R-REGION CDNA INSERTS IN RETROVIRAL VECTORS ARE COMPATIBLE WITH VIRUS-REPLICATION AND HIGH-LEVEL PROTEIN-SYNTHESIS FROM THE INSERT

Protein expression from retroviral vectors is often highest when the e xpressed cDNA is driven by the retroviral promoter. However, the typic al retroviral vector design places the cDNA downstream of the retrovir al packaging signal and far from the retroviral promoter. In an attemp t to improve protein production levels from cDNAs expressed in retrovi ral vectors, we inserted the MyoD or the purine nucleoside phosphoryla se (PNP) cDNAs into the R regions of both retroviral LTRs, close to th e retroviral promoter and just upstream of the polyadenylation signal present in each long terminal repeat (LTR). These R-region double-copy vectors could be produced in unrearranged form, although the titer wa s about seven-fold lower than that of typical vectors. R-region positi oning of the MyoD cDNA resulted in five-fold higher MyoD expression co mpared to MyoD expression in a typical vector, whereas PNP expression was not improved. Thus, R-region double-copy vectors provide an altern ative vector design that can improve protein expression from some cDNA s.