THE BIOSYNTHESIS OF LEUCINE. III. THE CONVERSION OF ALPHA-HYDROXY-BETA-CARBOXYISOCAPROATE TO ALPHA-KETOISOCAPROATE

Characterization of BLUF-photoreceptors present in Acinetobacter nosocomialis

Acinetobacter nosocomialis is a Gram-negative opportunistic pathogen, whose ability to cause disease in humans is well recognized. Blue light has been shown to modulate important physiological traits related to persistence and virulence in this microorganism. In this work, we characterized the three Blue Light sensing Using FAD (BLUF) domain-containing proteins encoded in the A. nosocomialis genome, which account for the only canonical light sensors present in this microorganism. By focusing on a light-modulated bacterial process such as motility, the temperature dependence of light regulation was studied, as well as the expression pattern and spectroscopic characteristics of the different A. nosocomialis BLUFs. Our results show that the BLUF-containing proteins AnBLUF65 and AnBLUF46 encode active photoreceptors in the light-regulatory temperature range when expressed recombinantly. In fact, AnBLUF65 is an active photoreceptor in the temperature range from 15°C to 37°C, while AnBLUF46 between 15°C to 32°C, in vitro. In vivo, only the Acinetobacter baumannii BlsA’s ortholog AnBLUF65 was expressed in A. nosocomialis cells recovered from motility plates. Moreover, complementation assays showed that AnBLUF65 is able to mediate light regulation of motility in A. baumannii ΔblsA strain at 30°C, confirming its role as photoreceptor and in modulation of motility by light. Intra-protein interactions analyzed using 3D models built based on A. baumannii´s BlsA photoreceptor, show that hydrophobic/aromatic intra-protein interactions may contribute to the stability of dark/light- adapted states of the studied proteins, reinforcing the previous notion on the importance of these interactions in BLUF photoreceptors. Overall, the results presented here reveal the presence of BLUF photoreceptors in A. nosocomialis with idiosyncratic characteristics respect to the previously characterized A. baumannii’s BlsA, both regarding the photoactivity temperature-dependency as well as expression patterns, contributing thus to broaden our knowledge on the BLUF family.

A closer look on the polyhydroxybutyrate- (PHB-) negative phenotype of Ralstonia eutropha PHB-4

The undefined poly(3-hydroxybutyrate)- (PHB-) negative mutant R. eutropha PHB^-4 was generated in 1970 by 1-nitroso-3-nitro-1-methylguanidine (NMG) treatment. Although being scientific relevant, its genotype remained unknown since its isolation except a recent first investigation. In this study, the mutation causing the PHA-negative phenotype of R. eutropha PHB^-4 was confirmed independently: sequence analysis of the phaCAB operon identified a G320A mutation in phaC yielding a stop codon, leading to a massively truncated PhaC protein of 106 amino acids (AS) in R. eutropha PHB^-4 instead of 589 AS in the wild type. No other mutations were observed within the phaCAB operon. As further mutations probably occurred in the genome of mutant PHB^-4 potentially causing secondary effects on the cells' metabolism, the main focus of the study was to perform a 2D PAGE-based proteome analysis in order to identify differences in the proteomes of the wild type and mutant PHB^-4. A total of 20 differentially expressed proteins were identified which provide valuable insights in the metabolomic changes of mutant PHB^-4. Besides excretion of pyruvate, mutant PHB^-4 encounters the accumulation of intermediates such as pyruvate and acetyl-CoA by enhanced expression of the observed protein species: (i) ThiJ supports biosynthesis of cofactor TPP and thereby reinforces the 2-oxoacid dehydrogenase complexes as PDHC, ADHC and OGDHC in order to convert pyruvate at a higher rate and the (ii) 3-isopropylmalate dehydrogenase LeuB3 apparently directs pyruvate to synthesis of several amino acids. Different (iii) acylCoA-transferases enable transfer reactions between organic acid intermediates, and (iv) citrate lyase CitE4 regenerates oxaloacetate from citrate for conversion with acetyl-CoA in the TCC in an anaplerotic reaction. Substantial amounts of reduction equivalents generated in the TCC are countered by (v) synthesis of more ubiquinones due to enhanced synthesis of MenG2 and MenG3, thereby improving the respiratory chain which accepts electrons from NADH and succinate.

