Monoallelic antigen expression in trypanosomes requires a stage-specific transcription activator

Monoallelic expression of a single gene family member underpins a molecular "arms race" between many pathogens and their host, through host monoallelic immunoglobulin and pathogen monoallelic antigen expression. In Trypanosoma brucei, a single, abundant, variant surface glycoprotein (VSG) covers the entire surface of the bloodstream parasite1 and monoallelic VSG transcription underpins their archetypal example of antigenic variation. It is vital for pathogenicity, only occurring in mammalian infectious forms1. Transcription of one VSG gene is achieved by RNA polymerase I (Pol I)2 in a singular nuclear structure: the expression site body (ESB)3. How monoallelic expression of the single VSG is achieved is incompletely understood and no specific ESB components are known. Here, using a protein localisation screen in bloodstream parasites, we discovered the first ESB-specific protein: ESB1. It is specific to VSG-expressing life cycle stages where it is necessary for VSG expression, and its overexpression activates inactive VSG promoters. This showed monoallelic VSG transcription requires a stage-specific activator. Furthermore, ESB1 is necessary for Pol I recruitment to the ESB, however transcript processing and inactive VSG gene exclusion ESB sub-domains do not require ESB1. This shows that the cellular solution for monoallelic transcription is a complex factory of functionally distinct and separably assembled sub-domains.