The nucleotide sequence of the aroF gene of Escherichia coli and the amino acid sequence of the encoded protein, the tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

Evolution of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase-encoding genes in the yeast Saccharomyces cerevisiae

The shikimate pathway resulting in three aromatic amino acids is initiated in different organisms by two and three 3-deoxy-d-arabino-heptulosonate-7-phosphate synthases, respectively. Aro3p and Aro4p are the yeast enzymes feedback-inhibited by phenylalanine and tyrosine, respectively. A yeast strain deficient in the general control transcriptional regulatory system of amino acid biosynthesis is unable to live in the presence of high amounts of phenylalanine and tyrosine. Here, we show that this yeast strain can be rescued by the expression of aroH from Escherichia coli encoding the tryptophan-regulated AroH as third isoenzyme. Yeast carrying Ec AroH as the only enzyme for the initial step of the shikimate pathway can grow in the absence of tryptophan. Without aromatic amino acids, this yeast strain survives only when the yeast ARO3 promoter instead of the ARO4 promoter drives E. coli aroH. The detailed analysis of Aro3p and Aro4p revealed a triple feedback control by tyrosine/phenylalanine and tryptophan. Dissecting this control allowed engineering of Aro4p S195A as an enzyme, which is inhibited like AroH only by tryptophan. In addition, Aro4p variants were constructed that show an equally strong inhibition by tyrosine and tryptophan (Aro4p P165G Q302R) and in which the regulation by tyrosine and tryptophan was reversed (Aro4p P165G). Our data suggest that yeast possesses only two instead of three isogenes encoding 3-deoxy-D-arabino-heptulosonate-7-phosphate synthases because both isoenzymes can be fine tuned by tryptophan as additional effector and because transcriptional regulation by the general control system can be induced as backup when aromatic amino acids in the environment are imbalanced.

(De)regulation of key enzyme steps in the shikimate pathway and phenylalanine-specific pathway of the actinomycete Amycolatopsis methanolica

Prephenate dehydratase (PDT), chorismate mutase (CM) and 3-deoxy-D-arabino-7-heptulosonate 7-phosphate (DAHP) synthase are key regulatory enzymes in aromatic amino acid biosynthesis in the actinomycete Amycolatopsis methanolica. Deregulated, feedback-control-resistant mutants were isolated by incubation of A. methanolica on glucose mineral agar containing the toxic analogue p-fluoro-DL-phenylalanine (pFPhe). Several of these mutants had completely lost PDT sensitivity to Phe inhibition and Tyr activation. Mutant characterization yielded new information about PDT amino acid residues involved in Phe and Tyr effector binding sites. A. methanolica wild-type cells grown on glucose mineral medium normally possess a bifunctional CM/DAHP synthase protein complex (with DS1, a plant-type DAHP synthase). The CM activity of this protein complex is feedback-inhibited by Tyr and Phe, while DS1 activity is mainly inhibited by Trp. Isolation of pFPhe-resistant mutants yielded two feedback-inhibition-resistant CM mutants. These were characterized as regulatory mutants, derepressed in (a) synthesis of CM, now occurring as an abundant, feedback-inhibition-resistant, separate protein, and (b) synthesis of an alternative DAHP synthase (DS2, an E. coli-type DAHP synthase), only inhibited by Tyr and Trp. DS1 and DS2 thus are well integrated in A. methanolica primary metabolism: DS1 and CM form a protein complex, which stimulates CM activity and renders it sensitive to feedback inhibition by Phe and Tyr. Synthesis of CM and DS2 proteins appears to be controlled co-ordinately, sensitive to Phe-mediated feedback repression.

Characterization of a new feedback-resistant 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase AroF of Escherichia coli

Tyrosine feedback-inhibits the 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase isoenzyme AroF of Escherichia coli. Here we show that an Asn-8 to Lys-8 substitution in AroF leads to a tyrosine-insensitive DAHP synthase. This mutant enzyme exhibited similar activities (v=30-40 U mg(-1)) and substrate affinities (K(m)(erythrose-4-phosphate)=0.5 mM, positive cooperativity with respect to phospho(enol)pyruvate) as the wild-type AroF, but showed decreased thermostability. An engineered AroF enzyme lacking the seven N-terminal residues also was tyrosine-resistant. These results strongly suggest that the N-terminus of AroF is involved in the molecular interactions occurring in the feedback-inhibition mechanism.

