Phosphate transport in Neurospora. Derepression of a high-affinity transport system during phosphorus starvation.

In addition to the constitutive, low-affinity phosphate-transport system described previously, Neurospora possesses a second, high-affinity system which is derepressed during phosphorus starvation. At pH 5.8, System ii has a K1/2 of about 3muM and a Jmax of 5.2 mmol/1 cell water per min. System ii reaches maximal activity after about 2 h of growth in phosphorus-free minimal medium. Its formation is blocked by cycloheximide and, once made, it appears to turn over rapidly. Addition of cycloheximide to fully derepressed cultures results in the decay of System ii with a t1/2 of 14 min, very similar to the turnoacteriol. 95, 959-966) for tryptophan transport in Neurospora. Thus, these transport systems appear to be regulated by a balance between synthesis and breakdown, as affected by intracellular pools of substrate or related compounds.     

Characterization and regulation of galactose transport in Neurospora crassa

Two galactose uptake systems were found in the mycelia of Neurospora crassa. In glucose-grown mycelia, galactose was transported by a low-affinity (Km = 400 mM) constitutive system which was distinct from the previously described glucose transport system I (R. P. Schneider and W. R. Wiley, J. Bacteriol. 106:479--486, 1971). In carbon-starved mycelia or mycelia incubated with galactose, a second galactose transport activity appeared which required energy, had a high affinity for galactose (Km = 0.7 mM), and was shown to be the same as glucose transport system II. System II also transported mannose, 2-deoxyglucose, xylose, and talose and is therefore a general monosaccharide transport system. System II was derepressed by carbon starvation, completely repressed by glucose, mannose, and 2-deoxyglucose, and partially repressed by fructose and xylose. Incubation with galactose yielded twice as much activity as starvation. This extra induction by galactose required protein synthesis, and represented an increase in activity of system II rather than the induction of another transport system. Glucose, mannose, and 2-deoxyglucose caused rapid degradation of preexisting system II; fructose and xylose caused a slower degradation of activity.     

Hormonal regulation of the system a amino acid transport adaptive response mechanism in a kidney epithelial cell line (MDCK)

When mamalian cells are starved for amino acids, the activity of the A amino acid transport system increases, a phenomenon called adaptive regulation. We have examined the effects of those factors which support Madin-Darby canine kidney (MDCK) cell growth in a defined medium on the derepression of System A activity. Of the five factors which supported MDCK cell growth, insulin was found to be an absolute requirement for derepression. In contrast, PGE₁ was a negative controlling factor for the transport system. Growth of MDCK cells in the absence of PGE₁ resulted in elevated System A activity which derepressed poorly upon amino acid starvation. Kinetic analysis of α-(methylamino) isobutyric acid (mAIB) uptake as a function of substrate concentration showed that the elevated A activity observed when cells were grown in the absence of PGE₁ was kinetically similar to the activity induced by starvation for amino acids. Transport of mAIB by amino-acid-fed cells grown in the presence of PGE₁ was characterized by a linear Eadie-Hofstee graph and by a relatively low Vmax. Transport by cells starved for amino acids or by cells grown in the absence of PGE₁ was characterized by biphasic kinetics for mAIB transport and by elevated Vmax values. An influence of growth factors on the inactivation of derepressed A activity was also observed. In the presence of cycloheximide the rate of loss of A activity in amino-acid-starved cells was 1/4–1/2 that of amino-acid-fed cells. Insulin slowed inactivation in the absence of most amino acids in a protein-synthesis-independent manner, but insulin did not influence the more rapid inactivation observed in amino-acid-fed cells. These results indicate that the level of System A activity observed in response to regulation by amino acids represents a balance between carrier synthesis and inactivation, which can be positively or negatively influenced by growth factors.     

