mRNAs Containing the Unstructured 59 Leader Sequence of Alfalfa Mosaic Virus RNA 4 Translate Inefficiently

Poliovirus infection is accompanied by translational control that precludes translation of 5-capped mRNAs and facilitates translation of the uncapped poliovirus RNA by an internal initiation mechanism. Previous reports have suggested that the capped alfalfa mosaic virus coat protein mRNA (AlMV CP RNA), which contains an unstructured 5 leader sequence, is unusual in being functionally active in extracts prepared from poliovirus-infected HeLa cells (PI-extracts). To identify the cis-acting nucleotide elements permitting selective AlMV CP expression, we tested capped mRNAs containing structured or unstructured 5 leader sequences in addition to an mRNA containing the poliovirus internal ribosome entry site (IRES). Translations were performed with PI-extracts and extracts prepared from mock-infected HeLa cells (MI-extracts). A number of control criteria demonstrated that the HeLa cells were infected by poliovirus and that the extracts were translationally active. The data strongly indicate that translation of RNAs lacking an internal ribosome entry site, including AlMV CP RNA, was severely compromised in PI-extracts, and we find no evidence that the unstructured AlMV CP RNA 5 leader sequence acts in cis to bypass the poliovirus translational control. Nevertheless, cotranslation assays in the MI-extracts demonstrate that mRNAs containing the unstructured AlMV CP RNA 5 untranslated region have a competitive advantage over those containing the rabbit a-globin 5 leader. Previous reports of AlMV CP RNA translation in PI-extracts likely describe inefficient expressio