Delineation of the regulated Variant Surface Glycoprotein gene expression site domain of Trypanosoma brucei

The African trypanosome Trypanosoma brucei is protected in the bloodstream of the mammalian host by a dense Variant Surface Glycoprotein (VSG) coat. Although an individual cell has hundreds of VSG genes, the active VSG is transcribed in a mutually exclusive fashion from one of about twenty telomeric VSG expression sites. Expression sites are regulated domains flanked by 50 bp repeat arrays and extensive tracts of repetitive elements. We have integrated exogenous rDNA and expression site promoters upstream of the 50 bp repeats of the VO2 VSG expression site. Transcription from both types of exogenous promoter is downregulated and comparable to promoters targeted into the VSG Basic Copy arrays. We show that the upstream exogenous rDNA promoter escapes VSG expression site control, as switching the downstream VO2 VSG expression site on and off does not affect its activity. Therefore, the 50 bp repeat arrays appear to be the boundary of the regulated expression site domain.