Derepressed leucine transport activity in Escherichia coli

Effects of polychlorinated biphenyls on macromolecular synthesis by a heterotrophic marine bacterium

Growth rates and final cell yields of a polychlorinated biphenyl (PCB)-sensitive pseudomonad isolated from the open ocean were reduced in a dose-dependent manner by 10 to 100 mug of Aroclor 1254 per liter, a commercial mixture of PCB isomers added to its culture medium. Effects on growth rates were detected within 1 h (approximately one doubling time) of treatment. By 4 h posttreatment, the amounts of deoxyribonucleic acid and ribonucleic acid per cell in exponentially growing populations treated with sublethal doses of Aroclor were detectably lower than in appropriate controls. Corresponding cell protein values were slightly higher than in controls. Selective degradation of cell proteins or nucleic acids was not detected in cells whose growth was totally suppressed for 4 h by PCBs. Cells whose growth rate was inhibited 20 to 50% by Aroclor synthesized protein at normal rates for periods in excess of 5 h from the time the chlorinated hydrocarbons were added. In contrast, rates per cell of adenine uptake and adenine incorporation into deoxyribonucleic acid and total nucleic acids by the cells treated with PCBs were significantly lower than in control cells. Intracellular adenine pools of cells whose growth was inhibited to 20% of the control rate by PCBs were 30% smaller and appeared to require a longer interval to equilibrate than those of untreated cells. This may indicate impaired transport and/or efflux of this nucleic acid precursor through the membrane of affected cells. Inhibition of nucleic acid synthesis in this sensitive bacterium by PCBs could explain the observed inhibitory effects of the chlorinated hydrocarbons on its growth.

The leucine-responsive regulatory protein, a global regulator of metabolism in Escherichia coli

The leucine-responsive regulatory protein (Lrp) regulates the expression of more than 40 genes and proteins in Escherichia coli. Among the operons that are positively regulated by Lrp are operons involved in amino acid biosynthesis (ilvIH, serA)), in the biosynthesis of pili (pap, fan, fim), and in the assimilation of ammonia (glnA, gltBD). Negatively regulated operons include operons involved in amino acid catabolism (sdaA, tdh) and peptide transport (opp) and the operon coding for Lrp itself (lrp). Detailed studies of a few members of the regulon have shown that Lrp can act directly to activate or repress transcription of target operons. A substantial fraction of operons regulated by Lrp are also regulated by leucine, and the effect of leucine on expression of these operons requires a functional Lrp protein. The patterns of regulation are surprising and interesting: in some cases activation or repression mediated by Lrp is antagonized by leucine, in other cases Lrp-mediated activation or repression is potentiated by leucine, and in still other cases leucine has no effect on Lrp-mediated regulation. Current research is just beginning to elucidate the detailed mechanisms by which Lrp can mediate such a broad spectrum of regulatory effects. Our view of the role of Lrp in metabolism may change as more members of the regulon are identified and their regulation characterized, but at this point Lrp seems to be important in regulating nitrogen metabolism and one-carbon metabolism, permitting adaptations to feast and to famine.

Lrp, a leucine-responsive protein, regulates branched-chain amino acid transport genes in Escherichia coli

We investigated the relationship between two regulatory genes, livR and lrp, that map near min 20 on the Escherichia coli chromosome. livR was identified earlier as a regulatory gene affecting high-affinity transport of branched-chain amino acids through the LIV-I and LS transport systems, encoded by the livJ and livKHMGF operons. lrp was characterized more recently as a regulatory gene of a regulon that includes operons involved in isoleucine-valine biosynthesis, oligopeptide transport, and serine and threonine catabolism. The expression of each of these livR- and lrp-regulated operons is altered in cells when leucine is added to their growth medium. The following results demonstrate that livR and lrp are the same gene. The lrp gene from a livR1-containing strain was cloned and shown to contain two single-base-pair substitutions in comparison with the wild-type strain. Mutations in livR affected the regulation of ilvIH, an operon known to be controlled by lrp, and mutations in lrp affected the regulation of the LIV-I and LS transport systems. Lrp from a wild-type strain bound specifically to several sites upstream of the ilvIH operon, whereas binding by Lrp from a livR1-containing strain was barely detectable. In a strain containing a Tn10 insertion in lrp, high-affinity leucine transport occurred at a high, constitutive level, as did expression from the livJ and livK promoters as measured by lacZ reporter gene expression. Taken together, these results suggest that Lrp acts directly or indirectly to repress livJ and livK expression and that leucine is required for this repression. This pattern of regulation is unusual for operons that are controlled by Lrp.

