End Processing Research Articles

Identifying human pre-mRNA cleavage and polyadenylation factors by genome-wide CRISPR screens using a dual fluorescence readthrough reporter.

Messenger RNA precursors (pre-mRNA) generally undergo 3' end processing by cleavage and polyadenylation (CPA), which is specified by a polyadenylation site (PAS) and adjacent RNA sequences and regulated by a large variety of core and auxiliary CPA factors. To date, most of the human CPA factors have been discovered through biochemical and proteomic studies. However, genetic identification of the human CPA factors has been hampered by the lack of a reliable genome-wide screening method. We describe here a dual fluorescence readthrough reporter system with a PAS inserted between two fluorescent reporters. This system enables measurement of the efficiency of 3' end processing in living cells. Using this system in combination with a human genome-wide CRISPR/Cas9 library, we conducted a screen for CPA factors. The screens identified most components of the known core CPA complexes and other known CPA factors. The screens also identified CCNK/CDK12 as a potential core CPA factor, and RPRD1B as a CPA factor that binds RNA and regulates the release of RNA polymerase II at the 3' ends of genes. Thus, this dual fluorescence reporter coupled with CRISPR/Cas9 screens reliably identifies bona fide CPA factors and provides a platform for investigating the requirements for CPA in various contexts.

Role of Rmd9p in 3′-end processing of mitochondrial 15S rRNA in Saccharomyces cerevisiae

Ribosome biogenesis, involving processing/assembly of rRNAs and r-proteins is a vital process. In Saccharomyces cerevisiae mitochondria, ribosomal small subunit comprises 15S rRNA (15S). While the 15S 5′-end processing uses Ccm1p and Pet127p, the mechanisms of the 3′-end processing remain unclear. We reveal involvement of Rmd9p in safeguarding/processing 15S 3′-end. Rmd9p deficiency results in a cleavage at a position 183 nucleotides upstream of 15S 3′-end, and in the loss of the 3′-minor domain. Rmd9p binds the 3′-end spacer sequence in 15S precursor, and a genetic interaction between rmd9 and dss1 indicates that Rmd9p regulates/limits mtEXO activity during the 3′-end spacer processing.

Molecular basis of human polyA polymerase recruitment by mPSF.

3' end processing of most eukaryotic pre-mRNAs is a crucial co-transcriptional process that generally involves the cleavage and polyadenylation of the precursor transcripts. Within the human 3' end processing machinery, the 4-subunit mammalian polyadenylation specificity factor (mPSF) recognizes the polyadenylation signal (PAS) in the pre-mRNA and recruits the polyA polymerase α (PAPOA) to it. To shed light on the molecular mechanisms of PAPOA recruitment to mPSF, we used a combination of cryogenic-electron microscopy (cryo-EM) single-particle analysis, computational structure prediction and in vitro biochemistry to reveal an intricate interaction network. A short linear motif in the mPSF subunit FIP1 interacts with the structured core of human PAPOA, with a binding mode that is evolutionary conserved from yeast to human. In higher eukaryotes, however, PAPOA contains a conserved C-terminal motif that can interact intramolecularly with the same residues of the PAPOA structured core used to bind FIP1. Interestingly, using biochemical assay and cryo-EM structural analysis, we found that the PAPOA C-terminal motif can also directly interact with mPSF at the subunit CPSF160. These results show that PAPOA recruitment to mPSF is mediated by two distinct intermolecular connections and further suggest the presence of mutually exclusive interactions in the regulation of 3' end processing.

