Studies on Repression of the Histidine Operon, Ii. The Role of the First Enzyme in Control of the Histidine System

Kovach, John S.; Phang, James M.; Ference, Michael; Goldberger, Robert F.

doi:10.1073/pnas.63.2.481

Cited by 60 publications

(43 citation statements)

References 21 publications

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“…Such a possibility is not without precedence. For example in Salmonella typhimurium, charged histidyl t-RNA both inhibits and represses the formation of ATP phosphoribosyl transferase, the enzyme at the branch point in the pathway leading to histidine (10). The apparent discrepancy between the concentration dependence of the effect of itaconate in vivo and in vitro could be the result of transport or of metabolic modification to less inhibitory forms.…”

Section: Discussionmentioning

confidence: 99%

Enzyme Profiles in Seedling Development and the Effect of Itaconate, an Isocitrate Lyase-directed Reagent

Khan¹,

McFadden²

1979

Plant Physiol.

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Changes in levels of isocitrate lyase, malate synthase, and catalase have been investigated during germination of flax (Liu,m usitatissiumn L.) in the presence and absence of itaconate. Germination was accompanied by a rapid increase in these enzymes during the first 3 days. The presence of 38 miHmolar itaconate inhibited the incidence of seed germination and the growth of embryo axes as well as Plant Materials. The following were used: flax (Linum usitatissimum), corn (Zea mays), soybean (Glycine hispida), green gram (Vigna glabra), black eyed bean (Vigna sinensis), fenugreek (Trigonella foenumgraecum), alfalfa (Medicago sativa), lentil (Lens culinaris), and wheat (Triticum aestivum L.).Germination and Seedling Growth. Flax seeds variety Varansi local from an Indian cultivar was used for detailed studies. Seeds were germinated essentially as described by Khan et al. (9). Seeds were surface-sterilized for 5 s in 0.01% (w/v) HgCl2 solution, washed with sterile distilled H20, and transferred to sterile Petri dishes lined with moistened filter paper and incubated at 25 C.For the treatment with itaconate, seeds were soaked in an appropriate concentration of the inhibitor for 6 h at 4 C, and the seeds then transferred to Petri dishes containing 10 ml of test solution. Respective controls received the same treatment, except that water was used instead of inhibitor solution. The zero time of the germination period in all of these experiments was 6 h after the initiation of seed imbibition. In separate experiments we have shown that no marked change in enzyme activities was observed during the imbibition period at 4 C. For enzyme assays done on seeds incubated in the presence or absence of itaconate, seeds were selected at random prior to germination. Postgermination assays were conducted only on seeds in which the embryo axis had emerged.Preparation of Extracts. For the preparation of crude enzyme extract, seedlings were harvested and washed with deionized H20 followed by glass-distilled H20. The

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Section: Discussionmentioning

confidence: 99%

Enzyme Profiles in Seedling Development and the Effect of Itaconate, an Isocitrate Lyase-directed Reagent

Khan¹,

McFadden²

1979

Plant Physiol.

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“…In the case of repression of biosynthetic enzymes, the situation is not as well understood, although there has been evidence that control might be exerted at the level of both transcription and translation (22), with strong evidence for translational control (23). There is additional evidence that repression might involve tRNAs (24,25), aminoacyl-tRNA synthetases (26)(27)(28), and completed biosynthetic enzymes (1)(2)(3)(4)(5)(6)(7). The best evidence that a completed enzyme might be involved in the repression of other enzymes of the same pathway has been obtained in the histidine pathway of S. typhimurium, where it has been shown that the binding site for feedback inhibition of the first enzyme of the histidine pathway is also involved in repression (3).…”

Section: Discussionmentioning

confidence: 99%

Involvement of Threonine Deaminase in Multivalent Repression of the Isoleucine-Valine Pathway in Saccharomyces cerevisiae

Bollon

Magee

1971

Proc. Natl. Acad. Sci. U.S.A.

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A strain (MIAR33) of Saccharomyces cerevisiae containing a threonine deaminase [L-threonine hydrolyase (deaminating) EC 4.2.1.161 with decreased feedback sensitivity has been shown to have a specific activity of acetohydroxy acid synthetase higher than that of the parent strain (MDIll) when both are grown on minimal medium. When strain MAR33 is grown on minimal medium supplemented only with isoleucine, the specific activity of the synthetase is reduced to that found in the parent strain. Another strain, D106-lA, contains a nonsense mutation in the middle of the gene for threonine deaminase. When this strain is grown on minimal medium containing appropriate supplements (which include a nonrepressing concentration of isoleucine), or on minimal medium supplemented with isoleucylglycine (which acts as a limiting source of isoleucine), acetohydroxy acid synthetase remains repressed. Leucine limitation causes partial derepression. With the reversion of the nonsense mutation, either intragenically or via a suppressor for the mutation, partial derepression of the synthetase returns. When D106-lA is diploidized with either M15, a mutant lacking the synthetase, or MD9, a strain containing the enzyme, normal, partially derepressed, values for this enzyme are found. This indicates that threonine deaminase is necessary for derepression, and that it possibly acts as an inducer.The involvement of a biosynthetic enzyme of a specific aminoacid pathway in repression of other enzymes of the same pathway has been considered by several investigators (1-7).Our possession of several mutants affected in the allosteric properties of threonine deaminase permitted us to explore the involvement of this enzyme in the repression of acetohydroxy acid synthetase in Saccharomyces cerevisiae. Threonine deaminase and acetohydroxy acid synthetase are the first two enzymes in the isoleucine-valine biosynthetic pathway. The properties of these two enzymes in S. cerevisiae have been examined (8-10). Regulation of the isoleucinevaline pathway has been examined in S. cerevisiae (11, 12), in Escherichia coli (13-15), and in Salmonella typhimurium (16). All of these organisms show multivalent repression of at least some of the enzymes of the pathway. Full repression occurs only in the presence of all three aminio acids: isoleucine, valine, and leucine. Starvation for one of these compounds causes derepression of the enzymes of the pathway, even in the presence of an excess of the other two ( 1). METHODS AND RESULTSThtreonine deaminase mutated in its allosteric properties and the repression of acetohydroxy acid synthetase MAR33, a haploid strain of S. cerevisiae, contains a single mutation that renders threonine deaminiase 100-fold less sensitive to isoleucine inhibition than the enzyme of MD11 (the parent strain) (17). Parent and mutant strains were grown inl minimal and repressing medium, which is minimal medium supplemented with isoleucine, valine, and leucine, each at 5 mM\. The cultures were made permeable with toluene, and the acetohydroxy a...

