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 to promote abdominal development, but must be excluded from the anterior to permit head and thorax development [1,2]. Spatial restriction of Nos is accomplished by selective translation of the 4% of nos mRNA localized to the posterior pole and translational repression of the remaining unlocalized mRNA [3-5]. Repression is mediated by a 90-nucleotide translational control element (TCE) in the nos 3' untranslated region (UTR) and the TCE-binding protein Smaug [4,6,7], but the molecular mechanism is unknown. We used sucrose density gradient sedimentation to ascertain whether unlocalized nos mRNA is excluded from polysomes and therefore repressed during translational initiation. Surprisingly, a significant percentage of nos mRNA was found to be associated with polysomes, even in mutants in which all nos mRNA is unlocalized and repressed. Using a regulated Drosophila cell-free translation system, we showed that ribosomes contained within these polysomes are capable of elongation in vitro, under conditions in which synthesis of Nos protein is repressed. Thus, synthesis of ectopic Nos protein is inhibited by a novel regulatory mechanism that does not involve a stable arrest of the translation cycle.
Clark
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Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
Wyckoff
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Gavis
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D020413:3' Untranslated Regions / Q000235:genetics; D000818:Animals; D002474:Cell-Free System; D004330:Drosophila / Q000196:embryology* / Q000235:genetics*; D029721:Drosophila Proteins; D004625:Embryo, Nonmammalian; D018507:Gene Expression Regulation, Developmental; D019476:Insect Proteins / Q000235:genetics*; D009024:Morphogenesis; D011132:Polyribosomes / Q000378:metabolism; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000032:analysis; D016601:RNA-Binding Proteins
D020413:3' Untranslated Regions; D029721:Drosophila Proteins; D019476:Insect Proteins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; C070574:nos protein, Drosophila