Accumulation of thallous ions (Tl+) as a measure of the electrical potential difference across the cytoplasmic membrane of bacteria

Minimization of extracellular space as a driving force in prokaryote association and the origin of eukaryotes

BACKGROUND

Internalization-based hypotheses of eukaryotic origin require close physical association of host and symbiont. Prior hypotheses of how these associations arose include chance, specific metabolic couplings between partners, and prey-predator/parasite interactions. Since these hypotheses were proposed, it has become apparent that mixed-species, close-association assemblages (biofilms) are widespread and predominant components of prokaryotic ecology. Which forces drove prokaryotes to evolve the ability to form these assemblages are uncertain. Bacteria and archaea have also been found to form membrane-lined interconnections (nanotubes) through which proteins and RNA pass. These observations, combined with the structure of the nuclear envelope and an energetic benefit of close association (see below), lead us to propose a novel hypothesis of the driving force underlying prokaryotic close association and the origin of eukaryotes.

RESULTS

Respiratory proton transport does not alter external pH when external volume is effectively infinite. Close physical association decreases external volume. For small external volumes, proton transport decreases external pH, resulting in each transported proton increasing proton motor force to a greater extent. We calculate here that in biofilms this effect could substantially decrease how many protons need to be transported to achieve a given proton motor force. Based as it is solely on geometry, this energetic benefit would occur for all prokaryotes using proton-based respiration.

CONCLUSIONS

This benefit may be a driving force in biofilm formation. Under this hypothesis a very wide range of prokaryotic species combinations could serve as eukaryotic progenitors. We use this observation and the discovery of prokaryotic nanotubes to propose that eukaryotes arose from physically distinct, functionally specialized (energy factory, protein factory, DNA repository/RNA factory), obligatorily symbiotic prokaryotes in which the protein factory and DNA repository/RNA factory cells were coupled by nanotubes and the protein factory ultimately internalized the other two. This hypothesis naturally explains many aspects of eukaryotic physiology, including the nuclear envelope being a folded single membrane repeatedly pierced by membrane-bound tubules (the nuclear pores), suggests that species analogous or homologous to eukaryotic progenitors are likely unculturable as monocultures, and makes a large number of testable predictions.

REVIEWERS

This article was reviewed by Purificación López-García and Toni Gabaldón.

Evidence for Na(+) influx via the NtpJ protein of the KtrII K(+) uptake system in Enterococcus hirae

The ntpJ gene, a cistron located at the tail end of the vacuolar-type Na(+)-ATPase (ntp) operon of Enterococcus hirae, encodes a transporter of the KtrII K(+) uptake system. We found that K(+) accumulation in the ntpJ-disrupted mutant JEM2 was markedly enhanced by addition of valinomycin at pH 10. Studies of the membrane potential (DeltaPsi; inside negative) by 3, 3'-dihexyloxacarbocyanine iodide fluorescence revealed that the DeltaPsi was hyperpolarized at pH 10 in JEM2; the DeltaPsi values of the parent strain ATCC 9790 and JEM2, estimated by determining the equilibrium distribution of K(+) or Rb(+) in the presence of valinomycin, were -118 and -160 mV, respectively. DeltaPsi generation at pH 10 was accomplished by an electrogenic Na(+) efflux via the Na(+)-ATPase, whose levels in the two strains were quite similar. Na(+) uptake driven by an artificially imposed DeltaPsi (inside negative) was missing in JEM2, suggesting that NtpJ mediates Na(+) movement in addition to K(+) movement. Finally, the growth of JEM2 arrested in K(+)-limited high-Na(+) medium at pH 10 was restored by addition of valinomycin. These results suggest that NtpJ mediates electrogenic transport of K(+) as well as Na(+), that it likely mediates K(+) and Na(+) cotransport, and that Na(+) movement via NtpJ is the major Na(+) reentry pathway at high pH values.

