Cell wall precursors are required to organize the chlamydial division septum.

Members of the Chlamydiales order are major bacterial pathogens that divide at mid-cell, without a sequence homologue of the FtsZ cytokinetic tubulin and without a classical peptidoglycan cell wall. Moreover, the spatiotemporal mechanisms directing constriction in Chlamydia are not known. Here we show that the MreB actin homologue and its conserved regulator RodZ localize to the division furrow in Waddlia chondrophila, a member of the Chlamydiales order implicated in human miscarriage. RodZ is recruited to the septal site earlier than MreB and in a manner that depends on biosynthesis of the peptidoglycan precursor lipid II by the MurA enzyme. By contrast, crosslinking of lipid II peptides by the Pbp3 transpeptidase disperses RodZ from the septum. Altogether, these findings provide a cytological framework for understanding chlamydial cytokinesis driven by septal cell wall synthesis

Vitesses de réaction de dissolution et précipitation au voisinage de l'interface oxydo-réducteur dans un lac méromictique : le lac Pavin (Puy de Dôme, France)

Une étude à l'échelle centimétrique de l'interface redox situé à la limite entre mixolimnion et monimolimnion d'un lac méromictique (le lac Pavin) a permis d'observer très finement l'évolution de la concentration d'un certain nombre d'éléments chimiques. Nous avons choisi de présenter ici des résultats concernant 5 éléments qui présentent des comportements très contrastés : le rubidium, le fer, le baryum, le vanadium et le manganèse. La comparaison avec un élément conservatif, le sodium, montre que Rb est conservatif, que Fe, Ba et V sont précipités et que Mn est dissous dans cette zone.Une modélisation de ces concentrations en vue de préciser à quelle profondeur et avec quelle vitesse se produisent les réactions concernant ces éléments nécessite la détermination des paramètres de transport au voisinage de cet interface.Une représentation analytique des concentrations de sodium permet de calculer le coefficient de diffusion turbulente Kz en fonction de la profondeur. Au voisinage de l'interface redox, ce coefficient est très petit (0,0017m2/jour) et inférieur au coefficient de diffusion thermique moléculaire.Les concentrations des éléments étudiés ont pu être représentés avec précisions par des polynômes en fonction de la concentration en sodium.Cela permet d'estimer les vitesses des réactions de précipitation dissolution en fonction de la profondeur. Le rubidium n'est affecté par aucune réaction. Le fer précipite entre 63 et 65 m, le baryum entre 68 et 72 m tandis que le vanadium précipite à la fois dans ces 2 zones. Le manganèse réagit dans une zone très étroite : il est précipité entre 61,5 et 62 m et dissous entre 62,8 et 63,1 m.Une étude similaire de tous les éléments majeurs (y compris pH et COD) pourrait permettre d'élucider les processus qui conduisent à ces comportements complexes.Lake Pavin, French Massif Central, is the main meromictic lake in France and has been extensively studied from more than 50 years. The upper part (mixolimnion) at a depth of less than about 60 m behaves as an oligotrophic lake and is oxic during the major part of the year. The lower layer (monimolimnion) has a higher salinity and is permanently anoxic. Unlike the top of the mixolimnion, element concentrations in the monimolimnion can be considered at steady state. The boundary between mixolimnion and monimolimnion is a redox interface. At this interface, an important number of both chemical and biochemical reactions occur. This boundary, where element concentrations vary greatly, was studied at the centimeter scale between 58 and 64 m depth. The present paper is focused on five elements showing very different behaviour: rubidium, iron, manganese, vanadium and barium. Sodium was used as a reference element. Sodium and rubidium concentrations had similar patterns: a progressive increase began at 61 m depth and the maximal gradient was located at 63 m. They continue to increase towards the bottom of the lake. Iron concentrations were low (< 1 µmol/L) at a depth less than 62.8 m and increased very sharply below this depth. Manganese concentrations were very low in the mixolimnion(<0.01 µmol/L), exhibited a peak between 62.4 and 63.5 m depth (up to 60 µmol/L at 63 m) and reached a value of about 30 µmol/L at 85 m. Barium concentrations began to increase only at depths