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 temperature on the expression of wild-type and thermostable mutants of kanamycin nucleotidyltransferase in Escherichia coli.

The expression of kanamycin nucleotidyltransferase (KNTase) in Escherichia coli results in different forms of the protein, depending on the temperature; soluble active enzyme is synthesized at 23 degrees C but the protein is mostly aggregated and inactive in inclusion bodies when made at 37 degrees C. However, active enzyme can be recovered by solubilization of the inclusion bodies with 8 M urea followed by dilution of the denaturant, indicating that the polypeptide is not damaged covalently but is present in a misfolded state. The availability of thermostable mutants of KNTase allows a test of the hypothesis that formation of inclusion bodies when proteins are highly expressed in E. coli is due to the accumulation of a folding intermediate that is prone to temperature-dependent aggregation. Because these mutants were isolated by cloning the KNTase gene into the thermophile Bacillus stearothermophilus and selecting for kanamycin resistance at high growth temperatures, they must be thermostable for both synthesis and activity and must have folding intermediates that are less susceptible to the formation of aggregates. Indeed, whereas decreasing the temperature from 37 to 23 degrees C increased the KNTase specific activity 10-fold in cells expressing the wild-type enzyme, this change resulted in only a 2.1-fold increase for the TK1 (Asp80----Tyr) mutant and a 1.7-fold increase for the TK101 (Asp80----Tyr and Thr130----Lys) double mutant. The strategy of cloning in thermophiles and selecting or screening for mutants that fold correctly to yield biological activity at high growth temperatures may be useful in overcoming the problem of the insolubility of some proteins when expressed in heterologous hosts.     

Cation transport in Escherichia coli. VIII. Potassium transport mutants

Analysis of K transport mutants indicates the existence of four separate K uptake systems in Escherichia coli K-12. A high affinity system called Kdp has a Km of 2 muM, and Vmax at 37 degrees C of 150 mumol/g min. This system is repressed by growth in high concentrations of K. Two constitutive systems, TrkA and TrkD, have Km's of 1.5 and 0.5 mM and Vmax's of 550 and 40 at 37 and 30 degrees C, respectively. Mutants lacking all three of these saturable systems take up K slowly by a process, called TrkF, whose rate of transport is linearly dependent on K concentration up to 105 mM. On the whole, each of these systems appears to function as an independent path for K uptake since the kinetics of uptake when two are present is the sum of each operating alone. This is not true for strains having both the TrkD and Kdp systems, where presence of the latter results in K uptake which saturates at a K concentration well below 0.1 mM. This result indicates some interaction between these systems so that uptake now has the affinity characteristic of the Kdp system. All transport systems are able to extrude Na during K uptake. The measurements of cell Na suggest that growing cells of E. coli have very low concentrations of Na, considerably lower than indicated by earlier studies.     

Flagellar formation in Escherichia coli electron transport mutants.

Mutants of Escherichia coli lacking ubiquinone or heme have been tested for motility and found to be essentially immotile. The loss of motility is identified with the loss of flagellum synthesis.     

O6-methylguanine-DNA methyltransferase in wild-type and ada mutants of Escherichia coli.

O(6)-Methylguanine-DNA methyltransferase is induced in Escherichia coli during growth in low levels of N-methyl-N'-nitro-N-nitrosoguanidine. We have developed a sensitive assay for quantitating low levels of this activity with a synthetic DNA substrate containing 3H-labeled O(6)-methylguanine as the only modified base. Although both wild-type and adaptation-deficient (ada) mutants of E. coli contained low but comparable numbers (from 13 to 60) of the enzyme molecules per cell, adaptation treatment caused a significant increase of the enzyme in the wild type but not in the ada mutants, suggesting that the ada mutation is in a regulatory locus and not in the structural gene for the methyltransferase.     

