Tat-SF: A Cellular Factor for Regulation of Transcriptional Elongation by HIV Tat

Deregulation of transcriptional elongation can cause the development of certain malignancies. HIV-1 Tat has been used as a model system to study the control of elongation by RNA polymerase ii. Tat stimulates HIV- 1 transcription by recognizing the TAR RNA structure near the 5 end of the nascent viral transcript and by recruiting the human transcription elongation factor P-TEFb to the HIV-1 promoter. Tat functions in human and primate cells, but not in cells of other species. To investigate whether human P-TEFb, composed of Cdk9 and CycT1, contributes to the species-specific Tat function, we generated and examined the activities of a panel of human-rodent hybrid P-TEFb complexes that consist of Cdk9 and CycT1 derived from the two different species. We found that P-TEFb complexes containing human CycT1 complexed with either human or rodent Cdk9 supported Tat transactivation and interacted with the Tat activation domain and TAR RNA to form ribonucleoprotein complexes. While a stable dimer containing rodent CycT1 and human Cdk9 was capable of mediating basal HIV-1 elongation, it failed to interact with Tat and to mediate Tat transactivation, indicating that the abilities of P-TEFb to support basal elongation and Tat activation can be separated. Our data indicated that the specific interaction of human P-TEFb with TatTAR mostly through CycT1 is crucial for P-TEFb to mediate a Tat- specific and species-restricted activation of HIV-1 transcription. Moreover, the cyclin-box of CycT1 and its immediate flanking region are responsible for the specific P-TEFbTat interaction.