MESSENGER-RNAS CONTAINING THE UNSTRUCTURED 5'-LEADER SEQUENCE OF ALFALFA MOSAIC-VIRUS RNA-4 TRANSLATE INEFFICIENTLY IN LYSATES FROM POLIOVIRUS-INFECTED HELA-CELLS

Poliovirus infection is accompanied by translational control that prec ludes translation of 5'-capped mRNAs and facilitates translation of th e uncapped poliovirus RNA by an internal initiation mechanism. Previou s reports have suggested that the capped alfalfa mosaic virus coat pro tein mRNA (AIMV CP RNA), which contains an unstructured 5' leader sequ ence, is unusual in being functionally active in extracts prepared fro m poliovirus-infected HeLa cells (PI-extracts). To identify the eis-ac ting nucleotide elements permitting selective AIMV CP expression, we t ested capped mRNAs Containing structured or unstructured 5' leader seq uences in addition to an mRNA containing the poliovirus internal ribos ome entry site (IRES). Translations were performed with PI-extracts an d extracts prepared from mock-infected HeLa cells (MI-extracts). A num ber 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 ri bosome entry site, including AIMV CP RNA, was severely compromised in PI-extracts, and we find no evidence that the unstructured AIMV CP RNA 5' leader sequence acts in cia to bypass the poliovirus translational control. Nevertheless, cotranslation assays in the MI-extracts demons trate that mRNAs containing the unstructured AIMV CP RNA 5' untranslat ed region have a competitive advantage over those containing the rabbi t ol-globin 5' leader. Previous reports of AIMV CP RNA translation in PI-extracts likely describe inefficient expression that can be explain ed by residual cap-dependent initiation events, where AIMV CP RNA tran slation is competitive because of a diminished quantitative requiremen t for initiation factors.