Mutations in a 5' region of the osk gene disrupt both Osk protein function and osk mRNA translational activation. Matt Kanke, Goheun Kim, Young-Hee Ryu, Paul M. Macdonald. Molecular Cell and Developmental Biology, University of Texas at Austin, Austin, TX.

   Proper regulation of oskar (osk) mRNA translation is essential for axial patterning. osk is synthesized in the nurse cells and subsequently localized to the posterior of the oocyte in a translationally repressed state. Upon posterior localization, the two Osk isoforms, Long Osk and Short Osk, begin to accumulate. Repression of osk mRNA translation has been extensively characterized and relies on defined elements in the osk 3 UTR as well as known regulatory factors. Translational activation is less well understood. The current, long standing model proposes that a 5 regulatory element acts to override repression specifically at the posterior pole of the oocyte. The proposed element lies in the 5' part of the osk mRNA that is either a protein coding region (of Long Osk) or 5' UTR (for production of Short Osk). Mutations in the element may have consequences from change in an RNA element or from change in the Osk protein sequence. We have used osk and osk::GFP transgenes to evaluate the role of the 5' region. Characterization of osk::GFP transgenes shows that the Osk aminoterminal domain confers anchoring on the fusion protein, and that mutations in the presumed regulatory element both disrupt protein anchoring and reduce protein levels. The anchoring defect shows that a protein domain is altered; reduced protein levels could result from either protein or RNA changes. To definitively test for the presence of an RNA translational activation element we used osk transgenes in which Long Osk is not produced, and mutations in the proposed 5' element only affect the 5' UTR. We find that there is indeed a translational activation element in the osk 5' region, and that this element is essential for Short Osk expression.