Expression, purification, and biochemical characterization of an NAD+-dependent homoserine dehydrogenase from the symbiotic Polynucleobacter necessarius subsp. necessarius.

Homoserine dehydrogenase (HSD), encoded by the hom gene, is a key enzyme in the aspartate pathway, which reversibly catalyzes the conversion of l-aspartate ß-semialdehyde to l-homoserine (l-Hse), using either NAD(H) or NADP(H) as a coenzyme. In this work, we presented the first characterization of the HSD from the symbiotic Polynucleobacter necessaries subsp. necessarius (PnHSD) produced in Escherichia coli. Sequence analysis showed that PnHSD is an ACT domain-containing monofunctional HSD with 436 amnio acid residues. SDS-PAGE and Western blot demonstrated that PnHSD could be overexpressed in E. coli BL21(DE3) cell as a soluble form by using SUMO fusion technique. It could be purified to apparent homogeneity for biochemical characterization. Size-exclusion chromatography revealed that the purified PnHSD has a native molecular mass of ∼160 kDa, indicating a homotetrameric structure. The oxidation activity of PnHSD was studied in this work. Kinetic analysis revealed that PnHSD displayed an up to 1460-fold preference for NAD+ over NADP+, in contrast to its homologs. The purified PnHSD displayed maximal activity at 35 °C and pH 11. Similar to its NAD+-dependent homolog, neither NaCl and KCl activation nor L-Thr inhibition on the enzymatic activity of PnHSD was observed. These results will contribute to a better understanding of the coenzyme specificity of the HSD family and the aspartate pathway of P. necessarius.

Biochemical characterization and redesign of the coenzyme specificity of a novel monofunctional NAD+-dependent homoserine dehydrogenase from the human pathogen Neisseria gonorrhoeae.

Gonorrhoea, caused by Neisseria gonorrhoeae, is a major global public health concern. Homoserine dehydrogenase (HSD), a key enzyme in the aspartate pathway, is a promising metabolic target against pathogenic infections. In this study, a monofunctional HSD from N. gonorrhoeae (NgHSD) was overexpressed in Escherichia coli and purified to >95% homogeneity for biochemical characterization. Unlike the classic dimeric structure, the purified recombinant NgHSD exists as a tetramer in solution. We determined the enzymatic activity of recombinant NgHSD for l-homoserine oxidation, which revealed that this enzyme was NAD+ dependent, with an approximate 479-fold (kcat/Km) preference for NAD+ over NADP+, and that optimal activity for l-homoserine oxidation occurred at pH 10.5 and 40 °C. At 800 mM, neither NaCl nor KCl increased the activity of NgHSD, in contrast to the behavior of several reported NAD+-independent homologs. Moreover, threonine did not markedly inhibit the oxidation activity of NgHSD. To gain insight into the cofactor specificity, site-directed mutagenesis was used to alter coenzyme specificity. The double mutant L45R/S46R, showing the highest affinity for NADP+, caused a shift in coenzyme preference from NAD+ to NADP+ by a factor of ~974, with a catalytic efficiency comparable with naturally occurring NAD+-independent homologs. Collectively, our results should allow the exploration of drugs targeting NgHSD to treat gonococcal infections and contribute to the prediction of the coenzyme specificity of novel HSDs.

Proteomic analysis of a hom-disrupted, cephamycin C overproducing Streptomyces clavuligerus.