Sugar transport in Coprinus cinereus

Two transport systems for glucose were detected: a high affinity system with a K_m of 27 μM, and a low affinity system with a K_m of 3.3 mM. The high affinity system transported glucose, 2-deoxy-d-glucose (K_m = 26 μM), 3-O-methylglucose (K_m = 19 μM), d-glucosamine (K_m = 652 μM), d-fructose (K_m = 2.3 mM) and l-sorbose (K_m = 2.2 mM). All sugars were accumulated against concentration gradients. The high affinity system was strongly or completely inhibited by N-ethylmaleimide, quercetin, 2,4-dinitrophenol and sodium azide. The system had a distinct pH optimum (7.4) and optimum temperature (45°C). The low affinity system transported glucose, 2-deoxy-d-glucose (K_m = 7.5 mM), and 3-O-methylglucose (K_m = 1.5 mM). Accumulation again occurred against a concentration gradient. The low affinity system was inhibited by N-ethylmaleimide, quercetin and 2,4-dinitrophenol, but not by sodium azide. The rate of uptake by the low affinity system was constant over a wide temperature range (30–50°C) and was not much affected by pH; but as the pH of the medium was altered from 4.5 to 8.9 a co-ordinated increase in affinity for 2-deoxy-d-glucose (from 52.1 mM to 0.3 mM) and decrease in maximum velocity (by a factor of five) occurred. Both uptake systems were present in sporelings germinated in media containing sodium acetate as sole carbon source. Only the low affinity system could initially be demonstrated in glucose-grown tissue, although the high affinity system was restored by starvation in glucose-free medium. The half-time for restoration of high affinity activity was 3.5 min and the process was unaffected by cycloheximide. Addition of glucose to an acetate-grown culture inactivated the high affinity system with a half-life of 5–7.5 s. Addition of cycloheximide to an acetate-grown culture caused decay of the high affinity system with a half-life of 80 min. Regulation is thus thought to depend on modulation of protein activity rather than synthesis, and the kinetics of glucose, 2-deoxy-d-glucose and 3-O-methylglucose uptake would be consistent with there being a single carrier showing negative co-operativity.     

Potassium transport in Neurospora. Evidence for a multisite carrier at high pH

At low extracellular pH (4–6), net uptake of potassium by Neurospora is a simple exponential process which obeys Michaelis kinetics as a function of [K]_o. At high pH, however, potassium uptake becomes considerably more complex, and can be resolved into two distinct exponential components. The fast component (time constant = 1.2 min) is matched quantitatively by a rapid loss of sodium; it is attributed to ion exchange within the cell wall, since it is comparatively insensitive to low temperature and metabolic inhibitors. By contrast, the slower component (time constant = 10.9 min) is inhibited markedly at 0°C and by CN and deoxycorticosterone, and is thought to represent carrier-mediated transport of potassium across the cell membrane. This transport process exhibits sigmoid kinetics as a function of [K]_o; the data can be fitted satisfactorily by two different two-site models (one involving a carrier site and a modifier site, the other an allosteric model). Either of these models could also accommodate the simple Michaelis kinetics at low pH.     

Potassium-proton symport inNeurospora: kinetic control by pH and membrane potential

Summary Active transport of potassium in K⁺-starvedNeurospora was previously shown to resemble closely potassium uptake in yeast,Chlorella, and higher plants, for which K⁺ pumps or K⁺/H⁺-ATPases had been proposed. ForNeurospora, however, potassium-proton cotransport was demonstrated to operate, with a coupling ratio of 1 H⁺ to 1 K⁺ taken inward so that K⁺, but not H⁺, moves against its electrochemical gradient (Rodriguez-Navarro et al.,J. Gen. Physiol. 87:649–674).In the present experiments, the current-voltage (I–V) characteristic of K⁺–H⁺ cotransport in spherical cells ofNeurospora has been studied with a voltage-clamp technique, using difference-current methods to dissect it from other ion-transport processes in theNeurospora plasma membrane. Addition of 5-200 M K⁺ to the bathing medium causes 10–150 mV depolarization of the unclamped membrane, and yields a sigmoidI–V curve with a steep slope (maximal conductance of 10–30 S/cm²) for voltages of –300 to –100 mV, i.e., in the normal physiologic range. Outside that range the apparentI–V curve of the K⁺-H⁺ symport saturates for both hyperpolarization and depolarization. It fails to cross the voltage axis at its predicted reversal potential, however, an effect which can be attributed to failure of theI–V difference method under reversing conditions.In the absence of voltage clamping, inhibitors—such as cyanide or vanadate—which block the primary proton pump inNeurospora also promptly inhibit K⁺ transport and K⁺-H⁺ currents. But when voltage clamping is used to offset the depolarizing effects of pump blockade, the inhibitors have no immediate effect on K⁺-H⁺ currents. Thus, the inhibition of K⁺ transport usually observed with these agents reflects the kinetic effect of membrane depolarization rather than any direct chemical action on the cotransport system itself.Detailed study of the effects of [K⁺]_o and pH_o on theI–V curve for K⁺-H⁺ symport has revealed that increasing membrane potential systematicallydecreases the apparent affinity of the transporter for K⁺, butincreases affinity for protons (K _m range: for [K⁺]_o, 15–45 M; for [H⁺]_o, 10–35 nM). This behavior is consistent with two distinct reaction-kinetic models, in which (i) a neutral carrier binds K⁺ first and H⁺ last in the forward direction of transport, or (ii) a negatively charged carrier (–2) binds H⁺ first and K⁺ last.     

