Postillumination adenosine triphosphate synthesis in Rhodospirillum rubrum chromatophores. I. Conditions for maximal yields.

The very low level of postillumination ATP synthesis in chromatophores was markedly stimulated when permeant anions (thiocyanate or perchlorate) or permeant cations (potassium in the presence of valinomycin) were added to the light stage. Although these compounds stimulated also light-induced proton uptake in chromatophores the pH dependence of both photoreactions was different. Proton uptake peaked at pH 6.5 while the amount of postillumination ATP was maximal when the light stage was carried out around pH 7.7. The increased yield of ATP at the more alkaline pH could not be explained by a slower decay of the high energy state at this pH, since the decay rate was faster at pH 7.7 than at pH 6.5. The proton concentration gradient which is maintained across the chromatophore membrane in the light was also found to increase when the external pH was raised from 6.0 to 8.0. Only a minimal amount of postillumination ATP was formed when this gradient was below 2.1 pH units, but above this value the ATP yield rose steeply as a function of the increasing pH gradient. In light of these results it is suggested that in order to obtain a high yield of postillumination ATP synthesis in chromatophores two conditions are required: the particles have to be loaded with a sufficient number of protons and a light-induced pH gradient above a certain threshold value has to be maintained across their membrane. The low yield of postillumination ATP in chromatophores and the increase obtained by adding permeating ions, is thus explained by similar variations in the extent of the pH gradient, which exceeded the threshold value only in the presence of the permeating ions.     

The protonmotive force in bovine heart submitochondrial particles. Magnitude, sites of generation and comparison with the phosphorylation potential.

1. The magnitude of the protonmotive force in respiring bovine heart submitochondrial particles was estimated. The membrane-potential component was determined from the uptake of S14CN-ions, and the pH-gradient component from the uptake of [14C]methylamine. In each case a flow-dialysis technique was used to monitor uptake. 2. With NADH as substrate the membrane potential was approx. 145mV and the pH gradient was between 0 and 0.5 unit when the particles were suspended in a Pi/Tris reaction medium. The addition of the permeant NO3-ion decreased the membrane potential with a corresponding increase in the pH gradient. In a medium containing 200mM-sucrose, 50mM-KCl and Hepes as buffer, the total protonmotive force was 185mV, comprising a membrane potential of 90mV and a pH gradient of 1.6 units. Thus the protonmotive force was slightly larger in the high-osmolarity medium. 3. The phosphorylation potential (= deltaG0' + RT ln[ATP]/[ADP][Pi]) was approx. 43.1 kJ/mol (10.3kcal/mol) in all the reaction media tested. Comparison of this value with the protonmotive force indicates that more than 2 and up to 3 protons must be moved across the membrane for each molecule of ATP synthesized by a chemiosmotic mechanism. 4. Succinate generated both a protonmotive force and a phosphorylation potential that were of similar magnitude to those observed with NADH as substrate. 5. Although oxidation of NADH supports a rate of ATP synthesis that is approximately twice that observed with succinate, respiration with either of these substrates generated a very similar protonmotive force. Thus there seemed to be no strict relation between the size of the protonmotive force and the phosphorylation rate. 6. In the presence of antimycin and/or 2-n-heptyl-4-hydroxyquinoline N-oxide, ascorbate oxidation with either NNN'N'-tetramethyl-p-phenylenediamine or 2,3,5,6-tetramethyl-p-phenylenediamine as electron mediator generated a membrane potential of approx. 90mV, but no pH gradient was detected, even in the presence of NO3-. These data are discussed with reference to the proposal that cytochrome oxidase contains a proton pump.     

Comparison of permeant ion uptake and carotenoid band shift as methods for determining the membrane potential in chromatophores from Rhodopseudomonas sphaeroides Ga.

1. A comparison was made of two methods for estimating the membrane potential in chromatophores from Rhodopseudomonas sphaeroides Ga. Illuminated chromatophores generated a potential that is apparently much larger when estimated on the basis of the red-band shift of carotenoids rather than from the extent of uptake of the permeant SCN- ion. 2. In contrast, when the chromatophores were oxidizing NADH or succinate the uptake of SCN- indicated a larger membrane potential than was estimated from the carotenoid band shift. 3. The extent of SCN- uptake and the carotenoid-band shift respond differently to changes in the ionic composition of the reaction medium. 4. The effects of antimycin on the carotenoid band shift and SCN- uptake are reported. 5. It is concluded that the carotenoid band shift and the uptake of SCN- are responding to different aspects of the energized state.     