Insertion of an uncharged polypeptide into the mitochondrial inner membrane does not require a trans-bilayer electrochemical potential: effects of positive charges

Mitochondria with a ruptured outer membrane exhibited impaired import into this membrane of an outer membrane fusion protein containing the signal-anchor sequence of Mas70p. However, the Mas70p signal-anchor efficiently targeted and inserted the protein directly into exposed regions of the inner membrane. Import into the inner membrane was dependent on delta psi and this dependence was due to the presence of the positively-charged amino acids located at positions 2, 7, and 9 of the signal-anchor. In contrast to wild-type signal-anchor, mutants lacking the positively-charged residues mediated import into the inner membrane in both the presence and absence of delta psi. The results suggest two conclusions: (1) delta psi-dependent import of the signal-anchor sequence was due exclusively to an effect of delta psi on the positively-charged domain of the signal-anchor, rather than to an effect of delta psi on a property of the inner membrane import machinery; (2) in the absence of delta psi, the positively-charged domain of the signal-anchor prevented the otherwise import-competent signal-anchor from inserting into the membrane. This suggests that the positively-charged domain leads import across the inner membrane, and that delta psi is required to vectorially clear this domain in order to allow the distal region of the signal-anchor to enter the translocation pathway. The implications of these findings on the mechanism of import into the mitochondrial inner membrane and matrix are discussed.

Potassium/proton antiport system of growing Enterococcus hirae at high pH

The cytoplasmic pH (pHin) of Enterococcus hirae growing at pH 9.2 was maintained at about 8.1. Membrane-permeating amines such as ammonia alkalinized the pHin from 8.1 to 9.0 at a high concentration and induced K+ extrusion. The pHin alkalinization was transient; the pHin fell from 9.0 to the original value of pH 8.1, at which point K+ extrusion ceased, and remained constant. Cells accumulated ammonium ion to an extent stoichiometrically equivalent to the K+ loss. This bacterium continued to grow well under this condition. These results suggest that the pHin-responsive primary K+/H+ antiport system (Y. Kakinuma, and K. Igarashi, J. Biol. Chem. 263:14166-14170, 1988) works for the pHin regulation of this organism growing at a high pH.

Altered topoisomerase activities may be involved in the regulation of DNA supercoiling in aerobic-anaerobic transitions in Escherichia coli

This study uncovers a new mechanism of regulation of DNA supercoiling operative in vivo upon an aerobic-anaerobic transition in Escherichia coli. Exponentially growing aerobic batch cultures were subjected to a shift to anaerobic conditions. The ratio [ATP]/[ADP] remained essentially constant at 8.5 in the aerobic culture and after a transition to anaerobiosis while DNA supercoiling increased noticeably upon anaerobiosis. This result indicated that the mechanism of regulation of DNA supercoiling by the [ATP]/[ADP] ratio was not operative. The increase in DNA supercoiling was followed by a large decrease in the DNA-relaxing activity of topoisomerase I while gyrase activity remained relatively constant. This decrease in the activity of topoisomerase I is likely to be responsible for the increase in DNA supercoiling.

A signal-anchor sequence selective for the mitochondrial outer membrane

pOMD29 is a hybrid protein containing the NH2-terminal topogenic sequence of a bitopic, integral protein of the outer mitochondrial membrane in yeast, OMM70, fused to dihydrofolate reductase. The topogenic sequence consists of two structural domains: an NH2-terminal basic region (amino acids 1-10) and an apolar region which is the predicted transmembrane segment (amino acids 11-29). The transmembrane segment alone was capable of targeting and inserting the hybrid protein into the outer membrane of intact mitochondria from rat heart in vitro. The presence of amino acids 1-10 enhanced the rate of import, and this increased rate depended, in part, on the basic amino acids located at positions 2, 7, and 9. Deletion of a large portion of the transmembrane segment (amino acids 16-29) resulted in a protein that exhibited negligible import in vitro. Insertion of pOMD29 into the outer membrane was not competed by import of excess precursor protein destined for the mitochondrial matrix, indicating that the two proteins may have different rate-limiting steps during import. We propose that the structural domains within amino acids 1-29 of pOMD29 cooperate to form a signal-anchor sequence, the characteristics of which suggest a model for proper sorting to the mitochondrial outer membrane.

