Abstract 2263 A repressive compartment silences subtelomeric chromatin in trypanosomes

Chromatin compartmentalization is essential for the monogenic expression of variant surface glycoprotein (VSG) genes in African trypanosomes. Trypanosomes express one out of 2,500 VSG genes at a time from telomeric expression sites (ESs). VSGs form a homogenous surface coat essential for parasite evasion of host antibodies by antigenic variation. Trypanosomes periodically switch the VSG expressed by transcriptional switching among telomeric ESs or by VSG gene recombination. Most silent VSG genes are subtelomeric and silenced by mechanisms still not understood. We found that subtelomeric chromatin is organized within repressive compartments essential for silencing VSG genes. Chromatin capture (Pore-C) experiments showed significant enrichment in interactions of silent subtelomeres, whereas the actively transcribed subtelomeric VSG gene interacts with highly transcribed chromosome loci. Fluorescence in situ hybridization and microscopy analysis confirmed telomeric and subtelomeric chromatin compartmentalization. In vivo crosslinking and mass spectrometry interaction analysis of the phosphatidylinositol phosphate 5-phosphatase (PIP5Pase) protein, which regulates VSG gene silencing, revealed a network of interactions involved in compartment assembly and gene repression. Genetic mutations that affect PIP5Pase activity resulted in the reorganization of subtelomeric chromatin, followed by transcription of silent subtelomeric genes and the switching in VSG gene expression. The data indicate that subtelomeric chromatin assembles into repressive compartments that coordinates the repression of hundreds of VSG genes. The compartments are essential for coordinated expression of surface antigens during antigenic variation. This work was funded by Canadian Institutes of Health Research (IC); The Natural Sciences and Engineering Research Council of Canada (IC); Fonds de Recherche du Québec - Nature et Technologie (IC); Canada Foundation for Innovation (IC); and FRQNT-Ukraine postdoctoral fellowship (OK).