Structure of a regulatory 3' untranslated region from Trypanosoma brucei.

The African trypanosome Trypanosoma brucei multiplies in mammalian extracellular fluids (bloodstream forms) and in the midgut of Tsetse flies (procyclic forms). The control of gene expression that is necessary to survive in these two environments operates almost exclusively at the posttranscriptional level, and the sequences responsible are located in the 3' untranslated regions of the mRNAs. The major surface proteins of procyclic trypanosomes, EP1, EP2, EP3, and GPEET, are not expressed in bloodstream forms. The 3' untranslated regions of these four mRNAs are not very similar, but all contain 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 with CMCT (introduced by electroporation). Results indicate that the 3' UTR can 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 parts of the 3' UTR. Domain I contains three leadzymes that do not conform to the previously reported consensus.