Enzymatic Synthesis of TNA Protects DNA Nanostructures

Xeno-nucleic acids (XNAs) are synthetic genetic polymers with improved biological stabilities and offer powerful molecular tools such as aptamers and catalysts. However, XNA application has been hindered by a very limited repertoire of tool enzymes, particularly those that enable de novo XNA synthesis. Here we report that terminal deoxynucleotide transferase (TdT) catalyzes untemplated threose nucleic acid (TNA) synthesis at the 3' terminus of DNA oligonucleotide, resulting in DNA-TNA chimera resistant to exonuclease digestion. Moreover, TdT-catalyzed TNA extension supports one-pot batch preparation of biostable chimeric oligonucleotides, which can be used directly as staple strands during self-assembly of DNA origami nanostructures (DONs). Such TNA-protected DONs show enhanced biological stability in the presence of exonuclease I, DNase I and fetal bovine serum. This work not only expands the available enzyme toolbox for XNA synthesis and manipulation, but also provides a promising approach to fabricate DONs with improved stability under the physiological condition.

Conformational preferences of guanine-containing threose nucleic acid building blocks in B3LYP studies

In this paper, detailed and systematic gas-phase B3LYP conformational studies of four monomers of threose nucleic acid (TNA) with guanine attached at the C1' atom and bearing different substituents (OH, OP(=O)OH2 and OCH3) in the C2' and C3' positions of the α-l-threofuranose moiety are presented. All exocyclic single-bond (χ, ε and γ) rotations, as well as the ν0-ν4 endocyclic torsion angles, were taken into consideration. Three (threoguanosines TG1-TG3) or two (TG4) energy minima were found for the rotation about the χ torsion angle. The syn orientation (the A rotamer family) is strongly privileged in geometries TG1 and TG2, whereas the anti orientation (the C rotamer family) and the syn orientation are observed to be in equilibrium (with populations of 56% and 44%, respectively) for TG3. In the case of TG4, the high-anti orientation (the B rotamer family) turned out to be by far the most favourable, with the contribution exceeding 90% in equilibrium. Such a preference can be attributed to the inability of H-bonding between sugar and nucleobase and possibly because of the steric strains. The low-energy conformers of TG1-TG4 occupy the northeastern (P ∼ 40°) and/or southern (P ∼ 210°) parts of the pseudorotational wheel, which fits the A- and B-type DNA helices quite well. Additionally, in the case of TG4, some relatively stable geometries have the furanoid ring in conformation lying on the northwestern part of the pseudorotational wheel (P ∼ 288°).

α-l-Threose Nucleic Acids as Biocompatible Antisense Oligonucleotides for Suppressing Gene Expression in Living Cells

Because of the chemical simplicity of α-l-threose nucleic acid (TNA) and its ability to exchange genetic information between itself and RNA, it has attracted significant interest as the RNA ancestor. We herein explore the biological properties and evaluate the potency of sequence-designed TNA polymers to suppress the gene expression in living environments. We found that sequence-specific TNA macromolecules exhibit strong affinity and specificity toward the complementary RNA targets, are highly biocompatible and nontoxic in a living cell system, and readily enter a number of cell lines without using transfecting agents. Particularly, TNA exhibited much stronger enzymatic resistance toward fetal bovine serum or human serum as compared to traditional antisense oligonucleotides, which means that the intrinsic structure of TNA is thoroughly resistant to biological degradation. Importantly, the efficacy of the TNA molecule with green fluorescent protein (GFP) target sequence (anti-GFP TNAs) as antisense agents was first demonstrated in living cells in which these polymers revealed high antisense activity in terms of the degree of inhibition of GFP gene expression. The GFP gene inhibition studies in HeLa and HEK293 cells characterize sequence-controlled TNA as a functional biomaterial and a valuable alternative to traditional antisense oligonucleotides such as peptide nucleic acids, phosphorodiamidate morpholino oligomers, and locked nucleic acids for a wide range of applications in drug discovery and life science research. Additionally, we also first reported the cost-efficient approach to synthesize the four TNA phosphoramidite monomers using 2-cyanoethyl N, N, N', N'-tetraisopropylphosphoramidite as a key reagent. Furthermore, by increasing the frequency of the deblocking and coupling reactions together with extending their reaction time in each synthesis cycle, sequence-controlled TNAs can be easily synthesized in a quantitative yield and high purity.

