The critical factor that triggers the release of paused Pol II into productive elongation at almost all genes of multicellular organisms is P-TEFb, which is composed of the catalytic Cdk9 and a regulatory CycT1, T2a or T2b subunits (Figure 1, bottom). To accomplish the goal, P-TEFb phosphorylates the C-terminal domain of Pol II at Serine 2 residues (Ser2-P), which is the defining mark active of Pol II elongation, to tether additional elongation, RNA maturation and chromatin modifying factors to Pol II. At the same time, P-TEFb counteracts the actions of multi-subunit Pol II pause-inducing factors NELF and DSIF. Here, phosphorylation of the NELF-E subunit evicts NELF from Pol II, and phosphorylation of the Spt5 subunit of DSIF transforms it into a positive elongation factor, promoting the Pol II pause release.
Befitting the general role of P-TEFb in Pol II transcription, its kinase activity is kept under a tight control by the inhibitory 7SK snRNP complex (Figure 2). In fact, up to 90% of P-TEFb can be sequestered inside 7SK snRNP, in which kinase activity of CDK9 is repressed by dimeric P-TEFb inhibitors HEXIM1 and HEXIM2. In addition to these proteins, canonical 7SK snRNP is composed of the following core 7SK snRNP components: the non-coding 7SK snRNA, La-related protein family member LARP7 and the 7SK γ-methylphosphate capping enzyme MePCE. While 7SK snRNA functions as an RNA scaffold of the snRNP, LARP7 and MePCE promote 7SK stability by binding 3’ end of 7SK and capping 5’ end of 7SK, respectively.
Importantly, P-TEFb-dependent Pol II pause release is frequently dysregulated in cancers, particularly in those addicted to c-MYC and translocations of mixed-lineage leukemia (MLL) gene. Hence, understanding fundamental biology of P-TEFb will provide foundation for anti-cancer therapies of the future.
These are some of the questions that we are currently addressing:
- What are molecular mechanisms that drive the release of P-TEFb from 7SK snRNP?
- Does P-TEFb activation play a prominent role in response to genotoxic stress?
- Could we exploit the knowledge of P-TEFb biology for therapeutic purposes?