Multimetabolite Control of a Biosynthetic Pathway by Sequential Metabolites

Growth-coupled production of L-isoleucine in Escherichia coli via metabolic engineering

L-isoleucine, an essential amino acid, is widely used in the pharmaceutical and food industries. However, the current production efficiency is insufficient to meet the increasing demands. In this study, we aimed to develop an efficient L-isoleucine-producing strain of Escherichia coli. First, accumulation of L-isoleucine was achieved by employing feedback-resistant enzymes. Next, a growth-coupled L-isoleucine synthetic pathway was established by introducing the metA-metB-based α-ketobutyrate-generating bypass, which significantly increased L-isoleucine production to 7.4 g/L. Upon employing an activity-improved cystathionine γ-synthase mutant obtained from adaptive laboratory evolution, L-isoleucine production further increased to 8.5 g/L. Subsequently, the redox flux was improved by bypassing the NADPH-dependent aspartate aminotransferase pathway and employing the NADH-dependent pathway and transhydrogenase. Finally, L-isoleucine efflux was enhanced by modifying the transport system. After fed-batch fermentation for 48 h, the resultant strain, ISO-12, reached an L-isoleucine production titer of 51.5 g/L and yield of 0.29 g/g glucose. The strains developed in this study achieved a higher L-isoleucine production efficiency than those reported previously. These strategies will aid in the development of cell factories that produce L-isoleucine and related products.

Multivariate modular metabolic engineering for enhanced L-methionine biosynthesis in Escherichia coli

BACKGROUND

L-Methionine is the only bulk amino acid that has not been industrially produced by the fermentation method. Due to highly complex and strictly regulated biosynthesis, the development of microbial strains for high-level L-methionine production has remained challenging in recent years.

RESULTS

By strengthening the L-methionine terminal synthetic module via site-directed mutation of L-homoserine O-succinyltransferase (MetA) and overexpression of metA^fbr, metC, and yjeH, L-methionine production was increased to 1.93 g/L in shake flask fermentation. Deletion of the pykA and pykF genes further improved L-methionine production to 2.51 g/L in shake flask fermentation. Computer simulation and auxotrophic experiments verified that during the synthesis of L-methionine, equimolar amounts of L-isoleucine were accumulated via the elimination reaction of cystathionine γ-synthetase MetB due to the insufficient supply of L-cysteine. To increase the supply of L-cysteine, the L-cysteine synthetic module was strengthened by overexpression of cysE^fbr, serA^fbr, and cysDN, which further increased the production of L-methionine by 52.9% and significantly reduced the accumulation of the byproduct L-isoleucine by 29.1%. After optimizing the addition of ammonium thiosulfate, the final metabolically engineered strain MET17 produced 21.28 g/L L-methionine in 64 h with glucose as the carbon source in a 5 L fermenter, representing the highest L-methionine titer reported to date.

CONCLUSIONS

In this study, a high-efficiency strain for L-methionine production was derived from wild-type Escherichia coli W3110 by rational metabolic engineering strategies, providing an efficient platform for the industrial production of L-methionine.

Structural and Functional Characterization of Mycobacterium tuberculosis Homoserine Transacetylase

Mycobacterium tuberculosis (Mtb) lacking functional homoserine transacetylase (HTA) is compromised in methionine biosynthesis, protein synthesis, and in the activity of multiple essential S-adenosyl-l-methionine-dependent enzymes. Additionally, deficient mutants are further disarmed by the toxic accumulation of lysine due to a redirection of the metabolic flux toward the lysine biosynthetic pathway. Studies with deletion mutants and crystallographic studies of the apoenzyme have, respectively, validated Mtb HTA as an essential enzyme and revealed a ligandable binding site. Seeking a mechanistic characterization of this enzyme, we report crucial structural details and comprehensive functional characterization of Mtb HTA. Crystallographic and mass spectral observation of the acetylated HTA intermediate and initial velocity studies were consistent with a ping-pong kinetic mechanism. Wild-type HTA and its site-directed mutants were kinetically characterized with a panel of natural and alternative substrates to understand substrate specificity and identify critical residues for catalysis. Titration experiments using fluorescence quenching showed that both substrates─acetyl-CoA and l-homoserine─engage in a strong and weak binding interaction with HTA. Additionally, substrate inhibition by acetyl-CoA and product inhibition by CoA and O-acetyl-l-homoserine were proposed to form the basis of a feedback regulation mechanism. By furnishing key mechanistic and structural information, these studies provide a foundation for structure-based design efforts around this attractive Mtb target.

