Elimination of subtelomeric repeat sequences exerts little effect on telomere essential functions in Saccharomyces cerevisiae.

Telomeres, which are chromosomal end structures, play a crucial role in maintaining genome stability and integrity in eukaryotes. In the baker's yeast Saccharomyces cerevisiae, the X- and Y'-elements are subtelomeric repetitive sequences found in all 32 and 17 telomeres, respectively. While the Y'-elements serve as a backup for telomere functions in cells lacking telomerase, the function of the X-elements remains unclear. This study utilized the S. cerevisiae strain SY12, which has three chromosomes and six telomeres, to investigate the role of X-elements (as well as Y'-elements) in telomere maintenance. Deletion of Y'-elements (SY12YΔ), X-elements (SY12XYΔ+Y), or both X- and Y'-elements (SY12XYΔ) did not impact the length of the terminal TG1-3 tracks or telomere silencing. However, inactivation of telomerase in SY12YΔ, SY12XYΔ+Y, and SY12XYΔ cells resulted in cellular senescence and the generation of survivors. These survivors either maintained their telomeres through homologous recombination-dependent TG1-3 track elongation or underwent microhomology-mediated intra-chromosomal end-to-end joining. Our findings indicate the non-essential role of subtelomeric X- and Y'-elements in telomere regulation in both telomerase-proficient and telomerase-null cells and suggest that these elements may represent remnants of S. cerevisiae genome evolution. Furthermore, strains with fewer or no subtelomeric elements exhibit more concise telomere structures and offer potential models for future studies in telomere biology.

Effect of China's long-term care insurance on health outcomes of older disabled people: role of institutional care.

Since the public long-term care insurance (LTCI) system was piloted in Chengdu, China, in October 2017, there has been considerable growth of LTC institutions in China. This study aimed to evaluate the health value effect of LTCI in older patients with severe disabilities in an LTC institution. This prospective study was based on data from 985 severe disability patients with or without LTCI from October 2017 to May 2021 in the Eighth People's Hospital, Chengdu, China. The Cox proportional hazard model estimated LTCI's health value, including survival probability and risk of pneumonia/pressure ulcers. Subgroup analysis was performed for sex, age, Charlson Comorbidity Index (CCI), and the number of drugs. In the analysis, 519 and 466 patients in LTCI and non-LTCI groups were included, respectively. In adjusted Cox analyses, the LTCI group had a significantly elevated survival rate compared with the non-LTCI groups at 12 months (P < .001, hazard ratio (HR) = 1.758, 95% confidence interval (CI) 1.300-2.376). At 40 months, the adjusted survival rate was 62.6% in the LTCI group, which was significantly higher (53.7%; P = .003, HR = 1.438, 95% CI 1.131-1.831). The subgroups of patients aged 60 to 79 years (interaction P = .007) and with CCI ≥ 3 (interaction P = .026) were more significantly associated with survival improvement than those aged >80 years and with CCI< 3. The LTCI group was also at lower risk for hospital-acquired pneumonia (P = .016, HR 0.622, 95% CI 0.422-0.917) and pressure ulcers (P = .008, HR 0.695, 95% CI 0.376-0.862). The improved survival of LTCI remained stable in sensitivity analyses. For older patients with severe disabilities, in a LTC institution, LTCI significantly improved their health profile and longevity after a year, suggesting the large role and development potentiality of institution care in the LTCI system of China.

Swc4 protects nucleosome-free rDNA, tDNA and telomere loci to inhibit genome instability.

In the baker's yeast Saccharomyces cerevisiae, NuA4 and SWR1-C, two multisubunit complexes, are involved in histone acetylation and chromatin remodeling, respectively. Eaf1 is the assembly platform subunit of NuA4, Swr1 is the assembly platform and catalytic subunit of SWR1-C, while Swc4, Yaf9, Arp4 and Act1 form a functional module, and is present in both NuA4 and SWR1 complexes. ACT1 and ARP4 are essential for cell survival. Deletion of SWC4, but not YAF9, EAF1 or SWR1 results in a severe growth defect, but the underlying mechanism remains largely unknown. Here, we show that swc4Δ, but not yaf9Δ, eaf1Δ, or swr1Δ cells display defects in DNA ploidy and chromosome segregation, suggesting that the defects observed in swc4Δ cells are independent of NuA4 or SWR1-C integrity. Swc4 is enriched in the nucleosome-free regions (NFRs) of the genome, including characteristic regions of RDN5s, tDNAs and telomeres, independently of Yaf9, Eaf1 or Swr1. In particular, rDNA, tDNA and telomere loci are more unstable and prone to recombination in the swc4Δ cells than in wild-type cells. Taken together, we conclude that the chromatin associated Swc4 protects nucleosome-free chromatin of rDNA, tDNA and telomere loci to ensure genome integrity.