New insights into the metal center of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

Metal binding properties for a series of metal-substituted forms of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, DAHPS(Tyr), have been followed by UV-vis and EPR spectroscopy. The results show that there are two metal species present at pH = 7.0 and these are coordinated in a distorted metal binding site with a mixed nitrogen and oxygen donor atom coordination set. There is no spectroscopic evidence for strong M-S interactions in this system at any pH. Metal saturation occurs at a substoichiometric ratio of 0.8-0.85 metal/monomer, and the binding trends mirror previously published enzyme activity profiles. There is a conformational change for CuDAHPS under basic conditions, and equivalent protein handling for apoDAHPS leads to apparent loss of metal binding ability. Addition of the substrate PEP does not alter the UV-vis spectra, but there are small changes in the EPR spectra of CuDAHPS(Tyr). Further addition of the substrate analogue A5P has no effect on either spectra. Taken together, these results serve to link previous studies on enzyme activity with the recently determined X-ray crystal structure for DAHPS(Phe) and represent the first detailed spectroscopic characterization of the metal binding properties of DAHPS(Tyr).

Serine 187 is a crucial residue for allosteric regulation of Corynebacterium glutamicum 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase

Corynebacterium glutamicum 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase is sensitive to feedback inhibition by tyrosine. One feedback-insensitive mutant was obtained by in vitro chemical mutagenesis and the mutation was identified as a C-->G mutation at nucleotide 560 causing a Ser(187) to Cys(187) substitution. Replacing Ser(187) with cysteine, tyrosine or phenylalanine by site-directed mutagenesis not only reduced the enzymatic activity but also relieved its feedback inhibition by tyrosine, while Ser(187)Ala exhibited a comparable activity to that of wild-type enzyme and sensitized to allosteric regulation. The His(6)-tagged enzymes were expressed in Escherichia coli and purified to homogeneity by immobilized nickel-ion affinity chromatography. Kinetic analysis showed that tyrosine is a competitive inhibitor of phosphoenol pyruvate, one of the precursors for DAHP biosynthesis.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Substrate ambiguity of 3-deoxy-D-manno-octulosonate 8-phosphate synthase from Neisseria gonorrhoeae in the context of its membership in a protein family containing a subset of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthases

3-Deoxy-D-manno-octulosonate 8-phosphate (KDOP) synthase and 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase catalyze similar phosphoenolpyruvate-utilizing reactions. The genome of Neisseria gonorrhoeae contains one gene encoding KDOP synthase and one gene encoding DAHP synthase. Of the two nonhomologous DAHP synthase families known, the N. gonorrhoeae protein belongs to the family I assemblage. KDOP synthase exhibited an ability to replace arabinose-5-P with either erythrose-4-P or ribose-5-P as alternative substrates. The results of periodate oxidation studies suggested that the product formed by KDOP synthase with erythrose-4-P as the substrate was 3-deoxy-D-ribo-heptulosonate 7-P, an isomer of DAHP. As expected, this product was not utilized as a substrate by dehydroquinate synthase. The significance of the ability of KDOP synthase to substitute erythrose-4-P for arabinose-5-P is (i) recognition of the possibility that the KDOP synthase might otherwise be mistaken for a species of DAHP synthase and (ii) the possibility that the broad-specificity type of KDOP synthase might be a relatively vulnerable target for antimicrobial agents which mimic the normal substrates. An analysis of sequences in the database indicates that the family I group of DAHP synthase has a previously unrecognized membership which includes the KDOP synthases. The KDOP synthases fall into a subfamily grouping which includes a small group of DAHP synthases. Thus, family I DAHP synthases separate into two subfamilies, one of which includes the KDOP synthases. The two subfamilies appear to have diverged prior to the acquisition of allosteric-control mechanisms for DAHP synthases. These allosteric control specificities are highly diverse and correlate with the presence of N-terminal extensions which lack homology with one another.