Reversal of the orientation of an integral protein of the mitochondrial outer membrane

The NH2-terminus of the signal-anchor sequence of an integral, bitopic protein of the outer mitochondrial membrane was extended both in amino acid length (from 11 to 38 amino acids) and net charge (from +4 to +8)--changes that confer on the NH2-terminus characteristics of a strong matrix-targeting signal. The protein was inserted into the outer membrane but in an inverted orientation (Ncyto-Cin). These findings suggest that, in common with other membrane systems, the orientation of a protein in the outer mitochondrial membrane can be determined by a signal that causes retention of the NH2-terminus on the cytosolic side of the membrane.

Protein sorting between mitochondrial outer and inner membranes. Insertion of an outer membrane protein into the inner membrane

The amino terminal 29 amino acids of the outer mitochondrial membrane protein of yeast, OMM70 (MAS70), consisting of the targeting and membrane anchor domains, has been fused to a reporter protein, dihydrofolate reductase. The hybrid protein, designated pOMD29, was efficiently imported into the outer membrane of rat heart mitochondria by a process dependent on ATP and proteinase-sensitive components on the surface of the organelle, and in which the orientation of the native protein was retained. To determine if the protein translocation machinery of the inner membrane is also capable of recognizing and inserting pOMD29, direct access to the intermembrane space was provided to pOMD29 by selectively rupturing the mitochondrial outer membrane by osmotic shock. In this system, the outer membrane binding site for matrix-destined precursor proteins can be bypassed, and efficient import restored to proteinase-pretreated mitochondria. pOMD29 was imported into the inner membrane of osmotically-shocked mitochondria, mediated by protein components. The outer membrane orientation of pOMD29 was conserved when inserted into the inner membrane but, unlike the outer membrane, import into the inner membrane required delta psi. We conclude that the protein translocation machinery of the mitochondrial inner membrane is capable of recognizing and inserting a protein whose topogenic information otherwise results in insertion of the protein to the outer membrane. The significance of these findings for sorting of proteins between the mitochondrial inner and outer membranes is discussed.

Lowering of cytoplasmic pH is essential for growth of Streptococcus faecalis at high pH

The growth of Streptococcus faecalis at high pH was significantly stimulated by carbonate. In the absence of added carbonate the cells were unable to grow at a pH above 9.5, but in media containing 50 mM HCO3- they grew even at pH 10.5. Both rate and yield of growth at pH 9.5 were significantly stimulated by as little as 5 mM carbonate. The cytoplasmic pH in growing cells was maintained at about 7.8 to 8.2, whereas the medium pH ranged from 8.4 to 9.5. Nigericin and gramicidin D, ionophores which conduct protons, blocked growth at pH 9.5 but not at pH 7.5. These results indicate that lowering of the cytoplasmic pH is essential for the growth of this organism at high pH.

Impairment of Na+ transport across frog skin by Tl+: effects on turnover, area density and saturation kinetics of apical Na+ channels

Na+ transport across abdominal skin of the frogs, Rana temporaria and Rana esculenta was followed by measuring Na+-dependent short-circuit, current (INa) kinetics and INa fluctuations induced by triamterene, a diuretic. Exposure of the skin to serosal Tl+ led to a pronounced and irreversible drop in INa and INa-blocker noise. At low serosal Tl+ concentrations, we observed mainly a decrease in the apparent Michaelis constant for INa saturation while, at larger [Tl+], the maximal INa dropped irreversibly. Tl+ acts even when serosal Tl+ "transporters" like the Na+-K+ pump, or the K+ channel are nonfunctional. The rate constants for the triamterene/Na+ channel reaction were unchanged after Tl+ whereas the relaxation noise from channel blockage decreased in amplitude. Noise analysis in terms of a two-state blocking model suggested that Tl+ poisoning results in a small decrease in single-channel current through apical Na+ pathways, as well as in a drastic and irreversible drop in channel density. The impairment of Na+ transport by Tl+ can be attributed to the above cited concerted events at the level of the apical membrane.

