# P-TEFb
**P-TEFb** (positive transcription elongation factor b) is a protein complex, composed of the kinase cyclin-dependent kinase 9 (CDK9) and a regulatory T-type cyclin (cyclin T1, T2a, or T2b), that releases promoter-proximally paused [RNA polymerase II](https://www.edgepedia.org/rna-polymerase-ii) (Pol II) into productive elongation and thereby regulates the expression of most eukaryotic protein-coding genes.[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) • [2](https://link.springer.com/article/10.1007/S00018-021-03878-8) Its dysregulation contributes to inflammatory and neoplastic conditions, and the complex is central to HIV transcription.[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637)
| Key fact | Detail |
|---|---|
| Composition | CDK9 plus cyclin T1 (CycT1), T2a, or T2b; human cells also contain 42-kDa and 55-kDa CDK9 isoforms produced from different transcription start sites in the first exon[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/) |
| Core function | Phosphorylates Ser2 in the Pol II CTD and the pause factors DSIF and NELF, triggering NELF dissociation and entry into productive elongation[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) |
| Pausing context | Pol II pauses about 30–100 bases downstream of the transcription start site, stabilized by DSIF and NELF[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) |
| Inactive reservoir | In growing cells, P-TEFb is held inactive by reversible association with HEXIM1/2 in the 7SK snRNP; CDK7 phosphorylation of CDK9 releases active P-TEFb[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC11580798/pdf/nihms-2019956.pdf) |
| Resting cells | P-TEFb falls apart and cyclin T1 is degraded by the proteasome[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) |
| HIV dependence | CycT1 (87 kDa) binds the HIV-1 Tat activation domain, mediating high-affinity, loop-specific binding to TAR RNA[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC11994015/) |
| Disease link | MLL gene rearrangements with super elongation complex components recruit P-TEFb aberrantly and drive leukemic transcription; CDK9 inhibitors are in development for cancer and autoimmune disease[7](https://www.cell.com/article/S1097276522011637/pdf) |
## Discovery
P-TEFb was identified in the early 1990s as a nuclear factor that stimulates transcription elongation.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/) The starting observation was that cells treated with the nucleoside analog DRB (5,6-dichloro-1-β-D-ribofuranosyl benzimidazole), a general transcription inhibitor, continued to produce only short, capped RNAs, indicating a block at the elongation step rather than at initiation.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/) Work in a fly in vitro system showed that DRB did not inhibit Pol II itself but a factor required for its elongation, most likely a kinase; the factor was named P-TEFb and purified as a heterodimer whose smaller subunit, about 42 kDa, was identified as CDK9.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/) Its cyclin subunit, CycT1, was then co-purified with the HIV transactivator of transcription (Tat), making P-TEFb central to understanding viral replication.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/) Removing P-TEFb from a nuclear extract incubated with a DNA template abolished production of long RNA transcripts while leaving short RNAs unaffected.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/)
## Role in pause release
Shortly after initiation, Pol II comes under the control of negative elongation factors and enters abortive elongation.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC85478/) In metazoans, the polymerase pauses in the promoter-proximal region, roughly 30–100 bases downstream of the transcription start site, because two factors act cooperatively: DSIF (a heterodimer of Spt4 and Spt5) and NELF (negative elongation factor).[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) A cryo-EM structure of the paused complex at 3.2 Å resolution showed that NELF binds the polymerase funnel, bridges two mobile polymerase modules, and contacts the trigger loop, restraining the Pol II mobility required for pause release; the paused complex also contains a tilted DNA–RNA hybrid that impairs substrate binding.[10](https://www.nature.com/articles/s41586-018-0442-2)
**Pause release by phosphorylation.** Recruitment of P-TEFb to the paused polymerase converts the paused state into productive elongation. The CDK9 kinase phosphorylates NELF and the Spt5 subunit of DSIF, triggering NELF dissociation; DSIF remains bound and switches from a negative to a positive elongation factor, stimulating Pol II and recruiting other elongation factors.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) CDK9 also phosphorylates serines at position 2 (Ser2) in the heptapeptide repeats of the Pol II C-terminal domain (CTD), a known mark of active elongation, and can additionally phosphorylate Ser5 and Ser7.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) Beyond these targets, CDK9 phosphorylates other elongation factors, chromatin modifiers, and RNA processing factors, so P-TEFb broadly encourages productive elongation through multiple phosphorylation targets in the elongation complex.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/) The kinase activity of P-TEFb is required in all assays requiring P-TEFb function; targeted recruitment of either CDK9 or cyclin T1 to DNA targets activates transcription, and a kinase-dead CDK9 mutant shows no activation.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC85478/)
## Regulation of P-TEFb activity
Two opposing pools govern how much active P-TEFb a cell has. In growing cells, a fraction of P-TEFb is sequestered in the 7SK snRNP, a ribonucleoprotein containing the 7SK snRNA and the HEXIM1 or HEXIM2 proteins, which reversibly inhibit the complex.[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) Release into the active pool is promoted by CDK7, which phosphorylates CDK9 in the T-loop to release active P-TEFb from HEXIM1/2 and 7SK snRNA.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC11580798/pdf/nihms-2019956.pdf) Protein phosphatases [PP2B](https://www.edgepedia.org/pp2b) and PP1α also assist in the release of P-TEFb from 7SK snRNP by dephosphorylating CDK9 pThr186.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/) In resting cells, P-TEFb falls apart and cyclin T1 is degraded by the proteasome.[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) The system is self-regulating: one of the genes that responds immediately to P-TEFb release is its own inhibitor hexim1, and newly expressed HEXIM1 re-incorporates P-TEFb into 7SK snRNP.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7070488/) Conversely, inhibiting CDK9 promotes P-TEFb release and activation as a cellular stress response, which complicates the interpretation of CDK9 inhibitor treatment.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7070488/)
