Overlapping and distinct cis-acting requirements for oskar mRNA localization pathways

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 accumulation of oskar at the posterior. Little is known, however, about what features of oskar are required for the later localization phase, that occurs by diffusion with local entrapment and results in the accumulation of large oskar ribonucleoprotein granules by the end of oogenesis. Here we test whether cis- and trans-acting requirements for kinesin-dependent oskar transport, the structured oskar spliced localization element (SOLE) and the exon junction complex (EJC), are also required for late-phase localization. In contrast to kinesin-dependent motility, late-phase localization is independent of the EJC and depends not only on the structure but also the sequence of the SOLE. Late-phase localization also requires the oskar 3 UTR and we show that the 3 UTR is sufficient for ribonucleoprotein granule selectivity.