Magnesium transport in murine S49 lymphoma cells: pharmacology and divalent cation selectivity

The murine S49 lymphoma cell transports Mg2+ by a system distinct from systems responsible for Ca2+ influx (J. Physiol. London 337: 351-371, 1983). We have now determined the ability of various cations, anions, and drugs to modulate Mg2+ influx. Neither sulfate, nitrate, phosphate, nor bicarbonate altered Mg2+ influx. Among cations only T1+, Ba2+, Zn2+, Mn2+, Sc3+, and La3+ potently inhibited Mg2+ influx without causing obvious cell toxicity. Seventeen other cations were ineffective at maximal nontoxic concentrations. T1+ inhibition (Ki = 300 micron) is noncompetitive and apparently derives from its ability to dissipate membrane potential. The noncompetitive nature of and the rather poor inhibition constants for Ca2+ (Ki approximately equal to 5 mM) and Mn2+ (Ki = 200 micron) indicate that neither cation is an effective physiological antagonist of Mg2+ influx. Only Ba2+ exhibited competitive inhibition of Mg2+ influx (Ki = 1 mM). Cisplatin and Ca2+ channel antagonists also did not inhibit Mg2+ influx. These data further differentiate Mg2+ transport systems from those for Ca2+. In addition, the selectivity series for group IIa cation inhibition of influx (Mg2+ greater than Ba2+ much greater than Ca2+ greater than or equal to Sr2+) has not been observed previously in biological systems and is indicative of a very high anionic field strength at the Mg2+ recognition site.

Coupling of ATP synthesis and methane formation from methanol and molecular hydrogen in Methanosarcina barkeri

The addition of methanol to a cell suspension of Methanosarcina barkeri resulted in an increase of the intracellular ATP concentration from 1 nmol/mg to 10 nmol/mg protein and in the formation of a proton-motive force delta p of -130 mV. delta p consisted of more than 90% of the membrane potential delta psi. These values were similar under N2 and under H2. The addition of the uncoupler tetrachlorosalicylanilide to the above system under N2 led to a drastic decrease of both, the ATP concentration and the delta p and to a stop of methanogenesis. With methanol and H2, however, methane formation continued, although the effect of the uncoupler on the ATP pool and on delta p was a under N2. The proton-translocating ATPase inhibitor N,N'-dicyclohexylcarbodiimide caused a rapid exhaustion of the ATP pool and a discontinuation of methane synthesis, whereas delta p was unaffected. Inhibition of methane formation under these conditions could be relieved by the addition of the uncoupler tetrachlorosalicylanilide. These results demonstrate that methane formation according to the equation CH3OH + H2----H2----CH4 + H2O was coupled to ATP synthesis by a chemiosmotic mechanism and was under the control of delta psi: Methane formation only proceeded if the delta psi generated was used for ATP synthesis or if an uncoupler was present. Under N2, methane formation according to the equation 4CH3OH ----CO2 + 3CH4 + 2H2O was abolished by an uncoupler, because one step in the oxidation of methanol to 1 CO2 apparently depended on an energized state of the membrane.

Cadmium and thallous ion permeabilities through lipid bilayer membranes

Cadmium (Cd2+) and thallous ion (Tl+) permeabilities were measured in planar (Mueller-Rudin) lipid bilayer membranes made from diphytanoylphosphatidylcholine in decane. Permeabilities of the electroneutral Cl- complexes, measured with tracers (109Cd and 204Tl), were about 10(-8) cm X s-1 for CdCl2 and 10(-6) cm X s-1 for TlCl. Electrical conductance measurements showed that permeabilities to Cd2+ and Tl+ were approx. 10(-11) cm X s-1, similar to the Na+ permeability. The low permeabilities to both Cd2+ and CdCl2 are consistent with biological studies which suggest that Cd transport and toxicity are protein mediated and correlated with Cd2+, not CdCl2, concentration. However, the low bilayer permeability to Tl+ raises questions about recent reports that Tl+ is a lipid permeable cation in biological membranes and liposomes. An alternative explanation for the lipid permeable behavior of Tl+ is presented, based on the diffusion of TlCl and other complexes of Tl+ with inorganic and organic anions.

