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Chromatin features are thought to have a role in the epigenetic transmission of transcription states from one cell generation to the next. It is unclear how chromatin structure survives disruptions caused by genomic replication or whether chromatin features are instructive of the transcription state of the underlying gene. We developed a method to monitor budding yeast replication, transcription, and chromatin maturation dynamics on each daughter genome in parallel, with which we identified clusters of secondary origins surrounding known origins. We found a difference in the timing of lagging and leading strand replication on the order of minutes at most yeast genes. We propose a model in which the majority of old histones and RNA polymerase II (RNAPII) bind to the gene copy that replicated first, while newly synthesized nucleosomes are assembled on the copy that replicated second. RNAPII enrichment then shifts to the sister copy that replicated second. The order of replication is largely determined by genic orientation: If transcription and replication are codirectional, the leading strand replicates first; if they are counterdirectional, the lagging strand replicates first. A mutation in the Mcm2 subunit of the replicative helicase Mcm2-7 that impairs Mcm2 interactions with histone H3 slows down replication forks but does not qualitatively change the asymmetry in nucleosome distribution observed in the WT. We propose that active transcription states are inherited simultaneously and independently of their underlying chromatin states through the recycling of the transcription machinery and old histones, respectively. Transcription thus actively contributes to the reestablishment of the active chromatin state.

CDK8 and CDK19 form a highly conserved cyclin-dependent kinase subfamily that binds to and inhibits the essential transcription complex, Mediator, and is thought to also activate gene expression by phosphorylating the C-terminal domain (CTD) of RNA polymerase II. Cells lacking either CDK8 or CDK19 are viable and have somewhat limited transcriptional alterations, but how they regulate expression of different genes has not been explained, and whether one of the two kinases must be expressed to allow cell differentiation is unknown. Here, we find that CDK8 and CDK19 are largely functionally redundant for tissue-specific gene expression. Genetic deletion of CDK8 in mice does not affect normal intestinal homeostasis and efficient tumourigenesis, and CDK8 is not required in vivo in cells lacking the main PolII CTD kinase, CDK7. Individual knockout of genes encoding CDK8 or CDK19 in intestinal organoids has only limited effects on gene expression due to their extensive functional redundancy in control of gene expression. Surprisingly, although their combined deletion in organoids reduces longterm proliferative capacity, it is not lethal and allows differentiation. Nevertheless, either CDK8 or CDK19 is required to maintain expression of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) pathway. In double mutant organoids, the CFTR pathway is downregulated, leading to mucus accumulation and increased secretion by goblet cells. Pharmacological inhibition indicates that expression and function of the CFTR pathway is dependent on CDK8/19 kinase activity. We conclude that expression of the Mediator kinases is not essential for cell proliferation and differentiation, but they cooperate to regulate tissuespecific transcriptional programmes.