Recruitment to specific genes occurs through transcription factors, chromatin-associated proteins such as BRD4, and the super elongation complex (SEC), which contains various combinations of ELL or ELL2, EAF1, ENL or AF9, and [AFF1](https://www.edgepedia.org/aff1) or AFF4; P-TEFb is an integral component of this larger complex.[1](https://www.sciencedirect.com/science/article/pii/S1097276522011637) • [12](https://link.springer.com/article/10.1186/s12977-024-00639-w) Antagonism also operates at the elongation complex itself, where phosphatases including PP2A and PP4 dephosphorylate P-TEFb substrates on Pol II.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/)
## Roles beyond early elongation
CDK9 functions at later stages of the transcription cycle as well. A second P-TEFb-dependent elongation checkpoint operates near terminal poly(A) sites: polymerase that escapes promoter-proximal pausing despite CDK9 inhibitors can elongate but prematurely terminates near these sites, a termination linked to loss of association of P-TEFb, DSIF, and poly(A) factors with Pol II.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/) Beyond the poly(A) site, CDK9 phosphorylates the 5′-to-3′ exoribonuclease Xrn2 on Thr439, enhancing its cleavage of the RNA transcript from Pol II and promoting termination.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/) P-TEFb is also involved in splicing and 3′ end formation, and later elongation stages on some genes are handled by other transcriptional CDKs, such as [CDK12](https://www.edgepedia.org/cdk12) and CDK11, that also phosphorylate Ser2 in the CTD.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/)
## P-TEFb in HIV and disease
HIV-1 exploits P-TEFb directly. The viral Tat protein binds the CycT1 subunit through its activation domain, and this interaction mediates high-affinity, loop-specific binding of the complex to TAR RNA, the stem-loop structure at the 5′ end of HIV transcripts, recruiting P-TEFb to the viral promoter to drive elongation.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC11994015/) Ectopic expression of human CycT1 in murine cells, which are normally refractory to HIV-1 transcription and replication, rescues Tat activity.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC11994015/) Crystal structures of the CDK9–CycT1–AFF4–Tat–TAR complex show the tight interface between P-TEFb, Tat, and TAR RNA.[12](https://link.springer.com/article/10.1186/s12977-024-00639-w)
Because P-TEFb is required for expression of most protein-coding genes, as shown by the sensitivity of HeLa cell transcription to the CDK9 inhibitor flavopiridol, its dysregulation has broad consequences.[13](https://journals.asm.org/doi/10.1128/mmbr.00011-06) Not all genes depend on it: certain p53 pathway genes can bypass the requirement, and the intronless histone H2b and U2 snRNA genes do not require P-TEFb for elongation.[13](https://journals.asm.org/doi/10.1128/mmbr.00011-06) In acute leukemia, translocations between the MLL gene and SEC components recruit P-TEFb aberrantly and drive abnormal transcription elongation.[7](https://www.cell.com/article/S1097276522011637/pdf) Many chemical CDK9 inhibitors have been developed for cancer and autoimmune diseases with varying degrees of success in clinical trials, and P-TEFb dysregulation more generally contributes to inflammatory and neoplastic conditions.[7](https://www.cell.com/article/S1097276522011637/pdf)
## References
1. [P-TEFb: The master regulator of transcription elongation, Molecular Cell (2023)](https://www.sciencedirect.com/science/article/pii/S1097276522011637)
2. [CDK9 keeps RNA polymerase II on track, Cellular and Molecular Life Sciences (2021)](https://link.springer.com/article/10.1007/S00018-021-03878-8)
3. [P-TEFb: The master regulator of transcription elongation, PMC full text](https://pmc.ncbi.nlm.nih.gov/articles/PMC9898187/)
4. [Mechanisms and Functions of the RNA Polymerase II General Transcription Machinery during the Transcription Cycle, Biomolecules (2024)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10886972/)
5. [CDK9 inhibitors for the treatment of solid tumors](https://pmc.ncbi.nlm.nih.gov/articles/PMC11580798/pdf/nihms-2019956.pdf)
6. [The HIV-1 Transcriptional Program: From Initiation to Elongation Control](https://pmc.ncbi.nlm.nih.gov/articles/PMC11994015/)
7. [P-TEFb: The master regulator of transcription elongation, Cell press PDF](https://www.cell.com/article/S1097276522011637/pdf)
8. [CDK9: A Comprehensive Review of Its Biology, and Its Role as a Potential Target for Anti-Cancer Agents](https://pmc.ncbi.nlm.nih.gov/articles/PMC8143439/)
9. [P-TEFb, a Cyclin-Dependent Kinase Controlling Elongation by RNA Polymerase II](https://pmc.ncbi.nlm.nih.gov/articles/PMC85478/)
10. [Structure of paused transcription complex Pol II–DSIF–NELF, Nature (2018)](https://www.nature.com/articles/s41586-018-0442-2)
11. [P-TEFb as A Promising Therapeutic Target](https://pmc.ncbi.nlm.nih.gov/articles/PMC7070488/)
12. [The cell biology of HIV-1 latency and rebound, Retrovirology (2024)](https://link.springer.com/article/10.1186/s12977-024-00639-w)
13. [The Yin and Yang of P-TEFb Regulation, Microbiology and Molecular Biology Reviews (2006)](https://journals.asm.org/doi/10.1128/mmbr.00011-06)
---
*Topic: Encyclopedia › Life and health › Biological foundations › RNA and gene regulation › Transcription and gene regulation › RNA polymerases and transcription machinery › Transcription elongation factors and pausing (single home)*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license