Regulation of beta-galactoside phosphate accumulation in Streptococcus pyogenes by an expulsion mechanism

Streptococcus pyogenes pregrown on lactose took up glucose, lactose, or methyl beta-D-thiogalactopyranoside (MeSGal or TMG) by a phosphoenolpyruvate-dependent phosphotransferase system. MeSGal accumulated in the cell as MeSGal-phosphate (MeSGalP). Three effects were noted when various sugars were added to MeSGal preloaded cells: (i) no decrease in intracellular MeSGalP concentration after addition of fructose, sucrose, o-nitrophenyl-beta-D-galactoside, glycerol, 6-deoxyglucose, alpha-methyl D-glucoside, 2-deoxygalactose, glucose 1-phosphate, or glucose 6-phosphate; (ii) slow loss of preaccumulated MeSGalP evoked by lactose, 2-deoxy-D-glucose, or unlabeled MeSGal; and (iii) a short lag followed by extremely rapid expulsion of intracellular MeSGalP elicited by glucose or mannose and a slower expulsion elicited by glucosamine. The expelled compound was free MeSGal, indicating the involvement of dephosphorylation in the expulsion mechanism. Deoxyglucose inhibited the expulsion evoked by mannose, and prepoisoning of cells with fluoride or arsenate prevented the glucose-dependent expulsion. The expulsion is due to activation of an expulsion mechanism rather than to turnover of MeSGalP and leak of internal MeSGal with concomitant inhibition of MeSGal influx. The results suggest the need for phosphotransferase-dependent translocation of a preferential sugar or accumulation of the sugar catabolite for expulsion activation. The significance of the expulsion mechanism in synthesis regulation of enzymes involved in carbohydrate utilization is proposed.

Participation of the ring oxygen in sugar interaction with transporters at renal tubular surfaces

The pulse-injection indicator-dilution technique in vivo has been used to study the interaction of 5-thio-D-glucose and methyl-beta-D-thiogalactopyranoside with renal tubular surfaces in dog kidney. (i) 5-Thio-D-glucose and methyl-beta-D-thiogalactopyranoside have nor antiluminal interaction. (ii) 37 +/- 5% of 5-thio-D-glucose is extracted at the luminal surface relative to simultaneously filtered creatinine. (iii) Luminal extraction of 5-thio-D-glucose is blocked by preloading with D-glucose and phlorizin. (iv) Methyl-beta-D-thiogalactopyranoside in contrast to D-galactose has not luminal interaction. It is concluded that 5-thio-D-glucose shares the glucose transporter at the luminal surface of the proximal tubule. The data also suggest that the ring oxygen participates in the interaction of pyranosides with luminal and antiluminal membrane carriers. At the luminal surface, its absence is quantitatively important while at the antiluminal surface it is apparently essential for the sugar-transporter interaction.

The stimulatory effect of testosterone propionate and 17 beta-estradiol on 5'-methylthioadenosine phosphorylase activity in rat target tissues

The effect of castration and subsequent administration of 17 beta-estradiol and testosterone propionate on 5'-methylthioadenosine phosphorylase activity in rat target tissues was studied. Castration 34 days earlier resulted in a 95% reduction in ventral prostate 5'-methylthioadenosine phosphorylase activity and 16 days earlier in a 67% reduction in uterine 5'-methylthioadenosine phorphorylase activity. Four days of testosterone propionate administration stimulated ventral prostate 5'-methylthioadenosine phosphorylase activity 32% above castrate levels, which represented more than 50% of the intact control levels. 17 beta-Estradiol on the other hand stimulated uterine 5'-methylthioadenosine phosphorylase activity 35% above castrate controls within 24 h and with 3 days of continuous hormone treatment to within 97% of the intact control levels. However, castration and subsequent 17 beta-estradiol administration did not affect 5'-methylthioadenosine phosphorylase activity in rat liver and lung. Both prostate and uterine 5'-methylthioadenosine phosphorylase were shown to metabolize 5'-methylthioadenosine to 5-methylthioribose through a 5'-methylthioribose 1-phosphate intermediate. The data suggest aht 5'-methylthioadenosine is not allowed to accumulate in rat target tissues even under conditions which are known to stimulate polyamine synthesis.

