Figure 1

A simplified view of the multiple mechanisms of transcriptional interference implicated in HIV-1 postintegration latency. (a) HIV-1 integrates into the host cell genome predominantly in intronic regions of actively transcribed genes [55–57]. Transcriptional interference may lead to the establishment of latency by different mechanisms depending at least on the orientation of viral integration compared to the host gene. (b) steric hindrance: when proviral integration occurs in the same transcriptional orientation as the cellular host gene, "read-through" RNA polymerase II (RNAPII) transcription from the upstream promoter displaces key transcription factors (TFs) from the HIV-1 promoter [60] and prevents assembly of the pre-initiation complex on the viral promoter. The integrated virus is thought to be transcribed along with the other intronic regions of the cellular gene, but is then merely spliced out. HIV-1 transcription inhibition could be reversed by hindering the upstream transcription or by cooperatively activating viral transcription initiation and elongation; certain host transcription factors and/or viral activators, which bind strongly to their cognate site, could resist the "read-through" RNAPII passage [61]. This phenomenon was also observed following Tat-mediated transactivation of HIV-1 transcription [67]. (c) promoter occlusion: provirus integration in the opposite orientation compared to the host gene may lead to collisions of the elongating RNA polymerases from each promoter, resulting in a premature termination of transcription from the weaker or from both promoters. (d) enhancer trapping: an enhancer of one gene (the 5'LTR enhancer of HIV-1 in this case) is placed out of context near the promoter of a second gene (a cellular gene in this case) and acts on the transcriptional activity of this cellular promoter, thereby preventing the enhancer action on the 5'LTR.