Vendredi 18 décembre 2009 à 14h30

Bertrand SÉRAPHIN

IGBMC, Strasbourg

Moving... from pre-mRNA splicing to mRNA decay
ou
De l'épissage à la dégradation des ARN messagers

Invité par Antonin Morillon (01 69 82 38 82)

Résumé :

The presentation will summarize the research work performed at the CGM by the various team members. It will cover the switch of the main focus of the group from the analysis of pre-mRNA splicing to mRNA decay. Main results will be presented in the context of the recent developments of our understanding of mRNA decay and of tools dedicated for the analysis protein complexes involved in this process.

La présentation résumera le travail effectué au CGM par les différents membres de l’équipe. L’évolution de la thématique de l’équipe depuis l’analyse de l’épissage vers une étude de la dégradation des ARN messagers sera présentée. Les résultats marquants seront décrits dans le contexte des développements récents de notre compréhension de la dégradation des ARN messagers et de l’analyse des complexes protéiques impliqués dans le métabolisme des ARN.

Jeudi 17 décembre 2009 à 14h30

Salle de conférences, bâtiment 26
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Marie Laetitia BONNÉ

Etude de l'ADN translocase FtsK et de ses interactions avec les recombinases XerCD chez Escherichia coli

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Vendredi 11 décembre 2009 à 14h30

Manuel THÉRY

Physics of the Cytoskeleton and Morphogenesis
LPCV-CEA, Grenoble

Cell Shape and Mechanics Regulate Ciliogenesis in Quiescent Cells

Invité par Jean Cohen (01 69 82 43 73)

Résumé :

Regulation of the balance between cell growth and quiescence is fundamental to embryogenesis and tissue renewal. Cell cycle exit is tightly coupled to the growth of a primary cilium.
We analysed cell cycle exit and ciliogenesis in human epithelial cells and found that all cells exiting the cycle do not generate a primary cilium. Using adhesive micro-patterns of various sizes, we showed that the probability of generating a cilium was negatively correlated to the extension of cell spreading. In addition, most cells in sparse cultures on soft, deformable surfaces had primary cilia contrary to cultures on rigid substrates where few cells were ciliated. Both parameters affect acto-myosin contraction, which appeared to play a dual role: it supports the positioning of the basal body to the cell apex but hinders the extension of the primary cilium.
These results indicate that various physical parameters regulating the level of cell tension control defined stages during ciliogenesis in cell cycle arrested cells.

Vendredi 11 décembre 2009 à 9h30

Salle de conférences, bâtiment 26
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Lise MATHIEU

La biogenèse des complexes respiratoires mitochondriaux
chez la levure Saccharomyces cerevisiae

Soutenance de Thèse de Doctorat - Université Versailles / Saint-Quentin-en-Yvelines

Vendredi 4 décembre 2009 à 14h30

Tâm MIGNOT

Laboratoire de Chimie Bactérienne, Marseille

Dissecting a novel motility machinery in bacteria

Invité par Oliver Espéli (01 69 82 32 14)

Résumé :

In bacteria, studied motility machineries mostly include flagella and type-IV pili. However, many bacterial species have the capacity to move smoothly over solid surfaces; this motion, termed gliding motility, does not involve obvious extracellular organelles and has remained mysterious despite several decades of research.
We study the model bacterium Myxococcus xanthus. In this organism, two models have been proposed to explain gliding motility (A-motility). One model postulates that engines located at the back of the cells propel them forward.
A second model proposes instead, that the motility engines are located within fixed cytoskeleton-anchored focal adhesion complexes distributed periodically along the cell body. During this seminar, I will discuss ongoing cell biology and biophysics approaches to dissect the motility mechanism and its spatial regulation in the cell by dynamic protein oscillations.

Vendredi 27 novembre 2009 à 14h30

Professor Harald STENMARK

Centre for Cancer Biomedicine, Faculty of Medicine
University of Oslo

Endosomal sorting and its role in tumour suppression

Invité par Renaud Legouis (01 69 82 43 74)

Accès au site Web de son laboratoire :

http://folk.uio.no/stenmark/

Vendredi 20 novembre 2009 à 14h30

Martin THANBICHLER

Max Planck Institute for Terrestrial Microbiology, Marburg, Germany

Regulation of cell division by MipZ:
how to establish a protein gradient within a prokaryotic cell

Invité par François-Xavier Barre (01 69 82 33 24)

Résumé

In order to ensure successfull cell division, chromosome replication and segregation have to be tightly coordinated with the positioning and assembly of the cell division apparatus. In many bacteria this objective is achieved by the combined action of nucleoid occlusion and oscillatory waves of MinCDE, which inhibit FtsZ ring formation at the poles and thus restrict establishment of the division apparatus to midcell.
Caulobacter crescentus, which lacks homologues of these proteins, has recently been shown to coordinate formation of the FtsZ ring near midcell by means of a newly identified essential ATPase, called MipZ. At the beginning of the cell cycle, MipZ is localized to the stalked cell pole by interaction with the origin-bound ParB•parS complex, whereas FtsZ is found at the opposite pole. As soon as chromosome replication is initiated and the origin region is duplicated, a second MipZ•ParB•parS complex appears which rapidly traverses the cell to the other pole. As a consequence, a gradient of MipZ is established within the cell, with its concentration being highest at the poles and lowest at the cell center. Due to an inhibitory effect of MipZ on FtsZ polymerization, the polar FtsZ cluster disassembles and FtsZ relocates to midcell, where it initiates cell division by polymerizing into a stable FtsZ ring.
Using several mutant MipZ derivatives, we show that a dynamic equilibrium between the ATP-bound, dimeric state and the ADP-bound or nucleotide-free, monomic state is crucial for the function of MipZ. We demonstrate that MipZ does not only interact with FtsZ and ParB but is also able to bind unspecifically to the nucleoid. Based on our in vivo and in vitro data, we present a model that explains how ATP binding and hydrolysis modulate the interaction of MipZ with its different binding partners and thereby enable this ATPase to coordinate chromosome segregation with cell division in both a temporal and spatial manner.

