The human positive transcription elongation factor P-TEFb, consisting of a CDK9/cyclin T1 heterodimer, functions as both a general and an HIV-1 Tat-specific transcription factor. P-TEFb activates transcription by phosphorylating RNA polymerase (Pol) II, leading to the formation of processive elongation complexes. As a Tat cofactor, P-TEFb stimulates HIV-1 transcription by interacting with Tat and the transactivating responsive (TAR) RNA structure located at the 5' end of the nascent viral transcript. Here we identified 7SK, an abundant and evolutionarily conserved small nuclear RNA (snRNA) of unknown function, as a specific P-TEFb-associated factor. 7SK inhibits general and HIV-1 Tat-specific transcriptional activities of P-TEFb in vivo and in vitro by inhibiting the kinase activity of CDK9 and preventing recruitment of P-TEFb to the HIV-1 promoter. 7SK is efficiently dissociated from P-TEFb by treatment of cells with ultraviolet irradiation and actinomycin D. As these two agents have been shown to significantly enhance HIV-1 transcription and phosphorylation of Pol II (refs 6,7,8), our data provide a mechanistic explanation for their stimulatory effects. The 7SK/P-TEFb interaction may serve as a principal control point for the induction of cellular and HIV-1 viral gene expression during stress-related responses. Our studies demonstrate the involvement of an snRNA in controlling the activity of a Cdk-cyclin kinase.

y g by addition of nonrecombinant pCDNA3, as necessary. Results represent three t o six different transfected cultures, with the experiments performed on at least two different days. For dark conditions, plates of NIH 313 cells were wrapped in foil immediately after transfection until the time of harvesting (48 hours), except for a single brief medium change performed under dim red light (Kodak Safelight, Wratten Series 1A) 24 hours before harvesting. 18. E. A. Griffin Jr., D. Staknis, C. J. Weitz, unpublished data. (17) and light conditions were processed in parallel and incubated together. For light conditions, cells were illuminated (Fiber-Lite High Intensity Illuminator, Series 180; set at 10 relative illumination units) immediately after transfection until the time of harvesting. Plates were positioned 15 t o 30 cm from the tips of the twin fiberoptic guides. Transfected cells under dark

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