A ribosomal DNA promoter replacing the promoter of a telomeric VSG gene expression site can be efficiently switched on and off in T. brucei

Trypanosoma brucei survives in the mammalian blood-stream by regularly changing its variant surface glycoprotein (VSG) coat. The active VSG gene is located in a telomeric expression site, and coat switching occurs either by replacing the transcribed VSG gene or by changing the expression site that is active. To determine whether VSG expression site control requires promoter-specific sequences, we replaced the active VSG expression site promoter in bloodstream-form T. brucei with a ribosomal DNA (rDNA) promoter. These transformants were fully infective in laboratory animals, and the rDNA promoter, which is normally constitutively active, was efficiently inactivated and reactivated in the context of the VSG gene expression site. As there is no sequence similarity between the VSG expression site promoter and the rDNA promoter, VSG expression site control does not involve sequences specific to the VSG expression site promoter. We conclude that an epigenetic mechanism, such as telomeric silencing, is involved in VSG expression site control in bloodstream-form T. brucei.