Regulation of branched-chain amino acid transport in Escherichia coli

A study on L-threonine and L-serine uptake in Escherichia coli K-12

In the current study, we report the identification and characterization of the yifK gene product as a novel amino acid carrier in E. coli K-12 cells. Both phenotypic and biochemical analyses showed that YifK acts as a permease specific to L-threonine and, to a lesser extent, L-serine. An assay of the effect of uncouplers and composition of the reaction medium on the transport activity indicates that YifK utilizes a proton motive force to energize substrate uptake. To identify the remaining threonine carriers, we screened a genomic library prepared from the yifK-mutant strain and found that brnQ acts as a multicopy suppressor of the threonine transport defect caused by yifK disruption. Our results indicate that BrnQ is directly involved in threonine uptake as a low-affinity but high-flux transporter, which forms the main entry point when the threonine concentration in the external environment reaches a toxic level. By abolishing YifK and BrnQ activity, we unmasked and quantified the threonine transport activity of the LIV-I branched chain amino acid transport system and demonstrated that LIV-I contributes significantly to total threonine uptake. However, this contribution is likely smaller than that of YifK. We also observed the serine transport activity of LIV-I, which was much lower compared with that of the dedicated SdaC carrier, indicating that LIV-I plays a minor role in the serine uptake. Overall, these findings allow us to propose a comprehensive model of the threonine/serine uptake subsystem in E. coli cells.

Bioremediation of petroleum-contaminated saline soil by Acinetobacter baumannii and Talaromyces sp. and functional potential analysis using metagenomic sequencing

Microbial remediation is a potential remediation method for petroleum-contaminated soil. In order to explore the petroleum degradation mechanism by microorganisms, the oilfield soil was remedied by Acinetobacter baumannii combined with Talaromyces sp. The degradation mechanism was studied by analyzing soil microbial community and functional genes through metagenomics during the degradation process. The result showed the degradation rate of petroleum was 65.6% after 28 days. The concentration of petroleum decreased from 1220 mg/kg to 420 mg/kg. In the co-culture group, Acinetobacter baumannii became the dominant species, the annotated genes of it at the species level accounted for 7.34% while that of Talaromyces sp. accounted for only 0.34%. Meanwhile, the annotated genes of Bacillus, Halomonas, and Nitriliruptor at the genus level were up-regulated by 1.83%, 0.90%, and 0.71%, respectively. In addition, large functional genes were significantly up-regulated, including the peroxisome, P450 enzyme (CYP53, CYP116, CYP102, CYP645), and biofilm formulation, promoting the oxidation and hydroxylation, and catalyzing the epoxidation of aromatic and aliphatic hydrocarbons. Meanwhile, the degrading genes of alkanes and aromatic hydrocarbons were expressed promotionally, and degradation pathways were deduced. In conclusion, the inoculation of Acinetobacter baumannii combined with Talaromyces sp. accelerated the degradation of petroleum in oilfield soil and improved the growth of indigenous petroleum-degrading bacteria. Many functional genes related to petroleum degradation were promoted significantly. These results proved the co-culture of bacteria-fungi consortium contributes to the bioremediation of petroleum-contaminated soil.

Defining the metabolic requirements for the growth and colonization capacity of Campylobacter jejuni

During the last decade Campylobacter jejuni has been recognized as the leading cause of bacterial gastroenteritis worldwide. This facultative intracellular pathogen is a member of the Epsilonproteobacteria and requires microaerobic atmosphere and nutrient rich media for efficient proliferation in vitro. Its catabolic capacity is highly restricted in contrast to Salmonella Typhimurium and other enteropathogenic bacteria because several common pathways for carbohydrate utilization are either missing or incomplete. Despite these metabolic limitations, C. jejuni efficiently colonizes various animal hosts as a commensal intestinal inhabitant. Moreover, C. jejuni is tremendously successful in competing with the human intestinal microbiota; an infectious dose of few hundreds bacteria is sufficient to overcome the colonization resistance of humans and can lead to campylobacteriosis. Besides the importance and clear clinical manifestation of this disease, the pathogenesis mechanisms of C. jejuni infections are still poorly understood. In recent years comparative genome sequence, transcriptome and metabolome analyses as well as mutagenesis studies combined with animal infection models have provided a new understanding of how the specific metabolic capacity of C. jejuni drives its persistence in the intestinal habitat of various hosts. Furthermore, new insights into the metabolic requirements that support the intracellular survival of C. jejuni were obtained. Because C. jejuni harbors distinct properties in establishing an infection in comparison to pathogenic Enterobacteriaceae, it represents an excellent organism for elucidating new aspects of the dynamic interaction and metabolic cross talk between a bacterial pathogen, the microbiota and the host.

