In most eukaryotic chromosomes, DNA is organized in quasiperiodic arrays of nucleosomes connected by linker DNA segments, yet how these arrays fold inside cells remains unclear. Using cryo electron tomography of vitreous sections, we directly visualized the DNA filament, wrapped around and linking nucleosomal particles, in native flash frozen Drosophila melanogaster embryos. By tracing individual DNA linkers and modelling them as worm like chains, we reveal an irregular zig zag chromatin fold characterized by low curvature and high variability in linker length. Nucleosomes display rich conformational diversity, including open, closed, and gaping states, consistent with a dynamic liquid like chromatin behavior.
In addition, nucleosome like particles accommodating from less than one to three DNA gyres were observed, consistent with tetrasomes, hemisomes, hexasomes, and overlapping dinucleosomes in situ.
Taking advantage of the distinctive chromosome organization of Drosophila melanogaster embryos, where large constitutive heterochromatin domains segregate from more dispersed euchromatin and facultative heterochromatin regions, we localized these unusual particles within euchromatin and facultative heterochromatin nano compartments dispersed in the nucleoplasm. This suggests that structural transitions involving nucleosome assembly and disassembly and sliding may be associated with chromatin remodeling and transcription. The observation of these labile states highlights the potential of cryo electron tomography to address functionally relevant chromatin reorganization at the nanometer scale in situ.
Contact : amelie.leforestier@universite-paris-saclay.fr