7-phosphate Synthetase Research Articles

Heterobasidionspp, triggers non-specific defence responses in bark of Norway spruce

Norway spruce [Picea abies (L.) Karst.] is one of the economically most important conifer species in Europe. The major pathogen on Norway spruce is Heterobasidion parviporum (Fr.) Niemela & Korhonen. The completed genome sequence of H. irregulare opens up new possibilities to understand the interactions between Heterobasidion spp. and spruce. However to date there are no completed genome sequencing projects of conifer genomes available for complementing studies. To achieve a better understanding of induced transcriptional defence responses in Norway spruce upon Heterobasidion spp. attack, we compared transcriptional responses in bark to H. parviporum infection to the response to wounding using cDNA-AFLP and transcriptome sequencing. In an initial study bark samples were harvested at 3, 7 and 14 days post inoculation (dpi) and untreated bark was used as negative control. About 2500 transcribed derived fragments (TDFs) generated by cDNA-AFLP were screened. 199 TDFs were investigated further based on band intensity in the inoculated bark in relation to either untreated bark or wounded bark. Out of these, 119 TDFs had a putative homology and a consistent band intensity pattern between replications. A majority of these TDFs showed homology to genes known to associate with defence e.g. 3-deoxy-d-arabino-heptulosonate 7-phosphate synthetase (DAHP), Pathogenesis-related protein 1 (PR1), Lipoxygenase (LOX), ACC-synthase (ACS), ACC-oxidase (ACO) and Jasmonate ZIM-domain 1 (JAZ1). Many of these are found in Salicylic acid- or Jasmonic acid/ethylene-signalling pathways. The majority of the TDFs showed a similar expression pattern for all treatments but samples inoculated with H. parviporum generally showed an enhanced reaction (induction/repression) compared to wounding alone. Expression patterns were confirmed by qPCR in material treated with wounding and inoculation with H. parviporum or Phlebiopsis gigantea. Our data suggest that infection with H. parviporum in Norway spruce induces a broad defence, with many similarities to non-specific defence responses in angiosperms. Additionally signs of reallocation of carbon from primary to secondary metabolism were evident. With this information at hand we analysed four Norway spruce genotypes with either high or low susceptibility to Heterobasidion spp. [1] sampled 0, 5, 15 and 28 dpi with H. annosum. The bark phenol-composition was profiled in each sample. The 500,000 454-reads were assembled into 17,228 contigs that assembled in 14,364 putative transcript units (PTU) using the sequence assembler software Newbler™ (http://www.454.com). The assembled reference file was annotated with the software Blast2Go and the PTUs were submitted to differential expression analysis. Data on associations between gene expression levels and phenol composition in bark upon H. annosum inoculation and level of susceptibility will be presented.

A Single Ser-180 Mutation Desensitizes Feedback Inhibition of the Phenylalanine-Sensitive 3-Deoxy-D-Arabino-Heptulosonate 7-Phosphate (DAHP) Synthetase in Escherichia coli1

The Escherichia coli phenylalanine-sensitive DAHP (3-deoxy-D-arabino-heptulosonate 7-phosphate) synthetase (aroG product) is one of the DAHP synthetase isozymes that catalyze the first committed step in the biosynthesis of aromatic amino acids and vitamins. Through target-directed mutagenesis of the cloned aroG on a plasmid vector, followed by screening of phenylalanine-resistant colonies, we isolated a clone (pG908) showing feedback-insensitive mutation of DAHP synthetase. The mutations were identified as a T-->A mutation at nucleotide 22, and a C-->T mutation at nucleotide 539, causing a Leu-8 to Ile-8 mutation and a Ser-180 to Phe-180 substitution, respectively. The resulting enzyme exhibited comparable enzymatic activity to the wild type, but the degree of feedback inhibition had declined from approximately 60% to less than 10% in the presence of 20 mM phenylalanine in the assay medium. Replacement of Ile-8 by Leu, and substitution of Phe-180 with Ser, Asn and Cys, using site-directed mutagenesis, demonstrated that Ser-180 is a critical residue in the feedback inhibition of AroG. The result suggests that the major role played by Ser-180 may not involve a simple charge-size effect in the Phe-binding site of the enzyme molecule, but rather may involve more complicated molecular interactions occurring in the feedback inhibition mechanisms.

