Multiple -acting signals, some weak by necessity, collectively direct robust transport of mRNA to the oocyte.

Localization of mRNAs can involve multiple steps, each with its own -acting localization signals and transport factors. How is the transition between different steps orchestrated? We show that the initial step in localization of mRNA - transport from nurse cells to the oocyte - relies on multiple -acting signals. Some of these are binding sites for the translational control factor Bruno, suggesting that Bruno plays an additional role in mRNA transport. Although transport of mRNA is essential and robust, the localization activity of individual transport signals is weak. Notably, increasing the strength of individual transport signals, or adding a strong transport signal, disrupts the later stages of mRNA localization. We propose that the transport signals are weak by necessity; their weakness facilitates transfer of the mRNA from the oocyte transport machinery to the machinery for posterior localization.