END-PRODUCT REGULATION OF THE GENERAL AROMATIC-PATHWAY IN ESCHERICHIA COLI W

Influence of Amino Acid Feeding on Production of Calcimycin and Analogs in Streptomyces chartreusis

Streptomyces chartreusis NRRL 3882 produces the polyether ionophore calcimycin and a variety of analogs, which originate from the same biosynthetic gene cluster. The role of calcimycin and its analogs for the producer is unknown, but calcimycin has strong antibacterial activity. Feeding experiments were performed in chemically defined medium systematically supplemented with proteinogenic amino acids to analyze their individual effects on calcimycin synthesis. In the culture supernatants, in addition to known calcimycin analogs, eight so far unknown analogs were detected using LC-MS/MS. Under most conditions cezomycin was the compound produced in highest amounts. The highest production of calcimycin was detected upon feeding with glutamine. Supplementation of the medium with glutamic acid resulted in a decrease in calcimycin production, and supplementation of other amino acids such as tryptophan, lysine, and valine resulted in the decrease in the synthesis of calcimycin and of the known intermediates of the biosynthetic pathway. We demonstrated that the production of calcimycin and its analogs is strongly dependent on amino acid supply. Utilization of amino acids as precursors and as nitrogen sources seem to critically influence calcimycin synthesis. Even amino acids not serving as direct precursors resulted in a different product profile regarding the stoichiometry of calcimycin analogs. Only slight changes in cultivation conditions can lead to major changes in the metabolic output, which highlights the hidden potential of biosynthetic gene clusters. We emphasize the need to further study the extent of this potential to understand the ecological role of metabolite diversity originating from single biosynthetic gene clusters.

Substrate deactivation of phenylalanine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase by erythrose 4-phosphate

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS, EC 4.1.2.15) catalyzes the condensation of phosphoenolpyruvate (PEP) with erythrose 4-phosphate (E4P) to give DAH7P via an ordered sequential mechanism. In the absence of PEP (the first substrate to bind), E4P binds covalently to the phenylalanine-sensitive DAH7PS of Escherichia coli, DAH7PS(Phe), deactivating the enzyme. Activity is restored on addition of excess PEP but not if deactivation was carried out in the presence of sodium cyanoborohydride. Electrospray mass spectrometry indicates that a single E4P is bound to the protein. These data are consistent with a slow, reversible Schiff base reaction of the aldehydic functionality of E4P with a buried lysine. Molecular modeling indicates that Lys186, a residue at the base of the substrate-binding cavity involved in hydrogen bonding with PEP, is well placed to react with E4P forming an imine linkage that is substantially protected from solvent water.

Comparative action of glyphosate as a trigger of energy drain in eubacteria

Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa, each possessing a 5-enolpyruvylshikimate 3-phosphate synthase that is sensitive to inhibition by glyphosate [N-(phosphonomethyl)glycine], provide a good cross-section of organisms exemplifying the biochemical diversity of the aromatic pathway targeted by this potent antimicrobial compound. The pattern of growth inhibition, the alteration in levels of aromatic-pathway enzymes, and the accumulation of early-pathway metabolites after the addition of glyphosate were distinctive for each organism. Substantial intracellular shikimate-3-phosphate accumulated in response to glyphosate treatment in all three organisms. Both E. coli and P. aeruginosa, but not B. subtilis, accumulated near-millimolar levels of shikimate-3-phosphate in the culture medium. Intracellular backup of common-pathway precursors of shikimate-3-phosphate was substantial in B. subtilis, moderate in P. aeruginosa, and not detectable in E. coli. The full complement of aromatic amino acids prevented growth inhibition and metabolite accumulation in E. coli and P. aeruginosa where amino acid end products directly control early-pathway enzyme activity. In contrast, the initial prevention of growth inhibition in the presence of aromatic amino acids in B. subtilis was succeeded by progressively greater growth inhibition that correlated with rapid metabolite accumulation. In B. subtilis glyphosate can decrease prephenate concentrations sufficiently to uncouple the sequentially acting loops of feedback inhibition that ordinarily link end product excess to feedback inhibition of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase by prephenate. The consequential unrestrained entry is an energy-rich substrates into the aromatic pathway, even in the presence of aromatic amino acid end products, is an energy drain that potentially accounts for the inability of end products to fully reverse glyphosate inhibition in B. subtilis. Even in E. coli after glyphosate inhibition and metabolite accumulation were allowed to become fully established, a transient period where end products were capable of only partial reversal of growth inhibition occurred. The distinctive metabolism produced by dissimilation of different carbon sources also profound effects upon glyphosate sensitivity.

