Structurally Defective Galactose-binding Protein Isolated from a Mutant Negative in the β-Methylgalactoside Transport System of Escherichia coli

The active transport of carbohydrates by Escherichia coli

The active transport of carbohydrates by Escherichia coli is discussed with particular reference to (1) identification of an uptake process as 'active transport', (2) nature and control of transport proteins, and (3) mechanisms of energy transduction. (1) The use of substrate analogues, of mutants blocked in metabolism and of subcellular vesicles in the isolation of the transport process from interference by subsequent metabolic reactions is described. Criteria are outlined for establishing that the solute is taken up against a concentration gradient and that this is energy-dependent. Three types of poisons for energy systems that act primarily on respiration, on ATP formation and as uncoupling ('proton conducting') agents are considered. (2) Methods are described for the selection of mutants impaired in the active uptake of specific carbohydrates. (3) Results show that the uptake of galactose, D-fucose and arabinose by appropriate strains of E. coli is inducible, specific and accompanied by proton uptake. Such and other data support a model based on a chemiosmotic theory of active transport.

Signal integration in the galactose network of Escherichia coli

The gal regulon of Escherichia coli contains genes involved in galactose transport and metabolism. Transcription of the gal regulon genes is regulated in different ways by two iso-regulatory proteins, Gal repressor (GalR) and Gal isorepressor (GalS), which recognize the same binding sites in the absence of d-galactose. DNA binding by both GalR and GalS is inhibited in the presence of d-galactose. Many of the gal regulon genes are activated in the presence of the adenosine cyclic-3',5'-monophosphate (cAMP)-cAMP receptor protein (CRP) complex. We studied transcriptional regulation of the gal regulon promoters simultaneously in a purified system and attempted to integrate the two small molecule signals, d-galactose and cAMP, that modulate the isoregulators and CRP respectively, at each promoter, using Boolean logic. Results show that similarly organized promoters can have different input functions. We also found that in some cases the activity of the promoter and the cognate gene can be described by different logic gates. We combined the transcriptional network of the galactose regulon, obtained from our experiments, with literature data to construct an integrated map of the galactose network. Structural analysis of the network shows that at the interface of the genetic and metabolic network, feedback loops are by far the most common motif.

Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein

We describe a glucose sensor based on a mutant glucose/galactose binding protein (GGBP) and phase-modulation fluorometry. The GGBP from Escherichia coli was mutated to contain a single cysteine residue at position 26. When labeled with a sulfhydryl-reactive probe 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid, the labeled protein displayed a twofold decrease in intensity in response to glucose, with a dissociation constant near 1 microM glucose. The ANS-labeled protein displayed only a modest change in lifetime, precluding lifetime-based sensing of glucose. A modulation sensor was created by combining ANS26-GGBP with a long-lifetime ruthenium (Ru) metal-ligand complex on the surface of the cuvette. Binding of glucose changed the relative intensity of ANS26-GGBP and the Ru complex, resulting in a dramatic change in modulation at a low frequency of 2.1 MHz. Modulation measurements at 2.1 MHz were shown to accurately determine the glucose concentration. These results suggest an approach to glucose sensing with simple devices.

Lateral diffusion of proteins in the periplasm of Escherichia coli

We have introduced biologically active, fluorescently labeled maltose-binding protein into the periplasmic space of Escherichia coli and measured its lateral diffusion coefficient by the fluorescence photobleaching recovery method. Diffusion of this protein in the periplasm was found to be surprisingly low (lateral diffusion coefficient, 0.9 X 10(-10) cm2 s-1), about 1,000-fold lower than would be expected for diffusion in aqueous medium and almost 100-fold lower than for an equivalent-size protein in the cytoplasm. Galactose-binding protein, myoglobin, and cytochrome c were also introduced into the periplasm and had diffusion coefficients identical to that determined for the maltose-binding protein. For all proteins nearly 100% recovery of fluorescence was obtained after photobleaching, indicating that the periplasm is a single contiguous compartment surrounding the cell. These data have considerable implications for periplasmic structure and for the role of periplasmic proteins in transport and chemotaxis.

