Abstract
Shortening of the poly(A) tail (deadenylation) is the first and often rate-limiting step in the degradation pathway of most eukaryotic mRNAs and is also used as a means of translational repression, in particular in early embryonic development. The nanos mRNA is translationally repressed by the protein Smaug in Drosophila embryos. The RNA has a short poly(A) tail at steady state and decays gradually during the first 2-3 h of development. Smaug has recently also been implicated in mRNA deadenylation. To study the mechanism of sequence-dependent deadenylation, we have developed a cell-free system from Drosophila embryos that displays rapid deadenylation of nanos mRNA. The Smaug response elements contained in the nanos 3'-untranslated region are necessary and sufficient to induce deadenylation; thus, Smaug is likely to be involved. Unexpectedly, deadenylation requires the presence of an ATP regenerating system. The activity can be pelleted by ultracentrifugation, and both the Smaug protein and the CCR4.NOT complex, a known deadenylase, are enriched in the active fraction. The same extracts show pronounced translational repression mediated by the Smaug response elements. RNAs lacking a poly(A) tail are poorly translated in the extract; therefore, SRE-dependent deadenylation contributes to translational repression. However, repression is strong even with RNAs either bearing a poly(A) tract that cannot be removed or lacking poly(A) altogether; thus, an additional aspect of translational repression functions independently of deadenylation.
