A pol I transcriptional body associated with VSG mono-allelic expression in Trypanosoma brucei

In the mammalian host, African trypanosomes generate consecutive waves of p arasitaemia by changing their antigenic coat. Because this coat consists of a single type of variant surface glycoprotein (VSG), the question arises o f how a trypanosome accomplishes the transcription of only one of a multi-a llelic family of VSG expression site loci to display a single VSG type on t he surface at any one time(1). No major differences have been detected betw een the single active expression site and the cohort of inactive expression sites(2). Here we identify an extranucleolar body containing RNA polymeras e I (pol I) that is transcriptionally active and present only in the bloods tream form of the parasite. Visualization of the active expression site loc us by tagging with green fluorescent protein(3) shows that it is specifical ly located at this unique pol I transcriptional factory. The presence of th is transcriptional body in postmitotic nuclei and its stability in the nucl eus after DNA digestion provide evidence for a coherent structure. We propo se that the recruitment of a single expression site and the concomitant exc lusion of inactive loci from a discrete transcriptional body define the mec hanism responsible for VSG mono-allelic expression.