Bruno regulates mRNA translation by binding to multiple sequence motifs
Oskar (Osk) is a posterior body patterning determinant in Drosophila melanogaster oocytes. oskar (osk) mRNA is translationally repressed until it reaches the posterior of the oocyte where Osk protein accumulates. Translational repression of osk prior to posterior localization is mediated by the RNA binding protein, Bruno (Bru). To better define Bru binding sites, I performed in vitro selections using full length Bru and the fragments containing either the first two RRMs (RRM1+2) or the third RRM (RRM3+). The aptamers from the final round from each of the selections produced a multitude of overrepresented primary sequence motifs. Examples of each of these motifs were found in the 3’UTRs of the mRNAs that Bru is known to regulate during oogenesis. GFP reporter transgenes under the control of the UAS-Gal4 expression system were constructed with each class of the binding sites within the reporter transgenes’ 3’UTRs to test the motifs’ ability to repress the reporters in vivo. In a wildtype background, the GFP reporters containing the binding sites were translationally repressed. In the aret mutant background, the GFP levels of the repressed GFP reporters increased with reduced Bru activity, suggesting the transgenes’ repression is mediated by Bru. Three of the motifs isolated in the in vitro selections reside in the AB and C regions of the osk 3’UTR, and the three classes of sites were mutated in the AB and C regions. The mutated AB and C regions were used to assay for a reduction of Bru binding affinity for the mutant RNAs. Additionally, the mutations were incorporated into an osk genomic transgene that was introduced into an osk RNA null as well as an Osk protein null background. The mutations reduced Bru binding to the AB and C regions. The transgenes containing the mutated Bru binding sites could not fully rescue the osk RNA null phenotype but can fully rescue the Osk protein null phenotype, suggesting an osk transcript can regulate other osk mRNAs in trans.