Identification and characterization of EYD1, encoding an erythritol dehydrogenase in Yarrowia lipolytica and its application to bioconvert erythritol into erythrulose

In this study, gene YALI0F01650g has been isolated and characterized. Several experimental evidences suggest that the identified gene, renamed EYD1, encodes an erythritol dehydrogenase. An efficient bioreactor process for the bioconversion of erythritol into erythrulose was also developed. Using constitutive expression of EYD1 in a Y. lipolytica mutant containing a disrupted EYK1 gene, which encodes erythrulose kinase, erythrulose could be synthesized from erythritol at a rate of 0.116g/g_DCW.h and with a bioconversion yield of 0.64g/g.

Endothelial cell dynamics under pulsating flows: significance of high versus low shear stress slew rates (d(tau)/dt)

Shear stress modulates endothelial cell (EC) remodeling via realignment and elongation. We provide the first evidence that the upstroke slopes of pulsatile flow, defined as shear stress slew rates (positive d(tau)/dt), affect significantly the rates at which ECs remodel. We designed a novel flow system to isolate various shear stress slew rates by precisely controlling the frequency, amplitude, and time-averaged shear stress (tau(ave)) of pulsatile flow. Bovine aortic endothelial cell (BAEC) monolayers were exposed to three conditions: (1) pulsatile flow (1 Hz) at high slew rate (293 dyn/cm2 s), (2) pulsatile flow (1 Hz) at low slew rate (71 dyn/cm2s), and (3) steady laminar flow at d(tau)/dt = 0. All of the three conditions were operated at tau(ave) = 50 dyn/cm2. BAEC elongation and alignment were measured over 17 h. We were able to demonstrate the effects of shear stress slew rates ((tau)/dt) on EC remodeling at a fixed spatial shear stress gradient (d(tau)/dx). We found that pulsatile flow significantly increased the rates at which EC elongated and realigned, compared to steady flow at d(tau)/dt = 0. Furthermore, EC remodeling was faster in response to high than to low slew rates at a given tau(ave).

Sequence of a cellulase gene from the rumen anaerobe Ruminococcus flavefaciens 17

A cellulase gene (endA) was isolated from a library of Ruminococcus flavefaciens strain 17 DNA fragments inserted in pUC13. The endA product showed activity against acid-swollen cellulose, carboxymethyl-cellulose, lichenan, cellopentaose and cellotetraose, but showed no activity against cellotriose or binding to avicel. Nucleotide sequencing indicated an encoded product of 455 amino acids which showed significant sequence similarity (ranging from 56% to 61%) with three endoglucanases from Ruminococcus albus, and with Clostridium thermocellum endoglucanase E. Little relatedness was found with a cellodextrinase previously isolated from R. flavefaciens FD1.

Purification and properties of an endo-1,4-beta-glucanase translated from a Clostridium josui gene in Escherichia coli

An endoglucanase encoded by a gene of Clostridium josui was expressed in Escherichia coli and purified. The homogeneous enzyme, with a molecular weight of 39,000, revealed maximum endoglucanase activity at pH 7.2 to 7.5 and a temperature of 65 to 70 degrees C. The enzyme was stable at a temperature lower than 45 degrees C (the growth temperature of the bacterium) in the range of pH 4.5 to 9.0. The amino acid sequence of the enzyme at the N terminus was Val-Glu-Glu-Asp-Ser-Ser-His-Leu-Ile-Thr-Asn-Gln-Ala-Lys-Lys----. The enzyme hydrolyzed cellotetraose to cellobiose and then transferred cellobiose to the residual cellotetraose. The resulting cellohexaose was cleaved to cellotriose.

