The transition from transcription initiation into productive elongation is a rate-limiting, highly regulated step essential for gene activation. One key regulatory factor in this transition is the positive transcription elongation factor b (P-TEFb), which phosphorylates RNA Polymerase II and negative elongation factors at promoters to relieve the block in elongation. The role of P-TEFb in activating transcription elongation programs is undisputed, yet the mechanisms through which it aintains the active transcriptional state are poorly understood. Recent work by the D’Orso lab described a detailed molecular mechanism in which a novel transcriptional co-activator, the PPM1G phosphatase, regulates the assembly of a non-coding ribonucleoprotein complex (referred to as 7SK snRNP), which recruits P-TEFb and inactivates its kinase function. Although mechanisms of P-TEFb release from the snRNP are becoming clearer, how P-TEFb remains in the 7SK-unbound state to sustain transcription elongation programs remains unknown. In response to DNA damage signaling, nuclear factor (NF)-kB rapidly recruits PPM1G to its target genes where it dephosphorylates the P-TEFb kinase thereby promoting its release from the inactive 7SK snRNP complex and promoting transcription elongation. Subsequently, PPM1G binds 7SK RNA and the P-TEFb kinase inhibitor Hexim1 to prevent reassembly of P-TEFb onto the snRNP and sustain transcription. Once the damage is resolved, PPM1G is dislodged from the snRNP and P-TEFb recycled back to form an inactive kinase complex with the 7SK snRNP at the promoter thus preventing further transcription elongation. This study provides an unprecedented example of an enzyme that regulates transcriptional activation and maintenance in response to DNA damage signaling by formation of a protein-RNA complex.
Gudipaty SA1, McNamara RP1, Morton EL1, D’Orso I2.[expand title=”Show Affiliations”]
- Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.
- Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA [email protected]
Transcription elongation programs are vital for the precise regulation of several biological processes. One key regulator of such programs is the P-TEFb kinase, which phosphorylates RNA polymerase II (Pol II) once released from the inhibitory 7SK small nuclear ribonucleoprotein (snRNP) complex. Although mechanisms of P-TEFb release from the snRNP are becoming clearer, how P-TEFb remains in the 7SK-unbound state to sustain transcription elongation programs remains unknown. Here we report that the PPM1G phosphatase (inducibly recruited by nuclear factor κB [NF-κB] to target promoters) directly binds 7SK RNA and the kinase inhibitor Hexim1 once P-TEFb has been released from the 7SK snRNP. This dual binding activity of PPM1G blocks P-TEFb reassembly onto the snRNP to sustain NF-κB-mediated Pol II transcription in response to DNA damage. Notably, the PPM1G-7SK RNA interaction is direct, kinetically follows the recruitment of PPM1G to promoters to activate NF-κB transcription, and is reversible, since the complex disassembles before resolution of the program. Strikingly, we found that the ataxia telangiectasia mutated (ATM) kinase regulates the interaction between PPM1G and the 7SK snRNP through site-specific PPM1G phosphorylation. The precise and temporally regulated interaction of a cellular enzyme and a noncoding RNA provides a new paradigm for simultaneously controlling the activation and maintenance of inducible transcription elongation programs.
Copyright © 2015, American Society for Microbiology. All Rights Reserved.Go To Mol Cell Biol.