A four-nucleotide translation enhancer in the 3′-terminal consensus sequence of the nonpolyadenylated mRNAs of rotavirus

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Abstract
The 5′ cap and poly(A) tail of eukaryotic mRNAs work synergistically to enhance translation through a process that requires interaction of the cap-associated eukaryotic initiation factor, eIF-4G, and the poly(A)-binding protein, PABP. Because the mRNAs of rotavirus, and other members of the Reoviridae, contain caps but lack poly(A) tails, their translation may be enhanced through a unique mechanism. To identify translation-enhancement elements in the viral mRNAs that stimulate translation in vivo, chimeric RNAs were prepared that contained an open reading frame for luciferase and the 5′ and 3′ untranslated regions (UTRs) of a rotavirus mRNA or of a nonviral mRNA. Transfection of the chimeric RNAs into rotavirus-infected cells showed that the viral 3′ UTR contained a translation-enhancement element that promoted gene expression. The element did not enhance gene expression in uninfected cells and did not affect the stability of the RNAs. Mutagenesis showed that the conserved sequence GACC located at the 3′ end of rotavirus mRNAs operated as an enhancement element. The 3′-GACC element stimulated protein expression independently of the sequence of the 5′ UTR, although efficient expression required the RNA to contain a cap. The results indicate that the expression of viral proteins in rotavirus-infected cells is specifically up-regulated by the activity of a novel 4-nt 3′ translation enhancer (TE) common to the 11 nonpolyadenylated mRNAs of the virus. The 4-nt sequence of the rotavirus 3′ TE represents by far the shortest of any of the sequence enhancers known to stimulate translation.