Effect of gibberellic acid on berberine and tyrosine accumulation in Coptis japonica

The enhancement of berberine production by gibberellic acid (GA 3) in addition to cell cultures of Coptis japonica was investigated. Coptis cells cultured in the presence of GA 3 gave increases in sucrose uptake, activation of 3-deoxy- d-arabino-heptulosonic acid 7-phosphate (DAHP) synthetase, a putative regulatory enzyme of the shikimate pathway, and free tyrosine levels relative to the controls. The timing of this increase in cellular berberine corresponded closely to the time course of the activation of DAHP synthetase and free tyrosine accumulation. Furthermore, the addition of tyrosine to the medium stimulated berberine production, but only in the presence of GA 3. We propose that GA 3 treatment contributes to berberine production by activating primary metabolic steps involved in tyrosine biosynthesis.

Regulation of 3-deoxy-d-arabinoheptulosonic acid 7-phosphate synthetase inNocardia mediterranei

Regulation of 3-deoxy-d-arabinoheptulosonic acid 7-phosphate synthetase in<i>Nocardia mediterranei</i>

Regulation of 3-deoxy-?-arabinoheptulosonic acid 7-phosphate synthetase in Nocardia mediterranei

Regulation of 3-deoxy-?-arabinoheptulosonic acid 7-phosphate synthetase in Nocardia mediterranei

Enzymatic Basis for Overproduction of Tryptophan and Its Metabolites in Hansenula polymorpha Mutants

3-Deoxy-d-arabinoheptulosonate 7-phosphate (DAHP) synthetase and anthranilate synthetase are key regulatory enzymes in the aromatic amino acid biosynthetic pathway. The DAHP synthetase activity of Hansenula polymorpha was subject to additive feedback inhibition by phenylalanine and tyrosine but not by tryptophan. The synthesis of DAHP synthetase in this yeast was not repressed by exogenous aromatic amino acids, singly or in combinations. The activity of anthranilate synthetase was sensitive to feedback inhibition by tryptophan, but exogenous tryptophan did not repress the synthesis of this enzyme. Nevertheless, internal repression of anthranilate synthetase probably exists, since the content of this enzyme in H. polymorpha strain 3-136 was double that in the wild-type and less sensitive 5-fluorotryptophan-resistant strains. The biochemical mechanism for the overproduction of indoles by the 5-fluorotryptophan-resistant mutants was due primarily to a partial desensitization of the anthranilate synthetase of these strains to feedback inhibition by tryptophan. These results support the concept that inhibition of enzyme activities rather than enzyme repression is more important in the regulation of aromatic amino acid biosynthesis in H. polymorpha.

Regulation at metabolic branch points of aromatic amino acid biosynthesis in Brevibacterium flavum.

