleolus. The chromosomes present stacked series of nested arcs which are studied in

Biology of the Cell, 1999

In the Eukaryotic nucleus, chromatin forms a complex and dynamic structure which is still unsolve... more In the Eukaryotic nucleus, chromatin forms a complex and dynamic structure which is still unsolved despite years of investigation (Van Holde K. and Zlatanova 1. (1995) Chromatin Higher Order Structure: Chasing a Mirage ? J, Biol. Chem. 270, 8373-8376). So far, only the first level of DNA organisation into the nucleosomic filament and the structure of the nucleosome core particle itself (Luger K., MHder A.W., Richmond R.K., Sargent D.F. and Richmond T.J. (1997) Crystal structure of the nucleosome core particle at 2.8 A resolution. Nature, 389, 251-260) are well documented and characterised. The lack of understanding of the higher levels of folding reflects the complexity of the problem : chromatin would not be arranged into a definite stable structure but rather show fluctuations between a variety of states during the various stages of the cell cycle.

Biology of the Cell, 1998

s Trinoculaire ‘98 des Microscopies, Strasbourg-lllkirch, France, l-3 July 1998 285 A new nuclear... more s Trinoculaire ‘98 des Microscopies, Strasbourg-lllkirch, France, l-3 July 1998 285 A new nuclear protein in a Dinoflagellate: Light and TEM immunocytolocalization and cyclic expression Y. Bhaud, M.L. Gkraud, J. Ausseil, M. Albert, M.O. Soyer-Gobillard, and H. Moreau UMR CNRSUPMC 7628, ‘Modhles en Siologie Ceflulaire et Evofubve: Obsewatoire Ockwologiqoe, BP44, F-66651 Banyuls-sur-mer, France The permanently high level of DNA condensation of the dinoflagellate nuclear genome raises problems for replication and/or gene transcription, particularly with respect to the accessibility to the coding sequences of the replication/transcription machinery of these special protists. Novel regulation mechanisms and possibly special proteins could be involved, but, apart from the characterization of two nuclear basic proteins (SalaRovira et al., 1991, Ckromusomu, 100, 510-518). no molecular approach to characterize the quantitatively major nuclear proteins has been attempted previously. Dinoflagellate nuclei were isolated and nuclear proteins were extracted in three fractions, corresponding to increasing affinity of these proteins to genomic DNA. One fraction contained two major bands (48 and 46 kDa) and antibodies specific to this fraction revealed four bands by western-blot on protein extracts, including the 46and 4%kDa bands. The 48-kDa protein was detected cyclically during the cell cycle, in Gl phase but not in S and M phases. Immunocytolocalization on C.coknii cryosections for LM and quick frozen and cryosubstituted cells for TEM showed that the nucleus was immunoreactive only during the Gl phase : the nucleoplasm and the nucleolus were immunostained, while both the chromosome core and nuclear envelope were negative. A cDNA expression library of C. cohnii was screened with these antibodies, and two different open reading frames were isolated. The coding sequence of one of these was produced in E. co/i and appeared to correspond to the cyclically-expressed 48-kDa nuclear protein. No homologue of this sequence was found in the databases, but two regions were identified, one including two putative zinc finger repeats, and one coding for two potential W/W domains. The second coding sequence showed a low similarity to non-specific sterol carrier proteins. Chirality in nucleosome liquid crystalline phases Franqoise Livolant and Am&e Leforestier laboratoire de physique des Sofides, Bat 510, IJniversitG Paris-Sud, 91405 Orsay, France In concentrated solutions, nucleosome core particles (NCP) spontaneously organize into liquid crystalline phases, which nature depends on the NCP concentmtion and on the ionic environment. In these phases, the core particles pile up the one onto the others to form columns that themselves orient in pardllel and tend to form a 2-dimensionnal hexagonal lattice (Leforesticr A., Livolant F. (1997).Biophys. J. 73, 1771-1776) However. the chirality of the particles introduces deviations from a perfect hexagonal order. The expression of the competition chirality/2-d order is modulated with tbc concentration of the monovalent counterions prcscnt in the solution. In presence of high salt concentration (100 to 300 mM Nail), the sixfold symmetry of the 2-d array is preserved, but the growth of the liquid crystalline germs is assoeiatcd with a splitting into 6 branches that coil into a macroscopic left-handed helix. We pmposc an interpretative model that includes curvature of the columns, twist and splay bctwcen columns. Curvature can bc followed over long distance by polarizing microscopy examination of the germs. Twist and splay are visualized at the molecular scale by elcctmn microscopy of fccze-fracture replicas. In presence of low monovalent salt concentraiion (c 15 mM). the sixfold symmetry of the germs is lost. They form helical ribbons, tubes or helical rods, which morphologies complexify with the increase of the NCP concentration. Preliminary results suggest that these phases would cormspond to 2-D ordered columnar phases, with a distorted hexagonal lattice. On account of the existence of a twist angle between ncighbouring columns, the reticular planes of the array would tend tn curl into helical surfaces and hollow cylinders. The mecanisms involved in the formation of these stmcturcs are probably related to lhose previously described in lamellar phases of chiral amphiphiles (Helfrich W., Pmst J. (1988) Phys. Rev. 38.30653068; Selinger J.V., MacKintosh F.C.. Scbnur J.M. (1996)Pkys. Rev. E 53.3804-3818). Even though the precise nature of the influence of the ionic strength remains to te understood, the chirality of these supramolccular organizations may be involved in the helicity of some metaphase chromosomes (Ohnuki Y. (1968) Chronwsoma 25.402428; Boy de la Tour E., Laemmli U.K. (1988) Cell, 55,937.944; Houchmandzadeh et al, (1997) J. Cell Biol 139, l-12). Formation of pseudonucleoliand persistence of prenucleolar bodies at the end of mitosis in the absence of complete translocation of…

