Abstract
Localization of oskar mRNA includes two distinct phases: transport from nurse cells to the oocyte, a process typically accompanied by cortical anchoring in the oocyte, followed by posterior localization within the oocyte. Signals within the oskar 3’ UTR directing transport are individually weak, a feature previously hypothesized to facilitate exchange between the different localization machineries. We show that alteration of the SL2a stem-loop structure containing the oskar transport and anchoring signal (TAS) removes an inhibitory effect such that in vitro binding by the RNA transport factor, Egalitarian, is elevated as is in vivo transport from the nurse cells into the oocyte. Cortical anchoring within the oocyte is also enhanced, interfering with posterior localization. We also show that mutation of Staufen recognized structures (SRSs), predicted binding sites for Staufen, disrupts posterior localization of oskar mRNA just as in staufen mutants. Two SRSs in SL2a, one overlapping the Egalitarian binding site, are inferred to mediate Staufen-dependent inhibition of TAS anchoring activity, thereby promoting posterior localization. The other three SRSs in the oskar 3’ UTR are also required for posterior localization, including two located distant from any known transport signal. Staufen, thus, plays multiple roles in localization of oskar mRNA.
Highlights
Localization of mRNAs serves to target expression of encoded proteins to specific subcellular domains [1,2]
When localization of an mRNA involves more than one step, the mRNA must be released from the machinery directing the first step to allow the machinery for the step to perform its function
Prior studies suggested that the multiple signals in the oskar mRNA that mediate transport were individually weak by necessity, to facilitate action by the posterior localization machinery
Summary
Localization of mRNAs serves to target expression of encoded proteins to specific subcellular domains [1,2]. They can encode proteins, or they can function as noncoding RNAs (ncRNAs). At the earlier stages of oogenesis when osk ncRNA activity is required, the mRNA is efficiently transported from nurse cells (the sites of transcription) through ring canals into the oocyte; if this transport is disrupted, osk ncRNA activity is disrupted [7]. Both the coding and noncoding roles of osk mRNA require some form of localization. We refer to the two phases of osk mRNA localization as ‘transport’ (directed movement from nurse cells to oocyte) and ‘localization’ (directed movement within the oocyte to the posterior at later stages of oogenesis)
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