RNA polymerase II C-terminal domain phosphorylation patterns in Caenorhabditis elegans operons, polycistronic gene clusters with only one promoter

DAF-16/FOXO requires Protein Phosphatase 4 to initiate transcription of stress resistance and longevity promoting genes

In C. elegans, the conserved transcription factor DAF-16/FOXO is a powerful aging regulator, relaying dire conditions into expression of stress resistance and longevity promoting genes. For some of these functions, including low insulin/IGF signaling (IIS), DAF-16 depends on the protein SMK-1/SMEK, but how SMK-1 exerts this role has remained unknown. We show that SMK-1 functions as part of a specific Protein Phosphatase 4 complex (PP4^SMK-1). Loss of PP4^SMK-1 hinders transcriptional initiation at several DAF-16-activated genes, predominantly by impairing RNA polymerase II recruitment to their promoters. Search for the relevant substrate of PP4^SMK-1 by phosphoproteomics identified the conserved transcriptional regulator SPT-5/SUPT5H, whose knockdown phenocopies the loss of PP4^SMK-1. Phosphoregulation of SPT-5 is known to control transcriptional events such as elongation and termination. Here we also show that transcription initiating events are influenced by the phosphorylation status of SPT-5, particularly at DAF-16 target genes where transcriptional initiation appears rate limiting, rendering PP4^SMK-1 crucial for many of DAF-16’s physiological roles.

The transcription factor DAF-16/FOXO mediates a wide variety of aging-preventive responses by driving the expression of stress resistance and longevity promoting genes. Here the authors show that transcriptional initiation at many DAF-16/FOXO target genes requires the dephosphorylation of SPT-5 by Protein Phosphatase 4.

The Caenorhabditis elegans Ortholog of TDP-43 Regulates the Chromatin Localization of the Heterochromatin Protein 1 Homolog HPL-2

TDP-1 is the Caenorhabditis elegans ortholog of mammalian TDP-43, which is strongly implicated in the etiology of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). We discovered that deletion of the tdp-1 gene results in enhanced nuclear RNA interference (RNAi). As nuclear RNAi in C. elegans involves chromatin changes moderated by HPL-2, a homolog of heterochromatin protein 1 (HP1), we investigated the interaction of TDP-1 and HPL-2. We found that TDP-1 and HPL-2 interact directly and that loss of TDP-1 dramatically alters the chromatin association of HPL-2. We showed previously that deletion of the tdp-1 gene results in transcriptional alterations and the accumulation of double-stranded RNA (dsRNA). These molecular changes are replicated in an hpl-2 deletion strain, consistent with HPL-2 acting in consort with TDP-1 to modulate these aspects of RNA metabolism. Our observations identify novel mechanisms by which HP1 homologs can be recruited to chromatin and by which nuclear depletion of human TDP-43 may lead to changes in RNA metabolism that are relevant to disease.

Histone H3K9 and H4 Acetylations and Transcription Facilitate the Initial CENP-AHCP-3 Deposition and De Novo Centromere Establishment in Caenorhabditis elegans Artificial Chromosomes

Background

The centromere is the specialized chromatin region that directs chromosome segregation. The kinetochore assembles on the centromere, attaching chromosomes to microtubules in mitosis. The centromere position is usually maintained through cell cycles and generations. However, new centromeres, known as neocentromeres, can occasionally form on ectopic regions when the original centromere is inactivated or lost due to chromosomal rearrangements. Centromere repositioning can occur during evolution. Moreover, de novo centromeres can form on exogenously transformed DNA in human cells at a low frequency, which then segregates faithfully as human artificial chromosomes (HACs). How centromeres are maintained, inactivated and activated is unclear. A conserved histone H3 variant, CENP-A, epigenetically marks functional centromeres, interspersing with H3. Several histone modifications enriched at centromeres are required for centromere function, but their role in new centromere formation is less clear. Studying the mechanism of new centromere formation has been challenging because these events are difficult to detect immediately, requiring weeks for HAC selection.