Slow decrease in temperature produces readthrough transcripts in mammalian hibernation

Accumulating evidence suggests that various cellular stresses interfere with the end processing of mRNA synthesis and lead to the production of abnormally long transcripts, known as readthrough transcripts (RTTs), which extend beyond the termination sites. Small mammalian hibernators repeatedly enter a state referred to as deep torpor (DT), where the metabolic rate, respiration rate, and core body temperature become extremely low, which produces various types of cellular stresses and therefore induces RTTs. However, the types of stresses and processes around the DT that cause RTTs are unclear. In the present study, we showed that RTTs are produced from different gene loci in the livers of Syrian hamsters under DT and summer-like conditions. Moreover, in vitro analysis using hamster primary hepatocytes revealed that DT-specific RTTs are induced by a slow decline in temperature, as seen in body temperature in the entrance phase of DT, but not by rapid cold treatment or hypoxia. In addition, it was observed that RTTs were not elongated under a significantly cold temperature (4 °C). These results indicate that DT-specific RTTs are produced during the entrance phase of torpor by a slow decrease in body temperature.

U1 snRNP Biogenesis Defects in Neurodegenerative Diseases.

The U1 small ribonucleoprotein (U1 snRNP) plays a pivotal role in the intricate process of gene expression, specifically within nuclear RNA processing. By initiating the splicing reaction and modulating 3'-end processing, U1 snRNP exerts precise control over RNA metabolism and gene expression. This ribonucleoparticle is abundantly present, and its complex biogenesis necessitates shuttling between the nuclear and cytoplasmic compartments. Over the past three decades, extensive research has illuminated the crucial connection between disrupted U snRNP biogenesis and several prominent human diseases, notably various neurodegenerative conditions. The perturbation of U1 snRNP homeostasis has been firmly established in diseases such as Spinal Muscular Atrophy, Pontocerebellar hypoplasia, and FUS-mediated Amyotrophic Lateral Sclerosis. Intriguingly, compelling evidence suggests a potential correlation in Fronto-temporal dementia and Alzheimer's disease as well. Although the U snRNP biogenesis pathway is conserved across all eukaryotic cells, neurons, in particular, appear to be highly susceptible to alterations in spliceosome homeostasis. In contrast, other cell types exhibit a greater resilience to such disturbances. This vulnerability underscores the intricate relationship between U1 snRNP dynamics and the health of neuronal cells, shedding light on potential avenues for understanding and addressing neurodegenerative disorders.

Abstract 2874: Cancer metabolism in radiation resistance: Complementary roles of O-GlcNAc transferase and PARP1

Abstract The efficacy of image-guided radiotherapy (RT) has been linked to directing increased chromosomal double strand breaks (DSBs) to tumor cells while sparing surrounding normal tissue. DSBs can block cell cycle progression and promote mitotic catastrophe, cell death and senescence. End processing of persistent DSBs releases single strand DNA (ssDNA), driving cGAS/STING signaling and anti-tumor immune response. DSB repair, whether by non-homologous end-joining (NHEJ) or homologous recombination (HR), is a resistance mechanism, supporting cell survival, repopulation and immune evasion. Decades of effort to block DSB repair to enhance benefits of RT has yet to produce approved agents. Toxicity due to lack of cancer specificity may prevent the current generation of targeted agents from reaching the clinic. An answer may come from examining cancer metabolic reprogramming as a determinant of intrinsic radiation resistance. Along with other glucose metabolites, the Warburg effect increases N-acetyl glucosamine (GlcNAc) biosynthesis, which not only supports N-glycosylation but also modification of nucleocytoplasmic proteins by O-GlcNAc transferase (OGT). OGT-dependent O-GlcNAcylation confers radiation resistance, in part by accelerating DSB repair. OGT regulates γH2AX, 53BP1 and BRCA1 foci kinetics and suppresses ssDNA formation at DSB ends, limiting HR but favoring NHEJ for S/G2 DSB repair. Blocking OGT induces radiation sensitization in vitro and in vivo. In cells, OGT inhibition results in hyperresection, persistent RPA and RAD51 foci and accumulation of cytosolic DNA. One mediator may be the histone methyltransferase EZH2, which is rapidly recruited to deposit H3K27me3 at DSBs, promoting NHEJ repair. Irradiation of EZH2 KO cells results in hyperresection that cannot be suppressed by O-GlcNAcylation. Cancer cells may also accumulate NAD+, a substrate for PARP1 incorporated into poly-ADP-ribose to initiate DSB repair. Much like OGT inhibition, blocking PARP1 induces hyperresection and shifts repair toward HR and away from NHEJ. Importantly, increasing O-GlcNAcylation can suppress ssDNA formation even in PARP1 KO cells. In turn, inhibiting PARP1 enhances hyperesection in EZH2 KO cells, leading to a marked increase in cytosolic ssDNA. Our data assign complementary roles for EZH2 and PARP1 in DSB repair pathway choice by independently limiting 5' end resection. This directs DSBs to rapid, imprecise repair by NHEJ over slow and precise repair by HR. An implication is that PARP inhibitor resistance due to the Warburg effect may be mediated by EZH2. Driving hyperresection by targeting O-GlcNAcylation or H3K27 trimethylation along with PARP inhibition may saturate the capacity of HR and produce irreversible damage in cancer cells. Targeting impacts of metabolic reprogramming on DSB repair may not only enhance the therapeutic index of radiation but also sharpen synthetic lethality with HR deficiency. Citation Format: Elena E. Efimova, Sera Averbek, Donald J. Wolfgeher, Ding Wu, Yue Liu, Stephen J. Kron. Cancer metabolism in radiation resistance: Complementary roles of O-GlcNAc transferase and PARP1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2874.

