ENL initiates multivalent phase separation of the super elongation complex (SEC) in controlling rapid transcriptional activation

Abstract

Release of paused RNA polymerase II (Pol II) requires incorporation of the positive transcription elongation factor b (P-TEFb) into the super elongation complex (SEC), thus resulting in rapid yet synchronous transcriptional activation. However, the mechanism underlying dynamic transition of P-TEFb from inactive to active state remains unclear. Here, we found that the SEC components are able to compartmentalize and concentrate P-TEFb via liquid-liquid phase separation from the soluble inactive HEXIM1 containing the P-TEFb complex. Specifically, ENL or its intrinsically disordered region is sufficient to initiate the liquid droplet formation of SEC. AFF4 functions together with ENL in fluidizing SEC droplets. SEC droplets are fast and dynamically formed upon serum exposure and required for rapid transcriptional induction. We also found that the fusion of ENL with MLL can boost SEC phase separation. In summary, our results suggest a critical role of multivalent phase separation of SEC in controlling transcriptional pause release.

Chaos, Order and Systematics in Evolution of The Genetic Code

“…In this regard, we note that some human transcription factors include long polyglycine tracts. Human transcription is highly dependent on hydrogel (LLPS) compartments [13][14][15]18]. One example is the human androgen receptor, which includes a Gly23 tract.…”

“…For instance, LLPS compartments tend to be larger and easier to visualize in eukaryotic systems. As a critical phase of their evolution, Eukaryotes appear to have powerfully enriched LLPS systems by increasing the use of proteins that include intrinsically disordered regions with the potential for covalent modification and non-covalent bonding of diverse hydrogel components [13][14][15]17,18,32,33,132]. Examples of intrinsically disordered regions with these properties include histone tails and the carboxy-terminal domain of RNA polymerase II.…”

“…For instance, as RNA polymerase II traverses the transcription cycle, polymerase can move between LLPS compartments that support different phases of the cycle. Transcriptional super-enhancers that direct cell-specific gene regulation programs organize hydrogel compartments, and, in some cancers, super-enhancers become disorganized and sometimes fuse together [15,17]. The nucleolis is a hydrogel compartment for RNA polymerase I transcription and ribosome assembly [134].…”

“…Subsequently, another endosymbiotic event involving a cyanobacterium invading an alga gave rise to plants. Evolution of animal complexity required evolution of a promoter-proximal pausing mechanism of RNA polymerase II and the RNA polymerase II CTD (carboxy-terminal domain) [13][14][15]. The best descriptions of the key stages relate to evolution of biological coding, translation and transcriptional mechanisms.…”

“…It appears that Eukarya evolved a new use for hydrogels (liquid-liquid phase separation; LLPS) involving intrinsically disordered regions (IDRs) of proteins [13,17,32,33]. Histone tails and the carboxy-terminal domain (CTD) of RNA polymerase II are IDRs with regulated interactions and protein readers [13][14][15]. Other factors with IDRs cooperate in the transcription cycle, sequestering complexes in LLPS compartments.…”