Synthesis of the posterior determinant Nanos is spatially restricted by a novel cotranslational regulatory mechanism

Nanos (Nos) protein is required in the posterior of the Drosophila embryo t o promote abdominal development, but must be excluded from the anterior to permit head and thorax development [1,2], Spatial restriction of Nos is acc omplished by selective translation of the 4% of nos mRNA localized to the p osterior pole and translational repression of the remaining unlocalized mRN A [3-5]. Repression is mediated by a 90-nucleotide translational control el ement (TCE) in the nos 3' untranslated region (UTR) and the TCE-binding pro tein Smaug [4,6,7], but the molecular mechanism is unknown. We used sucrose density gradient sedimentation to ascertain whether unlocalized nos mRNA i s excluded from polysomes and therefore repressed during translational init iation, Surprisingly, a significant percentage of nos mRNA was found to be associated with polysomes, even in mutants in which all nos mRNA is unlocal ized and repressed. Using a regulated Drosophila cell-free translation syst em, we showed that ribosomes contained within these polysomes are capable o f elongation in vitro, under conditions in which synthesis of Nos protein i s repressed, Thus, synthesis of ectopic Nos protein is inhibited by a novel regulatory mechanism that does not involve a stable arrest of the translat ion cycle.