Rational Engineering of Homoserine O-Succinyltransferase from Escherichia coli for Reduced Feedback Inhibition by Methionine

Methionine is an essential amino acid in all living organisms and has been used in various industrial applications such as food and feed additives. However, inhibition of enzymes involved in methionine biosynthesis is considered to be a crucial bottleneck for an efficient bio-based methionine production process. Homoserine O-succinyltransferase fromEscherichia coli (EcHST) has been reported to be feedback inhibited by the final product methionine. To understand the regulation mechanism of the enzyme and generate a feedback-resistant mutant, we determined the crystal structure of EcHST and elucidated the binding site of homoserine and succinyl-CoA. The enzyme kinetic experiments of EcHST revealed that the enzyme is noncompetitively inhibited by methionine with a K_i value of 2.44 mM, and we also identified a putative inhibitor binding site located in the vicinity of the substrate binding site. We then generated the EcHST^T242A variant with reduced feedback inhibition with a K_i value of 17.40 mM.

Methionine

This review focuses on the steps unique to methionine biosynthesis, namely the conversion of homoserine to methionine. The past decade has provided a wealth of information concerning the details of methionine metabolism and the review focuses on providing a comprehensive overview of the field, emphasizing more recent findings. Details of methionine biosynthesis are addressed along with key cellular aspects, including regulation, uptake, utilization, AdoMet, the methyl cycle, and growing evidence that inhibition of methionine biosynthesis occurs under stressful cellular conditions. The first unique step in methionine biosynthesis is catalyzed by the metA gene product, homoserine transsuccinylase (HTS, or homoserine O-succinyltransferase). Recent experiments suggest that transcription of these genes is indeed regulated by MetJ, although the repressor-binding sites have not yet been verified. Methionine also serves as the precursor of S-adenosylmethionine, which is an essential molecule employed in numerous biological processes. S-adenosylhomocysteine is produced as a consequence of the numerous AdoMet-dependent methyl transfer reactions that occur within the cell. In E. coli and Salmonella, this molecule is recycled in two discrete steps to complete the methyl cycle. Cultures challenged by oxidative stress appear to experience a growth limitation that depends on methionine levels. E. coli that are deficient for the manganese and iron superoxide dismutases (the sodA and sodB gene products, respectively) require the addition of methionine or cysteine for aerobic growth. Modulation of methionine levels in response to stressful conditions further increases the complexity of its regulation.

Emergence of robust growth laws from optimal regulation of ribosome synthesis

Bacteria must constantly adapt their growth to changes in nutrient availability; yet despite large-scale changes in protein expression associated with sensing, adaptation, and processing different environmental nutrients, simple growth laws connect the ribosome abundance and the growth rate. Here, we investigate the origin of these growth laws by analyzing the features of ribosomal regulation that coordinate proteome-wide expression changes with cell growth in a variety of nutrient conditions in the model organism Escherichia coli. We identify supply-driven feedforward activation of ribosomal protein synthesis as the key regulatory motif maximizing amino acid flux, and autonomously guiding a cell to achieve optimal growth in different environments. The growth laws emerge naturally from the robust regulatory strategy underlying growth rate control, irrespective of the details of the molecular implementation. The study highlights the interplay between phenomenological modeling and molecular mechanisms in uncovering fundamental operating constraints, with implications for endogenous and synthetic design of microorganisms.

Bacterial methionine biosynthesis

Methionine is essential in all organisms, as it is both a proteinogenic amino acid and a component of the cofactor, S-adenosyl methionine. The metabolic pathway for its biosynthesis has been extensively characterized in Escherichia coli; however, it is becoming apparent that most bacterial species do not use the E. coli pathway. Instead, studies on other organisms and genome sequencing data are uncovering significant diversity in the enzymes and metabolic intermediates that are used for methionine biosynthesis. This review summarizes the different biochemical strategies that are employed in the three key steps for methionine biosynthesis from homoserine (i.e. acylation, sulfurylation and methylation). A survey is presented of the presence and absence of the various biosynthetic enzymes in 1593 representative bacterial species, shedding light on the non-canonical nature of the E. coli pathway. This review also highlights ways in which knowledge of methionine biosynthesis can be utilized for biotechnological applications. Finally, gaps in the current understanding of bacterial methionine biosynthesis are noted. For example, the paper discusses the presence of one gene (metC) in a large number of species that appear to lack the gene encoding the enzyme for the preceding step in the pathway (metB), as it is understood in E. coli. Therefore, this review aims to move the focus away from E. coli, to better reflect the true diversity of bacterial pathways for methionine biosynthesis.