Vendredi 13 novembre 2009 à 14h30

Frank C. P. HOLSTEGE

UMC Utrecht - The Netherlands

Expression-profile phenotypes for understanding regulatory circuitry

Invité par Domenico Libri (01 69 82 38 09)

Résumé

The availability of whole genome sequences and the parallel development of various high-throughput techniques is making it possible to analyze and understand regulatory processes in a systematic manner. In the long-term, this will lead to the development of genome control maps, exhaustive wiring diagrams which describe in intricate detail the contribution of every regulatory factor towards regulation of every single gene. We are systematically generating DNA microarray mRNA expression-profiles of targeted mutations in components of the signaling and transcription machinery in the yeast S. cerevisiae in order to uncover new regulatory mechanisms.
A previous pilot study which focused on the coregulatory Mediator complex has shown the feasibility of interpreting such expression-profiles for structure-function analyses, discovering new regulatory pathways, uncovering epistasis and pin-pointing the precise effects of regulatory protein modifications on transcription. We have increased the throughput of this approach by extensive robotic automation and have improved the accuracy and precision of microarray technology in various ways, using external control calibration standards to assess improvements.
The results of expression-profiling 800 components of the regulatory machinery will be presented. This includes profiles for all protein kinases, the ubiquitin machinery as well as various gene-specific and global transcription factors.

Vendredi 6 novembre 2009 à 14h30

Martin BARON

Wellcome Trust Centre for Cell-Matrix Research, Faculty of Life Sciences
The University of Manchester, UK

Deltex and endocytic Notch signal activation

Invité par Anne-Marie Pret (01 69 82 31 46)

Résumé

The E3 ubiquitin ligase Deltex can activate Notch in an endocytic-dependent manner. Our data suggest a model for this ligand-independent signalling mechanism. We propose that Deltex has two biochemical functions. Deltex directs Notch into an endocytic pathway and this has two possible end points. If Notch transits into the lysosome lumen, it is degraded. Alternatively a second activity of Deltex directs Notch to the limiting membrane of the lysosome, and degradation of the Notch extracellular domain allows subsequent Presenilin-mediated release of NICD.
Currently we are using cell culture and in vivo assays to identify components of the pathway and are performing structure/function analyses of Notch and Deltex to identify the mechanisms by which the different activities of Deltex are mediated. I will further discuss the developmental requirements for Deltex-regulated Notch signalling which suggest the pathway plays a role in providing robustness to the Notch signalling network in a variety of tissues including the embryo nervous system, the adult wing and during oogenesis.

Jeudi 5 novembre 2009 à 14h00

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Xavier MICHELET

Etude du rôle des protéines VPS de classe E au cours de l'endocytose et du développement
du nématode Caenorhabditis elegans

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Mardi 27 octobre 2009 à 14h00, salle de conférences, bâtiment 26

Christophe LAVELLE

Interdisciplinary Research Institute - USR 3078 - Lille

Chromatin: the DNA manager (and vice versa)

Invité par Claude Thermes (01 69 82 38 28)

Résumé

In eukaryotic and prokaryotic cells, DNA is organized in large supercoiled nucleoprotein complexes generally know as "chromatin". Through dynamics changes in structure resulting from chemical modifications and mechanical constraints imposed by numerous factors in vivo, chromatin plays a critical role in the regulation of DNA metabolism processes, including repair, replication and transcription.
Biophysical approaches such as molecular microscopy and single-molecule micromanipulation have greatly improved our understanding of the way chromatin dynamically packages our genome and participates in the regulation of gene expression. During this seminar, I will discuss how these recent data help us to better understand chromatin topology, structure and dynamics.

Vendredi 23 octobre 2009 à 14h30, salle de conférences, bâtiment 26

Svend PETERSEN-MARHT

DNA Editing Laboratory - Cancer Research UK - London Research Institute

DNA Deaminases - Beyond the Immune Connection

Invité par Bénédicte Michel (01 69 82 32 29)

Résumé

DNA deaminases, enzymes that actively deaminate cytosine in our genomes, are part of our immune defences. They induce DNA instability, leading to activation of DNA damage response pathways, and - under certain circumstances - result in error-prone DNA repair. We have began to explore the possibility that this type of active DNA alteration can also influence other important cellular aspects. Our recent work has uncovered that DNA deaminases can influence meiotic DNA recombination during germ cell formation, as well as alter the important epigenetic mark 5-methyl-cytosine on DNA. The seminar will cover these broad aspects, as well as their direct role in oncogenesis and how they are regulated at the molecular level.

Vendredi 9 octobre 2009 à 14h30

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Laetitia JEZEQUEL

Développement d'approches prédictives pour l'ingénierie des protéines par évolution dirigée
et application au développement d'une thérapie anticancéreuse

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Vendredi 2 octobre 2009 à 14h30

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Julia BERRETTA

Identification et caractérisation fonctionnelle d’un ARN non codant
impliqué dans la régulation du retrotransposon Ty1 chez Saccharomyces cerevisiae

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Jeudi 1er octobre 2009 à 14h30

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Rajani Kanth GUDIPATI

Transcription termination and degradation of RNA in Saccharomyces cerevisiae

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Jeudi 1er octobre 2009 à 11h30

Professor Nick J. PROUDFOOT

Sir William Dunn School of Pathology - University of Oxford

Transcriptional termination of RNA polymerase II: non-coding RNAs and roles in DNA repair and cell cycle progression

Invité par Antonin Morillon (01 69 82 38 82 - 36 38)

Résumé

Our studies on the molecular mechanism and biological significance of transcriptional termination in eukaryotes has opened up a surprisingly intricate array of mechanisms and function. In mammals, Pol II termination directly impacts on gene expression. In S. cerevisiae enormous trouble is taken to stop transcription at the ends of genes and thereby keep transcription from one gene separate from its neighbours. This also prevents transcripts from invading the DNA template, which otherwise result in R-loop formation and consequent DNA damage.
An unexpected twist comes from our studies in S. pombe. Here we show that convergent genes regulate termination so that in G1 cell cycle, termination is inefficient. This results in dsRNA formation with consequent RNA interference induced heterochromatin formation. This transient heterochromatin mark recruits cohesin in G2 which is required to fully align chromosomes prior to mitosis. Our recent studies show that RNAi genes are themselves convergent and are autoregulated by this G1 specific transient heterochromatin state.

