POLY(A)-TAIL-PROMOTED TRANSLATION IN YEAST - IMPLICATIONS FOR TRANSLATIONAL CONTROL

The cap structure and the poly(A) tail synergistically activate mRNA t ranslation in vivo. Recent work using Saccharomyces cerevisiae spherop lasts and a yeast cell-free translation system revealed that the poly( A) tail can function as an independent promotor for ribosome recruitme nt, to internal initiation sites within an mRNA. This raises the quest ion of how regulatory upstream open reading frames and translational r epressor proteins binding to the 5' UTR can function, as well as how r egulated polyadenylation can support faithful activation of protein sy nthesis. We investigated the function of the regulatory upstream open reading frame 4 from the yeast GCN 4 gene and the effect of IRP-1 bind ing to an iron-responsive element introduced into the 5' UTR of report er mRNAs. Both manipulations effectively block cap-dependent translati on, whereas ribosome recruitment promoted by the poly(A) tail under no ncompetitive conditions can efficiently bypass both blocks. We show th at the synergistic use of both, the cap structure and the poly-A tail enforced by mRNA competition reinstates the full extent of translation al control by both types of 5' UTR regulatory elements. With a view to wards regulated polyadenylation, we studied the function of poly(A) ta ils of defined length on the translation of capped mRNAs. We find that poly(A) tail elongation increases translational efficiency, particula rly under competitive conditions. Our results integrate recent finding s on the function of the poly(A) tail into an understanding of transla tional control.