Pluripotent stem cells (PSCs) transition between cell states in vitro, reflecting developmental changes in the early embryo. PSCs can be stabilized in the naive state by blocking extracellular differentiation stimuli, particularly FGF–MEK signalling. Here, we report that multiple features of the naive state in human and mouse PSCs can be recapitulated without affecting FGF–MEK signalling or global DNA methylation. Mechanistically, chemical inhibition of CDK8 and CDK19 (hereafter CDK8/19) kinases removes their ability to repress the Mediator complex at enhancers. CDK8/19 inhibition therefore increases Mediator-driven recruitment of RNA polymerase II (RNA Pol II) to promoters and enhancers. This efficiently stabilizes the naive transcriptional program and confers resistance to enhancer perturbation by BRD4 inhibition. Moreover, naive pluripotency during embryonic development coincides with a reduction in CDK8/19. We conclude that global hyperactivation of enhancers drives naive pluripotency, and this can be achieved in vitro by inhibiting CDK8/19 kinase activity. These principles may apply to other contexts of cellular plasticity., M.N.S. was funded by a Leverhulme Trust early career fellowship. Work in the laboratory of M.Z.-G. was funded by the Wellcome Trust (098287/Z/12/Z) and the European Research Council (ERC) (669198). I.C. was funded by the Secretaria d’Universitats i Recerca de la Generalitat de Catalunya and European Social Fund. I.A. and P.S. were supported by the Fondation pour la Recherche Medicale (DEQ20170336757), Infrastructure Nationale en Biologie et Santé INGESTEM (ANR-11-INBS-0009), IHU-B CESAME (ANR-10-IBHU-003), LabEx REVIVE (ANR-10-LABX-73), LabEx DEVweCAN (ANR-10-LABX-0061) and LabEx CORTEX (ANR-11-LABX-0042) of University of Lyon within the programme ‘Investissements d’Avenir’ (ANR-11-IDEX-0007). Research by J.P., S.M. and C.B.-A. was supported in part by a grant from the Spanish Ministry of Economy and Competitiveness (SAF2013-44267-R) and by the CNIO. Work in the laboratory of D.F. was funded by the Institut National du Cancer (PLBIO10-068 and PLBIO15-005) and the Ligue National Contre le Cancer (EL2018.LNCC/DF). Work in the laboratory of N.M. was funded by the ERC, under the European Union Horizon 2020 research and innovation programme (StG-2014–640525_REGMAMKID), the Spanish Association Against Cancer (AECC/LABAE16006), Carlos III Health Institute (Red TerCel, CardioCel, RD16/0011/0027), Ministry of Economy and Competitiveness (MINECO) projects SAF2017–89782-R, SAF2015–72617-EXP and RYC-2014–16242, and the CERCA/Government of Catalonia (2017 SGR 1306). Work in the laboratory of S.O. was funded by SAF2013–44866-R from MINECO Spain. Work in the laboratory of M.F.F. was funded by Plan Nacional de I+D+I 2013–2016/FEDER (PI15/00892, to M.F.F. and A.F.F.); the ISCIII-Subdireccion General de Evaluación y Fomento de la Investigación and Plan Nacional de I+D+I 2008–2011/FEDER (CP11/00131, to A.F.F.); IUOPA (to M.I.S.); and the Asturias Regional Government (GRUPIN14–052, to M.F.F.). The IUOPA is supported by the Obra Social Liberbank-Cajastur, Spain. Work in the laboratory of M.S. was funded by the CNIO, the IRB and by grants from Spanish Ministry of Economy co-funded by the European Regional Development Fund (SAF2017-82613-R), ERC (ERC-2014-AdG/669622), Botin Foundation, Banco Santander (Santander Universities Global Division), laCaixa Foundation and Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement of Catalonia (Grup de Recerca consolidat 2017 SGR 282).

International audience; The Mps1 kinase corrects improper kinetochore-microtubule attachments, thereby ensuring chromosome biorientation. Yet, its critical phosphorylation targets in this process remain largely elusive. Mps1 also controls the spindle assembly checkpoint (SAC), which halts chromosome segregation until biorientation is attained. Its role in SAC activation is antagonised by the PP1 phosphatase and involves phosphorylation of the kinetochore scaffold Knl1/Spc105, which in turn recruits the Bub1 kinase to promote assembly of SAC effector complexes. A crucial question is whether error correction and SAC activation are part of a single or separable pathways. Here, we isolate and characterise a new yeast mutant, mps1-3, that is severely defective in chromosome biorientation and SAC signalling. Through an unbiased screen for extragenic suppressors, we found that mutations lowering PP1 levels at Spc105 or forced association of Bub1 with Spc105 reinstate both chromosome biorientation and SAC signalling in mps1-3 cells. Our data argue that a common mechanism based on Knl1/Spc105 phosphorylation is critical for Mps1 function in error correction and SAC signalling, thus supporting the idea that a single sensory apparatus simultaneously elicits both pathways.

Hepatocellular carcinoma (HCC) is a common cancer with high mortality. The limited therapeutic options for advanced disease include treatment with Sorafenib, a multi-kinase inhibitor whose targets include the Mediator kinase CDK8. Since CDK8 has reported oncogenic activity in Wnt-dependent colorectal cancer, we investigated whether it is also involved in HCC. We find that CDK8 and its paralogue CDK19 are significantly overexpressed in HCC patients, where high levels correlate with poor prognosis. Liver-specific genetic deletion of CDK8 in mice is well supported and protects against chemical carcinogenesis. Deletion of either CDK8 or CDK19 in hepatic precursors had little effect on gene expression in exponential cell growth but prevented oncogene-induced transformation. This phenotype was reversed by concomitant deletion of TP53. These data support important and non-redundant roles for mediator kinases in liver carcinogenesis, where they genetically interact with the TP53 tumor suppressor.