Biology of the Cell, 1991

The ultrastructure of liquid crystalline phases of DNA raises numerous problems because of the st... more 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.

Biology of the Cell, 1996

Biochemistry, 2004

The goal of this work was to test the role of the histone tails in the emergence of attractive in... more 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 [

APS March Meeting Abstracts, 2019

Tissue and Cell, 1984

The fine structure of chromatin in sperm heads was investigated by different microscopic techniqu... more The fine structure of chromatin in sperm heads was investigated by different microscopic techniques: in viva examinations in the polarizing microscope, thin sections and freeze-fracture replicas observed by transmission electron microscopy. The freeze-fractured chromatin appears to be formed of superimposed lamellae, each one 330 A thick. These lamellae are parallel to tbe flattening plane of the sperm head. This situation was already described in other mammal spermatozoa and in particular in the bull and the rabbit. This work presents a new interpretation of this lamellated aspect. The chromatin structure of these spermatozoa is that of a cholesteric liquid crystal, This structure resembles that of a plywood, made of superimposed layers of parallel filaments, but instead of having a right angle between two successive layers, there is a progressive rotation and similar orientation occurs at each 180" rotation. The apparent lamellae result from cleavages due to freeze-fracture between levels of parallel filament orientation. The thickness of lamellae corresponds therefore to the half helicoidal pitch of the cholesteric liquid crystal. This model is consistent with our observations by polarizing microscopy. The lamellation is not visible in thin sections of stallion spermatozoa. There are however biochemical methods to deeondense chromatin and we are able to observe this lamellation in sections normal to the flattening plane of sperm heads. The methods used classically to decondense the sperm chromatin lead to extremely varied aspects which are discussed, some of them being closely related to the structure of cholesteric liquid crystals.

