THE SEQUENCE AND STRUCTURE OF THE 3'-ARM OF THE FIRST STEM-LOOP OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-3 TRANSACTIVATION RESPONSIVE REGION MEDIATE TAT-2 TRANSACTIVATION

Human immunodeficiency virus type 2 (HIV-2) cases AIDS, but generally after a much longer asymptomatic period than that which follows infect ion with HIV-1. At the molecular level, HIV-2 is much more closely rel ated to the simian immunodeficiency viruses than to HIV-1 and our prec ious studies have demonstrated that HIV-2 and HIV-1 enhancer stimulati on is mediated by different sets of cellular proteins following T-cell activation. Similar to HIV-1, HIV-2 encodes a transactivating protein , Tat, which appears to be necessary for viral replication and stimula tes viral transcriptional initiation and/or elongation. While Tat-1 bi nds to the RNA of the trans-activation responsive (TAR) region of HIV- 1 and HIV-2, cellular factors that bind to the RNA transcript are also necessary for Tat to function in vivo. Since almost all previous inve stigations of cellular cofactors for Tat had focused on HIV-1, we unde rtook studies aimed at understanding the interaction between the TAR R NA region of the HIV-2 promoter (TAR-2) and cellular proteins. By usin g extension inhibition analysis (toeprinting) and RNA electrophoretic mobility shift assays, we demonstrated binding of a nuclear factor(s) in T cells to the base of the promoter-proximal stem-loop structure. M utational analysis of this region revealed that both the sequence of t he 3' arm and the stem structure itself are important for activation o f the promoter by Tat-2. In contrast, the structure is necessary for a ctivation of TAR-2 by Tat-1 but the sequence is less important. These results suggest that a cellular factor interacts with the 3' arm of th e proximal stem-loop structure of TAR-2 and mediates Tat-2-induced inc reases in the level of HIV-2 transcripts.