The protonmotive force in phosphorylating membrane vesicles from Paracoccus denitrificans. Magnitude, sites of generation and comparison with the phosphorylation potential

1. The magnitude of the protonmotive force in phosphorylating membrane vesicles from Paracoccus denitrificans was estimated. The membrane potential component was determined from the uptake of S(14)CN(-), and the transmembrane pH gradient component from the uptake of [(14)C]methylamine. In each case a flow-dialysis technique was used to monitor uptake. 2. With NADH as substrate, the membrane potential was about 145mV and the pH gradient was below 0.5 pH unit. The membrane potential was decreased by approx. 15mV during ATP synthesis, and was abolished on addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone. In the presence of KCl plus valinomycin the membrane potential was replaced by a pH gradient of 1.5 units. 3. Succinate oxidation generated a membrane potential of approx. 125mV and the pH gradient was below 0.5 pH unit. Oxidation of ascorbate (in the presence of antimycin) with either 2,3,5,6-tetramethyl-p-phenylenediamine or NNN'N'-tetramethyl-p-phenylenediamine as electron mediator usually generated a membrane potential of approx. 90mV. On occasion, ascorbate oxidation did not generate a membrane potential, suggesting that the presence of a third energy-coupling site in P. denitrificans vesicles is variable. 4. With NADH or succinate as substrate, the phosphorylation potential (DeltaG(p)=DeltaG(0)'+RTln[ATP]/ [ADP][P(i)]) was approx. 53.6kJ/mol (12.8kcal/mol). Comparison of this value with the protonmotive force indicates that more than 3 protons need to be translocated via the adenosine triphosphatase of P. denitrificans for each molecule of ATP synthesized by a chemiosmotic mechanism. In the presence of 10mm-KNO(3) the protonmotive force was not detectable (<60mV) but DeltaG(p) was not altered. This result may indicate either that there is no relationship between the protonmotive force and DeltaG(p), or that for an unidentified reason the equilibration of SCN(-) or methylamine with the membrane potential and the pH gradient is prevented by NO(3) (-) in this system.     

Postillumination adenosine triphosphate synthesis in Rhodospirillum rubrum chromatophores. II. Stimulation by a K+ diffusion potential.

Addition of valinomycin, nonactin, or monactin plus KCl in the dark to preilluminated chromatophores induced the synthesis of a large amount of ATP. This stimulation of postillumination ATP synthesis by a dark-imposed K+ diffusion potential was different from the stimulation caused by addition of permeant anions or cations in the light, since it increases when the pH of the light stage decreased from 8.0 to 6.0. It was thus most pronounced when the chromatophores were preloaded with protons but the light-induced proton concentration gradient (deltapH) was low. Imposition of a Kplus diffusion potential resulted however in stimulation of ATP synthesis even when the light-induced deltapH was already above the threshold value required to initiate postillumination ATP synthesis. This situation was realized when valinomycin plus KCl were added in the dark to chromatophores preilluminated above pH 6.7 with thiocyanate as the permeant anion, and the amount of ATP formed was the sum of the yields obtained with each of these affectors by itself. On the other hand addition of thiocyanate together with valinomycin plus KCl in the dark led to inhibition of ATP synthesis. In this case the permeant anion could not affect the light-induced deltapH but it did eliminate the diffusion potential by decreasing the difference between the permeabilities of Kplus and the anion present in the reaction mixture.     

Comparison of the electrochemical proton gradient and phosphate potential maintained by Rhodospirillum rubrum chromatophores in the steady state.

A technique for the estimation of light-induced membrane potential in chromatophores is described. It is based on measurement of light-induced enhancement in fluorescence of 8-anilinonaphthalene sulfonic acid, which is calibrated by known K⁺ diffusion potentials. The electrochemical proton gradient (Δ_μH+^?) formed during lightinduced electron transport in Rhodospirillum rubrum chromatophores amounts to 250 mV, which is almost equally distributed between the membrane potential and the pH gradient as measured by changes in the fluorescence of anilinonaphthalene sulfonate and 9-amino acridine. Addition of the permeant anion, NaSCN, or of NH₄Cl reduces the overall Δ_μH+^? by less than 20% but changes its distribution between the pH gradient and the membrane potential so that with NaSCN it is composed mainly of the first and with NH₄Cl mainly of the second. Initiation of phosphorylation causes a drop of about 50 mV in the measured Δ_μH+^?. In the absence of salts, the drop is observed in both components, although two-thirds of it are reflected in the membrane potential. In the presence of NaSCN or NH₄Cl the 50-mV drop is exclusively recorded in the pH gradient or in the membrane potential, respectively. The steady-state phosphate potential maintained during electron transport was found to change in parallel to the Δ_μH+^?, but exceeded it by 60 to 80 mV when based on a stoichiometry of two protons translocated per ATP synthesized.     