BACKGROUND: Streptomyces clavuligerus is prolific producer of cephamycin C, a medically important antibiotic. In our former study, cephamycin C titer was 2-fold improved by disrupting homoserine dehydrogenase (hom) gene of aspartate pahway in Streptomyces clavuligerus NRRL 3585. OBJECTIVE: In this article, we aimed to provide a comprehensive understanding at the proteome level on potential complex metabolic changes as a consequence of hom disruption in Streptomyces clavuligerus AK39. METHODS: A comparative proteomics study was carried out between the wild type and its hom disrupted AK39 strain by 2 Dimensional Electrophoresis-Matrix Assisted Laser Desorption and Ionization Time-Of-Flight Mass Spectrometry (2DE MALDI-TOF/MS) and Nanoscale Liquid Chromatography- Tandem Mass Spectrometry (nanoLC-MS/MS) analyses. Clusters of Orthologous Groups (COG) database was used to determine the functional categories of the proteins. The theoretical pI and Mw values of the proteins were calculated using Expasy pI/Mw tool. RESULTS: "Hypothetical/Unknown" and "Secondary Metabolism" were the most prominent categories of the differentially expressed proteins. Upto 8.7-fold increased level of the positive regulator CcaR was a key finding since CcaR was shown to bind to cefF promoter thereby direcly controlling its expression. Consistently, CeaS2, the first enzyme of CA biosynthetic pathway, was 3.3- fold elevated. There were also many underrepresented proteins associated with the biosynthesis of several Non-Ribosomal Peptide Synthases (NRPSs), clavams, hybrid NRPS/Polyketide synthases (PKSs) and tunicamycin. The most conspicuously underrepresented protein of amino acid metabolism was 4-Hydroxyphenylpyruvate dioxygenase (HppD) acting in tyrosine catabolism. The levels of a Two Component System (TCS) response regulator containing a CheY-like receiver domain and an HTH DNA-binding domain as well as DNA-binding protein HU were elevated while a TetR-family transcriptional regulator was underexpressed. CONCLUSION: The results obtained herein will aid in finding out new targets for further improvement of cephamycin C production in Streptomyces clavuligerus.

Molecular and Enzymatic Features of Homoserine Dehydrogenase from Bacillus subtilis.

Homoserine dehydrogenase (HSD) catalyzes the reversible conversion of L-aspartate-4- semialdehyde to L-homoserine in the aspartate pathway for the biosynthesis of lysine, methionine, threonine, and isoleucine. HSD has attracted great attention for medical and industrial purposes due to its recognized application in the development of pesticides and is being utilized in the large scale production of L-lysine. In this study, HSD from Bacillus subtilis (BsHSD) was overexpressed in Escherichia coli and purified to homogeneity for biochemical characterization. We examined the enzymatic activity of BsHSD for L-homoserine oxidation and found that BsHSD exclusively prefers NADP+ to NAD+ and that its activity was maximal at pH 9.0 and in the presence of 0.4 M NaCl. By kinetic analysis, Km values for L-homoserine and NADP+ were found to be 35.08 ± 2.91 mM and 0.39 ± 0.05 mM, respectively, and the Vmax values were 2.72 ± 0.06 µmol/min-1 mg-1 and 2.79 ± 0.11 µmol/min-1 mg-1, respectively. The apparent molecular mass determined with size-exclusion chromatography indicated that BsHSD forms a tetramer, in contrast to the previously reported dimeric HSDs from other organisms. This novel oligomeric assembly can be attributed to the additional C-terminal ACT domain of BsHSD. Thermal denaturation monitoring by circular dichroism spectroscopy was used to determine its melting temperature, which was 54.8°C. The molecular and biochemical features of BsHSD revealed in this study may lay the foundation for future studies on amino acid metabolism and its application for industrial and medical purposes.

Candida albicans Hom6 is a homoserine dehydrogenase involved in protein synthesis and cell adhesion.

BACKGROUND/PURPOSE: Candida albicans is a common fungal pathogen in humans. In healthy individuals, C. albicans represents a harmless commensal organism, but infections can be life threatening in immunocompromised patients. The complete genome sequence of C. albicans is extremely useful for identifying genes that may be potential drug targets and important for pathogenic virulence. However, there are still many uncharacterized genes in the Candida genome database. In this study, we investigated C. albicans Hom6, the functions of which remain undetermined experimentally. METHODS: HOM6-deleted and HOM6-reintegrated mutant strains were constructed. The mutant strains were compared with wild-type in their growth in various media and enzyme activity. Effects of HOM6 deletion on translation were further investigated by cell susceptibility to hygromycin B or cycloheximide, as well as by polysome profiling, and cell adhesion to polystyrene was also determined. RESULTS: C. albicans Hom6 exhibits homoserine dehydrogenase activity and is involved in the biosynthesis of methionine and threonine. HOM6 deletion caused translational arrest in cells grown under amino acid starvation conditions. Additionally, Hom6 protein was found in both cytosolic and cell-wall fractions of cultured cells. Furthermore, HOM6 deletion reduced C. albicans cell adhesion to polystyrene, which is a common plastic used in many medical devices. CONCLUSION: Given that there is no Hom6 homologue in mammalian cells, our results provided an important foundation for future development of new antifungal drugs.

