Sur la résistance deRickettsiella melolonthae Krieg aux antibiotiques

Zusammenfassung Rickettsiella melolonthae Krieg unterscheidet sich deutlich von den andern Rickettsien durch seine Verh?ltnis mit Beziehung zu Antibiotika und zwar durch seine Resistenz gegen Tetracycline.

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Je remercie vivement le professeurP. Giroud de l'Institut Pasteur pour ses conseils et l'intérêt qu'il a manifesté à ces rechereches.     

La teneur en proline et la résistance des plantes aux sels

Application of sodium chloride to crop plants in the culture medium leads to an increase in their endogenous content of free proline. There is a certain concentration of sodium chloride above which the proline content of the plants strongly rises (critical point). A relationship between endogenous sodium and proline contents could be found. In salt-sensitive plants (wheat) the critical point lies below that of salt-tolerant plants (barley). The critical concentration is not changed by certain alterations of the culture medium. The determination of the critical point by means of measurement of the proline concentration served as a basis for the analysis of the salt tolerance of crop plants.      

Variations du taux d'adsorption du bactériophageϕ80 en rapport avec la résistance à la colicine B

Various mutants ofEscherichia coli selected by colicine B show a wide range of resistance spectra towards colicines B, I and V, phages T₁ andq 80, by mutation in locusbt ₁. Such relations of specificity of resistance seem to result from quantitative variations in the adsorption rate. Indeed, the ability to fix the phage 80, or its derivative the plaque-forming transducing phage 80pt ₁, decreases in correlation with the extent of the spectrum of cross-resistance in the mutants studied. It is never completely abolished even in mutants apparently resistant to this phage, as shown either directly by titration of infective centers or indirectly by the rate of transduction.     

Réactivité du plasmalemme végétal Etude cytochimique

Résumé Plusieurs méthodes cytochimiques utilisées convergent pour indiquer l'existence d'une réactivité du plasmalemme, qui le distingue des autres cytomembranes. 1. Des résultats positifs constants ont été obtenus avec deux sortes de méthodes. Certaines sont effectues sur pièces, utilisant la capture de métaux colloïdaux par les groupements électronégatifs. D'autres sont réalisées sur coupes ultrafines: technique dérivant de l'APS et méthode utilisant l'acide phosphotungstique à bas pH, dans différents milieux d'inclusion. 2. D'autres méthodes fournissent des résultats plus sporadiques, en particulier les techniques de détection d'ions inorganiques.Ces données cytochimiques tendent à indiquer l'existence d'une sorte de glycocalyx au niveau du plasmalemme végétal, évoquant celui des cellules animales. Il s'en distingue toutefois par certains caractères (absence d'action de la neuraminidase). La confrontation de ces résultats avec ceux obtenus après les tests d'activité enzymatique ou après des études histoautoradiographiques, confirme l'idée d'une organisation du plasmalemme, liée à son activité.Ces résultats fournissent des précisions sur les modalités de différenciation des membranes et sur leurs relations dans les phénomènes d'échange entre la cellule et le milieu extérieur (pinocytose, émission de vésicules sécrétrices).

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Reactivity of the plasmalemma. Cytochemical study

Summary Several cytochemical methods converge to indicate a reactivity of the plasmalemma, which makes it different from others cytomembranes. 1. Positive and constant results are obtained with two kinds of methods. Some are performed on specimen blocks, using the binding of colloidal metals with electronegative groups; some are performed on ultrathin sections: procedure derivating from PAS reaction, and method using phosphotungstic acid at a low pH in different embedding resins (epon, araldite, methacrylate, glycol-methacrylate). 2. Rather sporadic results are obtained with other procedures, specially those which demonstrate inorganic ions.These cytochemical data tend to indicate the existence, at the level of the plasmalemma, of a kind of glycocalyx similar to that described in animal cells. It is however different in some characteristics (neuraminidase has no effect on its reactivity).After a comparison of the results with those obtained from enzymatic tests or autoradiographic studies, the idea of a special activity of the plasmalemma is enhanced.These results give some informations on the ways the differenciation of cytomembranes occurs, and on their relations during exchanges between the cell and the exterior (pinocytosis and emission of secretory vesicles).

