Structural requirements for trans activation of human immunodeficiency virus type 1 long terminal repeat-directed gene expression by tat: importance of base pairing, loop sequence, and bulges in the tat-responsive sequence.

In order to elucidate the molecular mechanisms of action of the tat-responsive sequence, mutational analysis of the tat-responsive sequence was carried out. The most critical region comprised nucleotides +18 to +44 and included the 3-nucleotide bulge at positions +23 to +25, the loop sequence, and an intact stem. In addition, base pairing up to nucleotide +52 was required for the full magnitude of the trans-activation response. Single-nucleotide bulges at positions +5 to +17 were dispensable. Analysis of truncated and full-length transcripts demonstrated that a transcriptional antitermination model does not fully account for trans activation.