Crystal Structures of a Hyperthermophilic Archaeal Homoserine Dehydrogenase Suggest a Novel Cofactor Binding Mode for Oxidoreductases.

NAD(P)-dependent dehydrogenases differ according to their coenzyme preference: some prefer NAD, others NADP, and still others exhibit dual cofactor specificity. The structure of a newly identified archaeal homoserine dehydrogenase showed this enzyme to have a strong preference for NADP. However, NADP did not act as a cofactor with this enzyme, but as a strong inhibitor of NAD-dependent homoserine oxidation. Structural analysis and site-directed mutagenesis showed that the large number of interactions between the cofactor and the enzyme are responsible for the lack of reactivity of the enzyme towards NADP. This observation suggests this enzyme exhibits a new variation on cofactor binding to a dehydrogenase: very strong NADP binding that acts as an obstacle to NAD(P)-dependent dehydrogenase catalytic activity.

Structural basis for the catalytic mechanism of homoserine dehydrogenase.

Homoserine dehydrogenase (HSD) is an oxidoreductase in the aspartic acid pathway. This enzyme coordinates a critical branch point of the metabolic pathway that leads to the synthesis of bacterial cell-wall components such as L-lysine and m-DAP in addition to other amino acids such as L-threonine, L-methionine and L-isoleucine. Here, a structural rationale for the hydride-transfer step in the reaction mechanism of HSD is reported. The structure of Staphylococcus aureus HSD was determined at different pH conditions to understand the basis for the enhanced enzymatic activity at basic pH. An analysis of the crystal structure revealed that Lys105, which is located at the interface of the catalytic and cofactor-binding sites, could mediate the hydride-transfer step of the reaction mechanism. The role of Lys105 was subsequently confirmed by mutational analysis. Put together, these studies reveal the role of conserved water molecules and a lysine residue in hydride transfer between the substrate and the cofactor.

Metabolic and genetic factors affecting the productivity of pyrimidine nucleoside in Bacillus subtilis.

BACKGROUND: Cytidine and uridine are produced commercially by Bacillus subtilis. The production strains of cytidine and uridine were both derivatives from mutagenesis. However, the exact metabolic and genetic factors affecting the productivity remain unknown. Genetic engineering may be a promising approach to identify and confirm these factors. RESULTS: With the deletion of the cdd and hom genes, and the deregulation of the pyr operon in Bacillus subtilis168, the engineered strain produced 200.9 mg/L cytidine, 14.9 mg/L uridine and 960.1 mg/L uracil. Then, the overexpressed prs gene led to a dramatic increase of uridine by 25.9 times along with a modest increase of cytidine. Furthermore, the overexpressed pyrG gene improved the production of cytidine, uridine and uracil by 259.5%, 11.2% and 68.8%, respectively. Moreover, the overexpression of the pyrH gene increasesd the yield of cytidine by 40%, along with a modest augments of uridine and uracil. Lastly, the deletion of the nupC-pdp gene resulted in a doubled production of uridine up to 1684.6 mg/L, a 14.4% increase of cytidine to 1423 mg/L, and a 99% decrease of uracil to only 14.2 mg/L. CONCLUSIONS: The deregulation of the pyr operon and the overexpression of the prs, pyrG and pyrH genes all contribute to the accumulation of pyrimidine nucleoside compounds in the medium. Among these factors, the overexpression of the pyrG and pyrH genes can particularly facilitate the production of cytidine. Meanwhile, the deletion of the nupC-pdp gene can obviously reduce the production of uracil and simultaneously improve the production of uridine.

The contest for precursors: channelling L-isoleucine synthesis in Corynebacterium glutamicum without byproduct formation.

