The arom multifunctional enzyme from Neurospora crassa

Regulation of Plant Tannin Synthesis in Crop Species

Plant tannins belong to the antioxidant compound family, which includes chemicals responsible for protecting biological structures from the harmful effects of oxidative stress. A wide range of plants and crops are rich in antioxidant compounds, offering resistance to biotic, mainly against pathogens and herbivores, and abiotic stresses, such as light and wound stresses. These compounds are also related to human health benefits, offering protective effects against cardiovascular and neurodegenerative diseases in addition to providing anti-tumor, anti-inflammatory, and anti-bacterial characteristics. Most of these compounds are structurally and biosynthetically related, being synthesized through the shikimate-phenylpropanoid pathways, offering several classes of plant antioxidants: flavonoids, anthocyanins, and tannins. Tannins are divided into two major classes: condensed tannins or proanthocyanidins and hydrolysable tannins. Hydrolysable tannin synthesis branches directly from the shikimate pathway, while condensed tannins are derived from the flavonoid pathway, one of the branches of the phenylpropanoid pathway. Both types of tannins have been proposed as important molecules for taste perception of many fruits and beverages, especially wine, besides their well-known roles in plant defense and human health. Regulation at the gene level, biosynthesis and degradation have been extensively studied in condensed tannins in crops like grapevine (Vitis vinifera), persimmon (Diospyros kaki) and several berry species due to their high tannin content and their importance in the food and beverage industry. On the other hand, much less information is available regarding hydrolysable tannins, although some key aspects of their biosynthesis and regulation have been recently discovered. Here, we review recent findings about tannin metabolism, information that could be of high importance for crop breeding programs to obtain varieties with enhanced nutritional characteristics.

The shikimate dehydrogenase family: functional diversity within a conserved structural and mechanistic framework

Shikimate dehydrogenase (SDH) catalyzes the NADPH-dependent reduction of 3-deydroshikimate to shikimate, an essential reaction in the biosynthesis of the aromatic amino acids and a large number of other secondary metabolites in plants and microbes. The indispensible nature of this enzyme makes it a potential target for herbicides and antimicrobials. SDH is the archetypal member of a large protein family, which contains at least four additional functional classes with diverse metabolic roles. The different members of the SDH family share a highly similar three-dimensional structure and utilize a conserved catalytic mechanism, but exhibit distinct substrate preferences, making the family a particularly interesting system for studying modes of substrate recognition used by enzymes. Here, we review our current understanding of the biochemical and structural properties of each of the five previously identified SDH family functional classes.

Identification of a missing link in the evolution of an enzyme into a transcriptional regulator

The evolution of transcriptional regulators through the recruitment of DNA-binding domains by enzymes is a widely held notion. However, few experimental approaches have directly addressed this hypothesis. Here we report the reconstruction of a plausible pathway for the evolution of an enzyme into a transcriptional regulator. The BzdR protein is the prototype of a subfamily of prokaryotic transcriptional regulators that controls the expression of genes involved in the anaerobic degradation of benzoate. We have shown that BzdR consists of an N-terminal DNA-binding domain connected through a linker to a C-terminal effector-binding domain that shows significant identity to the shikimate kinase (SK). The construction of active synthetic BzdR-like regulators by fusing the DNA-binding domain of BzdR to the Escherichia coli SKI protein strongly supports the notion that an ancestral SK domain could have been involved in the evolutionary origin of BzdR. The loss of the enzymatic activity of the ancestral SK domain was essential for it to evolve as a regulatory domain in the current BzdR protein. This work also supports the view that enzymes precede the emergence of the regulatory systems that may control their expression.