A synthetic recursive "+1" pathway for carbon chain elongation

Nature uses four methods of carbon chain elongation for the production of 2-ketoacids, fatty acids, polyketides, and isoprenoids. Using a combination of quantum mechanical (QM) modeling, protein-substrate modeling, and protein and metabolic engineering, we have engineered the enzymes involved in leucine biosynthesis for use as a synthetic "+1" recursive metabolic pathway to extend the carbon chain of 2-ketoacids. This modified pathway preferentially selects longer-chain substrates for catalysis, as compared to the non-recursive natural pathway, and can recursively catalyze five elongation cycles to synthesize bulk chemicals, such as 1-heptanol, 1-octanol, and phenylpropanol directly from glucose. The "+1" chemistry is a valuable metabolic tool in addition to the "+5" chemistry and "+2" chemistry for the biosynthesis of isoprenoids, fatty acids, or polyketides.

Crystal structure of 3-isopropylmalate dehydrogenase in complex with NAD(+) and a designed inhibitor

Isopropylmalate dehydrogenase (IPMDH) is the third enzyme specific to leucine biosynthesis in microorganisms and plants, and catalyzes the oxidative decarboxylation of (2R,3S)-3-isopropylmalate to alpha-ketoisocaproate using NAD(+) as an oxidizing agent. In this study, a thia-analogue of the substrate was designed and synthesized as an inhibitor for IPMDH. The analogue showed strong competitive inhibitory activity with K(i)=62nM toward IPMDH derived from Thermus thermophilus. Moreover, the crystal structure of T. thermophilus IPMDH in a ternary complex with NAD(+) and the inhibitor has been determined at 2.8A resolution. The inhibitor exists as a decarboxylated product with an enol/enolate form in the active site. The product interacts with Arg 94, Asn 102, Ser 259, Glu 270, and a water molecule hydrogen-bonding with Arg 132. All interactions between the product and the enzyme were observed in the position associated with keto-enol tautomerization. This result implies that the tautomerization step of the thia-analogue during the IPMDH reaction is involved in the inhibition.

A lysine-tyrosine pair carries out acid-base chemistry in the metal ion-dependent pyridine dinucleotide-linked beta-hydroxyacid oxidative decarboxylases

This work reviews published structural and kinetic data on the pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases. The family of metal ion-dependent pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases can be divided into two structural families with the malic enzyme, which has an (S)-hydroxyacid substrate, comprising one subfamily and isocitrate dehydrogenase, isopropylmalate dehydrogenase, homoisocitrate dehydrogenase, and tartrate dehydrogenase, which have an (R)-hydroxyacid substrate, comprising the second subclass. Multiple-sequence alignment of the members of the (R)-hydroxyacid family indicates a high degree of sequence identity with most of the active site residues conserved. The three-dimensional structures of the members of the (R)-hydroxyacid family with structures available superimpose on one another, and the active site structures of the enzymes have a similar overall geometry of residues in the substrate and metal ion binding sites. In addition, a number of residues in the malic enzyme active site are also conserved, and the arrangement of these residues has a similar geometry, although the (R)-hydroxyacid and (S)-hydroxyacid family sites are geometrically mirror images of one another. The active sites of the (R)-hydroxyacid family have a higher positive charge density when compared to those of the (S)-hydroxyacid family, largely due to the number of arginine residues in the vicinity of the substrate alpha-carboxylate and one fewer carboxylate ligand to the divalent metal ion. Data available for all of the enzymes in the family have been considered, and a general mechanism that makes use of a lysine (general base)-tyrosine (general acid) pair is proposed. Differences exist in the mechanism for generating the neutral form of lysine so that it can act as a base.

Enzymology and evolution of the pyruvate pathway to 2-oxobutyrate in Methanocaldococcus jannaschii

The archaeon Methanocaldococcus jannaschii uses three different 2-oxoacid elongation pathways, which extend the chain length of precursors in leucine, isoleucine, and coenzyme B biosyntheses. In each of these pathways an aconitase-type hydrolyase catalyzes an hydroxyacid isomerization reaction. The genome sequence of M. jannaschii encodes two homologs of each large and small subunit that forms the hydrolyase, but the genes are not cotranscribed. The genes are more similar to each other than to previously characterized isopropylmalate isomerase or homoaconitase enzyme genes. To identify the functions of these homologs, the four combinations of subunits were heterologously expressed in Escherichia coli, purified, and reconstituted to generate the iron-sulfur center of the holoenzyme. Only the combination of MJ0499 and MJ1277 proteins catalyzed isopropylmalate and citramalate isomerization reactions. This pair also catalyzed hydration half-reactions using citraconate and maleate. Another broad-specificity enzyme, isopropylmalate dehydrogenase (MJ0720), catalyzed the oxidative decarboxylation of beta-isopropylmalate, beta-methylmalate, and d-malate. Combined with these results, phylogenetic analysis suggests that the pyruvate pathway to 2-oxobutyrate (an alternative to threonine dehydratase in isoleucine biosynthesis) evolved several times in bacteria and archaea. The enzymes in the isopropylmalate pathway of leucine biosynthesis facilitated the evolution of 2-oxobutyrate biosynthesis through the introduction of a citramalate synthase, either by gene recruitment or gene duplication and functional divergence.