L-Isoleucine is an essential amino acid, which is required as a pharma product and feed additive. Its synthesis shares initial steps with that of L-lysine and L-threonine, and four enzymes of L-isoleucine synthesis have an enlarged substrate specificity involved also in L-valine and L-leucine synthesis. As a consequence, constructing a strain specifically overproducing L-isoleucine without byproduct formation is a challenge. Here, we analyze for consequences of plasmid-encoded genes in Corynebacterium glutamicum MH20-22B on L-isoleucine formation, but still obtain substantial accumulation of byproducts. In a different approach, we introduce point mutations into the genome of MH20-22B to remove the feedback control of homoserine dehydrogenase, hom, and threonine dehydratase, ilvA, and we assay sets of genomic promoter mutations to increase hom and ilvA expression as well as to reduce dapA expression, the latter gene encoding the dihydrodipicolinate synthase. The promoter mutations are mirrored in the resulting differential protein levels determined by a targeted LC-MS/MS approach for the three key enzymes. The best combination of genomic mutations was found in strain K2P55, where 53 mM L-isoleucine could be obtained. Whereas in fed-batch fermentations with the plasmid-based strain, 94 mM L-isoleucine with L-lysine as byproduct was formed; with the plasmid-less strain K2P55, 109 mM L-isoleucine accumulated with no substantial byproduct formation. The specific molar yield with the latter strain was 0.188 mol L-isoleucine (mol glucose)(-1) which characterizes it as one of the best L-isoleucine producers available and which does not contain plasmids.

Effects of metal ions and hydrogen peroxide on the phenotype of yeast hom6Δ mutant.

UNLABELLED: HOM6 is a major gene in the aspartate pathway which leads to biosynthesis of threonine and methionine. The phenotypes of the gene deletion mutant (hom6∆) in a variety of cultural conditions have previously provided meaningful insights into the biological roles of HOM6 and its upstream intermediate metabolites. Here, we conducted a survey on a spectrum of metal ions for their effect on the aspartate pathway and broader sulphur metabolism. We show that manganese (Mn(2+) ) promoted the growth of hom6∆ under both anaerobic and aerobic conditions. Unexpectedly, 4 mmol l(-1) hydrogen peroxide (H2 O2 ), a dose normally causing temporary cell growth arrest, enhanced the growth of hom6∆ under the anaerobic condition only, while it had no effect on the wild type strain BY4743. We propose that Mn(2+) and H2 O2 promote the growth of hom6∆ by reducing the accumulation of the toxic intermediate metabolite-aspartate ß-semialdehyde, via directing the aspartate pathway to the central sugar metabolism-tricarboxylic acid cycle. SIGNIFICANCE AND IMPACT OF THE STUDY: This study focuses on the yeast strain which lacks homoserine dehydrogenase encoded by HOM6 gene in aspartate metabolism. The HOM6-deletion mutant (hom6Δ) was analysed in the context of varying environmental parameters such as metal ions and oxidants, under anaerobic and aerobic conditions. We demonstrated that both manganese and hydrogen peroxide can promote the growth of hom6Δ, with the latter exerting such effect only under anaerobic condition. The findings are relevant to the research areas of ageing and anti-fungal drug development. It highlights the importance of interactions between gene expression and environmental factors as well as culture conditions.

Rational design of allosteric regulation of homoserine dehydrogenase by a nonnatural inhibitor L-lysine.

Allosteric proteins, which can sense different signals, are interesting biological parts for synthetic biology. In particular, the design of an artificial allosteric enzyme to sense an unnatural signal is both challenging and highly desired, for example, for a precise and dynamical control of fluxes of growth-essential but byproduct pathways in metabolic engineering of industrial microorganisms. In this work, we used homoserine dehydrogenase (HSDH) of Corynebacterium glutamicum, which is naturally allosterically regulated by threonine and isoleucine, as an example to demonstrate the feasibility of reengineering an allosteric enzyme to respond to an unnatural inhibitor L-lysine. For this purpose, the natural threonine binding sites of HSD were first predicted and verified by mutagenesis experiments. The threonine binding sites were then engineered to a lysine binding pocket. The reengineered HSD only responds to lysine inhibition but not to threonine. This is a significant step toward the construction of artificial molecular circuits for dynamic control of growth-essential byproduct formation pathway for lysine biosynthesis.