Characterization of the arom gene in Rhizoctonia solani, and transcription patterns under stable and induced hypovirulence conditions

The quinate pathway is induced by quinate in the wild-type virulent Rhizoctonia solani isolate Rhs 1AP but is constitutive in the hypovirulent, M2 dsRNA-containing isolate Rhs 1A1. Constitutive expression of the quinate pathway results in downregulation of the shikimate pathway, which includes the pentafunctional arom gene in Rhs 1A1. The arom gene has 5,323 bp including five introns as opposed to a single intron found in arom in ascomycetes. A 199-bp upstream sequence has a GC box, no TATAA box, but two GTATTAGA repeats. The largest arom transcript is 5,108 nucleotides long, excluding the poly(A) tail. It contains an open reading frame of 4,857 bases, coding for a putative 1,618-residue pentafunctional AROM protein. A Kozak sequence (GCGCCATGG) is present between +127 and +135. The 5'-end of the arom mRNA includes two nucleotides (UA) that are not found in the genomic sequence, and are probably added post-transcriptionally. Size and sequence heterogeneity were observed at both 5'- and 3'-end of the mRNA. Northern blot and suppression subtractive hybridization analyses showed that presence of a low amount of quinate, inducer of the quinate pathway, resulted in increased levels of arom mRNA, consistent with the compensation effect observed in ascomycetes.

Biophysical and kinetic analysis of wild-type and site-directed mutants of the isolated and native dehydroquinate synthase domain of the AROM protein

Dehydroquinate synthase (DHQS) is the N-terminal domain of the pentafunctional AROM protein that catalyses steps 2 to 7 in the shikimate pathway in microbial eukaryotes. DHQS converts 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) to dehydroquinate in a reaction that includes alcohol oxidation, phosphate beta-elimination, carbonyl reduction, ring opening, and intramolecular aldol condensation. Kinetic analysis of the isolated DHQS domains with the AROM protein showed that for the substrate DAHP the difference in Km is less than a factor of 3, that the turnover numbers differed by 24%, and that the Km for NAD+ differs by a factor of 3. Isothermal titration calorimetry revealed that a second (inhibitory) site for divalent metal binding has an approximately 4000-fold increase in KD compared to the catalytic binding site. Inhibitor studies have suggested the enzyme could act as a simple oxidoreductase with several of the reactions occurring spontaneously, whereas structural studies have implied that DHQS participates in all steps of the reaction. Analysis of site-directed mutants experimentally test and support this latter hypothesis. Differential scanning calorimetry, circular dichroism spectroscopy, and molecular exclusion chromatography demonstrate that the mutant DHQS retain their secondary and quaternary structures and their ligand binding capacity. R130K has a 135-fold reduction in specific activity with DAHP and a greater than 1100-fold decrease in the kcat/Km ratio, whereas R130A is inactive.

The crystal structure of shikimate dehydrogenase (AroE) reveals a unique NADPH binding mode

Shikimate dehydrogenase catalyzes the NADPH-dependent reversible reduction of 3-dehydroshikimate to shikimate. We report the first X-ray structure of shikimate dehydrogenase from Haemophilus influenzae to 2.4-A resolution and its complex with NADPH to 1.95-A resolution. The molecule contains two domains, a catalytic domain with a novel open twisted alpha/beta motif and an NADPH binding domain with a typical Rossmann fold. The enzyme contains a unique glycine-rich P-loop with a conserved sequence motif, GAGGXX, that results in NADPH adopting a nonstandard binding mode with the nicotinamide and ribose moieties disordered in the binary complex. A deep pocket with a narrow entrance between the two domains, containing strictly conserved residues primarily contributed by the catalytic domain, is identified as a potential 3-dehydroshikimate binding pocket. The flexibility of the nicotinamide mononucleotide portion of NADPH may be necessary for the substrate 3-dehydroshikimate to enter the pocket and for the release of the product shikimate.

Expression of a hypovirulence-causing double-stranded RNA is associated with up-regulation of quinic acid pathway and down-regulation of shikimic acid pathway in Rhizoctonia solani

We reported previously that a 3.6-kb double-stranded RNA, designated as M2, is associated with hypovirulence in the Rhizoctonia solani isolate Rhs 1A1 and proposed that the M2-encoded putative polypeptide A (pA) might interfere with the regulation of the quinate and shikimate pathways. In this study, Western blot analysis showed that a protein band of the predicted size (83 kDa) binds antibodies specific to a pA epitope and is detectable in M2-containing but not in M2-lacking cultures. A mRNA, associated with Rhs 1A1 polysomes immunoprecipitated with anti-pA antibodies, has a sequence basically identical to that of the sense-strand of M2. The normally inducible quinate pathway was constitutively expressed, whereas the shikimate pathway was down-regulated in the M2-containing, hypovirulent Rhs 1A1. Finally, the relative concentration of phenylalanine, precursor of the virulence determinant phenylacetic acid, was correlated with the degree of pathogenicity in the virulent Rhs 1AP but not in the hypovirulent Rhs 1A1.

