Valine-sensitive acetohydroxy acid synthases in Escherichia coli K-12: unique regulation modulated by multiple genetic sites.

Summary Spontaneous mutants (146) of Escherichia coli K-12 were selected that were resistant to inhibition of growth by 1.2 mM L-valine (Val^r). The Val^r isolates, containing acetohydroxy acid synthase resistant to feedback inhibition by L-valine (AHAS^r), were classed according to cotransduction of the mutation with leu. Several mutations resulting in an AHAS^r phenotype were found to be cotransducible with glyA. However, no mutations causing a Val^r phenotype were linked to ilv. AHAS activity was more closely examined in representatives of three classes of mutants with Val^r linked to leu, labeled ilv-660, ilv-661, and ilv-662. The ilvE503 allele in E. coli K-12, known to cause a two- to three-fold derepression of AHAS, was found to affect regulation of synthesis of both valine-sensitive AHAS (AHAS^s) and AHAS^r in the mutants containing ilv-660 and ilv-661, whereas it affected repression of AHAS^s, only, in the mutant containing ilv-662. Further, both AHAS^s and AHAS^r in the ilv-661 mutant were repressed by valine, whereas valine did not repress AHAS^r synthesis in the strain carrying ilv-660 and only partially repressed AHAS^r in the strain carrying ilv-662. Unexpectedly, AHAS^r synthesis in strains carrying ilv-660 or ilv-662 was repressible by leucine. The ilv-660 locus appears to be similar in position to ilvH and encodes a product that confers valine-sensitivity upon AHAS activity in the wild-type E. coli K-12. The ilv-660 and ilv-662 loci may normally encode products that influence both the feedback sensitivity of AHAS and control of AHAS biosynthesis.     

Regulation of Transaminase C Synthesis in Escherichia coli: Conditional Leucine Auxotrophy

The regulation of synthesis of the valine-alanine-alpha-aminobutyrate transaminase (transaminase C) was studied in Escherichia coli mutants lacking the branched-chain amino acid transaminase (transaminase B). An investigation was made of two strains, CU2 and CU2002, each carrying the same transaminase B lesion but exhibiting different growth responses on a medium supplemented with branched-chain amino acids. Both had the absolute isoleucine requirement characteristic of ilvE auxotrophs, but growth of strain CU2 was stimulated by valine, whereas that of strain CU2002 was markedly inhibited by valine. Strain CU2002 behaved like a conditional leucine auxotroph in that the inhibition by valine was reversed by leucine. Results of enzymatic studies showed that synthesis of transaminase C was repressed by valine in strain CU2002 but not in strain CU2. Inhibition by valine in strain CU2002 appears to be the combined effect of repression on transaminase C synthesis and valine-dependent feedback inhibition of alpha-acetohydroxy acid synthase activity, causing alpha-ketoisovalerate (and hence leucine) limitation. The ilvE markers of strains CU2 and CU2002 were each transferred by transduction to a wild-type genetical background. All ilvE recombinants from both crosses resembled strain CU2002 and were inhibited by valine in the presence of isoleucine. Thus, strain CU2 carries an additional lesion that allows it to grow on a medium containing isoleucine plus valine. It is concluded that conditional leucine auxotrophy is characteristic of mutants carrying an ilvE lesion alone.     

Evidence for isoleucine as a positive effector of the ilvBN operon in Salmonella typhimurium

Concerted efforts were directed towards understanding the control of acetohydroxy acid synthase (AHAS) in the gyrB mutant hisU1820 of Salmonella typhimurium. A media shift from valine to valine plus isoleucine causes a dramatic 4 to 5 fold burst of AHAS valine sensitive activity which appears to be dependent on translation. DJ19, an isolated valine sensitive derivative of the gyrB mutant, maintains a dramatic increase in AHAS valine sensitive activity upon the addition of isoleucine to valine supplemented cultures, suggesting that the isoleucine effect is specific for valine sensitive AHAS. Evidence supports isoleucine as a positive effector on valine sensitive AHAS expression and that the gyrB mutation accentuates the isoleucine effect.     

