The 3'-terminal region of the mRNAs for VSG and procyclin can confer stage specificity to gene expression in Trypanosoma brucei.

The variant surface glycoprotein (VSG) and procyclin are the respective 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 expression of a reporter gene in an inverse manner, depending on the developmental form of the parasite. In the case of VSG mRNA, the 97 nt sequence upstream from the polyadenylation site is responsible for these effects. The regulation occurs through a variation of mRNA abundance which is not due to a change in primary transcription. In the bloodstream form this effect is manifested by an increase in RNA stability, whereas 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 posttranscriptional over transcriptional controls is probably linked to the organization of the trypanosome genome in polycistronic transcription units.