Light-induced proton permeability changes in retinal rod photoreceptor disk membranes.

We have used the membrane-permeant charged fluorescent dye, 3,3'-dipropylthiadicarbocyanine iodide (diS-C3[5]), to monitor electrical potentials across the membranes of isolated bovine disks. Calibration curves obtained from experiments where a potential was created across the disk membrane by a potassium concentration gradient and valinomycin showed an approximately linear relation between dye fluorescence and calculated membrane potential from 0 to -120 mV. Light exposure in the presence of the permeant buffer, imidazole, caused a rapid decay of the membrane potential to a new stable level. Addition of CCCP, a proton ionophore, in the dark produced the same effect as illumination. When the permeant buffer, imidazole, was replaced by the impermeant buffer, Hepes, neither light nor CCCP discharged the gradient. We interpret the changes in membrane potential measured upon illumination to be the result of a light-induced increase in the permeability of the disk membrane to protons. A permeant buffer is required to prevent the build-up of a pH gradient which would inhibit the sustained proton flow needed for an observable change in membrane potential.     

Isolation and purification of an active gamma-subunit of the F0.F1-ATP synthase from chromatophore membranes of Rhodospirillum rubrum. The role of gamma in ATP synthesis and hydrolysis as compared to proton translocation

We have earlier shown that extraction of Rhodospirillum rubrum chromatophores with LiCl removed completely the beta-subunit of their coupling factor ATPase complex leaving the other four subunits attached to the membrane (Philosoph, S., Binder, A., and Gromet-Elhanan, Z. (1977) J. Biol. Chem. 252, 8747-8752). Further treatment of these beta-less chromatophores with LiBr, under the described optimal conditions, resulted in specific removal of one additional subunit, the gamma-subunit, and both subunits were purified to homogeneity. The beta, gamma-less chromatophores as well as the beta-less ones lost their ATP-linked activities, but retained their light-induced proton uptake, resulting in the formation of an electrochemical gradient of protons composed of both a pH gradient and a membrane potential. These results indicate that the removed beta and gamma subunits cannot be an integral part of an H+ gate in the R. rubrum chromatophore membrane. Each of the removed subunits could bind to the beta, gamma-less chromatophores, but such separate reconstitution of either beta or gamma alone did not lead to restoration of any ATP-linked activity. ATP synthesis and hydrolysis could be restored to the same extent to these chromatophores by their reconstitution with both beta and gamma. It is thus concluded that the presence of both subunits is required for ATP synthesis as well as hydrolysis by the R. rubrum F0.F1 complex. The identical degree of elimination and restoration of ATP synthesis and hydrolysis upon removal and reconstitution of beta and gamma indicates that in R. rubrum at least, there seems to be no reason for suggesting the operation of different catalytic sites for the two activities.     

Electrochemical potential of protons in vesicles reconstituted from purified, proton-translocating adenosine triphosphatase.

Measurements were made of the difference in the electrochemical potential of protons (delta-mu H+) across the membrane of vesicles restituted from the ATPase complex (TF0.F1) purified from a thermophilic bacterium and P-lipids. Two fluorescent dyes, anilinonaphthalene sulfonate (ANS) and 9-aminoacridine (9AA) were used as probes for measuring the membrane potential (delta psi) and pH difference across the membrane (delta pH), respectively. In the presence of Tris buffer the maximal delta psi ans no delta pH were produced, while in the presence of the permeant anion NO-3 the maximal delta pH and a low delta psi were produced by the addition of ATP. When thATP concentration was 0.24 mm, the delta psi was 140-150 mV (positive inside) in Tris buffer, and the delta pH was 2.9-3.5 units (acidic inside) in the presence of NO-3. Addition of a saturating amount of ATP produced somewhat larger delta psi and delta pH values, and the delta -muH+attained was about 310mV. By trapping pH indicators in the vesicles during their reconstitution it was found that the pH inside the vesicles was pH 4-5 during ATP hydrolysis. The effects of energy transfer inhibitors, uncouplers, ionophores, and permeant anions on these vesicles were studied.     

