During development, translational control of mRNAs regulates gene expression. Translational control is usually achieved through binding of trans-acting factors to mRNA untranslated regions but, on , Mary Lou King and co-workers reveal a novel, structure-based mechanism for translational repression of Xenopus germline nanos1. Nanos translational repressors maintain primordial germ cell identity during development. nanos1 RNA is transcribed during early oogenesis and stored in germinal granules. Surprisingly, the researchers report that, unlike other mRNAs, nanos1 RNA translates poorly after injection into Xenopus oocytes. Thus, sequestration within germinal granules cannot explain translational control of nanos1 mRNA. Instead, they report, a secondary structural element immediately downstream of the mRNA start site is necessary and sufficient to repress the initiation of nanos1 translation through steric hindrance of ribosome scanning; insertion of 15 nucleotides between the start codon and this element relieves repression. Although structure-based translational regulation is common in prokaryotes it has not been observed before in eukaryotes and represents a new, developmentally important mode of nanos1 regulation.
