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  • 1
    ISSN: 1432-0614
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The operon encoding aspartokinase and aspartate semialdehyde dehydrogenase was cloned and sequenced from rifamycin-SV-producing Amycolatopsis mediterranei U32 previously. In the present work, these two genes were introduced into the auxotrophic Escherichia coli strain CGSC5074 (ask −) and E. coli X6118 (asd −), respectively. The A. mediterranei U32 aspartokinase and aspartate semialdehyde dehydrogenase genes can be functionally expressed in E. coli and the gene products are able to substitute for the E. coli enzymes. Histidine-tagged aspartokinase and aspartate semialdehyde dehydrogenase were partially purified from E. coli cellular extracts and their kinetic characteristics were studied. Both aspartokinase and aspartate semialdehyde dehydrogenase showed typical Michaelis-Menten type substrate saturation patterns. Aspartokinase has K m values of 3.4 mM for aspartate and 2.3 mM for ATP, while aspartate semialdehyde dehydrogenase has K m values of 1.25 mM for dl-aspartate semialdehyde and 0.73 mM for NADP, respectively. Aspartokinase was inhibited by l-threonine, l-lysine, and l-methionine, but not by l-isoleucine and diaminopimelate. Aspartate semialdehyde dehydrogenase was not inhibited by any of the end-product amino acids at a concentration of less than 5 mM. Hill plot analysis suggested that aspartokinase was subject to allosteric control by l-threonine. Repression of both aspartokinase and aspartate semialdehyde dehydrogenase gene transcription in A. mediterranei U32 by l-lysine, l-methionine, l-threonine, and l-isoleucine were found. The network of regulation of aspartokinase and aspartate semialdehyde dehydrogenase in rifamycin SV-producing A. mediterranei U32 is presented.
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  • 2
    ISSN: 1432-0606
    Keywords: Complexity ; Estimates by integrals ; Analytic centers ; Path-following method ; Interior-point method ; Linear programs ; 90C60 ; 90C05 ; 49M35 ; 68Q25
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper we study a particular class of primal-dual path-following methods which try to follow a trajectory of interior feasible solutions in primal-dual space toward an optimal solution to the primal and dual problem. The methods investigated are so-called first-order methods: each iteration consists of a “long” step along the tangent of the trajectory, followed by explicit recentering steps to get close to the trajectory again. It is shown that the complexity of these methods, which can be measured by the number of points close to the trajectory which have to be computed in order to achieve a desired gain in accuracy, is bounded by an integral along the trajectory. The integrand is a suitably weighted measure of the second derivative of the trajectory with respect to a distinguished path parameter, so the integral may be loosely called a curvature integral.
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  • 3
    ISSN: 1612-1112
    Keywords: Gas chromatography ; Retention values ; Adsorption ; Glass capillary columns
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Effect of adsorption on the retention values for some n-alkanes, n-alcohols and esters in glass capillary columns coated with PEG-20M was studied. Adsorption parameters for the used solutes were evaluated from the retention data.
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  • 4
    ISSN: 1612-1112
    Keywords: Column liquid chromatography ; Ion-exclusion ; Retention ; Adsorption effects ; Dicarboxylic acids
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Retention values and distribution coefficients for C4−C9 dicarboxylic acids on a column packed with cation exchange resin in the H+ form were determined, using dilute HCl solution as eluent. Based on the Donnan membrane equilibrium and adsorption equilibrium, adsorption coefficients were evaluated, and the adsorption contributions to the retention of these analytes were estimated.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Chromatographia 29 (1990), S. 248-250 
    ISSN: 1612-1112
    Keywords: General chromatographic theory ; Exponentially modified Gaussian peaks ; Gamma peaks ; Peak formation mechanisms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary It is shown that part of non-EMG peaks are gamma peaks. The describing function of a gamma peak is derived. The peak area equation and the formation mechanism of a gamma peak are discussed.