[3H]Leucine incorporation method as a tool to measure secondary production by periphytic bacteria associated to the roots of floating aquatic macrophyte

The present study assessed the application of [(3)H]Leucine incorporation into protein by periphytic bacteria associated with the roots of the floating aquatic macrophyte Eichornia crassipes. Basic assumptions underlying the method, such as linearity of leucine incorporation, saturation level of incorporation rates, incorporation into other macromolecules, specificity of incorporation for bacterial assemblages and [(3)H]Leucine degradation during samples storage were tested, and two procedures for extracting the incorporated leucine were compared. Both methods gave the same results, however, the hot TCA extraction method was less time consuming than the alkaline extraction method. Incorporation of [(3)H]Leucine was linear for up to 40 min. Saturation concentration of [(3)H]Leucine incorporation into protein was 1500 nM. An experiment with prokaryotic and eukaryotic inhibitors showed no significant [(3)H]Leucine incorporation into eukaryotes even in high leucine concentrations. No significant amounts of radiolabel were incorporated into other macromolecules. The maximum time of sample storage after the incubation is 15 days. The leucine incorporation method can be a reliable tool to measure bacterial production in the periphyton root-associated bacteria.

The lrp gene product regulates expression of lysU in Escherichia coli K-12

In Escherichia coli K-12, expression of the lysU gene is regulated by the lrp gene product, as indicated by an increase in the level of lysyl-tRNA synthetase activity and LysU protein in an lrp mutant. Comparison of the patterns of protein expression visualized by two-dimensional gel electrophoresis indicated that LysU is present at higher levels in an lrp strain than in its isogenic lrp+ parent. The purified lrp gene product was shown to bind to sites upstream of the lysU gene and to protect several sites against DNase I digestion. A region extending over 100 nucleotides, between 60 and 160 nucleotides upstream from the start of the lysU coding sequence, showed altered sensitivity to DNase I digestion in the presence of the Lrp protein. The extent of protected DNA suggests a complex interaction of Lrp protein and upstream lysU DNA.

Isolation of a cDNA clone for human threonyl-tRNA synthetase: amplification of the structural gene in borrelidin-resistant cell lines

A cDNA for threonyl-tRNA synthetase was isolated from a human placental cDNA lambda gt11 expression library by immunological screening, and its identity was confirmed by hybrid-selected mRNA translation. With this cDNA used as a hybridization probe, borrelidin-resistant Chinese hamster ovary cells that overproduced threonyl-tRNA synthetase were shown to have increased levels of threonyl-tRNA synthetase mRNA and gene sequences. Amplification of the gene did not appear to have been accompanied by any major structural reorganizations.

Isolation of a cDNA clone for human threonyl-tRNA synthetase: amplification of the structural gene in borrelidin-resistant cell lines

A cDNA for threonyl-tRNA synthetase was isolated from a human placental cDNA lambda gt11 expression library by immunological screening, and its identity was confirmed by hybrid-selected mRNA translation. With this cDNA used as a hybridization probe, borrelidin-resistant Chinese hamster ovary cells that overproduced threonyl-tRNA synthetase were shown to have increased levels of threonyl-tRNA synthetase mRNA and gene sequences. Amplification of the gene did not appear to have been accompanied by any major structural reorganizations.

Anaerobic and leucine-dependent expression of a peptide transport gene in Salmonella typhimurium

Using Mu d1-mediated lac operon fusions, we studied the transcriptional regulation of the genes encoding two peptide transport systems, the oligopeptide permease and the tripeptide permease. The four opp genes were found to be constitutively expressed, whereas the genes encoding the tripeptide permease are under a complex set of regulatory controls. Two loci, tppA and tppB, are required for tripeptide permease function. Locus tppA is shown to be a positive regulator of tppB expression. In addition, tppB expression is specifically induced by exogeneous leucine or by anaerobiosis. Anaerobic induction of tppB is independent of the fnr gene product which is required for the anaerobic expression of several respiratory enzymes. Thus, there must be at least two distinct pathways for the anaerobic regulation of gene expression.

