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Abstract

To maintain homeostasis, organisms have developed different mechanisms to cope with stress. One of the quickest responses to stress is the global attenuation of protein synthesis. This can be achieved by controlling the availability of transcripts by packaging them into higher order cytoplasmic ribonucleoprotein (RNP) granules. Among the best studied examples are processing bodies (P-bodies), which are known to harbour silenced mRNAs, and are implied to have a role in protecting maternal mRNAs during animal development. During Drosophila oogenesis, P-bodies form upon nutritional stress. It has been shown that RNP granule assembly is dependent on protein-protein interactions. In addition, it has been suggested that RNAs could play an important role in this process, providing a scaffold for RNA binding proteins (RBPs), thus promoting RNP granule formation. How RNA contributes to RNP granule assembly in vivo is not well explored. In Drosophila, oskar mRNA is one example of an mRNA with distinct coding and noncoding functions during animal development. Besides coding for Oskar protein, oskar mRNA is necessary for proper oogenesis progression and completion, karyosome formation and distribution of germline specific proteins. Furthermore, the 3’UTR of oskar has been shown to be important for oskar recruitment into transport particles during Drosophila oogenesis. The ability of oskar mRNA to oligomerize, thus concentrate P-body components, raised the question whether this molecule could play a role in the formation of P-bodies. In this work I showed that oskar mRNA is an integral part of Drosophila P-bodies and that the 3’UTR of oskar is sufficient to drive P-body formation. In addition, in oskar mRNA non-dimerizing mutants P-body assembly was impaired. Furthermore, I observed that introducing mutations in the oskar 3’UTR affected recruitment of oskar interactors such as the translational repressor, Bruno, also shown to be involved in the oligomerization of oskar mRNPs. The exact mechanism how oskar contributes to P-body formation is at the moment still an open question. However, here I propose a hypothetical model of how this mRNA could drive P-body assembly. This process would be driven by oskar mRNA dimer formation followed by the recruitment of specific RBPs in order to stabilize the dimer and further promote granule maturation. Coordinated and stepwise interactions between RNA molecules and RBPs could be crucial for proper P-body formation. To summarize, in this work I showed that in the in vivo setting of the Drosophila oocyte, RNA molecules have an important function in driving RNP granule biogenesis.