Efficient expression of drug-selectable genes in retroviral vectors under control of an internal ribosome entry site.

We describe a new retroviral vector system pSXLC/pHa that utilizes a putative internal ribosome entry site (IRES) from encephalomyocarditis virus downstream from a multicloning site to co-express drug-selectable markers with a second non-selectable cDNA in a eukaryotic expression vector. The positive drug-selectable marker, MDR1, and the positive-negative marker, herpes simplex virus thymidine kinase (HSV-TK), were successfully introduced and expressed in the pSXLC/pHa system. The pSXLC-MDR and pSXLC-TK vectors contain the drug-selectable genes under translational control of the IRES and multiple cloning sites upstream for insertion of second cDNAs which can be co-expressed in this system. The inserts of these pSXLC plasmids were designed for easy transfer to the pHa retrovirus vector which has a strong promoter from Harvey murine sarcoma virus. The IRES-MDR-carrying retroviral vector, pHa-MCS-IRES-MDR, conferred resistance to vincristine and adriamycin. The IRES-TK-containing vector, pHa-MCS-IRES-TK conferred HAT-resistance in TK-deficient cells and the transfectants showed hypersensitivity to ganciclovir. These "flexible" vectors should be useful for co-expression of genes for selectable gene transfer and for positive-negative (suicide) selections in vitro and in vivo.