Acetohydroxy acid synthase is a target for leucine containing peptide toxicity in Escherichia coli

Acetohydroxy acid synthase from a mutant resistant to leucine-containing peptides was insensitive to leucine inhibition. It is concluded that acetohydroxy acid synthase is a target for the toxicity of the high concentrations of leucine brought into Escherichia coli K-12 by leucine-containing peptides.

Toxicity of leucine-containing peptides in Escherichia coli caused by circumvention of leucine transport regulation

A variety of leucine-containing peptides (LCP), Phe-Leu, Gly-Leu, Pro-Leu, Ala-Leu, Ala-Leu-Lys, Leu-Phe-Ala, Leu-Leu-Leu, and Leu-Gly-Gly, inhibited the growth of a prototrophic strain of Escherichia coli K-12 at concentrations between 0.05 and 0.28 mM. Toxicity requires normal uptake of peptides. When peptide transport was impaired by mutations, strains became resistant to the respective LCP. Inhibition of growth occurred immediately after the addition of LCP, and was relieved when 0.4 mM isoleucine was added. The presence of Gly-Leu in the medium correlated with the inhibition of growth, and the bacteria began to grow at the normal rate 70 min after Gly-Leu became undetectable. Disappearance of the peptide corresponded with the appearance of free leucine and glycine in the medium. The concentration of leucine inside the LCP-treated bacteria was higher than that in the leucine-treated and the control cultures. We suggest that entry of LCP into the cells via peptide transport systems circumvents the regulation of leucine transport, thereby causing abnormality high concentrations of leucine inside the cells. This accumulation of leucine interferes with the biosynthesis of isoleucine and inhibits the growth of the bacteria.

Biochemistry and regulation of a second L-proline transport system in Salmonella typhimurium

This paper reports some biochemical characteristics of a second L-proline transport system in Salmonella typhimurium. In the accompanying paper, R. Menzel and J. Roth (J. Bacteriol. 141:1064--1070, 1980) have identified this system by showing that it is inactivated by mutations at the locus proP. We have found that it is an active transport system with an apparent Km for L-proline of 3 x 10(-4) M and a strict specificity for L-proline and some of its analogs. Unlike the L-proline transport system encoded in putP, this second system is induced by amino acid limitation.

Uptake and binding of riboflavin by membrane vesicles of Bacillus subtilis

Riboflavin uptake and membrane-associated riboflavin-binding activity have been investigated in Bacillus subtilis. The uptake and binding activity of the vitamin were found to be repressed coordinately by riboflavin present in the growth medium. The uptake or riboflavin has been shown to have properties of a carrier-mediated process, and membrane vesicles have been shown to demonstrate riboflavin counterflow and exchange. The membrane-associated binding activity for riboflavin has been solubilized with detergents, and a procedure for the partial purification of this component is described. The partially purified riboflavin-binding component has properties expected for a carrier involved in riboflavin uptake, as it shows saturation kinetics and is inhibited by riboflavin analogues. Evidence is also presented showing that reduced riboflavin binds to a greater extent than oxidized riboflavin, and the possible role of the reduced riboflavin in riboflavin uptake is discussed.

Nitrogen control of Salmonella typhimurium: co-regulation of synthesis of glutamine synthetase and amino acid transport systems

Nitrogen control in Salmonella typhimurium is not limited to glutamine synthetase but affects, in addition, transport systems for histidine, glutamine, lysine-arginine-ornithine, and glutamate-aspartate. Synthesis of both glutamine synthetase and transport proteins is elevated by limitation of nitrogen in the growth medium or as a result of nitrogen (N)-regulatory mutations. Increases in the amounts of these proteins were demonstrated by direct measurements of their activities, by immunological techniques, and by visual inspection of cell fractions after gel electrophoresis. The N-regulatory mutations are closely linked on the chromosome to the structural gene for glutamine synthetase, glnA: we discuss the possibility that they lie in a regulatory gene, glnR, which is distinct from glnA. Increases in amino acid transport in N-regulatory mutant strains were indicated by increased activity in direct transport assays, improved growth on substrates of the transport systems, and increased sensitivity to inhibitory analogs that are trnasported by these systems. Mutations to loss of function of individual transport components (hisJ, hisP, glnH, argT) were introduced into N-regulatory mutant strains to determine the roles of these components in the phenotype and transport behavior of the strains. The structural gene for the periplasmic glutamine-binding protein, glnH, was identified, as was a gene argT that probably encodes the structure of the lysine-arginine-ornithine-binding protein. Genes encoding the structures of the histidine- and glutamine-binding proteins are not linked to glnA or to each other by P22-mediated transduction; thus, nitrogen control is exerted on several unlinked genes.

