Specific threonine-4 phosphorylation and function of RNA polymerase II CTD during M phase progression

Defining gene ends: RNA polymerase II CTD threonine 4 phosphorylation marks transcription termination regions genome-wide

Defining the beginning of a eukaryotic protein-coding gene is relatively simple. It corresponds to the first ribonucleotide incorporated by RNA polymerase II (Pol II) into the nascent RNA molecule. This nucleotide is protected by capping and maintained in the mature messenger RNA (mRNA). However, in higher eukaryotes, the end of mRNA is separated from the sites of transcription termination by hundreds to thousands of base pairs. Currently used genomic annotations only take account of the end of the mature transcript - the sites where pre-mRNA cleavage occurs, while the regions in which transcription terminates are unannotated. Here, we describe the evidence for a marker of transcription termination, which could be widely applicable in genomic studies. Pol II termination regions can be determined genome-wide by detecting Pol II phosphorylated on threonine 4 of its C-terminal domain (Pol II CTD-T4ph). Pol II in this state pauses before leaving the DNA template. Up to date this potent mark has been underused because the evidence for its place and role in termination is scattered across multiple publications. We summarize the observations regarding Pol II CTD-T4ph in termination regions and present bioinformatic analyses that further support Pol II CTD-T4ph as a global termination mark in animals.

Thr4 phosphorylation on RNA Pol II occurs at early transcription regulating 3'-end processing

RNA polymerase II relies on a repetitive sequence domain (YSPTSPS) within its largest subunit to orchestrate transcription. While phosphorylation on serine-2/serine-5 of the carboxyl-terminal heptad repeats is well established, threonine-4's role remains enigmatic. Paradoxically, threonine-4 phosphorylation was only detected after transcription end sites despite functionally implicated in pausing, elongation, termination, and messenger RNA processing. Our investigation revealed that threonine-4 phosphorylation detection was obstructed by flanking serine-5 phosphorylation at the onset of transcription, which can be removed selectively. Subsequent proteomic analyses identified many proteins recruited to transcription via threonine-4 phosphorylation, which previously were attributed to serine-2. Loss of threonine-4 phosphorylation greatly reduces serine-2 phosphorylation, revealing a cross-talk between the two marks. Last, the function analysis of the threonine-4 phosphorylation highlighted its role in alternative 3'-end processing within pro-proliferative genes. Our findings unveil the true genomic location of this evolutionarily conserved phosphorylation mark and prompt a reassessment of functional assignments of the carboxyl-terminal domain.

Distinctive interactomes of RNA polymerase II phosphorylation during different stages of transcription

During eukaryotic transcription, RNA polymerase II undergoes dynamic post-translational modifications on the C-terminal domain (CTD) of the largest subunit, generating an information-rich PTM landscape that transcriptional regulators bind. The phosphorylation of Ser5 and Ser2 of CTD heptad occurs spatiotemporally with the transcriptional stages, recruiting different transcriptional regulators to Pol II. To delineate the protein interactomes at different transcriptional stages, we reconstructed phosphorylation patterns of the CTD at Ser5 and Ser2 in vitro. Our results showed that distinct protein interactomes are recruited to RNA polymerase II at different stages of transcription by the phosphorylation of Ser2 and Ser5 of the CTD heptads. In particular, we characterized calcium homeostasis endoplasmic reticulum protein (CHERP) as a regulator bound by phospho-Ser2 heptad. Pol II association with CHERP recruits an accessory splicing complex whose loss results in broad changes in alternative splicing events. Our results shed light on the PTM-coded recruitment process that coordinates transcription.

Characterization of factors that underlie transcriptional silencing in C. elegans oocytes

While it has been appreciated for decades that prophase-arrested oocytes are transcriptionally silenced on a global level, the molecular pathways that promote silencing have remained elusive. Previous work in C. elegans has shown that both topoisomerase II (TOP-2) and condensin II collaborate with the H3K9me heterochromatin pathway to silence gene expression in the germline during L1 starvation, and that the PIE-1 protein silences the genome in the P-lineage of early embryos. Here, we show that all three of these silencing systems, TOP-2/condensin II, H3K9me, and PIE-1, are required for transcriptional repression in oocytes. We find that H3K9me3 marks increase dramatically on chromatin during silencing, and that silencing is under cell cycle control. We also find that PIE-1 localizes to the nucleolus just prior to silencing, and that nucleolar dissolution during silencing is dependent on TOP-2/condensin II. Our data identify both the molecular components and the trigger for genome silencing in oocytes and establish a link between PIE-1 nucleolar residency and its ability to repress transcription.