Chromosome territory reorganization through artificial chromosome fusion is compatible with cell fate determination and mouse development.

Chromosomes occupy discrete spaces in the interphase cell nucleus, called chromosome territory. The structural and functional relevance of chromosome territory remains elusive. We fused chromosome 15 and 17 in mouse haploid embryonic stem cells (haESCs), resulting in distinct changes of territories in the cognate chromosomes, but with little effect on gene expression, pluripotency and gamete functions of haESCs. The karyotype-engineered haESCs were successfully implemented in generating heterozygous (2n = 39) and homozygous (2n = 38) mouse models. Mice containing the fusion chromosome are fertile, and their representative tissues and organs display no phenotypic abnormalities, suggesting unscathed development. These results indicate that the mammalian chromosome architectures are highly resilient, and reorganization of chromosome territories can be readily tolerated during cell differentiation and mouse development.

Kae1 of Saccharomyces cerevisiae KEOPS complex possesses ADP/GDP nucleotidase activity.

The KEOPS complex is an evolutionarily conserved protein complex in all three domains of life (Bacteria, Archaea, and Eukarya). In budding yeast Saccharomyces cerevisiae, the KEOPS complex (ScKEOPS) consists of five subunits, which are Kae1, Bud32, Cgi121, Pcc1, and Gon7. The KEOPS complex is an ATPase and is required for tRNA N6-threonylcarbamoyladenosine modification, telomere length maintenance, and efficient DNA repair. Here, recombinant ScKEOPS full complex and Kae1-Pcc1-Gon7 and Bud32-Cgi121 subcomplexes were purified and their biochemical activities were examined. KEOPS was observed to have ATPase and GTPase activities, which are predominantly attributed to the Bud32 subunit, as catalytically dead Bud32, but not catalytically dead Kae1, largely eliminated the ATPase/GTPase activity of KEOPS. In addition, KEOPS could hydrolyze ADP to adenosine or GDP to guanosine, and produce PPi, indicating that KEOPS is an ADP/GDP nucleotidase. Further mutagenesis characterization of Bud32 and Kae1 subunits revealed that Kae1, but not Bud32, is responsible for the ADP/GDP nucleotidase activity. In addition, the Kae1V309D mutant exhibited decreased ADP/GDP nucleotidase activity in vitro and shortened telomeres in vivo, but showed only a limited defect in t6A modification, suggesting that the ADP/GDP nucleotidase activity of KEOPS contributes to telomere length regulation.

Single-chromosome fission yeast models reveal the configuration robustness of a functional genome.

In eukaryotic organisms, genetic information is usually carried on multiple chromosomes. Whether and how the number and configuration of chromosomes affect organismal fitness and speciation remain unclear. Here, we have successfully established several single-chromosome fission yeast Schizosaccharomyces pombe strains, in which the three natural chromosomes have been fused into one giant chromosome in different orders. Chromosome fusions accompanied by the deletions of telomeres and centromeres result in the loss of chromosomal interactions and a drastic change of global chromosome organization, but alter gene expression marginally. The single-chromosome strains display little defects in cell morphology, mitosis, genotoxin sensitivity, and meiosis. Crosses between a wild-type strain and a single-chromosome strain or between two single-chromosome strains with different fusion orders suffer defective meiosis and poor spore viability. We conclude that eukaryotic genomes are robust against dramatic chromosomal reconfiguration, and stochastic changes in chromosome number and genome organization during evolution underlie reproductive isolation and speciation.

The importance of fecal nucleic acid detection in patients with coronavirus disease (COVID-19): A systematic review and meta-analysis.

