"Dynamic SUMOylation Controls RNA Polymerase I Extranucleolar Organization and Antigenic Variation in Trypanosoma brucei"

Antigenic variation in Trypanosoma brucei relies on strict monoallelic expression of variant surface glycoprotein (VSG) genes from a single telomeric expression site (ES), a process sustained by the extranucleolar RNA polymerase I (Pol I) transcriptional body known as the expression site body (ESB). Although the ESB is essential for VSG expression, the mechanisms governing its assembly and maintenance remain poorly understood. Here, we identify SUMOylation as a central regulator of ESB organization and demonstrate that the balance between SUMO conjugation and deconjugation determines the transcriptional state of VSG expression sites. Ectopic expression of the SUMO protease TbSENP disrupted the highly SUMOylated nuclear focus associated with the active-ES, displaced Pol I from its extranucleolar compartment, and markedly increased VSG in situ switching frequency, indicating that continuous SUMOylation is required to preserve ESB integrity. Conversely, targeted recruitment of the SUMO-conjugating enzyme TbUBC9 to a silent ES locally restored SUMOylation, induced de novo formation of an extranucleolar Pol I compartment, activated transcription of the corresponding telomeric VSG gene, and generated stable antigenic switchers expressing the new surface coat. Local SUMOylation preceded Pol I redistribution, supporting a model in which SUMO-dependent interactions nucleate assembly of a transcriptionally competent ESB. Together, our findings identify SUMOylation as both a structural and regulatory determinant of nuclear organization in T. brucei and suggest that dynamic SUMO homeostasis governs the assembly, maintenance, and remodeling of this specialized transcriptional body. Significance StatementAntigenic variation in Trypanosoma brucei depends on the monoallelic expression of Variant Surface Glycoprotein (VSG) genes from a specialized RNA polymerase I transcriptional compartment known as the Expression Site Body (ESB). However, the molecular signals that govern transitions between active and silent expression sites have remained unknown. We show that SUMOylation acts as a reversible molecular switch: disruption of SUMO homeostasis dismantles ESB organization and promotes VSG switching, whereas localized SUMOylation is sufficient to nucleate a functional transcriptional compartment and activate a silent VSG expression site. Our findings establish SUMOylation as a central regulator of nuclear architecture and antigenic variation.