Quantification of RNA Polymerase I transcriptional attenuation at the active VSG expression site in Trypanosoma brucei

The cell surface of the extracellular pathogen Trypanosoma brucei consists of a dense coat of variant surface glycoprotein (VSG), which enables the parasite to evade the immune system of the vertebrate host. Only one VSG gene from a large repertoire is expressed from a so-called bloodstream form expression site (BES) at a given timepoint. There are several BES in every parasite but only one is transcriptionally active. Other BES are silenced by transcriptional attenuation. Periodic activation of a previously-silenced BES results in differential VSG transcription and escape from the immune response. A process called antigenic variation. In contrast to gene transcription in other eukaryotes, the BES is transcribed by RNA polymerase I (Pol I). It was proposed that this highly-processive polymerase is needed to provide a sufficiently high transcription rate at the VSG gene. Surprisingly, we discovered a position-dependent Pol I activity and attenuation of transcriptional elongation also at the active BES. Transcription rates at the VSG gene appear to be comparable to Pol II-mediated transcription of house-keeping genes. Although these findings are in contradiction to the long-standing concept of continuously high transcription rates at the active BES in Trypanosoma brucei, they are complementary to recent groundbreaking findings about transcriptional regulation of VSG genes.