Evidence for the shikimate pathway in apicomplexan parasites

Parasites of the phylum Apicomplexa cause substantial morbidity, mortality and economic losses, and new medicines to treat them are needed urgently. The shikimate pathway is an attractive target for herbicides and antimicrobial agents because it is essential in algae, higher plants, bacteria and fungi, but absent from mammals. Here we present biochemical, genetic and chemotherapeutic evidence for the presence of enzymes of the shikimate pathway in apicomplexan parasites. In vitro growth of Toxoplasma gondii, Plasmodium falciparum (malaria) and Cryptosporidium parvum was inhibited by the herbicide glyphosate, a well-characterized inhibitor of the shikimate pathway enzyme 5-enolpyruvyl shikimate 3-phosphate synthase. This effect on T. gondii and P. falciparum was reversed by treatment with p-aminobenzoate, which suggests that the shikimate pathway supplies folate precursors for their growth. Glyphosate in combination with pyrimethamine limited T. gondii infection in mice. Four shikimate pathway enzymes were detected in extracts of T. gondii and glyphosate inhibited 5-enolpyruvyl shikimate 3-phosphate synthase activity. Genes encoding chorismate synthase, the final shikimate pathway enzyme, were cloned from T. gondii and P. falciparum. This discovery of a functional shikimate pathway in apicomplexan parasites provides several targets for the development of new antiparasite agents.

Control of metabolic flux through the quinate pathway in Aspergillus nidulans

The quinic acid ulitization (qut) pathway in Aspergillus nidulans is a dispensable carbon utilization pathway that catabolizes quinate to protocatechuate via dehydroquinate and dehydroshikimate(DHS). At the usual in vitro growth pH of 6.5, quinate enters the mycelium by means of a specific permease and is converted into PCA by the sequential action of the enzymes quinate dehydrogenase, 3-dehydroquinase and DHS dehydratase. The extent of control on metabolic flux exerted by the permease and the three pathway enzymes was investigated by applying the techniques of Metabolic Control Analysis. The flux control coefficients for each of the three quinate pathway enzymes were determined empirically, and the flux control coefficient of the quinate permease was inferred by use of the summation theorem. There measurements implied that, under the standard growth conditions used, the values for the flux control coefficients of the components of the quinate pathway were: quinate permease, 0.43; quinate dehydrogenase, 0.36; dehydroquinase, 0.18; DHS dehydratase, <0,03. Attempts to partially decouple quinate permease from the control over flux by measuring flux at pH 3.5 (when a significant percentage of the soluble quinate is protonated and able to enter the mycelium without the aid of a permease) led to an increase of approx. 50% in the flux control coefficient for dehydroquinase. Taken together with the fact that A. nidulans has a very efficient pH homeostasis mechanism, these experiments are consistent with the view that quinate permease exerts a high degree of control over pathway flux under the standard laboratory growth conditions at pH 6.5. The enzymes quinate dehydrogenase and 3-dehydroquinase have previously been overproduced in Escherichia coli, and protocols for their purification published. The remaining qut pathway enzyme DHS dehydratase was overproduced in E. coli and a purification protocol established. The purified DHS dehydratase was shown to have a K(m) of 530 microM for its substrate DHS and a requirement for bivalent metal cations that could be fulfilled by Mg(2+), Mn(2+) or Zn(2+). All three qut pathway enzymes were purified in bulk and their elasticity coefficients with respect to the three quinate pathway intermediates were derived over a range of concentrations in a core tricine/NaOH buffer, augmented with necessary cofactors and bivalent cations as appropriate. Using these empirically determined relative values, in conjunction with the connectivity theorem, the relative ratios of the flux control coefficients for the various quinate pathway enzymes, and how this control shifts between them, was determined over a range of possible metabolic concentrations. These calculations, although clearly subject to caveates about the relationswhip between kinetic measurements in vitro and the situation in vivo, were able to successfully predict the hiearchy of control observed under the standard laboratory growth conditions. The calculations imply that the hierarchy of control exerted by the quinate pathway enzymes is stable and relatively insensitive to changing metabolite concentrations in the ranges most likely to correspond to those found in vivo. The effects of substituting the type I 3-dehydroquinases from Salmonella typhi and the A. nidulans AROM protein (a pentadomain protein catalysing the conversion of 3-deoxy-D-arabinoheptulosonic acid 7-phosphate into 5-enolpyruvylshikimate 3 phosphate), and the Mycobacterium tuberculosis type II 3-dehydroquinase, in the quinate pathway were investigated and found to have an effect. In the case of S. typhi and A. nidulans, overproduction of heterologous dehydroquinase led to a diminuation of pathway flux caused by a lowering of in vivo quinate dehydrogenase levels increased above those of the wild type. We speculate that these changes in qu