Reconstitution of maltose chemotaxis in Escherichia coli by addition of maltose-binding protein to calcium-treated cells of maltose regulon mutants

Maltose chemotaxis was reconstituted in delta malE cells lacking maltose-binding protein (MBP). Purified MBP was introduced into intact cells during incubation with 250 mM CaCl2 in Tris-hydrochloride buffer at 0 degrees C. After removal of extracellular CaCl2 and MBP, chemotaxis was measured with tethered bacteria in a flow chamber or with free-swimming cells in a capillary assay. About 20% of tethered cells responded to 10(-4) M maltose; the mean response times were about half those of CaCl2-treated wild-type cells (100 s as opposed to 190 s). In capillary tests, the maltose response of reconstituted cells was between 15 and 40% of the aspartate response, about the same percentage as in wild-type cells. The best reconstitution was seen with 0.5 to 1 mM MBP in the reconstitution mixture, which is similar to the periplasmic MBP concentration estimated for maltose-induced wild-type cells. Strains containing large deletions of the malB region and malT mutants lacking the positive regulator gene of the mal regulon also could be reconstituted for maltose chemotaxis, showing that no product of the mal regulon other than MBP is essential for maltose chemotaxis.

Characterization of the mgl operon of Escherichia coli by transposon mutagenesis and molecular cloning

We used transposon insertion mutagenesis, molecular cloning, and a novel procedure for in vitro construction of polar and nonpolar insertion mutations to characterize the genetic organization and gene products of the beta-methylgalactoside (Mgl) transport system, which utilizes the galactose-binding protein. The data indicate that the mgl operon contained three genes, which were transcribed in the order mglB, mglA, and mglC. The first gene coded for the 31,000 Mr galactose-binding protein, which was synthesized as a 3,000-dalton-larger precursor form. The mglA product was a 50,000 Mr protein which was tightly associated with the membrane, and the mglC product was a 38,000 Mr protein which was apparently loosely associated with the membrane and was probably located on the internal face of the cytoplasmic membrane. Identification of gene products was facilitated by in vitro insertion of a fragment of Tn5 containing the gene conferring kanamycin resistance into a restriction site in the operon. The fragment proved to have a polar effect on the expression of promoter-distal genes only when inserted in one of the two possible orientations. The three identified gene products were necessary and apparently sufficient for transport activity, but only the binding protein was required for chemotaxis towards galactose. The transport system appeared to contain the minimum number of components for a binding protein-related system: a periplasmic recognition component, a transmembrane protein, and a peripheral membrane protein that may be involved in energy linkage.

Cloning of mglB, the structural gene for the galactose-binding protein of Salmonella typhimurium and Escherichia coli

From libraries of EcoRI fragments of Salmonella typhimurium and Escherichia coli DNA in lambda gt7, phages could be isolated that carry mglB, the structural gene of the galactose-binding protein as well as other mgl genes. Lysogenization of an E. coli mutant carrying a defective galactose-binding protein with lambda gt7 mglB (Salmonella) restores full galactose transport and galactose chemotaxis. Both the E. coli mutant protein as well as the wild-type Salmonella galactose-binding protein are synthesized in this strain. The EcoR1 fragments of both organisms carrying the mgl genes were 6 Kb long. They were subcloned into the multicopy plasmid pACYC184. The hybrid plasmid containing the Salmonella mgl DNA gives rise to the synthesis of large amounts of galactose-binding protein in the periplasm of E. coli. The protein can be precipitated by antibodies against the E. coli binding protein and is identical to the fully processed protein isolated from Salmonella typhimurium LT2. In vitro protein synthesis (Zubay-system) with either lambda gt7 mgl phages as well as the hybrid plasmid as DNA matrix produces the galactose-binding protein mainly in precursor form that is precipitable by specific antibodies.

