Figure 4

Mechanisms of transcriptional activation by the viral protein Tat. (A) In the absence of Tat, transcription from the HIV-1 5'LTR produces predominantly short mRNAs as a result of the activity of the negative elongation factor N-TEF, composed of NELF and DSIF, which binds to the hypophosphorylated RNA polymerase II and impedes transcriptional elongation. (B) Following the synthesis of the first molecules of Tat, this viral protein migrates to the nucleus. Tat then binds to the RNA hairpin TAR, located in the 5' region of all nascent HIV-1 transcripts and activates viral transcription by recruiting the positive elongation factor pTEFb, composed of Cdk9 and CyclT1. This recruitment is enhanced through Tat acetylation by PCAF on K28, located in the transactivation domain of the viral Tat protein. Cdk9 phosphorylates the CTD domain of RNAPII, leading to processive transcriptional elongation and to the dissociation of N-TEF. Acetylation of Tat on K50 by p300 and Gcn5 promotes the release of pTEFb [246], dissociation of Tat from TAR and its subsequent transfer to the elongating polymerase complex. Tat then recruits PCAF to the elongation complex. Tat also recruits the ATP-dependent remodeling complex SWI/SNF. Another model based on FRAP experiments propose that the Tat/pTEFb complex dissociates from the RNAPII complex following transcription initiation and undergoes subsequent cycles of association/dissociation [249]. (C) At the end of the elongation process, Tat deacetylation by the class III HDAC Sirtuin 1 allows its dissociation from RNAPII and from PCAF, and the recycling of Tat initiates a new cycle of transcriptional activation.