Results

DNA injected into the Caenorhabditis elegans gonad can concatemerize to form artificial chromosomes (ACs) in embryos, which first undergo passive inheritance, but soon autonomously segregate within a few cell cycles, more rapidly and frequently than HACs. Using this in vivo model, we injected LacO repeats DNA, visualized ACs by expressing GFP::LacI, and monitored equal AC segregation in real time, which represents functional centromere formation. Histone H3K9 and H4 acetylations are enriched on new ACs when compared to endogenous chromosomes. By fusing histone deacetylase HDA-1 to GFP::LacI, we tethered HDA-1 to ACs specifically, reducing AC histone acetylations, reducing AC equal segregation frequency, and reducing initial kinetochroe protein CENP-A^HCP−3 and NDC-80 deposition, indicating that histone acetylations facilitate efficient centromere establishment. Similarly, inhibition of RNA polymerase II-mediated transcription also delays initial CENP-A^HCP-3 loading.

Conclusions

Acetylated histones on chromatin and transcription can create an open chromatin environment, enhancing nucleosome disassembly and assembly, and potentially contribute to centromere establishment. Alternatively, acetylation of soluble H4 may stimulate the initial deposition of CENP-A^HCP−3-H4 nucleosomes. Our findings shed light on the mechanism of de novo centromere activation.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0185-1) contains supplementary material, which is available to authorized users.

Localization of RNAPII and 3' end formation factor CstF subunits on C. elegans genes and operons

Transcription termination is mechanistically coupled to pre-mRNA 3' end formation to prevent transcription much beyond the gene 3' end. C. elegans, however, engages in polycistronic transcription of operons in which 3' end formation between genes is not accompanied by termination. We have performed RNA polymerase II (RNAPII) and CstF ChIP-seq experiments to investigate at a genome-wide level how RNAPII can transcribe through multiple poly-A signals without causing termination. Our data shows that transcription proceeds in some ways as if operons were composed of multiple adjacent single genes. Total RNAPII shows a small peak at the promoter of the gene cluster and a much larger peak at 3' ends. These 3' peaks coincide with maximal phosphorylation of Ser2 within the C-terminal domain (CTD) of RNAPII and maximal localization of the 3' end formation factor CstF. This pattern occurs at all 3' ends including those at internal sites in operons where termination does not occur. Thus the normal mechanism of 3' end formation does not always result in transcription termination. Furthermore, reduction of CstF50 by RNAi did not substantially alter the pattern of CstF64, total RNAPII, or Ser2 phosphorylation at either internal or terminal 3' ends. However, CstF50 RNAi did result in a subtle reduction of CstF64 binding upstream of the site of 3' cleavage, suggesting that the CstF50/CTD interaction may facilitate bringing the 3' end machinery to the transcription complex.

TDP-1, the Caenorhabditis elegans ortholog of TDP-43, limits the accumulation of double-stranded RNA

Caenorhabditis elegans mutants deleted for TDP-1, an ortholog of the neurodegeneration-associated RNA-binding protein TDP-43, display only mild phenotypes. Nevertheless, transcriptome sequencing revealed that many RNAs were altered in accumulation and/or processing in the mutant. Analysis of these transcriptional abnormalities demonstrates that a primary function of TDP-1 is to limit formation or stability of double-stranded RNA. Specifically, we found that deletion of tdp-1: (1) preferentially alters the accumulation of RNAs with inherent double-stranded structure (dsRNA); (2) increases the accumulation of nuclear dsRNA foci; (3) enhances the frequency of adenosine-to-inosine RNA editing; and (4) dramatically increases the amount of transcripts immunoprecipitable with a dsRNA-specific antibody, including intronic sequences, RNAs with antisense overlap to another transcript, and transposons. We also show that TDP-43 knockdown in human cells results in accumulation of dsRNA, indicating that suppression of dsRNA is a conserved function of TDP-43 in mammals. Altered accumulation of structured RNA may account for some of the previously described molecular phenotypes (e.g., altered splicing) resulting from reduction of TDP-43 function.

RNA polymerase II transcription elongation and Pol II CTD Ser2 phosphorylation: A tail of two kinases

The transition between initiation and productive elongation during RNA Polymerase II (Pol II) transcription is a well-appreciated point of regulation across many eukaryotes. Elongating Pol II is modified by phosphorylation of serine 2 (Ser2) on its carboxy terminal domain (CTD) by two kinases, Bur1/Ctk1 in yeast and Cdk9/Cdk12 in metazoans. Here, we discuss the roles and regulation of these kinases and their relationship to Pol II elongation control, and focus on recent data from work in C. elegans that point out gaps in our current understand of transcription elongation.