Regulation of acetohydroxy acid synthase activities in adenyl cyclase-deficient strains of Escherichia coli K-12

Previous findings suggested that cyclic AMP was involved in the regulation of ilvB(AHASI) only and that ilvG (AHASII) and ilvHI (AHASIII) were not controlled by this nucleotide. In this study, derepression patterns of total AHAS activities (ilvB and ilvHI) in adenyl cyclase-negative strains (i.e. cya-) were substantially reduced as contrasted with AHAS activity observed for cya+ strains. Further, the parental strains (cya+) consistently exhibited higher levels of AHAS activity than mutant strains (cya-) during carbon and energy downshifts. Other data suggested that the valine derepression signal could not override the necessity for cya gene product to yield maximal derepression of AHAS gene activities. Cyclic AMP stimulated AHAS gene activities under both in vivo and in vitro assay conditions. Thus, these data provide evidence for an absolute requirement of cAMP for maximal expression of the genes encoding for AHAS activities of E. coli K-12.     

Metabolic Basis for the Isoleucine, Pantothenate or Methionine Requirement of ilvG Strains of Salmonella typhimurium

Salmonella typhimurium strain DU501, which was found to be deficient in acetohydroxy acid synthase II (AHAS II) and to possess elevated levels of transaminase B and biosynthetic threonine deaminase, required isoleucine, methionine, or pantothenate for growth. This strain accumulated α-ketobutyrate and, to a lesser extent, α-aminobutyrate. We found that α-ketobutyrate was a competitive substrate for ketopantoate hydroxymethyltransferase, the first enzyme in pantothenate biosynthesis. This competition with the normal substrate, α-ketoisovalerate, limited the supply of pantothenate, which resulted in a requirement for methionine. Evidence is presented to support the conclusion that the ambivalent requirement for either pantothenate or methionine is related to a decrease in succinyl coenzyme A, which is produced from pantothenate and which is an obligatory precursor of methionine biosynthesis. The autointoxification by endogenously produced α-ketobutyrate could be mimicked in wild-type S. typhimurium by exogenously supplied α-ketobutyrate or salicylate, a known inhibitor of pantothenate biosynthesis. The accumulation of α-ketobutyrate was initiated by the inability of the residual AHAS activity provided by AHAS I to efficiently remove the α-ketobutyrate produced by biosynthetic threonine deaminase. The accumulation of α-ketobutyrate was amplified by the action of transaminase B, which decreased the isoleucine pool by catalyzing the formation of α-keto-β-methylvalerate and aminobutyrate from isoleucine and α-ketobutyrate; this resulted in release of threonine deaminase from end product inhibition and unbridled production of α-ketobutyrate. Isoleucine satisfied the auxotrophic requirement of the AHAS II-deficient strain by curtailing the activity of threonine deaminase. Additional lines of evidence based on genetic and physiological experiments are presented to support the basis for the autointoxification of strain DU501 as well as other nonpolarigenic ilvG mutant strains.     

Multivalent regulation of isoleucine-valine transaminase in an Escherichia coli K-12 ilvA deletion strain.

In a strain of Escherichia coli K-12 lacking threonine deaminase, the enzyme converting alpha-ketoisovalerate and alpha-keto-beta-methylvalerate to valine and isoleucine, respectively, was multivalently repressed by valine, isoleucine, and leucine. This activity was due to transaminase B, specified by the ilvE structural gene.     

The acetohydroxy acid synthase III isoenzyme of Escherichia coli K-12: regulation of synthesis by leucine.

Acetohydroxy acid synthase III (AHAS III) is one of the three isoenzymes which catalyze the condensation reaction for the biosynthesis of the branched chain amino acids in $\text{Escherichia coli}$ K-12. The synthesis of this enzyme is repressed by leucine. As a consequence of this regulatory feature, strain PS1035, in which AHAS III is the only AHAS isoenzyme expressed, does not grow in minimal medium containing leucine. The other two branched chain amino acids, isoleucine and valine, do not have regulatory effects on AHAS III synthesis.     

