ELEMENTS IN THE 3'-UNTRANSLATED REGION OF PROCYCLIN MESSENGER-RNA REGULATE EXPRESSION IN INSECT FORMS OF TRYPANOSOMA-BRUCEI BY MODULATING RNA STABILITY AND TRANSLATION

Procyclins are the major surface glycoproteins of insect forms of Tryp anosoma brucei. We have previously shown that a conserved 16-mer in th e 3' untranslated region (UTR) of procyclin transcripts functions as a positive element in procyclic-form trypanosomes, A systematic analysi s of the entire 297-hase 3' UTR has now revealed additional elements w hich are involved in posttranscriptional regulation: a positive elemen t which requires the first 30 bases of the 3' UTR and at least one neg ative element between nucleotides 101 and 173 (the LII domain), Deleti on of either positive element resulted in a >8-fold reduction in the a mount of protein but only an similar to 2-fold decrease in the steady- state level of mRNA, suggesting that regulation also occurred at the l evel of translation, In contrast, deletion of LII caused a threefold i ncrease in the steady-state levels of both the mRNA and protein, LII-1 6-mer double deletions also gave high levels of expression, suggesting that the 16-mer functions as an antirepressor of the negative element rather than as an independent activator, All three elements have an e ffect on RNA turnover. When either positive element was deleted, the h alf-life (t(1/2)) of the mRNA was reduced from similar to 50 min (the t(1/2) of the wild-type 3' UTR) to <15 min, whereas removal of the LII element resulted in an increased t(1/2) of similar to 100 min, We pre sent a model of posttranscriptional regulation in which the negative d omain is counteracted by two positive elements which shield it from nu cleases and/or translational repressors.