Abstract 4356: An isoform-resolution transcriptomic atlas of colorectal cancer from long-read single-cell sequencing

Abstract Colorectal cancer (CRC), a complex and molecularly heterogenous disease that lacks robust predictive markers and targeted therapies, is the second leading cause of cancer death worldwide. In recent years, technological advances in short-read single-cell RNA sequencing (scRNA-seq) have been instrumental in deciphering tumor cell heterogeneities. However, most studies have focused on gene-level expression quantification without considering alterations in transcript structures arising from widespread alternative end processing and/or splicing which are frequently dysregulated in cancer. Here, we combined short- and long-read scRNA-seq of CRC patient samples to build the first isoform-resolution CRC transcriptomic atlas. We identified 394 dysregulated transcript structures in tumor epithelial cells, including 299 derived from the coupling of multiple alternative splicing events. Additionally, we characterized genes and isoforms associated with different colon epithelial lineages and tumor cell subpopulations exhibiting varying levels of stemness and differentiation, as well as their distinct prognostic implications. Finally, we built an algorithm by integrating mass spectrometry data and novel peptides derived from predicted open reading frames of recurrent tumor-specific transcripts to curate a panel of recurring neoepitopes. Overall, this study unveils the transcriptomic landscape of CRC and may drive the identification of neoantigens based on full-length transcriptomic atlases to aid future development of more universal neoantigen-based cancer vaccines. Citation Format: Zhongxiao Li, Bin Zhang, Jia Jia Chan, Hossein Tabatabaeian, Qing Yun Tong, Xiao Hong Chew, Xiaonan Fan, Charlene Chan, Patrick Driguez, Faith Cheong, Shi Wang, Bei En Siew, Ian Jse-Wei Tan, Kai-Yin Lee, Bettina Lieske, Wai-Kit Cheong, Dennis Kappei, Ker-Kan Tan, Xin Gao, Yvonne Tay. An isoform-resolution transcriptomic atlas of colorectal cancer from long-read single-cell sequencing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4356.

Structural basis of how MGME1 processes DNA 5' ends to maintain mitochondrial genome integrity.