In the biosynthetic pathway of aromatic amino acids of Brevibacterium flavum, ratios of each biosynthetic flow at the chorismate branch point were calculated from the reaction velocities of anthranilate synthetase for tryptophan and chorismate mutase for phenylalanine and tyrosine at steady state concentrations of chorismate. When these aromatic amino acids were absent, the ratio was 61, showing an extremely preferential synthesis of tryptophan. The presence of tryptophan at 0.01 mM decreased the ratio to 0.07, showing a diversion of the preferential synthesis to phenylalanine and tyrosine. Complete recovery by glutamate of the ability to synthesize the Millon-positive substance in dialyzed cell extracts confirmed that tyrosine was synthesized via pretyrosine in this organism. Partially purified prephenate aminotransferase, the first enzyme in the tyrosine-specific branch, had a pH optimum of 8.0 and Km's of 0.45 and 22 mM for prephenate and glutamate, respectively, and its activity was increased 1 5-fold by pyridoxal-5-phosphate. Neither its activity nor its synthesis was affected at all by the presence of the end product tyrosine or other aromatic amino acids. The ratio of each biosynthetic flow for tyrosine and phenylalanine at the prephenate branch point was calculated from the kinetic equations of prephenate aminotransferase and prephenate dehydratase, the first enzyme in the phenylalanine-specific branch. It showed that tyrosine was synthesized in preference to phenylalanine when phenylalanine and tyrosine were absent. Furthermore, this preferential synthesis was diverted to a balanced synthesis of phenylalanine and tyrosine through activation of prephenate dehydratase by the tyrosine thus synthesized. The feedback inhibition of prephenate dehydratase by phenylalanine was proposed to play a role in maintaining a balanced synthesis when supply of prephenate was decreased by feedback inhibition of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP*) synthetase, the common key enzyme. Overproduction of the end products in various regulatory mutants was also explained by these results.

Regulatory control of 3-deoxy-D-arabino-heptulosonic acid 7-phosphate synthetase in Streptomyces antibioticus.

Streptomyces antibioticus possesses a tryptophan-inhibitable 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthetase whose synthesis is also repressed by L-tryptophan. Studies of the DAHP synthetase obtained by ammonium sulfate fractionation of a crude extract derived from S. Antibioticus revealed that the enzymic activity was only partially inhibited by tryptophan. Inhibition of the DAHP synthetase activity was strongly pH dependent at values below 7.0. A number of tryptophan analogues was noted to inhibit the enzyme; by contrast, other aromatic amino acid end products failed to affect DAHP synthetase activity. Chorismic acid, a key intermediate in aromatic amino acid biosynthesis, was ineffective as an inhibitor when used alone; however, if supplied with L-tryptophan, a further reduction of DAHP synthetase activity (15--25%) was routinely observed.

Purification and Properties of Bifunctional 3-Deoxy-D-arabino-Heptulosonate 7-Phosphate Synthetase-Chorismate Mutase Component A from Brevibacterium flavum

3-Deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthetase of Brevibacterium flavum, which is the first enzyme in the aromatic amino acid biosynthetic pathway, was purified 540-fold to a disc-gel-electrophoretically homogeneous state. During the purification processes, component A of chorismate mutase, a common enzyme for phenylalanine and tyrosine biosynthesis, was purified together with DAHP synthetase in a constant ratio. It was concluded that DAHP synthetase and chorismate mutase component A form a bifunctional enzyme. The molecular weight of bifunctional DAHP synthetase-chorismate mutase component A was estimated to be 250,000 by gel-filtration. On SDS-polyacrylamide-gel electrophoresis, a single protein band was obtained and the molecular weight was estimated to be 55,000, suggesting that DAHP synthetase-chorismate mutase component A is an oligomer of identical subunits. 5, 5'-Dithio-bis(2-nitrobenzoic acid) (DTNB) reacted with 3 mol of sulfhydryl groups per mol of the subunit. One of the sulfhydryl groups was essential for the DAHP synthetase activity, whereas the chorismate mutase activity was not affected by DTNB at all. The optimum pH of the DAHP synthetase reaction was 6.5 with the purified preparation. The DAHP synthetase activity was stimulated 30% by chorismate mutase component B but not by chorismate, the substrate of chorismate mutase. Double-reciprocal plots of the reaction rate against erythrose-4-phosphate concentration gave straight lines with Km of 0.4 mM for erythrose-4-phosphate. Plots of the reaction rate against phosphoenolpyruvate concentration curved upwards. The Hill coefficient and S0.5 for phosphoenolpyruvate were estimated to be 4.1 and 0.4 mM, respectively. Inhibition by inorganic phosphate, one of the products, was noncompetitive for erythrose-4-phosphate and was not competitive for phosphoenol-pyruvate. Phenylalanine and tyrosine were competitive inhibitors for erythrose-4-phosphate (apparent K1, 0.066 mM) and mixed-type inhibitors for phosphoenolpyruvate. Tryptophan did not affect DAHP synthetase activity at all.

