AUUUA IS NOT SUFFICIENT TO PROMOTE POLY(A) SHORTENING AND DEGRADATIONOF AN MESSENGER-RNA - THE FUNCTIONAL SEQUENCE WITHIN AU-RICH ELEMENTSMAY BE UUAUUUA(U A)(U/A)/

AU-rich elements (AREs) in the 3' untranslated regions of several cyto kine and oncogene mRNAs have been shown to function as signals for rap id mRNA degradation, and it is assumed that the many other cytokine an d oncogene mRNAs that contain AU-rich sequences in the 3' untranslated region are similarly targeted for rapid turnover. We have used a chim eric gene composed mostly of growth hormone sequences with expression driven by the c-fos promoter to investigate the minimal sequence requi red to act as a functional destabilizing element and to monitor the ef fect of these sequences on early steps in the degradation pathway. We find that neither AUUUA, UAUUUA, nor AUUUAU can function as a destabil izing element. However, the sequence UAUUUAU, when present in three co pies, is sufficient to destabilize a chimeric mRNA. We propose that th is sequence functions by virtue of being a sufficient portion of the l arger sequence, UUAUUUA(U/A)(U/A), that we propose forms the optimal b inding site for a destabilizing factor. The destabilizing effect depen ds on the number of copies of this proposed binding site and their deg ree of mismatch in the first two and last two positions, with mismatch es in the AUUUA sequence not being tolerated. We found a strict correl ation between the effect of an ARE on degradation rate and the effect on the rate of poly(A) shortening, consistent with deadenylation being the first and rate-limiting step in degradation, and the step stimula ted by destabilizing AREs. Deadenylation was observed to occur in at l east two phases, with an oligo(A) intermediate transiently accumulatin g, consistent with the suggestion that the degradation processes may b e similar in yeast and mammalian cells. AREs that are especially U ric h and contain no UUAUUUA(U/A)(U/A) motifs failed to influence the degr adation rate or the deadenylation rate, either when downstream of subo ptimal destabilizing AREs or when alone.