Genetic analysis of the glutamate permease in Escherichia coli K-12

Emerging roles for ABC transporters as virulence factors in uropathogenic Escherichia coli

Urinary tract infections (UTI) account for a substantial financial burden globally. Over 75% of UTIs are caused by uropathogenic Escherichia coli (UPEC), which have demonstrated an extraordinarily rapid growth rate in vivo. This rapid growth rate appears paradoxical given that urine and the human urinary tract are relatively nutrient-restricted. Thus, we lack a fundamental understanding of how uropathogens propel growth in the host to fuel pathogenesis. Here, we used large in silico, in vivo, and in vitro screens to better understand the role of UPEC transport mechanisms and their contributions to uropathogenesis. In silico analysis of annotated transport systems indicated that the ATP-binding cassette (ABC) family of transporters was most conserved among uropathogenic bacterial species, suggesting their importance. Consistent with in silico predictions, we determined that the ABC family contributed significantly to fitness and virulence in the urinary tract: these were overrepresented as fitness factors in vivo (37.2%), liquid media (52.3%), and organ agar (66.2%). We characterized 12 transport systems that were most frequently defective in screening experiments by generating in-frame deletions. These mutant constructs were tested in urovirulence phenotypic assays and produced differences in motility and growth rate. However, deletion of multiple transport systems was required to achieve substantial fitness defects in the cochallenge murine model. This is likely due to genetic compensation among transport systems, highlighting the centrality of ABC transporters in these organisms. Therefore, these nutrient uptake systems play a concerted, critical role in pathogenesis and are broadly applicable candidate targets for therapeutic intervention.

H-NS controls metabolism and stress tolerance in Escherichia coli O157:H7 that influence mouse passage

BACKGROUND

H-NS is a DNA-binding protein with central roles in gene regulation and nucleoid structuring in Escherichia coli. There are over 60 genes that are influenced by H-NS many of which are involved in metabolism. To determine the significance of H-NS-regulated genes in metabolism and stress tolerance, an hns mutant of E. coli O157:H7 was generated (hns::nptI, FRIK47001P) and its growth, metabolism, and gastrointestinal passage compared to the parent strain (43895) and strain FRIK47001P harboring pSC0061 which contains a functional hns and 90-bp upstream of the open-reading frame.

RESULTS

The hns mutant grew slower and was non-motile in comparison to the parent strain. Carbon and nitrogen metabolism was significantly altered in the hns mutant, which was incapable of utilizing 42 carbon, and 19 nitrogen sources that the parent strain metabolized. Among the non-metabolized substrates were several amino acids, organic acids, and key metabolic intermediates (i.e., pyruvate) that limit carbon acquisition and energy generation. Growth studies determined that the parent strain grew in LB containing 14 to 15% bile or bile salts, while the hns mutant grew in 6.5 and 9% of these compounds, respectively. Conversely, log-phase cells of the hns mutant were significantly (p < 0.05) more acid tolerant than the parent strain and hns mutant complemented with pSC0061. In mouse passage studies, the parent strain was recovered at a higher frequency (p < 0.01) than the hns mutant regardless of whether log- or stationary-phase phase cells were orally administered.

CONCLUSION

These results demonstrate that H-NS is a powerful regulator of carbon and nitrogen metabolism as well as tolerance to bile salts. It is likely that the metabolic impairments and/or the reduced bile tolerance of the E. coli O157:H7 hns mutant decreased its ability to survive passage through mice. Collectively, these results expand the influence of H-NS on carbon and nitrogen metabolism and highlight its role in the ability of O157:H7 strains to respond to changing nutrients and conditions encountered in the environment and its hosts.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Roles of glutamate synthase, gltBD, and gltF in nitrogen metabolism of Escherichia coli and Klebsiella aerogenes

