TRANSCRIPTION ELONGATION-FACTOR P-TEFB IS REQUIRED FOR HIV-1 TAT TRANSACTIVATION IN-VITRO

P-TEFb is a key regulator of the process controlling the processivity- of RNA polymerase II and possesses a kinase activity that can phosphor ylate the carboxy-terminal domain of the largest subunit of RNA polyme rase II. Here we report the cloning of the small subunit of Drosophila P-TEFb and the finding that it encodes a Cdc2-related protein kinase. Sequence comparison suggests that a protein with 72% identity, PITALR E, could be the human homolog of the Drosophila protein. Functional ho mology was suggested by transcriptional analysis of an RNA polymerase II promoter with HeLa nuclear extract depleted of PITALRE. Because the depleted extract lost the ability to produce long DRB-sensitive trans cripts and this loss was reversed by the addition of purified Drosophi la P-TEFb, we propose that PITALRE is a component of human P-TEFb. In addition, we found that PITALRE associated with the activation domain of HIV-1 Tat, indicating that P-TEFb is a Tat-associated kinase (TAK). An in vitro transcription assay demonstrates that the effect of Tat o n transcription elongation requires P-TEFb and suggests that the enhan cement of transcriptional processivity by Tat is attributable to enhan ced function of P-TEFb on the HIV-1 LTR.