Determination of how miRNAs mediate repression in Drosophila and the essential role of the oskar mRNA in egg chamber development
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miRNAs are important regulators of gene expression. These small RNAs function throughout development and regulate translation of a number mRNAs. miRNAs exert their affect on translation as part of the RNP complex RISC. RISC can affect translation of transcripts at both the level of translation initiation, and post-initiation. Although mechanisms of repression mediated by miRNAs have been intensively studied, repression is not well characterized. In order to understand how miRNAs regulate translation in Drosophila, we first characterized miRNA-mediated repression in the ovary. We developed an ovarian assay sensitive to regulation by miRNAs and found that regulated transcripts localize to cytoplasmic puncta distinct from sponge bodies, cytoplasmic RNP structures consisting of proteins implicated in miRNA-mediated regulation. In addition, we devised a genetic screen to identify genes involved in miRNA-mediated regulation. Seven mutants were isolated from the screen, and two mutants were subsequently mapped to separate 1Mb genomic regions. Both these regions are devoid of genes implicated in miRNA-mediated regulation, suggesting our mutants identify novel components involved in repression. The oskar mRNA encodes for the Oskar protein, which is vital in establishing the posterior axis of the Drosophila embryo. In addition to its protein coding function, the osk mRNA has another essential role: it is required for egg chamber progression through oogenesis. This role of oskar is mediated by its 3ʼ UTR, but how it functions in this role is unknown. Here, we investigate the function of the 3ʼ UTR and discover that the well-defined BRE sequences are required for egg chamber progression through oogenesis. The BREs mediate translational repression of the highly regulated oskar mRNA and were previously defined by their ability to bind Bruno, which represses translation of the oskar mRNA. We also provide evidence that the osk BREs sequester Bruno, potentially inhibiting Bruno from binding and misregulating other mRNAs. Our results suggest a novel regulatory loop, where oskar sequesters and inhibits Bruno from misregulating mRNAs, and Bruno, in turn, regulates translation of the oskar mRNA.