Measurement of the electrochemical proton gradient in submitochondrial particles

The pH gradient and membrane potential of submitochondrial particles from bovine heart were estimated by the uptake of [14C]ethylamine and [36Cl]perchlorate, using filtration through a glass fiber prefilter and Millipore filter without washing to separate the vesicles from the medium. An external volume probe of [3H] sucrose was also used. Internal volume of the vesicles was measured by the extent of uptake of glucose, which equilibrates slowly across the membrane. The electrochemical potential gradient of H+ (delta micro H+) calculated from uptake of ethylamine and perchlorate, assuming the ions taken up were free in solution inside the vesicles, was 23 to 24 kJ/mol of H+ (240-250 mV) during respiration in the absence of ATP. The ratio of the free energy of ATP synthesis (delta GATP) to delta micro H+ was 2.2 to 2.3 during oxidative phosphorylation and only slightly higher during ATP hydrolysis indicating that the H+-translocating ATPase is close to equilibrium under both conditions. The nonintegral ratio suggests there is a systematic error in the measurement of delta micro H+. The value of delta micro H+ calculated from ion uptake could be too high if some of the ions taken up are bound to the membrane or concentrated into the electric double layer at the inner membrane-water interface. The effects of vesicle volume (varied osmotically) and permeant ions (which affect internal ionic strength and pH) on the ratio of delta GATP to delta micro H+ suggested that ion association with the membrane in fact caused significant overestimation of delta micro H+. Association of ethylammonium and perchlorate ions with unenergized submitochondrial particles was measured by centrifugation, in the presence of a high concentration of impermeant salt to minimize association with the external surface. The results were used to estimate the extent of binding during the ion uptake assays, and delta micro H+ was recalculated taking this binding into account. The resulting values were between 19 and 20 kJ/mol of H+ (197-207 mV) during respiration in the absence of ADP, and the ratio of delta GATP to delta micro H+ was about 3 during oxidative phosphorylation.     

Steady-state measurements of ΔpH and Δψ in Rhodospirillum rubrum chromatophores by two different methods. Comparison with phosphorylation potential

The pH gradient, ΔpH, and the membrane potential, Δψ, formed during light-induced electron transport in Rhodospirillum rubrum chromatophores were measured by two independent methods: (a) using specific electrodes to monitor light-dependent uptake of NH₄Cl and SCN^? at chromatophore concentrations of about 0.1 mg bacteriochlorophyll/ml and (b) using 9-aminoacridine and 8-anilinonaphthalenesulfonic acid as fluorescent probes for ΔpH and Δψ, respectively, at chromatophore concentrations of about 0.01 mg bacteriochlorophyll/ml. The light intensity was measured and set at a level which saturated the highest bacteriochlorophyll concentration used. The steady-state values obtained with each method under phosphorylating conditions were compared with the phosphorylation potential maintained by the chromatophores under identical conditions. The results indicate that under all conditions employed the ratio $\text{H}{}^{\mathrm{+}}\text{ATP}$ is greater than 2, and varies between 2.4 and 3.4 depending on the method used for estimation of the electrochemical proton gradient.     

The functional unit of electrical events and phosphorylation in chromatophores from Rhodopseudomonas sphaeroides

1. From electron micrographs of chromatophores from Rhodopseudomonas sphaeroides and from the estimated bacteriochlorophyll content of the sample a mean value of 4700 bacteriochlorophyll per chromatophore was estimated. The mean diameter of the chromatophore vesicles was 600 Å.2. The decay of the flash-induced electric potential across the chromatophore membrane measured by the carotenoid band shift was 20% accelerated by about one valinomycin molecule per 4700 bacteriochlorophyll, i.e. by one ionophore molecule per chromatophore.3. The inhibition of the flash-induced ATP formation by valinomycin followed a similar pattern to the accelerated decay of the electric potential.4. The single turnover flash yield of ATP synthesis gave a mean value of one ATP per 1470 bacteriochlorophyll or about 3 ATP per vesicle.5. With regard to the partitioning of the ionophore between the membrane (85%) and aqueous phase (15%) we conclude that one molecule of valinomycin per chromatophore is sufficient to begin to collapse the electrical potential and inhibit ATP synthesis. It is therefore suggested that the membrane potential is an essential component of the energized state which is used for phosphorylation.The results correspond to those obtained for the 100-fold larger vesicles in chloroplasts (thylakoids) where one molecule of ionophore is also sufficient to quench both events.     