Mutants of Escherichia coli and Klebsiella aerogenes that are deficient in glutamate synthase (glutamate-oxoglutarate amidotransferase [GOGAT]) activity have difficulty growing with nitrogen sources other than ammonia. Two models have been proposed to account for this inability to grow. One model postulated an imbalance between glutamine synthesis and glutamine degradation that led to a repression of the Ntr system and the subsequent failure to activate transcription of genes required for the use of alternative nitrogen sources. The other model postulated that mutations in gltB or gltD (which encode the subunits of GOGAT) were polar on a downstream gene, gltF, which is necessary for proper activation of gene expression by the Ntr system. The data reported here show that the gltF model is incorrect for three reasons: first, a nonpolar gltB and a polar gltD mutation of K. aerogenes both show the same phenotype; second, K. aerogenes and several other enteric bacteria lack a gene homologous to gltF; and third, mutants of E. coli whose gltF gene has been deleted show no defect in nitrogen metabolism. The argument that accumulated glutamine represses the Ntr system in gltB or gltD mutants is also incorrect, because these mutants can derepress the Ntr system normally so long as sufficient glutamate is supplied. Thus, we conclude that gltB or gltD mutants grow slowly on many poor nitrogen sources because they are starved for glutamate. Much of the glutamate formed by catabolism of alternative nitrogen sources is converted to glutamine, which cannot be efficiently converted to glutamate in the absence of GOGAT activity. Finally, GOGAT-deficient E. coli cells growing with glutamine as the sole nitrogen source increase their synthesis of the other glutamate-forming enzyme, glutamate dehydrogenase, severalfold, but this is still insufficient to allow rapid growth under these conditions.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Ordered binding model as a general mechanistic mechanism for secondary active transport systems

The mechanistic mechanism of secondary active transport processes has not been fully elucidated. Based on substrate binding studies dependent on coupling cation concentrations of the glutamate, melibiose, lactose and proline transport carriers in Escherichia coli, the ordered binding mechanism was proposed as the energy coupling mechanism of the transport systems. This ordered binding mechanism satisfactorily explained the properties of the secondary active transport systems. Thus, this mechanism as the general energy coupling mechanism for the transport systems is discussed.

Characterization of the Escherichia coli K12 gltS glutamate permease gene

The gltS gene is known to encode a sodium-dependent, glutamate-specific permease. We have localized the Escherichia coli K12 gltS gene with respect to the spoT gene, sequenced it, and recombined a null insertion-deletion allele into the chromosome without loss of viability. The gltS null allele gives a Glt- phenotype, i.e. it abolishes the ability of a gltCc host to grow on glutamate as sole carbon and nitrogen source and also confers alpha-methylglutamate resistance. A multicopy plasmid expressing the gltS gene can reverse the Glt- phenotype of gltS- or wild-type strains while other plasmids show host-dependent complementation patterns. Induction of gltS gene overexpression under control of isopropyl-beta-D-thiogalactoside (IPTG)-inducible promoters severely inhibits growth. The GltS protein is deduced to be a 42425 dalton hydrophobic protein with 2 sets of 5 possible integral protein domains, each flanking a central hydrophilic, flexible region.

Cloning and sequencing of a gene encoding a glutamate and aspartate carrier of Escherichia coli K-12

A gene encoding a carrier protein for glutamate and aspartate was cloned into Escherichia coli K-12 strain BK9MDG by using the high-copy-number plasmid pBR322. The gene (designated gltP) is probably identical to a gene recently cloned from E. coli B (Y. Deguchi, I. Yamato, and Y. Anraku, J. Bacteriol. 171:1314-1319). A 1.6-kilobase DNA fragment containing gltP was subcloned into the expression plasmids pT7-5 and pT7-6, and its product was identified by a phage T7 RNA polymerase-T7 promoter coupled system (S. Tabor and C. C. Richardson, Proc. Natl. Acad. Sci. USA 82:1074-1078) as a polypeptide with an apparent mass of 38 kilodaltons. A portion of the gltP polypeptide was associated with the cytoplasmic membrane. The nucleotide sequence of the 1.6-kilobase fragment was determined. It contained an open reading frame capable of encoding a highly hydrophobic polypeptide of 395 amino acids, containing four possible transmembrane segments. Uptake of glutamate and aspartate was increased 5.5- and 4.5-fold, respectively, in strains containing gltP plasmids. Glutamate uptake was insensitive to the concentration of Na+ and was inhibited by L-cysteate and beta-hydroxyaspartate. These results suggest that gltP is a structural gene for a carrier protein of the Na(+)-independent, binding-protein-independent glutamate-aspartate transport system.