Binding of n-alkyl beta-D-xylopyranosides and n-alkyl 1-thio-beta-D-xylopyranosides to beta-D-xylosidase from Bacillus pumilus PRL B12

The binding of D-xylose and of a series of n-alkyl beta-D-xylopyranosides and their 1-thio analogues to beta-D-xylosidase from B. pumilus PRL B12 has been investigated. The binding constants and thermodynamic equilibrium parameters delta H0 and delta S0 have been determined. The enzyme does not distinguish between alpha- and beta-D-xylopyranose. Although the enthalpy of binding of D-xylose is very favourable, the overall free-energy is small, due to a large decrease in entropy. Furthermore, all of the evidence available suggests that the aglycon group is bound by unspecific, hydrophobic forces. However, simple correlations between the binding parameters and the relative hydrophobicity of the compounds could not be found. Unexpectedly, no parallelism between binding of n-alkyl beta-D-xylopyranosides and the corresponding 1-thio derivatives was found.

5-methylthioribose 1-phosphate: a product of partially purified, rat liver 5'-methylthioadenosine phosphorylase activity

5'-Methylthioadenosine phosphorylase from rat liver has been purified 112-fold. A molecular weight of 90 000 for the enzyme was estimated from gel filtration on Sephadex G-150. The Km for 5'-methylthioadenosine was 4.7 . 10(-7) M, while the Km for phosphate was 2 . 10(-4) M. The products of the reaction were isolated and identified as adenine and 5-methylthioribose 1-phosphate. In addition to 5'-methylthioadenosine the nucleoside analogues 5'-ethylthioadenosine and 5'-n-propylthioadenosine also served as substrates for the enzyme. The 7-deaza analogue 5'-methylthiotubercidin was found to be an inhibitor of the reaction, but was inactive as a substrate.

A requirement for ATP for beta-galactoside transport by Bacillus alcalophilus

Lactose-grown cells of Bacillus alcalophilus actively transported methylthio-beta, D-galactoside (TMG) in a range of pH values from 7.5 to 10.5 with a pH optimum at 8.5. The TMG was accumulated in a chemically unmodified form, and cell extracts failed to catalyze either ATP or P-enolpyruvate-dependent phosphorylation of TMG. At pH 8.5, the lactose-grown cells exhibited a transmembrane proton gradient (deltapH) of 1.38 units, interior acid, and a transmembrane electrical potential (delta psi) of -132 mV. Accordingly, the total protonmotive force at this pH was very low, -51mV. Several lines of evidence indicate that the protonmotive force or delta psi did not directly energize TMG transport but, rather, that ATP was directly required: (a) in cells treated with arsenate so that the delta psi was unaffected and cellular ATP levels were markedly lowered, TMG transport was inhibited in proportion to the reduction of cellular ATP, while electrogenic alpha-aminoisobutyric acid transport was not; (b) when a valinomycin-induced potassium diffusion potential was established in starved cells, alpha-aminoisobutyric acid transport, but not TMG transport, was stimulated; and (c) in a series of experiments in which the delta psi was rapidly abolished by treatment with gramicidin, ATP levels declined slowly and the rate of TMG transport correlated directly with ATP levels rather than with the delta psi. Consumption of cellular ATP concomitant with TMG transport could be demonstrated.

Lactose carrier protein of Escherichia coli. Transport and binding of 2'-(N-dansyl)aminoethyl beta-D-thiogalactopyranoside and p-nitrophenyl alpha-d-galactopyranoside

