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The chromatin-modifying enzyme, Polycomb Repressive Complex 2 (PRC2), deposits the H3K27me3 epigenetic mark to negatively regulate expression at numerous target genes, and this activity has been implicated in embryonic development, cell differentiation, and various cancers. A biological role for RNA binding in regulating PRC2 histone methyltransferase activity is generally accepted, but the nature and mechanism of this relationship remains an area of active investigation. Notably, many in vitro studies demonstrate that RNA inhibits PRC2 activity on nucleosomes through mutually antagonistic binding, while some in vivo studies indicate that PRC2’s RNA-binding activity is critical for facilitating its biological function(s). Here we use biochemical, biophysical, and computational approaches to interrogate PRC2’s RNA and DNA-binding kinetics. Our findings demonstrate that PRC2-polynucleotide dissociation rates are dependent on the concentration of free ligand, indicating the potential for direct transfer between nucleic acid ligands without a free-enzyme intermediate. Direct transfer explains the variation in previously reported dissociation kinetics, allows reconciliation of prior in vitro and in vivo studies, and expands the potential mechanisms of RNA-mediated PRC2 regulation. Moreover, simulations indicate that such a direct transfer mechanism could be obligatory for RNA to recruit proteins to chromatin.

Many chromatin-binding proteins and protein complexes that regulate transcription also bind RNA. One of these, Polycomb repressive complex 2 (PRC2), deposits the H3K27me3 mark of facultative heterochromatin and is required for stem cell differentiation. PRC2 binds RNAs broadly in vivo and in vitro. Yet, the biological importance of this RNA binding remains unsettled. Here, we tackle this question in human induced pluripotent stem cells by using multiple complementary approaches. Perturbation of RNA–PRC2 interaction by RNase A, by a chemical inhibitor of transcription or by an RNA-binding-defective mutant all disrupted PRC2 chromatin occupancy and localization genome wide. The physiological relevance of PRC2–RNA interactions is further underscored by a cardiomyocyte differentiation defect upon genetic disruption. We conclude that PRC2 requires RNA binding for chromatin localization in human pluripotent stem cells and in turn for defining cellular state. Perturbation of RNA–PRC2 interaction in human pluripotent stem cells disrupts PRC2 chromatin occupancy and localization genome wide. PRC2–RNA interactions contribute to cardiomyocyte differentiation.

Biochemical reconstitution of PRC2 interactions with chromatinized templates demonstrates that protein-free linker DNA dominates the PRC2-nucleosome interaction, while RNA inhibits binding. Many studies have revealed pathways of epigenetic gene silencing by Polycomb repressive complex 2 (PRC2) in vivo , but understanding the underlying molecular mechanisms requires biochemistry. Here we analyze interactions of reconstituted human PRC2 with nucleosome complexes. Histone modifications, the H3K27M cancer mutation, and inclusion of JARID2 or EZH1 in the PRC2 complex have unexpectedly minor effects on PRC2–nucleosome binding. Instead, protein-free linker DNA dominates the PRC2–nucleosome interaction. Specificity for CG-rich sequences is consistent with PRC2 occupying CG-rich DNA in vivo . PRC2 preferentially binds methylated DNA regulated by its AEBP2 subunit, suggesting how DNA and histone methylation collaborate to repress chromatin. We find that RNA, known to inhibit PRC2 activity, is not a methyltransferase inhibitor per se . Instead, RNA sequesters PRC2 from nucleosome substrates, because PRC2 binding requires linker DNA, and RNA and DNA binding are mutually exclusive. Together, we provide a model for PRC2 recruitment and an explanation for how actively transcribed genomic regions bind PRC2 but escape silencing.

The CTC1-STN1-TEN1 (CST) complex is essential for telomere maintenance and resolution of stalled replication forks genome-wide. Here, we report the 3.0-angstrom cryo–electron microscopy structure of human CST bound to telomeric single-stranded DNA (ssDNA), which assembles as a decameric supercomplex. The atomic model of the 134-kilodalton CTC1 subunit, built almost entirely de novo, reveals the overall architecture of CST and the DNA-binding anchor site. The carboxyl-terminal domain of STN1 interacts with CTC1 at two separate docking sites, allowing allosteric mediation of CST decamer assembly. Furthermore, ssDNA appears to staple two monomers to nucleate decamer assembly. CTC1 has stronger structural similarity to Replication Protein A than the expected similarity to yeast Cdc13. The decameric structure suggests that CST can organize ssDNA analogously to the nucleosome’s organization of double-stranded DNA.

