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  • Springer  (2)
  • International Union of Crystallography (IUCr)  (1)
  • BMJ Publishing
  • 2000-2004  (3)
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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.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: As a representative natural porphyrin derivative, it is interesting to know the chemical form of lanthanide in chlorophyll a. We select the natural fern dicranopteris dichotoma, the most lanthanide-concentrated plant known, to isolate the chlorophyll a for our study. It is found by EXAFS that lanthanum substitutes in the magnesium position in chlorophyll and coordinates with the porphyrin ring. The lanthanum is seen to have a similar coordination structure to a sandwich-type lanthanide complex, with the La surrounded by eight nitrogen atoms from two porphyrin rings with average La-N bond length of 2.65 Å.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This paper presents evidence for long-term optical variability and colour behaviour of the blazar 3C 345 (1641+399, z=0.595). Our results show that the amplitude of the optical variations of 3C 345 has been only about 3 magnitudes from its photometric history; existent significant correlation between brightness and colours is found for 3C 345. Our recent CCD photometry of 3C 345 in May 1996 and May 1997 showed that they are in good agreement with our prediction of the optical variability period of about 10 years (see Zhang et al., 1998; Webb et al., 1988).
    Type of Medium: Electronic Resource
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