CHROMATIN DISRUPTION IN THE PROMOTER OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 DURING TRANSCRIPTIONAL ACTIVATION

Chromatin organization of eukaryotic promoters is increasingly recogni zed as an important factor in the regulation of transcription in vivo. To determine the role of chromatin in HIV-1 expression, we have exami ned the nucleosome organization of the promoter of HIV-1 under low and high transcription rates. Independently of the cell line examined, nu cleosomes are precisely positioned in the viral 5' long terminal repea t (5' LTR) and define two large nucleosome-free regions encompassing n t 200-450 and 610-720. A nucleosome positioned between these two regio ns, immediately after the transcription initiation site (nuc-1), is di srupted following TPA or TNF-alpha treatment. The disruption of nuc-1 from DNA is independent of DNA replication since it is completed in 20 min and independent of transcription as it is alpha-amanitin insensit ive. A model is proposed in which nuc-1 plays an organizing role in th e HIV-1 promoter to bring in close proximity factors bound to DNA in t he two nucleosome-free regions, upstream and downstream of the site of transcription initiation. These results define chromatin as an integr al component of the HIV-1 transcriptional regulatory machinery and ide ntify a chromatin transition associated with activation of viral gene expression.