Telomeres, the natural ends of linear chromosomes, comprise repeat-sequence DNA and associated proteins 1 . Replication of telomeres allows continued proliferation of human stem cells and immortality of cancer cells 2 . This replication requires telomerase 3 extension of the single-stranded DNA (ssDNA) of the telomeric G-strand ((TTAGGG) n ); the synthesis of the complementary C-strand ((CCCTAA) n ) is much less well characterized. The CST (CTC1–STN1–TEN1) protein complex, a DNA polymerase α-primase accessory factor 4 , 5 , is known to be required for telomere replication in vivo 6 – 9 , and the molecular analysis presented here reveals key features of its mechanism. We find that human CST uses its ssDNA-binding activity to specify the origins for telomeric C-strand synthesis by bound Polα-primase. CST-organized DNA polymerization can copy a telomeric DNA template that folds into G-quadruplex structures, but the challenges presented by this template probably contribute to telomere replication problems observed in vivo. Combining telomerase, a short telomeric ssDNA primer and CST–Polα–primase gives complete telomeric DNA replication, resulting in the same sort of ssDNA 3′ overhang found naturally on human telomeres. We conclude that the CST complex not only terminates telomerase extension 10 , 11 and recruits Polα–primase to telomeric ssDNA 4 , 12 , 13 but also orchestrates C-strand synthesis. Because replication of the telomere has features distinct from replication of the rest of the genome, targeting telomere-replication components including CST holds promise for cancer therapeutics. The Polα–primase-associated CST complex organizes telomeric C-strand DNA synthesis, and, in combination with telomerase, it carries out complete replication of the single-stranded DNA overhang found at human telomeres.

Using separation-of-function mutations of TPP1 that inhibit telomerase binding while maintaining telomere capping, a region on the surface of TPP1, the TEL patch, is identified and found to be required for both binding telomerase and enhancing its processivity. A TEL-ling patch on TPP1 telomere protein The human chromosomal telomere-binding protein TPP1 protein binds telomeric single-stranded DNA and has an essential role in protecting chromosome ends. It is believed to both recruit and stimulate telomerase, the enzyme that replicates chromosome ends. Thomas Cech and colleagues show, using separation-of-function mutants of TPP1, that telomerase binding and telomere capping are independent functions. These mutants identify a small patch of amino acids on the surface of TPP1—the TEL patch—that is required both for binding telomerase and promoting its action. Telomerase is overexpressed in many cancers, making it a prime target for drug development. As it has been proven difficult to inhibit telomerase itself, the TEL patch provides a promising new strategy for anticancer therapy. Human chromosome ends are capped by shelterin, a protein complex that protects the natural ends from being recognized as sites of DNA damage and also regulates the telomere-replicating enzyme, telomerase 1 , 2 , 3 . Shelterin includes the heterodimeric POT1–TPP1 protein, which binds the telomeric single-stranded DNA tail 4 , 5 , 6 , 7 , 8 , 9 . TPP1 has been implicated both in recruiting telomerase to telomeres and in stimulating telomerase processivity (the addition of multiple DNA repeats after a single primer-binding event) 9 , 10 , 11 , 12 , 13 , 14 . Determining the mechanisms of these activities has been difficult, especially because genetic perturbations also tend to affect the essential chromosome end-protection function of TPP1 (refs 15 , 16 , 17 ). Here we identify separation-of-function mutants of human TPP1 that retain full telomere-capping function in vitro and in vivo , yet are defective in binding human telomerase. The seven separation-of-function mutations map to a patch of amino acids on the surface of TPP1, the TEL patch, that both recruits telomerase to telomeres and promotes high-processivity DNA synthesis, indicating that these two activities are manifestations of the same molecular interaction. Given that the interaction between telomerase and TPP1 is required for telomerase function in vivo , the TEL patch of TPP1 provides a new target for anticancer drug development.

The human shelterin proteins associate with telomeric DNA to confer telomere protection and length regulation. They are thought to form higher-order protein complexes for their functions, but studies of shelterin proteins have been mostly limited to pairs of proteins. Here we co-express various human shelterin proteins and find that they form defined multi-subunit complexes. A complex harboring both TRF2 and POT1 has the strongest binding affinity to telomeric DNA substrates comprised of double-stranded DNA with a 3′ single-stranded extension. TRF2 interacts with TIN2 with an unexpected 2:1 stoichiometry in the context of shelterin (RAP1 2 :TRF2 2 :TIN2 1 :TPP1 1 :POT1 1 ). Tethering of TPP1 to the telomere either via TRF2–TIN2 or via POT1 gives equivalent enhancement of telomerase processivity. We also identify a peptide region from TPP1 that is both critical and sufficient for TIN2 interaction. Our findings reveal new information about the architecture of human shelterin and how it performs its functions at telomeres. The human shelterin complex protects telomere ends from being recognized as damaged DNA sites and regulates telomere length in conjunction with telomerase. Here the authors establish the stoichiometries of human shelterin complexes of various compositions and show shelterin provides dual pathways to stimulate telomerase processivity.