Genetic studies of mutants in a high-affinity methionine transport system in Salmonella typhimurium

A total of 30 metP mutations defective in the high-affinity methionine transport system were linked in P1 transduction to the zaf-1351::Tn10 insertion mutation at min 5-6 on the Salmonella typhimurium chromosome map. The relationship of metP to several other markers in this region was studied. Methionine transport was strongly inhibited by arsenate, suggesting that the metP system belongs to the shock-sensitive category and possesses a periplasmic binding protein. However, other experiments provided less clear cut evidence. Transport activity was only slightly reduced by osmotic shock; a methionine binding activity was detected in shock fluids from the wild-type strain, and although this activity was reduced by 50% in 3 frameshift mutants, mutants without any activity were not found. No differences were detected in the shock fluids of the 30 mutants when examined by SDS-polyacrylamide gel electrophoresis.

Leucine transport system in a facultatively alkalophilic Bacillus

Some characterizations of the leucine transport system in a facultative alkalophile, which is able to grow over a wide pH range from 7.0 to 10.5, were attempted. Although the direction of a transmembrane pH gradient of the bacterium below pH 8.2 is opposite to that above pH 8.2 (N. Koyama and Y. Nosoh (1985) Biochim. Biophys. Acta 812, 206-212), leucine transport is likely to be driven only by sodium electrochemical potential irrespective of the external pH. It was suggested that histidine and sulfhydryl groups in the leucine transporter are involved in the translocation mechanism and the pK value of the histidine residue involved is approximately 7.0.

Utilization of D-amino acids by dadR mutants of Salmonella typhimurium

Utilization of D-amino acids being substrates of D-amino acid dehydrogenase of Salmonella typhimurium was examined. The experiments were done with wild type strains and the mutants dadA missing the enzyme activity and dadR in which its synthesis is released from catabolite repression. Growth on D-tryptophan, D-histidine and D-methionine used as precursors of the L-amino acids was faster when the respective auxotrophs carried dadR mutations. The dadR mutants grew faster when D-or L-alanine was present as a sole source of nitrogen. Experiments with D-amino acid dehydrogenase in vitro provided evidence that D-tryptophan is its substrate with a very low affinity to the dehydrogenase.

Transport and utilization of D-methionine and other methionine sources in Escherichia coli

The transport and utilization of D-methionine was investigated in several strains of Escherichia coli K-12. Wild-type cells exhibit a single transport system with a Km of 1.16 muM. This activity exhibits a specificity similar to that of the uptake of L-methionine. The activity toward the D-isomer and the high-affinity uptake of L-methionine are lost in strains mutant in metD, along with the ability to utilize D-methionine as methionine source. Both activities respond identically to gene dosage of metD and are both restored in revertants or transductants. However, although L-methionine is a potent inhibitor of D-methionine uptake, D-methionine has little or no effect on the uptake of the L-isomer. No mutants altered in the uptake of only one of the two isomers were found in a screening. Regulation of both activities was similar in their response to the internal methionine pool, and some evidence was suggestive of partial repressive control of these activities. The evidence is most consistent with the role of the metD product as a common step for two methionine-specific uptake systems, but other gene products may represent the initial substrate binding sites. This system also appears to be involved in the uptake of N-acetyl methionine and methionine sulfoxide and methionine sulfoximine. The uptake of the keto analogue of methionine, alpha-keto-gamma-methiol butyrate, appears to be mediated by a separate system specific for alpha-keto straight-chain acids 5- to 6-carbon units in length.

Regulation of branched-chain amino acid transport in Escherichia coli

The repression and derepression of leucine, isoleucine, and valine transport in Escherichia coli K-12 was examined by using strains auxotrophic for leucine, isoleucine, valine, and methionine. In experiments designed to limit each of these amino acids separately, we demonstrate that leucine limitation alone derepressed the leucine-binding protein, the high-affinity branched-chain amino acid transport system (LIV-I), and the membrane-bound, low-affinity system (LIV-II). This regulation did not seem to involve inactivation of transport components, but represented an increase in the differential rate of synthesis of transport components relative to total cellular proteins. The apparent regulation of transport by isoleucine, valine, and methionine reported elsewhere was shown to require an intact leucine, biosynthetic operon and to result from changes in the level of leucine biosynthetic enzymes. A functional leucyl-transfer ribonucleic acid synthetase was also required for repression of transport. Transport regulation was shown to be essentially independent of ilvA or its gene product, threonine deaminase. The central role of leucine or its derivatives in cellular metabolism in general is discussed.

