STIMULI OF DIFFERENTIATION REGULATE RNA ELONGATION IN THE TRANSCRIPTION UNITS FOR THE MAJOR STAGE-SPECIFIC ANTIGENS OF TRYPANOSOMA-BRUCEI

In Trypanosoma brucei, the mutually exclusive expression of the major surface antigens, the variant surface glycoprotein (VSG) of the bloods tream form and procyclin of the procyclic form, is due to a stage-spec ific accumulation of the respective mRNAs, Through the targeting of a reporter construct in the procyclin promoter region, we show that inde pendently of any selection pressure, a relatively high level of transc ription (similar to 10%) occurs from the procyclin promoter in the blo odstream form, This transcription leads to the production of detectabl e amounts of polyadenylated mRNAs. However, RNA elongation in the proc yclin transcription unit is down-regulated at this stage, Transcriptio n elongation in the procyclin and VSG units is inversely controlled by the combination of factors which cause the differentiation of bloodst ream into procyclic forms in vitro. These factors include temperature, citrate/cis-aconitate and the incubation medium, Our results suggest that inverse regulations of primary transcription in the VSG and procy clin units are early events that underly the differentiation of the pa rasite.