Mapping of mglB, the structural gene of the galactose-binding protein of Escherichia coli

The tetracycline resistance transposon Tn10 was inserted into the E. coli chromosome near mglB550, a structural gene for the galactose-binding protein. P1 transductions established the position of these Tn10 insertions (zee-700, 701, 702::Tn10) close to the genes ptsF, fpk, cdd, mglB550, his, and gatA with 85%-95%, 85%, 36%, 20%-40%, 12%-15%, and 0.5% cotransduction frequency. Three factor crosses revealed the relative sequence of the genes as: mglB550, zee-700::Tn10, ptsF, fpk, cdd, his, gatA was found to be 1.3% cotransducible with mglB550. Two Tn10 insertions near gatA were isolated and characterized. One, zef-704::Tn10, was 3% cotransducible with fpk, 8% with mglB550, and 42% with gatA. The other, zef-703::Tn10, was 98% cotransducible with gatA but not with mglB550 or fpk. Neither of these two Tn10 insertions was cotransducible with cdd. Four factor crosses revealed the sequence gatA, zef-704::Tn10, mglB550, fpk. Neither zee-700::Tn10 nor zef-703::Tn10 showed an (0/300) cotransduction with either glpT or gyrA. The clockwise order of genes is then: his, cdd, fpk, ptsF, zee-700::Tn10, mglB550, zef-704::Tn10, gatA. With a fix-point for his at 44 min, fpk would be placed at 45 min and mglB550 at 45.5 min. During the course of this work we noticed that the cotransduction frequency between Tn10 insertions and nearby markers tended to increase when new P1 lysates were prepared from freshly reisolated strains. This may indicate loss of nonessential genes adjacent to Tn10 insertions. Using insertion zee-703::Tn10, we isolated deletions extending into an mgl gene other than mglB. Crosses between such a deletion mutant and an mglB550 mutant were done. The analysis of the periplasmic proteins of these as well as other transductants or recombinants involving the mglB550 or the mglB551 gene revealed the existence of strains synthesizing both the wild-type as well as the corresponding mutant protein. Strains containing both proteins exhibit either wild-type or mutant phenotype. These strains appeared unstable. Upon reisolation from purified stock cultures kept in glycerol at -20 degrees C, colonies could be isolated that carried only mutant or wild-type protein.

Relationship of Cell Envelope Stability to Substrate Capture in a Marine Psychrophilic Bacterium

Cells of a psychrophilic marine bacterium were found to take up a variety of amino acids from seawater. Some of the amino acids that were taken up were released when the cells were exposed to a hypotonic salt solution. The proportion that was released varied according to the amino acid. A pool of the amino acid arginine that was formed during very short periods of exposure of cells to the exogenously supplied amino acid was particularly sensitive to reductions in salinity. In general, exposure to hypotonic salt solutions also resulted in reduced amino acid uptake by the cells. Complete removal of seawater salts (SE treatment) produced obvious structural alterations in the cell envelope, resulting in an even greater reduction in amino acid uptake. Under these conditions, amino acid-binding components were released by the cells. Differential centrifugation and fluorescent antibody studies indicated that arginine-binding components are located on or near the surface of intact cells. The data suggest that substrate receptors were sensitive to reductions in seawater salt concentrations and that lesions at this level affected the organism's substrate uptake and retention capabilities.

Metabolite gene regulation: imidazole and imidazole derivatives which circumvent cyclic adenosine 3',5'-monophosphate in induction of the Escherichia coli L-arabinose operon

Imidazole, histidine, histamine, histidinol phosphate, urocanic acid, or imidazolepropionic acid were shown to induce the L-arabinose operon in the absence of cyclic adenosine 3',5'-monophosphate. Induction was quantitated by measuring the increased differential rate of synthesis of L-arabinose isomerase in Escherichia coli strains which carried a deletion of the adenyl cyclase gene. The crp gene product (cyclic adenosine 3',5'-monophosphate receptor protein) and the araC gene product (P2) were essential for induction of the L-arabinose operon by imidazole and its derivatives. These compounds were unable to circumvent the cyclic adenosine 3',5'-monophosphate in the induction of the lactose or the maltose operons. The L-arabinose regulon was catabolite repressed upon the addition of glucose to a strain carrying an adenyl cyclase deletion growing in the presence of L-arabinose with imidazole. These results demonstrated that several imidazole derivatives may be involved in metabolite gene regulation (23).

