Using a dual-luciferase reporter system in Jurkat and 293T cells, we showed that the presence of TAR alters HIV-1 frameshift efficiency. The addition of a high amount of TAR, in cis or in trans of the reporter mRNA, decreases the frameshift efficiency. This effect is related to an inhibition of PKR. Conversely, a low amount of TAR increases the frameshift efficiency, by activating PKR.