Effect of growth medium on the relative polypeptide composition of cellular outer membrane and released outer membrane material in Escherichia coli

When ratios of the major polypeptides of the outer membrane isolated from cells of Escherichia coli B grown in minimal medium containing either a single amino acid or several amino acids were compared, no difference was observed. However, the ratio of these polypeptides in outer membrane material released into the medium during logarithmic phase growth on these two media was markedly different.

The relA locus specifies a positive effector in branched-chain amino acid transport regulation

The regulation of branched-chain amino acid transport and periplasmic binding proteins was studied in Escherichia coli strains which were isogenic except for the relA locus, the gene for the "stringent factor," which is responsible for guanosine tetraphosphate synthesis. The strain containing the relA mutation could not be derepressed for the synthesis of leucine transport or binding proteins when shifted from a medium containing all 20 amino acids in excess to one in which leucine was limiting. The relA+ strain showed normal derepression under these conditions.

Release of a special fraction of the outer membrane from both growing and phage T4-infected Escherichia coli B

Growing Escherichia coli release envelope material into the medium. Upon infection with T4 phage increased amounts of this material are released and at a greater rate. In order to determine whether both inner and outer membranes are present in this material, and whether the material released by growing cells differs from that released by infected cells, we have examined the protein composition of envelope released by growing and T4-infected E. coli B. Our results show: (a) the protein composition of envelope released from growing or infected cells is similar, (b) the proteins present are representative of the outer membrane, (c) the major outer membrane protein of E. coli B, protein II, is deficient in the released material. We therefore conclude that the envelope material released from growing or infected E. coli represents a special fraction of the outer membrane. This finding is discussed in relation to outer membrane structure and function. In addition, data are presented on the differing outer membrane protein composition of substrains of E. coli B obtained from different laboratories.

Genetic separation of high- and low-affinity transport systems for branched-chain amino acids in Escherichia coli K-12

The Escherichia coli K-12 mutant strain AE4107 (livH::Mu) is defective in the high-affinity binding protein-mediated uptake system for L-leucine, L-valine, and L-isoleucine (LIV-I). We have used this strain to produce mutations in the residual LIV-II membrane-bound branched-chain amino acid uptake system. Mutants selected for their inability to utilize exogenous L-leucine were found to be defective in the LIV-II system and fell into two classes. One class, represented by strain AE410709 (livP9), showed a complete loss of saturable uptake for L-leucine, L-valine, and L-isoleucine up to 50 muM, and a second class, represented by strain AE4017012 (liv-12), showed a residual component of saturable leucine uptake with increased Km. These mutations, livP9 and liv-12, were closely linked and mapped in the 74 to 78 min region of the E. coli genetic map. Strains constructed so that they lacked both LIV-I and LIV-II transport systems excreted leucine. Strains of the genotype livH+ livP were found to have normal high-affinity binding protein-mediated transport (LIV-I and leucine specific), whereas the low-affinity (LIV-II) transport was completely missing. We concluded from these studies that the high-affinity binding protein-mediated transport systems (LIV-I and leucine specific) can operate independently of the membrane-bound LIV-II system.

Specificity and regulation of gamma-aminobutyrate transport in Escherichia coli

A specific gamma-aminobutyrate (GABA) transport system in Escherichia coli K-12 cells with a K(m) of 12 muM and a V(max) of 278 nmol/ml of intracellular water per min is described. Membrane vesicles contained d-lactate-dependent activity of the system. Mutants defective in GABA transport were isolated; they lost the ability to utilize GABA as a nitrogen source, although the activities of glutamate-succinylsemialdehyde transaminase (GSST) (EC 2.6.1.19) and succinylsemialdehyde dehydrogenase (SSDH) (EC 1.2.1.16), the enzymes that catalyze GABA utilization, remained as high as in the parental CS101B strain. The ability to utilize l-ornithine, l-arginine, putrescine, l-proline, and glycine as a nitrogen source was preserved in the mutants. The genetic lesions resulting in the loss of GABA transport, gabP5 and gabP9, mapped in the gab gene cluster in close linkage to gabT and gabD, the structural genes of GSST and SSDH, and to gabC, a gene controlling the utilization of GABA, arginine, putrescine, and ornithine. The synthesis of the GABA transport carrier is subject to dual physiological control by (i) catabolite repression and (ii) nitrogen availability. Experiments with glutamine synthetase (EC 6.3.1.2)-negative and with glutamine synthetase-constitutive strains strongly indicate that this enzyme is the effector in the regulation of GABA carrier synthesis by route (ii).

