Fig. 5

Model for H1 function in heterochromatin and euchromatin organization at the topological and molecular level. Graphical representation of H1 roles on chromatin organization at the cytological (spatial) and molecular level based on analyses reported in this study. Heterochromatin: H1 is dispensable for silencing and peripheral positioning of the vast majority of heterochromatic repeats but necessary for their condensation into compact chromocenter domains; yet a subset of transposable elements is directly affected by H1 and become derepressed in its absence (yellow box, -H1). This indicates both H1-independant and H1-dependent TE silencing controls. Euchromatin: top right panel, H1 is necessary to provide homogeneity in chromatin topology and spatial organization of chromatin domains. H1 depletion results in both larger gaps between nanodomains, possibly enabling increased accessibility, and irregular, high local compaction; this chromatin heterogeneity is reminiscent of H1-depleted pluripotent cells [9], cells with a loss of a SWI/SNF chromatin remodel function or undergoing tumorigenic reprogramming [57]. Concomitantly, H1-depleted chromatin displays increased mobility and poor maintenance of histone H3 lysine 4 (green) and more strongly lysine 27 (red) methylation. At the molecular level (lower panel), H1 provides distinct structural signatures (nucleosome coverage) at loci marked by distinct expression rates but is not epistatic to transcriptional control for a majority of them (H1-independent regulation); a subset of genes (ca 600 under a stringent cutoff), however, displays an H1-dependent control possibly involving transcriptional regulators directly influenced by H1