The elevated level of lactose carrier protein present in cytoplasmic membranes derived from Escherichia coli strain T31RT, which carries the Y gene of the lac operon on a plasmid vector (Teather, R. M., et al. (1978) Mol. Gen. Genet. 159, 239--248), has allowed the detection of a complex between the carrier and the fluorescent substrate 2'-(N-dansyl)-aminoethyl beta-D-thiogalactopyranoside (Dns2-S-Gal). Binding is accompanied by a 50-nm blue shift in the emission maximum of the dansyl residue. The complex (dissociation constant, KD = 30 micron) rapidly dissociates upon addition of competing substrates such as beta-D-galactopyranosyl 1-thio-beta-D-galactopyranoside or upon reaction with the thiol reagent p-chloromercuribenzenesulfonate. Binding of both Dns2-S-Gal and p-nitrophenyl alpha-D-galactopyranoside (alpha-NPG) occurs spontaneously in the absence of an electrochemical potential gradient across the membrane. Comparison of equilibrium binding experiments using Dns2-S-Gal or alpha-NPG and differential labeling of the carrier with radioactive amino acids shows that the carrier binds 1 mol of substrate per mol of polypeptide (molecular weight 30 000). In addition to specific binding to the lactose carrier, Dns2-S-gal binds unspecifically to lipid vesicles or membranes, as described by a partition coefficient, K = 60, resulting in a 25-nm blue shift in the emission maximum of the dansyl group. Both Dns2-S-Gal and alpha-NPG are not only bound by the lactose carrier but also transported across the membrane by this transport protein in cells and membrane vesicles. The fluorescence changes observed with dansylated galactosides in membrane vesicles in the presence of an electrochemical gradient (Schuldiner et al. (1975) J. Biol. Chem. 250, 1361--1370)) are interpreted as an increase in unspecific binding after translocation.

Interaction of the lacZ beta-galactosidase of Escherichia coli with some beta-D-galactopyranoside competitive inhibitors

1. The location of the bivalent metal cation with respect to bound competitive inhibitors in Escherichia coli (lacZ) beta-galactosidase was investigated by proton magnetic resonance. 2. Replacement of Mg(2+) by Mn(2+) enhances both longitudinal and transverse relaxation of the methyl groups of the beta-d-galactopyranosyltrimethylammonium ion, and of methyl 1-thio-beta-d-galactopyranoside; linewidths are narrowed by increasing temperature. 3. The Mn(2+) ion is located 8-9A (0.8-0.9nm) from the centroid of the trimethylammonium group and 9A (0.9nm) from the average position of the methylthio protons. 4. The effective charge at the active site was probed by measurement of competitive inhibition constants (K(i) (o) and K(i) (+) respectively) for the isosteric ligands, beta-d-galactopyranosylbenzene and the beta-d-galactopyranosylpyridinium ion. 5. The ratio of inhibition constants (Q=K(i) (+)/K(i) (o)) obtained with 2-(beta-d-galactopyranosyl)-naphthalene and the beta-d-galactopyranosylisoquinolinium ion at pH7 with Mg(2+)-enzyme was identical, within experimental error, with that obtained with the monocyclic compounds. 6. The variation of Q for Mg(2+)-enzyme can be described by Q=0.1(1+[H(+)]/4.17x10(-10))/1+[H(+)]/10(-8)). 7. This, in the theoretical form for a single ionizable group, is ascribed to the ionization of the phenolic hydroxy group of tyrosine-501. 8. The variation of Q for Mg(2+)-free enzyme is complex, probably because of deprotonation of the groups normally attached to Mg(2+) as well as tyrosine-501.

Na+-dependent methyl beta-thiogalactoside transport in Salmonella typhimurium

We have studied the role of sodium ions in methyl beta-thiogalactoside (TMG) transport via the melibiose permease (TMG II) in Salmonella typhimurium. TMG uptake via TMG II in anaerobic, straved and metabolically poisoned cells is dependent on an inward-directed Na+ gradient. Cells which have been partially depleted of endogenous substrates show H+ extrusion upon sodium-stimulated TMG influx. Measurements of the electrochemical H+ gradient in cells, starved in different ways for endogenous substrates, suggest that this proton extrusion is probably not linked to the actual translocation mechanism but is the result of metabolism induced by TMG plug Na+ uptake.

