ACTIVATION OF A HETEROLOGOUS PROMOTER BY HUMAN-IMMUNODEFICIENCY-VIRUSTYPE-1 TAT REQUIRES SP1 AND IS DISTINCT FROM THE MODE OF ACTIVATION BY ACIDIC TRANSCRIPTIONAL ACTIVATORS

We have previously shown that the Tat protein of the human immunodefic iency virus type 1 (HIV-1) is a modular transcriptional activator that can he targeted upstream of either a synthetic promoter or the intact HIV promoter to activate transcription. This activation was shown to be largely dependent on the presence of consensus binding sites for th e cellular transcription factor Sp1. Since the use of heterologous pro moters may provide further insight into Tat-mediated transactivation, we have analyzed the transactivation of the thymidine kinase promoter of herpes simplex virus by Tat and by the acidic transcriptional trans activator VP16. The effects of mutations of defined upstream promoter elements show that Tat transactivation is dependent on Sp1 binding sit es in a site-specific manner. In contrast, transactivation by the acid ic transactivator VP16 is completely independent of any of the defined promoter elements upstream of the TATA box. These results suggest tha t Tat and the classically defined modular acidic transcriptional activ ators have different modes of transactivation. In addition, the substi tution of the HIV-1 TATA box for the thymidine kinase TATA box substan tially increases Tat transactivation, indicating that Tat transactivat ion may also ultimately involve TATA box-associated cellular transcrip tion factors.