Comparative analysis of the QUTR transcription repressor protein and the three C-terminal domains of the pentafunctional AROM enzyme

The AROM protein is a pentadomain protein catalysing steps two to six in the prechorismate section of the shikimate pathway in microbial eukaryotes. On the basis of amino acid sequence alignments and the properties of mutants unable to utilize quinic acid as a carbon source, the AROM protein has been proposed to be homologous throughout its length with the proteins regulating transcription of the genes necessary for quinate catabolism. The QUTR transcription repressor protein has been proposed to be homologous with the three C-terminal domains of the AROM protein and one-fifth of the penultimate N-terminal domain. We report here the results of experiments designed to overproduce the QUTR and AROM proteins and their constituent domains in Escherichia coli, the purpose being to facilitate domain purification and (in the case of AROM), complementation of E. coli aro- mutations in order to probe the degree to which individual domains are stable and functional. The 3-dehydroquinate dehydratase domain of the AROM protein and the 3-dehydroquinate dehydratase-like domain of the QUTR spectroscopy and fluorescence emission spectroscopy. The CD spectra were found to be virtually superimposable. The fluorescence emission spectra of both domains had the signal from the tryptophan residues almost completely quenched, giving a tyrosine-dominated spectrum for both the AROM- and QUTR-derived domains. This unexpected observation was demonstrated to be due to a highly unusual environment provided by the tertiary structure, as addition of the denaturant guanidine hydrochloride gave a typical tryptophan-dominated spectrum for both domains. The spectroscopy experiments had the potential to refute the biologically-based proposal for a common origin for the AROM and QUTR proteins; however, the combined biophysical data are consistent with the hypothesis. We have previously reported that the AROM dehydroquinate synthase and 3-dehydroquinate dehydratase are stable and functional as individual domains, but that the 5-enol-pyruvylshikimate-3-phosphate synthase is only active as part of the complete AROM protein or as a bi-domain fragment with dehydroquinate synthase. Here we report that the aromA gene (encoding the AROM protein) of Aspergillus nidulans contains a 53 nt intron in the extreme C-terminus of the shikimate dehydrogenase domain. This finding accounts for the previously reported observation that the AROM protein was unable to complement aroE- (lacking shikimate dehydrogenase) mutations in E. coli. When the intron is removed the correctly translated AROM protein is able to complement the E. coli aroE- mutation. An AROM-derived shikimate dehydrogenase domain is, however, non-functional, but function is restored in a bi-domain protein with e-dehydroquinate dehydratase. This interaction is not entirely specific, as substitution of the 3-dehydroquinate dehydratase domain with the glutathione S-transferase protein partially restores enzyme activity. Similarly an AROM-derived shikimate kinase domain is non-functional, but is functional as part of the complete AROM protein, or as a bi-domain protein with 3-dehydroquinate dehydratase.

The characterisation of the shikimate pathway enzyme dehydroquinase from Pisum sativum

Peptides accounting for 157 residues of the bifunctional shikimate pathway enzyme, dehydroquinase/shikimate dehydrogenase, of Pisum sativum were sequenced. Three of the peptides were homologous to regions in Escherichia coli dehydroquinase and two to E. coli shikimate dehydrogenase. The pea dehydroquinase activity was inhibited by treatment with dehydroquinate plus sodium borohydride, establishing it as a type I dehydroquinase. Synthetic oligonucleotides designed from the amino acid sequence were used as PCR primers to amplify fragments of P. sativum cDNA. DNA sequence analysis showed that these amplified products were derived from dehydroquinase/shikimate dehydrogenase cDNA. The complete amino acid sequence of the dehydroquinase domain has been defined; it is homologous to all other type I dehydroquinases and is N-terminal.