Enzymic properties of phosphonic analogues of D-erythrose 4-phosphate

4-Deoxy-D- erythro tetrose 4-phosphonate and 4,5 dideoxy D- erythro pentose 5-phosphonate, the phosphonic analogues of D-erythrose 4-phosphate, have been prepared by oxidation of the corresponding analogues of glucose 6-phosphate and tested as substrates of 3-deoxy-D- arabino heptulosonate 7-phosphate synthetase, transaldolase and transketolase. Kinetic parameters of the reaction with the phosphonate analogues and the natural substrate have been compared.

Two Components of Chorismate Mutase in &lt;italic&gt;Brevibacterium flavum&lt;/italic&gt;

Chorismate mutase of Brevibacterium flavum, a common enzyme in phenylalanine and tyrosine biosynthesis, was separted into two different component, A and B, with molecular weights of 250,000 and 25,000, respectively, by ammonium sulfate fractionation or gel-filtration. Both components were essential for the enzymatic activity. In the presence of the reaction substrate, chorismate, the two components associated reversibly to give an active enzyme complex with a molecular weight of 320,000. Binding sites of the feedback inhibitors, phenylalanine and tyrosine, on the enzyme were localized on component A as determined by hybridization experiments with the wild-type and mutant components. Tyrosine repressed the synthesis of component B much more strongly than that of component A, while phenylalanine did not show any significant repressive effect on either component. The wild-type strain No. 2247 had four times more component A than component B. Elution patterns in gel, DEAE-cellulose or hydroxyapatite column chromatography as well as the disc-gel electrophoretic pattern of chorismate mutase component A and 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthetase activities completely overlapped, suggesting the presence of a bifunctional protein having the two activities. In accord with this suggestion, chorismate mutase as well as DAHP synthetase was insensitive to feedback inhibition by phenylalanine and tyrosine in all the 3-fluorophenylalanine-resistant mutants tested that excreted both phenylalanine and tyrosine. All the phenylalanine and tyrosine double auxotrophs defective in chorismate mutase lacked component B but not A.

Some properties of the initial four enzymes involved in shikimic acid biosynthesis in tea plant.

The initial four enzymes in shikimic acid biosynthesis, 3-deoxy-d-arabino-heptulosonic acid 7-phosphate synthetase (EC 4.1.2.15), 3-dehydroquinic acid synthetase, 3-dehydroquinate hydro-lyase (EC 4.2.1.10), and shikimate: NADP oxidoreductase (EC 1.1.1.25), were detected in cell-free extracts of tea plant. No significant difference in the enzyme activities was detected between tea cultivars of low and high fiavonoid contents. 3-Dehydroquinic acid synthetase was inhibited by gallic acid, (—)-epicatechin gallate, and (—)-epigallocatechin gallate, but not by l-phenyl alanine, l-tyrosine, l-tryptophan, protocatechuic acid, chlorogenic acid, trans-cinnamic acid, (—)-epicatechin, nor (—)-epigallocatechin. The activities of 3-deoxy-d-arabino-heptulosonic acid 7-phosphate synthetase and 3-dehydroquinate hydro-lyase were not affected by any of the above compounds. 3-Deoxy-d-arabino-heptulosonic acid 7-phosphate synthetase was a particulate-bound enzyme and 3-dehydroquinic acid synthetase was a soluble enzyme in tea...