A second type-I PKS gene cluster isolated from Streptomyces hygroscopicus ATCC 29253, a rapamycin-producing strain

Analysis of a 32.8-kb segment of DNA from the rapamycin (Rp) producer, Streptomyces hygroscopicus ATCC 29253, revealed a new type-I polyketide synthase (PKS) cluster consisting of four open reading frames (ORF 1-4), each encoding a single PKS module. The four ORFs are transcribed in the same direction and are flanked by several smaller ORFs (ORF 5-9), which may be related to the PKS cluster. The first PKS-containing ORF has a ligase domain at the N-terminus of the polypeptide. This domain has 55% aa identity to the CoA ligase domain of the Rp PKS (Schwecke et al., 1995. Proc. Natl. Acad. Sci. 92, 7839-7843) which is also encoded in this strain (Lowden et al., 1996. Angew. Chem. Int. Ed. Engl. 35, 2249-2251). ORF5 (340 aa) and ORF6 (924 aa) were found to be homologous to RapK (41% aa identity) and RapH (35% aa identity), which are hypothesized to be a pteridine-dependent dioxygenase and a regulatory protein, respectively (Molnar et al., 1996. Gene 169, 1-7). In addition, ORF7 (391 aa) was found to have up to 42% aa identity to a number of plant 3-deoxy-D-arabino-heptulosonate-7-phosphate synthases (DAHPS) and 47% aa identity to PhzF, a bacterial DAHPS involved in phenazine antibiotic synthesis. The proximity of the DAHPS-encoding gene to the PKS cluster containing a Rp-like ligase domain suggests that a derivative of shikimate may be used as the PKS starter. ORF8 (283 aa) was found to have homology (32% aa identity) to a Synechocystis sp. gene of unknown function. The N-terminal portion of ORF9 was found to be similar to a tetracycline 6-hydroxylase (34% aa identity) from Streptomyces aureofaciens.

Overexpression, purification, and characterization of tyrosine-sensitive 3-deoxy-D-arabino-heptulosonic acid 7-phosphate synthase from Escherichia coli

An overexpression system (pCR105) for DAHP synthase (Tyr) was constructed by cloning the aroF gene at the NdeI site of the pET-22b(+) translation vector, a plasmid expression vector that contains the T7 lac promoter. The enzyme was overexpressed, purified to > 90% purity (by SDS-polyacrylamide gel electrophoresis), and characterized. The protein was overexpressed at a level of 58% the total soluble cell protein (based on enzymatic activities). About 244 mg of pure enzyme was obtained from a 2-liter cell culture. So far, this is the highest yield reported for the isozyme DAHP synthase (Tyr). The enzyme showed a bell-shaped pH-activity profile, with a pH optimum at pH 7.0-7.5 and pK values of 6.10 and 8.92. Inhibition of the enzyme by tyrosine was specific with 50% inhibition observed at 9 microM tyrosine, pH 7.0. The specific activity of the enzyme increased with added metal and metal sensitivity increased with purity of the enzyme. Only substoichiometric amounts of Cu, Fe, and Zn were found in the pure enzyme and this result is consistent with sensitivity of the enzyme to added metal. Although treatment with EDTA inactivated the enzyme almost completely, the activity of the apoenzyme was restored to differing extents by a variety of metals including Mn2+, Cd2+, Co2+, Fe2+, Cu2+, Mg2+, and Zn2+. Both Fe2+ and Cu2+ only partially reactivated EDTA-treated enzyme. Reconstitution of EDTA-treated enzyme with either Cd2+ or Mn2+ gave 1 mol of metal per mole of enzyme monomer. KCN inactivated the enzyme to only 80% and added metals reactivated the CN-treated enzyme only to a small extent. These results confirm the importance of the metal in the enzymatic reaction.

