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Molecular and Cellular Biology, August 2000, p. 5897-5907, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Relief of Two Built-In Autoinhibitory Mechanisms in P-TEFb Is Required for Assembly of a Multicomponent Transcription Elongation Complex at the Human Immunodeficiency Virus Type 1 Promoter

Yick W. Fong and Qiang Zhou*

Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3206

Received 24 February 2000/Returned for modification 31 March 2000/Accepted 17 May 2000

Tat stimulation of human immunodeficiency virus type 1 (HIV-1) transcription requires Tat-dependent recruitment of human positive transcription elongation factor b (P-TEFb) to the HIV-1 promoter and the formation on the trans-acting response element (TAR) RNA of a P-TEFb-Tat-TAR ternary complex. We show here 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.

* Corresponding author. Mailing address: Department of Molecular and Cell Biology, 206 Stanley Hall, no. 3206, University of California, Berkeley, Berkeley, CA 94720. Phone: (510) 643-1697. Fax: (510) 643-9290. E-mail: qzhou{at}uclink4.berkeley.edu.

Molecular and Cellular Biology, August 2000, p. 5897-5907, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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