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“…However, the model, in its original form, has proven inadequate for the explanation of many of the facts surrounding repression and derepression of enzymes involved in biosynthetic pathways (3). Several authors have proposed alternative models of regulation for these systems (4)(5)(6)(7)(8)(9)(10). One such alternative is the autoregulatory model proposed by Hatfield and Burns (11) in 1970, which was formulated to explain the repression pattern observed for enzymes involved in biosynthesis of the branched-chain amino acids, visoleucine, r-valine, and L-leucine (Fig.…”

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confidence: 99%

Autoregulation: A Role for a Biosynthetic Enzyme in the Control of Gene Expression

Calhoun

Hatfield

1973

Proc. Natl. Acad. Sci. U.S.A.

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It was previously proposed, primarily on the basis of evidence in vitro, that L-threonine deaminase, the ilvA gene product, is required for repression of its own synthesis and for repression of the other genes in the ilv-ADE operon. In this communication, evidence in vivo is presented that supports this autoregulatory model. Further evidence is presented that suggests that L-threonine deaminase is also required for induction of the ilvC gene product. The autoregulatory model is presented in an expanded form to include recent evidence that L-threonine deaminase (EC 4.2.1.16) is a central element for repression of the ilvADE and ilvB operons, and for induction of the ilOX operon.The operator-relpressor model proposed by Jacob and Monod(1) in 1961 provided molecular biologists an insight into the possible mechanisms of regulation of gene expression. After the introduction of this model, a large number of experimental facts have accumulated that have proven the validity of the model, particularly with respect to the lactose utilization system for which it was originally formulated (2). Because of the simplicity and aesthetic appeal of the JacobMonod model, great energy was expended after its introduction to apply it to other catabolic and biosynthetic regulatory systems. However, the model, in its original form, has proven inadequate for the explanation of many of the facts surrounding repression and derepression of enzymes involved in biosynthetic pathways (3). Several authors have proposed alternative models of regulation for these systems (4-10). One such alternative is the autoregulatory model proposed by Hatfield and Burns (11) in 1970, which was formulated to explain the repression pattern observed for enzymes involved in biosynthesis of the branched-chain amino acids, visoleucine, r-valine, and L-leucine (Fig. 1). Repression of these enzvmes is multivalent; i.e., for repression of enzyme synthesis to occur, all three branched-chain amino acids must be present in excess (12). The model (Fig. 2) proposed that, depending upon the chemical environment in the cell, -threonine deaminase (EC 4.2.1.16), the ilvA gene product, can function as either a catalytically active enzyme or as a catalytically inactive, autoregulatory feedback repressor of the ilvADE operon. The sequence of events proposed in the autoregulatory model corresponds to the molecular transitions demonstrated in vitro (Fig. 2, steps 2-6) with purified enzyme preparations (13,14). It was demonstrated that addition of the L-threonine deaminase cofactor, pyridoxal phosphate, to purified prepara- tions of apoenzyme results in formation of an immature (enzymatically inactive) holoenzyme (Fig. 2, step 4). In the presence of either L-isoleucine or L-valine (maturation-inducing ligands) this immature holoenzyme is converted to its mature (enzymatically active) form (Fig. 2, step 5a); the simultaneous presence of both L-isoleucine and i-valine blocks this maturation process (Fig. 2, step 5b). It was also shown that immature holo-L-threonine deaminase ...

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Studies on Repression of the Histidine Operon, Ii. The Role of the First Enzyme in Control of the Histidine System

Cited by 60 publications

References 21 publications

Enzyme Profiles in Seedling Development and the Effect of Itaconate, an Isocitrate Lyase-directed Reagent

Enzyme Profiles in Seedling Development and the Effect of Itaconate, an Isocitrate Lyase-directed Reagent

Involvement of Threonine Deaminase in Multivalent Repression of the Isoleucine-Valine Pathway in Saccharomyces cerevisiae

Autoregulation: A Role for a Biosynthetic Enzyme in the Control of Gene Expression

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