Evidence for spreading in the lower Kam Group of the Yellowknife greenstone belt: Implications for Archaean basin evolution in the Slave Province

The Yellowknife greenstone belt is the western margin of an Archean turbidite-filled basin bordered on the east by the Cameron River and Beaulieu River volcanic belts (Henderson, 1981; Lambert, 1982). This model implies that rifting was entirely ensialic and did not proceed beyond the graben stage. Volcanism is assumed to have been restricted to the boundary faults, and the basin was floored by a downfaulted granitic basement. On the other hand, the enormous thickness of submarine volcanic rocks and the presence of a spreading complex at the base of the Kam Group suggest that volcanic rocks were much more widespread than indicated by their present distribution. Rather than resembling volcanic sequences in intracratonic graben structures, the Kam Group and its tectonic setting within the Yellowknife greenstone belt have greater affinities to the Rocas Verdes of southern Chile, Mesozoic ophiolites, that were formed in an arc-related marginal basin setting. The similarities of these ophiolites with some Archean volcanic sequences was previously recognized, and served as basis for their marginal-basin model of greenstone belts. The discovery of a multiple and sheeted dike complex in the Kam Group confirms that features typical of Phanerozoic ophiolites are indeed preserved in some greenstone belts and provides further field evidence in support of such a model

Silencing of Vlaro2 for chorismate synthase revealed that the phytopathogen Verticillium longisporum induces the cross-pathway control in the xylem

The first leaky auxotrophic mutant for aromatic amino acids of the near-diploid fungal plant pathogen Verticillium longisporum (VL) has been generated. VL enters its host Brassica napus through the roots and colonizes the xylem vessels. The xylem contains little nutrients including low concentrations of amino acids. We isolated the gene Vlaro2 encoding chorismate synthase by complementation of the corresponding yeast mutant strain. Chorismate synthase produces the first branch point intermediate of aromatic amino acid biosynthesis. A novel RNA-mediated gene silencing method reduced gene expression of both isogenes by 80% and resulted in a bradytrophic mutant, which is a leaky auxotroph due to impaired expression of chorismate synthase. In contrast to the wild type, silencing resulted in increased expression of the cross-pathway regulatory gene VlcpcA (similar to cpcA/GCN4) during saprotrophic life. The mutant fungus is still able to infect the host plant B. napus and the model Arabidopsis thaliana with reduced efficiency. VlcpcA expression is increased in planta in the mutant and the wild-type fungus. We assume that xylem colonization requires induction of the cross-pathway control, presumably because the fungus has to overcome imbalanced amino acid supply in the xylem

Chorismate mutase of Thermus thermophilus is a monofunctional AroH class enzyme inhibited by tyrosine

aroG, encoding the monofunctional chorismate mutase (TtCM) of the thermophilic gram-negative bacterium Thermus thermophilus, was cloned and its gene product characterized. TtCM was purified to homogeneity on an SDS polyacrylamide gel as a His-fusion protein with a deduced molecular mass of 15.8 kDa. The enzyme belongs to the rare group of AroH-type chorismate mutases which are mainly found in gram-positive bacteria of the Bacillus/ Clostridia group and have recently also been described for gram-negative organisms. The native molecular mass is consistent with a pseudo-alpha/beta barrel enzyme that is organized as a trimer. Comparison of the enzyme's structure with that of its mesophilic counterpart from Bacillus revealed an increase in hydrophilicity on the protein's surface, greater hydrophobicity in cavities within the protein, and greater restriction of conformational freedom, features that contribute to the thermal stability of this chorismate mutase. The kinetic data show Michaelis-Menten substrate saturation with a Km of 290 microM, and a kcat/ Km value of 180 s(-1) mM(-1). TtCM was inhibited by tyrosine with a Ki =34 microM, possibly in a competitive manner

Origin of Archean subcontinental lithospheric mantle: Some petrological constraints

International audienceThe longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the combination of these features that allow it to remain isolated from the convecting mantle. Highly forsteritic olivine is not normally produced during mantle melting. Possible explanations for its abundance in old Archean subcontinental lithospheric mantle include: (a) high-degree mantle melting in a plume or at an Archean ocean ridge; (b) accretion of this material to older lithosphere and its reworking in a subduction zone; (c) redistribution of material to eliminate high-density, low-viscosity lithologies. Following an evaluation of these models based on petrological and numerical modeling, we conclude that the most likely explanation is the accumulation of the residues of melting of one or more mantle plumes following by gravity-driven ejection of denser, Fe-rich components

The 1998 Ma Purtuniqophiolite: imbricated and metamorphosed oceanic crust in the Cape Smith Thrust Belt, northern Quebec

A sequence of rocks representing obducted oceanic crust (an ophiolite) has been identified in the Cape Smith Belt. The physical and chemical nature of this 1998 Ma ophiolite is similar to rocks from Phanerozoic ophiolites and modern oceans, implying that tectonic processes similar to those operating today were active nearly two billion years ago. At that time, northern Quebec may have been tectonically similar to the modern Red Sea. Résumé Une séquence de roches caractérisant une croûte océanique obductée (une ophiolite) a été identifée dans la bande du Cap Smith. Les aspects physiques et chimiques decette ophiolite datée à 1998 Ma sont semblables à ceux des ophiolites phanérozoïque et des océans modernes. L'implication est queles processus tectoniques modernes sont semblables à ceux qui étaient actifs il y a deux milliards d'années. A ce temps là, lenord du Québec était peutêtre similaire à lamer Rouge actuelle

