Mutational analysis of the trans-activation-responsive region of the human immunodeficiency virus type I long terminal repeat.

We used site-directed mutagenesis to delineate sequences within the human immunodeficiency virus type I (HIV-I) long terminal repeat (LTR) required for trans-activation by the viral tat gene product. We demonstrated that sequences 3' to LTR position +44 are dispensable for trans-activation but that almost all of the mutations tested located between positions -17 and +44 greatly reduced trans-activation at both the transcriptional and posttranscriptional levels. However, displacement of the HIV-I LTR trans-activation-responsive region (TAR) 3' by insertion of up to 32 base pairs between the LTR TATA box and cap site had little effect on trans-activation. An analysis of the DNase I hypersensitivity profile of the HIV-I LTR in transfected cultures suggested the presence of at least two DNase I-hypersensitive sites, including one which extends into the viral TAR element; however, neither of these sites appeared to be significantly affected by tat coexpression. These results allow more precise delineation of the sequences important for TAR function and suggest that the TAR may be recognized by a host-specific DNA-binding protein rather than by the tat protein directly.