Antifolate-induced depletion of intracellular glycine and purines inhibits thymineless death in E. coli

Despite the therapeutic importance of antifolates, the links between their direct antimetabolite activity and downstream consequences remain incompletely understood. Here we employ metabolomics to examine the complete metabolic effects of the antibiotic trimethoprim in E. coli. In rich media, trimethoprim treatment causes thymineless death. In minimal media, in contrast, trimethoprim addition results in rapid stoppage of cell growth and stable cell stasis. We show that initial impairment of cell growth is due to rapid depletion of glycine and associated activation of the stringent response. Long-term stasis is due to purine insufficiency. Thus, E. coli has dual systems for surviving folate depletion and avoiding thymineless death: a short-term response based on sensing of amino acids and a long-term response based on sensing of nucleotides.

Amino acid biosynthesis: new architectures in allosteric enzymes

This review focuses on the allosteric controls in the Aspartate-derived and the branched-chain amino acid biosynthetic pathways examined both from kinetic and structural points of view. The objective is to show the differences that exist among the plant and microbial worlds concerning the allosteric regulation of these pathways and to unveil the structural bases of this diversity. Indeed, crystallographic structures of enzymes from these pathways have been determined in bacteria, fungi and plants, providing a wonderful opportunity to obtain insight into the acquisition and modulation of allosteric controls in the course of evolution. This will be examined using two enzymes, threonine synthase and the ACT domain containing enzyme aspartate kinase. In a last part, as many enzymes in these pathways display regulatory domains containing the conserved ACT module, the organization of ACT domains in this kind of allosteric enzymes will be reviewed, providing explanations for the variety of allosteric effectors and type of controls observed.

Determination of the Escherichia coli S-nitrosoglutathione response network using integrated biochemical and systems analysis

During infection or denitrification, bacteria encounter reactive nitrogen species. Although the molecular targets of and defensive response against nitric oxide (NO) in Escherichia coli are well studied, the response elements specific to S-nitrosothiols are less clear. Previously, we employed an integrated systems biology approach to unravel the E. coli NO-response network. Here we use a similar approach to confirm that S-nitrosoglutathione (GSNO) primarily impacts the metabolic and regulatory programs of E. coli in minimal medium by reaction with homocysteine and cysteine and subsequent disruption of the methionine biosynthesis pathway. Targeting of homocysteine and cysteine results in altered regulatory activity of MetJ, MetR, and CysB, activation of the stringent response and growth inhibition. Deletion of metJ or supplementation with methionine strongly attenuated the effect of GSNO on growth and gene expression. Furthermore, GSNO inhibited the ArcAB two-component system. Consistent with the underlying nitrosative and thiol-oxidative chemistry, growth inhibition and the majority of the regulatory perturbations were dependent upon GSNO internalization by the Dpp dipeptide transporter. Contrastingly, perturbation of NsrR appeared to be a result of the submicromolar levels of NO released from GSNO and did not require GSNO internalization.

Effects of deregulation of methionine biosynthesis on methionine excretion in Escherichia coli

Several regulators of methionine biosynthesis have been reported in Escherichia coli, which might represent barriers to the production of excess l-methionine (Met). In order to examine the effects of these factors on Met biosynthesis and metabolism, deletion mutations of the methionine repressor (metJ) and threonine biosynthetic (thrBC) genes were introduced into the W3110 wild-type strain of E. coli. Mutations of the metK gene encoding S-adenosylmethionine synthetase, which is involved in Met metabolism, were detected in 12 norleucine-resistant mutants. Three of the mutations in the metK structural gene were then introduced into metJ and thrBC double-mutant strains; one of the resultant strains was found to accumulate 0.13 g/liter Met. Mutations of the metA gene encoding homoserine succinyltransferase were detected in alpha-methylmethionine-resistant mutants, and these mutations were found to encode feedback-resistant enzymes in a 14C-labeled homoserine assay. Three metA mutations were introduced, using expression plasmids, into an E. coli strain that was shown to accumulate 0.24 g/liter Met. Combining mutations that affect the deregulation of Met biosynthesis and metabolism is therefore an effective approach for the production of Met-excreting strains.