Mercredi 30 septembre 2009 à 14h00

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Nouara LHOCINE

Analyse de nouveaux régulateurs de la voie de transduction du signal IMD chez Drosophila melanogaster

Soutenance de Thèse de Doctorat - Université Paris-Diderot (Paris 7)

Lundi 28 septembre 2009 à 14h30

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Romain MERCIER

Organisation du chromosome d’Escherichia coli en macrodomaines :
identification et rôle du système site spécifique matS/MatP

Soutenance de Thèse de Doctorat - Université Paris Sud 11

Vendredi 25 septembre 2009 à 14h30

Maude GUILLIER

Institut de Biologie Physico-Chimique (IBPC) - UPR9073 - Paris

Contrôle post-transcriptionnel par des ARN non-codants chez E. coli :
de la régulation de la synthèse des protéines de membrane externe à celle des systèmes à deux composants

Invitée par Nara Figueroa-Bossi (01 69 82 38 11)

Résumé

Bacterial small RNAs are widespread regulators of gene expression. A large class of these regulators interact with the RNA chaperone Hfq and act by pairing with target-mRNAs.
OmrA and OmrB are two of these sRNAs, that display sequence similarity and whose function is at least partially redundant. They repress the synthesis of at least four outer membrane proteins and we have shown that the same region of OmrA/B, their very conserved 5’ end, is involved in the regulation of all targets tested so far. This allowed us to identify additional direct targets of these regulatory sRNAs by looking for complementarity between this region and the translation initiation region of mRNAs. Interestingly, one of these targets encodes the EnvZ-OmpR two component-system responsible for transcriptional activation of the omrA/B genes.

Jeudi 24 septembre 2009 à 11h30

Dominique FOURMY

Institut de Chimie des Substances Naturelles (ICSN) - Gif-sur-Yvette

Synthèse des protéines par le ribosome : approches structurales et dynamiques

Invité par Domenico Libri (01 69 82 36 63)

Résumé

Le ribosome est un moteur moléculaire qui avec l'aide de facteurs d'élongation se déplace sur l'ARNm. Ce mouvement appelé translocation est couplé à une activité ARN hélicase qui permet au ribosome de fondre les régions structurées de l'ARNm. L'interaction du ribosome avec de multiples facteurs protéiques ainsi que des mouvements au sein des sous-unités/domaines structuraux du ribosome font de la synthèse des protéines un mécanisme complexe et dynamique. Une étape cruciale dans l'étude du ribosome consiste à caractériser cette dynamique. Nous décrirons quelques résultats de nos études concernant le recodage du code génétique, le décalage de la phase de lecture induit par des structures de l'ARNm et le développement de nouveaux outils de fluorescence afin d'observer en temps réel la synthèse d'une protéine par un ribosome fixé sur une surface.

Lundi 21 septembre 2009 à 14h00

Salle de conférences, bâtiment 24
Centre de Génétique Moléculaire
CNRS Gif-sur-Yvette

Riyad EL KHOURY

Effets de mutations de la translocase ATP/ADP mitochondriale
sur la stabilité de l'ADN mitochondrial et la longévité chez le champignon filamentaux Podospora anserina

Soutenance de Thèse de Doctorat - Université Paris Sud 11

Lundi 14 septembre 2009 à 11h30

Tom COOPER

Department of Molecular and Cellular Biology - Baylor College of Medicine - Houston - Texas

An RNA gain of Function Disrupts Developmentally Regulated Alternative Splicing in Myotonic Dystrophy

Invité par Joëlle Marie (01 69 82 38 00)

Résumé

The recent realization that most human genes generate multiple protein isoforms via alternative splicing has revealed an extensive degree of regulation that remains to be explored. Our lab is interested in understanding the mechanisms of this regulation, from how RNA binding proteins regulate splicing of individual pre-mRNAs to the signaling events that coordinate splicing networks during development. We also study myotonic dystrophy, the most common form of adult onset muscular dystrophy, in which disrupted splicing regulation causes the primary features of the disease.
I am planning to talk about splicing elements and how mutations can cause disease through splicing - to put the talk in perspective. Then a very brief introduction to alternative splicing. Next I will talk about myotonic dystrophy and the mechanism by which expression of the expanded repeats cause disease: through sequestration of MBNL and activation of PKC. The next section emphasizes that the disease is caused by disruption of a program of developmentally regulated splicing and I will talk about what we know of this program - that half of the splicing events that are regulated during heart development are regulated by CUGBP1 and MBNL1.

Vendredi 11 septembre 2009 à 14h30

Arndt BENECKE

Systems Epigenomics Group, Institut des Hautes Études Scientifiques & Interdisciplinary Research Institute

Systematic analysis of genome activity in the context of biomedical research using geometric methods

Invité par Claude Thermes (01 69 82 38 28)

Résumé

In search of an efficient method for gene regulatory network inference in the context of biomedical research we have recently proposed a new framework for the systematic representation of a genome using probability profiles. These profiles are derived from empirical and functional genomics data, model predictions, and the genome sequence itself. Some insights into the applicability and the mathematical nature of such a representation are discussed. Furthermore, one or two examples of the analysis of transcriptome and epigenome activity in the immune system will be presented.

Vendredi 4 septembre 2009 à 14h30

Kathrin MARHEINEKE

Ecole Normale Supérieure  - Département de Biologie, Laboratoire de Génétique Moléculaire - Paris

Comment dupliquer fidèlement son génome ?
Programme spatio-temporel de la réplication de l’ADN dans des embryons du Xénope et dans des cellules de mammifères

Invitée par Bénédicte Michel (01 69 82 38 84)

Résumé

Il est essentiel pour le maintien de la stabilité génomique que toutes les séquences du génome soient répliquées avant l’entrée en mitose. La régulation de l’activation de milliers d’origines de réplication, pendant la phase S dans le génome des eucaryotes supérieurs, reste relativement mal comprise. Depuis longtemps une corrélation entre transcription et réplication précoce chez les vertébrés est connue. Dans les embryons précoces de Xénope, avant la transition mid-blastuléenne, la phase S est très courte et la réplication démarre à des intervalles rapprochés et sans spécificité de séquence, contrairement aux cellules différenciées. Nous avons montré par peignage moléculaire de l’ADN qu’il existe néanmoins dans le système in vitro de Xénope une régulation temporelle de la phase S. Cette régulation dépend de la kinase « checkpoint » ATR/ATM en absence ou présence d’un stress réplicatif, tout comme dans des cellules différenciées. De plus,  d’autres résultats suggèrent qu’il existe une régulation épigénétique de l’activation temporelle des origines au niveau des foyers de réplication, à un moment du développement ou la transcription zygotique est absente.  Dans des cellules de mammifères différenciées en culture, nous avons récemment montré le rôle des Y ARN comme co-facteurs de l’initiation  et caractérisé  le rôle de la polymérase translesionelle êta dans la réponse de type checkpoint de réplication.