Mitochondrial genome maintenance exonuclease 1 (MGME1) helps to ensure mitochondrial DNA (mtDNA) integrity by serving as an ancillary 5'-exonuclease for DNA polymerase γ. Curiously, MGME1 exhibits unique bidirectionality in vitro, being capable of degrading DNA from either the 5' or 3' end. The structural basis of this bidirectionally and, particularly, how it processes DNA from the 5' end to assist in mtDNA maintenance remain unclear. Here, we present a crystal structure of human MGME1 in complex with a 5'-overhang DNA, revealing that MGME1 functions as a rigid DNA clamp equipped with a single-strand (ss)-selective arch, allowing it to slide on single-stranded DNA in either the 5'-to-3' or 3'-to-5' direction. Using a nuclease activity assay, we have dissected the structural basis of MGME1-derived DNA cleavage patterns in which the arch serves as a ruler to determine the cleavage site. We also reveal that MGME1 displays partial DNA-unwinding ability that helps it to better resolve 5'-DNA flaps, providing insights into MGME1-mediated 5'-end processing of nascent mtDNA. Our study builds on previously solved MGME1-DNA complex structures, finally providing the comprehensive functional mechanism of this bidirectional, ss-specific exonuclease.

Splicing targeting drugs highlight intron retention as an actionable vulnerability in advanced prostate cancer

BackgroundAdvanced prostate cancer (PC) is characterized by insensitivity to androgen deprivation therapy and chemotherapy, resulting in poor outcome for most patients. Thus, advanced PC urgently needs novel therapeutic strategies. Mounting evidence points to splicing dysregulation as a hallmark of advanced PC. Moreover, pharmacologic inhibition of the splicing process is emerging as a promising option for this disease.MethodBy using a representative androgen-insensitive PC cell line (22Rv1), we have investigated the genome-wide transcriptomic effects underlying the cytotoxic effects exerted by three splicing-targeting drugs: Pladienolide B, indisulam and THZ531. Bioinformatic analyses were performed to uncover the gene structural features underlying sensitivity to transcriptional and splicing regulation by these treatments. Biological pathways altered by these treatments were annotated by gene ontology analyses and validated by functional experiments in cell models.ResultsAlthough eliciting similar cytotoxic effects on advanced PC cells, Pladienolide B, indisulam and THZ531 modulate specific transcriptional and splicing signatures. Drug sensitivity is associated with distinct gene structural features, expression levels and cis-acting sequence elements in the regulated exons and introns. Importantly, we identified PC-relevant genes (i.e. EZH2, MDM4) whose drug-induced splicing alteration exerts an impact on cell survival. Moreover, computational analyses uncovered a widespread impact of splicing-targeting drugs on intron retention, with enrichment in genes implicated in pre-mRNA 3’-end processing (i.e. CSTF3, PCF11). Coherently, advanced PC cells displayed high sensitivity to a specific inhibitor of the cleavage and polyadenylation complex, which enhances the effects of chemotherapeutic drugs that are already in use for this cancer.ConclusionsOur study uncovers intron retention as an actionable vulnerability for advanced PC, which may be exploited to improve therapeutic management of this currently incurable disease.

Resection of DNA double-strand breaks activates Mre11-Rad50-Nbs1- and Rad9-Hus1-Rad1-dependent mechanisms that redundantly promote ATR checkpoint activation and end processing in Xenopus egg extracts.

Sensing and processing of DNA double-strand breaks (DSBs) are vital to genome stability. DSBs are primarily detected by the ATM checkpoint pathway, where the Mre11-Rad50-Nbs1 (MRN) complex serves as the DSB sensor. Subsequent DSB end resection activates the ATR checkpoint pathway, where replication protein A, MRN, and the Rad9-Hus1-Rad1 (9-1-1) clamp serve as the DNA structure sensors. ATR activation depends also on Topbp1, which is loaded onto DNA through multiple mechanisms. While different DNA structures elicit specific ATR-activation subpathways, the regulation and mechanisms of the ATR-activation subpathways are not fully understood. Using DNA substrates that mimic extensively resected DSBs, we show here that MRN and 9-1-1 redundantly stimulate Dna2-dependent long-range end resection and ATR activation in Xenopus egg extracts. MRN serves as the loading platform for ATM, which, in turn, stimulates Dna2- and Topbp1-loading. Nevertheless, MRN promotes Dna2-mediated end processing largely independently of ATM. 9-1-1 is dispensable for bulk Dna2 loading, and Topbp1 loading is interdependent with 9-1-1. ATR facilitates Mre11 phosphorylation and ATM dissociation. These data uncover that long-range end resection activates two redundant pathways that facilitate ATR checkpoint signaling and DNA processing in a vertebrate system.