Probing the active site of homoserine trans-succinylase

Homoserine trans-succinylase is the first enzyme in methionine biosynthesis of Escherichia coli and catalyzes the activation of homoserine via a succinylation reaction. The in vivo activity of this enzyme is subject to tight regulation by several mechanisms, including repression and activation of gene expression, feedback inhibition, temperature regulation and proteolysis. This complex regulation reflects the key role of this enzyme in bacterial metabolism. Here, we demonstrate--using proteomics and high-resolution mass spectrometry--that succinyl is covalently bound to one of the two adjacent lysine residues at positions 45 and 46. Replacing these lysine residues by alanine abolished the enzymatic activity. These findings position the lysine residues, one of which is conserved, at the active site.

Inhibition of Escherichia coli growth by acetic acid: a problem with methionine biosynthesis and homocysteine toxicity

The mechanism by which methionine relieves the growth inhibition of Escherichia coli K-12 that is caused by organic weak acid food preservatives was investigated. In the presence of 8 mM acetate the specific growth rate of E. coli Frag1 (in MacIlvaine's minimal medium pH 6.0) is reduced by 50%. Addition of methionine restores growth to 80% of that observed in untreated controls. Similar relief was seen with cultures treated with either benzoate or propionate. Mutants with an elevated intracellular methionine pool were almost completely resistant to the inhibitory effects of acetate, suggesting that the methionine pool becomes limiting for growth in acetate-treated cells. Measurement of the intracellular concentrations of pathway intermediates revealed that the homocysteine pool is increased dramatically in acetate-treated cells, suggesting that acetate inhibits a biosynthetic step downstream from this intermediate. Supplementation of the medium with homocysteine inhibits the growth of E. coli cells. Acetate inhibition of growth arises from the depletion of the intracellular methionine pool with the concomitant accumulation of the toxic intermediate homocysteine and this augments the effect of lowering cytoplasmic pH.

Control of methionine biosynthesis in Escherichia coli by proteolysis

Most bacterial proteins are stable, with half-lives considerably longer than the generation time. In Escherichia coli, the few exceptions are unstable regulatory proteins. The results presented here indicate that the first enzyme in methionine biosynthesis - homoserine trans-succinylase (HTS) - is unstable and subject to energy-dependent proteolysis. The enzyme is stable in triple mutants defective in Lon-, HslVU- and ClpP-dependent proteases. The instability of the protein is determined by the amino-terminal part of the protein, and its removal or substitution by the N-terminal part of beta-galactosidase confers stability. The effect of the amino-terminal segment is not caused by the N-end rule, as substitution of the first amino acid does not affect the stability of the protein. HTS is the first biosynthetic E. coli enzyme shown to have a short half-life and may represent a group of biosynthetic enzymes whose expression is controlled by proteolysis. Alternatively, the proteolytic processing of HTS may be unique to this enzyme and could reflect its central role in regulating bacterial growth, especially at elevated temperatures.

Direct sulfhydrylation for methionine biosynthesis in Leptospira meyeri

A gene library of the Leptospira meyeri serovar semaranga strain Veldrat S.173 DNA has been constructed in a mobilizable cosmid with inserts of up to 40 kb. It was demonstrated that a Leptospira DNA fragment carrying metY complemented Escherichia coli strains carrying mutations in metB. The latter gene encodes cystathionine gamma-synthase, an enzyme which catalyzes the second step of the methionine biosynthetic pathway. The metY gene is 1,304 bp long and encodes a 443-amino-acid protein with a molecular mass of 45 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The deduced amino acid sequence of the Leptospira metY product has a high degree of similarity to those of O-acetylhomoserine sulfhydrylases from Aspergillus nidulans and Saccharomyces cerevisiae. A lower degree of sequence similarity was also found with bacterial cystathionine gamma-synthase. The L. meyeri metY gene was overexpressed under the control of the T7 promoter. MetY exhibits an O-acetylhomoserine sulfhydrylase activity. Genetic, enzymatic, and physiological studies reveal that the transsulfuration pathway via cystathionine does not exist in L. meyeri, in contrast to the situation found for fungi and some bacteria. Our results indicate, therefore, that the L. meyeri MetY enzyme is able to perform direct sulfhydrylation for methionine biosynthesis by using O-acetylhomoserine as a substrate.