Role of D-tryptophan oxidase in D-tryptophan utilization by Escherichia coli

Mutants of Escherichia coli K-12 that require L-tryptophan (trp) are normally unable to utilize D-tryptophan to fulfill their requirement. However, secondary mutations (dadR) that confer this ability can be isolated. In such strains two distinct enzymes are found to be produced at high levels: D-amino acid oxidase (EC 1.4.3.3) and D-tryptophan oxidase. A convenient assay procedure for D-tryptophan oxidase is described. The two enzymes could be distinguished on the basis of their sensitivity to inhibition by L-phenylalanine and L-tyrosine. Strains that were trp dadR could not grow with D-tryptophan in the presence of L-phenylalanine, but further mutations, Fyo, could be isolated that allowed growth under these conditions. Some of them were characterized by further increases in the level of D-tryptophan oxidase activity and a sharp decrease in D-amino acid oxidase. These kinds of Fyo mutations lay in or near the dadR gene. The substrate specificity of the two enzymes toward a large number of compounds was examined. The transamination of aromatic keto acids was investigated. In the wild-type strain only a single enzyme, transaminase A (EC 2.6.1.5), was found, and it was irreversibly activated when subjected to elevated temperatures. The present state of our knowledge on D-amino acid utilization in E. coli is summarized.

Binding proteins and membrane transport

The recent studies have clearly established two types of active transport systems. One type is membrane-bound and can be observed in membrane vesicles and the other type is osmotic-shock-sensitive and requires binding proteins to produce active transport. It appears that the membrane-bound systems derive cellular energy from the energy-rich membrane state which can be formed from respiration or ATP-hydrolysis, while the binding protein systems are more directly coupled to phosphate bond energy derived from glycolysis or oxidative phosphorylation. The following conclusions concerning the role of the binding proteins are offered: 1. The binding proteins are present in relatively large amounts (approximately 10(-6) or 10%-5) M) and appear to reside in the periplasmic space. 2. They do not appear to be involved in solute translocation steps, although they cantain a second binding site that could interact with membrane components. 3. The binding proteins appear to increase the affinity of the transport system for the solute by interacting with a membrane component. This may substrate for the membrane transport system.

Role of leucyl-tRNA synthetase in regulation of branched-chain amino-acid transport

The regulation of the transport of leucine, isoleucine, and valine in Escherichia coli B/r was studied in a mutant with a complete deletion of the leucine biosynthetic operon and a temperature-sensitive leucyl-tRNA synthetase [L-leucine:tRNALeu ligase (AMP-forming), EC 6.1.1.4]. Under conditions of excess leucine and a functional leucyl-tRNA synthetase transport activity was repressed. Shifting the culture to a temperature at which the activation of leucine to an appropriate tRNA species became growth-rate-limiting led to a large increase in the high-affinity transport of leucine, isoleucine, and valine (system LIV-I) while the uptake of histidine and proline was unchanged. A similar increase was observed for branched-chain amino-acid binding protein activity. The temperature change did not alter the transport activity for any of these substrates or the level of the binding proteins in an isogenic strain with a normal leucyl-tRNA synthetase. The increase in transport activity observed in the mutant was prevented by inhibitors of protein and RNA synthesis and probably represents an increase in the differential rate of synthesis of the protein(s) required for transport. These experiments demonstrate that the repression of branched-chain amino-acid transport involves the interaction of leucine with its aminoacyl-tRNA synthetase and its cognate leucyl-tRNA species.

Transport of L-4-azaleucine in Escherichia coli

The uptake of L-4-azaleucine was examined in Escherichia coli K-12 strains to determine the systems that serve for its accumulation. L-4=Azaleucine in radio-labeled form was synthesized and resolved by the action of hog kidney N-acylamino-acid amidohydrolase (EC 3.5.1.B) on the racemic alpha-N-acetyl derivative of DL-[dimethyl-14C]4-azaleucine. L-4-Azaleucine is taken up in E. coli by energy-dependent processes that are sensitive to changes in the pH and to inhibition by leucine and the aromatic amino acids. Although a single set of kinetic parameters was obtained by kinetic experiments, other evidence indicates that transport systems for both the aromatic and the branched-chain amino acids serve for azaleucine. Azaleucine uptake in strain EO317, with a mutation leading to derepression and constitutive expression of branched-chain amino acid (LIV) transport and binding proteins, was not repressed by growth with leucine as it was in parental strain EO300. Lesions in the aromatic amino acid transport system, aroP, also led to changes in the regulation of azaleucine uptake activity when cells were grown on phenylalanine. Experiments on the specificity of azaleucine uptake and exchange experiments with leucine and phenylalanine support the hypothesis that both LIV and aroP systems transport azaleucine. The ability of external azaleucine to exchange rapidly with intracellular leucine may be an important contributor to azaleucine toxicity. We conclude from these and other studies that at least four other process may affect azaleucine sensitivity: the level of branched-chain amino acid biosynthetic enzymes; the level of leucine, isoleucine, and valine transport systems; the level of the aromatic amino acid, aroP, uptake system; and, possibly, the ability of the cell to racemize D and L amino acids. The relative importance of these processes in azaleucine sensitivity under various conditions is not known precisely.