Effect of anionic detergents on wild-type and envelope mutants of Escherichia coli and Pseudomonas aeruginosa

Escherichia coli UB1005 (DCO), an envelope mutant (DC2), Pseudomonas aeruginosa 799 and an envelope mutant (799/61) were exposed to sodium deoxycholate (DOC), sarkosyl and sodium lauryl sulphate (SLS). DOC was the least effective lytic agent, but the two Ps. aeruginosa strains, especially 799/61, were more susceptible to DOC and sarkosyl than the E. coli ones. SLS was an efficient lysing agent, although Ps. aeruginosa 799 was the least susceptible of the four strains. DC2 was lysed more rapidly and to a greater extent than UB1005 by all three agents. The mutant strains, especially DC2, were more sensitive to selective media than the wild-type ones.     

Genetic analysis of Escherichia coli oligopeptide transport mutants.

The composition of the outer membrane channels formed by the OmpF and OmpC porins is important in peptide permeation, and elimination of these proteins from the Escherichia coli outer membrane results in a cell in which the primary means for peptide permeation through this cell structure has been lost. E. coli peptide transport mutants which harbor defects in genes other than the ompF/ompC genes have been isolated on the basis of their resistance to toxic tripeptides. The genetic defects carried by these oligopeptide permease-negative (Opp-) strains were found to map in two distinct chromosomal locations. One opp locus was trp linked and mapped to the interval between att phi 80 and galU. Complementation studies with F'123 opp derivatives indicated that this peptide transport locus resembles that characterized in Salmonella typhimurium as a tetracistronic operon (B. G. Hogarth and C. F. Higgins, J. Bacteriol. 153:1548-1551, 1983). The second opp locus, which we have designated oppE, was mapped to the interval between dnaC and hsd at 98.5 min on the E. coli chromosome. The differences in peptide utilization, sensitivity and resistance to toxic peptides, and the L-[U-14C]alanyl-L-alanyl-L-alanine transport properties observed with these Opp-E. coli strains demonstrated that the transport systems encoded by the trp-linked opp genes and by the oppE gene(s) have different substrate preferences. Mutants harboring defects in both peptide transport loci defined in this study would not grow on nutritional peptides except for tri-L-methionine, were totally resistant to toxic peptides, and would not actively transport L-[U-14C]alanyl-L-alanyl-L-alanine.     

Sucrose transport through maltoporin mutants of Escherichia coli.

Maltoporin (LamB) and sucrose porin (ScrY) reside in the bacterial outer membrane and facilitate the passive diffusion of maltodextrins and sucrose, respectively. To gain further insight into the determinants of solute specificity, LamB mutants were designed to allow translocation of sucrose, which hardly translocates through wild-type LamB. Three LamB mutants were studied. (a) Based on sequence and structure alignment of LamB with ScrY, two LamB triple mutants were generated (R109D, Y118D,D121F; R109N,Y118D,D121F) to mimic the ScrY constriction. The crystal structure of the first of these mutants was determined to be 3.2 A and showed an increased ScrY-like cross-section except for D109 that protrudes into the channel. (b) Based on this crystal structure a double mutant was generated by truncation of the two residues that obstruct the channel most in LamB (R109A,Y118A). Analysis of liposome swelling and in vivo sugar uptake demonstrated substantial sucrose permeation through all mutants with the double alanine mutant performing best. The triple mutants did not show a well-defined binding site as indicated by sugar-induced ion current noise analysis, which can be explained by remaining steric interference as deduced from the crystal structure. Binding, however, was observed for the double mutant that had the obstructing residues truncated to alanines.     

Effect of growth conditions on the synthesis of nitrate reductase components in chlorate resistant mutants of Escherichia coli K 12.

The chlorate resistant mutants of $\text{Escherichia coli}$ synthetize, in variable quantities, proteins which give immunocomplex with specific nitrate reductase antiserum. The biosynthesis of these cross reacting materials presents the same type of regulation as nitrate reductase of the wild type. C.R.M. biosynthesis is repressed by oxygen and even in the presence of nitrate, the oxygen inhibition is not reversed with chlorate mutants and wild type. With anaerobically grown cells, nitrate acts as an inducer and increases the amount of antibody-precipitable material, three times in mutants and even four times with Chl-E.     