5-Methylthioribose kinase. A new enzyme involved in the formation of methionine from 5-methylthioribose

The presence of a previously unidentified enzyme, tentatively designated 5-methylthioribose kinase, has been demonstrated in cell-free extracts of Enterobacter aerogenes. The enzyme catalyzes the ATP-dependent phosphorylation of 5-methylthioribose. ADP is one of the products of the reaction and, based on functional group analyses, the other product is 5-methylthioribose 1-phosphate. A 40-fold purified enzyme preparation has been obtained from a cell-free extract of E. aerogenes. Activity of the partially purified enzyme is totally dependent on the presence of a divalent cation and a sulfhydryl reagent. The substrate specificity of the enzyme is quite narrow, and the Km values for ATP and 5-methylthioribose are 7.4 X 10(-5) M and 8.1 X 10(-6) M, respectively. These results suggest that 5-methylthioribose kinase may be a primary enzyme involved in the recycling of the methylthio group of 5-methylthioribose back into methionine.

beta-D-xylosidase from Bacillus pumilus PRL B12: hydrolysis of aryl beta-D-xylopyranosides

The influence of substituents on the binding and hydrolysis of several substituted beta-D-xylopyranosides by beta-D-xylosidase from Bacillus pumilus PRL B12 has been investigated. From a comparison of the inhibition constants of 1-thio-beta-D-xylopyranosides with the apparent Michaelis-Menten constants of the substrates, it followed that the latter constants are good approximations of the true equilibrium constants. The influence of the substituent on the rate and activation parameters is small. The results are in agreement with, but do not prove, a one-step mechanism without the formation of a glycosyl-enzyme intermediate.

The protonmotive force and beta-galactoside transport in Bacillus acidocaldarius

The acidophilic and thermophilic bacterium, Bacillus acidocaldarius maintains a cytoplasmic pH between 5.85 and 6.31 over a range of external pH from 2.0 to 4.5. Consistently, the pH optimum of beta-galactosidase, as assayed in cell extracts, is between pH 6.0 and 6.5. An electrical potential (delta-psi), interior positive, is also maintained across the membrane. A delta-psi of approximately 34 mV was calculated from determinations of thiocyanate uptake by cells at pH 3.5. Addition of the proton conductor carbonyl cyanide m-chlorophenylhydrazone increased the delta-psi. Treatment of cells with valinomycin (in the absence of external potassium ions) or high concentrations of thiocyanate, to abolish the delta psi, resulted in collapse of the transmembrane proton gradient (delta pH). Active transport of methylthio-beta, D-galactoside occurred optimally at pH 3.5. Transport of the galactoside was inhibited by various compounds which could dissipate the transmembrane delta pH and by respiratory inhibitors. A decrease in the delta pH and an increase in the delta psi occurred upon addition of methylthio-beta, D-galactoside to cells of B. acidocaldarius. Thus the transport of this solute appears to involve an electrogenic symport with protons. The transport system is most active at 50 degrees C and shows little activity at 25 degrees C, although the delta pH is the same at the two temperatures. Gramicidin inhibits methylthio-beta, D-galactoside transport equally effectively at 50 degrees C and 25 degrees C, while nigericin inhibits only after a lag at 25 degrees C.

The biochemical and genetic basis for high frequency thiomethyl galactoside resistance in lambda,lambdadg lysogens of Escherichia coli

In a culture of Escherichia coli K12 gal (lambdadg), cells which form large colonies on agar plates containing galactose and thiomethyl beta-D-galactoside (TMG) appear at high frequency. These clones are resistant to growth inhibition by TMG on galactose minimal medium. Biochemical studies of the steady-state levels of galactokinase and UDPgalactose 4-epimerase suggest that the resistant clones have extra copies of the genes for the galactose-metabolizing enzymes. The mutation for TMG resistance is not located in either the bacterial or the bacteriophage genome, but is probably due to an aberrant association between cell and prophage DNA. Mapping the TMG-resistant characteristic by phage P1 indicates that TMG-resistant bacteria posses at least two GAL+ OPERONS, ONE OF WHICH IS COTRANSDUCIBLe with bio+. In addition, TMG-resistant bacteria behave like lambdadg polylysogens when challenged with the phage lambdaI90c17. From these genetic experiments we conclude that TMG-resistant bacteria arise by duplication of the lambdadg prophage. Finally, gal+ bacteria which carry a single, additional, lambdadg prophage are TMG-resistant. TMG resistance is probably a gal+ gene dosage effect.