Spectroscopic characterization of 2-isopropylmalic acid–aluminum(III) complex

Spectroscopic elucidation of a 2-isopropylmalic acid (2-iPMA)-aluminum(III) complex has been carried out using (1)H, (13)C and (27)Al NMR spectroscopy, diffusion-ordered NMR spectroscopy (DOSY) and electrospray ionization mass spectrometry (ESI-MS). 2-iPMA is secreted by Saccharomyces cerevisiae and can dissolve Al(III) in the culture medium. The (1)H chemical shift perturbation and (1)H DOSY clearly indicated the formation of the 2-iPMA-Al(III) complex. The measurements of (13)C and (27)Al NMR spectroscopy and ESI-MS demonstrated that the major form of a complex is comprised four 2-iPMA and two Al(III) species. This compound is expected to possess strong Al(III)-detoxification capability.

Secretion of an aluminum chelator, 2-isopropylmalic acid, by the budding yeast, Saccharomyces cerevisiae

An aluminum(III)-binding substance (ABS), that solubilizes Al(III) at neutral pH, was found to be secreted by Saccharomyces cerevisiae. A combination of anion exchange chromatography and preparative high performance liquid chromatography using an octadecylsilane (ODS) column separated ABS from the medium. The structure determination of ABS was performed using 1H and 13C NMR spectroscopy and heteronuclear multiple-bond correlation (HMBC) spectroscopy, and ABS was identified to be 2-isopropylmalic acid (2-iPMA). The structure was further confirmed using high resolution electrospray ionization mass spectrometry. Solubilization of otherwise sparingly soluble Al(III) by 2-iPMA at neutral pH indicated the binding of the compound with Al(III). This is supported by 27Al NMR spectrometry for a solution containing 10 mM Al(III) and 20 mM 2-iPMA at pH 6.6, where four Al(III) species were evident. Although the function of this compound is unclear, it might play a key role in Al detoxification.

Isoleucine biosynthesis in Leptospira interrogans serotype lai strain 56601 proceeds via a threonine-independent pathway

Three leuA-like protein-coding sequences were identified in Leptospira interrogans. One of these, the cimA gene, was shown to encode citramalate synthase (EC 4.1.3.-). The other two encoded alpha-isopropylmalate synthase (EC 4.1.3.12). Expressed in Escherichia coli, the citramalate synthase was purified and characterized. Although its activity was relatively low, it was strictly specific for pyruvate as the keto acid substrate. Unlike the citramalate synthase of the thermophile Methanococcus jannaschii, the L. interrogans enzyme is temperature sensitive but exhibits a much lower K(m) (0.04 mM) for pyruvate. The reaction product was characterized as (R)-citramalate, and the proposed beta-methyl-d-malate pathway was further confirmed by demonstrating that citraconate was the substrate for the following reaction. This alternative pathway for isoleucine biosynthesis from pyruvate was analyzed both in vitro by assays of leptospiral isopropylmalate isomerase (EC 4.2.1.33) and beta-isopropylmalate dehydrogenase (EC 1.1.1.85) in E. coli extracts bearing the corresponding clones and in vivo by complementation of E. coli ilvA, leuC/D, and leuB mutants. Thus, the existence of a leucine-like pathway for isoleucine biosynthesis in L. interrogans under physiological conditions was unequivocally proven. Significant variations in either the enzymatic activities or mRNA levels of the cimA and leuA genes were detected in L. interrogans grown on minimal medium supplemented with different levels of the corresponding amino acids or in cells grown on serum-containing rich medium. The similarity of this metabolic pathway in leptospires and archaea is consistent with the evolutionarily primitive status of the eubacterial spirochetes.

Protein engineering reveals ancient adaptive replacements in isocitrate dehydrogenase

Evolutionary analysis indicates that eubacterial NADP-dependent isocitrate dehydrogenases (EC 1.1.1.42) first evolved from an NAD-dependent precursor about 3.5 billion years ago. Selection in favor of utilizing NADP was probably a result of niche expansion during growth on acetate, where isocitrate dehydrogenase provides 90% of the NADPH necessary for biosynthesis. Amino acids responsible for differing coenzyme specificities were identified from x-ray crystallographic structures of Escherichia coli isocitrate dehydrogenase and the distantly related Thermus thermophilus NAD-dependent isopropylmalate dehydrogenase. Site-directed mutagenesis at sites lining the coenzyme binding pockets has been used to invert the coenzyme specificities of both enzymes. Reconstructed ancestral sequences indicate that these replacements are ancestral. Hence the adaptive history of molecular evolution is amenable to experimental investigation.

