Transcription directed by the HIV long terminal repeat in vitro

The long terminal repeat (LTR) of the AIDS virus (HIV) has been found to contain promoter sequences that are active in uninfected HeLa whole cell and nuclear extracts. Here we report that elements upstream of position -104 (start site +1) do not affect transcriptional activity in vitro whereas sequences between -104 and -57 are required for such activity. Using a reconstituted RNA polymerase II system, we demonstrate that a partially purified fraction containing Spl not only stimulates, as was previously reported, but is required for accurate initiation of transcription directed by the HIV LTR. In addition, based on a computerized analysis, we report the presence of a region in the HIV LTR (positions -151 to -80) that is similar to the 72 base pair enhancer element of SV40 and that includes a highly conserved segment also present in the cytomegalovirus enhancer. Moreover, the HIV and HTLV-I LTRs are shown to share a region of similarity that includes the 21 base pair motif found in the enhancers of the human and bovine T-lymphotropic viruses. The R region of the HIV LTR is found to have two extensive regions of dyad symmetry rather than one as was previously reported. The significance of these observations for HIV pathogenesis is discussed.