Chromosome compartment assembly is essential for subtelomeric gene silencing in trypanosomes

Genome three-dimensional organization is essential for the coordination of eukaryote gene expression. The chromosomes of the pathogen Trypanosoma brucei contain hundreds of silent variant surface glycoprotein (VSGs) genes in subtelomeric regions. However, T. brucei transcribes a single VSG gene and periodically changes the VSG expressed by transcriptional or recombination mechanisms, altering its surface coat to escape host antibodies by antigenic variation. We show that VSG-rich silent subtelomeric regions form distinct chromosome compartments from transcribed regions, with subtelomeric compartments of different chromosomes co-interacting. We uncovered chromatin-associating factors at the boundaries of transcribed and silent compartments. Among these, repressor activator-protein 1 (RAP1) marks the compartment boundaries and spreads over silent regions. Inactivation of phosphatidylinositol phosphate 5-phosphatase removed RAP1 from compartment boundaries and subtelomeric regions, disrupting compartment assembly and derepressing all VSG genes. The data show spatial segregation of repressed from transcribed chromatin and phosphoinositides regulation of silent compartment assembly and genome organization.