Secondary structure in the 5 '-leader or 3 '-untranslated region reduces protein yield but does not affect the functional interaction between the 5 '-cap and the poly(A) tail

The 5'-cap structure and poly(A) tail of eukaryotic mRNAs cooperate to prom ote translation initiation but whether this functional interaction benefits certain classes of mRNAs has not been investigated. In this study, we inve stigate whether a structured 5'-leader or 3'-untranslated region (UTR) affe cts the cap/poly(A) tail interaction. A structured leader reduced the degre e to which the 5'-cap promoted translation in plant cells and inhibited tra nslation from capped and uncapped mRNAs equally in yeast, Secondary structu re within the 3'-UTR reduced translational efficiency when adjacent to the stop codon but had little effect on the cap/poly(A) tail synergy. The funct ional interaction between the cap and poly(A) tail was as important for an mRNA with a structured leader or 3'-UTR as it was for an unstructured mRNA in either species, suggesting that these structures can reduce translation without affecting the functional interaction between the cap and poly(A) ta il. However, the loss of Xrn1p, the major 5' -->3' exoribonuclease in yeast , abolished cap-dependent translation and the functional interaction betwee n the cap and poly(A) tail, suggesting that the cap/poly(ii) tail synergy i s of particular importance under conditions of active RNA turnover. (C) 199 9 Federation of European Biochemical Societies.