THE 3'-TERMINAL REGION OF THE MESSENGER-RNAS FOR VSG AND PROCYCLIN CAN CONFER STAGE SPECIFICITY TO GENE-EXPRESSION IN TRYPANOSOMA-BRUCEI

The variant surface glycoprotein (VSG) and procyclin are the respectiv e major surface antigens of the bloodstream and the procyclic forms of Trypanosoma brucei. These proteins and their mRNAs are both the most abundant and absolutely characteristic of their respective life cycle stages. We show that the 3'-terminal region of these mRNAs regulates e xpression of a reporter gene in an inverse manner, depending on the de velopmental form of the parasite. In the case of VSG mRNA, the 97 nt s equence upstream from the polyadenylation site is responsible for thes e effects. The regulation occurs through a variation of mRNA abundance which is not due to a change in primary transcription, In the bloodst ream form this effect is manifested by an increase in RNA stability, w hereas in the procyclic form it seems to be related to a reduction in the efficiency of mRNA maturation. The 3'-end of VSG mRNA can obviate the 5- to 10-fold stimulation of transcription driven by the procyclin promoter during differentiation from the bloodstream to the procyclic form. The predominance of post-transcriptional over transcriptional c ontrols is probably linked to the organization of the trypanosome geno me in polycistronic transcription units.