# A factor integrating transcription and repression of surface antigen genes in African trypanosomes

## TL;DR

This study identifies a protein, ESBX, that helps control the expression of surface antigens in African trypanosomes, which is crucial for their ability to evade the immune system.

## Contribution

The paper discovers ESBX as the first protein required for both activating and silencing antigen genes in trypanosomes.

## Key findings

- ESBX is necessary for RNA Pol I localization to the ESB and active BES transcription.
- ESBX knockdown derepresses inactive BESs and causes transcriptomic defects similar to ESB1 and VEX2 knockdown.
- ESBX overexpression weakly activates inactive BESs without disrupting the active BES.

## Abstract

African trypanosomes require antigenic variation to evade the host immune system. Individual trypanosomes express one variant surface glycoprotein (VSG) surface antigen gene from one of the largest known antigen gene families among all pathogens. The expression site body (ESB) is a dedicated subnuclear compartment central to this elegant pathogenicity mechanism. It is involved in both active VSG expression activation and inactive VSG silencing. We identify ESBX as the first protein required for both functions, giving insight into the critical balance of activation and silencing necessary for antigenic variation. This establishes a framework for understanding monoallelic expression, providing molecular insights into how pathogens regulate antigen expression to evade host immunity.

Antigenic variation in Trypanosoma brucei (T. brucei) requires monoallelic expression of one variant surface glycoprotein (VSG) from one of the subtelomeric bloodstream form (BSF) expression sites (BESs). This transcription is unusually mediated by RNA polymerase I (RNA Pol I) and occurs in a specialized nuclear body, the expression site body (ESB). While factors promoting active BES transcription and silencing inactive BESs are known, how these opposing activities are integrated remains unknown. Here, we identify ESBX (Tb927.3.1660) as a BSF-specific ESB protein necessary for this coordination. We show that ESBX RNAi knockdown prevents RNA Pol I localizing to the ESB and reduces active BES transcription, while also derepressing inactive BESs with low processivity transcription. Conversely, ESBX overexpression weakly activates inactive BESs in a distinct manner from ESBX knockdown, leading to processive transcription, without disrupting the active BES or forming supernumerary ESBs. ESBX knockdown causes a similar transcriptomic defect to ESB1 and VEX2 knockdown combined, establishing ESBX as a key factor linking transcriptional activation of the active BES with inactive BES silencing through the VSG exclusion (VEX) phenomenon. This allows us to suggest models for understanding the establishment and maintenance of monoallelic expression critical for parasite immune evasion.