[Heterologous expression and characterization of L200F/D215K mutant of homoserine dehydrogenase from Corynebacterium pekinense AS1.299].

OBJECTIVE: To obtain a new homoserine dehydrogenase with better properties from Corynebacterium pekinense by the spatial structure transfromation. METHODS: Double mutants L200F/D215A, L200F/D215E, L200F/D215G and L200F/D215K were constructed by site-directed mutagenesis and expressed in E. coli BL21. L200F/D215K was characterized for its highest catalytic efficiency and compared with that of L200F. RESULTS: The Vmax of L200F/D215K was 36.92 U/mg, 1.24 times as that of L200F. The optimum reaction temperature of L200F/D215K was 37 degrees C, 2 degrees C higher than that of L200F. The optimum pH of L200F/D215K was 7.5, the same as that of L200F. The half-life time of L200F/D215K under optimum temperature was 4.16 h and was 1.12 times as that of L200F. Both L200F/D215K and L200F had good resistance to organic solvents and metal ions. CONCLUSION: Through the spatial structure transformation, the enzymatic activity was increased, and the enzymology properties was optimized.

Regulatory mechanisms after short- and long-term perturbed lysine biosynthesis in the aspartate pathway: the need for isogenes in Arabidopsis thaliana.

The aspartate-derived amino acid pathway in plants is an intensively studied metabolic pathway, because of the biosynthesis of the four essential amino acids lysine, threonine, isoleucine and methionine. The pathway is mainly controlled by the key regulatory enzymes aspartate kinase (AK; EC 2.7.2.4), homoserine dehydrogenase (HSDH; EC 1.1.1.3) and 4-hydroxy-tetrahydrodipicolinate synthase (EC 4.3.3.7), formerly referred to as dihydrodipicolinate synthase (DHDPS). They are encoded by isoenzyme families and it is not known why such families are evolutionarily maintained. To gain more insight into the specific roles and regulation of the isoenzymes, we inhibited DHDPS in Arabidopsis thaliana with the chemical compound (N,N-dimethylglycinatoboranyloxycarbonylmethyl)-dimethylamine-borane (DDAB) and compared the short-term effects on the biochemical and biomolecular level to the long-term adaptations in dhdps knockout mutants. We found that DHDPS2 plays a crucial role in controlling lysine biosynthesis, thereby stabilizing flux through the whole aspartate pathway. Moreover, DHDPS2 was also shown to influence the threonine level to a large extent. In addition, the lysine-sensitive AKs, AKLYS1 and AKLYS3 control the short- and long-term responses to perturbed lysine biosynthesis in Arabidopsis thaliana.

[Identification of the differences in the genes responsible for methionine biosynthesis in Bacillus anthracis strains and phylogeny-related species].

The comparative analysis of the gene sequences encoding the synthesis of enzymes responsible for the intermediary metabolism of methionine in Bacillus anthracis strains and in closely related bacterial species was carried out. Deletion of 42 nucleotides in the hom2 gene, which determines the homoserinedehydrogenase, is detected in all tested Bacillus anthracis strains. In the strains of other bacillar species hom2 gene mutation, which blocks up the tracts of methionine and threonine biosynthesis, was not identified. The single nucleotide polymorphism was determined in asd1, metX, and metH genes. It provides the identification of B. anthracis strains using sequencing technology.

Crystal structure of serine dehydrogenase from Escherichia coli: important role of the C-terminal region for closed-complex formation.