A novel type of coupling between proline and galactoside transport in Escherichia coli

Exit of thiomethylgalactoside (TMG) from preloaded cells induced the accumulation of proline. Likewise, proline exit stimulated TMG accumulation. Since a proton ionophore (carbonylcyanide-m-chlorophenylhydrazone) abolished these effects, a protonmotive force was implicated as the "intermediate" in the coupling reaction. The evidence suggests that the exit of TMG resulted in proton exit, which produced either a membrane potential (inside negative or a pH gradient (outside acid) or both. This inwardly directed protonmotive force provided the energy for proline entry and accumulation. Thus the energy coupling was not via a common transport protein but by proton movements which coupled the two separate H+-dependent transport processes.

Thiocyanate formation from glucosinolates: a study of the autolysis of allylglucosinolate in Thlaspi arvense L. seed flour extracts

Thlaspi arvense seed flour extracts converted [1′-14C]allylglucosinolate to [3-14C]allylthiocyanate. This result is discussed in terms of the mechanisms by which organic thiocyanates may be derived from glucosinolates.

Energy requirements for the transport of methylthio-beta-D-galactoside by Escherichia coli: measurement by microcalorimetry and by rates of oxygen consumption and carbon dioxide production

The energy cost for maintenance of gradients of methylthio-beta-d-galactoside in Escherichia coli was evaluated. Information was also obtained concerning the energy flow associated with gradient establishment under some circumstances. Energy flow was evaluated from transport-induced changes in the rate of heat evolution, oxygen consumption, and carbon dioxide production in metabolically active cells. Heats were measured with an isothermal calorimeter. Energy expenditure behavior was characterized by a transition that depended on the level of accumulation. The data for steady-state maintenance could be rationalized in terms of the Mitchell hypothesis, two models for influx and efflux, and a transition between them. At low levels of uptake, steady-state proton-methylthio-beta-d-galactoside (TMG) symport for influx and efflux occurred via a nonenergy-requiring exchange process. The only energy requirement was that necessary to pump back in any TMG exiting via a leakage pathway (model I). Above the transition, all influx occurred with proton symport, but all exit, leak and carrier mediated, occurred without proton symport (model II). The H(+)/TMG stoichiometric ratio computed for the region of model II applicability (carbon source present, high level of uptake) approached 1. This value agreed with that of other workers for downhill beta-galactoside flow, suggesting that the energy cost for both downhill and uphill flow was approximately the same. For low levels of uptake, initial establishment of the gradient was followed by a burst of metabolism that was much larger than that expected on the basis of the chemiosmotic hypothesis. In the absence of carbon source, the stimulation in respiration was sufficient to produce 13 times more protons than are apparently necessary to establish the gradient. The results indicate also that the nature of the biochemical process stimulated by TMG depends on its level of uptake. Insight into several aspects of the nature of these processes was provided through analysis of the heat, oxygen, and CO(2) data. The key factor controlling the transition in energy flow behavior is suggested to be rate of flux. The present data suggest that it occurs at a flux of approximately 120 nmol/min per mg of protein.

Sodium-dependent methyl 1-thio-beta-D-galactopyranoside transport in membrane vesicles isolated from Salmonella typhimurium

Membrane vesicles isolated from Salmonella typhimurium G-30 grown in the presence of melibiose catalyze methyl 1-thio-beta-D-galactopyranoside (TMG) transport in the presence of sodium or lithium, as shown initially with intact cells by Stock and Roseman (Stock, J., and Roseman, S. (1971), Biochem. Biophys. Res. Commun. 44, 132). TMG-dependent sodium uptake is also observed, but only when a potassium diffusion potential (interior negative) is induced across the vesicle membrane. Cation-dependent TMG accumulation varies with the electrochemical gradient of protons generated as a result of D-lactate oxidation, and the vesicles catalyze D-lactate-dependent sodium efflux in a manner which is consistent with the operation of a proton-sodium exchange mechanism. Although the stoichiometry between sodium and TMG appears to be 1:1 when transport is induced by a potassium diffusion potential, evidence is presented which indicates that the relationship may exceed unity under certain conditions. The results are explained in terms of a model in which TMG-sodium (lithium) symport is driven by an electrochemical gradient of protons which functions to maintain a low intravesicular sodium or lithium concentration through proton--sodium (lithium) antiport.

