The evolving role of histone H1 in shaping chromatin and epigenetic landscapes

Histone H1, long recognized for its fundamental role in stabilizing nucleosomes and compacting chromatin, is now emerging as a highly dynamic and versatile regulator essential for diverse nuclear processes. This review synthesizes recent advancements that move beyond H1's canonical structural functions, illuminating its intricate, often context-dependent, control over epigenetic modifications, gene expression, and 3D genome organization across eukaryotes. H1's ability to modulate chromatin accessibility and architecture, influenced by its local density, variant composition, and dynamic binding, dictates its species- and locus-specific impacts. H1 critically shapes DNA methylation landscapes and the deposition of key histone marks H3K27me3, often by affecting enzyme accessibility and inter-pathway dynamics. Its transcriptional impact transcends canonical transposable element silencing, extending to the selective fine-tuning of gene expression, with certain H1 variants even functioning as direct transcriptional activators. Regarding 3D genome organization, while H1's local density drives compartmentalization and influences epigenetic states in mammals, in Arabidopsis, it exhibits more complex, locus-specific roles including modulating telomere clustering, interstitial telomeric repeat insulation, and facilitating phase separation for heterochromatin foci assembly. Collectively, these findings establish histone H1 not merely as a structural backbone, but as a sophisticated regulator that intricately links chromatin's physical state to its functional outputs, profoundly impacting genome integrity, gene regulation, and cellular identity.