Structure and evolution of the magnetochrome domains: no longer alone

Magnetotactic bacteria (MTB) can swim along Earth's magnetic field lines, thanks to the alignment of dedicated cytoplasmic organelles. These organelles, termed magnetosomes, are proteolipidic vesicles filled by a 35–120 nm crystal of either magnetite or greigite. The formation and alignment of magnetosomes are mediated by a group of specific genes, the mam genes, encoding the magnetosome-associated proteins. The whole process of magnetosome biogenesis can be divided into four sequential steps; (i) cytoplasmic membrane invagination, (ii) magnetosomes alignment, (iii) iron crystal nucleation and (iv) species-dependent mineral size and shape control. Since both magnetite and greigite are a mix of iron (III) and iron (II), iron redox state management within the magnetosome vesicle is a key issue. Recently, studies have started pointing out the importance of a MTB-specific c-type cytochrome domain found in several magnetosome-associated proteins (MamE, P, T, and X). This magnetochrome (MCR) domain is almost always found in tandem, and this tandem is either found alone (MamT), in combination with a PDZ domain (MamP), a domain of unknown function (MamX) or with a trypsin combined to one or two PDZ domains (MamE). By taking advantage of new genomic data available on MTB and a recent structural study of MamP, which helped define the MCR domain boundaries, we attempt to retrace the evolutionary history within and between the different MCR-containing proteins. We propose that the observed tandem repeat of MCR is the result of a convergent evolution and attempt to explain why this domain is rarely found alone

Crystal structures of an Extracytoplasmic Solute Receptor from a TRAP transporter in its open and closed forms reveal a helix-swapped dimer requiring a cation for alpha-keto acid binding.

International audienceBACKGROUND: The import of solutes into the bacterial cytoplasm involves several types of membrane transporters, which may be driven by ATP hydrolysis (ABC transporters) or by an ion or H+ electrochemical membrane potential, as in the tripartite ATP-independent periplasmic system (TRAP). In both the ABC and TRAP systems, a specific periplasmic protein from the ESR family (Extracytoplasmic Solute Receptors) is often involved for the recruitment of the solute and its presentation to the membrane complex. In Rhodobacter sphaeroides, TakP (previously named SmoM) is an ESR from a TRAP transporter and binds alpha-keto acids in vitro. RESULTS: We describe the high-resolution crystal structures of TakP in its unliganded form and as a complex with sodium-pyruvate. The results show a limited "Venus flytrap" conformational change induced by substrate binding. In the liganded structure, a cation (most probably a sodium ion) is present and plays a key role in the association of the pyruvate to the protein. The structure of the binding pocket gives a rationale for the relative affinities of various ligands that were tested from a fluorescence assay. The protein appears to be dimeric in solution and in the crystals, with a helix-swapping structure largely participating in the dimer formation. A 30 A-long water channel buried at the dimer interface connects the two ligand binding cavities of the dimer. CONCLUSION: The concerted recruitment by TakP of the substrate group with a cation could represent a first step in the coupled transport of both partners, providing the driving force for solute import. Furthermore, the unexpected dimeric structure of TakP suggests a molecular mechanism of solute uptake by the dimeric ESR via a channel that connects the binding sites of the two monomers

MamA as a Model Protein for Structure-Based Insight into the Evolutionary Origins of Magnetotactic Bacteria

International audienceMamA is a highly conserved protein found in magnetotactic bacteria (MTB), a diverse group of prokaryotes capable of navigating according to magnetic fields - an ability known as magnetotaxis. Questions surround the acquisition of this magnetic navigation ability; namely, whether it arose through horizontal or vertical gene transfer. Though its exact function is unknown, MamA surrounds the magnetosome, the magnetic organelle embedding a biomineralised nanoparticle and responsible for magnetotaxis. Several structures for MamA from a variety of species have been determined and show a high degree of structural similarity. By determining the structure of MamA from Desulfovibrio magneticus RS-1 using X-ray crystallography, we have opened up the structure-sequence landscape. As such, this allows us to perform structural-and phylogenetic-based analyses using a variety of previously determined MamA from a diverse range of MTB species across various phylogenetic groups. We found that MamA has remained remarkably constant throughout evolution with minimal change between different taxa despite sequence variations. These findings, coupled with the generation of phylogenetic trees using both amino acid sequences and 16S rRNA, indicate that magnetotaxis likely did not spread via horizontal gene transfer and instead has a significantly earlier, primordial origin