Glutamate transport in wild-type and mutant strains of Escherichia coli

Halpern, Yeheskel S. (Hebrew University-Hadassah Medical School, Jerusalem, Israel), and Meir Lupo. Glutamate transport in wild-type and mutant strains of Escherichia coli. J. Bacteriol. 90:1288-1295. 1965.-Mutants of Escherichia coli able to grow on glutamate as their source of carbon showed glutamate dehydrogenase and glutamate-oxaloacetate transaminase activities similar to those possessed by the parent strain. The mutants took up glutamate at a much faster rate and showed a several-fold greater capacity for concentrating the amino acid than did the corresponding parent strains. Curvilinear double reciprocal plots of velocity of uptake versus glutamate concentration were obtained with the E. coli H strains. A break in the curve of glutamate uptake was observed with the E. coli K-12 strains when incubated in a glucose medium. It is suggested that these findings may be due to allosteric activation of glutamate permease by its substrate.     

Comparison of active mutants and wild-type aspartate transcarbamoylase of Escherichia coli

Two active mutants of aspartate transcarbamoylase from Escherichia coli have been purified from strains which produce large quantities of enzyme. Each enzyme was isolated from a different spontaneous revertant of a pyrimidine auxotrophic strain produced by mutagenesis with nitrogen mustard. Both enzymes exhibit allosteric properties with one having significantly less and the other more cooperativity than wild-type enzyme. Isolated catalytic subunits had different values of Km and Vmax. Studies on hybrids constructed from mutant catalytic and wild-type regulatory subunits (and vice versa) indicate that catalytic chains encoded by pyrB and not the regulatory chains encoded by pyrI were affected by the mutations. Differential scanning calorimetry experiments support these conclusions. Both mutant enzymes undergo ligand-promoted conformational changes analogous to those exhibited by wild-type enzyme: a 3% decrease in the sedimentation coefficient and a 5-fold increase in the reactivity of the sulfhydryl groups of the regulatory chains. Interactions between catalytic and regulatory chains in the mutants are weaker than those in the wild-type enzyme. The gross conformational changes of the mutants upon adding the bisubstrate ligand, N-(phosphonacetyl)-L-aspartate, in the presence of the substrate, carbamoylphosphate, and the activator, ATP, correlate with differences in cooperativity. The mutant with lower cooperativity is more readily converted from the low-affinity, compact, T-state to the high-affinity, swollen, R-state than is wild-type enzyme; this conversion for the more cooperative enzyme is energetically less favorable.     

Uridine and cytidine transport in Escherichia coli B and transport-deficient mutants.

Three mutants of Escherichia coli B which are defective in components of the transport system for uridine and uracil were isolated and utilized to study the mechanism of uridine transport. Mutant U- was isolated from a culture resistant to 77 micronM 5-fluorouracil. Mutant U-UR-, isolated from a culture of mutant U-, is resistant to 770 micronM 5-fluorouracil and 750 micronM adenosine. Mutant NUC- is resistant to 80 micronM showdomycin and has been reported previously. The characteristics of uridine transport by E. coli B and the mutants provide data supporting the following conclusions. The transport of adenosine, deoxyadenosine, guanosine, deoxyguanosine, adenine, or guanine by mutant U- and mutant U-UR- is identical with that in the parental strain. Uridine is transported by E. coli B as intact uridine. In addition, extracellular uridine is also rapidly cleaved to uracil and the ribose moiety. The latter is transported into the cells, whereas uracil appears in the medium and is transported by a separate uracil transport system. The entry of the ribose moiety of uridine is fast relative to the uracil and uridine transport processes. The Km values and the inhibitory effects of heterologous nucleosides for the transport of uridine and the ribose moiety of uridine are similar. Studies of cytidine uptake in the parental and mutant strains provide evidence that cytidine is transported by two independent systems, one of which is the same as that involved in the transport of intact uridine. Uridine inhibits but is not transported by the other system for cytidine transport. Evidence for the above conclusions was based on comparisons of the characteristics of [2-14C]uridine, [U-14C]uridine, and [2-14C]cytidine transport using E. coli B and the three transport mutants under conditions which measure initial rates. The nature of the inhibitory effects of heterologous nucleosides on the uridine transport processes and identification of extracellular components from radioactive uridine provides supportive data for the conclusions.     