The absorption of protons with alpha-methyl glucoside and alpha-thioethyl glucoside by the yeast N.C.Y.C. 240. Evidence against the phosphorylation hypothesis

1. When yeast N.C.Y.C. 240 was grown with maltose in a complex medium based on yeast extract and peptone, washed cell preparations fermented alpha-methyl glucoside much more slowly than maltose. 2. The yeast absorbed alpha-methyl[14C]glucoside from a 10mM solution in the presence of antimycin and iodoacetamide, producing [14C]glucose, which accumulated outside the cells. The yeast itself contained hexose phosphates, trehalose, alpha-methyl glucoside and other products labelled with 14C, but no alpha-methyl glucoside phosphate. 3. About 1 equiv. of protons was absorbed with each equivalent of alpha-methylglucoside, and 1 equiv. of K+ ions left the yeast. 4. alpha-Thioethyl glucoside was also absorbed along with protons. Studies by g.l.c. showed that the yeast concentrated the compound without metabolizing it. 5. The presence of trehalose, sucrose, maltose, L-sorbose, glucose or alpha-phenyl glucoside in each case immediately stimulated proton uptake, whereas fructose, 3-O-methylglucose and 2-deoxyglucose failed to do so. 6. The observations support the conclusion that alpha-thioethyl glucoside, alpha-methyl glucoside and maltose are substrates of one or more proton symports, whereas they seem inconsistent with the notion that the absorption of alpha-methyl glucoside involves the phosphorylation of the carbohydrate [Van Stevenick (1970) Biochim. Biophys. Acta 203, 376-384].

A protonmotive force as the source of energy for galactoside transport in energy depleted Escherichia coli

An artificially produced electrochemical potential difference for protons (portonmotive force) provided the energy for the transport of galactosides in Escherichia coli cells which were depleted of their endogenous energy reserves. The driving force for the entry of protons was provided by either a transmembrane pH gradient or a membrane potential. The pH gradient across the membrane was created by acidifying the external medium. The membrane potential (inside negative) was established by the outward diffusion of potassium (in the presence of valinomycin) or by the inward diffusion of the permeant thiocyanate ion. The magnitude of the electrochemical potential difference for protons agreed well with magnitude of the chemical potential difference of the lactose analog, thiomethylgalactoside. The observations are consistent with the view that the carrier-mediated entry of each galactoside molecule is accompanied by the entry of one proton.

Microcalorimetric study of the binding of thiodigalactoside to the lactose permease M protein of Escherichia coli

The energetics of the binding of thiodigalatoside onto vesicles of Escherichia coli containing M protein is described. The Kd determined from equilibrium dialysis was 5-10(-5) M. The enthalpy change (deltaH) was measured by calorimetry. The derived deltaG and deltaH values allowed estimation of the entropic change associated with the binding reaction. The control experiments were made with membranes from cells that were not induced for the lac system. All the experiments were carried out in presence of 10(-2) M sodium azide to prevent any concentration of thiodigalactoside into the vesicles. It was concluded that such membrane vesicles which are in a de-energized state are able to bine thiodigalactoside specifically with a Kd corresponding to the Km of the entry of beta-galactoside measured with intact, active cells.

Attachment of thioglycosides to proteins: enhancement of liver membrane binding

Thioglycosides of D-galactose, D-glucose, N-acetyl-D-glucosamine, and D-mannose were covalently attached to Aspergillus oryzae alpha-amylase, hen's eggs lysozyme, and bovine serum albumin by amidination, diazo coupling, and amide formation. The binding of the newly formed glycoproteins (neoglycoproteins) to rabbit liver membranes was measured, using asialoorosomucoid as a reference. Attachment of D-galactosides by any of the three methods enhanced binding by several orders of magnitude. Coupling of a comparable number of D-mannosides or N-acetyl-D-glucosaminides had little or no effect. Attachment of D-glucosides also enhanced binding but to a variable extent depending on the method of attachment. Thus, the behavior of neoglycoproteins toward rabbit liver membranes closely paralleled that of serum glycoproteins (Ashwell and Morell, 1974) with respect to sugar specificity.