Branched-chain amino acid transport regulation in mutants blocked in tRNA maturation and transcriptional termination

The regulation of branched-chain amino acid transport and binding protein biosynthesis was studied in Escherichia coli strains containing hisT (the structural gene for pseudouridine synthetase) and rho (the structural gene for the mRNA transcriptional termination factor rho) mutations. The results indicate that the hisT strain cannot be fully derepressed for transport and that the hisT rho double mutant is partially derepressed under excess leucine conditions, but cannot be further derepressed by leucine deprivation. These data are consistent with a model in which fully mature tRNALeu is required for derepression and in which rho interacts with tRNALeu in regulating transport by terminating transcription, especially in excess-leucine growth conditions.

Repression of Escherichia coli pyridine nucleotide transhydrogenase by leucine

Addition of 0.1% casein hydrolysate to a minimal growth medium decreased membrane-bound transhydrogenase activity in Escherichia coli by about 80%. Of the amino acids added individually to the growth medium, only leucine and, to a lesser extent, methionine and alanine were effective, alpha-Ketoisocaproate- and leucine-containing peptides repressed the activity, and leucine also repressed activity in adenyl cyclase-deficient and relaxed strains. Derepression of transhydrogenase followed the removal of leucine from the growth medium and was sensitive to rifampin and chloramphenicol. A phosphoglucoisomerase-deficient strain that was forced to use the hexose monophosphate shunt exclusively had normal levels of transhydrogenase, which was repressed by leucine. Transhydrogenase activity doubled in mutants lacking either of the shunt dehydrogenases but was still repressed by leucine. In strains constitutive for the leucine biosynthetic operon, transhydrogenase was repressed by leucine but in strains livR and lst R, with leucine transport resistant to leucine repression, transhydrogenase was not repressed by leucine. These data suggest that transhydrogenase may have a function in the transport of branched-chain amino acids. In a hisT strain (which has altered leucyl-tRNA), transhydrogeanse was at a repressed level without the addition of leucine, suggesting that leucyl-tRNA may be involved in the regulation.

Bacitracin production by the high-yielding mutant Bacillus licheniformis strain AL: stimulatory effect of L-leucine

The high-yielding mutant Bacillus licheniformis AL produced only small amounts of bacitracin in the chemically defined M2 medium. L-leucine markedly stimulated bacitracin production and restored the mutant strain to its place as a superior producer as compared to Bacillus licheniformis ATCC 10716. Leucine also stimulated the growth rate of the mutant. The stimulatory effect of leucine on bacitracin production is discussed in relation to control mechanisms and overproduction of antibiotics.

Regulation of aromatic amino acid transport systems in Escherichia coli K-12

The regulation of the aromatic amino acid transport systems was investigated. The common (general) aromatic transport system and the tyrosine-specific transport system were found to be subject to repression control, thus confirming earlier reports. In addition, tryosine- and tryptophan-specific transport were found to be enhanced by growth of cells with phenylalanine. The repression and enhancement of the transport systems was abolished in a strain carrying an amber mutation in the regulator gene tyrR. This indicates that the tyrR gene product, which was previously shown to be involved in regulation of aromatic biosynthetic enzymes, is also involved in the regulation of the aromatic amino acid transport systems.

Alpha-isopropylmalate synthase from Alcaligenes eutrophus H 16. III. Endproduct inhibition and its relief by valine and isoleucine

The alpha-isopropylmalate synthase (EC 4.1.3.12) from Alcaligenes eutrophus H 16 was inhibited by L-leucine and alpha-ketoisocaproate. The extent of inhibition was influenced by substrate- and inhibitor concentrations as well as by the pH. Intermediary plateaus, which always appeared in the inhibition curves, suggested cooperative effects. The maximal Hill coefficient was found to be two. At low concentrations of leucine the inhibition mechanism was of the competitive type with respect to substrate acetyl coenzyme A and of the noncompetitive type with respect to substrate alpha-ketoisovalerate. The inhibition was specifically relieved by the addition of valine or isoleucine. The anomalous effect of temperature on enzyme activity was diminished by leucine. The Arrhenius energy of the reaction increased from about 11 kcal/mole in the absence of leucine to about 18 kcal/mole in the presence of leucine. The further addition of valine reversed this effect. The physiological relevance of the alpha-ketoisocaproate-mediated inhibition is discussed.