Inhibition of post-transcriptional RNA processing by CDK inhibitors and its implication in anti-viral therapy

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle and RNA polymerase II mediated transcription. Several pharmacological CDK inhibitors are currently in clinical trials as potential cancer therapeutics and some of them also exhibit antiviral effects. Olomoucine II and roscovitine, purine-based inhibitors of CDKs, were described as effective antiviral agents that inhibit replication of a broad range of wild type human viruses. Olomoucine II and roscovitine show high selectivity for CDK7 and CDK9, with important functions in the regulation of RNA polymerase II transcription. RNA polymerase II is necessary for viral transcription and following replication in cells. We analyzed the effect of inhibition of CDKs by olomoucine II on gene expression from viral promoters and compared its effect to widely-used roscovitine. We found that both roscovitine and olomoucine II blocked the phosphorylation of RNA polymerase II C-terminal domain. However the repression of genes regulated by viral promoters was strongly dependent on gene localization. Both roscovitine and olomoucine II inhibited expression only when the viral promoter was not integrated into chromosomal DNA. In contrast, treatment of cells with genome-integrated viral promoters increased their expression even though there was decreased phosphorylation of the C-terminal domain of RNA polymerase II. To define the mechanism responsible for decreased gene expression after pharmacological CDK inhibitor treatment, the level of mRNA transcription from extrachromosomal DNA was determined. Interestingly, our results showed that inhibition of RNA polymerase II C-terminal domain phosphorylation increased the number of transcribed mRNAs. However, some of these mRNAs were truncated and lacked polyadenylation, which resulted in decreased translation. These results suggest that phosphorylation of RNA polymerase II C-terminal domain is critical for linking transcription and posttrancriptional processing of mRNA expressed from extrachromosomal DNA.

Coupling mRNA processing with transcription in time and space

Maturation of mRNA precursors often occurs simultaneously with their synthesis by RNA polymerase II (Pol II). The co-transcriptional nature of mRNA processing has permitted the evolution of coupling mechanisms that coordinate transcription with mRNA capping, splicing, editing and 3' end formation. Recent experiments using sophisticated new methods for analysis of nascent RNA have provided important insights into the relative amount of co-transcriptional and post-transcriptional processing, the relationship between mRNA elongation and processing, and the role of the Pol II carboxy-terminal domain (CTD) in regulating these processes.

3' end formation of pre-mRNA and phosphorylation of Ser2 on the RNA polymerase II CTD are reciprocally coupled in human cells

Pre-mRNA 3′ end formation is coupled to transcription via the RNA Pol II C-terminal domain (CTD). However, how transcription and pre-mRNA maturation are coordinated in humans is poorly understood. Here, West and colleagues show that Pol II pausing promotes Ser2p by Cdk12, which recruits CPA factor CstF77 and is required for optimal 3′ end processing. This study delineates a reciprocal relationship between early steps in poly(A) site processing and Pol II Ser2p that ensures efficient pre-mRNA 3′ end formation in human cells.

3′ end formation of pre-mRNAs is coupled to their transcription via the C-terminal domain (CTD) of RNA polymerase II (Pol II). Nearly all protein-coding transcripts are matured by cleavage and polyadenylation (CPA), which is frequently misregulated in disease. Understanding how transcription is coordinated with CPA in human cells is therefore very important. We found that the CTD is heavily phosphorylated on Ser2 (Ser2p) at poly(A) (pA) signals coincident with recruitment of the CstF77 CPA factor. Depletion of the Ser2 kinase Cdk12 impairs Ser2p, CstF77 recruitment, and CPA, strongly suggesting that the processes are linked, as they are in budding yeast. Importantly, we additionally show that the high Ser2p signals at the 3′ end depend on pA signal function. Down-regulation of CPA results in the loss of a 3′ Ser2p peak, whereas a new peak is formed when CPA is induced de novo. Finally, high Ser2p signals are generated by Pol II pausing, which is a well-known feature of pA site recognition. Thus, a reciprocal relationship between early steps in pA site processing and Ser2p ensures efficient 3′ end formation.