Structural role for DNA Ligase IV in promoting the fidelity of non-homologous end joining

Nonhomologous end joining (NHEJ), the primary pathway of vertebrate DNA double-strand-break (DSB) repair, directly re-ligates broken DNA ends. Damaged DSB ends that cannot be immediately re-ligated are modified by NHEJ processing enzymes, including error-prone polymerases and nucleases, to enable ligation. However, DSB ends that are initially compatible for re-ligation are typically joined without end processing. As both ligation and end processing occur in the short-range (SR) synaptic complex that closely aligns DNA ends, it remains unclear how ligation of compatible ends is prioritized over end processing. In this study, we identify structural interactions of the NHEJ-specific DNA Ligase IV (Lig4) within the SR complex that prioritize ligation and promote NHEJ fidelity. Mutational analysis demonstrates that Lig4 must bind DNA ends to form the SR complex. Furthermore, single-molecule experiments show that a single Lig4 binds both DNA ends at the instant of SR synapsis. Thus, Lig4 is poised to ligate compatible ends upon initial formation of the SR complex before error-prone processing. Our results provide a molecular basis for the fidelity of NHEJ.

Poly(A) tale: From A to A; RNA polyadenylation in prokaryotes and eukaryotes.

Most eukaryotic mRNAs and different non-coding RNAs undergo a form of 3' end processing known as polyadenylation. Polyadenylation machinery is present in almost all organisms except few species. In bacteria, the machinery has evolved from PNPase, which adds heteropolymeric tails, to a poly(A)-specific polymerase. Differently, a complex machinery for accurate polyadenylation and several non-canonical poly(A) polymerases are developed in eukaryotes. The role of poly(A) tail has also evolved from serving as a degradative signal to a stabilizing modification that also regulates translation. In this review, we discuss poly(A) tail emergence in prokaryotes and its development into a stable, yet dynamic feature at the 3' end of mRNAs in eukaryotes. We also describe how appearance of novel poly(A) polymerases gives cells flexibility to shape poly(A) tail. We explain how poly(A) tail dynamics help regulate cognate RNA metabolism in a context-dependent manner, such as during oocyte maturation. Finally, we describe specific mRNAs in metazoans that bear stem-loops instead of poly(A) tails. We conclude with how recent discoveries about poly(A) tail can be applied to mRNA technology. This article is categorized under: RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Processing > 3' End Processing RNA Turnover and Surveillance > Regulation of RNA Stability.

Hepatitis B virus infection disrupts homologous recombination in hepatocellular carcinoma by stabilizing resection inhibitor ADRM1.

Many cancers harbor homologous recombination defects (HRDs). A HRD is a therapeutic target that is being successfully utilized in treatment of breast/ovarian cancer via synthetic lethality. However, canonical HRD caused by BRCAness mutations do not prevail in liver cancer. Here we report a subtype of HRD caused by the perturbation of a proteasome variant (CDW19S) in hepatitis B virus-bearing (HBV-bearing) cells. This amalgamate protein complex contained the 19S proteasome decorated with CRL4WDR70 ubiquitin ligase, and assembled at broken chromatin in a PSMD4Rpn10- and ATM-MDC1-RNF8-dependent manner. CDW19S promoted DNA end processing via segregated modules that promote nuclease activities of MRE11 and EXO1. Contrarily, a proteasomal component, ADRM1Rpn13, inhibited resection and was removed by CRL4WDR70-catalyzed ubiquitination upon commitment of extensive resection. HBx interfered with ADRM1Rpn13 degradation, leading to the imposition of ADRM1Rpn13-dependent resection barrier and consequent viral HRD subtype distinguishable from that caused by BRCA1 defect. Finally, we demonstrated that viral HRD in HBV-associated hepatocellular carcinoma can be exploited to restrict tumor progression. Our work clarifies the underlying mechanism of a virus-induced HRD subtype.

