A short RNA stem–loop is necessary and sufficient for repression of gene expression during early logarithmic phase in trypanosomes

We have compared the transcriptomes of cultured procyclic Trypanosoma brucei cells in early and late logarithmic phases and found that ∼200 mRNAs were differentially regulated. In late log phase cells, the most upregulated mRNA encoded the nucleobase transporter NT8. The 3′ untranslated region (UTR) of NT8 contains a short stem–loop cis-element that is necessary for the regulation of NT8 expression in response to external purine levels. When placed in the 3′-UTR of an unregulated transcript, the cis-element is sufficient to confer regulation in response to purines. To our knowledge, this is the first example of a discrete RNA element that can autonomously regulate gene expression in trypanosomes in response to an external factor and reveals an unprecedented purine-dependent signaling pathway that controls gene expression in eukaryotes.