Pooled data from 2352 hospitalized coronavirus disease 2019 (COVID-19) patients with viral RNA in feces across 46 studies were analyzed and the pooled prevalence of fecal RNA was 46.8% (95% confidence interval [CI]: 0.383-0.554). The pooled analysis showed that the occurrence of total gastrointestinal (GI) symptoms was 28.5% (95% CI: 0.125-0.44) in COVID-19 patients with fecal RNA, that of both respiratory and GI symptoms was 21.9% (95% CI: 0.09-0.346), that of only GI symptoms was 19.8% (95% CI: 0.107-0.288), and that of only respiratory symptoms was 50.5%（95% CI: 0.267-0.744). The pooled data showed no significant difference in positive fecal RNA between severe and nonsevere cases (odds ratio = 2.009, p = 0.079, 95% CI: 0.922-4.378). During hospital admission, after samples from the respiratory system tested negative for viral RNA, 55.4% (95% CI: 0.418-0.669) of the patients with positive fecal RNA had persistent shedding of fecal RNA and pooled results from the other 4 studies including 848 discharged patients with nucleic acid-negative stool samples indicated that the occurrence of repositive stool swabs was 18.1% (95% CI: 0.028-0.335), that of repositive respiratory swabs was 22.8% (95% CI: 0.003-0.452), that of both repositive stool and respiratory swabs was 19.1% (95% CI: 0.019-0.363), and that of only repositive stool swabs was 9.6% (95% CI: 0.010-0.203). The digestive tract may be an important organ involved in COVID-19 infection and in the excretion of the virus. Because of the potential risk of fecal-oral transmission, giving emphasis on stool swab tests can help increase the detection rate of asymptomatic carriers and reduce missed diagnoses.

Swc4 positively regulates telomere length independently of its roles in NuA4 and SWR1 complexes.

Telomeres at the ends of eukaryotic chromosomes are essential for genome integrality and stability. In order to identify genes that sustain telomere maintenance independently of telomerase recruitment, we have exploited the phenotype of over-long telomeres in the cells that express Cdc13-Est2 fusion protein, and examined 195 strains, in which individual non-essential gene deletion causes telomere shortening. We have identified 24 genes whose deletion results in dramatic failure of Cdc13-Est2 function, including those encoding components of telomerase, Yku, KEOPS and NMD complexes, as well as quite a few whose functions are not obvious in telomerase activity regulation. We have characterized Swc4, a shared subunit of histone acetyltransferase NuA4 and chromatin remodeling SWR1 (SWR1-C) complexes, in telomere length regulation. Deletion of SWC4, but not other non-essential subunits of either NuA4 or SWR1-C, causes significant telomere shortening. Consistently, simultaneous disassembly of NuA4 and SWR1-C does not affect telomere length. Interestingly, inactivation of Swc4 in telomerase null cells accelerates both telomere shortening and senescence rates. Swc4 associates with telomeric DNA in vivo, suggesting a direct role of Swc4 at telomeres. Taken together, our work reveals a distinct role of Swc4 in telomere length regulation, separable from its canonical roles in both NuA4 and SWR1-C.

Opportunistic Infection in Hospitalised Patients with Inflammatory Bowel Disease.

OBJECTIVE: To investigate the incidence, clinical features, and risk factors of opportunistic infections in elderly patients with inflammatory bowel disease (IBD). STUDY DESIGN: Observational study. PLACE AND DURATION OF STUDY: Department of Digestive and Geriatrics Center, Sichuan University West China Hospital, China between January 2012 and January 2019. METHODOLOGY: Patients (≥18 years) with IBD were enrolled in this study. Clinical data from the infected elderly group (age ≥60 years), non-infected elderly group (age ≥60 years) and infected adult group (age: 18-59 years) were compared. Logistic regression analysis was used for risk factors associated with opportunistic infection. RESULTS:  A total of 8.9% (307/3,456) of patients with IBD had opportunistic infection. The opportunistic infection rate of elderly group was 16.5% (80/485), which was significantly higher than that of adult group (7.6%, 227/2,971, p <0.05). Compared with infected adult group or non-infected elderly group, infected elderly group had less fever and leukocytosis, but more hypoproteinemia and several activities (p <0.05). Cytomegalovirus and Epstein-Barr virus were the most common agents in elderly group and adult group, respectively. Multiple episodes (three or more) were more common in infected elderly group; the time of opportunistic infections was associated with systemic inflammatory reaction syndrome (SIRS, p <0.05). Logistic regression analysis showed that age ≥60 years, corticosteroids, immunosuppressive and biological agents were risk factors for opportunistic infections in patients with IBD. CONCLUSION: Hospitalised elderly IBD patients, receiving corticosteroids, immunosuppressive, and biological agents, are at higher risk for infection. The symptoms of opportunistic infections in elderly patients are atypical, but they are prone to multiple infections with poor prognosis. Key Words: Elderly patients, Inflammatory bowel disease, Opportunistic infection, Systemic inflammation reaction syndrome.

Clinical features of Epstein-Barr virus in the intestinal mucosa and blood of patients with inflammatory bowel disease.