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Abréviations f fossette sécrétrice (anfractuosité du plasmalemme contenant des polysaccharides) - G dictyosome (appareil de Golgi) - ls lomasome - m mitochondrie - mn membrane nucléaire - mt méat - n noyau - P paroi - pe plasmalemme (pellicule ectoplasmique) - pl plaste - ps plasmodesme - re reticulum endoplasmique - sl slime body (corps muqueux de tube criblé) - t tractus polysaccharidique reliant cytoplasme et paroi - v vésicule cytoplasmique - va vacuole - Grossissement sur chaque cliché, le trait représente 0,5 micron     

Rôle du changement de conformation du CD4 lors de la fusion VIH/cellule

It is well known that the gp120-gp41 complex undergoes a conformational change after CD4 binding. It is likely that CD4 undergoes a conformational change as well. Recently, a calculation of the normal modes of the two N-terminal domains of CD4 has shown that a hinge-bending motion of one of these domains with respect to the other may occur. In the present study, results obtained previously are verified with two other normal mode calculations, starting from crystallographic structures of different origin. A scheme describing the first steps of the process leading to cell infection by human immunodeficiency virus (HIV) is then proposed. It rests upon the idea that CD4 and gp120-gp41 conformational changes allow for bringing the cell and virus membranes closer to each other.     

Observations sur une nouvelle couche pariétale dans les cellules associées aux vaisseaux du Robinier et du Sycomore

Résumé Après avoir achevé leur différenciation, les cellules associées aux vaisseaux élaborent une nouvelle couche pariétale bien distincte des parois primaires et secondaires classiques. Cette couche est située uniquement le long de la paroi commune avec un vaisseau; il n'existe pas de formation semblable le long des parois communes à deux cellules vivantes du parenchyme. Plusieurs méthodes ont été utilisées pour déterminer l'origine, la structure et la composition chimique de cette couche; en particulier, un traitement par le PATAG en révèle la nature polysaccharidique. Son rôle n'a pas été défini avec certitude: il est possible que les substances pariétales qui la composent soient utilisées lors de la formation des thylles.Ce travail a été effectué dans le cadre de l'Equipe de Recherche Associée n 179 et de l'Institut de Biologie Végétale, Université Paris VI, France.

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Observations on a new wall layer in vessel associated cells ofRobinia andAcer

Summary In mature vessel associated cells, a new wall layer is built (after cell differentiation). This layer is quite different from primary and secondary wall and is deposited only on the cell side which abuts a vessel element; no similar structures are seen along the wall separating two living parenchyma cells. Several methods were used to study the origin, structure, and composition of this layer; in particular, polysaccharides are visualized with PATAG technique. The role of this layer is discussed: the new wall materials may be used later during tylose formation.

     

Le modèle du réplicon est-il applicable aux eucaryotes supérieurs?

Thirty-five years ago, the Replicon model was proposed by Jacob, Brenner and Cuzin to explain the regulation of the Escherichia coli DNA replication. In this model, a genetic element, the replicator, would function as a target for a positive-acting initiator protein to drive the initiation of replication. This simple idea has been extremely useful in providing a framework to explain how the initiation of DNA replication occurs in all organisms. The identification of autonomously replicating sequences (ARSs) in budding yeast was the first extension of the Replicon model to eukaryotic chromosomes. In the higher eukaryotes, many biochemically defined replication start sites have been identified; nevertheless there is little genetic data indicating that these sites contain DNA sequences that are essential for replication. Moreover, in early Xenopus or Drosophila embryos, specific DNA sequences are not required either for initiating DNA replication or for preventing rereplication within a single cell cycle. This apparently fundamental difference between replicators in yeast and metazoan embryos may be more superficial than initially thought. In fact, during the past several years, an eukaryotic initiator conserved from yeast to man and also present in embryonic cells, the origin recognition complex (ORC), has been characterized, suggesting that the initiation mechanism should be essentially the same in prokaryotes and eukaryotes. In addition, the efficient once-per-cell-cycle replication of DNA is ensured in eukaryotes by a simple two-step mechanism in which the assembly of stable prereplicative complexes (PreRCs) at origins precedes and is temporally separated from the firing of these origins. Regulation of this process by cyclin-dependent kinases ensures that when origins fire, the cell is no longer competent to form new PreRCs. Now, it is important to understand how these complexes are remodeled or disassembled during replication initiation to trigger the transition from a stable origin-bound complex to a mobile replication machine.     

Techniques de détection du VIH dans les milieux biologiques: Application à la recherche du virus dans le sperme

Growing numbers of HIV seronegative women want to have children with HIV infected partners by artificial insemination. The most sensitive assays for showing the presence of the virus are the coculture method and the DNA-PCR that are able to detect proviral load. HIV is detected from non spermatozoal mononuclear cells, seminal fluid but it was not found in spermatozoa fraction. But we know, by in vitro studies, that HIV can bound to and enter spermatozoa. Thus artificial insemination between HIV seropositive man and seronegative woman lead to a risk of contamination even with purified spermatozoa. Today, any virological assay is able to affirm that specimen is not infectious.