Mutants in transmission of chemotactic signals from two independent receptors of E. coli

We have characterized chemotactic mutants of E. coli that appear to be defective in a common linkage of two independent receptors to the central chemotactic components. The mutants do not respond to gradients of ribose or galactose and thus are called trg (taxis to ribose and galactose), after Ordal and Adler (1974b). These trg mutants are indistinguishable from their parent in tactic response to other attractants, swimming pattern, growth rates, and transport of ribose and galactose. The mutant cells contain the usual amounts of ribose and galactose receptors, and those proteins function normally in their other role, transport of their respective ligands. The mutations, generated by insertion of translocatable drug-resistance elements (transposons)8 are located near 31 min on the map of the E. coli chromosome, a locus far removed from the genes coding for the ribose and galactose receptors. Trg mutants do not resemble either specific receptor mutants or che mutants. The nature of the requirement for the trg product in the response to ribose and galactose is not defined, but evidence for interference of tactic signals from the ribose and galactose receptors (Strange and Koshland, 1976) supports the idea that the product functions directly in the transmission of tactic signals from the two receptors to the flagella.

Independent regulation of transport and biosynthesis of arginine in Escherichia coli K-12

From an arginine auxotrophic strain, a mutant was isolated which is able to utilize d-arginine as a source of l-arginine and shows a high sensitivity to inhibition of growth by canavanine. Transport studies revealed a four- to five-fold increased uptake of arginine and ornithine in cells from the mutant strain. The kinetics of entry of arginine and ornithine evidenced elevated maximal influx values for the arginine- and ornithine-specific transport systems. A close parallel between arginine transport activity and arginine binding activity with one arginine-specific binding periplasmic protein in the mutant strongly suggests that such binding protein is a component of the arginine-specific permease. The affinity between arginine and the binder, isolated from the mutant cells, as well as the electrophoretic mobility of the protein, remain unchanged. The enhanced transport activity of arginine and ornithine with mutant cells is insensitive to repression by arginine or ornithine, whereas the biosynthesis of arginine-forming enzymes is normally repressible. When transport activity was examined in strains with mutations leading to derepression of arginine biosynthesis, the regulation of arginine transport was found to be normal. These studies support the conclusion that arginine transport and arginine biosynthesis, in Escherichia coli K-12, are not regulated in a concerted manner, although both systems may have components in common.

Purification and properties of a periplasmic protein related to sn-glycerol-3-phosphate transport in Escherichia coli

Protein GLPT, a periplasmic protein previously recognized as closely related to the active transport of sn-glycerol-3-phosphate in Escherichia coli was isolated by the cold osmotic shock procedure. It was purified by Sephadex chromatography and isoelectric focussing. The purified protein does not exhibit any detectable binding activity toward sn-glycerol-3-phosphate. It has no activity as a glycerol phosphatase nor as a glycerol kinase. Polyacrylamide gel electrophoresis in the presence of dodecylsulfate of the protein subsequent to treatment in urea, boiling in dodecylsulfate and crosslinking indicates that it occurs as an oligomeric protein composed of four identical subunits of 40 000 molecular weight. Membrane vesicles of wild-type strains that contain protein GLPT in whole cells loose it during vesicle preparation. However, they still exhibit high transport activity toward sn-glycerol-3-phosphate. Membrane vesicles prepared from glp T mutants that may or may not contain protein GLPT do not transport sn-glycerol-3-phospahte. We conclude from these results that protein GLPT does not participate in the energy-dependent active transport through the cytoplasmic membrane but could be involved in facilitating the diffusion of sn-glycerol-3-phosphate through the outer layers of E. coli.

