RNA sequences required for the noncoding function of oskar RNA also mediate regulation of Oskar protein expression by...

The Drosophila oskar (osk) mRNA is unusual in having both coding and noncoding functions. As an mRNA, osk encodes a protein which is deployed specifically at the posterior of the oocyte. This spatially-restricted deployment relies on a program of mRNA localization and both repression and activation of translation, all dependent on regulatory elements located primarily in the 3' untranslated region (UTR) of the mRNA. The 3' UTR also mediates the noncoding function of osk, which is essential for progression through oogenesis. Mutations which most strongly disrupt the noncoding function are positioned in a short region (the C region) near the 3' end of the mRNA, in close proximity to elements required for activation of translation. We show that Bicoid Stability Factor (BSF) binds specifically to the C region of the mRNA. Both knockdown of bsf and mutation of BSF binding sites in osk mRNA have the same consequences: Osk expression is largely eliminated late in oogenesis, with both mRNA localization and translation disrupted. Although the C region of the osk 3' UTR is required for the noncoding function, BSF binding does not appear to be essential for that function.