Measurement of plasma membrane and mitochondrial potentials in sea urchin sperm. Changes upon activation and induction of the acrosome reaction

The relationship between the plasma membrane potential and activation of sperm motility and respiration, or induction of the acrosome reaction, was explored in sperm of the sea urchin Strongylocentrotus purpuratus. Plasma and mitochondrial membrane potentials were estimated by measuring the uptake of [14C]thiocyanate ( [14C]SCN-) and [3H]tetraphenylphosphonium ( [3H]TPP+) in intact sperm and sperm made permeant with digitonin. Mitochondrial potentials up to-185 mV were found, consistent with data for TPP+ uptake into mitochondria from other cell types. Values for TPP+ uptake corrected for mitochondrial accumulation and estimates of SCN- uptake both indicated that the plasma membrane potential was about -30 mV for actively respiring sperm in seawater and about -60 mV for quiescent sperm in Na+-free seawater. Activation of sperm motility and respiration induced by Na+ increased the intracellular pH and caused a depolarization of both the plasma membrane and mitochondrial potentials. However, membrane potential depolarization did not occur when the activation was induced by increased extracellular pH or by the peptide speract, although activation was always linked to increased intracellular pH. The acrosome reaction, on the other hand, was always associated with sperm plasma membrane potential depolarization, whether it was induced by the physiological effector from the egg surface or by several artificial triggering regimens. Thus, activation of respiration and motility is primarily controlled by increased intracellular pH (Christen, R., Schackmann, R. W., and Shapiro, B. M. (1982) J. Biol. Chem. 257, 14881-14890), whereas the acrosome reaction also requires depolarization of the plasma membrane potential.     

Delayed fluorescence from bacteriochlorophyll in Chromatium vinosum chromatophores.

Delayed fluorescence from bacteriochlorophyll in Chromatium vinosum chromatophores was studied at room temperature and under intermittent illuminations. The decay of delayed fluorescence was constituted of two components; a fast component decayed with a half time of about 8 ms, a slow one decayed in parallel with the reduction of photooxidized bacteriochlorophyll (P+) with a half time of 100-200 ms. The biphasic decay of delayed fluorescence indicated that a rapid equilibrium was established between the primary electron acceptor and the secondary acceptor. In the presence of o-phenanthroline, the time course of the decay of delayed fluorescence was identical with that of the reduction of P+ in reaction center-rich subchromatophore particles, although they did not necessarily coincide with each other in "intact" chromatophores. The intensity of the slow component was increased and the decay was accelerated at basic pH values. Reagents that dissipate the proton gradient across the chromatophore membranes such as carbonylcyanide m-chlorophenylhydrazone (CCCP) and nigericin accelerated the decay of the slow component. These effects are probably resulting from changes in internal pH of chromatophore vesicles. Reagents that dissipate the membrane potential such as CCCP and valinomycin decreased the intensity.     

Active amino acid transport in plasma membrane vesicles from Simian virus 40-transformed mouse fibroblasts. Characteristics of electrochemical Na+ gradient-stimulated uptake.

Selectively permeable membrane vesicles isolated from Simian virus 40-transformed mouse fibroblasts catalyzed Na+ gradient-coupled active transport of several neutral amino acids dissociated from intracellular metabolism. Na+-stimulated alanine transport activity accompanied plasma membrane material during centrifugation in discontinuous dextran 110 gradients. Carrier-mediated transport into the vesicle was demonstrated. When Na+ was equilibrated across the membrane, countertransport stimulation of L-[3H]alanine uptake occurred in the presence of accumulated unlabeled L-alanine, 2-aminoisobutyric acid, or L-methionine. Competitive interactions among neutral amino acids, pH profiles, and apparent Km values for Na+ gradient-stimulated transport into vesicles were similar to those previously described for amino acid uptake in Ehrlich ascites cells, which suggests that the transport activity assayed in vesicles is a component of the corresponding cellular uptake process. Both the initial rate and quasi-steady state of uptake were stimulated as a function of a Na+ gradient (external Na+ greater than internal Na+) applied artificially across the membrane and were independent of endogenous (Na+ + K+)-ATPase activity. Stimulation by Na+ was decreased when the Na+ gradient was dissipated by monensin, gramicidin D or Na+ preincubation. Na+ decreased the apparent Km for alanine, 2-aminoisobutyric acid, and glutamine transport. Na+ gradient-stimulated amino acid transport was electrogenic, stimulated by conditions expected to generate an interior-negative membrane potential, such as the presence of the permeant anions NO3- and SCN-. Na+-stimulated L-alanine transport was also stimulated by an electrogenic potassium diffusion potential (K+ internal greater than K+ external) catalyzed by valinomycin; this stimulation was blocked by nigericin. These observations provide support for a mechanism of active neutral amino acid transport via the "A system" of the plasma membrane in which both a Na+ gradient and membrane potential contribute to the total driving force.     