Over-expression of the yeast multifunctional arom protein

The pentafunctional arom protein of Saccharomyces cerevisiae is encoded by the ARO1 gene. Substantial elevation of the levels of the arom protein (25-fold) was achieved in yeast using a vector that exploited the ubiquitin-fusion cleavage system of yeast. However, attempts to express the N-terminal 3-dehydroquinate synthase domain (E1) or the internal 3-dehydroquinase domain (E2) using the same system did not succeed. The yeast arom protein was successfully purified from the over-expressing transformant, and was found to possess all five enzymatic activities in a ratio similar to that observed in crude cell extracts. The purified material consisted mainly of a polypeptide that co-migrated in SDS-PAGE with intact arom proteins from other species.

Overproduction of, and interaction within, bifunctional domains from the amino- and carboxy-termini of the pentafunctional AROM protein of Aspergillus nidulans

The pentafunctional AROM protein in Aspergillus nidulans and other fungi catalyses five consecutive enzymatic steps leading to the production of 5-enolpyruvylshikimate 3-phosphate (EPSP) in the shikimate pathway. The AROM protein has five separate enzymatic domains that have previously been shown to display a range of abilities to fold and function in isolation as monofunctional enzymes. In this communication, we report (1) the stable overproduction of a bifunctional protein containing the 3-dehydroquinate (DHQ) synthase and EPSP synthase activities in Escherichia coli to around 10% of the total cell protein; (2) that both the DHQ synthase and EPSP synthase activities in the overproduced fragment are enzymatically active as judged by their ability to complement aroA and aroB mutants of E. coli; (3) that the EPSP synthase domain is only enzymatically active when covalently attached to the DHQ synthase domain (the cis arrangement). When DHQ synthase and EPSP synthase are produced concomitantly by transcribing sequences encoding the individual domains from separate plasmids in the same bacterial cell (the trans arrangement) no overproduction or enzyme activity can be detected for the EPSP synthase domain; (4) the EPSP synthase domain can be stably overproduced as a fusion protein with glutathione S-transferase (GST), however the EPSP synthase in this instance is enzymatically inactive; (5) a protein containing an enzymatically inactive DHQ synthase domain in the cis arrangement with EPSP synthase domain is stably overproduced with enzymatically active EPSP synthase; (6) the two C-terminal domains of the AROM protein specifying the 3-dehydroquinase and shikimate dehydrogenase domains can be overproduced in A. nidulans using a specially constructed expression vector. This same bi-domain fragment however is not produced in E. coli when identical coding sequences are transcribed from a prokaryotic expression vector. These data support the view that multifunctional/multidomain proteins do not solely consist of independent units covalently linked together, but rather that certain individual domains interact to varying degrees to stabilise enzyme activity.

Inducible overproduction of the Aspergillus nidulans pentafunctional AROM protein and the type-I and -II 3-dehydroquinases from Salmonella typhi and Mycobacterium tuberculosis

The aroQ gene of Mycobacterium tuberculosis, encoding a type-II 3-dehydroquinase, and the aroD gene of Salmonella typhi, encoding a type-I 3-dehydroquinase, have been highly overexpressed in Escherichia coli using the powerful trc promoter contained within the expression vector pKK233-2. The M. tuberculosis type-II 3-dehydroquinase has been purified in bulk from overproducing strains of E. coli to greater than 95% homogeneity. The protein is extremely heat-stable, is active as a homododecamer and has the lowest reported Km value of any type-II 3-dehydroquinase. The pentafunctional aromA gene of Aspergillus nidulans has been overexpressed more than 120-fold in an A. nidulans aromA- qutB- double mutant from a truncated quinate-inducible qutE promoter, such that the AROM protein is visible as a significant fraction (approx. 6%) in cell-free crude extracts. The M. tuberculosis aroQ gene has been fused to the same truncated qutE promoter and shown to encode quinate-inducible 3-dehydroquinase activity that allows a qutE- mutant strain of A. nidulans to utilize quinate as sole carbon source.