Purification and properties of tyrosine-sensitive 3-deoxy-D-arabino-heptolosonate-7-phosphate synthetase of Escherichia coli K12

1. The tyrosine-sensitive allosteric first enzyme of the aromatic amino acid biosynthetic pathway, 3-deoxy-D-arabinoheptulosonate 7-phosphate synthetase (7-Phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose 4-phosphate-lyase (pyruvate phosphorylating), EC 4.1.2.15) has been purified from a mutant strain of Escherichia coli. 2. The enzyme activity was inhibited to 50% at 2-10(-5) M tyrosine and to 90% at 2-10(-4) M tyrosine concentration. At tyrosine concentrations lower than 2-10(-5) M a cooperative interaction between tyrosine binding sites was observed. 3. Co2+ increased the enzyme activity about 2-2.5-fold. The presence of Co2+ ions stabilized the enzyme. EDTA inhibited the enzyme activity, and this inhibition was reversed by Co2+. Tyrosine-sensitive DAHP synthetase seems to be a metal containing enzyme. 4. Kinetic experiments were carried out to study the catalytic action. Contrary to earlier suggestions it is concluded, that the reaction mechanism appears to be more complex--with either the ping-pong or sequential type predominating, depending on conditions.

Purification and properties of 3-deoxy-D-arabionheptulosonic acid-7-phosphate synthetase (trp) from Escherichia coli

The 3-deoxy-d-arabinoheptulosonic acid-7-phosphate synthetase which is subject to regulation by tryptophan has been partially purified from a strain of Escherichia coli K-12, in which this is the only functional form of this enzyme activity, and from a similar strain possessing a feedback-resistant form of the enzyme. Maximal observed inhibition by tryptophan of the feedback-sensitive enzyme was 56%. There was no evidence for cooperativity in the saturation of the enzyme with tryptophan or E4P. The molecular weights of the feedback-sensitive and feedback-resistant forms of the enzyme were the same (52,000), and no change was detected in the molecular weight of the feedback-sensitve enzyme in the presence of tryptophan. The effect of tryptophan analogues was tested to determine the nature of the tryptophan binding site. Treatment with ethylenediaminetetraacetic acid removed 80% of the activity of the feedback-sensitive enzyme. This activity was restored upon the addition of Co(2+) or Mn(2+). Neither treatment with ethylenediaminetetraacetic acid nor addition of Co(2+) or Mn(2+) affected the activity of the feedback-resistant enzyme.

Regulation of 3-deoxy-D-arabino-heptulosonic 7-phosphate acid synthetase activity in relation to the synthesis of the aromatic vitamins in Escherichia coli K-12

Both in vivo and in vitro experiments on wild-type Escherichia coli K-12 and mutant strains possessing only single 3-deoxy-d-arabino-heptulosonic 7-phosphate acid (DAHP) synthetase isoenzymes indicated that, under conditions when all three isoenzymes are fully repressed, sufficient chorismate is still formed for the synthesis of aromatic vitamins. Under repressed conditions both DAHP synthetase (phe) and (trp), but not DAHP synthetase (tyr), were shown to contribute to vitamin production.

Regulator gene controlling enzymes concerned in tyrosine biosynthesis in Escherichia coli

Mutants of Escherichia coli K-12 have been isolated in which several enzymes concerned with tyrosine biosynthesis are derepressed. These mutants were obtained from a parent strain possessing only a single 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase isoenzyme, DAHP synthetase (tyr), by selecting for resistance to the tyrosine analogue, 4-aminophenylalanine. The mutation responsible for this derepression has been mapped and the gene, which is not closely linked to aroF and tyrA, has been designated tyrR.

Inhibition of 3-deoxy-d-arabinoheptulosonic acid-7-phosphate synthetase (trp) in Escherichia coli

The growth of a strain of Escherichia coli K-12 which possesses only the single isoenzyme 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase (trp), and which makes this enzyme constitutively, is inhibited by tryptophan. The accumulation of DAHP by a derivative of this strain unable to convert DAHP to dehydroquinate is also inhibited by tryptophan. The enzymic activity of DAHP synthetase (trp) in the presence of CO(2+) is sensitive to inhibition by tryptophan. Mutant strains of E. coli have been isolated in which DAHP synthetase (trp) is feedback-resistant.