Changes in active transport, intracellular adenosine 5'-triphosphate levels, macromolecular syntheses, and glycolysis in an energy-uncoupled mutant of Escherichia coli

The temperature-sensitive Escherichia coli mutant ecfts metC (Lieberman and Hong, 1974), previously shown to be defective in the coupling of metabolic energy to active transport, is also altered in a wide variety of cellular activities at the nonpermissive temperature. These alterations include a lowering of intracellular adenosine 5'-triphosphate levels, an alteration of glucose metabolism such that large quantities of pyruvate and dihydroxyacetone phosphate are excreted into the medium, excretion of accumulated potassium ions, and a cessation of deoxyribonucleic acid, ribonucleic acid, and phospholipid synthesis. Since these effects closely mimic the action of colicins E1 and K on E. coli cells, the possibility that the ecf gene product is the primary biochemical target for these colicins is discussed.

Kinetic studies of inducer binding to lactose repressor protein

The kinetics of binding of the inducer, isopropyl-beta, D-thiogalactoside to lactose repressor from Escherichia coli was studied by stopped flow rapid mixing techniques. Three different spectral probes for measuring changes in the conformation of the repressor were used: ultraviolet absorbance, fluorescence, and a reporter group, 2-mercuri-4-nitrophenol, in the visible region. Repressor can be reacted with this mercurial to modify two of the three free sulfhydryl groups per monomer without loss of inducer or operator binding activities. The observed first order rate constant for the reaction of repressor with 2-chloromercuri-4-nitrophenol at pH 7.5 and 20 degrees was found to be on the order of 0.1 s-1, an unexpectedly slow rate for this type of reaction. Once bound to repressor, the nitrophenol serves as a chromophoric probe to monitor changes in the surrounding environment. The binding of inducer to repressor causes a change in the absorbance of the bound 2-mercuri-4-nitrophenol moiety and exhibits a second order rate of 3.2 x 10(4) M-1 s-1. Similar rates were obtained when binding of inducer is monitored by changes in either the ultraviolet absorbance of fluorescence of tryptophan residues. Since the same rate of spectral perturbation is observed for different regions of the primary structure of the protein, the conformational change produced in response to inducer binding appears to be translated rapidly throughout the protein molecule.

Recovery of the accumulation ability of thiomethyl-beta-galactoside in Escherichia coli after bacteriophage T4 infection

Effects of UV-irridiated and unirradiated T4 phage infection on the beta-galactoside accumulation ability in Eschericia coli have been examined by the use of 14C-labeled thiomethyl-beta-galactoside (TMG). Under conditions where a synchronous adsorption of phage takes place, the cellular ability for TMG accumulation is found to be largely inhibited immediately after phage adsorption, but it recovers with time to a new level, which is dependent on the multiplicity of infection. When cells are infected with UV-irradiated T4 at the same multiplicity as that of unirradiated phage, the cellular accumulation ability is more severely inhibited and there is no recovery from the inhibition. The recovery process in T4-infected cells is mostly sensitive to puromycin. These results suggest that the initial inhibition of the TMG accumulation ability is probably caused by the adsorption of phage coats, and the subsequent restoration occurs through the action of a phage-directed protein(s). In the recovery process, no new transport system appears to be involved. The restored ability of TMG accumulation is resistant to the action of superinfecting UV phage. However, different mechanisms appear to be operating in T4-infected cells for the establishment of resistance to ghosts and for the recovery from the phage coat-induced inhibition.

Thiocyanate ion formation in rapeseed meals

Meal prepared from unheated rapeseed (Brassica napus cv. Zephyr) showed the presence of t,iocyanate ion, while meal from heated seed of the same cultivar did not show detectable amounts. Unheated seed meal on autolysis, and heated seed meal on incubation with thioglucosidase, yielded increased amounts of thiocyanate ion. Various commercial rapeseed meals showed the presence of t,iocyanate ion only after enzyme incubation. Low glucosinolate, cv. Bronowski, and higher glucosinolate, cv. Zephyr on enzymic incubation yielded comparable amounts of thiocyanate ion, suggesting that the precursor responsible in the two varieties was the same and present in similar quantities. No formation of thiocyanate ion was observed on incubation of sinigrin with thioglucosidase. Rats dosed with heated meal, containing intact glucosinolate, showed a slight increase of thiocyanate ion in the urine as compared with control rats dosed with water, while a relatively large increase followed dosing with sinigrin. Rats dosed with meal containing free thiocyanate ion excreted the ingested thiocyanate ion almost quantitatively.

