To screen for the formation of multinucleated syncytium in virus infected cells, 2 × 10 4 cells in six-well tissue culture plates or 10 5 cells in 60 mm plates were infected with virus-containing medium in the presence of 4 ug/ml polybrene. After 2-4 days, the cells were examined by light microscopy using objective lenses of 4×-20× and photo- The presence of the TAR RNA structure alters the programmed -1 ribosomal frameshift efficiency of the human immunodeficiency virus type 1 (HIV-1) by modifying the rate of translation initiation HIV-1 uses a programmed -1 ribosomal frameshift to synthesize the precursor of its enzymes, Gag-Pol. The frameshift efficiency that is critical for the virus replication, is controlled by an interaction between the ribosome and a specific structure on the viral mRNA, the frameshift stimulatory signal. The rate of cap-dependent translation initiation is known to be altered by the TAR RNA structure, present at the 5′ and 3′ end of all HIV-1 mRNAs. Depending upon its concentration, TAR activates or inhibits the double-stranded RNA-dependent protein kinase (PKR). We investigated here whether changes in translation initiation caused by TAR affect HIV-1 frameshift efficiency. CD4+ T cells and 293T cells were transfected with a dual-luciferase construct where the firefly luciferase expression depends upon the HIV-1 frameshift. Translation initiation was altered by adding TAR in cis or trans of the reporter mRNA. We show that HIV-1 frameshift efficiency correlates negatively with changes in the rate of translation initiation caused by TAR and mediated by PKR. A model is presented where changes in the rate of initiation affect the probability of frameshifting by altering the distance between elongating ribosomes on the mRNA, which influences the frequency of encounter between these ribosomes and the frameshift stimulatory signal. The precursor of HIV-1 structural proteins, Gag, and the precursor of the viral enzymes, Pol, are translated from the full-length viral messenger RNA (mRNA). Gag is produced by conventional translation whereas Pol requires a programmed -1 ribosomal frameshift during the elongation step of translation, which generates the fusion protein Gag-Pol (1, reviewed in 2, 3) . Previous studies showed that a 2-to 20-fold increase in the Gag-Pol to Gag ratio prevents viral infectivity (4-7) and our group showed that a decrease in the frameshift efficiency as low as 30% severely impairs the replication of the virus in cultured cells (8) . The Gag-Pol to Gag ratio is therefore critical for viral infectivity and the programmed -1 frameshift that determines this ratio represents an interesting target for the development of novel antiretroviral agents against HIV-1.