Glucose transport in thymocyte plasma-membrane vesicles

Calf-thymocyte membrane vesicles, prepared by hypotonic lysis and homogenization, were isolated by standard centrifugal techniques designed for enrichment of plasma membrane. At 20°C, these vesicles equilibrated with d-glucose and 3-O-methyl-d-glucose more rapidly than with l-glucose. About 25% of the equilibrium d-sugar space (6 μl/mg protein) was very slowly penetrated by l-glucose ( ). The time course of d-sugar accumulation in excess of l-glucose accumulation indicated that this space equilibrated with d-glucose and 3-O-methyl-d-glucose with half-times of approximately 0.2–0.4 min. The remainder of the equilibrium d-sugar space (about 75%) appeared equally accessible to both glucose isomers ( to 5 min). This was confirmed in studies of efflux from preloaded vesicles, where the d-glucose space fell with a short half-time (0.2 min) to the l-glucose space, after which the two isomers exited with the same half-time. Addition of sucrose to increase osmolarity reduced both spaces (specific and non-specific) in a manner which indicated that little if any of the vesicle sugar was bound. This was confirmed by the fact that equilibrium glucose space was independent of glucose concentration and by the fact that vesicles immediately lost their sugar when diluted with water at 0°C. These data indicate the presence of two vesicle types, discriminant and indiscriminant as regards transport of the glucose isomers. Entry of d-glucose into the discriminant (stereospecific) vesicles was temperature sensitive (Q₁₀ > 2), saturable (K_m 2 mM), and was inhibited by phloretin (K_i < 200 μM), N-ethylmaleimide (K_i < 10 mM) and cytochalasin B (K_i < 2 μM), suggesting that these vesicles contain the plasma-membrane glucose carrier. Entry of l- and d-glucose into the indiscriminant vesicles showed none of these properties. The equilibrium-exchange K_m and V were about five times the entry K_m and V, indicating the substrate loading greatly facilitates carrier translocation, at least in the outward direction.     

Nitrate transport system in Neurospora crassa

Nitrate uptake in Neurospora crassa has been investigated under various conditions of nitrogen nutrition by measuring the rate of disappearance of nitrate from the medium and by determining mycelial nitrate accumulation. The nitrate transport system is induced by either nitrate or nitrite, but is not present in mycelia grown on ammonia or Casamino Acids. The appearance of nitrate uptake activity is prevented by cycloheximide, puromycin, or 6-methyl purine. The induced nitrate transport system displays a K_m for nitrate of 0.25 mM. Nitrate uptake is inhibited by metabolic poisons such as 2,4-dinitrophenol, cyanide, and antimycin A. Furthermore, mycelia can concentrate nitrate 50-fold. Ammonia and nitrite are non-competitive inhibitors with respect to nitrate, with K_i values of 0.13 and 0.17 mM, respectively. Ammonia does not repress the formation of the nitrate transport system. In contrast, the nitrate uptake system is repressed by Casamino Acids. All amino acids individually prevent nitrate accumulation, with the exception of methionine, glutamine, and alanine. The influence of nitrate reduction and the nitrate reductase protein on nitrate transport was investigated in wild-type Neurospora lacking a functional nitrate reductase and in nitrate non-utilizing mutants, nit-1, nit-2, and nit-3. These mycelia contain an inducible nitrate transport system which displays the same characteristics as those found in the wild-type mycelia having the functional nitrate reductase. These findings suggest that nitrate transport is not dependent upon nitrate reduction and that these two processes are separate events in the assimilation of nitrate.     

