Characterization of transmembrane movement of glucose and glucose analogs in Streptococcus mutants Ingbritt.
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The transmembrane movement of radiolabeled, nonmetabolizable glucose analogs in Streptococcus mutants Ingbritt was studied under conditions of differing transmembrane electrochemical potentials (delta psi) and pH gradients (delta pH). The delta pH and delta psi were determined from the transmembrane equilibration of radiolabeled benzoate and tetraphenylphosphonium ions, respectively. Growth conditions of S. mutants Ingbritt were chosen so that the cells had a low apparent phosphoenolpyruvate (PEP)-dependent glucose:phosphotransferase activity. Cells energized under different conditions produced transmembrane proton potentials ranging from -49 to -103 mV but did not accumulate 6-deoxyglucose intracellularly. An artificial transmembrane proton potential was generated in deenergized cells by creating a delta psi with a valinomycin-induced K+ diffusion potential and a delta pH by rapid acidification of the medium. Artificial transmembrane proton potentials up to -83 mV, although producing proton influx, could not accumulate 6-deoxyglucose in deenergized cells or 2-deoxyglucose or thiomethylgalactoside in deenergized, PEP-depleted cells. The transmembrane diffusion of glucose in PEP-depleted, KF-treated cells did not exhibit saturation kinetics or competitive inhibition by 6-deoxyglucose or 2-deoxyglucose, indicating that diffusion was not facilitated by a membrane carrier. As proton-linked membrane carriers have been shown to facilitate diffusion in the absence of a transmembrane proton potential, the results therefore are not consistent with a proton-linked glucose carrier in S. mutans Ingbritt. This together with the lack of proton-linked transport of the glucose analogs suggests that glucose transmembrane movement in S. mutans Ingbritt is not linked to the transmembrane proton potential. (+info)
Binding-protein-dependent lactose transport in Agrobacterium radiobacter.
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Agrobacterium radiobacter NCIB 11883 was grown in lactose-limited continuous culture at a dilution rate of 0.045/h. Washed cells transported [14C]lactose and [methyl-14C]beta-D-thiogalactoside, a nonmetabolisable analog of lactose, at similar rates and with similar affinities (Km for transport, less than 1 microM). Transport was inhibited to various extents by the uncoupling agent carbonyl cyanide p-trifluoromethoxyphenylhydrazone, by unlabeled beta-galactosides and D-galactose, and by osmotic shock. The accumulation ratio for methyl-beta-D-thiogalactoside was greater than or equal to 4,100. An abundant protein (molecular weight, 41,000) was purified from osmotic-shock fluid and shown by equilibrium dialysis to bind lactose and methyl-beta-D-thiogalactoside, the former with very high affinity (binding constant, 0.14 microM). The N-terminal amino acid sequence of this lactose-binding protein exhibited some homology with several other sugar-binding proteins from bacteria. Antiserum raised against the lactose-binding protein did not cross-react with two glucose-binding proteins from A. radiobacter or with extracts of other bacteria grown under lactose limitation. Lactose transport and beta-galactosidase were induced in batch cultures by lactose, melibiose [O-alpha-D-galactoside-(1----6)alpha-D-glucose], and isopropyl-beta-D-thiogalactoside and were subject to catabolite repression by glucose, galactose, and succinate which was not alleviated by cyclic AMP. We conclude that lactose is transported into A. radiobacter via a binding protein-dependent active transport system (in contrast to the H+ symport and phosphotransferase systems found in other bacteria) and that the expression of this transport system is closely linked to that of beta-galactosidase. (+info)
Characterization of the double mutant, Val-177/Asn-322, of the lactose permease.
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The double mutant, Val-177/Asn-322, was investigated with regard to its ability to transport H+ and galactosides. In downhill lactose transport assays, the wild-type strain had a Km value for lactose uptake of 0.9 mM and a Vmax of 0.65 mumol lactose/min.mg protein while the mutant had a significantly higher Km value of 1.9 mM but a similar Vmax of 0.49 mumol/min.mg protein. In spite of its moderate ability to transport lactose downhill, the Val-177/Asn-322 mutant exhibited the striking property of being completely defective in the uphill accumulation of lactose or methyl-beta-D-thiogalactopyranoside. Direct measurements of H+ transport, however, showed that the mutant's defect in active accumulation is not due to a defect in the ability to transport H+ with lactose or methyl-beta-D-thiogalactopyranoside. The Val-177/Asn-322 mutant strain had a H+:lactose stoichiometry of 0.84 which was similar to that measured in the wild-type strain (0.68). These results are discussed with regard to the role His-322 plays in H+ transport, active accumulation of sugars, and sugar recognition. (+info)
The role of the proton motive force and electron flow in solute transport in Escherichia coli.
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Transport of lactose and methyl beta-D-thiogalactopyranoside, a melibiose analogue, was studied in intact cells of Escherichia coli. A proton motive force could drive the translocation of these solutes via these two transport systems, but the initial rates and steady-state levels of solute accumulation increased upon initiation of electron transfer. When the absolute value of the proton motive force was decreased by ionophores the steady-state levels of lactose accumulation did not decrease as expected if thermodynamic equilibrium with the proton motive force had existed. Accumulation of lactose was also observed in the absence of any measurable proton motive force as long as electron transfer took place. Since both proton/lactose and sodium/methyl beta-D-thiogalactopyranoside symport showed the same characteristics, an explanation based on local proton diffusion pathways is unlikely. (+info)
Characterization and sequencing of the lac Y54-41 "uncoupled" mutant of the lactose permease.
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The Escherichia coli strain carrying the lac Y54-41 gene encodes a mutant lactose permease which carries out normal downhill transport of galactosides but is defective in uphill accumulation. In this study, the mutant lac Y54-41 gene was cloned onto the multicopy vector pUR270. As expected, the cloned gene was shown to express normal downhill transport activity but was markedly defective in the uphill transport of methyl-beta-D-thiogalactopyranoside. Direct measurements of H+ transport revealed that the mutant permease can transport H+ with methyl-beta-D-thiogalactopyranoside but at a significantly reduced capacity compared to the wild-type strain. However, under conditions where the mutant and wild-type strains both transport lactose at similar rates, no detectable H+ transport was observed in the mutant strain. The entire cloned lac Y54-41 gene was subjected to DNA sequencing, and a single base substitution was found which replaces glycine 262 in the protein with a cysteine residue. Inhibition experiments showed that the mutant permease is dramatically more sensitive to three different sulfhydryl reagents: N-ethylmaleimide, p-hydroxymericuribenzoate, and p-hydroxymercuriphenylsulfonic acid. However, the lactose analogue, thiodigalactoside, was only marginally effective at protecting against inhibition in the mutant strain. The results are consistent with the idea that the sulfhydryl reagents are inhibiting the mutant permease activity by reacting with cysteine 262. (+info)
Fast measurement of galactoside transport by lactose permease.
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Lactose permease of Escherichia coli was reconstituted into vesicles of dimyristoylphosphatidylcholine, and the rate of galactoside counterflow was measured in the millisecond time range. The turnover number and the half-saturation constant for transport agree with the values known for cells. This result demonstrates that lactose permease is the sole protein necessary for galactoside transport. Furthermore, lactose permease seems not to require a high level of negatively charged lipids or a certain degree of unsaturation of the lipid hydrocarbon chains. However, the lipids must be in the fluid state, because the transport rate drastically decreases below the lipid ordered fluid phase transition. (+info)
The protonmotive force and beta-galactoside transport in Bacillus acidocaldarius.
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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. (+info)