Inhibition of the alpha-L-arabinofuranosidase III of Monilinia fructigena by 1,4-dideoxy-1,4-imino-L-threitol and 1,4-dideoxy-1,4-imino-L-arabinitol

1. 1,4-Dideoxy-1,4-imino-L-threitol was synthesized and the synthesis of 1,4-dideoxy-1,4-imino-L-arabinitol was improved. 2. Both compounds are competitive inhibitors of Monilinia fructigena alpha-L-arabinofuranosidase III, the additional hydroxymethyl group in the arabinitol contributing about 17.8 kj/mol (4.25 kcal/mol) to the Gibbs free energy of binding. 3. The affinities (1/Ki) of both compounds vary with pH in a classical bell-shaped way, the pKa value being that of the acid-catalytic group on the enzyme [5.9; Selwood & Sinnott (1988) Biochem. J. 254, 899-901] and the pKb values being those of the free inhibitors, 7.6 and 7.8 respectively. 4. On the basis of these and literature data we suggest that efficient inhibition of a glycosidase at its pH optimum by an appropriate iminoalditol will be found when the pKa of the iminoalditol is below that of the acid-catalytic group of the target enzyme.

Prebiotic synthesis of imidazole-4-acetaldehyde and histidine

The prebiotic synthesis of imidazole-4-acetaldehyde and imidazole-4-glycol from erythrose and formamidine has been demonstrated as well as the prebiotic synthesis of imidazole-4-ethanol and imidazole-4-glycol from erythrose, formaldehyde and ammonia. The products were identified by TLC, HPLC, and LC-MS by comparison with authentic samples. The maximum yields of imidazole-4-acetaldehyde, imidazole-4-ethanol, and imidazole-4-glycol obtained in these reactions are 1.6, 5.4, 6.8% respectively, based on the erythrose. Imidazole-4-acetaldehyde would have been converted to histidine on the primitive earth by a Strecker synthesis, and several prebiotic reactions would convert imidazole-4-glycol and imidazole-4-ethanol to imidazole-4-acetaldehyde.

General methods for enriching aldoses with oxygen isotopes

Synthetic methods are described for enriching 4-, 5-, and 6-carbon aldoses with oxygen isotopes. The general approach includes exchange between H2(1)8O and the aldehyde group of an aldose, exchange of O-1 onto C-2 of both of the 2-epimeric aldoses formed by molybdate-resin epimerization, and chain extension using cyanide addition. These methods make possible the production of all 16 aldohexoses enriched at 5 of the 6 oxygen atoms, all 8 aldopentoses enriched at 4 of the 5 oxygen atoms, and the four aldotetroses enriched at 2 of the 4 oxygen atoms. The general applicability of these methods is illustrated by the synthesis of a group of 22 different, 18O-enriched, biologically important D-aldoses having 4, 5, and 6 carbon atoms. The group includes D-[1-, 2-, 3-, 4-, O]glucose, D-[1-, 2-, 3-, 4-, and 6-18O]mannose, D-[1-, 2-, 3-, and 5-18O]arabinose, D-[1- abd 2- 18O] erythorose, and D-[1- and 2-18O]threose. The g.l.c.-m.s. characterization of these sugars with respect to the position and degree of 18O-enrichment is reported. The potential of the methods for producing aldoses having oxygen labels at multiple positions, or aldoses labeled simultaneously with oxygen, hydrogen, and carbon isotopes is discussed.

Direct 1H n.m.r. determination of the stereochemical course of hydrolyses catalysed by glucanase components of the cellulase complex

The stereochemical courses of the hydrolyses catalysed by three glycosidases have been determined directly by 1H nmr. The anomeric configuration of the initially formed product was ascertained in each case by observation of the chemical shift and coupling constant of the anomeric proton at the new hemiacetal centre. Two of the enzymes investigated, an endo-glucanase and an exo-glucanase are components of the cellulase complex of Cellulomonas fimi. The third enzyme is the beta-glucosidase from almond emulsin. Two of these enzymes, the exo-glucanase and the almond beta-glucosidase catalysed hydrolysis with retention of anomeric configuration, in agreement with previous observations on the almond enzyme. The endo-glucanase catalysed hydrolysis with inversion of configuration, this result being confirmed by optical rotation measurements. This 1H nmr approach has several advantages over other techniques in that it is applicable to a wide variety of glycosidases and substrates and it is non-destructive, allowing recovery of the enzyme.

