Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte in duces the assembly of the pole plasm and therefore defines where the abdome n and germ cells form in the embryo. This localization is achieved by the t argeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along micr otubules, since its localization requires both an intact microtubule cytosk eleton and the plus end-directed motor kinesin I, but nothing is known abou t how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurk en mRNA localization, the organization of the microtubules, or subsequent s teps in pole plasm assembly. Surprisingly, most mutant embryos still form a n abdomen, indicating that oskar mRNA localization is partially redundant w ith the translational control. Barentsz protein colocalizes to the posterio r with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Bar entsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.