Molecular cloning of gltS and gltP, which encode glutamate carriers of Escherichia coli B

Two genes encoding distinct glutamate carrier proteins of Escherichia coli B were cloned into an E. coli K-12 strain by using a cosmid vector, pHC79. One of them was the gltS gene coding for a glutamate carrier of an Na+-dependent, binding protein-independent, and glutamate-specific transport system. The content of the glutamate carrier was amplified about 25-fold in the cytoplasmic membranes from a gltS-amplified strain. The gltS gene was located in a 3.2-kilobase EcoRI-MluI fragment, and the gene product was identified as a membrane protein with an apparent Mr of 35,000 in a minicell system. A gene designated gltP was also cloned. The transport activity of the gltP system in cytoplasmic membrane vesicles from a gltP-amplified strain was driven by respiratory substrates and was independent of the concentrations of Na+, K+, and Li+. An uncoupler, carbonylcyanide m-chlorophenylhydrazone, completely inhibited the transport activities of both systems, whereas an ionophore, monensin, inhibited only that of the gltS system. The Kt value for glutamate was 11 microM in the gltP system and 3.5 microM in the gltS system. L-Aspartate inhibited the glutamate transport of the gltP system but not that of the gltS system. Aspartate was taken up actively by membrane vesicles from the gltP-amplified strain, although no aspartate uptake activity was detected in membrane vesicles from a wild-type E. coli strain. These results suggest that gltP is a structural gene for a carrier protein of an Na+-independent, binding protein-independent glutamate-aspartate transport system.

Accumulation of amino acids and glucose in isolated epitheliocytes from the small intestine of chickens depending on both breed and sex

In an experiment with chickens breed differences were established in accumulation of 14C-lysine, methionine and glycine, as well as intra-line differences in glycine accumulation in intestinal epitheliocytes from Plymouth Rock chickens. Differences were found in both methionine and glycine accumulation, between B line and four-line hybrids as well. Methionine accumulation in epitheliocytes from the small intestine of male chickens was higher than in female chickens from A line, higher than accumulation of tryptophane in both A and C lines and accumulation of glucose in D line. In male chickens of four-line hybrid, glycine accumulation was higher than in females.

Biochemical and genetic characterization of L-glutamate transport and utilization in Salmonella typhimurium LT-2 mutants

Two systems for L-glutamate transport were found in Salmonella typhimurium LT-2 GltU+ (glutamate utilization) mutants. The first one is similar to the glt system previously described in Escherichia coli; by transductional analysis the structural gene, gltS, coding for the transport protein was located at minute 80 of the chromosome as part of the operon gltC-gltS, and its regulator, the gltR gene, near minute 90; the gltS gene product transports both L-glutamate and L-aspartate, is sodium independent, and is beta-hydroxyaspartate sensitive. The second transport system, whose structural gene was called gltF and is located at minute 0, was L-glutamate specific, sodium independent, and alpha-methylglutamate sensitive. Two aspartase activities occurred in S. typhimurium LT-2: the first one was present only in the GltU+ mutants, had a pH 6.4 optimum, was essential for both L-glutamate and L-aspartate metabolism, and mapped at minute 94, close to the ampC gene. The second one had a pH 7.2 optimum, could be induced by several amino acids, and thus may have a general role in nitrogen metabolism.

