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.

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... alysis of truncated and full-length transcripts demonstrated that a transcriptional antitermination model does not fully account for trans activation.