Defective transport in S-adenosylmethionine synthetase mutants of Escherichia coli

The transport of several metabolites is decreased in mutant strains of Escherichia coli (Met K, E4 and E40), which contain decreased levels of S-adenosylmethionine synthetase. The rates and extents of uptake for lysine, leucine, methionine, and α-methylglucoside in both whole cells and membrane vesicles isolated from these mutants are 2- to 10-fold lower than in corresponding preparations from wild-type cells, although proline uptake is normal. The addition of S-adenosylmethionine to cultures of strain E40 can partially restore the rate and extent of lysine uptake. Lysine transport is lower in mutant vesicles in the presence of either d-lactate, succinate, α-hydroxylbutyrate, or NADH even though these substrates are oxidized at rates comparable to those in wild-type vesicles. This suggests that the defect is not related to the ability of vesicles to oxidize electron donors, but is very likely related to the ability of mutant vesicles to couple respiration to lysine transport. In addition, temperature-induced efflux of α-methylglucoside phosphate and dinitrophenol-induced efflux of lysine are similar in both the mutant and wild-type membranes, indicating that the barrier properties of the membrane and the activity of the lysine carrier are normal.     

Temperature-sensitive mutants of Escherichia coli affecting beta-galactoside transport

Six different temperature-sensitive (ts) mutants have been isolated which have parental beta-galactoside permease levels at low temperatures but have decreased permease levels when grown at high temperatures. These mutants were derived from Escherichia coli ML308 (lacI(-)Y(+)Z(+)A(+)). After N-methyl-N'-nitro-N'-nitro-soguanidine mutagenesis, ampicillin was used to select for cells unable to grow on low lactose concentrations at 42 C. Temperature-sensitive mutants were assayed for galactoside permease activity after growth in casein hydrolysate medium at 25 or 42 C by measuring both radioactive methylthio-beta-d-galactoside uptake and in vivo o-nitrophenyl-beta-d-galactoside hydrolysis. The six conditional isolates have decreased levels of galactoside permease which are correlated with decreased growth rates at elevated temperatures. The low permease levels are not due to a temperature labile lacY gene product but rather to a temperature labile synthesis rate of functional permease. Some of the mutants exhibit a ts increase in permeability as shown by the increased leakage of intracellular beta-galactosidase and by the increased rate of in vivo o-nitrophenyl-beta-d-galactoside hydrolysis via the nonpermease mediated entry mechanism. Preliminary evidence indicates that transport in general is decreased in these mutants, yet there is some specificity in the mutational lesion since glucoside transport is unaffected. All these observations suggest that these mutants have ts alterations in membrane synthesis which results in pleiotropic effects on various membrane functions.     

Ferrous iron transport mutants in Escherichia coli K12

A ferrous iron transport system in Escherichia coli is described. Mutants in this transport system were isolated using the antibiotic streptonigrin. The gene locus feo (for ferrous iron transport) was mapped near pncA at 38.5 min on the genetic map of E. coli K12. The transport of ferrous iron was regulated by fur as the siderophore transport systems.     

Effect of antecedent growth conditions on sensitivity of Escherichia coli to chlorine dioxide.

Bacterial resistance to inactivation by antibacterial agents that is induced by the growth environment was studied. Escherichia coli was grown in batch culture and in a chemostat, and the following parameters were varied: type of substrate, growth rate, temperature, and cell density during growth. Low doses (0.75 mg/liter) of chlorine dioxide were used to inactivate the cultures. The results demonstrated that populations grown under conditions that more closely approximated natural aquatic environments were more resistant than those grown under commonly employed batch culture conditions. In particular, bacteria grown at submaximal rates were more resistant than their counterparts grown at mumax. The most resistant populations encountered in this study were those grown at D values of 0.02 h-1 and 0.06 h-1 at 25 degrees C. Growth at 15 degrees C led to greater resistance than did growth at 37 degrees C. The conditions that produced relatively resistant phenotypes were much closer to those found in most natural environments than are the typical conditions of batch culture methods. The importance of major physiological changes that can be induced by the antecedent growth environment is discussed in light of the possible modes of action of several disinfectants.