Genetic characterization of the glutamate dehydrogenase gene (gdhA) of Salmonella typhimurium

Salmonella typhimurium mutants, either devoid or glutamate dehydrogenase activity or having a thermolabile glutamate dehydrogenase protein, were used to identify the structural gene (gdhA) for this enzyme. Transductions showed that the mutations producing these phenotypes were linked to both the pncA and nit genes, placing the gdhA locus between 23 and 30 U on the S. typhimurium chromosome. Additional transductions with several Tn10 insertions established the gene order as pncA-gdhA-nit. Since few genetic markers exist in this region of the chromosome, Hfr strains were constructed to orient the pncA-gdhA-nit cluster with outside genes. Conjugation experiments provided evidence for the gene order pyrD-pncA-gdhA-nit-trp. To further characterize gdhA, we used Mu cts d1 (Apr lac) insertions in this gene to select numerous strains containing deletions with various endpoints. Transductions of these deletions with strains containing different gdh mutations and with a mutant having a thermolabile glutamate dehydrogenase protein permitted us to construct a deletion map of the gdhA region.

Regulation of amino acid transport in Thiobacillus thioparus

Amino acid transport in amino acid auxotrophs of Thiobacillus thioparus was enhanced during growth on rate-limiting amino acid concentration. A pleiotropic mutation enhanced general amino acid transport as manifested by higher values of Vmax of amino acid transport. Affinity constants remained unaltered. Mutants with enhanced transport properties did not show changes in oxidation of thiosulfate, did not oxidize various organic compounds, and did not increase the heterotrophic potential of T. thioparus. The mutations for enhanced transport caused increased synthesis of amino acid transport system components. A method for genetic transformation of T. thioparus is described.

Inhibition of growth by imidazol(on)e propionic acid: evidence in vivo for coordination of histidine catabolism with the catabolism of other amino acids

Imidazole propionic acid (ipa), a gratuitous inducer of the histidine-utilization (hut) system in Salmonella typhimurium, inhibits the organism's growth on succinate minimal medium. Induction of the hut system is necessary, but not sufficient, to cause inhibition. A study of the ability of single amino acids to relieve ipa-restricted growth suggests that insufficient glutamate is the cause of slow growth. The inhibition of growth by imidazolone propionic acid (iopa), an intermediate in the catabolism of histidine to glutamate, is similar to that by ipa. Studies using 2, 3, 5-triphenyl tetrazolium chloride plates to examine amino acid catabolism suggest that accumulation of ipa or iopa leads to inactivation of aspartate amino-transferase (AAT). This interpretation is supported by studies of an Escherichia coli mutant lacking AAT. The mutant grows poorly on succinate minimal medium, and the poor growth is relieved by the same amino acids that relieve ipa- and iopa-restricted growth. These and other findings are discussed in terms of coordination of the histidine-utilization system with enzymatic activities involved in the catabolism of other amino acids.

Escherichia coli mutants deficient in deoxyuridine triphosphatase

Mutants deficient in deoxyuridine triphosphatase (dUTPase) were identified by enzyme assays of randomly chosen heavily mutagenized clones. Five mutants of independent origin were obtained. One mutant produced a thermolabile enzyme, and it was presumed to have a mutation in the structural gene for dUTPase, designated dut. The most deficient mutant had the following associated phenotypes: less than 1% of parental dUTPase activity, prolonged generation time, increased sensitivity to 5'-fluorodeoxyuridine, increased rate of spontaneous mutation, increased rate of recombination (hyper-Rec), an inhibition of growth in the presence of 2 mM uracil, and a decreased ability to support the growth of phage P1 (but not T4 or lambda). This mutation also appeared to be incompatible with pyrE mutations. A revertant selected by its faster growth had regained dUTPase activity and lost its hyper-Rec phenotype. Many of the properties of the dut mutants are compatible with their presumed increased incorporation of uracil into DNA and the subsequent transient breakage of the DNA by excision repair.

Properties of an Escherichia coli K-12 mutant defective in the transport of arginine and ornithine

A canavanine-resistant mutant strain, defective in the transport of arginine and ornithine, was isolated and characterized. Experiments presented show that both the kinetics of influx and the steady state of accumulation of arginine and ornithine are affected by the mutation, whereas the activity of other related transport systems remains unchanged. On the basis of competitive studies, it is concluded that L-canavanine can inhibit efficiently the arginine-specific uptake system. D-Arginine appears to be a moderate inhibitor. None of the basic amino acid-binding proteins of the mutant strain showed detectable alterations in terms of quantity, physical properties, or affinity constants. Studies on the relationship between the number of transport carriers and the steady state of accumulation of arginine suggested the presence of a reduced number of membrane carriers in the mutant strain. It is proposed that the mutation affects a regulatory gene concerned with controlling the amount of membrane carriers produced, which are components of the arginine- and ornithine-specific uptake systems. The mutation maps at min 62 on the recalibrated linkage map of Escherichia coli K-12, in a locus closely linked or identical to argP.