The Phantom Mark: Enigmatic roles of phospho-Threonine 4 modification of the C-terminal domain of RNA polymerase II

The largest subunit of RNA polymerase II (Pol II) has an unusual carboxyl-terminal domain (CTD). This domain is composed of a tandemly repeating heptapeptide, Y1 S2 P3 T4 S5 P6 S7 , that has multiple roles in regulating Pol II function and processing newly synthesized RNA. Transient phosphorylation of Ser2 and Ser5 of the YS2 PTS5 PS repeat have well-defined roles in recruiting different protein complexes and coordinating sequential steps in gene transcription. As such, these phospho-marks encipher a molecular recognition code, colloquially termed the CTD code. In contrast, the contribution of phospho-Threonine 4 (pThr4/pT4) to the CTD code remains opaque and contentious. Fuelling the debate on the relevance of this mark to gene expression are the findings that replacing Thr4 with a valine or alanine has varied impact on cellular function in different species and independent proteomic analyses disagree on the relative abundance of pThr4 marks. Yet, substitution with negatively charged residues is lethal and even benign mutations selectively disrupt synthesis and 3' processing of distinct sets of coding and non-coding transcripts. Suggestive of non-canonical roles, pThr4 marked Pol II regulates distinct gene classes in a species- and signal-responsive manner. Hinting at undiscovered roles of this elusive mark, multiple signal-responsive kinases phosphorylate Thr4 at target genes. Here, we focus on this under-explored residue and postulate that the pThr4 mark is superimposed on the canonical CTD code to selectively regulate expression of targeted genes without perturbing genome-wide transcriptional processes. This article is categorized under: RNA Processing > 3' End Processing RNA Processing > Processing of Small RNAs RNA Processing > Splicing Regulation/Alternative Splicing.

Maintaining Transcriptional Specificity Through Mitosis

Virtually all cell types have the same DNA, yet each type exhibits its own cell-specific pattern of gene expression. During the brief period of mitosis, the chromosomes exhibit changes in protein composition and modifications, a marked condensation, and a consequent reduction in transcription. Yet as cells exit mitosis, they reactivate their cell-specific programs with high fidelity. Initially, the field focused on the subset of transcription factors that are selectively retained in, and hence bookmark, chromatin in mitosis. However, recent studies show that many transcription factors can be retained in mitotic chromatin and that, surprisingly, such retention can be due to nonspecific chromatin binding. Here, we review the latest studies focusing on low-level transcription via promoters, rather than enhancers, as contributing to mitotic memory, as well as new insights into chromosome structure dynamics, histone modifications, cell cycle signaling, and nuclear envelope proteins that together ensure the fidelity of gene expression through a round of mitosis.

A comprehensive analysis of prefoldins and their implication in cancer

Prefoldins (PFDNs) are evolutionary conserved co-chaperones, initially discovered in archaea but universally present in eukaryotes. PFDNs are prevalently organized into hetero-hexameric complexes. Although they have been overlooked since their discovery and their functions remain elusive, several reports indicate they act as co-chaperones escorting misfolded or non-native proteins to group II chaperonins. Unlike the eukaryotic PFDNs which interact with cytoskeletal components, the archaeal PFDNs can bind and stabilize a wide range of substrates, possibly due to their great structural diversity. The discovery of the unconventional RPB5 interactor (URI) PFDN-like complex (UPC) suggests that PFDNs have versatile functions and are required for different cellular processes, including an important role in cancer. Here, we summarize their functions across different species. Moreover, a comprehensive analysis of PFDNs genomic alterations across cancer types by using large-scale cancer genomic data indicates that PFDNs are a new class of non-mutated proteins significantly overexpressed in some cancer types.