Transcriptional regulation of gene expression in C. elegans

Protein coding gene sequences are converted to mRNA by the highly regulated process of transcription. The precise temporal and spatial control of transcription for many genes is an essential part of development in metazoans. Thus, understanding the molecular mechanisms underlying transcriptional control is essential to understanding cell fate determination during embryogenesis, post-embryonic development, many environmental interactions, and disease-related processes. Studies of transcriptional regulation in C. elegans exploit its genomic simplicity and physical characteristics to define regulatory events with single-cell and minute-time-scale resolution. When combined with the genetics of the system, C. elegans offers a unique and powerful vantage point from which to study how chromatin-associated proteins and their modifications interact with transcription factors and their binding sites to yield precise control of gene expression through transcriptional regulation.

RSR-2, the Caenorhabditis elegans ortholog of human spliceosomal component SRm300/SRRM2, regulates development by influencing the transcriptional machinery

Protein components of the spliceosome are highly conserved in eukaryotes and can influence several steps of the gene expression process. RSR-2, the Caenorhabditis elegans ortholog of the human spliceosomal protein SRm300/SRRM2, is essential for viability, in contrast to the yeast ortholog Cwc21p. We took advantage of mutants and RNA interference (RNAi) to study rsr-2 functions in C. elegans, and through genetic epistasis analysis found that rsr-2 is within the germline sex determination pathway. Intriguingly, transcriptome analyses of rsr-2(RNAi) animals did not reveal appreciable splicing defects but instead a slight global decrease in transcript levels. We further investigated this effect in transcription and observed that RSR-2 colocalizes with DNA in germline nuclei and coprecipitates with chromatin, displaying a ChIP-Seq profile similar to that obtained for the RNA Polymerase II (RNAPII). Consistent with a novel transcription function we demonstrate that the recruitment of RSR-2 to chromatin is splicing-independent and that RSR-2 interacts with RNAPII and affects RNAPII phosphorylation states. Proteomic analyses identified proteins associated with RSR-2 that are involved in different gene expression steps, including RNA metabolism and transcription with PRP-8 and PRP-19 being the strongest interacting partners. PRP-8 is a core component of the spliceosome and PRP-19 is the core component of the PRP19 complex, which interacts with RNAPII and is necessary for full transcriptional activity. Taken together, our study proposes that RSR-2 is a multifunctional protein whose role in transcription influences C. elegans development.

Footprints of a trypanosomatid RNA world: pre-small subunit rRNA processing by spliced leader addition trans-splicing

The addition of a capped mini-exon [spliced leader (SL)] through trans-splicing is essential for the maturation of RNA polymerase (pol) II-transcribed polycistronic pre-mRNAs in all members of the Trypanosomatidae family. This process is an inter-molecular splicing reaction that follows the same basic rules of cis-splicing reactions. In this study, we demonstrated that mini-exons were added to precursor ribosomal RNA (pre-rRNA) are transcribed by RNA pol I, including the 5' external transcribed spacer (ETS) region. Additionally, we detected the SL-5'ETS molecule using three distinct methods and located the acceptor site between two known 5'ETS rRNA processing sites (A' and A1) in four different trypanosomatids. Moreover, we detected a polyadenylated 5'ETS upstream of the trans-splicing acceptor site, which also occurs in pre-mRNA trans-splicing. After treatment with an indirect trans-splicing inhibitor (sinefungin), we observed SL-5'ETS decay. However, treatment with 5-fluorouracil (a precursor of RNA synthesis that inhibits the degradation of pre-rRNA) led to the accumulation of SL-5'ETS, suggesting that the molecule may play a role in rRNA degradation. The detection of trans-splicing in these molecules may indicate broad RNA-joining properties, regardless of the polymerase used for transcription.

CCDC-55 is required for larval development and distal tip cell migration in Caenorhabditis elegans

The Caenorhabditis elegans distal tip cells (DTCs) are an in vivo model for the study of developmentally regulated cell migration. In this study, we characterize a novel role for CCDC-55, a conserved coiled-coil domain containing protein, in DTC migration and larval development in C. elegans. Although animals homozygous for a probable null allele, ccdc-55(ok2851), display an early larval arrest, RNAi depletion experiments allow the analysis of later phenotypes and suggest that CCDC-55 is needed within the DTC for migration to cease at the end of larval morphogenesis. The ccdc-55 gene is found in an operon with rnf-121 and rnf-5, E3 ubiquitin ligases that target cell migration genes such as the β-integrin PAT-3. Genetic interaction studies using RNAi depletion and the deletion alleles rnf-121(ok848) and rnf-5(tm794) indicate that CCDC-55 and the RNF genes act at least partially in parallel to promote termination of cell migration in the adult DTC.