Branched-Chain Amino Acid Biosynthesis in Salmonella typhimurium: a Quantitative Analysis

We report here the first quantitative study of the branched-chain amino acid biosynthetic pathway in Salmonella typhimurium LT2. The intracellular levels of the enzymes of the pathway and of the 2-keto acid intermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multiple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitness of enterobacteria. In wild-type LT2, AHAS isozyme I provides most of the flux to valine, leucine, and pantothenate, while isozyme II provides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the low enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AHAS I only is limited during growth on glucose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine deaminase activity and an increased 2-ketobutyrate/2-ketoisovalerate ratio, which in turn interferes with the synthesis of coenzyme A and methionine. The regulation of threonine deaminase is also crucial in this regard. It is conceivable that, because of fundamental limitations on the specificity of enzymes, no single AHAS could possibly be adequate for the varied conditions that enterobacteria successfully encounter.     

Acetohydroxyacid synthase: a proposed structure for regulatory subunits supported by evidence from mutagenesis

Valine inhibition of acetohydroxyacid synthase (AHAS) plays an important role in regulation of biosynthesis of branched-chain amino acids in bacteria. Bacterial AHASs are composed of separate catalytic and regulatory subunits; while the catalytic subunits appear to be homologous with several other thiamin diphosphate-dependent enzymes, there has been no model for the structure of the small, regulatory subunits (SSUs). AHAS III is one of three isozymes in Escherichia coli. Its large subunit (encoded by ilvI) by itself has 3-5 % activity of the holoenzyme and is not sensitive to inhibition by valine. The SSU (encoded by ilvH) associates with the large subunit and is required for full catalytic activity and valine sensitivity. The isolated SSU binds valine. The properties of several mutant SSUs shed light on the relation between their structure and regulatory function. Three mutant SSUs were obtained from spontaneous Val(R) bacterial mutants and three more were designed on the basis of an alignment of SSU sequences from valine-sensitive and resistant isozymes, or consideration of the molecular model developed here. Mutant SSUs N11A, G14D, N29H and A36V, when reconstituted with wild-type large subunit, lead to a holoenzyme with drastically reduced valine sensitivity, but with a specific activity similar to that of the wild-type. The isolated G14D and N29H subunits do not bind valine. Mutant Q59L leads to a valine-sensitive holoenzyme and isolated Q59L binds valine. T34I has an intermediate valine sensitivity. The effects of mutations on the affinity of the large subunits for SSUs also vary. D. Fischer's hybrid fold prediction method suggested a fold similarity between the N terminus of the ilvH product and the C-terminal regulatory domain of 3-phosphoglycerate dehydrogenase. On the basis of this prediction, together with the properties of the mutants, a model for the structure of the AHAS SSUs and the location of the valine-binding sites can be proposed.     

Metabolic effects of inhibitors of two enzymes of the branched-chain amino acid pathway in Salmonella typhimurium.

The metabolic effects of inhibitors of two enzymes in the pathway for biosynthesis of branched-chain amino acids were examined in Salmonella typhimurium mutant strain TV105, expressing a single isozyme of acetohydroxy acid synthase (AHAS), AHAS isozyme II. One inhibitor was the sulfonylurea herbicide sulfometuron methyl (SMM), which inhibits this isozyme and AHAS of other organisms, and the other was N-isopropyl oxalylhydroxamate (IpOHA), which inhibits ketol-acid reductoisomerase (KARI). The effects of the inhibitors on growth, levels of several enzymes of the pathway, and levels of intermediates of the pathway were measured. The intracellular concentration of the AHAS substrate 2-ketobutyrate increased on addition of SMM, but a lack of correlation between increased ketobutyrate and growth inhibition suggests that the former is not the immediate cause of the latter. The levels of the keto acid precursor of valine, but not of the precursor of isoleucine, were drastically decreased by SMM, and valine, but not isoleucine, partially overcame SMM inhibition. This apparent stronger effect of SMM on the flux into the valine arm, as opposed to the isoleucine arm, of the branched-chain amino acid pathway is explained by the kinetics of the AHAS reaction, as well as by the different roles of pyruvate, ketobutyrate, and the valine precursor in metabolism. The organization of the pathway thus potentiates the inhibitory effect of SMM. IpOHA has strong initial effects at lower concentrations than does SMM and leads to increases both in the acetohydroxy acid substrates of KARI and, surprisingly, in ketobutyrate. Valine completely protected strain TV105 from IpOHA at the MIC. A number of explanations for this effect can be ruled out, so that some unknown arrangement of the enzymes involved must be suggested. IpOHA led to initial cessation of growth, with partial recovery after a time whose duration increased with the inhibitor concentration. The recovery is apparently due to induction of new KARI synthesis, as well as disappearance of IpOHA from the medium.     