Formations of electrochemical proton gradient and adenosine triphosphate in proteoliposomes containing purified adenosine triphosphatase and bacteriorhodopsin.

Proteoliposome vesicles containing both bacteriorhodopsin of Halobacterium halobium and H+-translocating ATPase [EC 3.6,1.3] of a thermophilic bacterium, PS3, (TF0-F1) were reconstituted by either the dialysis method or the sonication method. Generation of the electrochemical proton gradient (deltamuH+) in these vesicles was measured using 9-aminoacridine for estimation of the chemical (deltapH) component and 8-anilinonaphthalene sulfonate for the electrical (deltaphi) component). In illuminated bacteriorhodopsin-vesicles the deltamuH+ reached 180-190 mV when reconstituted by the dialysis method and 210-220 mV when reconstituted by the sonication method. Vesicles reconstituted from both TF0-F1 and bacteriorhodopsin by the dialysis method generated a deltapH+ of about 200 mV on addition of ATP, while vesicles prepared by the sonication method generated very little deltamuH+, if any. These vesicles generated similar deltamuH+ on illumination to that found in bacteriorhodopsin-vesicles. Using vesicles reconstituted from both TF0-F1 and bacteriorhodopsin by the dialysis method, light dependent ATP synthesis was measured in relation to deltamuH+ formation. It was necessary to generate a deltamuH+ of above 170 mV for demonstration of appreciable formation of ATP and the greater the deltamuH+, the faster the rate of ATP synthesis.     

Synthesis of pyrophosphate by chromatophores of Rhodospirillum rubrum in the light and by soluble yeast inorganic pyrophosphatase in water-organic solvent mixtures

Chromatophores of Rhodospirillum rubrum contain a membrane-bound pyrophosphatase that synthesizes pyrophosphate when an electrochemical H+ gradient is formed across the chromatophore membrane upon illumination. In this report it is shown that MgCl2 and Pi have different effects on the synthesis of pyrophosphate in the light depending on whether initial velocities or steady-state levels are examined. When the water activity of the medium is reduced by the addition of organic solvents, soluble yeast inorganic pyrophosphatase (no H+ gradient present) synthesizes pyrophosphate in amounts similar to those synthesized by the chromatophores in totally aqueous medium during illumination, (H+ gradient present). The pH, MgCl2 and Pi dependence for the synthesis of pyrophosphate by the chromatophores at steady-state is similar to that observed at equilibrium with the soluble enzyme in the presence of organic solvents. The possibility is raised that a decrease in water activity may play a role in the mechanism by which the energy derived from the electrochemical H+ gradient is used for the synthesis of pyrophosphate in chromatophores of R. rubrum.     

ATPase activity and anion transport across the peribacteroid membrane of isolated soybean symbiosomes

Addition of ATP to intact symbiosomes isolated from soybean nodules, resulted in generation of a membrane potential (positive inside) across the peribacteroid membrane (PBM). This energisation was monitored as oxonol fluorescence quenching. The rate of fluorescence quenching was inhibited by the inclusion of permeant anions in the reaction medium. Using this inhibition as a measure of anion uptake across the PBM, the presence of a phthalonate-sensitive dicarboxylate carrier on the PBM was confirmed. Following dissipation of the membrane potential by a permeant anion, a pH gradient, measured using [¹⁴C]methylamine uptake, was slowly established across the PBM. This pH was abolished by addition of an uncoupler but was insensitive to inhibitors of bacteroid respiration. The difference in pH between the external medium and the symbiosome interior was estimated to be in the range of 1–1.6 pH units. The magnitude in planta will depend on the concentrations of ATP and permeant anions in the cytosol of the host cell.Abbreviations PBM peribacteroid membrane - electrical membrane potential - MA methylamine The term symbiosome refers to the peribacteroid unit consisting of bacteroids enclosed in the host-derived peribacteroid membrane