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Molecular and Cellular Biology, October 2007, p. 6996-7006, Vol. 27, No. 20
0270-7306/07/$08.00+0     doi:10.1128/MCB.00975-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Transcription-Dependent Dissociation of P-TEFb-HEXIM1-7SK RNA Relies upon Formation of hnRNP-7SK RNA Complexes

Charlotte Barrandon,¹ François Bonnet,¹ Van Trung Nguyen,¹ Valérie Labas,^2, and Olivier Bensaude^1*

UMR 8541 CNRS, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris Cedex 05,¹ UMR 7637 CNRS, Ecole Supérieure de Physique et Chimie Industrielles, 75005 Paris, France²

Received 2 June 2007/ Returned for modification 21 June 2007/ Accepted 3 August 2007

The positive transcription elongation factor P-TEFb controls the elongation of transcription by RNA polymerase II. P-TEFb is inactivated upon binding to HEXIM1 or HEXIM2 proteins associated with a noncoding RNA, 7SK. In response to the inhibition of transcription, 7SK RNA, as well as HEXIM proteins, is released by an unknown mechanism and P-TEFb is activated. New partners of 7SK RNA were searched for as potential players in this feedback process. A subset of heterogeneous ribonuclear proteins, hnRNPs Q and R and hnRNPs A1 and A2, were thus identified as major 7SK RNA-associated proteins. The degree of association of 7SK RNA with these hnRNPs increased when P-TEFb-HEXIM1-7SK was dissociated following the inhibition of transcription or HEXIM1 knockdown. This finding suggested that 7SK RNA shuttles from HEXIM1-P-TEFb complexes to hnRNPs. The transcription-dependent dissociation of P-TEFb-HEXIM1-7SK complexes was attenuated when both hnRNPs A1 and A2 were knocked down by small interfering RNA. As hnRNPs are known to interact transiently with RNA while it is synthesized, hnRNPs released from nascent transcripts may trap 7SK RNA and thereby contribute to the activation of P-TEFb.

Published ahead of print on 20 August 2007.

Present address: Service de Spectrométrie de Masse, Unité de Physiologie de la Reproduction et des Comportements, UMR CNRS 6175 INRA, 37380 Nouzilly, France.

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