Nutritional control of mRNA translation in Drosophila larvae. Sabarish Nagarajan, Savraj Grewal. Souther Alberta Cancer Research Institute, University of Calgary, Calgary, Alberta, Canada.

   Dietary protein is a key determinant of growth in Drosophila larvae. Several studies have described changes in gene transcription, (particularly metabolic genes) that may mediate nutrient effects on growth. In contrast the role of nutrition-dependent translational regulation is less clear. We have used polysome profiling to study global and specific translation of mRNAs in response to altered nutrition in larvae. We find that bulk translation is markedly decreased within 2 hours after protein starvation, with a maximal decrease in translation observed between 18 and 96hrs post-starvation. We next looked at translation of specific mRNAs by using qRT-PCR anlaysis of polysome fractions. We examined translation of a selected set of genes whose overall transcript levels were highly upregulated, highly downregulated or unchanged following protein starvation. With all genes, we observed decreased translation upon starvation. These translationally repressed genes included transcription factors such as Myc, hence providing a mechanism by which nutrition can alter gene transcription. Interestingly, we also found that several translationally repressed mRNAs contain specific features such as putative IRESs or decreased 5UTR complexity, that were previously suggested to allow elevated translation upon starvation. Together our data suggest that while starvation induces strong increases and decreases in transcription, translational repression may be an additional key determinant of overall gene expression in starved animals. The current view, largely from work in mammalian cell culture, is that nutrient signaling stimulates mRNA translation by inhibiting 4EBP, a conserved translational repressor. We found that 4EBP null larvae show no change in translation in fed conditions and are unable to reverse the decrease in mRNA translation during starvation. These data argue that 4EBP-independent mechanisms must contribute to nutrition-dependent translational control in larvae. We are currently exploring other potential translational regulators that may mediate these effects of nutrient signaling.