1. Membrane vesicles of Escherichia coli K-12 CS7, a strain gentically derepressed for glutamate permease, maintain low aspartate transport activity, like that of prep

1. Membrane vesicles of Escherichia coli K-12 CS7, a strain gentically derepressed for glutamate permease, maintain low aspartate transport activity, like that of preparations of the wild-type parent. Growth of the parent CS101 on aspartate as the source of carnon or nitrogen results in derepression of both asparatate and glytamate transport. Growth of strain CS7 on aspartate derepresses aspartate transport to the same extent as in strains CS101, but only slightly increases the derepressed level of glutamate transport activity. 2. The affinity of the membrane transport system for glutamate is enhanced by sodium, while that for asparate is not. 3. Although the affinities for glutamate (23 muM) and aspartate (12 muM) are similar, aspartate does not inhibit glutamate transport, while glutamate competitively inhibits aspartate transport. 4. Aspartate transport, but not glutamate transport, is competitively inhibited by C4 dicarboxylic acids, whereas 2-oxoglutarate competitively inhibits glutamate transport, but not aspartate transport. 5. Competitive inhibition of L-aspartate transport by L-glutamate and by the 5-methyl ester of L-glutamate is abolished in the presence of 2-oxoglutarate. However, 2-oxoglutarate does not affect the competitive inhibition of L-aspartate transport by D-aspartate and by DL-threo-3-hydroxyaspartate. The relationship between the two dicarboxylic amino acid transport systems and the spatial characteristics of the aspartate carrier are discussed in the light of these findings.

Glutamate transport in membrane vesicles of the wild-type strain and glutamate-utilizing mutants of Escherichia coli

A highly specific energy-dependent glutamate transport system was demonstrated in membrane vesicles of glutamate-utilizing Escherichia coli K-12 mutants. The glutamate transport activity of membranes from the parent strain, unable to grow on glutamate, was very low. With ascorbate-phenazine methosulfate as the electron donor, mutant preparations displayed 17 to 20 times higher activity than did the wild type. However, the affinity of the mutant carrier for L-glutamate remained the same as in the parent strain. Comparative inhibition analysis of glutamate transport in whole cells and membrane vesicles and of in vitro binding of glutamate to a specific periplasmic-binding protein suggests that under certain conditions the latter may be a component of the E. coli K-12 glutamate transport system.

Effect of growth conditions on glutamate transport in the wild-type strain and glutamate-utilizing mutants of Escherichia coli

The effects of growth conditions on the glutamate transport activity of intact cells and membrane vesicles and on the levels of glutamate-binding protein in wild-type Escherichia coli K-12 CS101 and in two glutamate-utilizing mutants, CS7 and CS2TC, were studied. Growth of CS101 on aspartate as the sole source of carbon or nitrogen resulted in a severalfold increase in glutamate transport activity of intact cells and membrane preparations to levels characteristic of the operator-constitutive mutant CS7. The high glutamate transport activity of mutant CS7 was not depressed further by growth on aspartate. Synthesis of glutamate-binding protein was not enhanced by aspartate in either strain. Mutant CS2TC produces a heat-labile repressor of glutamate permease synthesis and is therefore able to grow on glutamate at 42 C but not at 30 C. CS2TC cells grown in a glycerol-minimal medium at the restrictive temperature (30 C) exhibit low glutamate transport activity. Growth on aspartate at 30 C results in derepressed synthesis of glutamate permease. Cells grown on glycerol at 42 C have high glutamate transport activity. No further derepression is obtained upon growth on aspartate. Growth of CS101 and CS7 in "rich broth" greatly reduces the levels of glutamate-binding protein but does not appreciably affect glutamate transport by whole cells or membrane preparations. The identity of the carrier and the role of the binding protein in glutamate transport are discussed in the light of these findings.

