Next, we investigated the effect of a small amount of TAR, which activates PKR and thus interferes with translation initiation (21) . We used stable 293T transfectants expressing a dual-luciferase HIV reporter. Stable transfectants expressing a (-1) or (0) dual-luciferase HIV reporter were transfected with pTAR, pTARibulge Ã or pTARuucg Ã in amounts ranging from 0 to 2.3 mg. Figure 4A shows the effect of wild-type TAR. In the presence of a small quantity of TAR, the frameshift efficiency increases to about 140% of its value in absence of TAR but with a larger quantity of TAR, the frameshift efficiency decreases to about 80%, a decrease comparable to that observed with a transient transfection of pDual-HIV ( Figure 2 ). As a control, we used stable 293T transfectants expressing Rluc and Fluc in-frame, separated by a linker instead of the HIV-1 frameshift region. The ratio of Fluc activity to Rluc activity in lysates from these transfectants was unchanged in the presence of pTAR (data not shown), confirming that changes in the Fluc to Rluc ratio observed with stable transfectants expressing the dual-luciferase HIV reporter are due to variations in the frameshift efficiency. When the stable 293T transfectants expressing the dual-luciferase HIV reporter were transfected with plasmids producing TAR mutants that cannot bind PKR, the frameshift efficiency was unaltered ( Figure 4B) . The effect of a low amount of TAR was also assessed by transient co-transfection of Jurkat cells with pDual-HIV and different quantities of pTAR, ranging from 0 to 2 mg, the ratio of pTAR to pDual-HIV being equal or inferior to 1:1. The frameshift efficiency also increases under the conditions corresponding to low amounts of TAR, the highest increase being $140% of the frameshift efficiency without TAR (data not shown).