Serine dehydrogenase from Escherichia coli is a homotetrameric enzyme belonging to the short-chain dehydrogenase/reductase (SDR) family. This enzyme catalyses the NADP(+)-dependent oxidation of serine to 2-aminomalonate semialdehyde. The enzyme shows a stereospecificity for ß-(3S)-hydroxy acid as a substrate; however, no stereospecificity was observed at the α-carbon. The structures of the ligand-free SerDH and SerDH-NADP(+)-phosphate complex were determined at 1.9 and 2.7 Å resolutions, respectively. The overall structure, including the catalytic tetrad of Asn106, Ser134, Tyr147 and Lys151, shows obvious relationships with other members of the SDR family. The structure of the substrate-binding loop and that of the C-terminal region were disordered in the ligand-free enzyme, whereas these structures were clearly defined in the SerDH-NADP(+) complex as a closed form. Interestingly, the C-terminal region was protruded from the main body and it formed an anti-parallel ß-sheet with another C-terminal region on the subunit that is diagonally opposite to that in the tetramer. It is revealed that the C-terminal region possesses the important roles in substrate binding through the stabilization of the substrate-binding loop in the closed form complex. The roles of the C-terminal region along with those of the residues involved in substrate recognition were studied by site-directed mutagenesis.

Systems-wide metabolic pathway engineering in Corynebacterium glutamicum for bio-based production of diaminopentane.

In the present work the Gram-positive bacterium Corynebacterium glutamicum was engineered into an efficient, tailor-made production strain for diaminopentane (cadaverine), a highly attractive building block for bio-based polyamides. The engineering comprised expression of lysine decarboxylase (ldcC) from Escherichia coli, catalyzing the conversion of lysine into diaminopentane, and systems-wide metabolic engineering of central supporting pathways. Substantially re-designing the metabolism yielded superior strains with desirable properties such as (i) the release from unwanted feedback regulation at the level of aspartokinase and pyruvate carboxylase by introducing the point mutations lysC311 and pycA458, (ii) an optimized supply of the key precursor oxaloacetate by amplifying the anaplerotic enzyme, pyruvate carboxylase, and deleting phosphoenolpyruvate carboxykinase which otherwise removes oxaloacetate, (iii) enhanced biosynthetic flux via combined amplification of aspartokinase, dihydrodipicolinate reductase, diaminopimelate dehydrogenase and diaminopimelate decarboxylase, and (iv) attenuated flux into the threonine pathway competing with production by the leaky mutation hom59 in the homoserine dehydrogenase gene. Lysine decarboxylase proved to be a bottleneck for efficient production, since its in vitro activity and in vivo flux were closely correlated. To achieve an optimal strain having only stable genomic modifications, the combination of the strong constitutive C. glutamicum tuf promoter and optimized codon usage allowed efficient genome-based ldcC expression and resulted in a high diaminopentane yield of 200 mmol mol(-1). By supplementing the medium with 1 mgL(-1) pyridoxal, the cofactor of lysine decarboxylase, the yield was increased to 300 mmol mol(-1). In the production strain obtained, lysine secretion was almost completely abolished. Metabolic analysis, however, revealed substantial formation of an as yet unknown by-product. It was identified as an acetylated variant, N-acetyl-diaminopentane, which reached levels of more than 25% of that of the desired product.

Bacillus methanolicus pyruvate carboxylase and homoserine dehydrogenase I and II and their roles for L-lysine production from methanol at 50 degrees C.

We here present the pyc gene encoding pyruvate carboxylase (PC), and the hom-1 and hom-2 genes encoding two active homoserine dehydrogenase (HD) proteins, in methylotrophic Bacillus methanolicus MGA3. In general, both PC and HD are regarded as key targets for improving bacterial L-lysine production; PC plays a role in precursor oxaloacetate (OAA) supply while HD controls an important branch point in the L-lysine biosynthetic pathway. The hom-1 and hom-2 genes were strongly repressed by L-threonine and L-methionine, respectively. Wild-type MGA3 cells secreted 0.4 g/l L-lysine and 59 g/l L-glutamate under optimised fed batch methanol fermentation. The hom-1 mutant M168-20 constructed herein secreted 11 g/l L-lysine and 69 g/l of L-glutamate, while a sixfold higher L-lysine overproduction (65 g/l) of the previously constructed classical B. methanolicus mutant NOA2#13A52-8A66 was accompanied with reduced L-glutamate production (28 g/l) and threefold elevated pyc transcription level. Overproduction of PC and its mutant enzyme P455S in M168-20 had no positive effect on the volumetric L-lysine yield and the L-lysine yield on methanol, and caused significantly reduced volumetric L-glutamate yield and L: -glutamate yield on methanol. Our results demonstrated that hom-1 represents one key target for achieving L-lysine overproduction, PC activity plays an important role in controlling L-glutamate production from methanol, and that OAA precursor supply is not a major bottleneck for L-lysine overproduction by B. methanolicus.

