Transcription is initiated on silent variant surface glycoprotein expression sites despite monoallelic expression in Trypanosoma brucei.

African trypanosomes survive the immune defense of their hosts by regularly changing their antigenic coat made of variant surface glycoprotein (VSG). The Trypanosoma brucei genome contains more than 1,000 VSG genes. To be expressed, a given VSG gene must be located in one of 15 telomeric regions termed "VSG expression sites" (ESs), each of which contains a polycistronic transcription unit that includes ES-associated genes. Only one ES is fully active at a time, so only one VSG gene is transcribed per cell. Although this monoallelic expression is controlled at the transcriptional level, the precise molecular mechanism for this control is not understood. Here we report that in single cells transcription is initiated on several ESs simultaneously, indicating that the monoallelic control is not determined only at transcription initiation, but also at further control steps such as transcription elongation or RNA processing.