An automated method for the quasi-continuous analysis of degradation and transfer products during the enzymatic hydrolysis of oligosaccharides

A method is presented which allows for the automated quasi-continuous analysis of the degradation and transfer products developing during the enzymatic hydrolysis of oligosaccharides. A liquid chromatographic system is integrated into the bypass of a small batch reactor which makes it possible to take oligomer spectra without any manual sample processing being necessary. The time intervals between analyses are substantially reduced by making use of an overlapping analysis technique. Postcolumn derivatization with an orcinol sulfuric acid reagent gives a high sensitivity for carbohydrates. The great potential of this method is demonstrated for the characterization of a beta-glucosidase (pI 8.4) from Trichoderma reesei QM 9414 and an alpha 1,4-glucan glucohydrolase from Aspergillus niger with cellotetraose and maltohexaose as examplary substrates.

Carbon-13-enriched carbohydrates: preparation of triose, tetrose, and pentose phosphates

Three-, four-, and five-carbon aldononitrile phosphates were prepared, purified, and catalyticlly reduced with palladium--barium sulfate (5%) to the corresponding aldose phosphates in high yields at pH 1.7 +/- 0.1 and atmopsheric pressure. DL-Glyceraldehyde 3-phosphate and the tetrose 4-phosphates were prepared with carbon-13 enrichment at C-1, while the pentose 5-phosphates were prepared with enrichment at C-1 and C-2. Preparations of glycolaldehyde phosphate and d-glyceraldehyde 3-phosphate by lead tetra-acetate oxidation of glycerol phosphate and fructose 6-phosphate, respectively, are described. The proportions of cyclic hemiacetals and linear gem-diol forms of the two- to five-carbon aldose phosphates in aqueous solution are reported. Carbon-13 chemical shifts and carbon--phosphorus and carbon--hydrogen coupling constants for the furanose phosphate ring and linear gem-diol phosphates are reported and discussed. d-[2(-13)C]Ribulose 1,5-bisphosphate and L-[3,4(-13)C]sorbose 1,6-bisphosphate were prepared enzymatically from D-[2(-13)C]ribose 5-phosphate and dl-[1(-13)C]glyceraldehyde 3-phosphate, respectively.

Effect of NADP+ and its analogs on the Rose Bengal-sensitized photoinactivation of D-erythrulose reductase from beef liver

Upon addition of NADP+, the rose bengal-sensitized photoinactivation of D-erythrulose reductase from beef liver is prevented to a remarkable extent. Adenosine 2',5'-diphosphate (2',5'-ADP) also has a protective effect, but to a lesser extent. On the other hand, 2'-AMP markedly enhances the photoinactivation. Other nucleotides which have no 2'-phosphoryl group, such as NAD+, 3'-AMP, 5'-AMP, ADP, and NMN, are ineffective. Further, only 2'-AMP derivatives (NADP+, 2',5'-ADP, and 2'-AMP) among these nucleotides were found to be potent competitive inhibitors of the enzyme with small Ki's (6--13 muM). Photooxidation of some methionine residues in the enzyme is prevented by the addition of NADP+ and accelerated in the presence of 2'-AMP. Photooxidation products(s) of 2'-AMP derivatives have no effect upon the enzymatic activity. Although NADP+ and 2'-AMP induce detectable conformational changes of the enzyme, the changes are not characteristic to the compounds. Based on these observations, we present a possible action mechanism of 2'-AMP derivatives on the photoinactivation of D-erythrulose reductase.

3-Deoxy-D-arabino-heptulosonic acid 7-phosphate synthase mutants of Salmonella typhimurium

The first committed step of aromatic amino acid biosynthesis in Salmonella typhimurium was shown to be catalyzed by three isoenzymes of 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthase. Mutations in each of the genes specifying the isoenzymes were isolated and mapped. aroG, the structural gene for the phenylalanine-inhibitable isoenzyme, was linked to gal, and aroH, the structural gene for the tryptophan-inhibitable isoenzyme, was linked to aroE. aroF, the structural gene for the tyrosine-inhibitable isoenzyme, was linked to pheA and tyrA, which specify the phenylalanine- and tyrosine-specific branch-point enzymes, respectively. The phenylalanine-inhibitable isoenzyme was the predominant DAHP synthase in wild-type cells, and only the tryosine-inhibitable isoenzyme was completely repressed, as well as inhibited, by low levels of its allosteric effector. The DAHP synthase isoenzymes were separated by chromatography on diethylaminoethyl-cellulose with a phosphate gradient which contained enolpyruvate phosphate to protect the otherwise unstable phenylalanine-inhibitable isoenzyme. No cross-inhibition of either the tyrosine- or phenylalanine-inhibitable isoenzyme was observed at inhibitor concentrations up to 1 mM. The tryptophan-inhibitable isoenzyme was partially purified from extracts of a strain lacking the other two isoenzymes and shown to be inhibited about 30% by 1 mM tryptophan. A preliminary study of interference by tryptophan in the periodate-thiobarbiturate assay for DAHP suggested a combined effect of tryptophan and erythrose 4-phosphate, or an aldehydic compound resulting from degradation of erythrose 4-phosphate by periodate.