Galactose transport in Salmonella typhimurium

We have studied the various systems by which galactose can be transported in Salmonella typhimurium, in particular the specific galactose permease (GP). Mutants that contain GP as the sole galactose transport system have been isolated, and starting from these mutants we have been able to select point mutants that lack GP. The galP mutation maps close to another mutation, which results in the constitutive synthesis of GP, but is not linked to galR. Growth of wild-type strains on glaactose induces GP but not the beta-methylgalactoside permease (MGP). Strains lacking GP are able to grow slowly on galactose, and MGP is induced; however, D-fucose is a much better inducer of MGP. Induction of GP or MGP is not prevented by a pts mutation, although this mutation changes the apparent Km of MGP for galactose. pts mutations have no effect on GP. GP has a rather broad specificity: galactose, glucose, mannose, fucose, 2-deoxygalactose, and 2-deoxyglucose are substrates, but only galactose and fucose can induce this transport system.

L-Arabinose transport and the L-arabinose binding protein of Escherichia coli

The active accumulation of L-arabinose by arabinose induced cultures of Escherichia coli is mediated by 2 independent transport mechanisms. One, specified by the gene locus araE, is membrane bound and possesses a relatively "low affinity". The other, specified in part by the genetic locus araF, contains as a functional component the L-arabinose binding protein and functions with a "high affinity" for the substrate. The L-arabinose binding protein has been purified, partially characterized, crystallized, and sequenced.

Properties of the entry and exit reactions of the beta-methyl galactoside transport system in Escherichia coli

The Km, Vmax, and Ki of the entry reaction were determined for three substrates of the beta-methyl galactoside transport system: D-galactose, D-glycerol-beta-D-galactoside, and beta-methyl-D-galactoside. Although the data for D-galactose and D-glycerol-beta-D-galactoside followed simple Michaelis-Menten kinetics, the results for beta-methyl-D-galactoside deviated from Michaelis-Menten kinetics in that the Ki for beta-methyl-D-galactoside inhibition of both of the other two substrates was 10-fold greater than the Km for beta-methyl-D-galactoside entry. Furthermore, two partial mgl- strains retain 56% of the parental level of the beta-methyl-D-galactoside entry reaction, but only 12% of the parental level of transport of the other two substrates. The exit reaction of beta-methyl-D-galactoside was shown to be first order. It was stimulated sixfold when the cells were provided with an energy source. This stimulation required adenosine 5'-triphosphate or a related compound. The exit reaction was not altered by mutations in any of the three cistrons which inactivate the beta-methyl-D-galactoside entry reaction, was not increased by growth in the presence of inducers of the entry reaction, and was not repressed by growth on glucose. The striking differences between the entry and exit reactions suggest that they either use different carriers or that none of the three cistrons which are currently known to code for components of the beta-methyl galactoside transport system code for its membrane carrier.

The periplasmic galactose receptor protein of Escherichia coli in relation to galactose chemotaxis

The periplasmic galactose receptor protein of E. coli is the common macromolecule in the initiation of two functions, chemotaxis and active transport. The substrates are glucose and galactose and the affinity for binding to the receptor protein is high (Kp -2 X 10(-8) M for glucose and 1 X 10(-7) M for galactose). A second binding site shows a 100-fold lower affinity. The high concentration of the galactose receptor protein in the periplasmic space tends to give retention through recapture of the ligands. The kinetic properties of the galactose receptor protein are, in general, in harmomy with the kinetics of chemotactic responses to spatial or temporal sugar gradients.

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.

Chemotaxis in bacteria

Bacterial chemotaxis, the movement of motile bacteria toward or away from chemicals, was discovered nearly a century ago by Engelmann (1) and Pfeffer (2,3). The subject was actively studied for about 50 years, but then there were very few reports until quite recently. For reviews of the literature up to about 1960, see Berg (4), Weibull (5), and Ziegler (6). The present review will restrict itself to the recent work on chemotaxis in Escherichia coli and Salmonella typhimurium. Some of this is also covered in Berg's review (4), and a review by Parkinson (7) should be consulted for a more complete treatment of the genetic aspects.

