Our results show that HIV-1 frameshift efficiency increases at a low concentration of TAR, when capdependent translation initiation is slowed down, whereas it decreases at a high concentration of TAR, when translation initiation is stimulated. These effects were shown to be dependent on PKR. A model is presented which relates the effects of TAR on frameshift efficiency to changes in the spacing between the elongating ribosomes on the mRNA caused by changes in the rate of translation initiation. Such changes affect the frequency of encounter between the ribosomes and the frameshift stimulatory signal.