A direct interaction between CPF and RNA Pol II links RNA 3′ end processing to transcription

Transcription termination by RNA polymerase II (RNA Pol II) is linked to RNA 3' end processing by the cleavage and polyadenylation factor (CPF or CPSF). CPF contains endonuclease, poly(A) polymerase, and protein phosphatase activities, which cleave and polyadenylate pre-mRNAs and dephosphorylate RNA Pol II to control transcription. Exactly how the RNA 3' end processing machinery is coupled to transcription remains unclear. Here, we combine invitro reconstitution, structural studies, and genome-wide analyses to show that yeast CPF physically and functionally interacts with RNA Pol II. Surprisingly, CPF-mediated dephosphorylation promotes the formation of an RNA Pol II stalk-to-stalk homodimer invitro. This dimer is compatible with transcription but not with the binding of transcription elongation factors. Disruption of the dimerization interface in cells causes transcription defects, including altered RNA Pol II abundance on protein-coding genes, tRNA genes, and intergenic regions. We hypothesize that RNA Pol II dimerization may provide a mechanistic basis for the allosteric model of transcription termination.

A Zebrafish Model of Parn-Deficiency Reveals Conserved Mechanisms Underlying Marrow Failure and Opportunities for Therapeutic Amelioration of Hematopoiesis

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome (IBMFS) and a telomere biology disorder that is characterized by physical malformations such as nail dystrophy, oral leukoplakia, reticular hyperpigmentation, multilineage cytopenia and immunodeficiency. Definitive treatment for the hematological complications is hematopoietic stem cell transplantation (HSCT), which is associated with high risk of organ toxicity and engraftment failure in this population. Further, DC patients are prone to developing myelodysplastic syndrome (MDS) and solid tumors. Germline mutations in the poly(A)-specific ribonuclease (PARN) gene are associated with Hoyeraal-Hreidarsson syndrome, the most severe form of DC. PARN encodes a deadenylase enzyme involved in the 3'-end processing of various RNAs including the telomerase RNA component (TERC). The clinical heterogeneity of phenotypes and the paucity of primary human samples necessitate the use of animal models such as zebrafish ( Danio rerio), given their highly conserved genetic program, to elucidate underlying biology. Zebrafish have a single PARN ortholog with 64% sequence identity at the amino acid level. Using CRISPR-Cas9 mutagenesis, we generated a 1.3 kb deletion in the zebrafish parn gene, predicted to result in premature protein truncation. To measure the hematopoietic output of the parn mutants, we performed whole-mount in situ hybridization (WISH) for key hematopoietic lineage markers. At 48 hours post-fertilization (hpf), we found a reduction in lcp1 + total leukocytes, mpx + neutrophils and hbbe3 + mature erythrocytes. Impaired erythroid differentiation was accompanied by an accumulation of gata1 + erythroid progenitors. This finding was further corroborated by o-dianisidine staining, showing reduced hemoglobinized erythrocytes. Thus, erythrocyte and leukocyte differentiation are compromised in our zebrafish parn mutant, phenocopying the multilineage cytopenia observed in DC patients. Flow cytometry of the whole kidney marrow (WKM, human bone marrow equivalent) at 12 months of age revealed an increase in hematopoietic stem and progenitor cells in the parn mutants. Giemsa staining of WKM touchpreps further identified an increase in myeloid-biased progenitors. Next, we investigated the consequences of Parn loss on telomerase activity. Using the telomere repeat amplification protocol, we found reduced telomerase activity in the WKMs of parn mutant zebrafish. Using 3' rapid amplification of cDNA ends, we observed longer ends in the terc amplicons of the parn mutants, reminiscent of defective deadenylation of human TERC RNA in the setting of PARN mutations. Exogenous thymidine supplementation has been shown to support telomere elongation in human cells. Following treatment of zebrafish embryos with thymidine nucleoside, we observed an improvement in erythrocyte numbers in the parn mutants by o-dianisidine staining at 48 hpf. Collectively, our preliminary findings suggest that telomerase-dependent mechanisms may contribute to the erythropenia seen in parn-mutant zebrafish. In summary, we generated a robust zebrafish model carrying a germline parn mutation that phenocopies the molecular and hematologic features seen in DC patients. Our model holds promise as a new in vivo tool for investigating DC-related disease mechanisms and as a platform for preclinical therapeutic screening.