Methionine production by microorganisms

Microbial production of methionine is reviewed with 73 references. The review describes different methionine-producing organisms, as well as analog-resistant regulatory mutants, their optimum cultural conditions and yields. The pathways of methionine biosynthesis and their regulation are discussed.

Roles of O-acetyl-L-homoserine sulfhydrylases in micro-organisms

O-Acetyl-L-homoserine sulfhydrylase (EC 4.2.99.10) is essential for certain micro-organisms, functioning as a homocysteine synthase in the pathway of methionine synthesis. It participates in an alternative pathway of L-homocysteine synthesis for those microbes in which homocysteine is synthesized mainly via cystathionine. The protein can also catalyze the de novo synthesis of L-cysteine and O-alkyl-L-homoserine in some microorganisms. The enzyme possibly recycles the methylthio group of methionine.

Methionine biosynthesis in Enterobacteriaceae: biochemical, regulatory, and evolutionary aspects

The genes coding for the enzymes involved in methionine biosynthesis and regulation are scattered on the Escherichia coli chromosome. All of them have been cloned and most have been sequenced. From the information gathered, one can establish the existence (upstream of the structural genes coding for the biosynthetic genes and the regulatory gene) of "methionine boxes" consisting of two or more repeats of an octanucleotide sequence pattern. The comparison of these sequences allows the extraction of a consensus operator sequence. Mutations in these sequences lead to the constitutivity of the vicinal structural gene. The operator sequence is the target of a DNA-binding protein--the methionine aporepressor--which has been obtained in the pure state, for which S-adenosylmethionine acts as the corepressor. Mutations in the corresponding gene lead to the constitutive expression of all the methionine structural genes. The physicochemical properties of the methionine aporepressor are being investigated.

Partial purification and some properties of homoserine O-acetyltransferase of a methionine auxotroph of Saccharomyces cerevisiae

A wild-type strain and six methionine auxotrophs of Saccharomyces cerevisiae were cultured in a synthetic medium supplemented with 0.1 mM L-cysteine or L-methionine and analyzed for the synthesis of homoserine O-acetyltransferase (EC 2.3.1.31). Among them, four mutant strains exhibited enzyme activity in cell extracts. Methionine added to the synthetic medium at concentrations higher than 0.1 mM repressed enzyme synthesis in two of these strains. The enzyme was partially purified (3,500-fold) from an extract of a mutant strain through ammonium sulfate fractionation and chromatography on columns of DEAE-cellulose, Phenyl-Sepharose C1-4B, and Sephadex G-150. The enzyme exhibited optimal pH at 7.5 for activity and at 7.8 for stability. The reaction product was ascertained to be O-acetyl-L-homoserine by confirming that it produced L-homocysteine in an O-acetyl-L-homoserine sulfhydrylase reaction. The Km for L-homoserine was 1.0 mM, and for acetyl coenzyme A it was 0.027 mM. The molecular weight of the enzyme was estimated to be approximately 104,000 by Sephadex G-150 column chromatography and 101,000 by sucrose density gradient centrifugation. The isoelectric point was at pH 4.0. Of the hydroxy amino acids examined, the enzyme showed reactivity only to L-homoserine. Succinyl coenzyme A was not an acyl donor. In the absence of L-homoserine, acetyl coenzyme A was deacylated by the enzyme, with a Km of 0.012 mM. S-Adenosylmethionine and S-adenosylhomocysteine slightly inhibited the enzyme, but methionine had no effect.

Biological properties and mode of action of clavams

The clavams valclavam and hydroxyethylclavam were both bacteriostatic and fungistatic. The molecular basis for growth inhibition of Escherichia coli was a non-competitive inhibition of homoserine-O-succinyltransferase (EC 2.3.1.46), thus blocking methionine biosynthesis. Eucaryotes such as Saccharomyces cerevisiae were inhibited by a different mode of action. Instead of interfering with methionine biosynthesis, the clavams inhibited the formation of RNA in living cells, although the RNA-polymerases of isolated yeast nuclei were not inhibited. The action of valclavam on E. coli was dependent on functional peptide transport systems.