Cloning and characterization of the gene encoding ATP-dependent phospho-enol-pyruvate carboxykinase in Trypanosoma cruzi: comparison of primary and predicted secondary structure with host GTP-dependent enzyme

The complete nucleotide (nt) sequence of the PEPCK gene encoding Trypanosoma cruzi phospho-enol-pyruvate carboxykinase (PEPCK; ATP dependent, EC 4.1.1.49) has been determined. The predicted primary sequence has 473 amino acids (aa) with a calculated molecular mass of 52.5 kDa. The ubiquitous spliced leader is present at nt position -60 from the AUG start codon in PEPCK mRNA; the coding region is followed by a long 3'-non-coding region of 777 nt. Northern and Southern blot analysis showed that the PEPCK mRNA is 2.7 kb long and that the PEPCK gene is polymorphic in T. cruzi, with more than one copy in the genome of the epimastigote form. Comparison of the available aa sequences of ATP(protozoa, yeast and bacteria)- and GTP(vertebrates, insects, helminths and fungi)-dependent PEPCKs showed that the former lack two characteristic, highly conserved regions present in the GTP-dependent enzymes: one is associated with the binding of PEP while the second is frequently labeled as 'catalytic' and contains a conserved Cys residue of unusual reactivity. On the other hand, two consensus sequences with conserved predicted secondary structure were identified in all PEPCKs, independent of their nt specificity; one of them is a divalent metal-binding site previously identified in pyruvate kinase by X-ray crystallographic studies.

The pheA/tyrA/aroF region from Erwinia herbicola: an emerging comparative basis for analysis of gene organization and regulation in enteric bacteria

Extensive knowledge exists in Escherichia coli about the contiguous pheA and aroF-tyrA operons which have opposite transcription orientations and are separated by a bidirectional transcription terminator. The corresponding structural genes and individual components of the terminator and attenuator from Erwinia herbicola have been analyzed from an evolutionary vantage point. A 7.5-kb DNA fragment from E. herbicola carrying the linked pheA, tyrA, and aroF genes was cloned by functional complementation of E. coli auxotrophic requirements. A 3,433-bp segment of DNA consisting of more than half of aroF, all of tyrA, and the entire phenylalanine operon (promoter, leader region encoding the leader peptide and containing the phe attenuator, and pheA) was sequenced. A bidirectional transcription terminator was positioned between the divergently transcribed pheA and tyrA. The adjacent aroF and tyrA genes share a common transcription orientation, consistent with their probable coexistence within an operon. However, tyrA can be expressed efficiently from an internal promoter which appears to lie within the 3' portion of aroF. The gene order is pheA tyrA aroF in E. herbicola, with the same tail-to-tail arrangement of transcription known to exist in E. coli. The pheL coding region of the phe operon was dominated by phenylalanine codons, seven of the 15 amino acid residues of the leader peptide being L-phenylalanine. The E. herbicola pheA and tyrA genes were 1,161 bp and 1,119 bp in length, respectively, and corresponded to deduced gene products having subunit molecular weights of 43,182 and 41,847. The deduced amino acid sequences of PheA and TyrA were homologous at their N-termini, consistent with a common evolutionary origin of the chorismate mutase domains present at the amino terminus of both PheA and TyrA. A detailed comparison of the E. coli and E. herbicola sequences was made. The pheA, tyrA, and aroF genes of E. herbicola exhibited high overall identity with the counterpart E. coli genes. Within the leader region of the phe operon, the leader peptide coding region was highly conserved. Although the 1:2 and 2':3' stems defining the pause structure and the antiterminator, respectively, were also highly conserved, RNA segment 4 of the attenuator terminator exhibited considerable divergence, as did the distal portion of the attenuator region. Within the span of attenuator region encoding the three stem-loop structures of mRNA secondary configuration, hot spots of base-residue divergence were localized to looped-out regions. No changes occurred which would simultaneously disrupt alternative pairing relationships of secondary configuration. The bidirectional terminator between pheA and tyrA has diverged very substantially.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning, primary structure and regulation of the ARO4 gene, encoding the tyrosine-inhibited 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Saccharomyces cerevisiae

In Saccharomyces cerevisiae, two differently regulated 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (DAHPS; EC 4.1.2.15) isoenzymes carry out the first step in the shikimate pathway. Mutations in both genes are necessary to cause aromatic amino acid (aa) auxotrophy, since one isoenzyme alone is sufficient to produce enough DAHP for normal growth of the cells. The phenylalanine-inhibited DAHPS is encoded by the previously isolated and characterized ARO3 gene. Here, we report the cloning and characterization of the ARO4 gene, encoding the second DAHPS, which is inhibited by tyrosine. The aa sequence of the ARO4 gene product reveals 76% similarity to the ARO3-encoded isoenzyme and 66 and 73% to the three DAHPS isoenzymes from Escherichia coli. ARO4 gene expression is regulated by the general control system of aa biosynthesis. As in the case of the ARO3 gene, a single GCN4-recognition element in the promoter is responsible for derepression of the ARO4 gene under aa starvation conditions. However, in contrast to the situation in the isogene, ARO3, GCN4 does not contribute to the basal level of ARO4 transcription under nonderepressing conditions.