Vendredi 28 août 2009 à 14h30

Professor Hironori NIKI

National Institute of Genetics, Mishima, Japan

A gene network of morphogenesis in the rod shaped bacterium

Invité par Frédéric Boccard (01 69 82 32 11)

Résumé

Voir site Web du laboratoire de H. Niki

Vendredi 24 juillet 2009 à 14h30

Agnieszka SEKOWSKA

Institut Pasteur

« Vintage metabolism », ou comment un sujet passé de mode conduit à de nouvelles découvertes

Invitée par Bénédicte Michel (01 69 82 38 84)

Résumé

Le travail exposé décrit les premiers éléments du métabolisme des polyamines chez B. subtilis. Ce métabolisme nous a conduit à analyser le devenir du produit accessoire de la réaction de biosynthèse de la spermidine, la méthylthioadénosine. Notre étude débouche alors sur l'analyse détaillée du métabolisme de cette molécule, et plus généralement sur l'étude des cycles métaboliques où intervient l'atome de soufre. Nous nous intéressons aussi aux incompatibilités métaboliques sur l'exemple de métabolisme de la sérine. Et finalement, un travail collaboratif avec des physiciens sur les comportements collectifs bactériens nous a permis de mettre en évidence un des mécanismes de répulsion à l'origine de la formation des colonies bactériennes.
Pour terminer, un projet concernant le devenir de macromolécules modifiées naturellement, ou à la suite des agressions environnementales, sera présenté. L'objectif de ce programme de travail est de mettre au jour quelques-uns des chemins métaboliques impliqués dans ces processus chez les bactéries, en nous fondant initialement sur les bactéries modèles.

Vendredi 17 juillet 2009 à 14h30

Hugues ROEST-CROLLIUS

Ecole Normale Supérieure, Paris

Evolution ancienne et récente dans les génomes de vertébrés

Invité par B. Séraphin (01 69 82 38 84)

Résumé

La génomique comparative a traditionnellement consisté à comparer les génomes d'espèces différentes pour y retrouver des similarités ou constater des différences. Nous avons appliqué ce paradigme à plus de 30 génomes de vertébrés pour reconstruire l'organisation des génomes ancestraux dont ils sont issus. Cette nouvelle ressource nous permet de considérer l'évolution de manière chronologique, en examinant les changements qui se sont produits entre des ancêtres successifs jusqu'à nos jours. Cette analyse met en évidence et permet de quantifier les deux forces principales qui gouvernent les réarrangements dans les génomes: les événements aléatoires et la sélection négative.
Grâce aux nouvelles technologies de séquençage à haut débit la génomique comparative s'étend également à la compraraison de génomes d'individus de la même espèce pour y découvrir des polymorphismes. Nous avons développé une nouvelle méthode pour identifier des événements de balayages sélectifs, c'est-à-dire de cas probables de sélection positive, dans des génomes individuels de quatres espèces de primates. Cela nous a permit de compararer les gènes qui ont récemment (moins de 100.000 générations) été sujets à des événements de sélection positive dans ces lignées et de mettre en évidence un phénomène encore mal connu : la récurrence de la sélection positive sur les mêmes gènes, indépendemment dans des lignées différentes.

Jeudi 9 juillet 2009 à 11h30

Alfonso JARAMILLO

SYNTH-BIO, Epigenomics Project, Université d'Evry

"Computational design in synthetic biology"

Invité par O. Espéli (01 69 82 32 14)

Résumé

We have developed a new computational design methodology for the de novo design of proteins and nucleic acids, and their networks. We will discuss our recent experimental validations where we have synthesized a new thermostable chorismate mutase with a minimal amino acid alphabet, a thioredoxin protein with additional esterase activity or peptides that bind to MHC-I proteins. Afterwards, we will present our automated procedure to design genetic circuits, composed of predefined genetic parts, having a desired time-response and a degree of robustness. We will then discuss our designs for new transcriptional devices having a targeted switching or oscillatory behavior. Finally, we will present another computational procedure we have created for de novo design of metabolic pathways using a retrosynthetic algorithm. This tool will allow grafting new bioproduction pathways into a given cellular chassis or to find new degradation pathways for bioremediation applications.

Mardi 7 juillet 2009 à 14h00

Rodney L. LEVINE

NIH, National, Heart, Lung, & Blood Institute, Bethesda, USA

What in the world is methionine doing in proteins?

Invité par L. Chavatte (01 69 82 32 13)

Résumé

The roles of methionine residues in proteins have not been well defined, but a consideration of available studies leads to the proposal that methionine, like cysteine, functions as an antioxidant and as a key component of a system for regulation of cellular metabolism.  Methionine is readily oxidized to methionine sulfoxide by many reactive species.  The oxidation of surface exposed methionines thus serves to protect other functional essential residues from oxidative damage.  Methionine sulfoxide reductases have the potential to reduce the residue back to methionine, increasing the scavenging efficiency of the system. Methionine sulfoxide reductase isozyme A is found in the cytosol (~2/3) and mitochondria (~1/3). 
However, only a single gene encodes the protein, and its open reading frame begins with a mitochondrial targeting sequence.  In the course of developing transgenic mice overexpressing msrA targeted to the mitochondria or cytosol, we found that the cytosolic form is produced by initiation at a second Kozak sequence found downstream from that which initiates synthesis of the mitochondrially targeted form.  The fractional Initiation at each two site is strongly influenced by the nucleotide present at the -3 position of the first initiation site.  Genetic control is not the only way to direct msrA to the mitochondria. 
We have also found that it translocates from the cytosol to the mitochondria in cells exposed to oxidative stress.  This has been observed in all cell lines tested so far, including embryonic fibroblasts from transgenic mice overexpressing the cytosolic form of msrA.  Translocation is rapid, and it reverses after removal of the oxidative stress.  Given that chronic oxidative stress has been proposed as a causal mechanism of aging, mitochondrial protection by msrA may influence the rate of aging.

Vendredi 3 juillet 2009 à 14h30

Anne PLESSIS

Institut Jacques Monod,
UMR7592 CNRS-Université Paris7-Denis Diderot

Protein trafficking and interactions in the Hedgehog signalling pathway

Invitée par J. Montagne (01 69 82 32 27)

Résumé

Les protéines Hedgehog (HH) jouent un rôle central dans le développement de nombreux animaux et leur dérégulation est à l’origine  de multiples cancers. HH contrôle la transcription de gènes cibles via le facteur de transcription à doigts à Zinc GLI/Ci. La réception et la transduction d’HH font appel aux protéines membranaires Patched et Smoothened qui contrôlent l’activité et la composition de complexes cytoplasmiques incluant GLI/Ci et la protéine kinase Fused.
Nos objectifs sont de comprendre les mécanismes de transduction du signal Hedegehog et d’intégrer ces événements dans un contexte développemental. Nos données basées sur des tests d’interactions protéiques et sur approches d’imagerie de fluorescence et de génétique mettent en lumière l’importance des modifications post-traductionnelles dans le trafic de Patched et Smoothened.

