Selective degradation of early-response-gene mRNAs: functional analyses of sequence features of the AU-rich elements.

The metabolic lifetime of mRNA can be specified by specific cis-acting elements within mRNA. One type of element is an adenylate- and uridylate-rich element (ARE) found in the 3' untranslated region of many highly unstable mRNAs for mammalian early-response genes (ERGs). Among the better-characterized members of the ERG family are certain genes encoding nuclear transcription factors. Of particular significance was the finding that their mRNAs decay rapidly with kinetics similar to those of c-fos mRNA. Our previous studies of the c-fos ARE-directed mRNA decay have revealed the existence in this ARE of two structurally distinct and functionally interdependent domains, termed domain I and domain II. We proposed that the c-fos ARE-directed decay is a two-step mechanism in which rapid shortening of the poly(A) tail leads to the decay of the mRNA body and further hypothesized that this is a general mechanism by which the ERG AREs mediate rapid mRNA degradation. To test this hypothesis and to further address the generality of the critical structural characteristics within the c-fos ARE, the RNA-destabilizing functions of more than 10 different AU-rich sequences from various nuclear transcription factor mRNAs have been tested. Consistent with the above-mentioned hypothesis is the observation that mRNAs carrying the functional AREs display a biphasic decay, which is characteristic of the proposed two-step mechanism. Our results indicated that the presence of AUUUA pentanucleotides in an AU-rich region does not always guarantee an RNA-destabilizing function for this region. Our results also led to the identification of a novel class of AU-rich destabilizing elements which contains no AUUUA pentanucleotide. The results of sequence comparison and functional tests revealed that a continuous U-rich sequence is a unique feature among the functional AREs. Finally, our experiments further showed that the c-fos ARE domain II has an RNA decay-enhancing ability upon its fusion to heterologous AU-rich regions and defined for the first time an RNA decay-enhancing element, which we termed the RDE element.