Purification and properties of glutamate binding protein from the periplasmic space of Escherichia coli K-12

Glutamate binding protein released from the periplasmic space of Escherichia coli K-12 by lysozyme-EDTA treatment was purified to homogeneity and its physical and chemical properties were studied. It is a basic protein with a pI of 9.1. Its molecular weight, determined in an analytical ultracentrifuge, and by gel filtration on Sephadex G-100 and dodecylsulphate acrylamide is 29 700, 27 800 and 32 000, respectively. The KD value for glutamate was 6.7 - 10- minus 6 M. L-Aspartate, reduced glutathione, G-glutamate-gamma-benzylester and L-glutamate-gamma-ethylester competitively inhibited glutamate binding with K-i; values of 7.8 - 10- minus 5, 1.1 - 10- minus 5, 1.0 - 10- minus 5 and 1.0 - 10- minus 5 M, respectively. Spheroplasts retained 40% of glutamate transport as compared to intact cells. The glutamate binding activity of a glutamate-utilizing strain (CS7), was 1.6 times as high as that of the glutamate non-utilizing parent strain (CS101). Similarly, the glutamate binding activity of a temperature conditional glutamate-utilizing mutant (CS2-TC) was 1.9 times higher when grown at the permissive temperature (42 degrees C) than when grown at the restrictive temperature (30 degrees C).

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

Homoserine is transported by a single system that it shares with alanine, isoleucine, leucine, phenylalanine, threonine, valine and perhaps cysteine, methionine, serine, and tyrosine. We investigated the regulation of this transport system and found that alanine, isoleucine, leucine, methionine, and valine each repress the homoserine-transporting system. From the concentration resulting in 50% repression of this transport system and the maximal amount of repression, we ranked the amino acids according to their effectiveness in repressing homoserine transport (in decreasing order): leucine>methionine>alanine>valine>isoleucine. The exponential rate of decrease in transport capacity after leucine addition equals the exponential growth rate of the culture, and protein synthesis is necessary for the derepression seen when leucine is removed. Threonine, in addition to using the above system, is transported by a second system shared with serine. We present further evidence for this serine-threonine transport system and show that it is not regulated like the homoserine-transporting system.

Sodium and potassium requirements for active transport of glutamate by Escherichia coli K-12

Active transport of glutamate by Escherichia coli K-12 requires both Na(+) and K(+) ions. Increasing the concentration of Na(+) in the medium results in a decrease in the K(m) of the uptake system for glutamate; the capacity is not affected. Glutamate uptake by untreated cells is not stimulated by K(+). K(+)-depleted cells show a greatly reduced capacity for glutamate uptake. Preincubation of such cells in the presence of K(+) fully restores their capacity for glutamate uptake when Na(+) ions are also present in the uptake medium. Addition of either K(+) or Na(+) alone restores glutamate uptake to only about 20% of its maximum capacity in the presence of both cations. Changes in K(+) concentration affect the capacity for glutamate uptake but have no effect on the K(m) of the glutamate transport system. Ouabain does not inhibit the (Na(+)-K(+))-stimulated glutamate uptake by intact cells or spheroplasts of E. coli K-12.

Mutants of Salmonella typhimurium able to utilize D-histidine as a source of L-histidine

Secondary mutants able to utilize d-histidine, dhu, were isolated in histidine auxotrophs of Salmonella typhimurium. Mutations of one class (dhuA) are closely linked with the hisP locus which codes for a component of histidine permease. The specific activity of l-histidine permeation was estimated as increased two- to seven-fold in dhuA mutants. The dhuB mutants which have not been mapped also had elevated specific activity of l-histidine permeation. The uptake of d-histidine, barely detectable in the parental strains, was prominent in dhuA mutants and showed an apparent Michaelis constant about 1,000-fold higher than that observed with l-histidine. No change was detected in the kinetics of l-histidine permeation. d- and l-histidine competed in the uptake process. Tertiary mutants which lost the ability to grow on d-histidine were isolated by ampicillin counter-selection in dhuA his(-) strains. All of them mapped in the dhuA hisP region. Most of them had all known properties of hisP mutants. It is inferred from these data that the dhuA mutations increase synthesis of components critical to d- and l-histidine permeation.

