| Abstract: | The effects of 5' proximal secondary structure in mRNA molecules on their translation and on their interaction with the eukaryotic initiation factors (eIF)-4F, eIF-4A, and eIF-4B have been examined. Secondary structures were generated in the 5' noncoding region of rabbit globin and reovirus mRNAs by means of hybridization with cDNA molecules. cDNAs hybridized to the first 15 bases downstream from the cap inhibited the translation of the mRNAs in both reticulocyte and wheat germ lysates. The degree of inhibition was directly related to the monovalent ion concentration and inversely related to reaction temperature. These hybrid structures also reduced the competitive ability of the messages. Hybrid structures beginning downstream from the first 15 bases did not inhibit the translation of beta-globin mRNA or reovirus s3 mRNA. None of the hybrid structures were detrimental to the interaction of the mRNAs with the 26-kDa cap binding protein of eIF-4F, as determined by chemical cross-linking assays. However, in the presence of ATP, hybrid structures immediately adjacent to the cap severely inhibited the cross-linking to the p46 subunit of eIF-4F or to additional eIF-4A or eIF-4B. In order to account for these observations, a two-step mechanism is proposed for the interaction of eIF-4F with the 5' end of an mRNA molecule. The first step involves a weak initial interaction of the p26 subunit with the cap. The second step requires the hydrolysis of ATP and results in the formation of a stable initiation factor-mRNA complex, which may involve eIF-4A and eIF-4B. This second step is inhibited by the presence of 5' proximal secondary structure. In any event, our results demonstrate that the effect of mRNA structure on translation rate depends strongly on its position with respect to the 5' end and that this effect is due at least in part to an inhibition of the action of initiation factors normally required for the unwinding of structure. |