Aromatic amino acid biosynthesis in the yeast Saccharomyces cerevisiae: a model system for the regulation of a eukaryotic biosynthetic pathway

This review focuses on the gene-enzyme relationships and the regulation of different levels of the aromatic amino acid biosynthetic pathway in a simple eukaryotic system, the unicellular yeast Saccharomyces cerevisiae. Most reactions of this branched pathway are common to all organisms which are able to synthesize tryptophan, phenylalanine, and tyrosine. The current knowledge about the two main control mechanisms of the yeast aromatic amino acid biosynthesis is reviewed. (i) At the transcriptional level, most structural genes are regulated by the transcriptional activator GCN4, the regulator of the general amino acid control network, which couples transcriptional derepression to amino acid starvation of numerous structural genes in multiple amino acid biosynthetic pathways. (ii) At the enzyme level, the carbon flow is controlled mainly by modulating the enzyme activities at the first step of the pathway and at the branch points by feedback action of the three aromatic amino acid end products. Implications of these findings for the relationship of S. cerevisiae to prokaryotic as well as to higher eukaryotic organisms and for general regulatory mechanisms occurring in a living cell such as initiation of transcription, enzyme regulation, and the regulation of a metabolic branch point are discussed.

Two promoters control the aroH gene of Escherichia coli

The aroH gene from Escherichia coli encodes 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) synthase (Trp), one of three isoenzymes which catalyse the first committed step in the biosynthesis of aromatic amino acids and vitamins. S1 mapping and primer extension analysis of in vivo transcripts revealed the presence of two nonoverlapping promoters for aroH. The more distal of these has been described previously and is negatively regulated by the trp repressor. The second promoter is active under conditions of growth in rich medium, and may be involved in ensuring sufficient levels of precursors for the biosynthesis of aromatic vitamins under these growth conditions.

The tyrosine repressor negatively regulates aroH expression in Escherichia coli

The levels of the tryptophan-sensitive isoenzyme of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase of Escherichia coli, encoded by the aroH gene, were elevated in tyrR and/or trpR mutants. The effect of tyrR and trpR lesions on aroH expression was confirmed by using a lacZ reporter system. The mutational elimination of either repressor led to a threefold increase in beta-galactosidase.

Purification and properties of tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase from Escherichia coli

The aroH gene of Escherichia coli, which encodes the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase isoenzyme of the common aromatic biosynthetic pathway, was cloned behind the tac promoter in expression plasmid pKK223-3. The enzyme was overexpressed, purified to homogeneity, and characterized. The native enzyme was found to be a dimeric metalloprotein containing 0.3 mol of iron per mol of subunit and variable amounts of zinc. The activity of the native enzyme was stimulated two- to threefold when assayed in the presence of Fe2+ ions. Pretreatment of the enzyme with Fe2+ also resulted in activation, accompanied by an equivalent increase in iron content. Treatment of the enzyme with chelating agents led to inactivation, which was fully reversed by the presence of Fe2+ in the assay mixture. The native enzyme exhibited a unique absorption profile, having a shoulder of absorbance on the aromatic band with a maximum around 350 nm and a broad, weak band with a maximum around 500 nm. Treatment of the enzyme with Fe2+ enhanced the absorbance at 350 nm and eliminated the band at 500 nm. Treatment with reducing agents caused the disappearance of both bands and destabilized the enzyme. Feedback regulation of the activity of the enzyme was specific for tryptophan, with maximum inhibition at about 70%.

Cloning of an aroF allele encoding a tyrosine-insensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

In Escherichia coli, genes aroF+, aroG+, and aroH+ encode isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthases that are feedback inhibited by tyrosine, phenylalanine, and tryptophan, respectively. A single base pair change in aroF causes a Pro-148-to-Leu-148 substitution and results in a tyrosine-insensitive enzyme.