Refined molecular hinge between allosteric and catalytic domain determines allosteric regulation and stability of fungal chorismate mutase

The yeast chorismate mutase is regulated by tyrosine as feedback inhibitor and tryptophan as crosspathway activator. The monomer consists of a catalytic and a regulatory domain covalently linked by the loop L220s (212–226), which functions as a molecular hinge. Two monomers form the active dimeric enzyme stabilized by hydrophobic interactions in the vicinity of loop L220s. The role of loop L220s and its environment for enzyme regulation, dimerization, and stability was analyzed. Substitution of yeast loop L220s in place of the homologous loop from the corresponding and similarly regulated Aspergillus enzyme (and the reverse substitution) changed tyrosine inhibition to activation. Yeast loop L220s substituted into the Aspergillus enzyme resulted in a tryptophan-inhibitable enzyme. Monomeric yeast chorismate mutases could be generated by substituting two hydrophobic residues in and near the hinge region. The resulting Thr-212→Asp–Phe-28→Asp enzyme was as stable as wild type, but lost allosteric regulation and showed reduced catalytic activity. These results underline the crucial role of this molecular hinge for inhibition, activation, quaternary structure, and stability of yeast chorismate mutase

Tectonic setting and element geochemistry of the Aikendaban Formation volcanic rocks in west Tianshan

The Aikendaban Formation is a red-colored, continental volcanic sequence, unconformably overlying on the Dahalajunshan Formation of Early Carboniferous age. It shows no regional deformation and metamorphism. Its main lithologies include trachy basalt, basaltic trachyandesite, trachyandesite and trachyte. The SiO2 contents range from 41. 69 to 65. 99%, lower than the average of upper continental crust (66%). Along with increasing SiO2 contents, w(Na2O + K2O) values increase, whereas SI values decrease, with TiO2 generally lower than 1. 3% and Al 2O3 ranging from 12.82 to 18. 37%. These petrochemical characters show that the Aikendaban Formation is a typical volcanic sequence of shoshonite series. SiO2 contents in basalts and basaltic trachyandesites are lower than the average of lower continental crust (54. 4%), suggesting that basalts and basaltic trachyandesites were originated from the mantle rather than the continental crust. On the contrary, the trachytes and trachyandesites, with SiO2>54. 4%, were likely to be resulted from the crust or related to intracrustal differentiation. ∑REE, LREE, Zr, Hf, Nb, Ta, Ba, Sr, Pb and Y contents and LaN/YbN ratios in basalts and basaltic trachyandesites are higher than their averages in rocks of the same kinds worldwide, whereas Cr, Co and Ni contents are lower than their averages in same kinds of rocks worldwide. This indicates that mantle source was enriched in large ion lithosphile elements (LILE) and incompatible elements. With Eu/Eu * <0.94 and variable Sr contents (from depletion to enrichment), the basalts and basaltic trachyandesites were probably derived from a heterogeneous mantle source resulted from return of upper crustal compositions related to subduction. Eu/Eu * values in trachytes average 0. 59, lower than 0. 65 which is supposed as average of upper continental crust. Sr is strongly depleted in trachytes. Meanwhile, the ∑REE, LREE, LREE /HREE, LaN/YbN and Ce/Ce * values in trachytes are lower than those in trachyandesites. Sm/Nd ratios average 0. 25, same as the average of lower continental crust (0.25). La, Ce, Nd and Sm are enriched relative to Y and Yb. Ba shows positive anomaly, while Nb, Ta and Hf show depletion relatively. All these characteristics indicate that the trachytes are originated from an intracrustal differentiation with plagioclase-crystalline fractionation. Among various lithologies, trachyandesites have highest ∑REE, LREE, Ba, Sr, Zr and Nb contents, highest LREE/HREE, LaN/Yb N, Ce/Ce * ,Zr/Y and Hf/Yb ratios; and lowest Sm /Nd ratios. This shows that LILE and HFSE have enriched in trachyandesites, suggesting that magma has experienced intense crystalline fractionation. With Eu/Eu *=0. 63, lower than the upper continental crust (0. 65), the trachyandesites have higher HREE contents than basalts and basaltic trachyandesites, suggesting that trachyandesites have evolved from basaltic and/or basaltic-trachyandesitic magmas by intracrustal differentiation. Study on lithologic sequence and geological synthesis shows that the Aikendaban Formation developed in extension-collapse stage of late collision, i. e. Early Permian in Tianshan Orogen. Extension of a collisional-orogenic lithosphere generally evolved from shallow crust-level to deep root-level (lower crust and lithospheric mantle). Eruption of basalts, probably coeval with underplating of mantle-derived magmas, can be used as indicator of the end of a collisional-orogenic event.link_to_subscribed_fulltex