Driving forces and current-voltage characteristics of amino acid transport in Riccia fluitans

The complete steady-state I–V relationship of α-aminoisobutyric acid transport across the plasmalemma of rhizoid cells from Riccia fluitans has been measured and analysed with special emphasis on α-aminoisobutyric acid equilibrium and saturation conditions. (A) The electrical data show that: (1) the amino acid-induced electrical current saturates after the addition of the amino acid, regardless of the concentration; (2) a steady state is reached 1–2 h after incubation in α-aminoisobutyric acid, but after less that 5 min in the presence of 1 mM CN⁻; (3) the steady-state I–V characteristic of α-aminoisobutyric acid transport is a sigmoid curve and fairly symmetric in current with respect to the voltage axis; and (4) the equilibrium potential is clearly a function of the amino acid accumulation ratio. It is suggested that the sigmoid curve represents the characteristic of carrier-mediated α-aminoisobutyric acid transport with a voltage-insensitive step, possibly the translocation of the unloaded carrier, rate-limiting. Since under normal conditions the voltage-sensitive rate constant k_oi is much greater than k_io, it is further suggested that the energy to drive this system is put into the transfer of positive charge from outside to the cytoplasm. (B) Accumulation ratios have been determined by inspection of current-voltage data, and additionally by compartmental analysis on green thalli from Riccia fluitans. Both methods give ratios far too low compared with the thermodynamically possible accumulation of about 10⁴. It is suggested that substantial leakages via different non-electrical pathways prevent equilibrium at steady state, and it is concluded that in such leaky systems the thermodynamic equilibrium condition is not suitable for estimating stoichiometries.     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

Two components of auxin transport

The transport of indoleacetic acid-1-¹⁴C out of sunflower stem sections has been analyzed by a compartmental analysis procedure in which the radioactivity moving out of the tissue (log per cent) is plotted against time. The analysis indicates that indoleacetic acid is transported via a fast transport system (t_½ of about 30 minutes) and a slow transport system (t_½ about 10 hours). While we do not know the sources of these two pools, by analogy with ion transport studies, the fast efflux is characteristic of transport from the cytoplasm across the plasmalemma and the slow efflux is characteristic of transport across the tonoplast and thus out of the vacuole. Both components of transport are inhibited by 2,3,5-triiodobenzoic acid.     

The kinetics of the active and de-energized transport of O-methyl glucose in Ustilago maydis

The kinetics of the uptake and efflux of 3-O-methyl-glucose in sporidia of Ustilago maydis were measured, both in active cells and in cells whose metabolic activity had been inhibited by azide and iodoacetate.The de-energized transport system proved to be carrier mediated with apparent affinity constants 13 ± 2 mM outside (K₀) and 18 ± 2 mM inside (K₁). The apparent maximum rate constants for the same system were 0.66 ± 0.05 mmol/l cell water per min for uptake (V₊ and 0.53 ± 0.04 mmol/l cell water per min for efflux (V_-. For the active system K₀ = 0.08 ± 0.01, K₁ > 40, V₊ = 9.7 ± 0.5 and V₋ 1.1 ± 0.9 (in equivalent units). These results are discussed in the context of the carrier mechanism as proposed by Regen and Morgan (Regen, D.M. and Morgan, H.E. (1964) Biochim. Biophys. Acta 79, 151–166).The antifungal compound carboxin had no effect on de-energized transport but was shown to decrease both K₀ and V₊ in the active system. Phloretin and phlorizin were also found to be without effect on de-energized cells but the former enhanced while the latter inhibited active uptake.     

Contrasting responses of sulphate and phosphate transport in barley (Hordeum vulgare L.) roots to protein-modifying reagents and inhibition of protein synthesis

