Markussen F H, Michon A M, Breitwieser W, Ephrussi A
Differentiation Program, European Molecular Biology Laboratory, Heidelberg, Germany.
Development. 1995 Nov;121(11):3723-32. doi: 10.1242/dev.121.11.3723.
At the posterior pole of the Drosophila oocyte, oskar induces a tightly localized assembly of pole plasm. This spatial restriction of oskar activity has been thought to be achieved by the localization of oskar mRNA, since mislocalization of the RNA to the anterior induces anterior pole plasm. However, ectopic pole plasm does not form in mutant ovaries where oskar mRNA is not localized, suggesting that the unlocalized mRNA is inactive. As a first step towards understanding how oskar activity is restricted to the posterior pole, we analyzed oskar translation in wild type and mutants. We show that the targeting of oskar activity to the posterior pole involves two steps of spatial restriction, cytoskeleton-dependent localization of the mRNA and localization-dependent translation. Furthermore, our experiments demonstrate that two isoforms of Oskar protein are produced by alternative start codon usage. The short isoform, which is translated from the second in-frame AUG of the mRNA, has full oskar activity. Finally, we show that when oskar RNA is localized, accumulation of Oskar protein requires the functions of vasa and tudor, as well as oskar itself, suggesting a positive feedback mechanism in the induction of pole plasm by oskar.
在果蝇卵母细胞的后极，osk基因诱导极质进行紧密定位组装。osk基因活性的这种空间限制一直被认为是通过osk基因mRNA的定位来实现的，因为RNA在前部的错误定位会诱导前部极质的形成。然而，在osk基因mRNA未定位的突变卵巢中不会形成异位极质，这表明未定位的mRNA是无活性的。作为理解osk基因活性如何局限于后极的第一步，我们分析了野生型和突变体中osk基因的翻译情况。我们发现，将osk基因活性靶向到后极涉及两个空间限制步骤，即mRNA依赖细胞骨架的定位和依赖定位的翻译。此外，我们的实验表明，通过使用不同的起始密码子产生了两种Oskar蛋白异构体。从mRNA的第二个符合读框的AUG翻译而来的短异构体具有完整的osk基因活性。最后，我们发现当osk基因RNA定位时，Oskar蛋白的积累需要vasa基因和tudor基因以及osk基因自身的功能，这表明在osk基因诱导极质形成过程中存在一种正反馈机制。