BACKGROUND: The role of Epstein-Barr virus (EBV) in inflammatory bowel disease (IBD) remains to be elucidated. The aim of this study was to investigate the presence of EBV in the blood and intestinal mucosa of patients with IBD and evaluate the association between EBV positivity and IBD. METHODS: Patients with IBD, hospitalized between January 2015 and April 2018, were enrolled. The EBV-DNA load in blood samples from each subject was analyzed by quantitative real-time polymerase chain reaction. EBV-encoded small-RNA 1 (EBER-1) was detected by in-situ hybridization in intestinal mucosa tissue sections of patients with IBD. RESULT: EBV-DNA was detected in 48 out of 568 patients with IBD (8.4%), and EBER-1 positivity was detected in 27 of these patients (56.3%). Refractory IBD and severe mucosal inflammation were more common in patients with detectable levels of EBER-1 than in those without; the number of EBER-1-positive cells positively correlated with mucosal inflammation (P value < 0.05). Age (≥60 years old) and use of azathioprine were risk factors for EBV infection. There was no significant difference in clinical remission rate and surgical rate between the EBER-1 positive group and EBER-1 negative group, antiviral group and the non-antiviral group, among IBD patients who tested positive for EBV-DNA. CONCLUSION: Elderly patients with IBD, treated with azathioprine, are more susceptible to EBV positivity. Further, EBV mucosal detection correlated with the severity of mucosal damage and refractoriness, but not prognosis.

Cdc13 is predominant over Stn1 and Ten1 in preventing chromosome end fusions.

Telomeres define the natural ends of eukaryotic chromosomes and are crucial for chromosomal stability. The budding yeast Cdc13, Stn1 and Ten1 proteins form a heterotrimeric complex, and the inactivation of any of its subunits leads to a uniformly lethal phenotype due to telomere deprotection. Although Cdc13, Stn1 and Ten1 seem to belong to an epistasis group, it remains unclear whether they function differently in telomere protection. Here, we employed the single-linear-chromosome yeast SY14, and surprisingly found that the deletion of CDC13 leads to telomere erosion and intrachromosome end-to-end fusion, which depends on Rad52 but not Yku. Interestingly, the emergence frequency of survivors in the SY14 cdc13Δ mutant was ~29 fold higher than that in either the stn1Δ or ten1Δ mutant, demonstrating a predominant role of Cdc13 in inhibiting telomere fusion. Chromosomal fusion readily occurred in the telomerase-null SY14 strain, further verifying the default role of intact telomeres in inhibiting chromosome fusion.

Glial-derived neurotrophic factor regulates enteric mast cells and ameliorates dextran sulfate sodium-induced experimental colitis.

BACKGROUND & AIMS: Although interactions between enteric glial cells (EGCs) and enteric mast cells have been demonstrated to play an important role in the pathogenesis of inflammatory bowel disease (IBD), the exact mechanisms by which EGCs regulate enteric mast cells are still unknown. The aims of this study were to investigate whether glial-derived neurotrophic factor (GDNF), which has been confirmed to be produced mostly by EGCs, might regulate enteric mast cells and ameliorate dextran sulfate sodium (DSS)-induced experimental colitis. METHODS: Recombinant adenoviral vectors encoding GDNF (Ad-GDNF) were administered intracolonically in experimental colitis induced by DSS. The disease activity index and histological score were measured. The expression of tumour necrosis factor-α (TNF-α), interleukin-6 and myeloperoxidase (MPO) activity were measured by ELISA assay. The expression of trypsin and ß-hexosaminidase were evaluated. GDNF specific receptor (GFR-α1/RET) was detected. The calcium reflux was tested by microplate reader. The expression p-JNK was analyzed by western blot assay. RESULTS: GDNF resulted in a significant inhibition of the activation of enteric mast cells by down-regulating JNK signal pathway, lessening intracellular calcium influx, and then reducing the degranulation as well as the expression of pro-inflammatory cytokines via combing with its receptor (GFR-α1/RET) in mast cells, and these inhibitory effects were abrogated by treatment with neutralizing antibody against GDNF. Moreover, the administration of GDNF led to an amelioration of experimental colitis. CONCLUSIONS: GDNF are able to regulate enteric mast cells and ameliorate experimental colitis. GDNF might be an important mediator of the cross-talk between EGCs and enteric mast cells, and GDNF might be a useful therapeutic drug for IBD.

Both combinatorial K4me0-K36me3 marks on sister histone H3s of a nucleosome are required for Dnmt3a-Dnmt3L mediated de novo DNA methylation.