Interconversion of components of the bacterial proton motive force by electrogenic potassium transport

The influence of K+ ions on the components of the transmembrane proton motive force (delta mu H+) in intact bacteria was investigated. In K+-depleted cells of the glycolytic bacterium STreptococcus faecalis the addition of K+ ions caused a depolarization of the membrane by about 60 mV. However, since the depolarization was compensated for by an increase in the transmembrane pH gradient (delta pH), the total proton motive force remained almost constant at about 120 mV. Half-maximal changes in the potential were observed at K+ concentrations at which the cells accumulated K+ ions extensively. In EDTA-treated, K+-depleted cells of Escherichia coli K-12, the addition of K+ ions to the medium caused similar, although smaller changes in the components of delta mu H+. Experiments with various E. coli K-12 K+ transport mutants showed that for the observed potential changes the cells required either a functional TrkA or Kdp K+ transport system. These data are interpreted to mean that the inward movement of K+ ions via each of these bacterial transport systems is electrogenic. Consequently, it leads to a depolarization of the membrane, which in its turn allows the cell to pump more protons into the medium.

Estimation with an ion-selective electrode of the membrane potential in cells of Paracoccus denitrificans from the uptake of the butyltriphenylphosphonium cation during aerobic and anaerobic respiration

1. Aerobic respiration by cells of Paracoccus dentrificans drives the uptake of the lipophilic cation butyltriphenylphosphonium. Anaerobiosis or addition of an uncoupler of oxidative phosphorylation (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) results in efflux of the cation. Changes in the concentration of butyltriphenylphosphonium in the suspension medium were measured by using an ion-selective electrode, the construction of which is described. 2. If the uptake of butyltriphenylphosphonium is used as an indicator of membrane potential, then at pH 7.3 an estimate of about 160 mV is obtained for cells of P. dentrificans respiring aerobically in 100 mM-Hepes [4-(2-hydroxyethyl)-1-piperazine-ethanesulphonic acid/NaOH or 100mM-NaH2PO4/NaOH. This potential, however, is decreased by more than 20 mV in reaction media containing a high concentration of phosphate (100 mM) together with at least 1 mM-K+. 3. Anaerobic electron transport with NO3-, NO2- or N2O as terminal electron acceptor generates a membrane potential of about 150mV in described suspension media. The presence of these species under aerobic conditions, moreover, has negligible effect upon the extent of uptake of butyltriphenylphosphonium normally driven by aerobic respiration. These data indicate that none of these molecules exert a significant uncoupling effect on the protonmotive force. 4. No 204Tl+ uptake into respiring cells was detected. This adds to the evidence that 204Tl+ is not a freely permeable cation in bacterial cells and therefore not an indicator of membrane potential as has been proposed. The absence of respiration-driven 204Tl+ uptake indicates that P. denitrificans cells grown under the conditions specified in the present work do not possess K+-transport systems of either the Kdp or TrkA types that have been described in Escherichia coli.