Regulation of L-cystine transport in Salmonella typhimurium

A kinetic analysis of L-cystine uptake in wild-type Salmonella typhimurium indicates the presence of at least two, and possibly three, separate transport systems. CTS-1 accounts for the majority of uptake at 20 muM L-cystine, with a Vmax of 9.5 nmol/min per mg and a Km of 2.0 muM; CTS-2 is a low-capacity, higher-affinity system with a Vmax of 0.22 nmol/min per mg and a Km of 0.05 muM; a third, nonsaturable process has been designated CTS-3. We find that wild-type CTS-1 levels are at least 11 times higher in sulfur-limited cells than in L-cystine-grown cells. Pleiotropic cysteine auxotrophs of the types cysE (lacking serine transacetylase) and cysB- (lacking a regulatory element of positive control) have very low levels of CTS-1 even when grown under conditions of sulfur limitation, which response is analogous to that previously observed for cysteine biosynthetic enzymes (N . M. Kredich, J. Biol. Chem. 246:3474-3484, 1971). CTS-1 is induced in cysE mutants by growth in the presence of O-acetyl-L-serine (the product of serine transacetylase), again paralleling the behavior of the cysteine biosynthetic pathway. Strain DW25, a prototrophic cysBc mutant, which is constitutive for cysteine biosynthesis, is also derepressed for CTS-1 when grown on L-cystine. Since CTS-1 is regulated by sulfur limitation, O-acetyl-L-serine, and the cysB gene product, the same three conditions controlling cysteine biosynthesis, we propose that this transport system is a part of the cysteine regulon.

Regulation of amino acid transport in Escherichia coli by transcription termination factor rho

Amino acid transport rates and amino acid binding proteins were examined in a strain containing the rho-120 mutation (formerly SuA), which has been shown to lower the rho-dependent, ribonucleic acid-activated adenosine triphosphatase activity to 9% of the rho activity in the isogenic wild-type strain. Tryptophan and proline transport, which occur by membrane-bound systems, were not altered. On the other hand, arginine, histidine, leucine, isoleucine, and valine transport were variably increased by a factor of 1.4 to 5.0. Kinetics of leucine transport showed that the LIV (leucine, isoleucine, and valine)-I (binding protein-associated) transport system is increased 8.5-fold, whereas the LIV-II (membrane-bound) system is increased 1.5-fold in the rho mutant under leucine-limited growth conditions. The leucine binding protein is increased fourfold under the same growth conditions. The difference in leucine transport in these strains was greatest during leucine-limited growth; growth on complex media repressed both strains to the same transport activity. We propose that rho-dependent transcriptional termination is important for leucine-specific repression of branched-chain amino acid transport, although rho-independent regulation, presumably by a corepressor-aporepressor-type mechanism, must also occur.

Role of transport systems in amino acid metabolism: leucine toxicity and the branched-chain amino acid transport systems

The livR locus, which leads to a trans-recessive derepression of branched-chain amino acid transport and periplasmic branched-chain amino acid-binding proteins, is responsible for greatly increased sensitivity toward growth inhibition by leucine, valine, and serine and, as shown previously, for increased sensitivity toward toxicity by branched-chain amino acid analogues, such as 4-azaleucine or 5',5',5'-trifluoroleucine. These phenotypes are similar to those of relA mutants; however, the livR mutants retain the stringent response of ribonucleic acid synthesis. However, an increase in the rate of transport or in the steady-state intracellular level of amino acids in the livR strain cannot completely account for this sensitivity. The ability of the LIV-I transport system to carry out exchange of pool amino acids for extracellular leucine is a major factor in leucine sensitivity. The previous finding that inhibition of threonine deaminase by leucine contributes to growth inhibition is confirmed by simulating the in vivo conditions using a toluene-treated cell preparation with added amino acids at levels corresponding to the internal pool. The relationship between transport systems and corresponding biosynthetic pathways is discussed and the general principle of a coordination in the regulation of transport and biosynthetic pathways is forwarded. The finding that the LIV-I transport system functions well for amino acid exchange in contrast to the LIV-II system provides another feature that distinguishes these systems in addition to previously described differences in regulation and energetics.