Lundi 29 juin 2009 à 14h30

Olivier ESPELI

Dynamique des chromosomes bactériens

Habilitation à Diriger les Recherches - Université Paris-Sud 11

Devant un jury constitué de :
Bénédicte Michel,
Ivan Matic,
Vincent Géli,
Patrice Polard,
Jean-Pierre Rousset,
Frédéric Boccard

Vendredi 26 juin 2009 à 14h00

Professor Benjamin J. BLENCOWE

Centre for Cellular and Biomolecular Research,
University of Toronto, Canada

Alternative Splicing Regulatory Networks

Invité par B. Séraphin (01 69 82 38 84)

Résumé

Site Web : http://www.utoronto.ca/intron

Vendredi 19 juin 2009 à 14h00

Carolyn K. SUZUKI

Department of Biochemistry and Molecular Biology, University of Medicine and Dentistry of New Jersey
New Jersey Medical School, Newark, USA

Multitasking in the mitochondrion by the Lon protease in protein
and mtDNA quality control

Invitée par B. Guiard (01 69 82 31 84)

Résumé

Human mitochondrial Lon is an ATP-dependent protease that selectively degrades misfolded as well as certain fully folded protein substrates, and also binds to mitochondrial DNA (mtDNA). Lon associates with mtDNA in a sequence-specific and strand-specific manner by preferentially binding to single stranded DNA sequences, which form parallel G-quartets. In living cells, Lon binds to a G-rich site within the control region of mtDNA where replication and transcription are initiated, in addition to other G-rich regions within the genome. We propose that the DNA binding property of Lon recruits the protease to the mitochondrial genome where it selectively degrades proteins involved in mtDNA maintenance. Indeed, Lon has been identified as a core protein component of mitochondrial nucleoids, which are complexes of maintenance proteins that function in mtDNA inheritance and segregation. We show that Lon selectively degrades Transcription factor A of mitochondria (TFAM or mtTFA), which is essential for both mtDNA transcription and replication. Previous work shows that TFAM protein levels determine mtDNA copy number. TFAM knockout in mice leads to loss of mtDNA, whereas TFAM overexpression upregulates mtDNA copy number. Interestingly, cells that have reduced mtDNA content have dramatically reduced levels of TFAM protein although transcript levels are unchanged. We demonstrate that mitochondrial Lon is responsible for the selective turnover of TFAM that is not bound to mtDNA. Blocking Lon-mediated proteolysis of TFAM leads to increased mtDNA copy number in cells that substantially lack mtDNA. Insights into the role of mitochondrial Lon in human health and disease will be discussed.

Vendredi 12 juin 2009 à 14h00

François LEULIER

Rupture de tolérance immunitaire innée :
Mécanismes et conséquences chez un organisme modèle invertébré

Habilitation à Diriger les Recherches - Université Paris-Sud 11

Devant un jury constitué de :
Frédéric Boccard
Jean Kanellopoulos
Jean-Marc Reichhart
Bruno Lemaitre
Philippe Sansonetti
François Schweisguth

Jeudi 11 juin 2009 à 11h30

Arthur GROSSMAN

Department of Plant Biology, Carnegie Institution for Science
Stanford, USA

Molecular and Genomic approaches to dissect the ways in which Chlamydomonas
senses and responses to its sulfur environment

Invité par A.-S. Steunou (01 69 82 31 53)

Résumé

Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the ~120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.
            Sulfur (S) is an essential element present in proteins, lipids, carbohydrates, electron carriers, various metabolites (some involved in the detoxification of heavy metals and xenobiotics), and signaling molecules. In the absence of sulfur (S), Chlamydomonas reinhardtii increases the abundance of several transcripts encoding proteins associated with S acquisition and assimilation, conserves S amino acids, and acclimates to suboptimal growth conditions. A positive regulator, SAC1 (for sulfur acclimation protein 1), and a negative regulator, SAC3, were shown to participate in the control of these processes. In this study, we investigated two allelic mutants (ars11 and ars44) affected in a gene encoding a SNRK2 (for SNF1-related protein kinase 2) kinase designated SNRK2.1. Like the sac1 mutant, both snrk2.1 mutants were deficient in the expression of S-responsive genes. Furthermore, the mutant cells bleached more rapidly than wild-type cells during S deprivation, although the phenotypes of ars11 and ars44 were not identical: ars11 exhibited a more severe phenotype than either ars44 or sac1. The phenotypic differences between the ars11 and ars44 mutants reflected distinct alterations of SNRK2.1 mRNA splicing caused by insertion of the marker gene. In contrast to the nonepistatic relationship between SAC3 and SAC1, characterization of the sac3 ars11 double mutant showed that SNRK2.1 is epistatic to SAC3. These data reveal the crucial regulatory role of SNRK2.1 in the signaling cascade critical for eliciting S deprivation responses in Chlamydomonas. The phylogenetic relationships and structures of the eight members of the SNRK2 family in Chlamydomonas are discussed

Voir site web de A. Grossman

Vendredi 5 juin 2009 à 14h30

Carla SALEH

Institut Pasteur

RNA interference: the antiviral immune system of insects

Invitée par A.-M. Pret (01 69 82 31 46)

Résumé

RNA interference (RNAi) is a conserved sequence-specific, gene-silencing mechanism that is induced by double-stranded RNA (dsRNA). One of the functions of this pathway is the defense against foreign nucleic acids: transposons and viruses. Previous work established that core components of the RNAi machinery, e.g. Ago2, Dcr2, R2D2, etc., are required for the antiviral effect. We have shown that uptake of exogenous dsRNA is essential to establish protective antiviral immunity in adult flies. Flies with mutations in genes responsible for uptake and/or intracellular transport of exogenous dsRNA are unable to control Drosophila C virus and Sindbis virus replication and they are hypersensitive to infection. The role of exogenous dsRNA in systemic antiviral immunity was further examined by inoculating dsRNA into flies, which completely prevents viral replication in a sequence specific manner. Importantly, mutants flies defective in dsRNA uptake were unable to mount an antiviral immune response after dsRNA inoculation. These findings establish the antiviral RNAi-response in arthropods as a process akin to adaptive immunity in vertebrates and underscore the necessity of the transport and uptake of dsRNA to confer effective antiviral immunity.