Valine inhibition of beta-isopropylmalate dehydrogenase takes part in the regulation of leucine biosynthesis in Candida maltosa

The beta-isopropylmalate (IPM) dehydrogenase (EC 1.1.1.85) of Candida maltosa, the third pathway-specific enzyme of leucine biosynthesis, was purified, some properties of the enzyme were studied and a novel regulatory pattern was found. The Km values of the enzyme were estimated to be 0.42 mM for beta-IPM and 0.34 mM for NAD+. It is demonstrated that the enzyme can be regulated by L-valine. The inhibition was competitive with respect to beta-IPM (Ki = 1.84 mM) and non-competitive with respect to NAD+ (Ki = 5.67 mM). Exogenous addition of L-valine to C. maltosa cells increased the intracellular pool of some intermediates of leucine biosynthesis (alpha-ketoisovalerate, alpha-IPM, beta-IPM), but has hardly influence on the leucine pool.

Manganese: its acquisition by and function in the lactic acid bacteria

The transition metal manganese is considered to be a minor micronutrient in both pro- and eukaryotes, usually being required from the environment at subnanomolar levels. Until recently, Mn was only known to function in cells as a cofactor for a few enzymatic reactions. A notable exception has been reported in many lactic acid bacterial species which require micromolar medium Mn levels for growth and contain up to 35 mM Mn. These high Mn concentrations are accompanied by the near or complete absence of intracellular iron and superoxide dismutase (SOD). Lacking hemes, Lactobacillus plantarum and related species contain a unique Mn-cofactored catalase as well as millimolar Mn(II) in a nonenzymic complex performing the function of the micromolar superoxide dismutase found in most other aerotolerant cells. The high Mn(II) levels are accumulated via an efficient active transport system and are stored intracellularly in a high molecular weight complex. Study of Lactobacillus plantarum has provided an interesting example of the substitution of Mn for Fe in several of the biological roles of Fe, an alternative mechanism of aerotolerance, and a better understanding of the unique biochemistry of the lactic acid bacteria.

Promoter mutation causing catabolite repression of the Salmonella typhimurium leucine operon

Two mutations that affect expression of the Salmonella typhimurium leu operon were investigated. leu operon DNA from these mutant strains was cloned, and nucleotide sequences of the leu control regions were determined. leu-500, which eliminates expression of all four leu genes simultaneously, is a point mutation in the -10 region of the leu promoter. leu-2012 is a point mutation within the -35 region of the leu promoter. leu-2012 suppressed leucine auxotrophy caused by leu-500 only when the medium contained a carbon source that does not cause catabolite repression. A cya mutation (adenylate cyclase deficiency) introduced into the leu-500 leu-2012 strain caused leu enzymes to be made only if cAMP was supplied exogenously. A leu-500 leu-2012 strain containing a crp mutation (cAMP receptor protein deficiency), on the other hand, could not make leu enzymes even in the presence of cAMP. In vitro transcription experiments demonstrated that the leu-2012 mutation created a new transcription initiation site. RNA polymerase utilized this site in vitro in the absence of added cAMP receptor protein and cAMP.

Transcription initiation sites of the leucine operons of Salmonella typhimurium and Escherichia coli

Evidence for a transcription attenuation site downstream from the leu promoter was obtained by transcription experiments in vitro. Most transcription initiated in vitro from leuP is terminated prematurely, resulting in the synthesis of a 160 nucleotide leader RNA. We define here the point at which transcription is initiated in vitro and in vivo and demonstrate that the site of premature termination is between the promoter and the first structural gene (leuA). Additional nucleotide sequences are presented that extend the known sequence 200 base-pairs upstream and 300 base-pairs downstream from leuP. The location of the promoter-proximal end of cistron leuA was deduced by comparing nucleotide sequence data with the sequence of the ten amino acids at the N-terminus of alpha-isopropylmalate synthase. To facilitate the isolation of quantities of material for sequencing experiments, the enzyme was isolated from a plasmid-containing strain, CV605, grown under conditions of leucine limitation. Under such conditions, about 20% of the total soluble protein of strain CV605 is alpha-isopropylmalate synthase and another 20% is beta-isopropylmalate dehydrogenase (leuB product).

