HIV TAT REPRESSES TRANSCRIPTION THROUGH SP1-LIKE ELEMENTS IN THE BASAL PROMOTER

MHC class I genes are potently repressed by HIV Tat, which transactiva tes the HIV LTR. Tat represses class I transcription by binding to com plexes associated with a novel promoter element, consisting of Sp1-lik e DNA binding sites. Transcription by other Sp1-dependent promoters, s uch as MDR1 and the minimal SV40 promoters, is also repressed by Tat, whereas the human p-actin promoter is neither activated by Sp1 nor rep ressed by Tat. Tat repression can be overcome by a strong enhancer ele ment. Thus, the SV40 72 bp enhancer element confers protection from Ta t-mediated repression on both the minimal SV40 promoter and the class I promoter. Surprisingly, Tat can activate the class I promoter in the presence of both the HIV TAR element and a strong upstream enhancer. These data demonstrate that Tat differentially affects Sp1-responsive promoters, depending on promoter architecture.