Differential flux through the quinate and shikimate pathways. Implications for the channelling hypothesis

The qutC gene encoding dehydroshikimate dehydratase has been constitutively overexpressed in Aspergillus nidulans from a range of 1-30-fold over the normal wild-type level. This overexpression leads to impaired growth in minimal medium which can be alleviated by the addition of aromatic amino acids to the medium. Overexpression of the qutC gene in mutant strains lacking protocatechuic acid (PCA) oxygenase leads to the build up of PCA in the medium, which can be measured by a simple assay. Measuring the rate of production of PCA in strains overproducing dehydroshikimate dehydratase and correlating this with the level of overproduction and impaired ability to grow in minimal medium lacking aromatic amino acids leads to the conclusion that (a) the metabolites 3-dehydroquinate and dehydroshikimate leak from the AROM protein at a rate comparable with the extent of flux catalysed by the AROM protein, (b) the AROM protein has a low-level channelling function probably as a result of the close juxtaposition of five active sites and (c) this channelling function is only physiologically significant under non-optimal conditions of nutrient supply and oxygenation, when the organism is in situ in its natural environment.

A comparison of the enzymological and biophysical properties of two distinct classes of dehydroquinase enzymes

This paper compares the biophysical and mechanistic properties of a typical type I dehydroquinase (DHQase), from the biosynthetic shikimate pathway of Escherichia coli, and a typical type II DHQase, from the quinate pathway of Aspergillus nidulans. C.d. shows that the two proteins have different secondary-structure compositions; the type I enzyme contains approx. 50% alpha-helix while the type II enzyme contains approx. 75% alpha-helix. The stability of the two types of DHQase was compared by denaturant-induced unfolding, as monitored by c.d., and by differential scanning calorimetry. The type II enzyme unfolds at concentrations of denaturant 4-fold greater than the type I and through a series of discrete transitions, while the type I enzyme unfolds in a single transition. These differences in conformational stability were also evident from the calorimetric experiments which show that type I DHQase unfolds as a single co-operative dimer at 57 degrees C whereas the type II enzyme unfolds above 82 degrees C and through a series of transitions suggesting higher orders of structure than that seen for the type I enzyme. Sedimentation and Mr analysis of both proteins by analytical ultracentrifugation is consistent with the unfolding data. The type I DHQase exists predominantly as a dimer with Mr = 46,000 +/- 2000 (a weighted average affected by the presence of monomer) and has a sedimentation coefficient s0(20,w) = 4.12 (+/- 0.08) S whereas the type II enzyme is a dodecamer, weight-average Mr = 190,000 +/- 10,000 and has a sedimentation coefficient, s0(20,w) = 9.96 (+/- 0.21) S. Although both enzymes have reactive histidine residues in the active site and can be inactivated by diethyl pyrocarbonate, the possibility that these structurally dissimilar enzymes catalyse the same dehydration reaction by the same catalytic mechanism is deemed unlikely by three criteria: (1) they have very different pH/log kcat. profiles and pH optima; (2) imine intermediates, which are known to play a central role in the mechanism of type I enzymes, could not be detected (by borohydride reduction) in the type II enzyme; (3) unlike Schiff's base-forming type I enzymes, there are no conserved lysine residues in type II amino acid sequences.

In vivo overproduction of the pentafunctional arom polypeptide in Aspergillus nidulans affects metabolic flux in the quinate pathway

