Translational control by messenger RNA competition for eukaryotic initiation factor 2

Abstract

Translation of globin mRNA in a micrococcal nuclease-treated reticulocyte lysate was studied in the presence of increasing amounts of Mengovirus RNA, under conditions in which the number of translation initiation events remains constant as judged by the transfer of label from N-formyl[35S]methionyl-tRNAf into protein. The translation of globin mRNA is progressively inhibited by low concentrations of Mengovirus RNA, free of detectable traces of double-stranded RNA, concomitant with the increasing synthesis of Mengovirus RNA-directed products. On a molar basis, Mengovirus RNA apparently competes about 35 times more effectively than globin mRNA for a critical component in translation. The competition is relieved by the addition of highly purified eukaryotic initiation factor 2 (eIF-2). Addition of eIF-2 does not stimulate overall protein synthesis, but shifts it in favor of globin synthesis. No stimulation of globin mRNA translation by eIF-2 is seen when Mengovirus RNA is absent. These experiments show that Mengovirus RNA competes, directly or indirectly, with globin mRNA for eIF-2. In direct binding experiments using isolated mRNA and eIF-2, Mengovirus RNA is shown to compete with globin mRNA for eIF-2 and to exhibit a 30-fold higher affinity for this factor. The binding of Mengovirus RNA to eIF-2 is much more resistant to increasing salt concentrations than is the binding of globin mRNA, again reflecting its high affinity. These results reveal a direct correlation between the ability of these mRNA species to compete in translation and their ability to bind to initiation factor eIF-2. They suggest that the affinity of a given mRNA species for eIF-2 is essential in determining its translation, relative to that of other mRNA species. Messenger RNA competition for eIF-2 may contribute significantly to the selective translation of viral RNA in infected cells.