Arbovirus-Derived piRNAs Exhibit a Ping-Pong Signature in Mosquito Cells

The siRNA pathway is an essential antiviral mechanism in insects. Whether other RNA interference pathways are involved in antiviral defense remains unclear. Here, we report in cells derived from the two main vectors for arboviruses, Aedes albopictus and Aedes aegypti, the production of viral small RNAs that exhibit the hallmarks of ping-pong derived piwi-associated RNAs (piRNAs) after infection with positive or negative sense RNA viruses. Furthermore, these cells produce endogenous piRNAs that mapped to transposable elements. Our results show that these mosquito cells can initiate de novo piRNA production and recapitulate the ping-pong dependent piRNA pathway upon viral infection. The mechanism of viral-piRNA production is discussed

Small RNA Profiling in Dengue Virus 2-Infected Aedes Mosquito Cells Reveals Viral piRNAs and Novel Host miRNAs

Contains fulltext : 171518.PDF (publisher's version ) (Open Access)In Aedes mosquitoes, infections with arthropod-borne viruses (arboviruses) trigger or modulate the expression of various classes of viral and host-derived small RNAs, including small interfering RNAs (siRNAs), PIWI interacting RNAs (piRNAs), and microRNAs (miRNAs). Viral siRNAs are at the core of the antiviral RNA interference machinery, one of the key pathways that limit virus replication in invertebrates. Besides siRNAs, Aedes mosquitoes and cells derived from these insects produce arbovirus-derived piRNAs, the best studied examples being viruses from the Togaviridae or Bunyaviridae families. Host miRNAs modulate the expression of a large number of genes and their levels may change in response to viral infections. In addition, some viruses, mostly with a DNA genome, express their own miRNAs to regulate host and viral gene expression. Here, we perform a comprehensive analysis of both viral and host-derived small RNAs in Aedes aegypti Aag2 cells infected with dengue virus 2 (DENV), a member of the Flaviviridae family. Aag2 cells are competent in producing all three types of small RNAs and provide a powerful tool to explore the crosstalk between arboviral infection and the distinct RNA silencing pathways. Interestingly, besides the well-characterized DENV-derived siRNAs, a specific population of viral piRNAs was identified in infected Aag2 cells. Knockdown of Piwi5, Ago3 and, to a lesser extent, Piwi6 results in reduction of vpiRNA levels, providing the first genetic evidence that Aedes PIWI proteins produce DENV-derived small RNAs. In contrast, we do not find convincing evidence for the production of virus-derived miRNAs. Neither do we find that host miRNA expression is strongly changed upon DENV2 infection. Finally, our deep-sequencing analyses detect 30 novel Aedes miRNAs, complementing the repertoire of regulatory small RNAs in this important vector species

Courage, peur, « stress » : l’appréhension des militaires français en opérations extrêmes au XVIIe siècle

Turenne aurait toujours éprouvé une appréhension avant le combat alors que certains de ses hommes n’en ressentaient aucune. La nature de l’ennemi dans les conflits périphériques en terra incognita, face à un adversaire « asymétrique » réputé féroce (le Turc, le Maure, affrontés trois fois entre 1664 et 1669) y a sa part ainsi que l’approvisionnement logistique ou le comportement des chefs. Les émotions irraisonnées (peur, panique, courage) peuvent décider de la bataille. Le soldat français combat l’Infidèle au nom de son roi et pour « l’honneur du nom chrétien ». Ce sentiment identitaire renforce-t-il son moral ? À travers les rapports officiels et les récits des témoins conservés à la Bibliothèque nationale, aux Archives nationales et au Service historique de la Défense, nous examinerons les éléments qui conditionnent la psychologie du militaire engagé en Hongrie, en Crète et en Afrique du Nord.Arnoux Pascal. Courage, peur, « stress » : l’appréhension des militaires français en opérations extrêmes au XVIIe siècle. In: Combattre à l’époque moderne. Actes du 136e Congrès national des sociétés historiques et scientifiques, « Faire la guerre, faire la paix », Perpignan, 2011. Paris : Editions du CTHS, 2013. pp. 63-72. (Actes des congrès nationaux des sociétés historiques et scientifiques, 136-6