Absence of acetohydroxy acid synthetase in a clinical isolate of Neisseria gonorrhoeae requiring isoleucine and valine

A clinical isolate of Neisseria gonorrhoeae with an unusual growth requirement for isoleucine and valine lacked the activity of acetohydroxy acid synthetase, one of the enzymes required for the biosynthesis of these amino acids. A spontaneous mutant which no longer required isoleucine and valine had acquired this enzymatic activity.

Cloning of an EcoRI-generated fragment of the leucine operon of Salmonella typhimurium

Recombinant plasmids carrying part of the leucine operon of Salmonella typhimurium were isolated following transformation of an Escherichia coli leucine auxotroph to prototrophy with a ligated mixture of EcoRI-treated Salmonella DNA and plasmid pSC101 DNA. Plasmids pCV11 and pCV13, containing a 3.4-10(6) dalton DNA fragment ligated to the vector, had the leu operon oriented in opposite directions. The orientation of the leu operon relative to plasmid genes was determined. The 3.4-10(6) dalton fragment was ligated in to the EcoRI site of plasmid pMB9 yielding plasmids pCV12 (orientation as in pCV11) and pCV14 (orientation as in pCV13). The results of enzyme assays and complementation tests indicated that these plasmids carry functional leuA, leuB, and leuC genes but not a functional leuD gene. Furthermore, the following results indicated that they have a functional leu control region and promoter. Expression of plasmid leu genes was markedly enhanced under conditions of leucine limitation whereas introduction of a leu promoter mutation into the operon oriented in either direction with respect to plasmid genes had a strong negative effect upon leu operon expression. Transcriptional readthrough from plasmid promoters, if it occurs at all, must be small in comparison with transcription initiated at the leu promoter. RNA was isolated from leucine auxotrophs grown under conditions of repression and derepression and from prototrophic strains derepressed for the leucine operon as a result of mutations in leuO, leuS, and flrB. The rate of synthesis of leu mRNA, measured by hybridization to plasmid pCV12 DNA, was proportional in each case to leu enzyme levels.

[Transaminase B from Escherichia coli. I.-Purification and first properties]

Transaminase B (EC.2.6.1.6.) from E. coli, the specific enzyme for branched-chain aminoacids, was obtained in a purity equal to or greater than 96 p. cent after an 800-fold purification, employing two different procedures. One of the procedures involved heating at 60degreesC. The apparent molecular weight of the enzyme was estimated by chromatography on Sephadex and gel electrophoresis to be close to 180,000. The protein is made up of 6 subunits of equal size, with one molecule of coenzyme in each. Its absorption spectrum shows bands at 335 and 415 nm, and was found to be almost insensitive to the pH of the medium between 4.6 and 9. Transaminase B is active on phenylalanine as well, although the reaction between L-phenylalanine and alpha-ketoglutarate is about 50 to 100 times slower than the analogous reaction using L-valine as an aminoacid. Three sets of data show that the phenylalanine aminotransferase activity associated with transaminase B is not an artefact due to a protein contaminant. 1) Activities displayed toward phenylalanine and valine cannot be resolved by different methods, including chromatography, gel electrophoresis, and electrofucussing. 2) The absorption spectrum of the enzyme is as strongly modified by phenylalanine as by valine. 3) A ketoglutarate-free reaction between phenylalanine, tyrosine or typtophane and an aliphatic alpha-ketoacid is catalyzed by the pure enzyme and follows a mechanism belonging to the usual ping pong type. The possible significance of this reaction as a regulatory device in the cell metabolism is briefly discussed.

Binding of alpha-ketoisovalerate to alpha-isopropylmalate synthase. Half-of-the-sites and all-of-the-sites availability

Binding of alpha-ketoisovalerate to alpha-isopropylmalate synthase (3-hydroxy-4-methyl-3-carboxyvalerate 2-oxo-3-methylbutyrate-lyase (CoA-acetylating), EC 4.1.3.12) from Salmonella thyphimurium has been studied by equilibrium dialysis. When alpha-ketoisovalerate is the only ligand present, no more than two sites per enzyme tetramer can be saturated under the conditions chosen. The binding is non-cooperative with a dissociation constant of 6.6+/- 0.4 muM. Binding of alpha-ketoisovalerate has also been studied in the presence of propionyl-CoA. This compound was selected because of its close similarity to the natural substrate acetyl-CoA. It is a competitive inhibitor with respect to acetyl-CoA while reacting only extremely sluggishly as as substrate itself. The presence of propionyl-CoA has a profound effect on alpha-ketoisovalerate binding. The number of sites available to alpha-ketoisovalerate increases to about four per tetramer. At the same time, the dissociation constant for alpha-ketoisovalerate increases approx. 4-fold. These results suggest that the active conformation of alpha-isopropylmalate synthase is not obtained unless both substrates are present. They also support the notion, based on previous studies with the feedback inhibitor L-leucine, that alpha-isopropylmalate synthase has a tendency to form "functional dimers".