Radiation chemistry of DNA, II. Strand breaks and sugar release by gamma-irradiation of DNA in aqueous solution. The effect of oxygen

From gamma-irradiated oxygenated aqueous solutions of DNA, 2-deoxy-tetrodialdose (1) and 2-deoxy-pentos-4-ulose (2) have been isolated as free sugars. The formation of 1 indicates that in the presence of oxygen the DNA strand is not only broken by mere phosphate ester cleavage but also by C--C-bond rupture in the sugar moiety. Such a process has not been encountered in deoxygenated solutions so far. The mechanism for the formation of 1 is discussed.

Inhibition of growth by erythritol catabolism in Brucella abortus

The growth of Brucella abortus (US-19) in a complex tryptose-yeast extract medium containing D-glucose is inhibited by 10 mM erythritol. The enzymes of the erythritol pathway, except for D-erythrulose 1-phosphate dehydrogenase (D-glycero-2-tetrulose 1-phosphate:nicotinamide adenine dinucleotide (NAD+) 4-oxidoreductase) were detected in the soluble and membrane fractions of cell extracts. Glucose catabolism by cell extracts was inhibited by erythritol, whereas, phosphorylated intermediates of the hexose monophosphate pathway were converted to pyruvic acid with oxygen consumption. Erythritol kinase (EC 2.7.1.27; adenosine 5'-triphosphate (ATP): erythritol 1-phosphotransferase) was found to be eightfold higher in activity than the hexokinase in cell extracts. In vivo, ATP is apparently consumed with the accumulation of D-erythrulose 1-phosphate (D-glycero-2-tetrulose 1-phosphate) and no substrate level phosphorylation. ATP levels dropped 10-fold in 30 min after addition of erythritol to log phase cells in tryptose-yeast extract medium with D-glucose as the carbon source. These data suggest bacteriostasis in the presence of erythritol results from the ATP drain caused by erythritol kinase.

The synthesis of 3-deoxyheptulosonic acid 7-phosphate

3-Deoxyheptulosonic acid 7-phosphate was obtained by a one-step chemical synthesis through condensation of oxalacetate with erythrose 4-phosphate. This reaction occurs at measurable rates only in the presence of a metal ion; Co2+ and Ni2+ are the most effective catalysts. The Co2+ catalyzed condensation of oxalacetate and erythrose 4-phosphate proceeds at room temperature and neutral pH. Since erythrose 4-phosphate can be replaced by any free aldehyde tested thus far, this type of a homogeneous catalysis opens new synthetic routes to alpha-keto-gamma-hydroxy-fatty acids and their derivatives.