Mechanism of mitochondrial transport of thallous ions

Rat liver mitochondria were found to swell under nonenergized conditions when suspended in media containing 30-40 mM TINO3. Respiration on succinate caused a rapid contraction of mitochondria swollen under nonenergized conditions. In the presence of thallous acetate, there was a rapid initial swelling under nonenergized conditions until a plateau was reached; respiration on succinate then caused a further swelling. Trace amounts of 204Tl (less than 100 microM) equilibrated fairly rapidly across the mitochondrial membrane. The influx of Tl+ was able to promote the decay not only of a valinomycin-induced K+-diffusion potential but also of respiration-generated fields in the inner membrane in accordance with the electrophoretic nature of Tl+ movement. Efflux of Tl+ showed a half-time of about 10 sec at 20 degrees C and was not affected appreciably by the energy state. Efflux was retarded by Mg2+ and by lowering the temperature. The data indicate that Tl+ when present at high concentrations, 30mM or more, distributes across the mitochondrial inner membrane both in response to electrical fields and to delta pH. In energized mitochondria the uptake of Tl+ would occur electrophoretically, while Tl+/H+ exchange would constitute a leak. In the presence of NO3-, the movements of Tl+ are determined by that of NO3-, indicating short-range coupling of electrical forces. At low concentrations of Tl+, 5 mM or less, there was no indication of a Tl+/H+ exchange, which appears to be induced by high concentrations of Tl+.

Effects of aerobiosis and nitrogen source on the proton motive force in growing Escherichia coli and Klebsiella pneumoniae cells

The electrochemical gradient of hydrogen ions, or proton motive force (PMF), was measured in growing Escherichia coli and Klebsiella pneumoniae in batch culture. The electrical component of the PMF (delta psi) and the chemical component (delta pH) were calculated from the cellular accumulation of radiolabeled tetraphenylphosphonium, thiocyanate, and benzoate ions. In both species, the PMF was constant during exponential phase and decreased as the cells entered stationary phase. Altering the growth rate with different energy substrates had no effect on the PMF. The delta pH (alkaline inside) varied with the pH of the culture medium, resulting in a constant internal pH. During aerobic growth in media at pH 6 to 7, the delta psi was constant at 160 mV (negative inside). The PMF, therefore, was 255 mV in cells growing at pH 6.3, and decreased progressively to 210 mV in pH 7.1 cultures. K. pneumoniae cells and two E. coli strains (K-12 and ML), including a mutant deficient in the H+-translocating ATPase and a pleiotropically energy-uncoupled mutant with a normal ATPase, had the same PMF during aerobic exponential phase. During anaerobic growth, however, both species had delta psi values equal to 0. Therefore, the PMF in anaerobic cells consisted only of the delta pH component, which was 75 mV or less in cells growing at pH 6.2 or greater. These data thus met the expectation that cells deriving metabolic energy from respiration have a PMF above a threshold value of about 200 mV when the ATPase functions in the direction of H+ influx and ATP synthesis; in fermenting cells, a PMF below a threshold value was expected since the enzyme functions in the direction of H+ extrusion and ATP hydrolysis. K. pneumoniae cells growing anaerobically had no delta psi whether the N source added was N2, NH+4 or one of several amino acids; the delta pH was unaffected. Therefore, any energy cost incurred by the process of nitrogen fixation could not be detected as an alteration of the proton gradient.

Effect of membrane potential on the passive transport of Tl+ in human red blood cells

It was earlier found that Tl+ can easily penetrate the red cell membrane. The main finding of this work is that Tl+ can be used for studying alterations in the membrane potential of human red blood cells exposed to various experimental conditions. It was shown that after inhibiting active transport by ouabain, both the rate of trans-membrane movement and the cell/medium distribution of Tl+ were in a good agreement with the expected changes in membrane potential. Alterations in membrane potential were induced by modifying the cation permeability of the red cell membrane and by varying the cation concentration gradient across the membrane, which were achieved: 1) by incubation in an electrolyte-free sucrose solution. 2) by addition of valinomycin or 3) by addition of propranolol. Changes in cation permeability were followed by means of 86Rb tracer. Hyperpolarization of the red cell membrane led to accelerated influx and retarded efflux of Tl+. The opposite effect was obtained by depolarization. Quantification of the results was made using the Nernst equation and the cell/medium concentration ratio of Tl+ at equilibrium. The calculations show that the membrane potential of the propranolol-treated cells increased to about -20 mV, negative inside. The mechanism of the propranolol effect is briefly discussed.

