RAPID DEGRADATION OF AU-RICH ELEMENT (ARE) MESSENGER-RNAS IS ACTIVATED BY RIBOSOME TRANSIT AND BLOCKED BY SECONDARY STRUCTURE AT ANY POSITION-5' TO THE ARE

The 3' noncoding region (NCR) AU-rich element (ARE) selectively confer s rapid degradation on many mRNAs via a process requiring translation of the message. The role of cotranslation in destabilization of ARE mR NAs was examined by insertion of translation-blocking stable secondary structure at different sites in test mRNAs containing either the gran ulocyte-macrophage colony-stimulating factor (GM-CSF) ARE or a control sequence. A strong (-80 kcal/moI [1 kcal = 4.184 kJ) but not a modera te (-30 kcal/mol) secondary structure prevented destabilization of mRN As when inserted at any position upstream of the ARE, including in the 3' NCR. Surprisingly, a strong secondary structure did not block rapi d mRNA decay when placed immediately downstream of the ARE. Studies ar e also presented showing that the turnover of mRNAs containing control or ARE sequences is not altered by insertion of long (1,000-nucleotid e) intervening segments between the stop codon and the ARE or between the ARE and poly(A) tail. Characterization of ARE-containing mRNAs in polyadenylated and whole cytoplasmic RNA fractions failed to find evid ence for decay intermediates degraded to the site of strong secondary structure from either the 5' or 3' end. From these and other data pres ented, this study demonstrates that complete translation of the coding region is essential for activation of rapid mRNA decay controlled by the GM-CSF ARE and that the structure of the 3' NCR can strongly influ ence activation. The results are consistent with activation of ARE-med iated decay by possible entry of translation-linked decay factors into the 3' NCR or translation-coupled changes in 3' NCR ribonucleoprotein structure or composition.