Quasicrystals, model sets, and automatic sequences

We survey mathematical properties of quasicrystals, first from the point of view of harmonic analysis, then from the point of view of morphic and automatic sequences. Nous proposons un tour d'horizon de propri\'et\'es math\'ematiques des quasicristaux, d'abord du point de vue de l'analyse harmonique, ensuite du point de vue des suites morphiques et automatiques

The crystal structure of the ring-hydroxylating dioxygenase from Sphingomonas CHY-1

International audienceThe ring-hydroxylating dioxygenase (RHD) from Sphingomonas CHY-1 is remarkable due to its ability to initiate the oxidation of a wide range of polycyclic aromatic hydrocarbons (PAHs), including PAHs containing four- and five-fused rings, known pollutants for their toxic nature. Although the terminal oxygenase from CHY-1 exhibits limited sequence similarity with well characterized RHDs from the naphthalene dioxygenase family, the crystal structure determined to 1.85 Å by molecular replacement revealed the enzyme to share the same global α3β3 structural pattern. The catalytic domain distinguishes itself from other bacterial non-heme Rieske iron oxygenases by a substantially larger hydrophobic substrate binding pocket, the largest ever reported for this type of enzyme. While residues in the proximal region close to the mononuclear iron atom are conserved, the central region of the catalytic pocket is shaped mainly by the side chains of three amino acids, Phe350, Phe404 ad Leu356, which contribute to the uniform trapezoidal form of the pocket. Two flexible loops, LI and LII, exposed to the solvent seem to control the substrate access to the catalytic pocket and control the pocket length. Compared with other naphthalene dioxygenases residues Leu223 and Leu226, on loop LI, are moved toward the solvent, thus elongating the catalytic pocket by at least 2 Å. An 11 Å long water channel extends from the interface between the α and β subunits to the catalytic site. The comparison of these structures with other known oxygenases suggests that the broad substrate specificity presented by the CHY-1 oxygenase is primarily due to the large size and particular topology of its catalytic pocket and provided the basis for the study of its reaction mechanism

Characterization of a naphthalene dioxygenase endowed with an exceptionally broad substrate specificity toward polycyclic aromatic hydrocarbons

International audienceIn Sphingomonas CHY-1, a single ring-hydroxylating dioxygenase is responsible for the initial attack of a range of polycyclic aromatic hydrocarbons (PAHs) composed of up to five rings. The components of this enzyme were separately purified and characterized. The oxygenase component (ht-PhnI) was shown to contain one Rieske-type [2Fe-2S] cluster and one mononuclear Fe center per alpha subunit, based on EPR measurements and iron assay. Steady-state kinetic measurements revealed that the enzyme had a relatively low apparent Michaelis constant for naphthalene (Km= 0.92 ± 0.15 µM), and an apparent specificity constant of 2.0 ± 0.3 µM-1 s-1. Naphthalene was converted to the corresponding 1,2-dihydrodiol with stoichiometric oxidation of NADH. On the other hand, the oxidation of eight other PAHs occurred at slower rates, and with coupling efficiencies that decreased with the enzyme reaction rate. Uncoupling was associated with hydrogen peroxide formation, which is potentially deleterious to cells and might inhibit PAH degradation. In single turnover reactions, ht-PhnI alone catalyzed PAH hydroxylation at a faster rate in the presence of organic solvent, suggesting that the transfer of substrate to the active site is a limiting factor. The four-ring PAHs chrysene and benz[a]anthracene were subjected to a double ring-dihydroxylation, giving rise to the formation of a significant proportion of bis-cis-dihydrodiols. In addition, the dihydroxylation of benz[a]anthracene yielded three dihydrodiols, the enzyme showing a preference for carbons in positions 1,2 and 10,11. This is the first characterization of a dioxygenase able to dihydroxylate PAHs made up of four and five rings

Challenges for the conservation of threatened plants and natural habitats in French overseas territories

The French overseas territories represent a broad range of climatic and biogeographical conditions, from sub-arctic to equatorial, resulting in a richness, diversity and uniqueness of their floras that mirrors that of the global flora. These territories range widely in area (from small oceanic islands to large continental regions) and human population densities, leading to more or less pronounced anthropogenic impacts on natural ecosystems. Threats to the French overseas flora are generally the same (habitat destruction, biological invasions, species overexploitation, climate change) but with varying importance depending on the territory. Conservation measures, including the establishment of lists of protected species or habitats and new protected areas, as well as habitat restoration, species reintroduction or population reinforcement of threatened species, also vary depending on the territory, in relation with their legal status, and the concern of local populations and authorities toward the conservation of their natural heritage. We underline the necessity and urgency to better know and conserve this flora (more than 10 000 vascular plants, including 3480 endemics, and with 685 protected species) of not only national but also regional and global importance.Les collectivités françaises d’outre-mer sont situées dans des contextes climatiques et biogéographiques très diversifiés, allant de conditions sub-arctiques à équatoriales, ce qui détermine des richesses, diversités et originalités floristiques bien différentes, mais largement complémentaires à une échelle mondiale. Ces territoires présentent par ailleurs des superficies variables, allant de petites îles océaniques à de grandes régions continentales, et des densités de population humaine très contrastées, se traduisant par des impacts plus ou moins prononcés des perturbations anthropiques sur les écosystèmes naturels. Les menaces sur la flore ultramarine sont globalement de même nature (destruction des habitats, invasions biologiques, surexploitation d’espèces, changements climatiques), mais d’importance variable selon les territoires. Les actions de conservation de la flore et des habitats naturels initiées, comme l’établissement de listes d’espèces ou d’habitats protégés, la mise en place d’aires protégées, les opérations de restauration d’habitats et de réintroduction ou renforcement de populations d’espèces menacées, sont très variables selon les situations dans les collectivités, en relation avec des différences de statuts législatifs et de sensibilité des populations et autorités locales par rapport à la protection de leur patrimoine naturel. Nous soulignons la nécessité et l’urgence de mieux connaître et sauvegarder la flore ultramarine française (plus de 10 000 plantes vasculaires dont 3480 endémiques strictes, avec 685 espèces menacées et protégées) d’importance nationale, mais également régionale et global