Regulation of mRNA accumulation by a human immunodeficiency virus trans-activator protein.

HIV LTR-directed expression is markedly stimulated in trans by coexpression of a region of the HIV genome encoding a portion of the tat reading frame. Transient expression assay analysis reveals that trans-activation of LTR-directed expression results primarily from an increase in mRNA accumulation. Deletion analysis of the LTR indicates that upstream promoter and enhancer elements are dispensible for trans-activation, while sequences 3' of the RNA start site displaying strict orientation and position dependence are required. These sequences, contained in the 5' leader of all HIV transcripts, form a stable stem-loop structure with twofold symmetry in the cognate mRNA. Analysis of mutations in the trans-acting region demonstrates that the trans-activator is the protein product of the tat gene, identified biochemically in HIV-infected and transfected cells as an Mr 15,000 polypeptide. We discuss possible mechanisms whereby the interaction of p15tat with the dyad element promotes the accumulation of LTR-directed mRNA.