Activity of synthetic tat peptides in human immunodeficiency virus type 1 long terminal repeat-promoted transcription in a cell-free system.

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Abstract
The tat protein encoded by the human immunodeficiency virus type 1 is a potent trans-activator of gene expression from the viral long terminal repeat. The domains that are essential for trans-activation, a Pro-Xaa3-Pro triad, a cysteine-rich metal-binding sequence motif, and a cluster of basic residues, are present within the N-terminal 57 residues of tat. To determine the structural requirements for tat function and the role of metal binding at the transcription level alone, tat-(1-86) (full-length tat peptide), tat-(1-57), and tat-(1-47) were chemically synthesized. These peptides as well as the Cd2+ and Zn2+ complexes of tat-(1-86) and tat-(1-57) were evaluated for stimulation of transcription from the human immunodeficiency virus type 1 long terminal repeat by using cell-free in vitro methods. All three peptides produced a 7- to 9-fold increase over the basal level of transcription at a peptide concentration of 0.4 microM. Interestingly, at 4 microM, both tat-(1-57) and tat-(1-86) inhibited even the basal level of transcription. In contrast, tat-(1-47), which lacks the basic domain (residues 49-57), exhibited full stimulatory activity at 4 microM. Our data suggest, therefore, that the basic region may be responsible for the observed inhibitory activity of tat-(1-86) and tat-(1-57). Furthermore, binding to Zn2+ and not to Cd2+ ions only slightly augments (approximately 2-fold) the activity of the tat peptides.