FILIP1L Loss Is a Driver of Aggressive Mucinous Colorectal Adenocarcinoma and Mediates Cytokinesis Defects through PFDN1

Aneuploid mucinous colorectal adenocarcinoma (MAC) is an aggressive subtype of colorectal cancer with poor prognosis. The tumorigenic mechanisms in aneuploid MAC are currently unknown. Here we show that downregulation of Filamin A-interacting protein 1-like (FILIP1L) is a driver of MAC. Loss of FILIP1L increased xenograft growth, and, in colon-specific knockout mice, induced colonic epithelial hyperplasia and mucin secretion. The molecular chaperone prefoldin 1 (PFDN1) was identified as a novel binding partner of FILIP1L at the centrosomes throughout mitosis. FILIP1L was required for proper centrosomal localization of PFDN1 and regulated proteasome-dependent degradation of PFDN1. Importantly, increased PFDN1, caused by downregulation of FILIP1L, drove multinucleation and cytokinesis defects in vitro and in vivo, which were confirmed by time-lapse imaging and 3D cultures of normal epithelial cells. Overall, these findings suggest that downregulation of FILIP1L and subsequent upregulation of PFDN1 is a driver of the unique neoplastic characteristics in aggressive aneuploid MAC. SIGNIFICANCE: This study identifies FILIP1L as a tumor suppressor in mucinous colon cancer and demonstrates that FILIP1L loss results in aberrant stabilization of a centrosome-associated chaperone protein to drive aneuploidy and disease progression.

A combinatorial view of old and new RNA polymerase II modifications

The production of mRNA is a dynamic process that is highly regulated by reversible post-translational modifications of the C-terminal domain (CTD) of RNA polymerase II. The CTD is a highly repetitive domain consisting mostly of the consensus heptad sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Phosphorylation of serine residues within this repeat sequence is well studied, but modifications of all residues have been described. Here, we focus on integrating newly identified and lesser-studied CTD post-translational modifications into the existing framework. We also review the growing body of work demonstrating crosstalk between different CTD modifications and the functional consequences of such crosstalk on the dynamics of transcriptional regulation.

Methods for the analysis of transcriptome dynamics

The transcriptome is the complete set of transcripts in a cell or tissue and includes ribosomal RNA (rRNA), messenger RNA (mRNA), transfer RNA (tRNA), and regulatory noncoding RNA. At steady-state, the transcriptome results from a compensatory variation of the transcription and decay rate to maintain the RNA concentration constant. RNA transcription constitutes the first stage in gene expression, and thus is a major and primary mode of gene expression control. Nevertheless, regulation of RNA decay is also a key factor in gene expression control, involving either selective RNA stabilization or enhanced degradation. Transcriptome analysis allows the identification of gene expression alterations, providing new insights regarding the pathways and mechanisms involved in physiological and pathological processes. Upon perturbation of cell homeostasis, rapid changes in gene expression are required to adapt to new conditions. Thus, to better understand the regulatory mechanisms associated with gene expression alterations, it is vital to acknowledge the relative contribution of RNA synthesis and decay to the transcriptome. To the toxicology field, the study of gene expression regulation mechanisms can help identify the early and mechanistic relevant cellular events associated with a particular response. This review aims to provide a critical comparison of the available methods used to analyze the contribution of RNA transcription and decay to gene expression dynamics. Notwithstanding, an integration of the data obtained is necessary to understand the entire repercussions of gene transcription changes at a system-level. Thus, a brief overview of the methods available for the integration and analysis of the data obtained from transcriptome analysis will also be provided.

The RNA helicase UPF1 associates with mRNAs co-transcriptionally and is required for the release of mRNAs from gene loci

UPF1 is an RNA helicase that is required for nonsense-mediated mRNA decay (NMD) in eukaryotes, and the predominant view is that UPF1 mainly operates on the 3'UTRs of mRNAs that are directed for NMD in the cytoplasm. Here we offer evidence, obtained from Drosophila, that UPF1 constantly moves between the nucleus and cytoplasm by a mechanism that requires its RNA helicase activity. UPF1 is associated, genome-wide, with nascent RNAs at most of the active Pol II transcription sites and at some Pol III-transcribed genes, as demonstrated microscopically on the polytene chromosomes of salivary glands and by ChIP-seq analysis in S2 cells. Intron recognition seems to interfere with association and translocation of UPF1 on nascent pre-mRNAs, and cells depleted of UPF1 show defects in the release of mRNAs from transcription sites and their export from the nucleus.

Noncanonical CTD kinases regulate RNA polymerase II in a gene-class-specific manner

Phosphorylation of the carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) governs stage-specific interactions with different cellular machines. The CTD consists of Y₁S₂P₃T₄S₅P₆S₇ heptad repeats, and sequential phosphorylations of Ser7, Ser5 and Ser2 occur universally across Pol II-transcribed genes. Phosphorylation of Thr4, however, appears to selectively modulate transcription of specific classes of genes. Here, we identify 10 new Thr4 kinases from different kinase structural groups. Irreversible chemical inhibition of the most active Thr4 kinase, Hrr25, reveals a novel role for this kinase in transcription termination of specific class of noncoding snoRNA genes. Genome-wide profiles of Hrr25 reveal a selective enrichment at 3ʹ regions of noncoding genes that display termination defects. Importantly, phospho-Thr4 marks placed by Hrr25 are recognized by Rtt103, a key component of the termination machinery. Our results suggest that these uncommon CTD kinases selectively place phospho-Thr4 marks to regulate expression of targeted genes.