The shikimate pathway and the quinic acid utilisation (QUT) pathway of Aspergillus nidulans and other fungi share the two common metabolic intermediates, 3-dehydroquinic acid (DHQ) and dehydroshikimic acid (DHS), which are interconverted by two isoenzymes, catabolic 3-dehydroquinase, (cDHQase) and biosynthetic dehydroquinase, (bDHQase). bDHQase is one of five consecutive enzymatic activities associated with the pentafunctional arom protein encoded by the complex AROM locus, whereas cDHQase is encoded by the single-function QUTE gene, one of seven genes comprising the QUT gene cluster in A. nidulans, which is required for the catabolism of quinate to protocatechuate. We addressed the question of how much (if any) leakage there is of the two common substrates between the two pathways, by increasing the concentration of the arom protein in vivo by means of recombinant DNA technology. We demonstrated that constitutive overproduction of the arom protein by 12-fold in the presence of quinate inhibits germination of conidiospores, but showed that 12-fold quinate-inducible overproduction of arom protein does not have this effect. In addition we showed that a qutE mutant (lacking cDHQase) can grow with quinic acid as sole carbon source whtn the arom protein is overproduced fivefold. The data are most simply interpreted as simple competition for common substrates by the enzymes of the two pathways and demonstrate that any channelling function of the arom protein in vivo is relatively leaky.

Domain structure and interaction within the pentafunctional arom polypeptide

The AROM locus of Aspergillus nidulans specifies a pentafunctional polypeptide catalysing five consecutive steps leading to the production of 5-enolpyruvylshikimate 3-phosphate in the shikimate pathway. Aided by oligonucleotide-mediated site-directed mutagenesis, the whole AROM locus and various overlapping subfragments from within it have been fused to the powerful hybrid trc promoter in the Escherichia coli plasmid pKK233-2. Expression of these subfragments in appropriate aro mutants of E. coli has (a) allowed the delineation of functional domains within the arom polypeptide, (b) shown that the arom polypeptide falls in two independently folding and functioning regions, the N-terminal half specifying 3-dehydroquinate (DHQ) synthase and EPSP synthase and the C-terminus specifying shikimate kinase, biosynthetic 3-dehydroquinase (DHQase) and shikimate dehydrogenase, and (c) strongly suggested an interaction between the DHQ synthase and EPSP synthase domains to stabilise the EPSP synthase activity. In addition an isoenzyme of biosynthetic DHQase, catabolic DHQase, encoded by the QUTE gene of A. nidulans has been transcribed from the trc promoter and upon isopropyl-thio-beta-D-galactoside induction produces up to 20% of the total soluble cell protein.

Chorismate synthase. Pre-steady-state kinetics of phosphate release from 5-enolpyruvylshikimate 3-phosphate

The pre-steady-state kinetics of phosphate formation from 5-enolpyruvylshikimate 3-phosphate catalysed by Escherichia coli chorismate synthase (EC 4.6.1.4) were studied by a rapid-acid-quench technique at 25 degrees C at pH 7.5. No pre-steady-state 'burst' or 'lag' phase was observed, showing that phosphate is released concomitant with the rate-limiting step of the enzyme. The implications of this result for the mechanism of action of chorismate synthase are discussed.

The Saccharomyces cerevisiae ARO1 gene. An example of the co-ordinate regulation of five enzymes on a single biosynthetic pathway

The ARO1 gene of Saccharomyces cerevisiae encodes the arom multifunctional enzyme. Specific inhibitors of amino acid biosynthesis have been used to obtain evidence that expression of a cloned ARO1 gene is regulated in response to amino acid limitation. Northern blot analysis and sequence studies indicate that ARO1 is regulated by the well characterised S. cerevisiae 'general control' mechanism. This provides a very economical means of simultaneously tailoring the synthesis of five shikimate pathway enzymes to the needs of the cell.

The overexpression, purification and complete amino acid sequence of chorismate synthase from Escherichia coli K12 and its comparison with the enzyme from Neurospora crassa

The enzyme chorismate synthase was purified in milligram quantities from an overproducing strain of Escherichia coli. The amino acid sequence was deduced from the nucleotide sequence of the aroC gene and confirmed by determining the N-terminal amino acid sequence of the purified enzyme. The complete polypeptide chain consists of 357 amino acid residues and has a calculated subunit Mr of 38,183. Cross-linking and gel-filtration experiments show that the enzyme is tetrameric. An improved purification of chorismate synthase from Neurospora crassa is also described. Cross-linking and gel-filtration experiments on the N. crassa enzyme show that it is also tetrameric with a subunit Mr of 50,000. It is proposed that the subunits of the N. crassa enzyme are larger because they contain a diaphorase domain that is absent from the E. coli enzyme.