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 parasite 1 and monoallelic VSG transcription underpins their archetypal example of antigenic variation. It is vital for pathogenicity, only occurring in mammalian infectious forms 1 . 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.