Transcriptional Activation of the Integrated Chromatin-Associated Human Immunodeficiency Virus Type 1 Promoter

Abstract

The regulation of human immunodeficiency virus type 1 (HIV-1) gene expression involves a complex interplay between cellular transcription factors, chromatin-associated proviral DNA, and the virus-encoded transactivator protein, Tat. Here we show that Tat transactivates the integrated HIV-1 long terminal repeat (LTR), even in the absence of detectable basal promoter activity, and this transcriptional activation is accompanied by chromatin remodeling downstream of the transcription initiation site, as monitored by increased accessibility to restriction endonucleases. However, with an integrated promoter lacking both Sp1 and NF-B sites, Tat was unable to either activate transcription or induce changes in chromatin structure even when it was tethered to the HIV-1 core promoter upstream of the TATA box. Tat responsiveness was observed only when Sp1 or NF-B was bound to the promoter, implying that Tat functions subsequent to the formation of a specific transcription initiation complex. Unlike Tat, NF-B failed to stimulate the integrated transcriptionally silent HIV-1 promoter. Histone acetylation renders the inactive HIV-1 LTR responsive to NF-B, indicating that a suppressive chromatin structure must be remodeled prior to transcriptional activation by NF-B. Taken together, these results suggest that Sp1 and NF-B are required for the assembly of transcriptional complexes on the integrated viral promoter exhibiting a continuum of basal activities, all of which are fully responsive to Tat.

Prostratin Antagonizes HIV Latency by Activating NF-κB

“…Nonetheless, RelA is required to promote efficient transcriptional elongation by the RNA polymerase II complex, probably through its ability to recruit the pTEFb complex that phosphorylates the C-terminal heptapeptide repeat of RNA polymerase II. Indeed, in the absence of Sp1, RelA is probably less efficient in driving LTR transcription (39). Determining the full range of transcription factors bound to the transcriptionally repressed and activated LTR in vivo will be instrumental in enhancing our understanding of the molecular events that govern HIV latency.…”

Tat Acetyl-acceptor Lysines Are Important for Human Immunodeficiency Virus Type-1 Replication

“…Previously, it has been shown that the two-exon form of Tat activates an integrated LTR more efficiently than the Tat one-exon form (50). In the context of chromatin, Tat also has to overcome the chromatin repression exerted by the nucleosomal architecture of the integrated provirus (51,52). In this respect, Tat has been shown to disrupt the repressive nucleosome 1 (nuc1) to activate the transcription from integrated LTR (52).…”

“…In the context of chromatin, Tat also has to overcome the chromatin repression exerted by the nucleosomal architecture of the integrated provirus (51,52). In this respect, Tat has been shown to disrupt the repressive nucleosome 1 (nuc1) to activate the transcription from integrated LTR (52). Thus, the inability of Tat K28R to activate an integrated LTR and, consequently, the reduced replication of viruses carrying this mutation may be due to the loss of its interaction with PCAF.…”

Silencing suppressors: viral weapons for countering host cell defenses

“…HIV-1 Tat is a multi-functional protein (El Kharroubi et al, 1998;Isel and Karn, 1999;Liu et al, 2002;Epie et al, 2005). In infected cells, Tat not only increases initiation of HIV-1 transcription but can also help to maintain the transcription process (Herrmann and Rice, 1995;Yang et al, 1997;Kiernan et al, 1999).…”