Purification and properties of tyrosine-sensitive 3-deoxy- d-arabino-heptolosonate-7-phosphate synthetase of Escherichia coli K12

1. 1.|The tyrosine-sensitive allosteric first enzyme of the aromatic amino acid biosynthetic pathway, 3-deoxy- d-arabinoheptulosonate 7-phosphate synthetase (7-Phospho-2-keto-3-deoxy- d- arabino-heptonate d-erythrose 4-phosphatelyase (pyruvate phosphorylating), EC 4.1.2.15.) has been purified from a mutant strain of Escherichia coli. 2. 2.|The enzyme activity was inhibited to 50% at 2 · 10 −5 M tyrosine and to 90% at 2 · 10 −4 M tyrosine concentration. At tyrosine concentrations lower than 2 · 10 −5 M a cooperative interaction between tyrosine binding sites was observed. 3. 3.|Co 2+ increased the enzyme activity about 2–2.5-fold. The presence of Co 2+ ions stabilized the enzyme. EDTA inhibited the enzyme activity, and this inhibition was reversed by Co 2+. Tyrosine-sensitive DAHP synthetase seems to be a metal containing enzyme. 4. 4.|Kinetic experiments were carried out to study the catalytic action. Contrary to earlier suggestions it is concluded, that the reaction mechanism appears to be more complex — with either the ping-pong or sequential type predominating, depending on conditions.

Mis-regulation of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthetase does not account for growth inhibition by phenylalanine in Agmenellum quadruplicatum.

The growth of the blue-green bacterium, Agmenellum quadruplicatum, is inhibited in the presence of l-phenylalanine. This species has a single, constitutively synthesized 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase. l-Phenylalanine inhibits DAHP synthetase non-competitively with respect to both substrate reactants. Other aromatic amino acids do not inhibit the activity of DAHP synthetase. A common expectation for branch-point enzymes such as DAHP synthetase is a balanced pattern of feedback control by all of the ultimate end products. It seemed likely that growth inhibition might equate with defective regulation within the branched aromatic pathway. Accordingly, the possibility was examined that mis-regulation of DAHP synthetase by l-phenylalanine in wild-type cells causes starvation for precursors of the other aromatic end products. However, the molecular basis for growth inhibition cannot be attributed to l-phenylalanine inhibition of DAHP synthetase for the following reasons: (i) DAHP synthetase enzymes from l-phenylalanine-resistant mutants are more, rather than less, sensitive to feedback inhibition by l-phenylalanine. (ii) Shikimate not only fails to antagonize inhibition, but is itself inhibitory. (iii) Neither the sensitivity nor the completeness of l-phenylalanine inhibition of the wild-type enzyme in vitro appears sufficient to account for the potent inhibition of growth in vivo by l-phenylalanine. The dominating effect of l-phenylalanine in the control of DAHP synthetase appears to reflect a mechanism that prevents rather than causes growth inhibition by l-phenylalanine. The alteration of the control of DAHP synthetase in mutants selected for resistance to growth inhibition by l-phenylalanine did indicate that the cause for this metabolite vulnerability can be localized within the aromatic amino acid pathway. Apparently, an aromatic intermediate (between shikimate and the end products) accumulates in the presence of l-phenylalanine, causing toxicity by some unknown mechanism. It is concluded that phenylpyruvate, potentially formed by transamination of l-phenylalanine, is an unlikely cause of growth inhibition. Although several significant questions remain unanswered, our results suggest that single-effector control of DAHP synthetase, the first regulatory enzyme activity of a branched pathway, may be more appropriate than it would seem a priori.

