HIV TAT PROTEIN-REQUIREMENTS FOR TRANSACTIVATION AND REPRESSION OF TRANSCRIPTION ARE SEPARABLE

The HIV Tat protein, primarily characterized as a transcriptional acti vator of the viral long terminal repeat (LTR), is also a potent repres sor of major histocompatibility complex (MHC) class I transcription. I n the present study, we demonstrate that these two functional activiti es are distinct and mediated by discrete, but overlapping, structural domains of Tat. Tat repressor activity depends on C-terminal sequences , whereas transactivation depends on N-terminal sequences; both functi ons require core sequences. The repressor activity requires a domain e ncompassing the region encoded by the second exon of the Tat gene, beg inning at amino acid 73, with a C-terminal limit between amino acids 8 0 and 83. Tat repressor function also depends on the presence of a lys ine at position 41, located within the core of the protein. Tat repres sor activity is independent of two N-terminal domains essential for tr ansactivation: the acidic segment and the cysteine-rich region. Conver sely, Tat transactivation is independent of the second exon-encoded re gion of Tat. As further support for this novel model of separable Tat functions, we show that in murine fibroblasts, Tat represses class I p romoter activity, but does not transactivate the HIV LTR. We propose t hat distinct structural domains mediate the two functionally distinct activities associated with the Tat protein.