A nucleosome contains two copies of each histone H2A, H2B, H3 and H4. Histone H3 K4me0 and K36me3 are two key chromatin marks for de novo DNA methylation catalyzed by DNA methyltransferases in mammals. However, it remains unclear whether K4me0 and K36me3 marks on both sister histone H3s regulate de novo DNA methylation independently or cooperatively. Here, taking advantage of the bivalent histone H3 system in yeast, we examined the contributions of K4 and K36 on sister histone H3s to genomic DNA methylation catalyzed by ectopically co-expressed murine Dnmt3a and Dnmt3L. The results show that lack of both K4me0 and K36me3 on one sister H3 tail, or lack of K4me0 and K36me3 on respective sister H3s results in a dramatic reduction of 5mC, revealing a synergy of two sister H3s in DNA methylation regulation. Accordingly, the Dnmt3a or Dnmt3L mutation that disrupts the interaction of Dnmt3aADD domain-H3K4me0, Dnmt3LADD domain-H3K4me0, or Dnmt3aPWWP domain-H3K36me3 causes a significant reduction of DNA methylation. These results support the model that each heterodimeric Dnmt3a-Dnmt3L reads both K4me0 and K36me3 marks on one tail of sister H3s, and the dimer of heterodimeric Dnmt3a-Dnmt3L recognizes two tails of sister histone H3s to efficiently execute de novo DNA methylation.

KEOPS complex promotes homologous recombination via DNA resection.

KEOPS complex is one of the most conserved protein complexes in eukaryotes. It plays important roles in both telomere uncapping and tRNA N6-threonylcarbamoyladenosine (t6A) modification in budding yeast. But whether KEOPS complex plays any roles in DNA repair remains unknown. Here, we show that KEOPS complex plays positive roles in both DNA damage response and homologous recombination-mediated DNA repair independently of its t6A synthesis function. Additionally, KEOPS displays DNA binding activity in vitro, and is recruited to the chromatin at DNA breaks in vivo, suggesting a direct role of KEOPS in DSB repair. Mechanistically, KEOPS complex appears to promote DNA end resection through facilitating the association of Exo1 and Dna2 with DNA breaks. Interestingly, inactivation of both KEOPS and Mre11/Rad50/Xrs2 (MRX) complexes results in synergistic defect in DNA resection, revealing that KEOPS and MRX have some redundant functions in DNA resection. Thus we uncover a t6A-independent role of KEOPS complex in DNA resection, and propose that KEOPS might be a DSB sensor to assist cells in maintaining chromosome stability.

Tel1 and Rif2 oppositely regulate telomere protection at uncapped telomeres in Saccharomyces cerevisiae.

It has been well documented that Tel1 positively regulates telomere-end resection by promoting Mre11-Rad50-Xrs2 (MRX) activity, while Rif2 negatively regulates telomere-end resection by inhibiting MRX activity. At uncapped telomeres, whether Tel1 or Rif2 plays any role remains largely unknown. In this work, we examined the roles of Tel1 and Rif2 at uncapped telomeres in yku70Δ and/or cdc13-1 mutant cells cultured at non-permissive temperature. We found that deletion of TEL1 exacerbates the temperature sensitivity of both yku70Δ and cdc13-1 cells. Further epistasis analysis indicated that MRX and Tel1 function in the same pathway in telomere protection. Consistently, TEL1 deletion increases accumulation of Exo1-dependent telomeric single-stranded DNA (ssDNA) at uncapped telomeres, which stimulates checkpoint-dependent cell cycle arrest. Moreover, TEL1 deletion in yku70Δ cells facilitates Rad51-dependent Y' recombination. In contrast, RIF2 deletion in yku70Δ cells decreases the accumulation of telomeric ssDNA after 8 h of incubation at the non-permissive temperature of 37 °C and suppresses the temperature sensitivity of yku70Δ cells, likely due to the increase of Mre11 association at telomeres. Collectively, our findings indicate that Tel1 and Rif2 regulate telomere protection at uncapped telomeres via their roles in balancing MRX activity in telomere resection.

Rad6-Bre1 mediated histone H2Bub1 protects uncapped telomeres from exonuclease Exo1 in Saccharomyces cerevisiae.