Regulation of Aromatic Amino Acid Biosynthesis in Higher Plants: II. 3-DEOXY-ARABINO-HEPTULOSONIC ACID 7-PHOSPHATE SYNTHETASE FROM CAULIFLOWER

Abstract 3-Deoxy-arabino-heptulosonic acid 7-phosphate synthetase, which catalyzes the condensation of P-enolpyruvate and erythrose-4-P to yield 3-deoxy-arabino-heptulosonic acid 7-phosphate has been purified 1000-fold from cauliflower florets. The enzyme exhibited an activity optimum around pH 6.6 and a stability optimum around pH 7.7. Optimum enzyme stability was achieved in a buffer of pH 7.7, containing a reducing agent, MnCl2, and a polyol compound such as glycerol or 1,3-propanediol. Loss of synthetase activity occurred rapidly in preparations with low protein concentration, but could be prevented by addition of bovine serum albumin. The reaction catalyzed by the enzyme was linear both with time and protein concentration. The synthetase exhibited high specificity for the substrates erythrose-4-P and P-enolpyruvate. An activation energy of 17,700 cal per mole was calculated for the enzymatic reaction. Strong inhibition by sulfhydryl reagents indicated that the synthetase contains a —SH group which is essential for activity. Inhibition by 5,5'-dithiobis(2-nitrobenzoic acid) was reversed by dithiothreitol. Inhibition of activity by a number of metal chelators indicates that the enzyme contains a metal ion important for activity. Of eight cations tested, Mn2+ was most active in reversing EDTA inhibition; Ni2+, Co2+, Zn2+, and Cu2+ inhibited enzyme activity. Although the addition of Mn2+ was effective in stabilizing the enzyme, its addition was not necessary for demonstrating enzyme activity. Double reciprocal plots of velocity versus concentration of P-enolpyruvate were biphasic. S0.5 values for erythrose-4-P of 0.24 and 0.14 mm were calculated for high and low concentrations of P-enolpyruvate, respectively. The corresponding S0.5 values for P-enolpyruvate were 0.30 and 0.025 mm at high and low concentrations of P-enolpyruvate, respectively. Inhibition by erythrose-4-P occurred at concentrations greater than 1 mm but the inhibition was reversible by P-enolpyruvate. Gel chromatography of partially purified enzyme preparations gave evidence for the existence of several forms of the enzyme with different molecular weights. The enzyme was not inhibited by chorismate, phenylalanine, tyrosine, or tryptophan.

Regulation of Aromatic Amino Acid Biosynthesis in Bacillus subtilis 168: PURIFICATION, CHARACTERIZATION, AND SUBUNIT STRUCTURE OF THE BIFUNCTIONAL ENZYME 3-DEOXY-d-ARABINOHEPTULOSONATE 7-PHOSPHATE SYNTHETASE-CHORISMATE MUTASE

Abstract A bifunctional enzyme, consisting of 3-deoxy-d-arabinoheptulosonate 7-phosphate synthetase and chorismate mutase, has been purified to homogeneity from Bacillus subtilis. Ultracentrifugation revealed a single symmetrical peak of protein. A single protein band also was observed when the homogeneous preparation was subjected to polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate at pH 7.0. These data suggest that the protein consists of identical subunits. Following treatment of the purified protein with dimethyl suberimidate, four protein bands could be clearly distinguished upon electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. These four bands correspond to the monomer, dimer, trimer, and tetramer forms of the enzyme, as calculated from their molecular weights. The molecular weight of the monomer is about 38,500 as determined by its mobility following electrophoresis on polyacrylamide in the presence of sodium dodecyl sulfate. This number corresponds closely to the minimum chemical molecular weight as calculated from the amino acid composition. The enzyme can exist as a tetramer of molecular weight 160,000 as determined by superfine Sephadex G-200 thin layer chromatography. Metal ions and chemical inhibitors affect the two enzyme activities quite differently, Cd+2 stimulates 3-deoxy-d-arabinoheptulosonate 7-phosphate synthetase without any effect on chorismate mutase, while p-hydroxymercuribenzoate inhibits 3-deoxy-d-arabinoheptulosonate 7-phosphate synthetase but not chorismate mutase.

Impaired regulation of aromatic amino acid synthesis in a mutant resistant to p-fluorophenylalanine.

