A CONSERVED STEM-LOOP STRUCTURE IN THE 3' UNTRANSLATED REGION OF PROCYCLIN MESSENGER-RNAS REGULATES EXPRESSION IN TRYPANOSOMA-BRUCEI

African trypanosomes that cycle between mammalian hosts and the tsetse fly vector must be poised to survive in different environments. The c ontrol of stage-specific gene expression is undoubtedly one of the key s to successful adaptation, but no regulatory elements have been defin ed to date. Procyclins (also known as procyclic acidic repetitive prot eins) are specifically expressed on the surface of procyclic and epima stigote forms in the fly. Procyclin genes are already transcribed in b loodstream forms, but stable mRNA, and later the protein, are first de tected when the parasites begin to differentiate into procyclic forms. We have now identified a region of 16 bases that forms part of a pred icted stem-loop structure in the 3' untranslated regions of different procyclin mRNAs; both the sequence and the secondary structure of this 16-mer appear to be required for efficient translation of a reporter gene in procyclic forms. The level of steady-state mRNA, its polyadeny lylation, and its distribution in the cell are all unaffected by the p resence or absence of this element. Deletion of the 16-mer alone reduc es expression more than removal or reversal of the entire 3' untransla ted region and flanking region, suggesting that there are additional n egative regulatory elements in the same 3' untranslated region.