The SIR complex (Silent Information Regulator) is the building block of heterochromatic structures that silence underlying genes. It is well established that the silenced state is epigenetically inherited but it is not known how the SIR complex is maintained through cell divisions in optimal or variable growth conditions. The biological function of heterochromatin located in subtelomeric regions is also unclear since heterochromatin coverage appears to be limited to a few kbps near chromosome ends and the expression of subtelomeric genes is only marginally affected in the absence of the SIR complex. We use a three pronged approach to address these questions. First, nanopore-MetID, an in vivo foot printing technique similar to DamID that uses nanopore sequencing technology, identified thousands of new transient contacts between Sir3 and euchromatic genes that are not detectable by ChIP-seq and revealed a previously undocumented low-density mode of Sir3 binding to subtelomeric regions that extends 25kbps downstream of subtelomeric SIR nucleation sites. Second, our measurements of genome-wide Sir3 exchange rates after exit from starvation show that heterochromatin is a highly dynamic structure in optimal growth conditions. Third, “spike-in” RNA-seq time course experiments in the same conditions reveal that Sir3 modulates global mRNA levels in correlation with fluctuations in nutrient availability. We now propose that subtelomeric regions serve as Sir3 hubs from which Sir3 diffuses down the chromosome arm and transiently contacts euchromatic genes in its path. We hypothesize that contacts between Sir3 and actively transcribed genes facilitate the removal of stalled transcription complexes and allow for optimal genome-wide transcription, which gives wt cells a competitive advantage oversir3Δcells when nutrients are limited.

International audience; Chromatin is thought to carry epigenetic information from one generation to the next, although it is unclear how such information survives the disruptions of nucleosomal architecture occurring during genomic replication. Here, we measure a key aspect of chromatin structure dynamics during replication-how rapidly nucleosome positions are established on the newly replicated daughter genomes. By isolating newly synthesized DNA marked with 5-ethynyl-2'-deoxyuridine (EdU), we characterize nucleosome positions on both daughter genomes of S. cerevisiae during chromatin maturation. We find that nucleosomes rapidly adopt their mid-log positions at highly transcribed genes, which is consistent with a role for transcription in positioning nucleosomes in vivo. Additionally, experiments in hir1Delta mutants reveal a role for HIR in nucleosome spacing. We also characterized nucleosome positions on the leading and lagging strands, uncovering differences in chromatin maturation dynamics at hundreds of genes. Our data define the maturation dynamics of newly replicated chromatin and support a role for transcription in sculpting the chromatin template.

Nuclear actin influences transcription in a manner dependent on its dynamics of polymerisation and nucleocytoplasmic translocation. Using human somatic cells and transcriptionally-silent Xenopus egg extracts, we show that actin dynamics is also required for DNA replication. We identify many actin regulators in replicating nuclei from Xenopus egg extracts, and show that in human cells, nuclear actin filaments form in early G1 and disassemble prior to S-phase. In either system, treatments that stabilise nuclear actin filaments abrogate nuclear transport and initiation of DNA replication. Mechanistically, actin directly binds RanGTP-importin complexes and disruption of its dynamics hinders cargo release. This prevents both nuclear pore complex (NPC) formation and active nuclear transport, which we show is required throughout DNA replication. Nuclear formin activity is required for two further steps: loading of cyclin-dependent kinase (CDK) and proliferating cell nuclear antigen (PCNA) onto chromatin and initiation of DNA replication. Thus, actin dynamics and formins are involved in several nuclear processes essential for cell proliferation.