Jeudi 28 mai 2009 à 11h30

Véronique ARLUISON

Laboratoire Jean Perrin, Université Denis Diderot (Paris 7)

Auto-assembly as a new regulatory mechanism of noncoding RNA

Invitée par L. Chavatte (01 69 82 32 13)

Résumé

RNA has the remarkable property of serving both as information storage (like DNA) and as a catalyst (like proteins). Beside the ubiquitous messenger RNAs serving as temporary copies of genes for protein synthesis, numerous non coding RNAs (ncRNAs) are found in cells, including small interfering (siRNA) and micro RNA (miRNA) in eukaryotes and small noncoding RNA (sRNA) in prokaryotes. In E. coli, there are around 100 known sRNAs of 50–400 nucleotides long that affect cell viability in numerous ways (e.g. resistance to oxidants and UV irradiation, control of cell division), often by interfering with translation by base pairing with the mRNA around the ribosome-binding site.
Interactions between different RNA molecules are quite common and many biological functions involved the assembly of RNA (e.g. dimerization of retroviral RNA). However, long RNA polymers have so far only been observed with artificial sequences, leading to RNA tectonics with the aim of designing RNA-based nanobiomaterials. We recently presented evidence that polymer of a natural ncRNA may in fact exist in the bacterial cell. Two sRNAs from E.  coli (DsrA -87 nt -and GcvB -206 nt), both known to control the translation and the turnover of some mRNAs, show striking auto-assembly properties, as assessed with various techniques (including gel electrophoresis and molecular microscopy). We propose that the formation of such polymers could be involved in the regulation of ncRNA concentration in vivo or in a quality control mechanism used by cells to eliminate misfolded RNAs. RNA auto-assembly could thus represent a new mean to regulate RNA-related metabolism.

Jeudi 7 mai 2009 à 11h30

Gabor JUHASZ

Department of Anatomy, Cell and Developmental Biology,
Eotvos Lorand University, Budapest

Role and regulation of autophagy in Drosophila

Invité par Jacques Montagne (01 69 82 32 27)

Résumé

Autophagy (cellular self-digestion by lysosomes) is activated in response to starvation in all eukaryotic cells, and ensures survival by recycling dispensable cellular constituents for re-use in synthetic processes. In animals, autophagy has been adopted to serve additional functions beyond starvation survival that include regulation of lifespan, stress responses, cellular homeostasis, cell death, immunity, cancer, neurodegeneration diseases, obesity and lipid metabolism, and more, so autophagy research has a great medical relevance.
Drosophila is one of the best models to study autophagy. Rapidly growing larval polyploid tissues like the fat body, the analogue of our liver and white fat, are programmed to undergo autophagy during starvation or metamorphosis, this way providing energy and nutrients to diploid cells that will make the adult fly by the end of the pupal stage. I will present our recent results on the genetic regulation of autophagy in Drosophila, and how this process is involved in aging, cell growth and cell death, metamorphosis, and stress responses.

Jeudi 30 avril 2009 à 11h30

Jean-Paul VINCENT

National Institute for Medical Research, London

Growth control and cell fate specification by the Wingless gradient

Invité par F. Agnès (01 69 82 43 85)

Résumé

Morphogens are signaling molecules that contribute to pattern formation by forming a concentration gradient and controlling target gene expression in a dose-dependent manner Recently, morphogens have also been shown to regulate tissue growth. We study both activities in imaginal discs of Drosophila, epithelia sacs that give rise to appendages such as the wings. Two signalling molecules can be considered the cardinal wing organisers: Dpp, which patterns the Anterior/Posterior (A/P) axis and Wingless, which spreads from the Dorso/Ventral (D/V) boundary.
Our work focuses on Wingless, the main Drosophila member of the Wnt family of lipoglycoproteins, a family of proteins associated with numerous cancers in humans. I will give a brief overview of how Wingless spreads within tissues and then discuss specifically how the Wingless gradient is read and interpreted by the cells of the prospective wing. Our results suggest that cells are able to compare their level of signalling with that of their neighbors and that the relative levels of signalling are crucial both for cell fate specification as well as growth.

Jeudi 16 avril 2009 à 11h30

M. Teresa TEIXEIRA

Laboratoire de Biologie Moléculaire de la Cellule, CNRS/ENS Lyon, Lyon

Telomeres: from the "DNA end replication problem" to the control of proliferation of eukaryotic cells

Invitée par A. Morillon (01 69 82 38 82 - 36 38)

Résumé

Telomeres are essential nucleoprotein complexes that protect eukaryotic chromosome ends from fusion and degradation. During DNA replication and segregation, telomeres face specific problems including a progressive shortening due to the so-called "DNA-end replication problem", that leads to replicative senescence. Telomerase is a specialized reverse transcriptase that counteracts telomere losses by synthetizing telomeric repeats from its template RNA.
We will show that telomerase biogenesis involves several cellular compartments and that telomerase activity is regulated by telomeres themselves that can adopt two states: a telomerase-extendable and non-extendable states. We will also present our studies on replicative senescence. Using a telomerase-negative yeast bearing a single critically short telomere, we demonstrated that length of the shortest telomere in the cell is a major determinant of the onset of replicative senescence.
We have also observed a pre-senescence state that requires a specific pathway of homologous recombination. We propose a model of senescence in two steps where the very first short telomere is initially maintained by sister chromatid recombination and then switches to a signalling state leading to senescence. This mechanism provides an explanation for the maintenance of genome stability as cells stop to proliferate in response to telomerase repression in aging tissues of multicellular Eukaryotes.