Anacystis nidulans mutants resistant to aromatic amino acid analogues

Three classes of mutants of Anacystis nidulans were selected on the basis of resistance to fluorophenylalanine and 2-amino-3-phenylbutanoic acid. The most frequent type exhibited DAHP synthetase (7-phospho-2-keto-3-deoxy-D-arabino-heptonate-D-erythrose-4-phosphate-lyase [pyruvate phosphorylating], EC 4.1.2.15) activity identical to that of the parental strain. The second type was characterized by extremely low levels of the activity. The third type had a DAHP synthetase showing decreased sensitivity to inhibition by L-tyrosine. The enzyme was purified 140-fold from wild-type and feedback-insensitive strains, and the kinetics of the reaction was examined. The activity of the wild-type enzyme was inhibited 75% in the presence of 2.0 X 10-3 M tyrosine, and the altered enzyme was inhibited 10%. The following apparent constants were obtained from kinetic studies with partially purified wild-type enzyme: S0.5 for D-erythrose-4-phophate equal to 7.1 X 10-4 M; S0.5 for phosphoenolpyruvate equal to 1.4 X 10-4 M. Inhibition by tyrosine was mixed with respect to binding of both D-erythrose-4-phosphate and phosphoenolpyruvate. In addition, tyrosine promoted cooperative interactions in the binding of phosphoenolpyruvate. For the altered enzyme the following apparent constants were obtained: S0.5 for D-erythrose-4-phosphate equal to 7.1 X 10-4 M; S0.5 for phosphoenolpyruvate equal to 2.9 X 10-4 M. Inhibition by tyrosine was mixed with respect to D-erythrose-4-phosphate and competitive with respect to phosphoenolpyruvate. Tyrosine did not promote cooperative effects in the binding of phosphoenolpyruvate to the altered enzyme.

Aromatic amino acid biosynthesis in Alcaligenes eutrophus H16. I. Properties and regulation of 3-deoxy-d-arabino heptulosonate 7-phosphate synthase

Properties and regulation of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHP-synthase), EC4.1.2.15, from Alcaligenes eutrophus H16 were investigated. DAHP synthase was unstable during manipulations such as dialysis, dilution, ammonium sulfate fractionation, chromatography on DEAE-cellulose or Sephadex G-200. For kinetic measurements Sephadex G-25 treated crude extracts were used. The enzyme was not affected by thiol reagents, EDTA or divalent metal ions. The activation energy, deltaH, amounted to 16100 cal/mole. Between pH 7.2 and pH 8.2 there was little change of enzyme activity. The Km-values for the two substrates were found to be 0.043 mM phosphoenolpyruvate and 0.055 mM erythrose-4-phosphate. DAHP-synthase was inhibited by 0.5 mM phenylalanine for 60% and by 0.5 mM tyrosine for 20%. In the presence of both amino acids cumulative inhibition occurred amounting to about 70%. No other amino acid exerted inhibitory effects. A repression of DAHP-synthase by the aromatic amino acids was not observed. Some other strains of hydrogen bacteria were included in this study. The DAHP synthase from strain 12/60/X and Corynebacterium autotrophicum 7C was unregulated. The enzyme from strain 33/X was subject to retro-tyrosine inhibition and from strain 3/2, H1 and H20 were subject to cumulative inhibition.

Specificity of glycerol kinase

The activity of a number of alcohols was examined as substrates or inhibitors of glycerol kinase (ATP-glycerol phosphotransferase; EC 2.7.1.30) from Candida mycoderma. On the basis of these and other results, a modified model is proposed to account for the substrate specificity of the enzyme.

Feedback-insensitive mutants of the gene for the tyrosine-inhibited DAHP synthetase in yeast

A yeast strain lacking the phenylalanine-sensitive DAHP synthetase exhibits a tyrosine-inhibited phenotype. Reversions from this phenotype were selected. They were the result of mutations affecting the tyrosine-sensitive DAHP synthetase which became insensitive to feedback inhibition, as shown by genetic and enzymatic studies. The feedback-insensitive mutations were located at five different sites scattered on the map of the aro4 gene.

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.

Regulatory gene of phenylalanine biosynthesis (pheR) in Salmonella typhimurium

4-Fluorophenylalanine-resistant mutants of Salmonella typhimurium were isolated in which synthesis of chorismate mutase P-prephenate dehydratase (specified by pheA) was highly elevated. Transduction analysis showed that the mutation affecting pheA activity was not linked to pheA, and conjugation and merodiploid analysis indicated that it was in the 95- to 100-min region of the Salmonella chromosome. Evidence is presented for the hypothesis that the mutation responsible for constitutivity of chorismate mutase P-prephenate dehydratase occurred in pheR, a gene specifying a cytoplasmic product that affected pheA. pheR mutants were found to carry a second mutation, tyrO. The tyrO mutation acts cis to cause increased levels of the tyrosine biosynthetic enzymes 3-deoxy-d-arabinoheptulosonate 7-phosphate synthetase (tyr) and prephenate dehydrogenase, but it has no effect on regulation of pheA.