Antigen Ki-67 is a nuclear protein expressed in proliferating mammalian cells. It is widely used in cancer histopathology but its functions remain unclear. Here, we show that Ki-67 controls heterochromatin organisation. Altering Ki-67 expression levels did not significantly affect cell proliferation in vivo. Ki-67 mutant mice developed normally and cells lacking Ki-67 proliferated efficiently. Conversely, upregulation of Ki-67 expression in differentiated tissues did not prevent cell cycle arrest. Ki-67 interactors included proteins involved in nucleolar processes and chromatin regulators. Ki-67 depletion disrupted nucleologenesis but did not inhibit pre-rRNA processing. In contrast, it altered gene expression. Ki-67 silencing also had wide-ranging effects on chromatin organisation, disrupting heterochromatin compaction and long-range genomic interactions. Trimethylation of histone H3K9 and H4K20 was relocalised within the nucleus. Finally, overexpression of human or Xenopus Ki-67 induced ectopic heterochromatin formation. Altogether, our results suggest that Ki-67 expression in proliferating cells spatially organises heterochromatin, thereby controlling gene expression., SCOPUS: ar.j, info:eu-repo/semantics/published

One of the major goals of evo-developmentalists is to understand how the genetic mechanisms controlling embryonic development have evolved to create the current diversity of bodyplans that we encounter in the animal kingdom. Tyrosine kinase receptors (RTKs) are transmembrane receptors present in all metazoans known to control several developmental processes. They act via the activation of various cytoplasmic signaling cascades, including the mitogen-activated protein kinase (MAPK), the PI3K/Akt, and the phospholipase C-gamma (PLCgamma)/protein kinase C (PKC) pathways. In order to address the evolution of these three pathways and their involvement during embryogenesis in chordates, we took advantage of the complete genome sequencing of a key evolutionarily positioned species, the cephalochordate amphioxus, and searched for the complete gene set of the three signaling pathways. We found that the amphioxus genome contains all of the most important modules of the RTK-activated cascades, and looked at the embryonic expression of two genes selected from each cascade. Our data suggest that although the PI3K/Akt pathway may have ubiquitous functions, the MAPK and the PLCgamma/PKC cascades may play specific roles in amphioxus development. Together with data known in vertebrates, the expression pattern of PKC in amphioxus suggests that the PLCgamma/PKC cascade was implicated in neural development in the ancestor of all chordates.

L'evo-devo est une jeune discipline dont le but est d'essayer d'expliquer l'evolution morphologique des organismes par la modification des mecanismes developpementaux et des reseaux de genes. Une des questions majeures de cette discipline est l'origine des vertebres. Il semble desormais admis que les vertebres sont issus d'un ancetre chorde invertebre, et plusieurs modeles au sein des representants vivants des chordes sont utilises actuellement pour repondre a cette question. Le petit monde de l'evo-devo s'interessant a l'apparition des vertebres est actuellement en pleine ebullition avec l'arrivee des sequences completes de plusieurs genomes permettant des analyses comparatives de plus en plus fiables, et avec le developpement de modeles "non classiques" auxquels il est desormais possible d'appliquer les techniques necessaires a l'etude fine du developpement embryonnaire. L'un de ces modeles est l'amphioxus (genre Branchiostoma ) dont nous allons decrire ici les caracteristiques faisant de lui un sympathique "filet d'anchois eclairant l'evolution des chordes" (Garcia-Fernandez, 2006a, b).

Cyclin-dependent kinases (CDKs) are the main regulators of cell cycle progression in eukaryotes. The role and regulation of canonical CDKs, such as the yeast (Saccharomyces cerevisiae) Cdc2 or plant CDKA, have been extensively characterized. However, the function of the plant-specific CDKB is not as well understood. Besides being involved in cell cycle control, Arabidopsis (Arabidopsis thaliana) CDKB would integrate developmental processes to cell cycle progression. We investigated the role of CDKB in Ostreococcus (Ostreococcus tauri), a unicellular green algae with a minimal set of cell cycle genes. In this primitive alga, at the basis of the green lineage, CDKB has integrated two levels of regulations: It is regulated by Tyr phosphorylation like cdc2/CDKA and at the level of synthesis-like B-type CDKs. Furthermore, Ostreococcus CDKB/cyclin B accounts for the main peak of mitotic activity, and CDKB is able to rescue a yeast cdc28ts mutant. By contrast, Ostreococcus CDKA is not regulated by Tyr phosphorylation, and it exhibits a low and steady-state activity from DNA replication to exit of mitosis. This suggests that from a major role in the control of mitosis in green algae, CDKB has evolved in higher plants to assume other functions outside the cell cycle.

