Genome-wide maps of nucleolus interactions reveal distinct layers of repressive chromatin domains

Eukaryotic chromosomes are folded into hierarchical domains, enabling the organization of the genome into functional compartments. Nuclear periphery and nucleolus are two nuclear landmarks thought to contribute to repressive chromosome architecture. However, while the role of nuclear lamina (NL) in genome organization has been well documented, the function of the nucleolus remains under-investigated due to the lack of methods for genome-wide maps of nucleolar associated domains (NADs). Here we established a method based on a Dam-fused engineered nucleolar histone H2B that marks DNA contacting the nucleolus. NAD-maps of ESCs and neural progenitors revealed layers of genome compartmentalization with distinct, repressive chromatin states based on the interaction with the nucleolus, NL, or both. NADs showed higher H3K9me2 and lower H3K27me3 content than regions exclusively interacting with NL. Upon ESC differentiation, chromosomes around the nucleolus acquire a more compact, rigid architecture whereas NADs specific for ESCs decrease their interaction strength within the repressive B-compartment strength, unlocking neural genes from repressive nuclear environment. The methodologies here developed will make possible to include the contribution of the nucleolus in future studies investigating the relationship between nuclear space and genome function.