Functional Characterization of a Non-AUUUA AU-Rich Element from the c-jun Proto-Oncogene mRNA: Evidence for a Novel Class of AU-Rich Elements

AU-rich RNA-destabilizing elements (AREs) found in the 3' untranslated regions of many labile mRNAs encoding proto-oncoproteins and cytokines generally contain (i) one or more copies of the AUUUA pentanucleotide and (ii) a high content of uridylate and sometimes also adenylate residues. Recently, we have identified a potent ARE from the 3' untranslated region of c-jun proto-oncogene mRNA that does not contain the AUUUA motif. In an attempt to further our understanding of the general principles underlying mechanisms by which AREs direct rapid and selective mRNA degradation, in this study we have characterized the functionally important structural features and properties of this non-AUUUA ARE. Like AUUUA-containing AREs, this non-AUUUA ARE directs rapid shortening of the poly(A) tail as a necessary first step for mRNA degradation. It can be further dissected into three structurally and functionally distinct regions, designated domains I, II, and III. None of three domains alone is able to significantly destabilize the stable beta-globin mRNA. The two unlinked domains, I and III, together are necessary and sufficient for specifying the full destabilizing function of this non-AUUUA ARE. Domain II appears functionally dispensable but can partially substitute for domain I. Domain swaps made between the c-jun non-AUUUA and the c-fos AUUUA-containing AREs reveal that the two AREs, while sharing no sequence homology, appear to contain sequence domains that are structurally distinct but functionally overlapping and exchangeable. These data support the idea that the ultimate destabilizing function of an individual ARE is determined by its own unique combination of structurally distinct and functionally interdependent domains. Our polysome profile studies show tha the destabilizing function of the c-jun non-AUUUA ARE does not require any active transit by ribosomes of the mRNA bearing it, further corroborating that the destabilizing function of AREs is not tightly coupled to ongoing translation by ribosomes. Moreover, unlike AUUUA-containing AREs, the c-jun ARE is insensitive to blockage of its effects by addition of transcription inhibitors. Thus, our data provide further evidence for the existence of a novel class of ARE with unique properties.