An internal ribosome entry segment promotes translation of the simian immunodeficiency virus genomic RNA

The retroviral genomic RNA is the messenger for the synthesis of the group- specific antigen (gag) and polymerase precursors of the major structural pr oteins and enzymes of the virion. The 5'-untranslated leader of the simian immunodeficiency virus (SIV) genomic RNA is formed of highly structured dom ains involved in key steps of the viral life cycle. Thus, the presence of s table RNA structures between the 5'-cap and the gag start codon are thought to strongly inhibit scanning of a 43 S preinitiation ribosomal complex. Th is prompted us to look for an alternative to the canonical ribosome scannin g. By using a standard bicistronic assay in the rabbit reticulocyte lysate, we show that the SIVmac 5'-leader contains an internal ribosome entry segm ent (IRES) and that gene expression driven by this IRES is stimulated upon cleavage of eukaryotic initiation factor 4G. Deletion analysis revealed tha t the sequence between the major splice donor and the gag AUG codon is requ ired for IRES activity. DNA transfection and viral transduction experiments in both NIH-3T3 and COS-7 cells confirmed that translation driven by the S IV leader is IRES-dependent and thus insensitive to the immunosuppressant r apamycin. Identification of an IRES in SIV is of particular interest for th e understanding of lentivirus replication and also for the design of novel lentiviral vectors suitable for gene transfer.