greater than 65-67 m. Vanadium concentrations in the mixolimnion were about 14 nmol/L, decreased to a minimum below the detection limit at 62.2 m and then increase drastically (150 nmol/L at 85 m).In order to derive the accurate location of the chemical reactions and an estimation of their rates from the concentration profiles, knowledge of the transport parameters was needed. As advection can be considered to be negligible, the major parameter of interest is the vertical eddy diffusion coefficient Kz. Na is assumed to be unreactive in the studied layer. Its concentrations can be represented by an analytical function      Cmax - Cmin                 Cmax + CminC = ___________ * th [P(z)] + ___________             2                                    2with P(z)=0.0016 * (z-zo)3 - 0.0493 * (z-zo)2 + 0.5735 * (z-zo) - 0.4811This allows the determination of the coefficient Kz.Kz = λ ch2 [P(z)]/ [P'(z)]λ is determined from the value of Kz at 85 m depth previously obtained from an hydrodynamic study of the lake (Aeshbach-Hertig et al., 1999). This coefficient is about 0.1 m2/day at the bottom of the monimolimnion. It is very low at the redox interface (0.0017 m2/day), far below the molecular thermal diffusion coefficient. It increases very sharply at the bottom of the mixolimnion. The Kz profile is in fair agreement with the results obtained from the earlier hydrodynamic study. A quantitative study of the dissolution-precipitation reactions at the center of the lake at depths between 55 and 85 m can then be undertaken. The 55 m limit corresponds to a depth where inputs of fresh water can occur. The 85 m limit is about 7 m above the bottom of the lake. Below this depth important inputs from the pore waters occur which are not taken into account by the present modeling. Concentrations of Rb, Fe, Ba and V can be accurately represented by polynomial functions of the Na concentration. The parameter u=th[P(z)] represents the concentrations of these 4 elements by polynomials :          NX(u) = Σ an * un          0The rate of dissolution-precipitation for each element as a function of depth can be derived.                                       NR = - λ [P'(z)] * ch-2[P(z)] Σ ann(n-1) * thn-2[P(z)]                                        0Rb concentrations are a linear function of the Na ones and therefore rubidium is not reactive. Fe concentrations can be related to sodium concentrations by a parabolic relationship. From this relationship, it can be derived that strong iron precipitation occurs in the 63 - 65 m depth layer. V concentrations are related to sodium ones by a 4th degree polynomial. It can be derived that V deposition occurs at depths of 63-65 m and at 70 m.Ba precipitates around 70 m depth. Mn concentrations are represented by [Mn]=a0 +a1 u + b1 exp[-(z-z°)2/z*2] and the derivation shows that Mn is strongly dissolved between 62.8 and 63 m and precipitated just above. These results are in good agreement with a previous study of particles fluxes derived from sediment trap analysis (Viollier et al, 1997).This study shows the complexity of this interface and more comprehensive studies including all major elements, dissolved organic carbon (DOC) and pH are needed

Disassembly of a Medial Transenvelope Structure by Antibiotics during Intracellular Division.

Chlamydiales possess a minimal but functional peptidoglycan precursor biosynthetic and remodeling pathway involved in the assembly of the division septum by an atypical cytokinetic machine and cryptic or modified peptidoglycan-like structure (PGLS). How this reduced cytokinetic machine collectively coordinates the invagination of the envelope has not yet been explored in Chlamydiales. In other Gram-negative bacteria, peptidoglycan provides anchor points that connect the outer membrane to the peptidoglycan during constriction using the Pal-Tol complex. Purifying PGLS and associated proteins from the chlamydial pathogen Waddlia chondrophila, we unearthed the Pal protein as a peptidoglycan-binding protein that localizes to the chlamydial division septum along with other components of the Pal-Tol complex. Together, our PGLS characterization and peptidoglycan-binding assays support the notion that diaminopimelic acid is an important determinant recruiting Pal to the division plane to coordinate the invagination of all envelope layers with the conserved Pal-Tol complex, even during osmotically protected intracellular growth

