The α-globin mRNA contains a C-rich stability element (CRE) in its 3′ untranslated region (3′ UTR) which is critical for the stability of this long-lived mRNA. A protein complex, termed the α-complex, forms on the CRE and has been shown to contribute to stabilization of the mRNA by at least two mechanisms, first by interacting with the poly(A)-binding protein (PABP) to prevent deadenylation, and second by protecting the mRNA from attack by an erythroid endoribonuclease. In this report, we demonstrate that the α-globin 3′ UTR can confer stability on a heterologous mRNA in cells, and this stability is dependent on the α-complex. Moreover, the stability was exclusively detected with cytoplasmic mRNA, suggesting that the regulation of α-globin mRNA stability is a cytoplasmic event. An additional mechanism by which the α-complex can confer stability on an RNA in vitro was also identified and shown to involve inhibition of 3′ to 5′ exonucleolytic degradation. Furthermore, using an in vitro mRNA decay system, we were able to follow the demise of the α-globin RNA and demonstrate that the decay was initiated by deadenylation followed by 3′-to-5′ decay carried out by the exosome and ultimately hydrolysis of the residual cap structure.

The cap structure and the poly(A) tail synergistically activate mRNA translation in vivo. Recent work using Saccharomyces cerevisiae spheroplasts and a yeast cell-free translation system revealed that the poly(A) tail can function as an independent promotor for ribosome recruitment, to internal initiation sites within an mRNA. This raises the question of how regulatory upstream open reading frames and translational repressor proteins binding to the 5′ UTR can function, as well as how regulated polyadenylation can support faithful activation of protein synthesis. We investigated the function of the regulatory upstream open reading frame 4 from the yeast GCN 4 gene and the effect of IRP-1 binding to an iron-responsive element introduced into the 5′ UTR of reporter mRNAs. Both manipulations effectively block cap-dependent translation, whereas ribosome recruitment promoted by the poly(A) tail under noncompetitive conditions can efficiently bypass both blocks. We show that the synergistic use of both, the cap structure and the poly-A tail enforced by mRNA competition reinstates the full extent of translational control by both types of 5′ UTR regulatory elements. With a view towards regulated polyadenylation, we studied the function of poly(A) tails of defined length on the translation of capped mRNAs. We find that poly(A) tail elongation increases translational efficiency, particularly under competitive conditions. Our results integrate recent findings on the function of the poly(A) tail into an understanding of translational control.