Separate regulation of transport and biosynthesis of leucine, isoleucine, and valine in bacteria

Since both transport activity and the leucine biosynthetic enzymes are repressed by growth on leucine, the regulation of leucine, isoleucine, and valine biosynthetic enzymes was examined in Escherichia coli K-12 strain EO312, a constitutively derepressed branched-chain amino acid transport mutant, to determine if the transport derepression affected the biosynthetic enzymes. Neither the iluB gene product, acetohydroxy acid synthetase (acetolactate synthetase, EC 4.1.3.18), NOR THE LEUB gene product, 3-isopropylmalate dehydrogenase (2-hydroxy-4-methyl-3-carboxyvalerate-nicotinamide adenine dinucleotide oxido-reductase, EC 1.1.1.85), were significantly affected in their level of derepression or repression compared to the parental strain. A number of strains with alterations in the regulation of the branched-chain amino acid biosynthetic enzymes were examined for the regulation of the shock-sensitive transport system for these amino acids (LIV-I). When transport activity was examined in strains with mutations leading to derepression of the iluB, iluADE, and leuABCD gene clusters, the regulation of the LIV-I transport system was found to be normal. The regulation of transport in an E. coli strain B/r with a deletion of the entire leucine biosynthetic operon was normal, indicating none of the gene products of this operon are required for regulation of transport. Salmonella typhimurium LT2 strain leu-500, a single-site mutation affecting both promotor-like and operator-like function of the leuABCD gene cluster, also had normal regulation of the LIV-I transport system. All of the strains contained leucine-specific transport activity, which was also repressed by growth in media containing leucine, isoleucine and valine. The concentrated shock fluids from these strains grown in minimal medium or with excess leucine, isoleucine, and valine were examined for proteins with leucine-binding activity, and the levels of these proteins were found to be regulated normally. It appears that the branched-chain amino acid transport systems and biosynthetic enzymes in E. coli strains K-12 and B/r and in S. typhimurium strain LT2 are not regulated together by a cis-dominate type of mechanism, although both systems may have components in common.

Biosynthesis of branched-chain amino acids in yeast: effect of carbon source on leucine biosynthetic enzymes

The three enzymes in the leucine biosynthetic pathway of yeast do not exhibit coordinate repression and derepression in response to the carbon source available in the culture medium. Growth in an acetate medium results in derepression of the first enzyme in the pathway, alpha-isopropylmalate synthase, and repression of the second two enzymes, alpha-isopropylmalate isomerase and beta-isopropylmalate dehydrogenase, relative to the levels found in glucose-grown cells. The role of endogenous leucine pools as a mediator of these differences was investigated. The leucine pools did not differ significantly between acetate-grown and glucose-grown cells. However, an elevated endogenous leucine pool, caused by exogenous leucine in the growth medium, did decrease the rate of decay of alpha-isopropylmalate synthase activity observed when acetate-grown cells were shifted to glucose. Evidence is provided suggesting that an elevated endogenous leucine pool may increase the in vivo stability of alpha-isopropylmalate synthase under several different conditions. Studies on the kinetics of alpha-isopropylmalate synthase decay in vivo and sensitivity to leucine inhibition indicate that there are two classes of the enzyme in acetate-grown yeast cells.

Pleiotropy of hisT mutants blocked in pseudouridine synthesis in tRNA: leucine and isoleucine-valine operons

The hisT gene codes for an enzyme responsible for the conversion of uridine to pseudouridine (Psi) in the anticodon region of many tRNA species in Salmonella typhimurium. We have previously shown that a hisT mutant has tRNA(His) which lacks pseudouridine in this region and as a consequence has an altered chromatographic behavior. We show here a similar alteration in chromatographic behavior of all tRNA(Leu) and one tRNA(Ile) species from a hisT mutant. By contrast, tRNA(Val), which contains no pseudouridine except for the one in the TPsiCG sequence, is chromatographically unaltered in a hisT mutant. The absence of pseudouridine in the anticodon region of tRNA in hisT mutants has been previously shown to cause derepression of the histidine operon. We show here that in hisT mutants the regulation of the leucine and the isoleucine and valine operons is also affected: the enzymes of these operons are refractory to repression by the branched chain amino acids. However, there is no difference between hisT and wild type in the pattern of derepression caused by isoleucine or valine limitation and only a slight difference in the enzyme levels in cells grown on minimal medium. The alteration in the regulation of branched chain amino acid operons may also explain why hisT mutants are resistant to inhibition of growth by the amino acid analogues 5,5,5-trifluoroleucine, beta-hydroxyleucine, and norleucine and by the oligopeptides glycylglycylnorleucine and norleucylnorleucine.