Physica A: Statistical Mechanics and its Applications, 1991

In vitro, pure DNA forms multiple liquid crystalline phases when the polymer concentration is inc... more In vitro, pure DNA forms multiple liquid crystalline phases when the polymer concentration is increased: precholesteric organization, cholesteric phase and columnar hexagonal phase. Similar organizations of ehromatin can be found in vivo: hexagonal packing in bacteriophages and certain sperm heads, cholesteric organization in dinoflagellate chromosomes, bacterial nucleoids and mitochondrial DNA, helical-shaped chromosomes in many species. The different forms of condensed chromatin seem to be related to different local concentrations of DNA. In the highly condensed forms, chromatin is inactive and the double stranded DNA molecule is linear with small amounts of associated proteins. A more detailed analysis is presented in the case of cholesteric structures (helical pitch, defects) in polarizing microscopy and in electron microscopy. Ditfe.rences observed in vitro and in vivo are probably related to the length of the DNA molecule and to the presence of proteins associated to DNA in the chromosomes.

Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1992

ABSTRACT

Liquid Crystals, 1994

ABSTRACT

Journal of Molecular Biology, 1999

We investigate the effect of the addition of a trivalent cation, spermidine, to dilute solutions ... more 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 cryoelectron 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 identi®ed 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.

Journal of Molecular Biology, 2008

The transfer of the bacteriophage genome from the capsid into the host cell is a key step of the ... more The transfer of the bacteriophage genome from the capsid into the host cell is a key step of the infectious process. In bacteriophage T5, DNA ejection can be triggered in vitro by simple binding of the phage to its purified Escherichia coli receptor FhuA. Using electrophoresis and cryo-electron microscopy, we measure the extent of DNA ejection as a function of the external osmotic pressure. In the high pressure range (7-16 atm), the amount of DNA ejected decreases with increasing pressure, as theoretically predicted and observed for λ and SPP1 bacteriophages. In the low and moderate pressure range (2-7 atm), T5 exhibits an unexpected behavior. Instead of a unique ejected length, multiple populations coexist. Some phages eject their complete genome, whereas others stop at some nonrandom states that do not depend on the applied pressure. We show that contrarily to what is observed for the phages SPP1 and λ, T5 ejection cannot be explained as resulting from a simple pressure equilibrium between the inside and outside of the capsid. Kinetics parameters and/or structural characteristics of the ejection machinery could play a determinant role in T5 DNA ejection.

Journal of Molecular Biology, 1991

I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase o... more I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase of DNA. Freeze-fracture methods reveal that this phase is a lamellar structure, each layer (30 to 40 A thick) composed of DNA molecules aligned in parallel. Numerous defects can be seen in the structure, and their nature is determined. I show that they are mainly screw dislocations of both handedness. By this method it is possible to follow individual double-stranded DNA molecules in this highly packed structure. I show, moreover, that there is a local twist between DNA molecules along the screw dislocation lines and that this twist can be either right-handed or left-handed. The interest of such ultrastructural analysis is discussed in relation to the understanding of chromatin structure.

Journal of Molecular Biology, 2010

Double-stranded DNA bacteriophage genomes are densely packaged into capsids until the ejection is... more Double-stranded DNA bacteriophage genomes are densely packaged into capsids until the ejection is triggered upon interaction of the tail with the bacterial receptor. Using cryo-electron microscopy, we describe the organization of the genome in the full capsid of T5 and show how it undergoes a series of phase transitions upon progressive ejection when the encapsidated DNA length decreases. Monodomains of hexagonally crystallized DNA segments initially form a three-dimensional lattice of defects. The structure turns liquid crystalline (twodimensional hexagonal and then cholesteric) and finally isotropic. These structures suggest a mechanism in which defects of the full capsid would initiate the ejection and introduce the necessary fluidity to relax the constrained mosaic crystal to let the genome start flowing out of the capsid.

Chromosoma, 1989

Freeze-fracture-etch replicas of concentrated DNA solutions which appeared, by polarized light mi... more Freeze-fracture-etch replicas of concentrated DNA solutions which appeared, by polarized light microscopy, to be in a cholesteric-like liquid crystalline state were examined by high resolution transmission electron microscopy (TEM). Individual DNA molecules were resolvable, and the microscopic morphologies observed for such replicas confirmed the cholesteric organization of DNA molecules in this liquid crystalline state. Furthermore, replica morphologies were strikingly similar to TEM images of dinoflagellate chromosomes in both thin section and freeze-etch replicas, providing strong support for the cholesteric DNA packing model proposed for the organization of DNA in these chromosomes by Bouligand and Livolant.