Characterisation of the enzyme activities involved in the valine biosynthetic pathway in a valine-producing strain of Corynebacterium glutamicum

The enzyme activities of the valine biosynthetic pathway and their regulation have been studied in the valine-producing strain, Corynebacterium glutamicum 13032DeltailvApJC1ilvBNCD. In this micro-organism, this pathway might involve up to five enzyme activities: acetohydroxy acid synthase (AHAS), acetohydroxy acid isomeroreductase (AHAIR), dihydroxyacid dehydratase and transaminases B and C. For each enzyme, kinetic parameters (optimal temperature, optimal pH and affinity for substrates) were determined. The first enzyme of the pathway, AHAS, was shown to exhibit a weak affinity for pyruvate (K(m)=8.3 mM). It appeared that valine and leucine inhibited the three first steps of the pathway (AHAS, AHAIR and DHAD). Moreover, the AHAS activity was inhibited by isoleucine. Considering the kinetic data collected during this work, AHAS would be a key enzyme for further strain improvement intending to increase the valine production by C. glutamicum.     

Enzymes of the Isoleucine-Valine Pathway in Acinetobacter

Regulation of four of the enzymes required for isoleucine and valine biosynthesis in Acinetobacter was studied. A three- to fourfold derepression of acetohydroxyacid synthetase was routinely observed in two different wild-type strains when grown in minimal medium relative to cells grown in minimal medium supplemented with leucine, valine, and isoleucine. A similar degree of synthetase derepression was observed in appropriately grown isoleucine or leucine auxotrophs. No significant derepression of threonine deaminase or transaminase B occurred in either wild-type or mutant cells grown under a variety of conditions. Three amino acid analogues were tested with wild-type cells; except for a two- to threefold derepression of dihydroxyacid dehydrase when high concentrations of aminobutyric acid were added to the medium, essentially the same results were obtained. Experiments showed that threonine deaminase is subject to feedback inhibition by isoleucine and that valine reverses this inhibition. Cooperative effects in threonine deaminase were demonstrated with crude extracts. The data indicate that the synthesis of isoleucine and valine in Acinetobacter is regulated by repression control of acetohydroxyacid synthetase and feedback inhibition of threonine deaminase and acetohydroxyacid synthetase.     

Effects of amino acids on Thiobacillus acidophilus. I. Growth studies with special reference to valine.

The heterotrophic growth of Thiobacillus acidophilus was inhibited by branched-chain amino acids; valine, isoleucine, and leucine. The inhibition by valine and leucine were partially reversed by isoleucine, and the inhibition by isoleucine was partially reversed by valine. Inhibitions by methionine or threonine were partially reversed when both amino acids were present in the growth medium. Inhibition by tyrosine was increased by phenylalanine or tryptophan. Cystine completely inhibited growth. Other amino acids tested produced little or no inhibition. Acetohydroxy acid synthetase (AHAS) activity was demonstrated in crude extracts of T. acidophilus. In crude extracts the optimum pH was 8.5 with a shift to 9.0 in the presence of valine. Valine was the only branched-chain amino acid which inhibited the AHAS activity. The presence of only one peak of AHAS activity upon centrifugation in linear glycerol density gradients demonstrated that the AHAS activity sediments as one component.     