The uptake of sulphate into roots of barley seedlings is highly sensitive to phenylglyoxal (PhG), an arginine-binding reagent. Uptake was inhibited by >80% by a 1-h pre-treatment of roots with 0.45 mol · m^–3 PhG. Inhibition was maximal in pre-treatment solutions buffered between pH 4.5 and 6.5. Phosphate uptake, measured simultaneously by double-labelling uptake solutions with ³²P and ³⁵S, was less susceptible to inhibition by PhG, particularly at pH <6.5, and was completely insensitive to the less permeant reagent p-hydroxyphenylglyoxal (OH-PhG) administered at 1 mol · m^–3 at pH at 5.0 or 8.2; sulphate uptake was inhibited in -S plants by 90% by OH-PhG-treatment. Root respiration in young root segments was unaffected by OH-PhG pre-treatment for 1 h and inhibited by only 17% after 90 min pre-treatment. The uptake of both ions was inhibited by the dithiol-specific reagent, phenylarsine oxide even after short exposures (0.5–5.0 min). Sulphate uptake was more severely inhibited than that of phosphate, but in both cases inhibition could be substantially reversed by 5 min washing of treated roots by 5 mol · m^–3 dithioerythritol. After longer pre-treatment (50 min) with phenylarsine oxide, inhibition of the ion fluxes was not relieved by washing with dithioerythritol. Inhibition of sulphate influx by PhG was completely reversed by washing the roots for 24 h with culture solution lacking the inhibitor. The reversal was dependent on protein synthesis; less than 20% recovery was seen in the presence of 50 mmol · m^–3 cycloheximide. Sulphate uptake declined rapidly when -S roots were treated with cycloheximide. In the same roots the phosphate influx was little affected, small significant inhibitions being seen only after 4 h of treatment. Respiration was depressed by only 20% in apical and by 31% in basal root segments by cycloheximide pre-treatment for 2 h. Similar rates of collapse of the sulphate uptake and insensitivity of phosphate uptake were seen when protein synthesis was inhibited by azetidine carboxylic acid, p-fluorophenylalanine and puromycin. Considering the effects of all of the protein-synthesis inhibitors together leads to the conclusion that the sulphate transporter itself, or some essential sub-component of the uptake system, turns over rapidly with a half-time of about 2.5 h. The turnover of the phosphate transporter is evidently much slower. The results are discussed in relation to strategies for identifying the transport proteins and to the regulation of transporter activity during nutrient stress.Abbreviations CAP chloramphenicol - CHM cycloheximide - DTE dithioerythritol - OH-PhG p-hydroxyphenylglyoxal - PhAsO phenylarsine - PhG phenylglyoxal Paper dedicated to the memory of the late Ken Treharne who did much to encourage this collaboration.D.T.C. gratefully acknowledges a fellowship provided by Le Ministére des Etrangers during his stay in Montpellier.     

Characterization ofl-leucine transport in the opportunistic yeastsMalassezia furfur andMalassezia pachydermatis

The lipophilic yeastsMalassezia furfur andM. pachydermatis show an initial rapid uptake ofl-leucine followed by slower steady-state rates. At least two independent transport systems forl-leucine were present in both species. The high-affinity system forM. furfur had a K_T of 0.047 µM with a J_max of 222 fM/min/10⁶ cells (65 pM/min/mg dry weight), whereas forM. pachydermatis the K_T was 0.067 µM with a J_max of 709 fM/min/10⁶ cells (89 pM/min/dry weight). The low-affinity system forM. furfur had a K_T of 646 µM with a J_max of 1.62 pM/min/10⁶ cells (0.5 nM/min/mg dry weight) and that ofM. pachydermatis had a K_T of 3.3 µM with a J_max of 9.97 pM/min/10⁶ cells (1.3 nM/min/mg dry weight). Both transport systems were energy-dependent. Cells incubated with Tween 80 showedl-leucine uptake via both transport systems. Cells incubated with a combination of glucose (1%) and Tween 80 (0.01%) showed decreased transport rates for the high-affinity system for both species as compared with cells incubated only with glucose. The low-affinity transport system of both species in the presence of glucose plus Tween 80 showed an initial rapid uptake followed by greater efflux than influx ofl-leucine.l-Leucine demonstrated binding to Tween 80, but the major effect of Tween 80 on membrane transport inMalassezia appears to be on the efflux of transported molecules.     