Chromosoma, 1980

Dinoflagellate chromosomes observed in thin section show regular patterns which have been shown t... more Dinoflagellate chromosomes observed in thin section show regular patterns which have been shown to correspond to a liquid crystalline helicoidal arrangement of DNA. Peripheral DNA filaments form a system of loops in the surrounding nucleoplasm. When such chromosomes (studied in Prorocentrum micans) are in presence of water, they extend considerably and form a double helical bundle. At the periphery of

Chromosoma, 1978

By using the optical properties of birefringence of DNA, the arrangement of these molecules has b... more By using the optical properties of birefringence of DNA, the arrangement of these molecules has been studied in Dinoflagellate chromosomes and Dipteran polytene chromosomes. These latter are used, here, as a reference material. These observations have been made under a polarizing microscope on intact and stretched chromosomes.--Intact Dinoflagellate chromosomes show a positive birefringence, in contrast with polytene chromosomes bands which are negatively birefringent. From these observations one can deducd the preferential orientation of DNA filaments, in Dinoflagellates, normal to the chromosome axis, and in polytene chromosomes parallel to the same axis.--After stretching, these two kinds of chromosomes are negatively birefringent. In both cases, DNA molecules have been aligned along the stretch axis.--In Dinoflagellate chromosomes the passage from a positive to a negative birefringence is realized without any isotropic stage. The intermediary state presents a biaxial structure.

Chromosoma, 1978

The Dinoflagellate Prorocentrum micans has been studied in classical and high voltage transmissio... more The Dinoflagellate Prorocentrum micans has been studied in classical and high voltage transmission electron microscopy, with the help of a goniometric stage. The general structure of the nucleus is analysed with special reference to the links observed between chromosomes and the nuclear envelope, the nucleoplasm and the nucleolus. The chromosomes present stacked series of nested arcs which are studied in

Biology of the Cell, 1999

In the Eukaryotic nucleus, chromatin forms a complex and dynamic structure which is still unsolve... more In the Eukaryotic nucleus, chromatin forms a complex and dynamic structure which is still unsolved despite years of investigation (Van Holde K. and Zlatanova 1. (1995) Chromatin Higher Order Structure: Chasing a Mirage ? J, Biol. Chem. 270, 8373-8376). So far, only the first level of DNA organisation into the nucleosomic filament and the structure of the nucleosome core particle itself (Luger K., MHder A.W., Richmond R.K., Sargent D.F. and Richmond T.J. (1997) Crystal structure of the nucleosome core particle at 2.8 A resolution. Nature, 389, 251-260) are well documented and characterised. The lack of understanding of the higher levels of folding reflects the complexity of the problem : chromatin would not be arranged into a definite stable structure but rather show fluctuations between a variety of states during the various stages of the cell cycle.

