MicroRNAs inhibit protein synthesis in Drosophila by preventing the assembly of translation machinery, according to a study in this week's Nature. The researchers used a novel in vitro system to show that microRNAs can inhibit the initiation of protein synthesis."They have a nice system for repression, it seems, in an in vitro extract, and that's been what everybody's wanted," said Rachel Green of Johns Hopkins University in Baltimore, Md., who was not involved in the work.MicroRNAs regulate gene expression in plants and animals by repressing protein translation or degrading messenger RNA (mRNA) molecules, but their precise mechanism of action has been controversial, said senior author Matthias Hentze of the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany. Studies in Caenorhabditis elegans and human cells have shown that polysomes -- ribosomal complexes involved in translating mRNA into protein -- appear to be present on mRNAs repressed by microRNAs, suggesting that...

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