A non-coding role for trypanosome VSG transcripts in allelic exclusion

Bloodstream-form African trypanosomes display mono-telomeric expression of a Variant Surface Glycoprotein (VSG) gene in an inter-chromosomally bridged transcription and splicing compartment, such that the dominant gene produces 10,000 times more transcript than excluded genes. Antigenic variation, whereby parasites switch to express other VSGs, then underpins a robust host immune evasion strategy. Specific chromatin and RNA-associated factors are required to maintain VSG exclusion, but our understanding of the mechanisms involved remains incomplete. Here we show that the VSG transcript impacts allelic competition. We induced either translation blockade by recruiting MS2 coat protein to the active VSG 5'-untranslated region, or VSG transcript depletion using RNA interference. Neither perturbation substantially compromised exclusion of native VSGs, as determined by transcriptomic analyses. In contrast, exclusion of a VSG transgene was specifically compromised when the native transcript was transiently depleted. Notably, while both perturbations blocked cytokinesis, an additional round of DNA replication and mitosis was observed when the transcript, known to be stabilized by a bloodstream-form specific cyclin-like F-box protein, was translationally blocked. We conclude that the VSG transcript is a bi-functional coding and non-coding RNA that participates in allelic competition, and possibly also in cell cycle control, to establish exclusion.