Occlusion of the HIV poly(A) site.

To investigate the selective use of poly(A) sites in the 3' long terminal repeat (LTR) but not the 5' LTR of retroviruses, we have studied the poly(A) site of the human immunodeficiency virus (HIV-1). Using hybrid HIV/alpha-globin gene constructs, we demonstrate that the HIV poly(A) site is inactive or occluded when adjacent to an active promoter, either the homologous HIV promoter or the alpha-globin gene promoter. Furthermore, this occlusion of the HIV poly(A) site occurs over a considerable distance of up to at least 500 bp. In contrast, two nonretroviral poly(A) sites [alpha-globin and a synthetic poly(A) site] are active when close to a promoter. We also show that a short fragment of approximately 60 nucleotides containing the HIV poly(A) site is fully active when placed at the 3' end of the human alpha-globin gene or within the rabbit beta-globin gene. This result rules out the requirement of more distant upstream elements for the activity of the HIV poly(A) site, as has been suggested for other viral poly(A) sites. Finally, we show that the GT-rich downstream region of the HIV poly(A) site confers poly(A) site occlusion properties on a synthetic poly(A) site. This result focuses attention on this more variable part of a poly(A) site in retroviruses as a possible general signal for poly(A) site occlusion

Tat-dependent occlusion of the HIV poly(A) site.

Retroviruses must ensure that poly(A) signals in the 3' LTR are highly active, while identical signals in the 5' LTR are inactive (occluded). In the case of HIV-1, both promoter proximity in the 5' LTR and U3 sequences in the 3' LTR may contribute to this regulation. We have discovered a novel regulatory mechanism for poly(A) site occlusion in HIV-1. When transcription initiation from the HIV promoter is activated by Tat, the HIV poly(A) site is specifically occluded, while other poly(A) sites are unaffected by Tat. Nucleotide signals associated with this Tat effect are immediately adjacent to the AAUAAA sequence of the HIV-1 poly(A) signal. These data suggest that elongating RNA polymerase II, activated by Tat specifically occludes the HIV poly(A) site