The anaphase promoting complex or cyclosome is the ubiquitin-ligase that targets destruction box-containing proteins for proteolysis during the cell cycle. Anaphase promoting complex or cyclosome and its activator (the fizzy and fizzy-related) proteins work together with ubiquitin-conjugating enzymes (UBCs) (E2s). One class of E2s (called E2-C) seems specifically involved in cyclin B1 degradation. Although it has recently been shown that mammalian E2-C is regulated at the protein level during the cell cycle, not much is known concerning the expression of these genes. Arabidopsis encodes two genes belonging to the E2-C gene family (called UBC19 andUBC20). We found that UBC19 is able to complement fission yeast (Schizosaccharomyces pombe)UbcP4-140 mutant, indicating that the plant protein can functionally replace its yeast ortholog for protein degradation during mitosis. In situ hybridization experiments were performed to study the expression of the E2-C genes in various tissues of plants. Their transcripts were always, but not exclusively, found in tissues active for cell division. Thus, the UBC19/20 E2s may have a key function during cell cycle, but may also be involved in ubiquitylation reactions occurring during differentiation and/or in differentiated cells. Finally, we showed that a translational fusion protein between UBC19 and green fluorescent protein localized both in the cytosol and the nucleus in stable transformed tobacco (Nicotiana tabacumcv Bright Yellow 2) cells.

Sm core proteins play an essential role in the formation of small nuclear ribonucleoprotein particles (snRNPs) by binding to small nuclear RNAs and participating in a network of protein interactions. The two-hybrid system was used to identify SmE interacting proteins and to test for interactions between all pairwise combinations of yeast Sm proteins. We observed interactions between SmB and SmD3, SmE and SmF, and SmE and SmG. For these interactions, a direct biochemical assay confirmed the validity of the results obtained in vivo. To map the protein-protein interaction surface of Sm proteins, we generated a library of SmE mutants and investigated their ability to interact with SmF and/or SmG proteins in the two-hybrid system. Several classes of mutants were observed: some mutants are unable to interact with either SmF or SmG proteins, some interact with SmG but not with SmF, while others interact moderately with SmF but not with SmG. Our mutational analysis of yeast SmE protein shows that conserved hydrophobic residues are essential for interactions with SmF and SmG as well as for viability. Surprisingly, we observed that other evolutionarily conserved positions are tolerant to mutations, with substitutions affecting binding to SmF and SmG only mildly and conferring a wild-type growth phenotype.

We report the sequence of a 35,600 bp fragment covering the PET123 region on the right arm of chromosome XV from Saccharomyces cerevisiae. This region contains 19 possible open reading frames (ORFs) of which 16 are non-overlapping ORFs. Eight ORFs correspond to the SPP2, SMP3, PDR5, NFI1, PUP1, PET123 and MTR10 loci, described previously. Two ORFs correspond to yeast homologues of genes from other organisms: O3530 is a member of the large ribosomal subunit protein L13 family and O3560 (SME1 gene) is a 94-codon ORF and is a homologue of the mammalian SmE spliceosomal core protein. Three ORFs (O3513, O3521, O3548) present significant similarities to proteins of unknown function and three ORFs (O3510, O3536, O3545) lack homology to sequences within the databases screened.

We have investigated chromosomal segregation during meiosis in a cross between two polymorphic haploid laboratory strains of Saccharomyces cerevisiae, FL100 and GRF18. These two strains have large chromosome-length polymorphisms for chromosomes I and III allowing for easy scoring of parental chromosomes after meiotic segregation. Chromosome III in the FL100 strain was 35 kb shorter than chromosome III in GRF18, while FL100 chromosome I was 40 kb larger than chromosome I in GRF18. Segregation analysis of chromosomes I and III in 50 tetrads showed an apparent association between chromosomes I and III, whereas only the original parental association of chromosomes I and III was found in the spores. By hybridization with chromosome-specific probes we have shown that the polymorphisms are due to a large translocation from chromosome III onto chromosome I in FL100. The translocated fragment is larger than 80 kb and was mapped between Ty and HML. In nine tetrads analyzed, chromosome-length polymorphisms which did not segregate according to Mendelian law were observed.