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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Current genetics 21 (1992), S. 191-196 
    ISSN: 1432-0983
    Keywords: Continuous culture ; Amino acid analogs ; Overproducer yeast mutants
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Mutants resistant to ethionine, a toxic analog of methionine, were selected by subjecting yeast cells to competition experiments under continous culture, controlled by pH, with a wide range of increasing ethionine concentrations. The mutants accumulated up to over 30 mM methionine and were able to grow in ethionine concentrations from 0.5 to 50 mM, whereas the wild-type strain stopped growing at 0.1 mM ethionine, and its free methionine pool was 0.2 mM. From ethionine-resistant mutants, strains able to overproduce threonine were isolated by selecting either in continuous or in batch cultures, the latter supplemented with 0.1–5 mM hydroxynorvaline a toxic analog of threonine. These mutants accumulated up to over 200 mM of threonine (the internal pool of threonine in the wild-type was around 5 mM), grew in minimal medium with growth rates similar to that of the wild-type, accumulated the analogs at internal concentrations proportional to the external, and accumulated other amino acids such as homoserine, aspartate, isoleucine and S-adenosyl-methionine.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-0991
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The activity of three enzymes, aspartokinase, homoserine dehydrogenase, and homoserine kinase, has been studied in the industrial strainSaccharomyces cerevisiae IFI256 and in the mutants derived from it that are able to overproduce methionine and/or threonine. Most of the mutants showed alteration of the kinetic properties of the enzymes aspartokinase, which was less inhibited by threonine and increased its affinity for aspartate, and homoserine dehydrogenase and homoserine kinase, which both lost affinity for homoserine. Furthermore, they showed in vitro specific activities for aspartokinase and homoserine kinase that were higher than those of the wild type, resulting in accumulation of aspartate, homoserine, threonine, and/or methionine/S-adenosyl-methionine (Ado-Met). Together with an increase in the specific activity of both aspartokinase and homoserine kinase, there was a considerable and parallel increase in methionine and threonine concentration in the mutants. Those which produced the maximal concentration of these amino acids underwent minimal aspartokinase inhibition by threonine. This supports previous data that identify aspartokinase as the main agent in the regulation of the biosynthetic pathway of these amino acids. The homoserine kinase in the mutants showed inhibition by methionine together with a lack or a reduction of the inhibition by threonine that the wild type undergoes, which finding suggests an important role for this enzyme in methionine and threonine regulation. Finally, homoserine dehydrogenase displayed very similar specific activity in the mutants and the wild type in spite of the changes observed in amino acid concentrations; this points to a minor role for this enzyme in amino acid regulation.
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  • 3
    ISSN: 1432-0991
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Ethionine is the toxic S-ethyl analog of the essential amino acid methionine. Whereas in prokaryotes the ethionine just competes with the methionine, in eukaryotes it can also be transformed into S-adenosyl-ethionine (Ado-Eth), competing with the S-adenosyl-methionine (Ado-Met). When the Ado-Met synthetase activity was studied in strains defective in either of the two isoenzymes, the one coded by theSAM1 gene was totally unable to convert ethionine into Ado-Eth and was inhibited by the analog, whereas the enzyme coded by theSAM2 gene was able to bind ethionine and was not inhibited by it. This has allowed the development of a procedure to measure Ado-Met synthetase and differentiate between the two isoenzymes present inSaccharomyces cerevisiae.
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Biotechnology progress 10 (1994), S. 372-376 
    ISSN: 1520-6033
    Source: ACS Legacy Archives
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1520-6033
    Source: ACS Legacy Archives
    Topics: Process Engineering, Biotechnology, Nutrition Technology
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  • 6
    ISSN: 1432-2048
    Keywords: Key words: Acyl-acyl carrier protein thioesterase –Helianthus (seed formation) – Lipid biosynthesis – Mutant (sunflower) – Palmitic acid mutant – Seed formation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. During sunflower (Helianthus annuus L.) seed formation there was an active period of lipid biosynthesis between 12 and 28 days after flowering (DAF). The maximum in-vitro acyl-acyl carrier protein (ACP) thioesterase activities (EC 3.1.2.14) were found at 15 DAF, preceding the largest accumulation of lipid in the seed. Data from the apparent kinetic parameters, V max and K m, from seeds of 15 and 30 DAF, showed that changes in acyl-ACP thioesterase activity are not only quantitative, but also qualitative, since, although the preferred substrate was always oleoyl-ACP, the affinity for palmitoyl-ACP decreased, whereas that for stearoyl-ACP increased with seed maturation. Bisubstrate assays carried out at 30 DAF seemed to indicate that the total activity found in mature seeds is due to a single enzyme with 100/75/15 affinity for oleoyl-ACP/stearoyl-ACP/palmitoyl-ACP. In contrast, at 15 DAF, enzymatic data together with partial sequences from cDNAs indicated the presence of at least two enzymes with different properties, a FatA-like thioesterase, with a high affinity for oleoyl-ACP, plus a FatB-like enzyme, with preference for long-chain saturated fatty acids, both being expressed during the active lipid biosynthesis period. Competition assays carried out with CAS-5, a mutant with a higher content of palmitic acid in the seed oil, indicated that a modified FatA-type thioesterase is involved in the mutant phenotype.
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  • 7
    ISSN: 1432-2048
    Keywords: Key words: Acyl-acyl carrier protein thioesterase ; Fatty acid synthetase II ; Helianthus (seed) ; β-keto-acyl-acyl carrier protein synthetase II ; Mutant fatty acid ; Palmitic acid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. Two high-palmitic acid sunflower (Helianthus annuus L.) mutants, CAS-5 and CAS-12, have been biochemically characterised. The enzymatic activities found to be responsible for the mutant characteristics are β-keto-acyl-acyl carrier protein synthetase II (KASII; EC 2.3.1.41) and acyl-acyl carrier protein thioesterase (EC 3.1.2.14). Our data suggest that the high-palmitic acid phenotype observed in both mutant lines is due to the combined effect of a lower KASII activity and a higher thioesterase activity with respect to palmitoyl-acyl carrier protein (16:0-ACP). The level of the latter enzyme appeared to be insufficient to hydrolyse the produced 16:0-ACP completely. As a consequence of this, three new fatty acids appear: palmitoleic acid (16:1 Δ9), asclepic acid (18:1 Δ11), and palmitolinoleic acid (16:2 Δ9 Δ12). These fatty acids should be synthesised from palmitoyl-ACP or a derivative by the action of the stearoyl-ACP desaturase, fatty acid synthetase II and oleoyl-phosphatidylcholine desaturase, respectively.
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  • 8
    ISSN: 1573-6784
    Source: Springer Online Journal Archives 1860-2000
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary Derivatization of primary amino acids with orthophthalaldehyde and β-mercaptoethanol forms derivatives that can be detected by absorbance at 340 nm. These were separated by reverse-phase high performance liquid chromatography (HPLC). A step- or more complex gradient (acetonitrile/phosphate buffer gradient) was used. The effects of several parameters (pH, ionic strength, etc) were characterized and used to design a rapid separation of the amino acids commonly found in physiological fluids. The method described is rapid, sensitive and precise as sensitivity limits are about 25 pmol and the separation time, injection to injection, is 16 min.
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