Construction and physical mapping of plasmids containing the MetA gene of Escherichia coli K-12

Plasmids containing the metA gene of E. coli K-12 were constructed in vitro using pBR322 as the cloning vehicle and lambda metA transducing phage as the source of metA DNA. EcoRI digests of pBR322 and lambda metA20 were joined by ligase and plasmids carrying the metA gene were selected after transformation in a metA deletion strain. Recombinant DNA molecules contained one pBR322 fragment and one lambda metA20 fragment of 12.2 kb which was present in either of two possible orientations. Plasmids constructed by BamHI digestion of lambda metA2 contained a single bacterial DNA fragment of 5.8 kb inserted in the tet gene. Insertion of the metA fragment led to loss of resistance to tetracycline in one orientation and partial resistance in the opposite orientation.

The regulation of methionine biosynthesis and metabolism in plants and bacteria

The amino acids biosynthetically derived from asparate including methionine are all essential in the diet of monogastric animals. Most of this requirement is met by plant foods. The methionine biosynthetic pathways in plants and bacteria are outlined and compared. Regulation in bacterial systems is by a combination of repression and feedback inhibition whereas in plants repression is unimportant. Several enzymes in the branched pathway to methionine in plants are regulated by feedback inhibition; others are yet to be investigated. In plants may amino acid biosynthetic enzymes are localized in plastids and this is also likely for methionine biosynthesis. Methionine occupies an important position in cellular metabolism where the processes of one-carbon transfer via S-adenosylmethionine, protein synthesis, protein initiation and ethylene synthesis are interlocked. Attempts to increase the levels of free methionine have been made by selecting for plant mutants resistant to lysine plus threonine. One dominant mutation causes elevation of free amino acid levels in vegetative tissues but also has undesirable side-effects. The potential of such approaches is discussed.

Isolation of a metK mutant with a temperature-sensitive S-adenosylmethionine synthetase

An Escherichia coli metK mutant, designated metK110, was isolated among spontaneous ethionine-resistant organisms selected at 42 degrees C. The S-adenosylmethionine synthetase activity of this mutant was present at lower levels than in the corresponding wild-type strain and was more labile than the wild-type enzyme when heated or dialyzed. A mixture of mutant and wild-type enzyme preparations had an activity equal to the sum of the component activities. These facts strongly suggest that the mutated gene in this strain is the structural gene for this enzyme. Genetic mapping experiments placed the metK110 mutation near or at the site of other known metK mutants (i.e., 63 min), confirming its designation as a metK mutant. A revised gene order has been established for this region, i.e., metC glc speC metK speB serA.

Repression of the tyrosine, lysine, and methionine biosynthetic pathways in a hisT mutant of Salmonella typhimurium

A comparison was made of the repressibility of certain enzymes in the tyrosine, methionine, and lysine biosynthetic pathways in wild-type Salmonella typhimurium and a hisT mutant. The results show that (i) tyrosine represses the synthesis of the tyrosine-sensitive 3-deoxy-D-arabino-heptulsonic acid 7-phosphate synthetase and the tyrosine aminotransferase to the same extent in a hisT mutant as in wild type and (ii) there is no detectable alteration in the extent to which methionine represses O-succinylhomoserine synthetase or in the extent to which lysine represses the lysine-sensitive beta-aspartokinase as a result of the hisT mutation.

Mechanism of inhibition of Chromatium D growth by L-methionine. Regulation of L-threonine biosynthesis by the intracellular level of S-adenosylmethionine

(1) An unusual accumulation of S-adenosyl-L-methionine in Chromatium D was associated with a marked growth inhibition by L-methionine. The inhibition was overcome by L-isoleucine, L-leucine, L-phyenylalanine, L-threonine, L-valine and putrescien. Based on their effects, these compounds are classified into 3 types. (2) L-Isoleucine, L-leucine, L-phyenylalanine and L-valine (Type I) inhibited the L-methionine uptake and consequently prevented the bacterium from the unusual accumulation of S-adenosyl-L-methionine even in the presence of L-methionine in the medium. Putrescine (Type II) stimulated the consumption of S-adenosyl-L-methionine, but did not influence the L-methionine uptake. Hence, the effect of putrescine would be explained by the action to diminish the intracellular level of S-adenosyl-L-methionine. L-Threonine (Type III) neither inhibited the L-methionine uptake nor affected the content of S-adenoxyl-L-methionine due to the addition of L-methionine. (3) The specific activity of homoserine kinase (EC 2.7.1.39) was greatly lowered by the addition of L-methionine under conditions in which Chromatium D unusually accumulates S-adenoxyl-L-methionine. Homoserine dehydrogenase (EC 1.1.1.3) activity was inhbitied by S-adenosyl-L-methionine (50% inhibition index, 3.5 mM). These facts strongly suggest that the growth inhibition by L-methionine is associated with the L-threonine deficiency caused by the unusual accumulation of S-adenosyl-L-methionine.

