To further confirm that inhibiting PKR decreases HIV-1 frameshift efficiency, a plasmid expressing Tat, a HIV-1 viral protein, was co-transfected with the dualluciferase plasmids. In addition to its well-characterized transactivation effect on transcription of the viral mRNAs by binding to TAR, Tat influences translation by inhibiting PKR, either directly by binding this kinase or indirectly by blocking the binding of TAR to PKR (32, 33) . We used a Tat mutant (Tat Ã ) that can bind TAR and inhibit PKR but cannot transactivate transcription, and, thereby, that does not affect mRNA levels (24) . Jurkat cells were co-transfected with the plasmid coding for this Tat mutant and with pDual-HIV, pDual-HIV-TAR or pDual-HIV-50TAR. In the presence of Tat Ã , the frameshift efficiency was decreased to approximately 60% of its value in absence of Tat Ã ( Figure 3C ). The decrease with Tat Ã was the same, whether TAR was present or not, which suggests that Tat Ã and TAR both act via the same mechanism, the inhibition of PKR.