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.

Transport of biosynthetic intermediates: homoserine and threonine uptake in Escherichia coli

Although amino acid transport has been extensively studied in bacteria during the past decade, little is known concerning the transport of those amino acids that are biosynthetic intermediates or have multiple fates within the cell. We have studied homoserine and threonine as examples of this phenomenon. Homoserine is transported by a single system which it shares with alanine, cysteine, isoleucine, leucine, phenylalanine, threonine, tyrosine, and valine. The evidence for this being the sole system for homoserine transport is (i) a linear double-reciprocal plot showing a homoserine K(m) of 9.6 x 10(-6) M, (ii) simultaneous reduction by 85% of homoserine and branched-chain amino acid uptake in a mutant selected for its inability to transport homoserine, and (iii) simultaneous reduction by 94% of the uptake of homoserine and the branched-chain amino acids by cells grown in millimolar leucine. Threonine, in addition to sharing the above system with homoserine, is transported by a second system shared with serine. The evidence for this second system consists of (i) incomplete inhibition of threonine uptake by any single amino acid, (ii) only 70% loss of threonine uptake in the mutant unable to transport homoserine, and (iii) only 40% reduction of threonine uptake when cells are grown in millimolar leucine. In this last case, the remaining threonine uptake can only be inhibited by serine and the inhibition is complete.

Multiplicity of isoleucine, leucine, and valine transport systems in Escherichia coli K-12

The kinetics of isoleucine, leucine, and valine transport in Escherichia coli K-12 has been analyzed as a function of substrate concentration. Such analysis permits an operational definition of several transport systems having different affinities for their substrates. The identification of these transport systems was made possible by experiments on specific mutants whose isolation and characterization is described elsewhere. The transport process with highest affinity was called the "very-high-affinity"process. Isoleucine, leucine, and valine are substrates of this transport process and their apparent K(m) values are either 10(-8), 2 x 10(-8), or 10(-7) M, respectively. Methionine, threonine, and alanine inhibit this transport process, probably because they are also substrates. The very-high-affinity transport process is absent when bacteria are grown in the presence of methionine, and this is due to a specific repression. Methionine and alanine were also found to affect the pool size of isoleucine and valine. Another transport process is the "high-affinity" process. Isoleucine, leucine, and valine are substrates of this transport process, and their apparent K(m) value is 2 x 10(-6) M for all three. Methionine and alanine cause very little or no inhibition, whereas threonine appears to be a weak inhibitor. Several structural analogues of the branched-chain amino acids inhibit the very-high-affinity or the high-affinity transport process in a specific way, and this confirms their existence as two separate entities. Three different "low-affinity" transport processes, each specific for either isoleucine or leucine or valine, show apparent K(m) values of 0.5 x 10(-4) M. These transport processes show a very high substrate specificity since no inhibitor was found among other amino acids or among many branched-chain amino acid precursors or analogues tried. The evolutionary significance of the observed redundancy of transport systems is discussed.

Multiplicity of leucine transport systems in Escherichia coli K-12

The major component of leucine uptake in Escherichia coli K-12 is a common system for l-leucine, l-isoleucine, and l-valine (LIV-I) with a Michaelis constant (K(m)) value of 0.2 muM (LIV-I system). The LIV-binding protein appears to be associated with this system. It now appears that the LIV-I transport system and LIV-binding protein also serve for the entry of l-alanine, l-threonine, and possibly l-serine. A minor component of l-leucine entry occurs by a leucine-specific system (L-system) for which a specific leucine-binding protein has been isolated. A mutant has been obtained that shows increased levels of the LIV-I transport activity and increased levels of both of the binding proteins. Another mutant has been isolated that shows only a major increase in the levels of the leucine-specific transport system and the leucine-specific binding protein. A third binding protein that binds all three branched-chain amino acids but binds isoleucine preferentially has been identified. The relationship of the binding proteins to each other and to transport activity is discussed. A second general transport system (LIV-II system) with a K(m) value of 2 muM and a relatively low V(max) can be observed in E. coli. The LIV-II system is not sensitive to osmotic shock treatment nor to growth of cells in the presence of leucine. This high K(m) system, which is specific for the branched-chain amino acids, can be observed in membrane vesicle preparations.