Structure of a regulatory 3 ' untranslated region from Trypanosoma brucei

The African trypanosome Trypanosoma brucei multiplies in mammalian extracel lular fluids (bloodstream forms) and in the midgut of Tsetse flies (procycl ic forms). The control of gene expression that is necessary to survive in t hese two environments operates almost exclusively at the posttranscriptiona l level, and the sequences responsible are located in the 3' untranslated r egions of the mRNAs. The major surface proteins of procyclic trypanosomes, EP1, EP2, EP3, and GPEET, are not expressed in bloodstream forms. The 3' un translated regions of these four mRNAs are not very similar, but all contai n a conserved 26mer sequence that is required for developmental regulation. We have analyzed the conformation of the EP1 3' UTR in vitro by enzymatic digestion and lead hydrolysis, and in vivo by modification with DMS and wit h CMCT (introduced by electroporation). Results indicate that the 3' UTR ca n be divided into three domains. Domains I and III, at the 5' and 3' ends, form stable structures, but the central domain (domain II), which includes the 26mer, has no stable interactions either within itself, or with other p arts of the 3' UTR. Domain I contains three leadzymes that do not conform t o the previously reported consensus.