Formation of aromatic amino acid pools in Escherichia coli K-12

Phenylalanine, tyrosine, and tryptophan were taken up into cells of Escherichia coli K-12 by a general aromatic transport system. Apparent Michaelis constants for the three amino acids were 4.7 x 10(-7), 5.7 x 10(-7), and 4.0 x 10(-7)m, respectively. High concentrations (> 0.1 mm) of histidine, leucine, methionine, alanine, cysteine, and aspartic acid also had an affinity for this system. Mutants lacking the general aromatic transport system were resistant to p-fluorophenylalanine, beta-2-thienylalanine, and 5-methyltryptophan. They mapped at a locus, aroP, between leu and pan on the chromosome, being 30% cotransducible with leu and 43% cotransducible with pan. Phenylalanine, tyrosine, and tryptophan were also transported by three specific transport systems. The apparent Michaelis constants of these systems were 2.0 x 10(-6), 2.2 x 10(-6), and 3.0 x 10(-6)m, respectively. An external energy source, such as glucose, was not required for activity of either general or specific aromatic transport systems. Azide and 2,4-dinitrophenol, however, inhibited all aromatic transport, indicating that energy production is necessary. Between 80 and 90% of the trichloroacetic acid-soluble pool formed from a particular exogenous aromatic amino acid was generated by the general aromatic transport system. This contribution was abolished when uptake was inhibited by competition by the other aromatic amino acids or by mutation in aroP. Incorporation of the former amino acid into protein was not affected by the reduction in its pool size, indicating that the general aromatic transport system is not essential for the supply of external aromatic amino acids to protein synthesis.

Gene controlling L-glutamic acid decarboxylase synthesis in Escherichia coli K-12

Genetically related Escherichia coli K-12 strains were found to differ widely in their l-glutamic acid decarboxylase (GAD) activity. This variation is due to differences in the amount of GAD produced by the different cultures, rather than to the appearance of altered enzymes differing in catalytic activity. A regulatory gene, gadR, which controls the amount of GAD was mapped on the E. coli K-12 chromosome. A strain with a lesion in the structural gene for GAD is described.

Mutants of Escherichia coli K-12 with an altered glutamyl-transfer ribonucleic acid synthetase

Three streptomycin-suppressible lethal mutants of Escherichia coli K-12 have been shown to possess structurally altered glutamyl-transfer ribonucleic acid (tRNA) synthetases. Each mutant synthetase displays a K(m) value for glutamate which is 10-fold higher than the parental value, and the mutations reside in two widely separate loci on the genetic map. Mixing of the mutant extracts in pairs gave no indication of in vitro complementation. All three enzymes charge the minor tRNA(glu) fraction identically, but one (EM 120) charges the major fraction at a twofold lower rate than do the other two (EM 102 and EM 111). Possible explanations for the existence of the two synthetase loci are presented.

Sodium-stimulated transport of glutamate in Escherichia coli

Wild-type Escherichia coli B grew poorly on glutamate as the sole carbon source, except at very high concentrations of the amino acid. The addition of sodium ion markedly stimulated the growth. It had the same effect in a mutant of E. coli B selected for the ability to grow at low glutamate concentrations. Sodium ion also potentiated growth inhibition by analogues of glutamate. The uptake of glutamate by nongrowing cells of the mutant was markedly stimulated by sodium ion in the presence of an energy source, chloramphenicol, and arsenite, which retarded glutamate degradation.

Genetic and physiological analysis of glutamate decarboxylase in Escherichia coli K-12

No correlation was found between glutamate decarboxylase (GAD) activity and the ability of Escherichia coli K-12 strains to grow on glutamate. A gene, gad, determining GAD activity maps near gltC, which controls glutamate permease.