Gene expression in the trypanosomatid Leishmania
Stanton, Julie Dangremond
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Leishmania species are the etiological agents of the disease leishmaniasis. Over 12 million people are infected with leishmaniasis, and ten percent of the world’s population is at risk of contracting the disease. Trypanosomatids exert little control at the level of transcription because most promoters are active by default. As a result these organisms rely heavily on post-transcriptional mechanisms for regulating gene expression. The untranslated regions (UTRs) of mRNA may contribute to this type of control. The purpose of this study was to investigate the effects of 5’ UTRs on gene expression in Leishmania. We focused on the AUG-proximal region (APR), defined as the nucleotides at positions -3, -2, and -1 upstream of the translation initiation codon. Eight 12-nucleotide sequences differing in the APR were studied in the context of a 180-nucleotide Leishmania 5’ UTR. Using stable transfectants of Leishmania tropica, up to a 6000-fold difference in reporter protein expression could be observed. Steady state levels of reporter mRNA did not reflect the dramatic variation in protein abundance, indicating that APRs act post-transcriptionally. To explore the possibility that APRs acted on translation, their effect on ribosome recruitment was examined. The protein expression from some APRs correlated with the extent of polysome formation on reporter mRNA. We conclude that APRs can affect translation in Leishmania. To further study the role of the 5’ UTR, tri-nucleotides in the APR of 200 Leishmania major genes were analyzed. Some tri-nucleotides were observed up to 7.724 x 10 5 times more than expected if selection was random. Twenty-five percent of the tri-nucleotides were absent from the data set, a 6-fold higher frequency than expected. We conclude that selection of tri-nucleotides in the APR of Leishmania 5’ UTRs is not random. Using nearest neighbor analysis, patterns containing frequently used and rare tri-nucleotides were found. Together, our experimental and genomic data indicate that APRs have functional significance in Leishmania.