Homologous expression of aspartokinase (ask) gene in Streptomyces clavuligerus and its hom-deleted mutant: effects on cephamycin C production.

In this study, the effect of homologous multiple copies of the ask gene, which encodes aspartokinase catalyzing the first step of the aspartate pathway, on cephamycin C biosynthesis in S. clavuligerus NRRL 3585 and its hom mutant was investigated. The intracellular pool levels of aspartate pathway amino acids accorded well with the Ask activity levels in TB3585 and AK39. When compared with the control strain carrying vector alone without any gene insert, amplification of the ask gene in the wild strain resulted in a maximum of 3.1- and 3.3-fold increase in specific, 1.7- and 1.9-fold increase in volumetric cephamycin C production when grown in trypticase soy broth (TSB) and a modified chemically defined medium (mCDM), respectively. However, expression of multicopy ask gene in a hom-deleted background significantly decreased cephamycin C yields when the cells were grown in either TSB or mCDM, most probably due to physiological disturbance resulting from enzyme overexpression and high copy number plasmid burden in an auxotrophic host, respectively.

Threonine-insensitive homoserine dehydrogenase from soybean: genomic organization, kinetic mechanism, and in vivo activity.

Aspartate kinase (AK) and homoserine dehydrogenase (HSD) function as key regulatory enzymes at branch points in the aspartate amino acid pathway and are feedback-inhibited by threonine. In plants the biochemical features of AK and bifunctional AK-HSD enzymes have been characterized, but the molecular properties of the monofunctional HSD remain unexamined. To investigate the role of HSD, we have cloned the cDNA and gene encoding the monofunctional HSD (GmHSD) from soybean. Using heterologously expressed and purified GmHSD, initial velocity and product inhibition studies support an ordered bi bi kinetic mechanism in which nicotinamide cofactor binds first and leaves last in the reaction sequence. Threonine inhibition of GmHSD occurs at concentrations (K(i) = 160-240 mM) more than 1000-fold above physiological levels. This is in contrast to the two AK-HSD isoforms in soybean that are sensitive to threonine inhibition (K(i) approximately 150 microM). In addition, GmHSD is not inhibited by other aspartate-derived amino acids. The ratio of threonine-resistant to threonine-sensitive HSD activity in soybean tissues varies and likely reflects different demands for amino acid biosynthesis. This is the first cloning and detailed biochemical characterization of a monofunctional feedback-insensitive HSD from any plant. Threonine-resistant HSD offers a useful biotechnology tool for manipulating the aspartate amino acid pathway to increase threonine and methionine production in plants for improved nutritional content.

Targeted disruption of homoserine dehydrogenase gene and its effect on cephamycin C production in Streptomyces clavuligerus.

The aspartate pathway of Streptomyces clavuligerus is an important primary metabolic pathway which provides substrates for beta-lactam synthesis. In this study, the hom gene which encodes homoserine dehydrogenase was cloned from the cephamycin C producer S. clavuligerus NRRL 3585 and characterized. The fully sequenced open reading frame encodes 433 amino acids with a deduced M (r) of 44.9 kDa. The gene was heterologously expressed in the auxotroph mutant Escherichia coli CGSC 5075 and the recombinant protein was purified. The cloned gene was used to construct a plasmid containing a hom disruption cassette which was then transformed into S. clavuligerus. A hom mutant of S. clavuligerus was obtained by insertional inactivation via double crossover, and the effect of hom gene disruption on cephamycin C yield was investigated by comparing antibiotic levels in culture broths of this mutant and in the parental strain. Disruption of hom gene resulted in up to 4.3-fold and twofold increases in intracellular free L-lysine concentration and specific cephamycin C production, respectively, during stationary phase in chemically defined medium.