The spatial partitioning of the transcriptome in the cell is an important form of gene-expression regulation. Here, we address how intron retention influences the spatio-temporal dynamics of transcripts from two clinically relevant genes: TERT (Telomerase Reverse Transcriptase) pre-mRNA and TUG1 (Taurine-Upregulated Gene 1) lncRNA. Single molecule RNA FISH reveals that nuclear TERT transcripts uniformly and robustly retain specific introns. Our data suggest that the splicing of TERT retained introns occurs during mitosis. In contrast, TUG1 has a bimodal distribution of fully spliced cytoplasmic and intron-retained nuclear transcripts. We further test the functionality of intron-retention events using RNA-targeting thiomorpholino antisense oligonucleotides to block intron excision. We show that intron retention is the driving force for the nuclear compartmentalization of these RNAs. For both RNAs, altering this splicing-driven subcellular distribution has significant effects on cell viability. Together, these findings show that stable retention of specific introns can orchestrate spatial compartmentalization of these RNAs within the cell. This process reveals that modulating RNA localization via targeted intron retention can be utilized for RNA-based therapies. RNA localization plays an important role in transcriptome regulation. The majority of TERT transcripts are detected in the nucleus and TUG1 lncRNAs in both the nucleus and cytoplasm. Here, the authors combine single-cell RNA imaging, antisense oligonucleotides and splicing analyses to show that retention of specific introns drives stable compartmentalization of TERT and TUG1 transcripts in the nucleus, and that splicing of TERT retained introns is mitotically regulated.

Biogenesis of human telomerase requires its RNA subunit (hTR) to fold into a multi-domain architecture that includes the template-pseudoknot (t/PK) and the three-way junction (CR4/5). These hTR domains bind the telomerase reverse transcriptase (hTERT) protein and are essential for telomerase activity. Here, we probe hTR structure in living cells using dimethyl sulfate mutational profiling with sequencing (DMS-MaPseq) and ensemble deconvolution analysis. Approximately 15% of the steady state population of hTR has a CR4/5 conformation lacking features required for hTERT binding. The proportion of hTR CR4/5 folded into the primary functional conformation is independent of hTERT expression levels. Mutations that stabilize the alternative CR4/5 conformation are detrimental to telomerase assembly and activity. Moreover, the alternative CR4/5 conformation is not found in purified telomerase RNP complexes, supporting the hypothesis that only the primary CR4/5 conformer is active. We propose that this misfolded portion of the cellular hTR pool is either slowly refolded or degraded, suggesting that kinetic RNA folding traps studied in vitro may also hinder ribonucleoprotein assembly in vivo. Telomerase assembles on a multi-domain RNA scaffold that must adopt a specific structure to function. Here, the authors reveal an alternative RNA conformation in living cells that disrupts telomerase activity, suggesting RNA misfolding may challenge proper telomerase assembly.

Methyltransferase PRC2 (Polycomb Repressive Complex 2) deposits histone H3K27 trimethylation to establish and maintain epigenetic gene silencing. PRC2 is precisely regulated by accessory proteins, histone post-translational modifications, and, particularly, RNA. Research on PRC2-associated RNA has mostly focused on the tight-binding G-quadruplex (G4) RNAs, which inhibit PRC2 enzymatic activity in vitro and in cells, a mechanism explained by our recent cryo-EM structure showing G4 RNA-mediated PRC2 dimerization. However, PRC2 binds a wide variety of RNA sequences, and it remained unclear how diverse RNAs beyond G4 associate with and regulate PRC2. Here, we show that variations in RNA sequence elicit disparate effects on PRC2 function. A G-rich RNA lacking consecutive G’s and an atypical G4 structure called a pUG-fold mediate PRC2 dimerization nearly identical to that induced by G4 RNA. In contrast, pyrimidine-rich RNAs, including a motif identified by CLIP-seq in cells, do not induce PRC2 dimerization and instead bind PRC2 monomers with retention of methyltransferase activity. Only RNAs that dimerize PRC2 compete with nucleosome binding and inhibit PRC2 methyltransferase activity. Thus, PRC2 binds many different RNAs with similar affinity; however, the functional effect on enzymatic activity depends entirely on the sequence of the bound RNA, a conclusion potentially applicable to any RNA-binding protein with a large transcriptome. PRC2 binds many RNAs, but how RNA sequence determines PRC2 regulation has remained unclear. Here, the authors show that G-rich RNAs promote PRC2 dimerization and inhibition, whereas pyrimidine-rich RNAs bind a monomeric complex without disrupting enzymatic activity.