TRANSCRIPTIONAL ACTIVATION OF THE INTEGRATED CHROMATIN-ASSOCIATED HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 PROMOTER

The regulation of human immunodeficiency virus type 1 (HIV-1) gene exp ression involves a complex interplay between cellular transcription fa ctors, chromatin-associated proviral DNA, and the virus-encoded transa ctivator protein, Tat. Here we show that Tat transactivates the integr ated HIV-1 long terminal repeat (LTR), even in the absence of detectab le basal promoter activity and this transcriptional activation is acco mpanied by chromatin remodelling downstream of the transcription initi ation site, as monitored by increased accessibility to restriction end onucleases. However, with an integrated promoter lacking both Sp1 and NF-kappa B sites, Tat was unable to either activate transcription or i nduce changes in chromatin structure even when it was tethered to the HIV-1 core promoter upstream of the TATA box. Tat responsiveness was o bserved only when Sp1 or NF-kappa B was bound to the promoter, implyin g that Tat functions subsequent to the formation of a specific transcr iption initiation complex. Unlike Tat, NF-kappa B failed to stimulate the integrated transcriptionally silent HIV-1 promoter. Histone acetyl ation renders the inactive HIV-1 LTR responsive to NF-kappa B, indicat ing that a suppressive chromatin structure must be remodelled prior to transcriptional activation by NF-kappa B. Taken together, these resul ts suggest that Sp1 and NF-kappa B are required for the assembly of tr anscriptional complexes on the integrated viral promoter exhibiting a continuum of basal activities, all of which are fully responsive to Ta t.