Deadenylation of interferon‐β mRNA is mediated by both the AU‐rich element in the 3′‐untranslated region and an instability sequence in the coding region

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Abstract
Viral infection of fibroblastic and endothelial cells leads to the transient synthesis of interferon‐β (IFN‐β). The down‐regulation of IFN‐β synthesis after infection results both from transcriptional repression of the IFN‐β gene and rapid degradation of mRNA. As with many cytokine mRNAs, IFN‐β mRNA contains an AU‐rich element (ARE) in its 3′‐untranslated region (UTR). AREs are known to mediate mRNA deadenylation and destabilization. Depending on the class of ARE, deadenylation was shown to occur through synchronous or asynchronous mechanisms. In this study, we analysed IFN‐β mRNA deadenylation in natural conditions of IFN‐β synthesis, e.g., after viral infection. We show that human IFN‐β mRNA follows an asynchronous deadenylation pathway typical of a mRNA containing a class II ARE. A deletion analysis of the IFN‐β natural transcript demonstrates that poly(A) shortening can be mediated by the ARE but also by a 32 nucleotide‐sequence located in the coding region, that was identified previously as an instability determinant. In fact, these elements are able to act independently as both of them have to be removed to abrogate mRNA deadenylation. Our data also indicate that deadenylation occurs independently of mRNA translation. Moreover, we show that deadenylation of IFN‐β mRNA is not under the control of viral infection as IFN‐β mRNA derived from a constitutively expressed gene cassette is deadenylated in absence of viral infection. Finally, an unidentified nuclear event appears to be a prerequisite for IFN‐β mRNA deadenylation as IFN‐β mRNA introduced directly into the cytoplasm does not undergo deadenylation. In conclusion, our study demonstrates that IFN‐β mRNA poly(A) shortening is under the control of two cis‐acting elements recruiting a deadenylating machinery whose activity is independent of translation and viral infection but might require a nuclear event.