Thallium interaction with the gastric (K, H)-ATPase

The gastric (K, H)-ATPase has been shown to catalyze an electroneutral H+ for K+ exchange. Tl+ is able to substitute for K+ as an activating cation in the hydrolytic reaction with an apparent dissociation constant of 90 microM as compared to about 870 microM for K+. The ability of Tl+ to participate in transport is shown by the development of pH gradients in the presence of Tl+ following addition of ATP to gastric vesicles and by the ATP-dependent efflux of Tl+ from gastric vesicles. Inhibition of hydrolysis is observed at pH 7.4 with external Tl+ concentrations above 3.0 mM. This inhibition of hydrolysis is correlated with inhibition of pH-gradient formation. The inhibition of transport activity is partially relieved by a decrease in medium pH. This inhibitory effect is attributed to Tl+ binding at an external, low affinity cation site. In contrast to rubidium chloride, at high Tl+ concentrations, following the initial Tl+ efflux, there is reuptake of the cation. This rapid uptake is attributed to lipid-dependent Tl+ entry pathways. The vesicles exhibit a high permeability to thallium nitrate demonstrating a half-time (t1/2) for uptake of about 1.0 min in contrast to 46 min for rubidium chloride. In both gastric vesicles or liposomes, external Tl+ concentrations in excess of 1 to 4 mM are able to dissipate intravesicular proton gradients. Thus, although Tl+ is able to activate the gastric ATPase by mimicking K+, the permeability of this cation in lipid bilayers tends to uncouple H+ transport at concentrations high enough to generate detectable proton gradients.

Ouabain-sensitive thallium fluxes in smooth muscle of rabbit uterus

1. Rabbit myometrium accumulates Tl in a time-dependent fashion and the majority of the uptake of Tl is ouabain-sensitive. 2. In normal chloride-containing media, the uptake of Tl, though ouabain-sensitive, is less than in chloride-free media and this difference is due to a greater ouabain-insensitive uptake. 3. The ouabain-insensitive uptake in normal chloride-containing media is reduced by furosemide and furosemide also reduces total uptake in this solution. In chloride-free media, however, furosemide is without effect on total or ouabain-sensitive uptake. 4. In chloride-free media, the uptake of Tl against Tl concentration is sigmoidal, suggesting that more than one Tl ion is being transported at a time. 5. Tl was capable of substituting for K in electrogenic Na pumping; it was approximately twice as effective as K and the inhibitory effect of Tl was blocked by ouabain. 6. Tl efflux can be explained by a simple two-compartment model in both normal and chloride-free solutions. 7. The uptake of Tl was inhibited by alkali cations with the order of potency being Tl+ > K+ > Rb+ > Cs+ at 10 mM and Tl+ > K4 = Rb+ > Cs+ at 5 mM ion concentrations. 8. It is concluded that Tl enters the smooth muscle of rabbit uterus by diffusion, active ouabain-sensitive transport and active chloride- and furosemide-sensitive transport.

Kinetic and steady-state investigations of solute accumulation in bacterial membranes by continuously monitoring the radioactivity in the effluent of flow-dialysis experiments

The flow-dialysis technique for studies of solute accumulation by membrane preparations has been made more suitable for routine measurements by recording continuously the radioactivity in the effluent of a flow-dialysis vessel with a homogeneous flow-monitoring device for beta-emitters. This modification not only decreases the time and cost of a flow-dialysis experiment but also allows the investigator to react directly on the outcome of his experiments. Analysis of the kinetics of this automated flow-dialysis system shows that this technique can also be used for the determination of the rate of uptake of solutes into bacterial membranes. This has been confirmed in Rhodopseudomonas sphaeroides by comparing the results of Rb+ and inorganic phosphate uptake studies performed by automated flow-dialysis and by the conventional filtration procedure. This application has the limitation that solute uptake has to proceed linearly for a period of about five times the half-time of the response of the flow-dialysis system. The two described applications make automated flow-dialysis very well-suited for experiments on the bioenergetics and regulation of solute uptake into bacterial membranes. Both driving force and rate of solute uptake can now be determined in one experiment.