Telomere maintenance in African trypanosomes

Telomere maintenance is essential for genome integrity and chromosome stability in eukaryotic cells harboring linear chromosomes, as telomere forms a specialized structure to mask the natural chromosome ends from DNA damage repair machineries and to prevent nucleolytic degradation of the telomeric DNA. In Trypanosoma brucei and several other microbial pathogens, virulence genes involved in antigenic variation, a key pathogenesis mechanism essential for host immune evasion and long-term infections, are located at subtelomeres, and expression and switching of these major surface antigens are regulated by telomere proteins and the telomere structure. Therefore, understanding telomere maintenance mechanisms and how these pathogens achieve a balance between stability and plasticity at telomere/subtelomere will help develop better means to eradicate human diseases caused by these pathogens. Telomere replication faces several challenges, and the “end replication problem” is a key obstacle that can cause progressive telomere shortening in proliferating cells. To overcome this challenge, most eukaryotes use telomerase to extend the G-rich telomere strand. In addition, a number of telomere proteins use sophisticated mechanisms to coordinate the telomerase-mediated de novo telomere G-strand synthesis and the telomere C-strand fill-in, which has been extensively studied in mammalian cells. However, we recently discovered that trypanosomes lack many telomere proteins identified in its mammalian host that are critical for telomere end processing. Rather, T. brucei uses a unique DNA polymerase, PolIE that belongs to the DNA polymerase A family (E. coli DNA PolI family), to coordinate the telomere G- and C-strand syntheses. In this review, I will first briefly summarize current understanding of telomere end processing in mammals. Subsequently, I will describe PolIE-mediated coordination of telomere G- and C-strand synthesis in T. brucei and implication of this recent discovery.

IntS6 and the Integrator phosphatase module tune the efficiency of select premature transcription termination events

The metazoan-specific Integrator complex catalyzes 3' end processing of small nuclear RNAs (snRNAs) and premature termination that attenuates the transcription of many protein-coding genes. Integrator has RNA endonuclease and protein phosphatase activities, but it remains unclear if both are required for complex function. Here, we show IntS6 (Integrator subunit 6) over-expression blocks Integrator function at a subset of Drosophila protein-coding genes, although having no effect on snRNAs or attenuation of other loci. Over-expressed IntS6 titrates protein phosphatase 2A (PP2A) subunits, thereby only affecting gene loci where phosphatase activity is necessary for Integrator function. IntS6 functions analogous to a PP2A regulatory B subunit as over-expression of canonical B subunits, which do not bind Integrator, is also sufficient toinhibit Integrator activity. These results show that the phosphatase module is critical at only a subset of Integrator-regulated genes and point to PP2A recruitment as a tunable step that modulates transcription termination efficiency.

Mutations in yeast Pcf11, a conserved protein essential for mRNA 3' end processing and transcription termination, elicit the Environmental Stress Response.