Gauging the Shadow Sector with SO(3)

We examine the phenomenology of a low-energy extension of the Standard Model, based on the gauge group SU(3) x SU(2) x U(1) x SO(3), with SO(3) operating in the shadow sector. This model offers $\nu_{e} \to \nu_{s}$ and $\nu_{\mu} \to \nu_{\tau}$ oscillations as the solution of the solar and atmospheric neutrino problems. Moreover, it provides a neutral heavy shadow lepton X that could play the role of a cold dark matter particle.Comment: 8 page

Une sonde photométrique pour l'analyse in situ : Principe, méthode, premiers essais

Certains composés dissous ne sont pas stables une fois prélevés hors de leur milieu. Pour éviter que l'information ne se perde entre le prélèvement et l'analyse, il est nécessaire d'effectuer cette dernière in situ. La solution que nous présentons, consiste à développer une réaction colorimétrique en profondeur; la cellule photométrique est immergée et reliée à un spectrophotomètre en surface, par 2 fibres optiques (fig. 1a, b, c). Cependant, lors d'un essai préliminaire, nous avons observé que, dans le circuit de mélange de la sonde, les proportions entre réactif et échantillon ne sont pas constantes. Ces variations de débits sont corrigées par des mesures à deux longueurs d'onde (λ1 et λ2)* et par l'adjonction d'un colorant auxiliaire ne perturbant pas la réaction calorimétrique. L'étalonnage se fait directement sur la cellule photométrique : dans un diagramme Absorbance à λ1 = f (Absorbance à λ2) (fig. 2), on place une droite d'étalonnage et des points particuliers. Les règles de mélange sont vérifiées indépendamment de toute réaction chimique avec différentes solutions d'hélianthine dans un tampon à pH 7 et du rouge de chlorophénol à la place du réactif (fig. 4 et 5). En outre nous utilisons le rouge de chlorophénol, jaune sous forme acide, comme colorant auxiliaire pour le dosage du fer total dans un premier essai in situ (lac d'Aydat, Puy de Dôme, France). Les résultats sont comparés à ceux obtenus par prélèvements et analyses au laboratoire (fig. 6). L'accord est satisfaisant. L'incorporation au système présenté, d'une pompe osmotique devrait permettre, avec cet appareillage simple, des mesures pendant plusieurs mois sans intervention.Various dissolved compounds are mot stables in surface conditions. We realized a prototype to collect chemical data related to redox sensitive species without any contact with the atmospheric oxygen.The principle of this probe for in situ measurements is to produce colorimetric reaction in depth. A photometric cell and a horizontal coiled glass tube for fluid mixing are immersed and connected with two optic fibers to a spectrophotometer on boat (fig. la, b, c). Reagent is injected continuously from surface and sample is sucked up with a peristaltic pump through a tubular filter.Every species which can be analysed by colorimetric method should be determined, in deep river or in lake, with this simple equipment.Nevertheless, during preliminary trial, we detected a lack of reproducibility in the mixing ratio of the sample with the complexing agent. The problem is solved by adding an auxiliary dye with reagent and measuring optical densities at two different wavelengths (λ1 and λ2). In the system, with a the sample proportion, absorbance A at λ is expressed as :A=ɛe∙l∙Ce∙α+ɛr∙l∙Cr∙(1-α)We suggest to calibrate directly the cell of the probe and work in a calibration graph. It is built with first, marking on an A1λ1=f(A2λ2)graph (fig. 2), the « pivot » point (P) (when α= 0), second, plotting the « calibration curve »A2=A1 (ɛe2/ɛe1)(when α= 1), third, plotting the different S1 (A1i, A2i) measured from standards. Therefore, if sample signal Re at two wavelengths is plotted in this graph, by joining P and Re, the straight line intersects with calibration curve at C. On this curve, interpolation of C between two standards determine the concentration of the analyte.Experimental verification of the mixing rule has been clone independently of chemical reaction, with different heliantine solutions in pH 7 buffer as samples and chlorophenol red as reagent, bath in laboratory and at 15 meters depth (Beffes lake, France) (fig. 4 and 5).Furthermore, chlorophenol red, previously tested, is used as auxiliary dye for total iron measurement, in Aydat lake (Puy de Dôme, France) for a first in situ trial.Results are compared to those got from oceanographic bottle sampling and laboratory analysis (fig. 6). Data from the probe are in good agreement with data from the laboratory method.Next development of this chemical sensor will consist in adding to the system an osmotic pump which should allow measurements without intervention during several months

