Pol II phosphorylation regulates a switch between transcriptional and splicing condensates

The synthesis of pre-mRNA by RNA polymerase II (Pol II) involves the formation of a transcription initiation complex, and a transition to an elongation complex. The large subunit of Pol II contains an intrinsically disordered C-terminal domain that is phosphorylated by cyclin-dependent kinases during the transition from initiation to elongation, thus influencing the interaction of the C-terminal domain with different components of the initiation or the RNA-splicing apparatus. Recent observations suggest that this model provides only a partial picture of the effects of phosphorylation of the C-terminal domain. Both the transcription-initiation machinery and the splicing machinery can form phase-separated condensates that contain large numbers of component molecules: hundreds of molecules of Pol II and mediator are concentrated in condensates at super-enhancers, and large numbers of splicing factors are concentrated in nuclear speckles, some of which occur at highly active transcription sites. Here we investigate whether the phosphorylation of the Pol II C-terminal domain regulates the incorporation of Pol II into phase-separated condensates that are associated with transcription initiation and splicing. We find that the hypophosphorylated C-terminal domain of Pol II is incorporated into mediator condensates and that phosphorylation by regulatory cyclin-dependent kinases reduces this incorporation. We also find that the hyperphosphorylated C-terminal domain is preferentially incorporated into condensates that are formed by splicing factors. These results suggest that phosphorylation of the Pol II C-terminal domain drives an exchange from condensates that are involved in transcription initiation to those that are involved in RNA processing, and implicates phosphorylation as a mechanism that regulates condensate preference

Mediator Condensates Localize Signaling Factors to Key Cell Identity Genes

The gene expression programs that define the identity of each cell are controlled by master transcription factors (TFs) that bind cell-type-specific enhancers, as well as signaling factors, which bring extracellular stimuli to these enhancers. Recent studies have revealed that master TFs form phase-separated condensates with the Mediator coactivator at super-enhancers. Here, we present evidence that signaling factors for the WNT, TGF-β, and JAK/STAT pathways use their intrinsically disordered regions (IDRs) to enter and concentrate in Mediator condensates at super-enhancers. We show that the WNT coactivator β-catenin interacts both with components of condensates and DNA-binding factors to selectively occupy super-enhancer-associated genes. We propose that the cell-type specificity of the response to signaling is mediated in part by the IDRs of the signaling factors, which cause these factors to partition into condensates established by the master TFs and Mediator at genes with prominent roles in cell identity

Coactivator condensation at super-enhancers links phase separation and gene control

Super-enhancers (SEs) are clusters of enhancers that cooperatively assemble a high density of the transcriptional apparatus to drive robust expression of genes with prominent roles in cell identity. Here we demonstrate that the SE-enriched transcriptional coactivators BRD4 and MED1 form nuclear puncta at SEs that exhibit properties of liquid-like condensates and are disrupted by chemicals that perturb condensates. The intrinsically disordered regions (IDRs) of BRD4 and MED1 can form phase-separated droplets, and MED1-IDR droplets can compartmentalize and concentrate the transcription apparatus from nuclear extracts. These results support the idea that coactivators form phase-separated condensates at SEs that compartmentalize and concentrate the transcription apparatus, suggest a role for coactivator IDRs in this process, and offer insights into mechanisms involved in the control of key cell-identity genes

Enhancers and phase separation in the control of gene expression

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biology, 2020Cataloged from student-submitted PDF of thesis.Includes bibliographical references.Gene regulation underlies the control of cell identity, development, and disease. Transcription of genes is regulated by DNA elements called enhancers, which are bound by transcription factors and coactivators, leading to the recruitment of RNA polymerase II and the production of RNA. Enhancers are thought to loop to specific gene promoters to stimulate transcription, but the mechanisms that cause enhancers to selectively loop to specific gene promoters is not well understood. In this thesis, I first describe new insights into enhancer-promoter loop specificity from studies examining the mechanisms that allow tumor-specific super-enhancers to loop to the MYC oncogene in diverse cancer types (Schuijers and Manteiga et al., 2018). While conducting these studies, it was proposed that super-enhancers and the factors associated with them form liquid-liquid phase-separated condensates.Following this proposal, I contributed to collaborative studies that strongly supported this model (Boija et al., 2018; Sabari et al., 2018, see Appendix I and II of this thesis). This model of transcription led me to ask how key transcriptional components could be recruited into super-enhancer condensates. I performed studies showing that the interaction of RNA polymerase II with these condensates involves the large heptapeptide repeat of the C-terminal domain (CTD) of the enzyme. Furthermore, these studies provided evidence that phosphorylation of the CTD, which is associated with the initiation to elongation transition, weakens these interactions, thus facilitating the transition of RNA polymerase II into different condensates involved in co-transcriptional splicing of the nascent transcript (Guo and Manteiga et al., 2019).These studies provide new insights into the mechanisms of enhancer-promoter interaction, roles for the RNA polymerase II CTD in the enzyme's partitioning into nuclear condensates, and a role for phosphorylation in switching the nuclear condensate partitioning behavior of RNA polymerase II.by John C. Manteiga.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Biolog

Transcription Factors Activate Genes through the Phase-Separation Capacity of Their Activation Domains

Gene expression is controlled by transcription factors (TFs) that consist of DNA-binding domains (DBDs) and activation domains (ADs). The DBDs have been well characterized, but little is known about the mechanisms by which ADs effect gene activation. Here, we report that diverse ADs form phase-separated condensates with the Mediator coactivator. For the OCT4 and GCN4 TFs, we show that the ability to form phase-separated droplets with Mediator in vitro and the ability to activate genes in vivo are dependent on the same amino acid residues. For the estrogen receptor (ER), a ligand-dependent activator, we show that estrogen enhances phase separation with Mediator, again linking phase separation with gene activation. These results suggest that diverse TFs can interact with Mediator through the phase-separating capacity of their ADs and that formation of condensates with Mediator is involved in gene activation.National Institutes of Health (U.S.) (Grant GM123511)National Institutes of Health (U.S.) (Grant GM117370)Swedish Research Council (Postdoctoral Fellowship VR 2017-00372)Damon Runyon Cancer Research Foundation ( Fellowship 2309-17

Partitioning of cancer therapeutics in nuclear condensates

The nucleus contains diverse phase-separated condensates that compartmentalize and concentrate biomolecules with distinct physicochemical properties. Here, we investigated whether condensates concentrate small-molecule cancer therapeutics such that their pharmacodynamic properties are altered. We found that antineoplastic drugs become concentrated in specific protein condensates in vitro and that this occurs through physicochemical properties independent of the drug target. This behavior was also observed in tumor cells, where drug partitioning influenced drug activity. Altering the properties of the condensate was found to affect the concentration and activity of drugs. These results suggest that selective partitioning and concentration of small molecules within condensates contributes to drug pharmacodynamics and that further understanding of this phenomenon may facilitate advances in disease therapy.11Nsciescopu