Induction of D.melanogaster germ cell formation depends on local activity of the maternal determinant Oskar at the posterior of the embryo. When Oskar is mis-expressed at the anterior, posterior structures develop ectopically, while absence of Oskar leads to loss of abdominal structures and germ cells. Spatial restriction of Oskar protein is achieved by enriching oskar mRNA in the oocyte, localising it to the posterior pole and by maintaining the mRNA translationally silencing during transport. The 3’UTR of oskar RNA contains elements for its translational repression and is, together with splicing, also required for posterior localisation. Intronless reporters bearing the oskar 3’UTR can also localise, however this is an indirect process involving hitch-hiking of the reporter RNA with endogenous oskar mRNA, to the posterior pole. We have analysed the molecular basis of the hitch-hiking process and tested its potential implication in regulation of endogenous oskar RNA in vivo. In vitro the oskar 3’UTR RNA forms RNA-RNA dimers via a specific RNA dimerisation domain. In vivo, this dimerisation domain is necessary - though not sufficient - for efficient hitch-hiking of 3’UTR-containing reporters with endogenous oskar mRNA to the posterior of the oocyte. In contrast, the dimerisation domain is not essential for oskar mRNA localisation. Surprisingly however, offspring of females expressing oskar RNA mutated in its dimerisation domain display patterning defects suggesting Oskar protein over-expression. Consistent with this, we found that in this oskar mutant the mRNA is prematurely and ectopically translated. This ectopic translation is suppressed when dimerisation is restored by co-expressing an oskar RNA bearing compensatory mutations that in vitro restore RNA dimerisation. My work thus revealed a direct role for RNA-RNA interaction in translational repression of oskar mRNA