The Pcf11 protein is an essential subunit of the large complex that cleaves and polyadenylates eukaryotic mRNA precursor. It has also been functionally linked to gene-looping, termination of RNA Polymerase II (Pol II) transcripts, and mRNA export. We have examined a poorly characterized but conserved domain (amino acids 142-225) of the Saccharomyces cerevisiae Pcf11 and found that while it is not needed for mRNA 3' end processing or termination downstream of the poly(A) sites of protein-coding genes, its presence improves the interaction with Pol II and the use of transcription terminators near gene promoters. Analysis of genome-wide Pol II occupancy in cells with Pcf11 missing this region, as well as Pcf11 mutated in the Pol II CTD Interacting Domain, indicates that systematic changes in mRNA expression are mediated primarily at the level of transcription. Global expression analysis also shows that a general stress response, involving both activation and suppression of specific gene sets known to be regulated in response to a wide variety of stresses, is induced in the two pcf11 mutants, even though cells are grown in optimal conditions. The mutants also cause an unbalanced expression of cell wall-related genes that does not activate the Cell Wall Integrity pathway but is associated with strong caffeine sensitivity. Based on these findings, we propose that Pcf11 can modulate the expression level of specific functional groups of genes in ways that do not involve its well-characterized role in mRNA 3' end processing.

Ubiquitin-proteasome system regulation of a key gene regulatory factor, Paf1C

Paf1 (Polymerase-associated factor 1) complex (Paf1C) is evolutionarily conserved from yeast to humans, and facilitates transcription elongation as well as co-transcriptional histone covalent modifications and mRNA 3’-end processing. Thus, Paf1C is a key player in regulation of eukaryotic gene expression. Paf1C consists of Paf1, Cdc73, Ctr9, Leo1 and Rtf1 in both yeast and humans, but it has an additional component, Ski8, in humans. The abundances of these components regulate the assembly of Paf1C and/or its functions, thus implying the mechanisms involved in regulating the abundances of the Paf1C components in altered gene expression and hence cellular pathologies. Towards finding the mechanisms associated with the abundances of the Paf1C components, we analyzed here whether the Paf1C components are regulated via targeted ubiquitylation and 26S proteasomal degradation. We find that the Paf1C components except Paf1 do not undergo the 26S proteasomal degradation in both yeast and humans. Paf1 is found to be regulated by the ubiquitin-proteasome system (UPS) in yeast and humans. Alteration of such regulation changes Paf1’s abundance, leading to aberrant gene expression. Intriguingly, while the Rtf1 component of Paf1C does not undergo the 26S proteasomal degradation, it is found to be ubiquitylated, suggesting that Rtf1 ubiquitylation could be engaged in Paf1C assembly and/or functions. Collectively, our results reveal distinct UPS regulation of the Paf1C components, Paf1 and Rtf1, in a proteolysis-dependent and -independent manners, respectively, with functional implications.

Algorithm for Adaptive Control of Spring Endsprocessing by Plasma Cutting

A brief overview and issue general state of the demandand the need for the production of springs for the rolling stock of the RAO Russian Railways are given. Among the existing technological methods, plasma cutting was reasonably chosen and applied as a progressive machining method, as a means to increase the production output. General characteristics of this method as an alter native in the processing of metal products are presented. The existing solution of the plasma cutting method application for spring end processing is described. A new block diagram of automated production control for spring end processing by a set of equipment consisting of: Kawasaki industrial robotic arm, Kjellbreg plasma cutting equipment, auxiliary equipment and tools is presented. A new experimental control system for the spring production is proposed. A well-developed algorithm for adaptive control of large-sized spring end processing by the plasma cutting method is presented, taking into account the individual geometric characteristics of the work pieces being processed. A measuring stand, billet measurement schemes and a program for processing measurement results have been developed. Experimental data of spring machined end surface deviation from the plane perpendicular to the spring axis, under constant parameters of the plasma cutting process are obtained. The factors stipulating the need for individual approach to the processing of spring ends are identified. The requirements for the initial data of processed products are formulated. The valuechangeregularity of technological parameters for plasma cutting due toworkpiece geometric characteristics is presented. A fast tabular method for cutting current calculation as a significant technological parameter is proposed. A software implementation of the algorithm for adaptive control of the machining process in the high-level language Visual Studio C++ has been developed. The analysis of the obtained testing and debugging results is carried out. Conclusion sare for mulated.