Summary: Coprinus lagopus is inhibited by the analogue of phenylalanine, p-fluorophenyl-alanine (pFPA). Mutant strain f 7, selected for growth on pFPA, is a slow grower which is not inhibited by the analogue. Over a prolonged period of growth, the amount of [14C] (pFPA incorporated into the trichloroacetic acid-insoluble fraction of resistant f 7 mycelium is significant, but is less than 20 % of the amount incorporated into similar fractions of the mycelium of the parental sensitive strain tc1. Resistant f 7 grown on minimal medium, excretes into the medium p-hydroxy-phenylpyruvic acid, phenylpyruvic acid and a number of unidentified phenolic compounds. The sensitive tci strain grown on minimal medium does not excrete, but when grown on media supplemented with phenylalanine or shikimate excretes traces of these compounds. The total biosynthesis of phenylalanine over 24 h by sensitive strain tc1 is greatly reduced by the presence of exogenously supplied phenylalanine. No such reduction in synthesis is observed in the resistant mutant f 7. It is inferred that the mutation impairs the regulation of phenylalanine biosynthesis, probably by the loss of feedback control at either the 3-deoxy-d-amftmo-heptulo-sonate 7-phosphate (DAHP) synthetase or chorismate mutase.

Tyrosine and phenylalanine biosynthesis in Escherichia coli K-12: complementation between different tyrR alleles.

Dominance tests in diploids have confirmed that the product of the tyrR gene is involved in a negative control system affecting the synthesis of both 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthetase (tyr) and DAHP synthetase (phe). Some tyrR mutants are derepressed for the synthesis of both DAHP synthetase (tyr) and (phe), whereas others are derepressed for the synthesis of DAHP synthetase (tyr) but overrepressed for the synthesis of DAHP synthetase (phe). Complementation tests between these alleles confirm that they are in the same cistron. The allele causing overrepression of enzyme synthesis is dominant over both the wild type and the derepressing allele in diploids.

Regulation of tyrosine and phenylalanine biosynthesis in Escherichia coli K-12: properties of the tyrR gene product.

A spontaneous amber tyrR mutant has been isolated in which constitutive synthesis of 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthetase (tyr) and DAHP synthetase (phe) is suppressible by supC(-), supD(-), supF(-) and supU(-). This finding suggests the tyrR gene product is a protein. Derepression of DAHP synthetase (phe) in this and in seven other spontaneous tyrR mutants and in four Mu-1-induced tyrR mutants provides further evidence for the involvement of the tyrR gene product in phenylalanine biosynthesis. Evidence that the tyrR product is a component of repressor, rather than an enzyme involved in its synthesis or modification, comes from a study of a temperature-sensitive tyrR mutant. This mutant is of the thermolabile type, since derepression occurs rapidly and in the presence and absence of growth.

TyrR, a regulatory gene of tyrosine biosynthesis in Salmonella typhimurium.

4-Fluorophenylalanine-resistant mutants of Salmonella typhimurium were isolated in which tyrosine pathway enzymes were not repressed by l-tyrosine. The mutants produced elevated levels of 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthetase (tyr) and chorismate mutase T-prephenate dehydrogenase, and these enzymes as well as transaminase A were not repressed by high concentrations of tyrosine. Genetic analysis revealed that a mutation in a gene designated tyrR was responsible for the constitutivity of the tyrosine pathway enzymes in strains SG1, SG7, and SG9, and that tyrR was linked to pyrF. In strain SG1 a mutation had also occurred in aroF, the structural gene for DAHP synthetase (tyr), resulting in loss of sensitivity of this enzyme to end-product inhibition. There appeared to be no relationship between loss of feedback inhibition and loss of end-product repression, since derivative strains of SG1 that carried only the tyrR mutation behaved like the singly mutated tyrR strains, SG7 and SG9, in showing high constitutive levels of tyrosine-specific enzymes that were not repressed by tyrosine.