Columbia revisited: Paleomagnetic results from the 1790 Ma colider volcanics (SW Amazonian Craton, Brazil)

In an attempt to improve our understanding of the Paleoproterozoic geodynamic evolution, a paleomagnetic study was performed on 10 sites of acid volcanic rocks of the Colider Suite, southwestern Amazonian Craton. These rocks have a well-dated zircon U-Pb mean age of 1789 +/- 7 Ma. Alternating field and thermal demagnetization revealed northern (southern) directions with moderate to high upward (downward) inclinations. Rock magnetism experiments and magnetic mineralogy show that this characteristic magnetization is carried by Ti-poor magnetite or by hematite that replaces magnetite by late-magmatic cleuteric alteration. Both magnetite and hematite carry the same characteristic component. The mean direction (Dm = 183.0 degrees, Im = 53.5 degrees, N = 10, alpha(95) = 9.8 degrees, K = 25.2) yielded a paleomagnetic pole located at 298.8 degrees E, 63.3 degrees S (alpha(95) = 10.2 degrees, K = 23.6), which is classified with a quality factor Q = 5. Paleogeographic reconstructions using this pole and other reliable Paleoproterozoic poles suggest that Laurentia, Baltica, North China Craton and Amazonian Craton were located in laterally contiguous positions forming a large continental mass at 1790 Ma ago. This is reinforced by geological evidence which support the existence of the supercontinent Columbia in Paleoproterozoic times. (C) 2008 Elsevier B.V. All rights reserved.

Exact Search Directions for Optimization of Linear and Nonlinear Models Based on Generalized Orthonormal Functions

A novel technique for selecting the poles of orthonormal basis functions (OBF) in Volterra models of any order is presented. It is well-known that the usual large number of parameters required to describe the Volterra kernels can be significantly reduced by representing each kernel using an appropriate basis of orthonormal functions. Such a representation results in the so-called OBF Volterra model, which has a Wiener structure consisting of a linear dynamic generated by the orthonormal basis followed by a nonlinear static mapping given by the Volterra polynomial series. Aiming at optimizing the poles that fully parameterize the orthonormal bases, the exact gradients of the outputs of the orthonormal filters with respect to their poles are computed analytically by using a back-propagation-through-time technique. The expressions relative to the Kautz basis and to generalized orthonormal bases of functions (GOBF) are addressed; the ones related to the Laguerre basis follow straightforwardly as a particular case. The main innovation here is that the dynamic nature of the OBF filters is fully considered in the gradient computations. These gradients provide exact search directions for optimizing the poles of a given orthonormal basis. Such search directions can, in turn, be used as part of an optimization procedure to locate the minimum of a cost-function that takes into account the error of estimation of the system output. The Levenberg-Marquardt algorithm is adopted here as the optimization procedure. Unlike previous related work, the proposed approach relies solely on input-output data measured from the system to be modeled, i.e., no information about the Volterra kernels is required. Examples are presented to illustrate the application of this approach to the modeling of dynamic systems, including a real magnetic levitation system with nonlinear oscillatory behavior.

Inhibition of Trypanosoma brucei glucose-6-phosphate dehydrogenase by human steroids and their effects on the viability of cultured parasites

Dehydroepiandrosterone ( DHEA) is known as an intermediate in the synthesis of mammalian steroids and a potent uncompetitive inhibitor of mammalian glucose-6-phosphate dehydrogenase (G6PDH), but not the enzyme from plants and lower eukaryotes. G6PDH catalyzes the first step of the pentose-phosphate pathway supplying cells with ribose 5-phosphate, a precursor of nucleic acid synthesis, and NADPH for biosynthetic processes and protection against oxidative stress. In this paper we demonstrate that also G6PDH of the protozoan parasite Trypanosoma brucei is uncompetitively inhibited by DHEA and epiandrosterone (EA), with K(i) values in the lower micromolar range. A viability assay confirmed the toxic effect of both steroids on cultured T. brucei bloodstream form cells. Additionally, RNAi mediated reduction of the G6PDH level in T. brucei bloodstream forms validated this enzyme as a drug target against Human African Trypanosomiasis. Together these findings show that inhibition of G6PDH by DHEA derivatives may lead to the development of a new class of anti-trypanosomatid compounds. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Cytological Characterization of YpsB, a Novel Component of the Bacillus subtilis Divisome

Cell division in bacteria is carried out by an elaborate molecular machine composed of more than a dozen proteins and known as the divisome. Here we describe the characterization of a new divisome protein in Bacillus subtilis called YpsB. Sequence comparisons and phylogentic analysis demonstrated that YpsB is a paralog of the division site selection protein DivIVA. YpsB is present in several gram-positive bacteria and likely originated from the duplication of a DivIVA-like gene in the last common ancestor of bacteria of the orders Bacillales and Lactobacillales. We used green fluorescent protein microscopy to determine that YpsB localizes to the divisome. Similarly to that for DivIVA, the recruitment of YpsB to the divisome requires late division proteins and occurs significantly after Z-ring formation. In contrast to DivIVA, however, YpsB is not retained at the newly formed cell poles after septation. Deletion analysis suggests that the N terminus of YpsB is required to target the protein to the divisome. The high similarity between the N termini of YpsB and DivIVA suggests that the same region is involved in the targeting of DivIVA. YpsB is not essential for septum formation and does not appear to play a role in septum positioning. However, a ypsB deletion has a synthetic effect when combined with a mutation in the cell division gene ftsA. Thus, we conclude that YpsB is a novel B. subtilis cell division protein whose function has diverged from that of its paralog DivIVA.