{"id":3438,"date":"2021-10-06T17:34:34","date_gmt":"2021-10-06T15:34:34","guid":{"rendered":"https:\/\/equipes.lps.u-psud.fr\/sobio\/?page_id=3438"},"modified":"2022-03-14T19:26:30","modified_gmt":"2022-03-14T18:26:30","slug":"chromatin-and-nucleosomes","status":"publish","type":"page","link":"https:\/\/equipes.lps.u-psud.fr\/sobio\/chromatin-and-nucleosomes\/","title":{"rendered":"Chromatin and nucleosomes"},"content":{"rendered":"\n

Liquid crystalline nucleosomes in vitro<\/h3>\n\n\n\n

We design in vitro model systems of condensed chromatin, explore their physico-chemical properties and establish phase diagrams. <\/p>\n\n\n\n

Nucleosome core particles are formed by 145\u2013147 bp DNA wrapped into 1.65 turn of a left-handed superhelix around a histone octamer (two copies of H2A, H2B, H3 and H4). We explore their self assembly, varying different parameters : (i) the osmotic pressure, (ii) monovalent salt concentration, (iii) the presence of di or multivalent cations. Under physico-chemical conditions that reproduce those found in the living cell, nucleosomes form multiple amorphous, liquid crystalline and crystalline phases that can be related to the poly-ampholyte and geometric characteristics of the particle. All phases also present complex mesoscopic chiral organisations with multiple levels of helicity.<\/p>\n\n\n\n

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Phase diagram of nucleosome core particles in the presence of monovalent counterions. <\/p>\n\n\n\n

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Nucleosome conformational variability in vitro and in situ<\/h3>\n\n\n\n

Using cryo electron microscopy of vitreous sections we analyse the conformation of native nucleosomes in vitro (purified particles solubilised at physiologically relevant concentrations) and in situ within interphase nuclei. 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.<\/p>\n\n\n\n

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(A, B) CEMOVIS of liquid crystalline lamella-columnar phase: nucleosomes stack into columns arranged in a triangular lattice. Top (A) and side (B) views are compared to the canonical crystallographic structure (cryo-EM image simulation from PDBID 1EQZ. (C) Distance between DNA gyres of the nucleosome measured in high and low added salt solutions compared to the canonical structure. Open conformations (\u201cgaping\u201d nucleosome sketched in yellow) prevail in low salt environment.<\/figcaption><\/figure>\n\n\n\n

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Local organisation of the eukaryotic chromosome: the CRYOCHROM<\/a> project<\/h3>\n\n\n\n

Eukaryotic chromosomes are complex polymorphic and dynamic objects. Centimeters to meters of DNA condense into a micron-scale nucleus though multiple levels of structural organisation, while interacting with a multitude of factors driving functional compartmentalization. Their basic beads-on-string nucleosome structure plays a key role in major cellular processes from regulation of gene expression and epigenetics to DNA replication and repair, cell differentiation and aging. <\/p>\n\n\n\n

Using cryo electron microscopy and tomography of vitreous sections,we unveil the conformation and local order of nucleosomes and chromatin in the nuclear context, in particular in the interphase nucleus. Following an integrated structural approach we aim to relate chromatin structure and functional compartmentalization and benchmark chromatin in active and repressed states. <\/p>\n\n\n\n

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Multiscale structural analysis of eukaryotic chromatin in situ in relationship with chromatin functional state<\/figcaption><\/figure>\n\n\n\n

CRYOCHROM<\/a> is a multidisciplinary collaborative project funded by the ANR, in partnership with M. Eltsov<\/a> (IGBMC, Illkirch), S. Jonic<\/a> (IMPMC, Sorbonne Universit\u00e9) and J.-M. Victor <\/a>(LPTMC, Sorbonne Universit\u00e9). <\/p>\n\n\n\n

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Contact<\/h4>\n\n\n\n

Am\u00e9lie Leforestier: amelie.leforestier@universite-paris-saclay.fr<\/p>\n\n\n\n

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Related publications<\/h4>\n\n\n\n