Functional Contributions of Prefoldin to Gene Expression

Prefoldin is a co-chaperone that evolutionarily originates in archaea, is universally present in all eukaryotes and acts as a co-chaperone by facilitating the supply of unfolded or partially folded substrates to class II chaperonins. Eukaryotic prefoldin is known mainly for its functional relevance in the cytoplasmic folding of actin and tubulin monomers during cytoskeleton assembly. However, the role of prefoldin in chaperonin-mediated folding is not restricted to cytoskeleton components, but extends to both the assembly of other cytoplasmic complexes and the maintenance of functional proteins by avoiding protein aggregation and facilitating proteolytic degradation. Evolution has favoured the diversification of prefoldin subunits, and has allowed the so-called prefoldin-like complex, with specialised functions, to appear. Subunits of both canonical and prefoldin-like complexes have also been found in the nucleus of yeast and metazoan cells, where they have been functionally connected with different gene expression steps. Plant prefoldin has also been detected in the nucleus and is physically associated with a gene regulator. Here we summarise information available on the functional involvement of prefoldin in gene expression, and discuss the implications of these results for the relationship between prefoldin structure and function.

The RNA polymerase II CTD "orphan" residues: Emerging insights into the functions of Tyr-1, Thr-4, and Ser-7

The C-terminal domain (CTD) of the RNA polymerase II largest subunit consists of a unique repeated heptad sequence of the consensus Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. An important function of the CTD is to couple transcription with RNA processing reactions that occur during the initiation, elongation, and termination phases of transcription. During this transcription cycle, the CTD is subject to extensive modification, primarily phosphorylation, on its non-proline residues. Reversible phosphorylation of Ser2 and Ser5 is well known to play important and general functions during transcription in all eukaryotes. More recent studies have enhanced our understanding of Tyr1, Thr4, and Ser7, and what have been previously characterized as unknown or specialized functions for these residues has changed to a more fine-detailed map of transcriptional regulation that highlights similarities as well as significant differences between organisms. Here, we review recent findings on the function and modification of these three residues, which further illustrate the importance of the CTD in precisely modulating gene expression.

Structural insight into recognition of phosphorylated threonine-4 of RNA polymerase II C-terminal domain by Rtt103p

Phosphorylation patterns of the C‐terminal domain (CTD) of largest subunit of RNA polymerase II (called the CTD code) orchestrate the recruitment of RNA processing and transcription factors. Recent studies showed that not only serines and tyrosines but also threonines of the CTD can be phosphorylated with a number of functional consequences, including the interaction with yeast transcription termination factor, Rtt103p. Here, we report the solution structure of the Rtt103p CTD‐interacting domain (CID) bound to Thr4 phosphorylated CTD, a poorly understood letter of the CTD code. The structure reveals a direct recognition of the phospho‐Thr4 mark by Rtt103p CID and extensive interactions involving residues from three repeats of the CTD heptad. Intriguingly, Rtt103p's CID binds equally well Thr4 and Ser2 phosphorylated CTD. A doubly phosphorylated CTD at Ser2 and Thr4 diminishes its binding affinity due to electrostatic repulsion. Our structural data suggest that the recruitment of a CID‐containing CTD‐binding factor may be coded by more than one letter of the CTD code.

The code and beyond: transcription regulation by the RNA polymerase II carboxy-terminal domain

The carboxy-terminal domain (CTD) extends from the largest subunit of RNA polymerase II (Pol II) as a long, repetitive and largely unstructured polypeptide chain. Throughout the transcription process, the CTD is dynamically modified by post-translational modifications, many of which facilitate or hinder the recruitment of key regulatory factors of Pol II that collectively constitute the 'CTD code'. Recent studies have revealed how the physicochemical properties of the CTD promote phase separation in the presence of other low-complexity domains. Here, we discuss the intricacies of the CTD code and how the newly characterized physicochemical properties of the CTD expand the function of the CTD beyond the code.