The ancient roots of nicotianamine: diversity, role, regulation and evolution of nicotianamine-like metallophores

International audienceNicotianamine(NA)isametabolitesynthesized by allplantsinwhichitisinvolved inthehomeostasisofdifferentmicronutrientssuchasiron,nickelorzinc.Insomeplantsitalsoserves as a precursor of phytosiderophores that are used for extracellular ironscavenging.PreviousworkshavealsoestablishedthepresenceofNA infilamentousfungiandsomemosseswhereasananalogueofNAwasinferredinanarchaea.Morerecently,opine-typemetallophoreswithhomologytoNAwereuncoveredinbacteria, especiallyinhuman pathogens such as Staphylococcus aureus, Pseudomonas aeruginosa or Yersiniapestis, synthesizing respectively staphylopine, pseudopalineand yersinopine.Here, wereviewthecurrentstateofknowledgeregardingthediscovery,biosynthesis,functionandregulationofthesemetallophores.WealsodiscussthegenomicenvironmentofthecntLgene,whichishomologous totheplantNAsynthase(NAS)gene,andplaysacentralroleinthesynthesisofNA-like metallophores.Thisreveals alargediversityofbiosynthetic,export and import pathways.Using sequence similarity networks, we uncovered thatthese metallophores are widespread in numerous bacteria thriving in very differentenvironments,suchasthose livingatthehost-pathogeninterfacebutalsointhe soil. Weadditionally establishedaphylogenyoftheNAS/cntLgeneand,asaresult,weproposethatthisgene isanancientgeneandNA, oritsderivatives, isan ancientmetallophore thatplayedaprominentroleinmetalacquisition ormetalresistance.Indeed,ourphylogeneticanalysis suggests an evolutionary model where the possibility to synthesize thismetallophore waspresentearlyintheapparitionoflife, althoughit waslaterlostbymostliving microorganisms, unless facing metal starvation such as at the host-pathogeninterface or in some soils. According to our model, NA then re-emerged as a centralmetabolitefor metalhomeostasis in fungi,mossesandallknownhigherplants

