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

摘要

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-stimulation of HIV-1 transcription requires Tat-dependent recruitment of the human transcription elongation factor P-TEFb to the HIV-1 promoter and the formation on the TAR RNA element a P-TEFb:Tat:TAR ternary complex. We recently found that the P-TEFb heterodimer of Cdk9/cyclin T1 is intrinsically incapable of forming a stable complex with Tat and TAR due to two built-in autoinhibitory mechanisms in P- TEFb. Both mechanisms exert little effect on the P-TEFb:Tat interaction, but prevent the P-TEFb:Tat complex from binding to TAR RNA The first autoinhibition arises from the unphosphorylated state of Cdk9 which establishes a P-TEFb conformation unfavorable for TAR-recognition. Autophosphorylation of Cdk9 overcomes this inhibition by inducing conformational changes in P-TEFb, thereby exposing a region in cyclin T1 for possible TAR-binding. An intramolecular interaction between the N- and C-terminal regions of cyclin T1 sterically blocks the P-TEFb:TAR interaction and constitutes the second autoinhibitory mechanism. This inhibition is relieved by the binding of the C-terminal region of cyclin T1 to the transcription elongation factor Tat-SF1 and perhaps other cellular factors. Upon release from the intramolecular interaction, the C-terminal region also interacts with RNA polymerase II and is required for HIV-1 transcription, suggesting its role in bridging the P-TEFb:Tat:TAR complex and the basal elongation apparatus. These data reveal novel control mechanisms for the assembly of a multicomponent transcription elongation complex at the HIV-1 promoter.