Regulation of sugar transport in Neurospora crassa

Sugar uptake systems in Neurospora crassa are catabolically repressed by glucose. Synthesis of a low K(m) glucose uptake system (system II) in Neurospora is derepressed during starvation for an externally supplied source of carbon and energy. Fasting also results in the derepression of uptake systems for fructose, galactose, and lactose. In contrast to the repression observed when cells were grown on glucose, sucrose, or fructose, system II was not repressed by growth on tryptone and casein hydrolysate. System II was inactivated in the presence of 0.1 m glucose and glucose plus cycloheximide but not by cycloheximide alone. Inactivation followed first-order kinetics with a half-time of 40 min. The addition of glycerol to the uptake medium had no significant effect on the kinetics of 3-0-methyl glucose uptake, suggesting that the system was not feedback inhibitable by catabolites of glycerol metabolism.     

Transport of glutamine inXenopus laevis oocytes: Relationship with transport of other amino acids

Summary We have investigated transport of the amino acid glutamine across the surface membranes of prophase-arrestedXenopus laevis oocytes. Glutamine accumulation was linear with time for 30 min; it was stereospecific with aK _m of 0.12±0.02mm andV _max of 0.92±0.17 pmol/oocyte · min forl-glutamine. Transport ofl-glutamine was Na⁺-dependent, the cation not being replaceable with Li⁺, K⁺, choline, tris(hydroxymethyl)-aminomethane (Tris), tetramethylammonium (TMA) or N-methyld-glucamine NMDG); external Cl^– appeared to be necessary for full activation of Na⁺-dependent glutamine transport. Two external Na⁺ may be required for the transport of one glutamine molecule.l-glutamine transport (at 50 m glutamine) was inhibited by the presence of other amino acids:l-alanine,d-alanine,l-leucine,l-asparagine andl-arginine (about 60% inhibition at 1mm);l-histidine,l-valine and glycine (25 to 40% inhibition at 1mm);l-serine,l-lysine,l-phenylalanine andl-glutamate (45 to 55% inhibition at 10mm). N-methylaminoisobutyric acid (meAIB) had no effect at 10mm, but 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) inhibited Na⁺/glutamine transport by about 50% at 10mm.l-glutamine was a competitive inhibitor of the Na⁺-dependent transport ofl-alanine,d-alanine andl-arginine; this evidence is consistent with the existence of a single system transporting all four amino acids. Glutamine uptake in oocytes appears to be catalyzed by a transport system distinct from the cotransport Systems A, ASC, N and Gly, although it resembles System B^0,+.     

Electrically silent anion transport through bilayer lipid membrane induced by tributylin and triethyllead

Summary The method of the measurement of the nonelectrogenic fluxes of hydrogen (or hydroxyl) ions (J _H) based on the local proton gradients formation in the unstirred layers near a bilayer lipid membrane (BLM) is applied for recording the nonelectrogenic anion/OH^– exchange on BLM induced by tributyltin (TBT) and a novel carrier (Hager, A., Moser, I., & Berthold, W. 1987.Z. Naturforsch.,42C1116–1120), triethyllead (TEL). This method has been used previously for measuring the cation fluxes through BLM. TBT and TEL are shown to be equally efficient in the induction of Cl^–/OH^– exchange.J _H induced by TBT is constant at 4J _H decreases at pH<4 and pH>7. Both ionophores have a transport sequence: I^–> Br^–>Cl^–>F^–. The quatitative measurements reveal that TEL better discriminates these four anions than TBT. It is concluded that this method may prove helpful in a search and study of anion/OH^–-exchangers isolated from natural membranes.     

Coregulation of electron transport through PS I by Cyt b 6 f,excitation capture by P700 and acceptor side reduction. Time kinetics and electron transport requirement

Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b ₆ f complex (k_q, s^–1) was obtained from the postillumination P700⁺ reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830_m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Y_m) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of k_q, D830_m and Y_m under rate-limiting CO₂ and O₂ concentrations applied after air (21% O₂, 310 ppm CO₂) was investigated. The amplitude of the downregulation of k_q (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm^–2 s^–1 and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO₂ and O₂. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O₂ leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830_m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of k_q. The variation of Y_m had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b ₆ f, excitation capture efficiency by P700, and by acceptor side reduction.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - D830 difference of the 830 nm signal from the dark level - ETR electron transport rate - PAD photon absorption density nmol cm^–2 s^–1 - PFD incident photon flux density, nmol cm^–2 s^–1 - PS I Photosystem I - PS II Photosystem II - PQH₂ plastoquinol - P700 Photosystem I donor pigment - Y quantum yield of PS I electron transport, rel. un.