Measurements of membrane potentials in Escherichia coli K-12 inner membrane vesicles with the safranine method

The use of safranine, a positively-charged dye, as a probe for the determination of membrane potentials in Escherichia coli vesicles has been studied. 1. Shifts in the spectrum of safranine were observed during induction of potassium ion diffusion potentials with valinomycin or during oxidation of formate by vesicles prepared from cells of E. coli K-12 or ML 308-225 subjected to anaerobic growth with nitrate. The extent of the valinomycin-dependent spectral change correlated linearly with the magnitude of the K+ equilibrium potential, as calculated from the Nernst equation, from 50 to 160 mV (interior negative). The formate-induced changes could also be calibrated by increasing the concentration of potassium in the presence of valinomycin, after the formation of formate-dependent responses. In this case, results identical to those obtained with the first method were obtained. 2. O2 or nitrate-dependent oxidation of formate resulted in a membrane potential of the order of 170 mV. The oxidation of ascorbate-reduced N-methylphenazonium methosulphate resulted in a potential of similar magnitude, but anaerobically with nitrate only a small but definite potential was formed. 3. The water-soluble quinones, duroquinone and menadione, could produce membrane potentials when used in their oxidized or reduced forms in the presence of formate or nitrate (or oxygen). 2-Hydroxy-1,4-naphthoquinone was not only ineffective but was found to be inhibitory. 4. N,N'-dicyclohexylcarbodiimide at suitable concentrations increased the rate of formation and the extent of membrane potentials induced by respiration or by artificial means.

Ionic control of germination of Blastocladiella emersonii zoospores

Encystment and germination of Blastocladiella emersonii zoospores involve a rapid and radical transformation of the motile but nongrowing spore into a sessile, growing germling. Certain inorganic ions, notably 50 mM KCl, are efficient inducers of germination. By use of the carbocyanine dye DiO-C6-(3), we found that KCl depolarizes the plasma membrane of zoospores and noted good correlation between depolarization and subsequent germination. Zoospores avidly accumulated K+ ions from the medium, attaining an internal concentration of over 50 mM and a concentration gradient of 2,500. Sodium ions, by contrast, were expelled. Internal K+ was required for normal germination but its function is not known. Zoospores also took up considerable amounts of calcium; most of this was associated with the external surface and appeared to be necessary for maintenance of zoospore integrity. KCl (50 mM) and other salts displaced surface calcium but this was not in itself sufficient to induce germination. The calcium ionophore A23187, in the presence of external calcium, was an effective inducer of germination, suggesting a possible role for cytosolic calcium in triggering the transformation. We propose that the first step in the induction of germination by salts is depolarization of the plasma membrane; subsequent events require the intervention of cytoplasmic signals.

Membrane H+ conductance of Streptococcus lactis

Membrane conductance to H+ was measured in the anaerobic bacterium Streptoccus lactis by a pulse technique employing a low driving force (0.1 pH unit; 6 mV). Over the pH range of 3.7 to 8.5, a constant value for passive H+ conductance was observed, corresponding to 0.2 mumol of H+/s per p/ unit per g, dry weight (1.6 microS/cm2 of surface area). The pH insensitivity of this low basal H+ conductance supports the idea that a circulation of protons can mediate highly efficiency engery transductions across the membranes of bacteria.

Movement of thallous ion across the ascites cell membrane

The movement of thallous ion (Tl+) across the ascites cell membrane has been characterized. Analogous to previous findings for 86Rb (used as a tracer for K+), 204Tl+-influx could be resolved into three components: a ouabain-inhibitable "pump" flux, a passive flux, and a furosemide- or NO-3-sensitive "exchange" flux. Although Tl+ moved approximately nine times faster across the membrane than K+, the pump/leak ratio was equal for the two ions. This suggests that the pump- and leak-pathways share a common rate-limiting step. The exchange mechanism was shown to provide close coupling between the Tl+- and K+-gradients.