Biology of the Cell, 1998

s Trinoculaire ‘98 des Microscopies, Strasbourg-lllkirch, France, l-3 July 1998 285 A new nuclear... more s Trinoculaire ‘98 des Microscopies, Strasbourg-lllkirch, France, l-3 July 1998 285 A new nuclear protein in a Dinoflagellate: Light and TEM immunocytolocalization and cyclic expression Y. Bhaud, M.L. Gkraud, J. Ausseil, M. Albert, M.O. Soyer-Gobillard, and H. Moreau UMR CNRSUPMC 7628, ‘Modhles en Siologie Ceflulaire et Evofubve: Obsewatoire Ockwologiqoe, BP44, F-66651 Banyuls-sur-mer, France The permanently high level of DNA condensation of the dinoflagellate nuclear genome raises problems for replication and/or gene transcription, particularly with respect to the accessibility to the coding sequences of the replication/transcription machinery of these special protists. Novel regulation mechanisms and possibly special proteins could be involved, but, apart from the characterization of two nuclear basic proteins (SalaRovira et al., 1991, Ckromusomu, 100, 510-518). no molecular approach to characterize the quantitatively major nuclear proteins has been attempted previously. Dinoflagellate nuclei were isolated and nuclear proteins were extracted in three fractions, corresponding to increasing affinity of these proteins to genomic DNA. One fraction contained two major bands (48 and 46 kDa) and antibodies specific to this fraction revealed four bands by western-blot on protein extracts, including the 46and 4%kDa bands. The 48-kDa protein was detected cyclically during the cell cycle, in Gl phase but not in S and M phases. Immunocytolocalization on C.coknii cryosections for LM and quick frozen and cryosubstituted cells for TEM showed that the nucleus was immunoreactive only during the Gl phase : the nucleoplasm and the nucleolus were immunostained, while both the chromosome core and nuclear envelope were negative. A cDNA expression library of C. cohnii was screened with these antibodies, and two different open reading frames were isolated. The coding sequence of one of these was produced in E. co/i and appeared to correspond to the cyclically-expressed 48-kDa nuclear protein. No homologue of this sequence was found in the databases, but two regions were identified, one including two putative zinc finger repeats, and one coding for two potential W/W domains. The second coding sequence showed a low similarity to non-specific sterol carrier proteins. Chirality in nucleosome liquid crystalline phases Franqoise Livolant and Am&e Leforestier laboratoire de physique des Sofides, Bat 510, IJniversitG Paris-Sud, 91405 Orsay, France In concentrated solutions, nucleosome core particles (NCP) spontaneously organize into liquid crystalline phases, which nature depends on the NCP concentmtion and on the ionic environment. In these phases, the core particles pile up the one onto the others to form columns that themselves orient in pardllel and tend to form a 2-dimensionnal hexagonal lattice (Leforesticr A., Livolant F. (1997).Biophys. J. 73, 1771-1776) However. the chirality of the particles introduces deviations from a perfect hexagonal order. The expression of the competition chirality/2-d order is modulated with tbc concentration of the monovalent counterions prcscnt in the solution. In presence of high salt concentration (100 to 300 mM Nail), the sixfold symmetry of the 2-d array is preserved, but the growth of the liquid crystalline germs is assoeiatcd with a splitting into 6 branches that coil into a macroscopic left-handed helix. We pmposc an interpretative model that includes curvature of the columns, twist and splay bctwcen columns. Curvature can bc followed over long distance by polarizing microscopy examination of the germs. Twist and splay are visualized at the molecular scale by elcctmn microscopy of fccze-fracture replicas. In presence of low monovalent salt concentraiion (c 15 mM). the sixfold symmetry of the germs is lost. They form helical ribbons, tubes or helical rods, which morphologies complexify with the increase of the NCP concentration. Preliminary results suggest that these phases would cormspond to 2-D ordered columnar phases, with a distorted hexagonal lattice. On account of the existence of a twist angle between ncighbouring columns, the reticular planes of the array would tend tn curl into helical surfaces and hollow cylinders. The mecanisms involved in the formation of these stmcturcs are probably related to lhose previously described in lamellar phases of chiral amphiphiles (Helfrich W., Pmst J. (1988) Phys. Rev. 38.30653068; Selinger J.V., MacKintosh F.C.. Scbnur J.M. (1996)Pkys. Rev. E 53.3804-3818). Even though the precise nature of the influence of the ionic strength remains to te understood, the chirality of these supramolccular organizations may be involved in the helicity of some metaphase chromosomes (Ohnuki Y. (1968) Chronwsoma 25.402428; Boy de la Tour E., Laemmli U.K. (1988) Cell, 55,937.944; Houchmandzadeh et al, (1997) J. Cell Biol 139, l-12). Formation of pseudonucleoliand persistence of prenucleolar bodies at the end of mitosis in the absence of complete translocation of…

Biology of the Cell, 1991

The ultrastructure of liquid crystalline phases of DNA raises numerous problems because of the st... more 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.

Biology of the Cell, 1996

Biochemistry, 2004

The goal of this work was to test the role of the histone tails in the emergence of attractive in... more 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 [