HIV-1 tat associates with a ctd kinase that is the human homolog of drosophila P-TEFb

Tat greatly stimulates transcription from the viral promoter located in the long terminal repeat (LTR) of the HIV genome. It binds to the downstream trans-activation response (TAR) element and increases the processivity of poly morases that otherwise would terminate prematurely. Tat-transactivation is sensitive to DRB and requires the CTD (carboxy terminal domain) of the largest subunit of RNA polymerase II. Tat associates with a serine/threonine kinase through its activation domain. The Tat associated kinase (TAX) is a CTD kinase that hyperphosphorylates the C'TD and is sensitive to DRB. We have partially purified TAK from HeLa cells and shown that it is the human homolog of Drosophila P-TEFb. P-TEFb is a positive transcription elongation factor required to overcome abortive elongation. Purified Drosophila P-TEFb consists of two polypeptide chains. The small subunit was cloned and found to have similarity to cyclin dependent kinases. Its human homolog, identified in a sequence database, exhibits 70% identity to the Drosophila protein. The correspondence of TAK with human P-TEFb is evidenced by: (!) re action with specific antibodies that deplete P-TEFb activity simultaniously with TAK, (2) specificity for binding to the activation domain of Tat, (3) (auto)phosphorylation of the small subunit, and (4) inhibition of phosphorylation by the drugs DRB and H-8. These findings are consistent with a model in which TAR RNA recruits via Tat a CTD kinase that controls the processivity of RNA polymerase.