Localization of oskar mRNA to the posterior of the Drosophila oocyte is essential for abdominal patterning and germline development. oskar localization is a multi-step process involving temporally and mechanistically distinct transport modes. Numerous cis-acting elements and trans-acting factors have been identified that mediate earlier motor-dependent transport steps leading to an initial accumulation of oskar at the posterior. Little is known, however, about the requirements for the later localization phase, which depends on cytoplasmic flows and results in the accumulation of large oskar ribonucleoprotein granules, called founder granules, by the end of oogenesis. Using super-resolution microscopy, we show that founder granules are agglomerates of smaller oskar transport particles. In contrast to the earlier kinesin-dependent oskar transport, late-phase localization depends on the sequence as well as on the structure of the spliced oskar localization element (SOLE), but not on the adjacent exon junction complex deposition. Late-phase localization also requires the oskar 3' untranslated region (3' UTR), which targets oskar to founder granules. Together, our results show that 3' UTR-mediated targeting together with SOLE-dependent agglomeration leads to accumulation of oskar in large founder granules at the posterior of the oocyte during late stages of oogenesis. In light of previous work showing that oskar transport particles are solid-like condensates, our findings indicate that founder granules form by a process distinct from that of well-characterized ribonucleoprotein granules like germ granules, P bodies, and stress granules. Additionally, they illustrate how an individual mRNA can be adapted to exploit different localization mechanisms depending on the cellular context.

中文翻译：

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通过核糖核蛋白颗粒中的聚集来定位 oskar mRNA。

oskar mRNA 定位于果蝇卵母细胞后部对于腹部图案形成和种系发育至关重要。奥斯卡定位是一个多步骤过程，涉及时间和机械上不同的运输模式。已鉴定出许多顺式作用元件和反式作用因子介导早期运动依赖性转运步骤，导致 oskar 在后部的初始积累。然而，人们对后期定位阶段的要求知之甚少，该阶段取决于细胞质流动并导致在卵子发生结束时积累大的奥斯卡核糖核蛋白颗粒（称为创始人颗粒）。使用超分辨率显微镜，我们发现创始人颗粒是较小的奥斯卡运输颗粒的聚集体。与早期的驱动蛋白依赖性 oskar 运输相反，后期定位取决于序列以及剪接 oskar 定位元件 (SOLE) 的结构，但不取决于相邻外显子连接复合物的沉积。后期定位还需要 oskar 3' 非翻译区 (3' UTR)，该区域将 oskar 定位为创始颗粒。总之，我们的结果表明，3' UTR 介导的靶向与 SOLE 依赖性团聚一起导致卵子发生后期 oskar 在卵母细胞后部的大创始人颗粒中积累。鉴于先前的工作表明奥斯卡传输颗粒是固体状凝聚物，我们的研究结果表明，创始人颗粒的形成过程不同于充分表征的核糖核蛋白颗粒（如胚芽颗粒、P 小体和应激颗粒）。此外，它们还说明了如何根据细胞环境调整单个 mRNA 以利用不同的定位机制。