Regulation of homocysteine biosynthesis in Salmonella typhimurium

The regulation of the homocysteine branch of the methionine biosynthetic pathway in Salmonella typhimurium has been reexamined with the aid of a new assay for the first enzyme. The activity of this enzyme is subject to synergistic feedback inhibition by methionine plus S-adenosylmethionine. The synthesis of all three enzymes of the pathway is regulated by noncoordinate repression. The enzymes are derepressed in metJ and metK regulatory mutants, suggesting the existence of regulatory elements common to all three. Experiments with a methionine/vitamin B(12) auxotroph (metE) grown in a chemostat on methionine or vitamin B(12) suggested that the first enzyme is more sensitive to repression by methionine derived from exogenous than from endogenous sources. metB and metC mutants grown on methionine in the chemostat did not show hypersensitivity to repression by exogenous methionine. Therefore, it appears that the metE chemostat findings are peculiar to the phenotype of this mutant; such evidence suggests a possible role for a functional methyltetrahydrofolate-homocysteine transmethylase in regulating the synthesis of the first enzyme. Thus there appear to be regulatory elements which are common to the repression of all three enzymes, as well as some that are unique to the first enzyme. The nature of the corepressor is not known, but it may be a derivative of S-adenosylmethionine. metJ and metK mutants of Salmonella have a normal capacity for S-adenosylmethionine synthesis but may be blocked in synthesis or utilization of a corepressor derived from it.

Growth rate of Escherichia coli at elevated temperatures: reversible inhibition of homoserine trans-succinylase

The preceding paper (10) showed that the growth of Escherichia coli is slowed, without killing, at 40 to 45 C, and that in the several strains tested the cause is a decrease in the activity of homoserine trans-succinylase. These temperatures are now shown to inhibit the enzyme directly, in crude extracts and after partial purification. The effect is rapid and is immediately reversible, unlike the progressive and slowly reversible changes of conventional denaturation.

Regulation of S-adenosylmethionine synthetase in Escherichia coli

Addition of methionine to the growth medium of Escherichia coli K-12 leads to a reduction in the specific activity of S-adenosylmethionine (SAM) synthetase. Thus the enzyme appears to be repressible rather than inducible. Mutant strains (probably metJ(-)) are constitutive for SAM synthetase as well as for the methionine biosynthetic enzymes, suggesting that the regulatory systems for these enzymes have at least some elements in common. Cells grown to stationary phase in complete medium, which have low specific activities of the enzymes, were routinely used for derepression experiments. The lag in growth and derepression when these cells are incubated in minimal medium is shortened by threonine. Ethionine, norleucine, and alpha-methylmethionine are poor substrates or nonsubstrates for SAM synthetase and are ineffective repressors. Selenomethionine, a better substrate for SAM synthetase than methionine, is also slightly more effective at repression than methionine. Although SAM is considered to be a likely candidate for the corepressor in the control of the methionine biosynthetic enzymes, addition of SAM to the growth medium does not cause repression. Measurement of SAM uptake shows that too little is taken into the cells to have a significant effect, even if it were active in the control system.

Inhibition of growth of Escherichia coli and of homoserine O-transsuccinylase by alpha-methylmethionine

The methionine analogue, alpha-methylmethionine, inhibits bacterial growth, but its action is overcome by methionine, homocysteine, and cystathionine. The effect of the analogue on growth is attributed to its ability to mimic methionine as a feed-back inhibitor of the first enzyme specific to methionine biosynthesis. This conclusion is based on the findings that (i) alpha-methylmethionine inhibits excretion of O-succinylhomoserine, the product of the first enzyme, by a methionine auxotroph unable to convert succinylhomoserine to cystahionine, and that (ii) the enzyme homoserine O-transsuccinylase is inhibited by alpha-methylmethionine in extracts of Escherichia coli. alpha-Methylmethionine also inhibits methionyl-ribonucleic acid synthetase in extracts, but this inhibition probably does not affect growth.