Jeudi 9 avril 2009 à 11h30 ANNULÉ ET REPORTÉ

Véronique ARLUISON

Laboratoire Jean Perrin, Université Denis Diderot (Paris 7)

Auto-assembly as a new regulatory mechanism of noncoding RNA

Invitée par L. Chavatte (01 69 82 32 13)

Résumé

RNA has the remarkable property of serving both as information storage (like DNA) and as a catalyst (like proteins). Beside the ubiquitous messenger RNAs serving as temporary copies of genes for protein synthesis, numerous non coding RNAs (ncRNAs) are found in cells, including small interfering (siRNA) and micro RNA (miRNA) in eukaryotes and small noncoding RNA (sRNA) in prokaryotes. In E. coli, there are around 100 known sRNAs of 50–400 nucleotides long that affect cell viability in numerous ways (e.g. resistance to oxidants and UV irradiation, control of cell division), often by interfering with translation by base pairing with the mRNA around the ribosome-binding site.
Interactions between different RNA molecules are quite common and many biological functions involved the assembly of RNA (e.g. dimerization of retroviral RNA). However, long RNA polymers have so far only been observed with artificial sequences, leading to RNA tectonics with the aim of designing RNA-based nanobiomaterials. We recently presented evidence that polymer of a natural ncRNA may in fact exist in the bacterial cell. Two sRNAs from E.  coli (DsrA -87 nt -and GcvB -206 nt), both known to control the translation and the turnover of some mRNAs, show striking auto-assembly properties, as assessed with various techniques (including gel electrophoresis and molecular microscopy). We propose that the formation of such polymers could be involved in the regulation of ncRNA concentration in vivo or in a quality control mechanism used by cells to eliminate misfolded RNAs. RNA auto-assembly could thus represent a new mean to regulate RNA-related metabolism.

Vendredi 3 avril 2009 à 14h30

Rodney ROTHSTEIN

Dept. Genetics & Development, Columbia University, New York

Yeast as a budding stem cell? - Kinetochore asymmetry defines a single yeast lineage

Invité par O. Espéli (01 69 82 32 14)

Résumé

"Adult stem cells” are present in most human organs and serve to repair and replace damaged tissue. Whereas “embryonic stem cells” are capable of creating any and all cell types, adult stem cells produce only the tissue from which they are derived. For example, bone marrow contains adult blood stem cells, which can be used to treat b-thalassemia patients. Adult stem cells must divide asymmetrically to maintain a pool of stem cells like themselves, while also giving rise to new cells to replenish those within the organ. Although microbes often divide asymmetrically, the asymmetry is present in all cells; in contrast adult stem cells only divide asymmetrically in one lineage of cells directly descended from the original stem cell.
For the first time, we have identified an asymmetric cell division in baker’s yeast that is confined to a single lineage, as seen in adult stem cells. We observed that the kinetochore proteins, which separate chromosomes during cell division, divide asymmetrically, but only in a single lineage. Since kinetochore proteins separate DNA during cell division, this asymmetry suggests a mechanism for the asymmetric distribution of the genetic material. Such a division is of interest since it has been suggested that adult stem cells undergo asymmetric chromosome segregation. After DNA replication, one of the two strands of each chromosome is new and may have unwanted mutations introduced during the copying process. To keep only error-free chromosomes, the stem cells may selectively keep old DNA strands for themselves – thus avoiding the accumulation of mutations.
Our findings suggest that the fundamental mechanisms of asymmetric divisions seen in adult stem cells originated in much more primitive organisms. By studying such organisms, we can learn about the mechanism of asymmetric stem cell divisions and identify conserved factors that define asymmetrically-dividing cells, which may aid in the identification of adult stem cell populations in humans. These stem cells may be useful therapeutically to restore organ function (e.g., islet stem cells in the pancreas for diabetes treatment or neuronal stem cells for treating many brain and spinal cord disorders).

Jeudi 2 avril 2009 à 11h30

Alex GOULD

MRC National Institute for Medical Research, Mill Hill, London

Hepatocyte-like cells in Drosophila: from formation to foie gras

Invité par J. Montagne (01 69 82 32 27)

Résumé

Efforts to harness the power of Drosophila genetics to model human fat metabolism have been limited by the lack of information on how and where insect lipids are processed. We have identified the oenocyte as a Drosophila cell type that is induced from the embryonic ectoderm by EGF Receptor signalling. Oenocytes appear to be functionally related to mammalian hepatocytes in that, during fasting, they accumulate lipid released from fat tissue. Tissue-specific manipulations of the Slimfast amino-acid transporter, the Lsd2 fat-storage regulator and the Brummer lipase indicate that oenocytes act downstream of adipose tissue. In turn, oenocytes are required for depleting stored lipid from adipose tissue during fasting. In addition, they specifically express more than 20 different lipid-metabolic proteins, including a P450 hydroxylase regulating triacylglycerol composition.
I will discuss the roles that oenocytes may play in regulating growth, development and feeding behaviour and compare and contrast some of their functions to those of the mammalian liver.

Jeudi 26 mars 2009 à 11h30

Mohammed MOUDJOU

INRA, Unité de Virologie et Immunologie Moléculaires,
Equipe Infection à Prions, Centre de Jouy-en-Josas

Etude des maladies à Prions : exemple de la tremblante du mouton

Invité par C. Klotz (01 69 82 43 92)

Résumé

Les encéphalopathies spongiformes transmissibles (EST), appelées aussi maladies à Prions, sont des maladies neurodégénératives fatales touchant l'homme et les animaux. Elles se caractérisent par l’accumulation dans le tissu nerveux et certains tissus lymphoïdes d’une protéine, la PrPsc (scrapie), conformère pathologique d’une glycoprotéine membranaire, la PrPc (cellulaire). La conversion de la PrPc en PrPsc est considérée comme le phénomène majeur responsable de ces maladies. La PrPsc est le seul marquer moléculaire identifié de l’agent infectieux.
L'existence au sein d'une même espèce de souches de prions ayant des propriétés biologiques distinctes est un phénomène dont le déterminisme moléculaire reste encore inconnu. L’utilisation de lignées de souris transgéniques exprimant la PrP ovine a permis au laboratoire d’étudier la diversité des souches ovines à partir d’isolats de terrain. De plus, ce travail a permis d’identifier une nouvelle souche de prion ovin qui touche les moutons qui jusqu’à une date récente étaient considérés comme résistants à la tremblante classique.
Le laboratoire développe également des systèmes cellulaires permissifs aux prions ovins afin d’étudier la biologie de l’agent infectieux. Un exemple de ces études sera abordé dans le séminaire. D’autres parts, la production et la caractérisation d’anticorps monoclonaux dirigés contre la PrP ovine a permis d’obtenir des anticorps anti-PrP dont les spécificités glycotypiques et allèliques sont originales.