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  • 6
    ISSN: 1432-1432
    Keywords: phe operon ; tyr operon ; Attenuation ; Secondary structure ; Nucleotide sequence ; Erwinia herbicola ; Chorismate mutase ; Prephenate dehydatase ; Prephenate dehydrogenase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Extensive knowledge exists inEscherichia coli about the contiguouspheA andaroF-tyrA operons which have opposite transcription orientations and are separated by a bidirectional transcription terminator. The corresponding structural genes and individual components of the terminator and attenuator fromErwinia herbicola have been analyzed from an evolutionary vantage point. A 7.5-kb DNA fragment fromE. herbicola carrying the linkedpheA, tyrA, andaroF genes was cloned by functional complementation ofE. coli auxotrophic requirements. A 3,433-bp segment of DNA consisting of more than half ofaroF, all oftyrA, and the entire phenylalanine operon (promoter, leader region encoding the leader peptide and containing thephe attenuator, andpheA) was sequenced. A bidirectional transcription terminator was positioned between the divergently transcribedpheA andtyrA. The adjacentaroF andtyrA genes share a common transcription orientation, consistent with their probable coexistence within an operon. However,tyrA can be expressed efficiently from an internal promoter which appears to lie within the 3′ portion ofaroF. The gene order ispheA tyrA aroF inE. herbicola, with the same tail-to-tail arrangement of transcription known to exist inE. coli. ThepheL coding region of the phe operon was dominated by phenylalanine codons, seven of the 15 amino acid residues of the leader peptide beingl-phenylalanine. TheE. herbicola pheA andtyrA genes were 1,161 by and 1,119 by in length, respectively, and corresponded to deduced gene products having subunit molecular weights of 43,182 and 41,847. The deduced amino acid sequences ofPheA andTyrA were homologous at their N-termini, consistent with a common evolutionary origin of the chorismate mutase domains present at the amino terminus of bothPheA andTyrA. A detailed comparison of theE. coli andE. herbicola sequences was made. ThepheA, tyrA, andaroF genes ofE. herbicola exhibited high overall identity with the counterpartE. coli genes. Within the leader region of thephe operon, the leader peptide coding region was highly conserved. Although the 1:2 and 2′:3′ stems defining the pause structure and the antiterminator, respectively, were also highly conserved, RNA segment 4 of the attenuator terminator exhibited considerable divergence, as did the distal portion of the attenuator region. Within the span of attenuator region encoding the three stern-loop structures of mRNA secondary configuration, hot spots of base-residue divergence were localized to looped-out regions. No changes occurred which would simultaneously disrupt alternative pairing relationships of secondary configuration. The bidirectional terminator betweenpheA andtyrA has diverged very substantially. Much of the promoter region and the untranslated region between the promoter and thepheL coding region also differed considerably between the two organisms.
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  • 7
    ISSN: 1573-4846
    Keywords: metal particles ; surface plasma resonance ; coating films ; composites
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Control of the optical properties of gel-derived oxide films containing fine metal particles is described. The duration of the aging of Si(OC2H5)4-derived sols and the amount of water for hydrolyzing Si(OC2H5)4 were found to greatly affect the size and the shape of Au particles formed in the silica matrix, and accordingly the optical absorption of the Au/SiO2 composite films. Employing dielectric media with high refractive indices like TiO2 was shown to shift the absorption peak of Au particles to longer wavelengths. Pd/TiO2 and Pt/TiO2 composite films showed absorption in the visible region.
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  • 8
    ISSN: 1573-2746
    Keywords: adhesion ; metal/ceramic bonding ; quantum mechanical computations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Adhesive energetics and interfacial electronic structures have been computed from first principles for the Cu/Al2O3 interface. Recent transmission electron microscopy results of Cu grown by molecular beam epitaxy on Al2O3(0001) were helpful in modelling the interfacial atomic structure. We found that Al2O3(0001) relaxation effects can lower the work of adhesion W ad by over a factor of 3. Our computed W ad value is in reasonably good agreement with experiment, being somewhat larger, as expected from our assumption of a coherent interface. One might begin to understand this metal/ceramic adhesion as a competition between Cu and Al for oxide formation, which is easily won by Al. However this simple picture is complicated by several indications of a significant metallic/covalent component to the Cu/Al2O3 adhesive bond.
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  • 9
    ISSN: 1573-2878
    Keywords: Large-scale linear programming ; interior-point methods ; Dantzig–Wolfe decomposition ; algorithmic complexity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper deals with an algorithm incorporating the interior-point method into the Dantzig–Wolfe decomposition technique for solving large-scale linear programming problems. The algorithm decomposes a linear program into a main problem and a subproblem. The subproblem is solved approximately. Hence, inexact Newton directions are used in solving the main problem. We show that the algorithm is globally linearly convergent and has polynomial-time complexity.
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  • 10
    ISSN: 1573-2878
    Keywords: Interior-point methods ; monotone variational inequality problems ; rate of convergence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper offers an analysis on a standard long-step primal-dual interior-point method for nonlinear monotone variational inequality problems. The method has polynomial-time complexity and its q-order of convergence is two. The results are proved under mild assumptions. In particular, new conditions on the invariance of the rank and range space of certain matrices are employed, rather than restrictive assumptions like nondegeneracy.
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