1,720,960 research outputs found
Site directed mutagenesis studies on the iron-sulfur flavoenzyme glutamate synthase : an enexpectedly complex self-regulated enzyme
No full textMycobacterium tuberculosis glutamyl tRNA synthetase and glutamyl-tRNA reductase
No full textTuberculosis is currently a major cause of mortality in both developing and industrialized countries with eight million people developing active tuberculosis and with two million dying from the disease every year. Difficulties to treat tuberculosis are mainly due to M. tuberculosis multidrug-resistant strains and the limited number of antitubercular agents [1]. In light of the dependence of M. tuberculosis on heme-containing enzymes [2], we have identified glutamyl-tRNA synthetase (GluRS) and glutamyl-tRNA reductase (GluTR) as potential targets for new drug design. GluRS is not only an essential enzyme because it provides Glu-tRNA for protein biosynthesis, but also because it forms with GluTR and glutamate 1-semialdehyde aminomutase the path leading to the synthesis of δ-aminolevulinic acid (ALA), the first common precursor of tetrapyrrole biosynthesis [3, 4].
The M. tuberculosis gltX gene encoding Mt-GluRS has been cloned, sequenced, and used to construct a plasmid for its overproduction in E. coli BL21(DE3) cells. Soluble recombinant protein was obtained in large amounts and purified to homogeneity. The catalytic properties of Mt-GluRS are being investigated using the well characterized E. coli GluRS as a reference, in order to highlight peculiar properties of the M. tuberculosis enzyme. The purified native protein and its seleno-methionine derivative have also been subjected to extensive crystallization trials. Crystals that diffracted to greater than 2.5 Ǻ were obtained. However, a twinning phenomenon and the small size of single crystals did not allow the resolution of Mt-GluRS structure. In order to gain insight on the enzyme structure and conformational flexibility, which may hamper crystallization, we are using a combination of dynamic light scattering, limited proteolysis, analytical gel filtration as well as small-angle X-ray scattering.
Investigation of Mt-GluTR is hampered by the low levels of soluble protein that can be produced in E. coli cells transformed with plasmids containing the putative hemA gene (Rv0509), as found for GluTR from various sources. Indeed, up to now, only a few GluTR have been characterized [5-7]. However, the red phenotype of E. coli cells indicates that Rv0509 is indeed the hemA gene. Recently, significant amounts of soluble Mt-GluTR have been obtained by producing chimeric proteins resulting from the fusion of Mt-GluTR and either a GyrA intein-Chitin Binding Domain fragment or the Maltose Binding Protein. In these cases, the coexpression of E. coli chaperon proteins DnaK, DnaJ and GrpE and low temperature greatly enhanced the amount of soluble protein we could obtain. On the basis of these results attempts to obtain stable and catalytically active preparations of the enzyme are being carried out
Studies on Mycobacterium tuberculosis glutamyl tRNA synthetase : a novel antitubercular target
No full textInvestigation of the structural properties and oligomerization state of the complex enzyme glutamate synthase by atomic force microscopy
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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