Regulation of the Salmonella typhimurium aroF gene in Escherichia coli

The Salmonella typhimurium aroF gene, encoding the tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, was localized to a chromosomal PstI fragment by Southern blotting with an Escherichia coli aroF probe. This fragment was cloned by screening a plasmid library for complementation of an E. coli aroF mutant. The nucleotide sequence of S. typhimurium aroF was determined and compared with its E. coli homolog. The nucleotide sequences are 85.1% identical, and the corresponding amino acid sequences are 96.1% identical. The E. coli genes encoding the three DAHP synthase isoenzymes are evolutionarily more distant from one another than are the homologous aroF genes of E. coli and S. typhimurium. The S. typhimurium aroF regulatory region contains three imperfect palindromes, two upstream of the promoter and one overlapping the promoter, that are nearly identical to operators aroFo1, aroFo2, and TyrR box 1 of E. coli. The aroFo1 and aroFo2 sequences of the two organisms are each separated by three turns of the DNA helix with no sequence similarity. The 5' ends of the aroF transcripts for both organisms contain untranslated regions with potential stem-loop structures. Translational fusions of the aroF regulatory regions to lacZ were constructed and then introduced in single copy into the E. coli chromosome. beta-Galactosidase assays for tyrR-mediated regulation of aroF-lacZ expression revealed that the E. coli TyrR repressor apparently recognizes the operators of both organisms with about equal efficiency.

Purification and properties of the 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (phenylalanine-inhibitable) of Saccharomyces cerevisiae

The phenylalanine-inhibitable 3-deoxy-D-arabino-heptulosonate-7-phosphate (dHp1P) synthase from Saccharomyces cerevisiae has been purified to apparent homogeneity by a 1250-fold enrichment of the enzyme activity present in wild-type crude extracts, employing an overproducing strain. The estimated molecular mass of 42 kDa corresponds to the calculated molecular mass of 42.13 kDa deduced from the previously determined primary sequence. Gel filtration indicates that the active enzyme is a monomer. The enzyme is an Fe protein and is inactivated by EDTA in a reaction which is reversible by several bivalent metal ions. The Michaelis constant of the enzyme is 18 microM for phosphoenolpyruvate (P-pyruvate) and 130 microM for erythrose 4-phosphate (Ery4P) and the rate constant was calculated as 10 s-1. Inhibition by phenylalanine is competitive with respect to erythrose 4-phosphate and non-competitive to phosphoenolpyruvate, with a Ki of 10 microM.

Mutational analysis of the catalytic and feedback sites of the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase of Escherichia coli

The nucleotide sequence of aroH, the structural gene for the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase [DAHPS(Trp)], is presented, and the deduced amino acid sequence of AroH is compared with that of the tyrosine-sensitive (AroF) and phenylalanine-sensitive (AroG) DAHPS isoenzymes. The high degree of sequence similarity among the three isoenzymes strongly indicates that they have a common evolutionary origin. In vitro chemical mutagenesis of the cloned aroH gene was used to identify residues and regions of the polypeptide essential for catalytic activity and for tryptophan feedback regulation. Missense mutations leading either to loss of catalytic activity or to feedback resistance were found interspersed throughout the polypeptide, suggesting overlapping catalytic and regulatory sites in DAHPS(Trp). We conclude that the specificity of feedback regulation of the isoenzymes was probably acquired by the duplication and divergent evolution of an ancestral gene, rather than by domain recruitment.

Chorismate mutase isoenzymes from Sorghum bicolor: immunological characterization

Highly purified fractions of chorismate mutase 1 and 2 from etiolated seedlings of Sorghum bicolor were used as the antigen for antibody production in BALB/c mice. Tests for antigen-antibody complex formation were made by immunodiffusion, immunoprecipitation, and enzyme-linked immunosorbent assay (ELISA). These tests indicated the presence of specific antibodies for each isoenzyme in their antisera. However, in the same tests, no cross-reaction was found between chorismate mutase 1 and 2 and their antisera. This indicates no immunological similarity between the two isoenzymes of chorismate mutase from sorghum.