Chaplygin Gas Cosmology - Unification of Dark Matter and Dark Energy

The models that unify dark matter and dark energy based upon the Chaplygin gas fail owing to the suppression of structure formation by the adiabatic speed of sound. Including string theory effects, in particular the Kalb-Ramond field, we show how nonadiabatic perturbations allow a successful structure formation.Comment: 7 pages, presented by N. B. at IRGAC 2006, Barcelona, 11-15 July 2006, typos corrected, concluding paragraph slightly expanded, final version, accepted in J. Phys. A, special issu

FtsZ-independent septal recruitment and function of cell wall remodelling enzymes in chlamydial pathogens.

The nature and assembly of the chlamydial division septum is poorly defined due to the paucity of a detectable peptidoglycan (PG)-based cell wall, the inhibition of constriction by penicillin and the presence of coding sequences for cell wall precursor and remodelling enzymes in the reduced chlamydial (pan-)genome. Here we show that the chlamydial amidase (AmiA) is active and remodels PG in Escherichia coli. Moreover, forward genetics using an E. coli amidase mutant as entry point reveals that the chlamydial LysM-domain protein NlpD is active in an E. coli reporter strain for PG endopeptidase activity (ΔnlpI). Immunolocalization unveils NlpD as the first septal (cell-wall-binding) protein in Chlamydiae and we show that its septal sequestration depends on prior cell wall synthesis. Since AmiA assembles into peripheral clusters, trimming of a PG-like polymer or precursors occurs throughout the chlamydial envelope, while NlpD targets PG-like peptide crosslinks at the chlamydial septum during constriction

Remobilisation of uranium from contaminated freshwater sediments by bioturbation

International audiencePrevious studies have demonstrated that benthic macro-invertebrate bioturbation can influence the remobilization of uranium initially associated with freshwater sediments resulting in a high release of this pollutant through the overlying water column. Giving the potential negative effects on aquatic biocenosis and the global ecological risk, it appeared crucial to improve our current knowledge concerning the uranium biogeochemical behaviour in sediments. The present study aimed to assess the biogeochemical modifications induced by Tubifex tubifex (Annelida, Clitellata, Tubificidae) bioturbation within the sediment permitting to explain such a release of uranium. To reach this goal, uranium distribution between solid and solute phases of a reconstructed benthic system (i.e. in mesocosms) inhabited or not by T. tubifex worms was assessed in a 12 day laboratory experiment. Thanks notably to fine resolution (mm-scale) measurements (e.g. DET gels probes for porewater, bioaccumulation in worms) of uranium and main chemical species (iron, sulfate, nitrate, nitrite), this work permitted (i) to confirm that the removal of bottom sediment particles to the surface through the digestive tract of worms greatly favours the oxidative loss of uranium in the water column, and (ii) to demonstrate that both uranium contamination and bioturbation of T. tubifex substantially influence major microbial-driven biogeochemical reactions in sediments (e.g. stimulation of denitrification, sulfate-reduction and iron dissolutive reduction). This study provides the first demonstration of biogeochemical modifications induced by bioturbation in freshwater uranium-contaminated sediments