EXON2 OF HIV-2 TAT CONTRIBUTES TO TRANSACTIVATION OF THE HIV-2 LTR BYINCREASING BINDING-AFFINITY TO HIV-2 TAR RNA

Human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) express r elated Tat proteins that are encoded in two exons. Tat proteins bind d irectly to the TAR RNA element contained in the 5' ends of viral trans cripts and thereby stimulate transcription through an as yet unidentif ied mechanism. We have investigated the functional significance of exo n2 of the HIV-2 Tat protein by examining properties of proteins consis ting of exon1 alone or exon1+2. In transactivation assays in vivo, exo n2 modestly increased HIV-2 Tat stimulation of transcription from the HIV-2 long terminal repeat (LTR) but had no effect on transcription fr om the HIV-1 LTR. in HeLa cells, exon2 increased transactivation of th e HIV-2 LTR by approximately three-fold, while in COS and Jurkat cells this value was less than two-fold. In binding assays in vitro, exon2 increased the binding affinity of the HIV-2 Tat protein to HIV-2 TAR R NA. Results with GAL4 fusion proteins and a synthetic promoter contain ing GAL4 DNA binding sites indicated that exon2 does not contribute to the HIV-2 Tat activation domain. These observations suggest that exon 2 of HIV-2 Tat contributes to transactivation of the HIV-2 LTR by incr easing the binding affinity to HIV-2 TAR RNA.