The role of Bicoid Stability Factor in oskar mRNA function and regulation, and the mechanisms for oskar mRNA transport to the oocyte
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My dissertation is separated into two subjects. First, I examined the role of Bicoid Stability Factor (BSF) in oskar (osk) regulation. Second, I studied cis-acting elements involved in osk mRNA transport to the oocyte during early oogenesis. Oskar (Osk) is a body patterning determinant in Drosophila and is highly concentrated at the posterior pole of the oocyte. This spatially-restricted deployment relies on a coordinated program of osk mRNA localization and translational regulation, all dependent on cis-acting regulatory elements located primarily in the 3’ UTR of the mRNA. Notably, some of these elements, as well as sequences required for a noncoding role of osk mRNA, are clustered in a short region (the C region) near the 3’ end of the osk mRNA. To better understand the role of the C region, I searched for proteins that bind specifically to this region and I found BSF. Binding assays to mutant RNAs suggested that BSF does not act in the noncoding function of osk mRNA. To test for a role for BSF in regulation of Osk protein expression, I used two complementary approaches, reducing the BSF level or disrupting BSF binding to the osk mRNA. Both generated similar results: a reduction or loss of posterior Osk protein and osk mRNA in late oogenesis and early embryogenesis, while the level of osk mRNA was not affected. My work suggests that BSF could act in a late phase of osk mRNA localization or translational activation. Localization of osk mRNA to the posterior pole of the oocyte is achieved by multiple transport steps. One is mRNA transport from the nurse cells to the oocyte. Although cis-acting elements including the oocyte entry signal (OES) in the osk mRNA 3’ UTR have been implicated in mRNA oocyte transport, the precise mechanisms remain unknown. Here, I show that the clusters of Bru binding sites in the osk mRNA 3’ UTR required for translational regulation confer oocyte transport on a reporter mRNA. However, neither Bru sites nor the OES are essential for oocyte transport of osk mRNA. This suggests that there are multiple mechanisms redundantly acting in oocyte transport.