On the role of isoleucyl-tRNA synthetase in multivalent repression

An isoleucine auxotroph of Salmonella typhimurium was derived from a merodiploid strain (containing the F-14 episome from Escherichia coli) that contained two copies of the structural genes concerned with isoleucine and valine biosynthesis. A haploid derivative, strain TU6001, having the same growth properties as the original merodiploid mutant was found to have normal biosynthetic enzymes and an altered isoleucyl-tRNA synthetase. The K _m for isoleucine was increased by about 200-fold over that for the wild-type enzyme. All five enzymes in the isoleucine and valine biosynthetic pathway were derepressed relative to wild-type enzyme levels. A partial revertant of strain TU6001 was isolated which had properties that were intermediate between those of the mutant and the wild type (i.e., intermediate growth dependence on exogenous isoleucine, intermediate activity of isoleucyl-tRNA synthetase, and intermediate derepression of biosynthetic enzymes). The properties of strain TU6001 were demonstrated to be simultaneously transferable by transduction (using P_LT22 H₄ bacteriophage) of a single genetic locus, linked to pyr A, which has been designated ilv S. It is concluded that some function of the isoleucyl-tRNA synthetase is important in repression of the isoleucine and valine biosynthetic enzymes.Supported by grant GM 12522 from the National Institute of General Medical Sciences, U.S. Public Health Service. J. M. B. received a U.S. Public Health Service Postdoctoral Fellowship 1-F02-GM-30, 650-02.     

Deregulated branched-chain amino acid synthesis in a Nicotiana plumbaginifolia cell line resistant to valine

A Nicotiana plumbaginifolia cell line able to grow in the presence of high doses of valine was isolated following -rays mutagenesis. The selected clone, named D5R5, showed a growth rate higher than that of wild-type. It was less sensitive also to an equimolar mixture of the three branched-chain amino acids, but did not display cross-resistance to isoleucine and leucine. The increased tolerance was due to neither a reduced valine uptake, nor a modification in the level or sensitivity to feed-back inhibition by valine of the first common enzyme (and the main regulative site) in isoleucine, leucine and valine synthesis, acetohydroxyacid synthase (AHAS). When wild-type cells were fed with valine or equimolar mixtures of the three aminoacids, a decrease in AHAS level was found. On the contrary, the level of extractable AHAS activity from D5R5 cells was significantly less affected by similar treatments, suggesting that some alteration in enzyme modulation mechanism(s) could account for valine resistance.Abbreviations AHAS acetohydroxyacid synthase - BCAA branched-chain amino acid - FAD flavin adenine dinucleotide - ILV equimolar mixture of isoleucine, leucine and valine - TPP thiamine pyrophosphate     

A role for a pyridoxne derivative in the multivalent repression of the isoleucine and valine biosynthetic enzymes

Starvation of a pdx mutant of Escherichia coli strain B in the presence of repressing levels of isoleucine, valine and leucine leads to a derepression of the normally repressible ilv genes. The derepression of the ilvA gene under these conditions results in the accumulation of apothreonine deaminase. Addition of pyridoxine leads to a sudden increase in threonine deaminase activity, and to restoration of repression. The pyridoxine component needed for the repression signal is probably not threonine deaminase but, more likely, some transient (“immature”) form of the enzyme.     

Arabidopsis Acetohydroxyacid Synthase Expressed in Escherichia coli Is Insensitive to the Feedback Inhibitors

Acetohydroxyacid synthase (AHAS), the first enzyme unique to the biosynthesis of isoleucine, leucine, and valine, is the target enzyme for several classes of herbicides. The AHAS gene from Arabidopsis thaliana, including the chloroplast transit peptide, was cloned into the bacterial expression plasmid pKK233-2. The resulting plasmid was used to transform an AHAS-deficient Escherichia coli strain MF2000. The growth of the MF2000 strain of E. coli was complemented by the functional expression of the Arabidopsis AHAS. The AHAS protein was processed to a molecular mass of 65 kilodaltons that was similar to the mature protein isolated from Arabidopsis seedlings. The AHAS activity extracted from the transformed E. coli cells was inhibited by imidazolinone and sulfonylurea herbicides. AHAS activity extracted from Arabidopsis is inhibited by valine and leucine; however, this activity was insensitive to these feedback inhibitors when extracted from the transformed E. coli.