Genetic analysis of the leucine region in Escherichia coli B-r: gene-enzyme assignments

Genetic mapping by transduction and conjugation using F(-) and F' strains carrying either point mutations in the l-arabinose or leucine regions or ara-leu fusion-deletion mutations has resulted in a detailed genetic map of the arabinose-leucine region of Escherichia coli B/r. These studies have identified four genes in the leucine region having the same order as found in Salmonella typhimurium: ara... leuDCBA.

Genetic fine structure of the leucine operon of Escherichia coli K-12

The order of mutational sites in 10 independently isolated leucine auxotrophys of Escherichia coli K-12 was determined by three-point reciprocal transductions. The sites of mutation mapped in linear sequence in a cluster; all leucine auxotrophic mutations were cotransducible with mutations in the arabinose operon. The mutations were assigned to four complementation groups by abortive transduction tests, designated D, C, B, and A, reading in a clockwise direction from the arabinose operon. Enzyme analyses showed that strains with a mutation in gene A lacked alpha-isopropylmalate synthetase activity (EC 4.1.3), and those with a mutation in gene B lacked beta-isopropylmalate dehydrogenase activity (EC 1.1.1). It is concluded that the gross structure of the leucine operon in E. coli is closely similar to, if not identical with, the gross structure of the leucine operon in Salmonella typhimurium.

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.

Alpha-isopropylmalate synthase from yeast: purification, kinetic studies, and effect of ligands on stability

alpha-Isopropylmalate synthase, the first specific enzyme in leucine biosynthesis, was purified approximately 100-fold from extracts of Saccharomyces sp. (strain 60615), the most effective step being specific elution with the feedback inhibitor leucine from a hydroxyapatite column. In the early steps of purification, special care was taken to protect the synthase against proteolytic activities. The apparent molecular weight of the enzyme as determined from gel filtration on a calibrated column was 137,000 in the absence and 121,000 in the presence of leucine. Inhibition by leucine was specific and strongly pH-dependent, with the leucine concentration necessary for half-maximal inhibition increasing about 10-fold as the pH increased from 7.5 to 8.5. Within this pH range, catalytic activity remained almost unchanged. The apparent K(m) values for the two substrates were found to be 16 mum for alpha-ketoisovalerate and 9 mum for acetyl-coenzyme A. K(+) was required for full activity, the apparent K(a) value being 2 mm. Leucine inhibition was of the mixed type, resulting in decreased V(max) and increased apparent K(m) values forboth substrates. Whereas no cooperative effects were observed with either substrate, positive cooperativity was seen with leucine in the presence of saturating substrate concentrations. Leucine and, to a lesser extent, alpha-ketoisovalerate stabilized the purified enzyme against heat-inactivation. The presence of acetyl-coenzyme A, on the other hand, accelerated the inactivation. In subsequent experiments, coenzyme A was recognized as the actual inactivating ligand, being effective even at lower temperatures and in concentrations which were estimated to be in the range of the enzyme concentration.

Recognition of an Escherichia operator by a Salmonella repressor

Escherichia coli leu(+) episomes were transferred to Salmonella typhimurium. The levels of two leucine-forming enzymes were normal (low) in the resulting merodiploids, suggesting that the putative Samonella leucine repressor recognizes the Escherichia leucine operator.

Effect of 4-azaleucine upon leucine metabolism in Salmonella typhimurium

dl-4-Azaleucine (5 x 3(-3)m) added to exponentially growing cells of Salmonella typhimurium resulted in an abrupt cessation of growth lasting 4 to 8 hr followed by a resumption of division. The transitory nature of inhibition was not due to the instability or modification of the analogue or to a derepression of leucine-forming enzymes. Of many compounds tested, leucine served most efficiently to reverse 4-azaleucine-induced inhibition. Inhibition of growth can be explained by the fact that 4-azaleucine inhibits alpha-isopropylmalate synthase, the first enzyme unique to leucine biosynthesis. The analogue was a poor inhibitor of both the transamination of alpha-ketoisocaproate to leucine and the charging of leucine to transfer ribonucleic acid. With a leucine auxotroph starved for leucine, the analogue was incorporated into protein specifically in place of leucine. Such incorporation was accompanied by the death of almost all of the cells.

