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The mammalian DNA replication timing (RT) program is crucial for the proper functioning and integrity of the genome. The best-known mechanism for controlling RT is the suppression of late origins of replication in heterochromatin by RIF1. Here, we report that in antigen-activated, hypermutating murine B lymphocytes, RIF1 binds predominantly to early-replicating active chromatin and promotes early replication, but plays a minor role in regulating replication origin activity, gene expression and genome organization in B cells. Furthermore, we find that RIF1 functions in a complementary and non-epistatic manner with minichromosome maintenance (MCM) proteins to establish early RT signatures genome-wide and, specifically, to ensure the early replication of highly transcribed genes. These findings reveal additional layers of regulation within the B cell RT program, driven by the coordinated activity of RIF1 and MCM proteins. Here the authors show that in activated B cells, RIF1 primarily binds early-replicating active chromatin and promotes early replication. RIF1 and MCM proteins establish early replication timing signatures genome-wide and ensure early replication of highly transcribed genes.

Active enhancers are frequently transcribed, yet the regulatory role of enhancer transcription remains debated. Here, we depleted the RNA polymerase II pausing and elongation factor Spt5 in activated mouse B cells and found that approximately 50% of enhancer–gene pairs showed co-regulated transcription, consistent with a potential functional requirement for enhancer transcription. In particular, Spt5 depletion led to loss of super-enhancer–promoter physical interaction and gene expression at the immunoglobulin heavy-chain locus ( Igh ), abrogating antibody class switch recombination. This defect correlated strictly with loss of enhancer transcription but did not affect acetylation of histone H3 at lysine 27, chromatin accessibility and occupancy of Mediator and cohesin at the enhancer. Strikingly, CRISPRa-mediated rescue of enhancer transcription in Spt5-depleted cells restored Igh gene expression. Our work suggests that Spt5-mediated enhancer transcription underlies the physical and functional interaction between a subset of active enhancers and their target promoters. Depletion of RNA polymerase II pausing and elongation factor Spt5 in B cells indicates that ~50% of enhancer–gene pairs show co-regulated transcription. CRISPRa-mediated rescue of enhancer transcription in Spt5-depleted cells restores Igh gene expression.

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) is a regulatory hub for transcription and RNA processing. Here, we identify PHD-finger protein 3 (PHF3) as a regulator of transcription and mRNA stability that docks onto Pol II CTD through its SPOC domain. We characterize SPOC as a CTD reader domain that preferentially binds two phosphorylated Serine-2 marks in adjacent CTD repeats. PHF3 drives liquid-liquid phase separation of phosphorylated Pol II, colocalizes with Pol II clusters and tracks with Pol II across the length of genes. PHF3 knock-out or SPOC deletion in human cells results in increased Pol II stalling, reduced elongation rate and an increase in mRNA stability, with marked derepression of neuronal genes. Key neuronal genes are aberrantly expressed in Phf3 knock-out mouse embryonic stem cells, resulting in impaired neuronal differentiation. Our data suggest that PHF3 acts as a prominent effector of neuronal gene regulation by bridging transcription with mRNA decay. Here the authors identify PHF3 SPOC domain as a reader of the phosphorylated RNA polymerase II (Pol II) C-terminal domain. They show that PHF3 clusters with Pol II complexes in cells, drives phase separation of Pol II in vitro, and regulates neuronal gene expression and neuronal differentiation.

Somatic hypermutation (SHM) of immunoglobulin variable (V) regions modulates antibody-antigen affinity is initiated by activation-induced cytidine deaminase (AID) on single-stranded DNA (ssDNA). Transcription is essential for SHM and AID target genes harbor activating chromatin marks and RNA polymerase II (Pol II) stalling, leading to the model that these features favor higher rates of mutagenesis. However, whether such relationships exist within V regions is undetermined. Here, we directly compared SHM and nascent transcription across four V regions and 275 non-immunoglobulin SHM targets at single-nucleotide resolution using precision run-on sequencing (PRO-seq). Although locales of Pol II enrichment and zones of Pol II stalling were detected within V regions, their correlation with SHM was not statistically significant. Moreover, SHM was robust against major reductions of activating epigenetic marks and transcription. This data suggests that SHM patterns and spectra are established independently of specific local nascent transcriptional features.Competing Interest StatementThe authors have declared no competing interest.Footnotes* The text has been revised in several sections for clarity. Figure S7-8 and Tables have been revised. Author affiliations updated.

Conflicts between transcription and cohesin-mediated loop extrusion can majorly influence 3D chromatin architecture but whether these structural changes affect biological function is unknown. Here, we show that a critical step in antibody class switch recombination (CSR) in activated B cells, namely, the juxtaposition (synapsis) of donor and acceptor switch (S) recombination sequences at the immunoglobulin heavy chain locus (Igh), occurs at the interface of a de novo topologically associating domain (TAD) boundary formed via transcriptional activity at acceptor S regions. Using Tri-C to capture higher-order multiway chromatin conformations, we find that synapsis occurs predominantly in the proximity of distal 3′ CTCF-binding sites and that this multiway conformation is abolished upon downregulation of transcription and loss of the TAD boundary at the acceptor S region. Thus, an insulating de novo TAD boundary created by conflict between transcription and loop extrusion plays a direct role in the mechanism of CSR. Competing Interest Statement The authors have declared no competing interest.

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) is a regulatory hub for transcription and RNA processing. Here, we identify PHD-finger protein 3 (PHF3) as a new CTD-binding factor that negatively regulates transcription and mRNA stability. The PHF3 SPOC domain preferentially binds to CTD repeats phosphorylated on Serine-2 and PHF3 tracks with Pol II across the length of genes. PHF3 competes with TFIIS for Pol II binding through its TFIIS-like domain (TLD), thus inhibiting TFIIS-mediated rescue of backtracked Pol II. PHF3 knock-out or PHF3 SPOC deletion in human cells result in gene upregulation and a global increase in mRNA stability, with marked derepression of neuronal genes. Key neuronal genes are aberrantly expressed in Phf3 knock-out mouse embryonic stem cells, resulting in impaired neuronal differentiation. Our data suggest that PHF3 is a prominent effector of neuronal gene regulation at the interface of transcription elongation and mRNA decay.

The mammalian DNA replication timing (RT) program is crucial for the proper functioning and integrity of the genome. The best-known mechanism for controlling RT is the suppression of late origins of replication in heterochromatin by RIF1. Here, we report that in antigen-activated B lymphocytes, RIF1 binds predominantly to early-replicating active chromatin, regulates early origin firing and promotes early replication. RIF1 has a minor role in gene expression and genome organization in B cells. Furthermore, we find that RIF1 functions in a complementary and non-epistatic manner with minichromosome maintenance (MCM) proteins to establish early RT signatures genome-wide and, specifically, to ensure the early replication of highly transcribed genes. These findings reveal new layers of regulation within the B cell RT program, driven by the coordinated activity of RIF1 and MCM proteins.