HIV‐1 Tat protein promotes formation of more‐processive elongation complexes.

The Tat protein of HIV‐1 trans‐activates transcription in vitro in a cell‐free extract of HeLa nuclei. Quantitative analysis of the efficiency of elongation revealed that a majority of the elongation complexes generated by the HIV‐1 promoter were not highly processive and terminated within the first 500 nucleotides. Tat trans‐activation of transcription from the HIV‐1 promoter resulted from an increase in processive character of the elongation complexes. More specifically, the analysis suggests that there exist two classes of elongation complexes initiating from the HIV promoter: a less‐processive form and a more‐processive form. Addition of purified Tat protein was found to increase the abundance of the more‐processive class of elongation complex. The purine nucleoside analog, 5,6‐dichloro‐1‐beta‐D‐ribofuranosylbenzimidazole (DRB) inhibits transcription in this reaction by decreasing the efficiency of elongation. Surprisingly, stimulation of transcription elongation by Tat was preferentially inhibited by the addition of DRB.