Mutants of Salmonella typhimurium with an altered leucyl-transfer ribonucleic acid synthetase

Two trifluoroleucine-resistant mutants of Salmonella typhimurium, strains CV69 and CV117, had an altered leucyl-transfer ribonucleic acid (tRNA) synthetase. The mutant enzymes had higher apparent K(m) values for leucine (ca. 10-fold) and lower specific activities (ca. twofold) than the parent enzyme when tested in crude extracts. Preparations of synthetase purified ca. 60-fold from the parent and strain CV117 differed sixfold in their leucine K(m) values. In addition, the mutant enzyme was inactivated faster than the parent enzyme at 50 C. The growth rates of strains CV69 and CV117 at 37 C were not significantly different from that of the parent, whereas at 42 C strain CV69 grew more slowly than the parent. Leucine-, valine-, and isoleucine-forming enzymes were partially derepressed when the mutants were grown in minimal medium; the addition of leucine repressed these enzymes to wild-type levels. During growth in minimal medium, the proportion of leucine tRNA that was charged in the mutants was about 75% of that in the parent. The properties of strain CV117 were shown to result from a single mutation located near gal at minute 18 on the genetic map. These studies suggest that leucyl-tRNA synthetase is involved in repression of the enzymes required for the synthesis of branched-chain amino acids.

Fidelity of initiation of protein synthesis after premature chain termination in polarity mutants

beta-Isopropylmalate dehydrogenase, the product of the second cistron of the leucine operon in Salmonella typhimurium, produced by strains bearing nonsense or frameshift mutations in the first cistron of the operon was shown to be homogeneous as judged by electrophoretic and immunological techniques. Amino terminal analyses suggest that the enzyme produced by the mutant strains is identical with the wild-type enzyme. This view is supported by the observation that a nonsense mutant strain beta-isopropylmalate dehydrogenase copurifies with the wild-type enzyme. The results suggest that the uncoupling of normal chain termination and reinitiation does not interfere with the fidelity of subsequent polypeptide chain initiation in a polycistronic messenger ribonucleic acid.

Oxidation D-malic and beta-alkylmalic acids wild-type and mutant strains of Salmonella typhimurium and by Aerobacter aerogenes

A mutant strain of Salmonella typhimurium (SL 1634 dml-51) capable of growth on d-malate as sole carbon source was shown to produce d-malic enzyme. This enzyme was absent in the parent wild-type strain which was unable to grow on d-malate. Growth of the mutant on d-malate also resulted in a greatly increased level of beta-isopropylmalic enzyme compared with its level in the wild-type strain grown on citrate or l-malate. The d-malic and beta-isopropylmalic enzymes, both of which catalyze a nicotinamide adenine dinucleotide- and Mg(++)-dependent oxidative decarboxylation of their respective substrates, were shown to be distinct enzymes by selective inhibition with erythro-dl-beta-hydroxyaspartate and by other methods. Cell extracts of the mutant strain also oxidized dl-beta-methyl-, dl-beta-ethyl-, dl-beta-propyl- and dl-betabeta-dimethylmalates, in order of decreasing activity. dl-beta-Methyl-malate was shown to be oxidized by both the d-malic and the beta-isopropylmalic enzymes, whereas the oxidation of the other beta-alkylmalates appeared to be effected exclusively by the beta-isopropylmalic enzyme. beta-Isopropylmalic enzyme activity was induced by d-malate but not by l-malate, showing that it behaved as a d-malictype enzyme. Growth of Aerobacter aerogenes on d-malate, which caused induction of d malic enzyme, resulted in only a small increase in the activity of beta-isopropylmalic enzyme.

Regulation of branched-chain amino acid biosynthesis in Salmonella typhimurium: isolation of regulatory mutants

5',5',5'-Trifluoro-dl-leucine inhibited the activity of alpha-isopropylmalate synthetase (the initial enzyme unique to leucine biosynthesis) as well as the growth of Salmonella typhimurium. Mutants of S. typhimurium resistant to the analogue were isolated and characterized. In most cases, they overproduced and excreted leucine or leucine, valine, and isoleucine as a result of an alteration in the regulation of branched-chain amino acid biosynthesis. Biochemical and genetic tests allowed the mutants to be grouped into three classes: I, a moderately large group (13%) which had high, constitutive leucine biosynthetic enzyme levels and mutant sites linked to the leucine operon (operator constitutive); II, a single mutant in which the mutant site was linked to the leucine operon and in which alpha-isopropylmalate synthetase was not inhibited by leucine (feedback negative); III, a majority type which had constitutive levels of leucine, valine, and isoleucine biosynthetic enzymes and mutant sites unlinked to the leucine operon. Mutants of class I provide important evidence for the concept of an operon organization of genes involved in leucine biosynthesis. The properties of class III mutants indicate that there is some element involved in regulation which is common to the three pathways.