Comparative regulation of isoenzymic 3-deoxy-D-arabino-heptulosonate 7-phosphate synthetases in microorganisms

The independent control of regulatory isoenzymes by different metabolites constitutes one well-known pattern of control in branched metabolic pathways. This pattern was previously found to be widely distributed in the aromatic amino acid pathway of microorganisms in the case of the first enzyme of the sequence, 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase. The comparative stability of the isoenzymes as well as the effect of aromatic amino acids in the growth medium upon the levels of the individual isoenzymes were shown for Salmonella typhimurium. Several lines of evidence are discussed to demonstrate the strong reliance of Escherichia coli upon the phenylalanine-sensitive isoenzyme for the ordinary biosynthetic needs of wild-type strains. The frequent occurrence of "dominant" isoenzyme species which resist repressive effects of the inhibitory end products was noted. The lack of an obligatory correlation of the level of an isoenzyme activity and the synthesis of the end product which specifically controls its activity is used to discount the possibility that each isoenzyme might feed a unique and separate metabolic pool of end-product precursor. An isoenzymic DAHP synthetase sensitive to feedback inhibition by low levels of tryptophan was fractionated from tyrosine- and phenylalanine-sensitive isoenzymes in cell-free extracts of Neurospora crassa.

Comparative control of a branch-point enzyme in microorganisms

Thirty-two genera of microorganisms were identified with one of six distinctive control patterns for the enzyme 3-deoxy-d-arabino-heptulosonate-7-phosphate synthetase. These patterns included sequential feedback inhibition, isoenzyme feedback inhibition, cumulative feedback inhibition, and three (apparent) simple one-effector patterns. Documentation is provided of an overwhelming tendency for control patterns to be strongly conserved among the member species of the various genera that were examined.

Genetic and biochemical analysis of the isoenzymes concerned in the first reaction of aromatic biosynthesis in Escherichia coli

Mutant strains of Escherichia coli K-12 were isolated possessing mutations which affected the tyrosine-inhibitable 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase, the phenylalanine-inhibitable DAHP synthetase, or the tryptophan-repressible DAHP synthetase. The mutations causing the loss of each of these activities have been mapped and are widely separated from each other on the E. coli chromosome. Chromatography on diethylaminoethyl cellulose columns allowed the recognition of four peaks of activity.

Distribution and function of genes concerned with aromatic biosynthesis in Escherichia coli

Pittard, James (School of Microbiology, University of Melbourne, Victoria, Australia), and B. J. Wallace. Distribution and function of genes concerned with aromatic biosynthesis in Escherichia coli. J. Bacteriol. 91:1494-1508. 1966.-A number of mutant strains of Escherichia coli K-12, which are blocked in the biosynthesis of the aromatic amino acids, were examined biochemically to determine their particular enzymatic deficiencies. The mutations carried by these strains were mapped by use of the methods of conjugation and transduction. Structural genes for five of the enzymes of the common pathway leading to chorismate and for the two enzymes converting chorismate to phenylpyruvate and p-hydroxyphenylpyruvate, respectively, were identified. Unlike the genes of the tryptophan operon most of these genes are distributed over widely separated regions of the chromosome.

Tryptophan- and indole-excreting prototrophic mutant of Escherichia coli

Lim, P. G. (Massachusetts Institute of Technology, Cambridge), and R. I. Mateles. Tryptophan- and indole-excreting prototrophic mutant of Escherichia coli. J. Bacteriol. 87:1051-1055. 1964.-A mutant of Escherichia coli K-12, capable of excreting 350 mg of indole and 50 mg of tryptophan per liter when grown on minimal medium, was found to have a level of 3-deoxy-d-arabino-heptulosonic acid-7-phosphate (DAHP) synthetase 60% higher than the parent, and to have a 10- to 15-fold elevation of the levels of enzymes in the tryptophan branch of the pathway for aromatic amino acid biosynthesis. Contrary to what previous investigators found in E. coli W, the presence of a tyrosine-repressible component of DAHP synthetase sensitive to end-product inhibition by tyrosine could not be demonstrated in either strain K-12 or the mutant. The mutant strain is an example of a microorganism which excretes biosynthetic end products solely because of genetic derepression, as opposed to most previously reported amino acid accumulators which require a combination of genetic and physiological manipulation to achieve derepression.