SummaryTrypanosomes are outstanding examples of the importance of mRNA metabolism in the regulation of gene expression, as these unicellular eukaryotes mostly control protein synthesis by posttranscriptional mechanisms. Here, we show that mRNA metabolism in these organisms involves recruitment of mRNAs and proteins to microscopically visible ribonucleoprotein granules in the cytoplasm. These structures engage transcripts that are being translated and protect mRNAs from degradation. Analysis of the protein composition of trypanosomal mRNA granules indicated that they contain orthologous proteins to those present in P bodies and stress granules from metazoan organisms. Formation of mRNA granules was observed after carbon-source deprivation of parasites in axenic culture. More important, mRNA granules are formed naturally in trypanosomes present in the intestinal tract of the insect vector. We suggest that trypanosomes make use of mRNA granules for transient transcript protection as a strategy to cope with periods of starvation that they have to face during their complex life cycles.

Trypanosomatids are protozoan micro-organisms that cause serious health problems in humans and domestic animals. In addition to their medical relevance, these pathogens have novel biological structures and processes. From nuclear DNA transcription to mRNA translation, trypanosomes use unusual mechanisms to control gene expression. For example, transcription by RNAPII (RNA polymerase II) is polycistronic, and only a few transcription initiation sites have been identified so far. The sequences present in the polycistronic units code for proteins having unrelated functions, that is, not involved in a similar metabolic pathway. Owing to these biological constraints, these micro-organisms regulate gene expression mostly by post-transcriptional events. Consequently, the function of proteins that recognize RNA elements preferentially at the 3' UTR (untranslated region) of transcripts is central. It was recently shown that mRNP (messenger ribonucleoprotein) complexes are organized within post-transcriptional operons to co-ordinately regulate gene expression of functionally linked transcripts. In the present chapter we will focus on particular characteristics of gene expression in the so-called TriTryp parasites: Trypanosoma cruzi, Trypanosoma brucei and Leishmania major.