Characterization of lactose-fermenting revertants from lactose-negative Streptococcus lactis C2 mutants

Partial lactose-fermenting revertants from lactose-negative (lac(-)) mutants of Streptococcus lactis C2 appeared on a lawn of lac(-) cells after 3 to 5 days of incubation at 25 C. The revertants grew slowly on lactose with a growth response similar to that for cryptic cells. In contrast to lac(+)S. lactis C2, the revertants were defective in the accumulation of [(14)C]thiomethyl-beta-d-galactoside, indicating that they were devoid of a transport system. Hydrolysis of o-nitrophenyl-beta-d-galactoside-6-phosphate by toluene-treated cells confirmed the presence of phospho-beta-d-galactosidase (P-beta-gal) in the revertant. However, this enzyme was induced only when the cells were grown in the presence of lactose; galactose was not an inducer. In lac(+)S. lactis C2, enzyme induction occurred in lactose- or galactose-grown cells. The revertants were defective in EII-lactose and FIII-lactose of the phosphoenolpyruvate-dependent phosphotransferase system. Galactokinase activity was detected in cell extracts of lac(+)S. lactis C2, but the activity was 9 to 13 times higher in extracts from the revertant and lac(-), respectively. This suggested that the lac(-) and the revertants use the Leloir pathway for galactose metabolism and that galactose-1-phosphate rather than galactose-6-phosphate was being formed. This may explain why lactose, but not galactose, induced P-beta-gal in the revertants. Because the revertant was unable to form galactose-6-phosphate, induction could not occur. This compound would be formed on hydrolysis of lactose phosphate. The data also indicate that galactose-6-phosphate may serve not only as an inducer of the lactose genes in S. lactis C2, but also as a repressor of the Leloir pathway for galactose metabolism.

Crambe thioglucoside glucohydrolase (EC 3.2.3.1): separation of a protein required for epithiobutane formation

Sonicated extract of crambe seed meal prepared in the presence of ferrous ion and dithiothreitol enzymatically converts epi-progoitrin to glucose, HSO4−, and a mixture of 1-cyano-2-hydroxy-3,4-epithiobutanes (50–70%) and 1-cyano-2-hydroxy-3-butene (30–50%). A fraction of the extract precipitating between 60 and 70% saturated ammonium sulfate contains thioglucosidase that converts epi-progoitrin essentially to 1-cyano-2-hydroxy-3-butene. Chromatography (on cross-linked dextran) of a 40–60% ammonium sulfate fraction leads to separation of a proteinaceous material (s20 = 2.6 S) that does not hydrolyze epi-progoitrin but, in the presence of thioglucosidase, promotes the formation of 1-cyano-2-hydroxy-3,4-epithiobutanes in amounts proportional to those from crude seed meal extract.

Proton-coupled accumulation of galactoside in Streptococcus lactis 7962

When cells of the anaerobe Streptococcus lactis 7962 are deprived of their normal fermentable energy source, active transport of galactosides is completely abolished although the membrane carriers are still capable of facilitating the equilibration of sugars across the cell membrane. In these nonmetabolizing cells it was possible to test the Mitchell hypothesis of obligatory coupling of proton movement with sugar transport. This hypothesis was supported by alkalinization of the medium observed when thiomethylgalactoside was added to a lightly buffered suspension of S. lactis cells. Conversely, addition of protons resulted in active transport of thiomethylgalactoside. Accumulation of thiomethylgalactoside to a concentration more than 20-times that in the external medium was induced by suddenly exposing cells to a medium at pH 6; no accumulation of thiomethylgalactoside was observed with cells exposed to pH 8.Active transport of thiomethylgalactoside occurred in the absence of energy metabolism when S. lactis cells were treated with valinomycin. This ionophore allowed intracellular K(+) to flow out, thus imposing a membrane potential (inside negative). This potential resulted in a proton uptake and an associated active transport of galactoside. The membrane potential was measured from the distribution ratio (inside/outside) of K(+) in the presence of valinomycin. The pH gradient was measured from the distribution ratio of [(14)C]methylamine. The protonmotive force, calculated from the membrane potential and the pH gradient, was found to be directly related to the accumulation of galactoside, in accordance with the chemiosmotic hypothesis.