The binding of maltose to 'virgin' maltose-binding protein is biphasic

The biphasic binding properties of the galactose-binding and maltose-binding proteins of Escherichia coli may be important in the functioning of these proteins as recognition components of chemoreceptors. However, Richarme and Kepes [Eur. J. Binding curve of the galactose-binding protein may be the result of isotopic dilution, during equilibrium dialysis, by unlabeled ligand retained by the binding throughout purification. Here the binding of maltose to maltose-binding protein which has never previously been exposed to sugar ('virgin' binding protein) is shown to be biphasic. This implies that the unusual binding properties are attributable to the maltose-binding protein itself.

Pediococcus cerevisiae mutant with altered transport of folates

A Pediococcus cerevisiae mutant that actively accumulated folate (PteGlu), in contrast to the wild-type, was also found to exhibit changes in the pattern of uptake of 5-methyl-tetrahydrofolate (5-CH3-H4PteGlu) and amethopterin. Most of the 5-CH3-H4PteGlue accumulated through a glucose- and temperature-dependent process, and a concentrative uptake was also found in gluocse-starved cells and in cells incubated at OC. About 75% of the accumulated 5-CH3-H4PteGlu exchanged with amethopterin. In contrast to the wild type, the mutant accumulated both diastereoisomers of 5-CH3-H4PteGlue by glucose-dependent and glucose-independent processes. Amethopterin and PteGlue competitively inhibited the uptake in both processes, with an apparent lower affinity of the carrier for PteGlu than for the analogue. p-Chloromercuribenzoate strongly inhibited the uptake (75%). The p-chloromercuribenzoate-nonsusceptible and temperature-independent uptake was also competed by amethopterin. Metabolic poisons like sodium azide, potassium fluoride, iodoacetate, and 2,4-dimitrophenol inhibited the glucose-dependent process. Uptake, in the absence of glucose, was enhanced by sodium azide and potassium fluoride.

The "hidden ligand" of the galactose-binding protein

Following tryptophan fluorescence of the galactose-binding during dissociation of the ligand it has been found that glucose dissociates with a half life of less than 5 s. Similarly, fast dissociation was also observed by following release of radioactively labelled glucose from Sepharose-coupled galactose-binding protein upon dilution. Accordingly, a previous claim that the galactose-binding protein contains glucose as a non-dissociable "hidden ligand" [G. Richarme and A. Kepes (1974) Eur. J. Biochem. 45, 127-133] has to be reinterpreted

Maltose chemoreceptor of Escherichia coli

Strains carrying mutations in the maltose system of Escherichia coli were assayed for maltose taxis, maltose uptake at 1 and 10 muM maltose, and maltose-binding activity released by osmotic shock. An earlier conclusion that the metabolism of maltose is not necessary for chemoreception is extended to include the functioning of maltodextrin phosphorylase, the product of malP, and the genetic control of the maltose receptor by the product of malT is confirmed. Mutants in malF and malK are defective in maltose transport at low concentrations as well as high concentrations, as previously shown, but are essentially normal in maltose taxis. The product of malE has been previously shown to be the maltose-binding protein and was implicated in maltose transport. Most malE mutants are defective in maltose taxis, and all those tested are defective in maltose transport at low concentrations. Thus, as previously suggested, the maltose-binding protein probably serves as the recognition component of the maltose receptor, as well as a component of the transport system. tsome malE mutants release maltose-binding activity and are tactic toward maltose, although defective in maltose transport, implying that the binding protein has separate sites for interaction with the chemotaxis and transport systems. Some mutations in lamB, whose product is the receptor for the bacteriophage lamba, cause defects in maltose taxis, indicating some involvement of that product in maltose reception.

Source of energy for the Escherichia coli galactose transport systems induced by galactose

The beta-methyl-galactoside- and galactose-specific transport systems of Escherichia coli were shown by experiments involving inhibitors and the use of an adenosine triphosphatase mutant strain to utilize adenosine 5'-triphosphate or a related compound to drive active transport. These systems were shown to be unable to use the activated-membrane state. The galactose-specific transport system was shown to behave most like a member of the binding-protein class of transport systems by its response to osmotic shock and vesicle formation. These results extended to two sugar transport systems: the correlation between the source of energy and class of transport system found by Berger (1973) for amino acid transport systems. That is, binding-protein systems utilized adenosine 5'-triphosphate whereas membrane-bound systems utilized the activated-membrane state to drive active transport.

