Recruitment of cyclin T1/P-TEFb to an HIV type I long terminal repeat promoter proximal RNA target is both necessary and sufficient for full activation of transcription

Transcriptional activation of the HIV type 1 (HIV-1) long terminal repeat ( LTR) promoter element by the viral Tat protein is an essential step in the HIV-1 life cycle. Tat function is mediated by the TAR RNA target element en coded within the LTR and is known to require the recruitment of a complex c onsisting of Tat and the cyclin T1 (CycT1) component of positive transcript ion elongation factor b (P-TEFb) to TAR. Here, we demonstrate that both TAR and Tat become entirely dispensable for activation of the HIV-1 LTR promot er when CycT1/P-TEFb is artificially recruited to a heterologous promoter p roximal RNA target. The level of activation observed was indistinguishable from the level induced by Tat and was neither inhibited nor increased when Tat was expressed in trans. Activation by artificially recruited CycT1 depe nded on the ability to bind the CDK9 component of P-TEFb. In contrast, alth ough binding to both Tat and TAR was essential for the ability of CycT1 to act as a Tat cofactor, these interactions became dispensable when CycT1 was directly recruited to the LTR. Importantly activation of the LTR both by T at and by directly recruited CycT1 was found to be at the level of transcri ption elongation. Together, these data demonstrate that recruitment of CycT 1/P-TEFb to the HIV-1 LTR is fully sufficient to activate this promoter ele ment and imply that the sole role of the Tat/TAR axis in viral transcriptio n is to permit the recruitment of CycT1/P-TEFb.