Histone H2B lysine 123 mono-ubiquitination (H2Bub1), catalyzed by Rad6 and Bre1 in Saccharomyces cerevisiae, modulates chromatin structure and affects diverse cellular functions. H2Bub1 plays roles in telomeric silencing and telomere replication. Here, we have explored a novel role of H2Bub1 in telomere protection at uncapped telomeres in yku70Δ and cdc13-1 cells. Deletion of RAD6 or BRE1, or mutation of H2BK123R enhances the temperature sensitivity of both yku70Δ and cdc13-1 telomere capping mutants. Consistently, BRE1 deletion increases accumulation of telomeric single-stranded DNA (ssDNA) in yku70Δ and cdc13-1 cells, and EXO1 deletion improves the growth of yku70Δ bre1Δ and cdc13-1 bre1Δ cells and decreases ssDNA accumulation. Additionally, deletion of BRE1 exacerbates the rate of entry into senescence of yku70Δ mre11Δ cells with telomere defects, and increases the recombination of subtelomeric Y' element that is required for telomere maintenance and survivor generation. Furthermore, Exo1 contributes to the abrupt senescence of yku70Δ mre11Δ bre1Δ cells, and Rad51 is essential for Y' recombination to generate survivors. Finally, deletion of BRE1 or mutation of H2BK123R results in nucleosome instability at subtelomeric regions. Collectively, this study provides a mechanistic link between H2Bub1-mediated chromatin structure and telomere protection after telomere uncapping.

Creating a functional single-chromosome yeast.

Eukaryotic genomes are generally organized in multiple chromosomes. Here we have created a functional single-chromosome yeast from a Saccharomyces cerevisiae haploid cell containing sixteen linear chromosomes, by successive end-to-end chromosome fusions and centromere deletions. The fusion of sixteen native linear chromosomes into a single chromosome results in marked changes to the global three-dimensional structure of the chromosome due to the loss of all centromere-associated inter-chromosomal interactions, most telomere-associated inter-chromosomal interactions and 67.4% of intra-chromosomal interactions. However, the single-chromosome and wild-type yeast cells have nearly identical transcriptome and similar phenome profiles. The giant single chromosome can support cell life, although this strain shows reduced growth across environments, competitiveness, gamete production and viability. This synthetic biology study demonstrates an approach to exploration of eukaryote evolution with respect to chromosome structure and function.

Yeast KEOPS complex regulates telomere length independently of its t6A modification function.

In Saccharomyces cerevisiae, the highly conserved Sua5 and KEOPS complex (including five subunits Kae1, Bud32, Cgi121, Pcc1 and Gon7) catalyze a universal tRNA modification, namely N6-threonylcarbamoyladenosine (t6A), and regulate telomere replication and recombination. However, whether telomere regulation function of Sua5 and KEOPS complex depends on the t6A modification activity remains unclear. Here we show that Sua5 and KEOPS regulate telomere length in the same genetic pathway. Interestingly, the telomere length regulation by KEOPS is independent of its t6A biosynthesis activity. Cytoplasmic overexpression of Qri7, a functional counterpart of KEOPS in mitochondria, restores cytosolic tRNA t6A modification and cell growth, but is not sufficient to rescue telomere length in the KEOPS mutant kae1Δ cells, indicating that a t6A modification-independent function is responsible for the telomere regulation. The results of our in vitro biochemical and in vivo genetic assays suggest that telomerase RNA TLC1 might not be modified by Sua5 and KEOPS. Moreover, deletion of KEOPS subunits results in a dramatic reduction of telomeric G-overhang, suggesting that KEOPS regulates telomere length by promoting G-overhang generation. These findings support a model in which KEOPS regulates telomere replication independently of its function on tRNA modification.

Independent manipulation of histone H3 modifications in individual nucleosomes reveals the contributions of sister histones to transcription.

Histone tail modifications can greatly influence chromatin-associated processes. Asymmetrically modified nucleosomes exist in multiple cell types, but whether modifications on both sister histones contribute equally to chromatin dynamics remains elusive. Here, we devised a bivalent nucleosome system that allowed for the constitutive assembly of asymmetrically modified sister histone H3s in nucleosomes in Saccharomyces cerevisiae. The sister H3K36 methylations independently affected cryptic transcription in gene coding regions, whereas sister H3K79 methylation had cooperative effects on gene silencing near telomeres. H3K4 methylation on sister histones played an independent role in suppressing the recruitment of Gal4 activator to the GAL1 promoter and in inhibiting GAL1 transcription. Under starvation stress, sister H3K4 methylations acted cooperatively, independently or redundantly to regulate transcription. Thus, we provide a unique tool for comparing symmetrical and asymmetrical modifications of sister histone H3s in vivo.