Jeudi 5 mars 2009 à 11h30

Professor Fernando DE LA CRUZ

Departamento de Biología Molecular, Instituto de Biomedicina y Biotecnología de Cantabria
Universidad de Cantabria, Spain

Plasmid R388 as a model for systems biology

Invité par B. Michel (01 69 82 32 29)

Jeudi 19 février 2009 à 11h30

Chantal IOBBI-NIVDOL

Laboratoire de Chimie Bactérienne IFR88-CNRS 3, Marseille

Une nouvelle famille de chaperons multifonctionnels au centre de la biogenèse des molybdoenzymes

Invitée par S. Ouchane (01 69 82 31 65)

Résumé

La biogenèse des métalloprotéines et des molybdoenzymes en particulier est un processus complexe qui nécessite en général l’intervention de chaperons dédiés. Au cours de ce séminaire, nous définirons la superfamille TorD dont les membres sont des chaperons spécifiques de molybdoenzymes procaryotes. Bien que peu homologues entre eux, ces chaperons présentent une structure tridimensionnelle conservée et caractéristique. Nous aborderons également les différents rôles joués par ces chaperons. Ainsi, ces chaperons interviennent dans la protection de l’apoenzyme partenaire en interagissant avec son extrémité N-terminale. De plus, ces chaperons sont directement impliqués lors de l’étape de maturation en maintenant l’apoenzyme dans un état compétent pour l’acquisition du cofacteur. Enfin, ces chaperons agissent comme une plate forme sur laquelle a lieu la dernière étape de biosynthèse du cofacteur à molybdène qui sera inséré au niveau du site catalytique de l’enzyme.

Vendredi 13 février 2009

Pierre KERNER

Etude de l'évolution du système nerveux chez les animaux :
neurogenèse comparative et phylogénomique

Soutenance de Thèse de Doctorat - Université Paris-Sud 11

Devant un jury constitué de :
Pierre Capy, Président
Jean-François Brunet, Rapporteur
Max Telford, Rapporteur
Sylvie Rétaux, Examinatrice
Reiner Veitia, Examinateur
Michel Vervoort, Directeur de thèse

Jeudi 12 février 2009 à 15h00, salle de conférences, bâtiment 26

Julien MOUYSSET

Centre for Molecular Neurobiology Hamburg (ZMNH)

A Novel Role for the C. elegans CDC-48/UFD-1/NPL-4 complex in DNA Replication

Invité par R. Legouis (01 69 82 43 74)

Jeudi 12 février 2009 à 11h30

Gilles FISCHER

Unité de Génétique Moléculaire des Levures, Institut Pasteur

Plasticité chromosomique des génomes de levures

Invité par L. Sperling (01 69 82 32 09)

Résumé

Le groupe des levures hemiascomycètes est le phylum eucaryote dans lequel le plus grand nombre de génomes ont été séquencés (une trentaine actuellement). Nous avons découvert que la diversité évolutive présente au sein de ce groupe est très importante puisqu’elle est comparable à la diversité trouvée au sein de l’embranchement des Chordés. Il en résulte que l’étude des mécanismes d’évolution des génomes chez les levures peut se révéler riche d’enseignement pour la compréhension de l’évolution des génomes eucaryotes dans leur ensemble. Nous aborderons, au cours de ce séminaire, l'étude de la plasticité des chromosomes sous deux aspects différents. D'une part, nous présenterons les mécanismes moléculaires qui sont à l'origine de certains réarrangements chromosomiques, telles les duplications d'ADN chez Saccharomyces cerevisiae. D'autre part, nous discuterons de la variabilité de la composition en bases des génomes à travers l'exemple du génome de Lachancea kluyveri qui présente une hétérogénéité surprenante de son contenu en bases G+C le long de ses chromosomes.

Jeudi 5 février 2009 à 11h30

Mauro MODESTI

Institut de Biologie Structurale et Microbiologie, Marseille

Visualisation de la dynamique d’interaction de la recombinase humaine RAD51 fluorescente
avec des molécules d’ADN uniques

(le séminaire sera donné en Anglais)

Invité par O. Espéli (01 69 82 32 14)

Résumé :

DNA strand exchange reactions driven by ATP-dependent recombinases (RecA in bacteria or RAD51 in eukaryotes) are central to genome maintenance mechanisms such as DNA double-strand break repair and the rescue of blocked or collapsed DNA replication forks. The recombinase forms a right-handed helical nucleoprotein filament around DNA. The extended, ATP coordinated filament assembled on single-stranded DNA is considered to be the active structure able to promote homology search in an homologous DNA duplex and catalyze strand exchange. Although much has been learned on the structure and biochemical activities of recombinase filaments, the detailed understanding of its mechanism of action requires quantitative determination of its dynamic properties. Since bulk phase or ensemble analysis, as well as static imaging, only provide partial information on complex dynamic processes, we are proceeding by direct real time visualization of fluorescent RAD51 during its interaction with single DNA molecules held and manipulated by means of optical traps.
Here, I will present an analysis of the interaction of RAD51 with double-stranded DNA. I will further introduce current work aimed at analyzing RAD51 interaction with single-stranded DNA, bare or coated with fluorescent RPA (a heterotrimeric single-stranded DNA binding protein).

Jeudi 22 janvier 2009 à 11h30

Martin OTT

Technische Universität Kaiserslautern

Biogenesis of mitochondrially encoded proteins: A critical role for Mrpl36

Invité par N. Bonnefoy (01 69 82 31 75)

Résumé

The respiratory chain of mitochondria allows eukaryotic cells to use oxidative phosphorylation as a highly efficient way to generate ATP. This oxidative phosphorylation machinery is a mosaic of nuclear and mitochondrially encoded components. The processes by which synthesis and assembly of the various subunits are coordinated remain largely elusive. During evolution, many subunits of the mitochondrial ribosome acquired additional protein domains pointing at specific properties or functions of the translation machinery in mitochondria. Here, we analyzed the role of the C-terminal region of Mrpl36, a protein associated with the large subunit of the mitochondrial ribosome. This mitochondria-specific domain is not required for protein synthesis per se and mutants lacking this domain can still synthesize proteins. Importantly, these translation products fail to be properly assembled into respiratory chain complexes and are rapidly degraded. Overexpression of Mrpl36 conversely even increases efficiency of mitochondrial translation. Our data suggest that Mrpl36 plays a critical role during translation that determines the rate of respiratory chain assembly. This important function appears to be carried out by a stabilizing activity of Mrpl36 on the interaction between large and small ribosomal subunits which could influence accuracy of protein synthesis.

Jeudi 15 janvier 2009 à 11h30

Anca SEGALL

Dept Biology, San Diego State University, USA

DNA repair as a novel antibiotic/chemotherapeutic target:
cellular responses to peptides that trap Holliday junctions

Invitée par N. Figueroa-Bossi (01 69 82 38 11)