Selection procedure for mutants defective in the beta-methylgalactoside transport system of Escherichia coli utilizing the compound 2R-glyceryl-beta-D-galactopyranoside

A procedure has been devised that allows selection of mutants defective in the beta-methylgalactoside transport system (mgl) of Escherichia coli. This procedure utilizes the compound 2R-glyceryl-beta-d-galactopyranoside (glycerylgalactoside), which is known to be transported by only two transport system in E. coli, namely, the lactose and the beta-methylgalactoside transport systems. Mutants lacking glycerol-3-phosphate dehydrogenase (glpD) are sensitive to glycerol. Similarly, mutants lacking uridine diphosphate-galactose-4-epimerase (galE) are sensitive to galactose. Glycerylgalactoside is an inducer of the lactose operon and also a substrate for beta-galactosidase. Thus, a mgl(+)glpD galE lacY strain will not grow in the presence of glycerylgalactoside owing to accumulated glycerol-3-phosphate, galactose-1-phosphate, and uridine diphosphate-galactose. We have constructed such a strain and shown that mgl mutants can be obtained by selecting for those that grow in the presence of glycerylgalactoside.

Properties of mutants in galactose taxis and transport

beta-Methylgalactoside (mgl) permease mutants of Escherichia coli, which are defective in three genes, mglA, mglB, and mglC, were assayed for galactose taxis and galactose transport. The mglB product is the galactose-binding protein. Previous evidence, supported by our new findings, shows that the galactose-binding protein is the recognition component for galactose taxis as well as for galactose transport. Most mutants defective in mglB showed strong effects on both chemotaxis and transport; however, a couple showed effects chiefly on one process or the other, thus allowing a separation of chemotaxis and transport. The mglA and mglC products have not yet been identified, but they must be components of the galactose transport machinery since mutants defective in mglA or mglC, or both, showed strongly reduced transport. Although some of these mutants showed little chemotaxis, most gave close to wild-type chemotactic responses. Thus, transport is not required for galactose taxis. The bacteria detect changes in the fraction of binding protein associated with galactose, not changes in the rate of transport.

Isolation and complementation of mutants in galactose taxis and transport

By using a new screening method, we have obtained 43 new Escherichia coli mutants defective in ring formation on galactose swarm plates, which score for defects in chemotaxis or transport. They were complemented and compared with mutations previously known to lie in the galactose-binding protein or the beta-methylgalactoside (mgl) permease, or both. The mutations were all found to lie in three genes, called mglA, mglB, and mglC. mglB codes for the gene specifying the binding protein. Based on co-transduction experiments, mglA, mglB, and mglC lie close to each other on the bacterial chromosome.

Basic amino acid transport in Escherichia coli: properties of canavanine-resistant mutants

A mutant of Escherichia coli strain CanR 22 has been isolated which is resistant to growth inhibition by canavanine, an analogue of arginine. The properties of this strain and of another canavanine-resistant mutant, JC182-5 (isolated by Celis et al. [5]), were studied. The mutation is pleiotropic in that it results in a reduction in the activity of two distinct permeases, the arginine-specific and lysine-arginine-ornithine transport systems. The lesion maps at min 56 of the E. coli linkage map, at or near the argP locus. Although strain CanR 22 excretes arginine, this excretion appears to result from reduced ability to concentrate arginine, rather than the loss of transport ability being the result of excretion. This conclusion is based on findings with a canavanine-resistant strain auxotrophic for arginine, which exhibits transport properties similar to those of the prototrophic strains. Additionally, growth in the presence of arginine or ornithine results in a repression of the activity of the two basic amino acid transport systems. Neither the arginine-specific nor the lysine-arginine-ornithine binding proteins of the mutant cells show significant alterations in terms of amount, physical properties, or kinetic parameters. These observations lead to the proposal of a model for the two basic amino acid transport systems in which two carrier proteins with different specificities interact with a common energy coupling mechanism. A lesion in the gene (or one of the genes) for this coupling mechanism can confer canavanine resistance.