Differential RNA elongation controls the variant surface glycoprotein geneexpression sites of Trypanosoma brucei

The protozoan parasite Trypanosoma brucei develops antigenic variation to e scape the immune response of its host. To this end, the trypanosome genome contains multiple telomeric expression sites competent for transcription of variant surface glycoprotein genes, but as a rule only a single antigen is expressed at any time. We used reverse transcription-PCR (RT-PCR) to analy se transcription of different segments of the expression sites in different variant clones of two independent strains of T. brucei. The results indica ted that RNA polymerase is installed and active at the beginning of many, i f not all, expression sites simultaneously, but that a progressive arrest o f RNA elongation occurs in all but one site. This defect is linked to ineff icient RNA processing and RNA release from the nucleus. Therefore, function al transcription in the active site appears to depend on the selective recr uitment of a RNA elongation/processing machinery.