THE NONAMER UUAUUUAUU IS THE KEY AU-RICH SEQUENCE MOTIF THAT MEDIATESMESSENGER-RNA DEGRADATION

Labile mRNAs that encode cytokine and immediate-early gene products of ten contain AU-rich sequences within their 3' untranslated region (UTR ). These AU-rich sequences appear to be key determinants of the short half-lives of these mRNAs, although the sequence features of these ele ments and the mechanism by which they target mRNAs for rapid decay hav e not been fully defined, We have examined the features of AU-rich ele ments (AREs) that are crucial for their function as determinants of mR NA instability in mammalian cells by testing the ability of various mu tant c-fos AREs and synthetic AREs to direct rapid mRNA deadenylation and decay when inserted within the 3' UTR of the normally stable beta- globin mRNA. Evidence is presented that the pentamer AUUUA, which prec iously was suggested to be the minimal determinant of instability pres ent in mammalian AREs, cannot direct rapid mRNA deadenylation and deca y. Instead, the nonamer UUAUUUAUU is the elemental AU-rich sequence mo tif that destabilizes mRNA. Removal of one uridine residue from either end of the nonamer (UUAUUUAU or UAUUUAUU) results in a decrease of po tency of the element, while removal of a uridine residue from both end s of the nonamer (UAUUUAU) eliminates detectable destabilizing activit y. The inclusion of an additional uridine residue at both ends of the nonamer (UUUAUUUAUUU) does not further increase the efficacy of the el ement. Taken together, these findings suggest that the nonamer UUAUUUA UU is the minimal AU-rich motif that effectively destabilizes mRNA. Ad ditional ARE potency is achieved by combining multiple copies of this nonamer in a single mRNA 3' UTR Furthermore, analysis of poly(A) short ening rates for ARE-containing mRNAs reveals that the UUAUUUAUU sequen ce also accelerates mRNA deadenylation and suggests that the UUAUUUAUU motif targets mRNA for rapid deadenylation as an early step in the mR NA decay process.