ETUDE STRUCTURALE DE LA PROTEINE HASA, UN HEMOPHORE IMPLIQUE DANS UNE VOIE ORIGINALE D'ACQUISITION DU FER

LE FER EST UN ELEMENT ESSENTIEL POUR LA PLUPART DES ORGANISMES VIVANTS ; ASSOCIE A DES METALLOPROTEINES, IL INTERVIENT DANS DES MECANISMES AUSSI VARIES QUE LA PHOTOSYNTHESE, LA RESPIRATION OU LE TRANSPORT D'OXYGENE. LES BACTERIES ONT DEVELOPPE PLUSIEURS VOIES D'ACQUISITION DE CET ELEMENT EN FONCTION DES CHANGEMENTS DU MILIEU EXTERIEUR. EN CAS DE CONTACT AVEC UN HOTE UNE SOURCE POTENTIELLE DE FER SE TROUVE DANS LES ERYTHROCYTES QUI CONTIENNENT JUSQU'A 300 MG.ML 1 D'HEMOGLOBINE, UNE PROTEINE CONTENANT UN COFACTEUR A FER : L'HEME. QUELQUES BACTERIES, DONT SERRATIA MARCESCENS, SONT CAPABLES D'UTILISER LE FER DE L'HEME DE L'HEMOGLOBINE PAR L'INTERMEDIAIRE DE LA SECRETION D'UN HEMOPHORE APPELE HASA. CELUI-CI CAPTE L'HEME LIBRE COMME L'HEME LIE A L'HEMOGLOBINE, AFIN DE L'ACHEMINER VERS LE RECEPTEUR DE LA MEMBRANE EXTERNE (HASR) LEQUEL, EN RETOUR, ASSURE SON TRANSPORT VERS L'INTERIEUR DE LA CELLULE. LE MECANISME PAR LEQUEL LA PROTEINE HASA EST SECRETEE FAIT APPEL A UN TRANSPORTEUR DE TYPE ABC. NOUS AVONS DETERMINE LA STRUCTURE TRIDIMENSIONNELLE DU COMPLEXE HEME / HASA DANS TROIS FORMES CRISTALLINES. UN FEUILLET LARGEMENT COURBE, SUR LEQUEL VIENNENT INTERAGIR LES QUATRE HELICES , FORME LE CORPS DE LA PROTEINE. A L'INTERFACE ENTRE CES DEUX PARTIES DE LA STRUCTURE EST LOCALISE LE SITE DE FIXATION DE L'HEME. CETTE FIXATION SE FAIT PAR L'INTERMEDIAIRE DE DEUX LIGANDS AXIAUX : L'HISTIDINE 32 ET LA TYROSINE 75. LA GEOMETRIE DU SITE DE FIXATION DE L'HEME NOUS PERMET D'EXPLIQUER DEUX DES PRINCIPALES PROPRIETES BIOPHYSIQUES DE LA PROTEINE HASA : D'UNE PART LE POTENTIEL REDOX TRES BAS DE L'HEME (550 MV) ET D'AUTRE PART L'AFFINITE RELATIVE DE L'HEME POUR LA PROTEINE QUI DOIT ETRE A LA FOIS ELEVEE (POUR CAPTER L'HEME) ET MODULABLE (POUR POUVOIR RENDRE L'HEME AU RECEPTEUR HASR). L'ABSENCE DE DENSITE ELECTRONIQUE POUR LES QUATORZE DERNIERS RESIDUS IMPORTANTS POUR LA SECRETION INDIQUE QUE LE MOTIF RELATIVEMENT CONSERVE ELLAA EST EXPOSE AU SOLVANT ET EST FLEXIBLE. CETTE MOBILITE POURRAIT ETRE UN PRE-REQUIS DANS LE PROCESSUS DE SECRETION PAR LA VOIE ABC-DEPENDANTE. LA STRUCTURE TRIDIMENSIONNELLE DE L'HEMOPHORE HASA DEFINIT LES BASES STRUCTURALES DES MECANISMES IMPLIQUES DANS LA CAPTATION DE L'HEME. DES EXPERIENCES DE MUTAGENESE DIRIGEE PERMETTRONT D'OBTENIR D'AVANTAGE D'INFORMATIONS CONCERNANT CETTE VOIE ORIGINALE D'ACQUISITION DU FER.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Pratiques d’enseignement de la lecture au CP et acquisitions des élèves

Teaching reading in the first year of school is a complex activity : it requires that one take into account both the dynamics of the class as a group as well as the specificity of each student. It also requires that sequences be organised in such a way that the different components come into play when learning to read (word recognition, identification, sight-reading, comprehension) can be covered during different activities (collective vs. individual). This article seeks to describe the practices used by teachers of first-year classes during a sequence of discovery of a text. Based on observations during this situation, the analysis of teacher-student interactions enabled us to define the specificities of class management and course content during different reading phases. We then turn to estimate the impact of these practices on students’ acquisition of reading skills.Cela demande de prendre en compte à la fois la dynamique de la classe en tant que groupe et la spécificité de chacun des élèves. Cela demande aussi d’organiser une séquence de manière à ce que les composantes qui entrent en jeu dans l’apprentissage de la lecture (reconnaissance des mots, identification, déchiffrage, compréhension) puissent être abordées au cours d’activités différentes (collectives vs individuelles). Cet article s’attache à décrire les pratiques que des enseignants de CP mettent en oeuvre pendant une séquence de découverte d’un texte. Sur la base d’observations en situation, l’analyse des interactions maître-élèves a permis de cerner les spécificités de la gestion de la classe et des contenus au cours de phases de lecture différentes. Dans un second temps, nous avons estimé l’impact des pratiques mises au jour sur les acquisitions des élèves.Arnoux Michèle, Bressoux Pascal, Lima Laurent. Pratiques d’enseignement de la lecture au CP et acquisitions des élèves. In: Les dossiers des sciences de l'éducation, N°19, 2008. Les pratiques d’enseignement-apprentissage : état des lieux. pp. 119-140