Induction of the phosphoenolpyruvate: hexose phosphotransferase system associated with relative anaerobiosis in an obligate aerobe.

Arthrobacter pyridinolis possesses alternative transport systems for D-fructose: a respiration-coupled transport system whereby D-fructose transport occurs with concomitant oxidation of L-malate, and a phosphoenolpyruvate: D-fructose phosphotransferase system. Studies of D-fructose uptake by whole cells in the presence and absence of cyanide demonstrate that respiration-coupled transport is used almost exclusively during the first half of logarithmic growth, after which it accounts for only 15-20% of D-fructose uptake. Phosphotransferase levels are low during log phase, peak during late log, and then slowly decline. In a mutant of A. pyridinolis which requires delta-aminolevulinic acid for growth, the growth rate, cell cytochrome content, and activity of the respiration-coupled transport system increased with increasing concentrations of delta-aminolevulinic acid up to 50 microgram/ml. By contrast, phosphotransferase activity was highest in cells grown on limiting delta-aminolevulinic acid. L-Malate, which stimulates respiration-coupled transport, repressed the phosphotransferase system. The respiratory activity and the ability to release CO2 from internalized d-fructose was consistently low in D-fructose-grown cells. A cyanide-resistant cytochrome, tentatively identified as cytochrome d, appeared in the late exponential phase of growth. Isocitrate lyase activity, required for aerobic growth of this organism, declined markedly during the late exponential phase. Thus the phosphotransferase system is maximally induced, in this obligate aerobe, under conditions of relative anaerobiosis during which metabolism is primarily fermentative.     

Natural paucity of anaplerotic enzymes: basis for dependence of Arthrobacter pyridinolis on L-malate for growth.

Previous work has shown that in Arthrobacter pyridinolis the transport systems for glucose and several amino acids are respiration coupled, with malate oxidation occurring concomitantly with transport. The requisite malate has to be supplied exogenously, so that growth on glucose or certain amino acids only occurs if malate is also present in the medium. These and other data suggested that A. pyridinolis might be deficient in anaplerotic enzymes, which maintain intracellular levels of dicarboxylic acids. A comparative study was undertaken of anaplerotic enzymes in A. pyridinolis and in a closely related species, A. crystallopoietes, which has respiration-coupled transport of glucose but can grow on glucose without added malate. The paucity of anaplerotic enzymes in A. pyridinolis and its probable relationship to the malate requirement for growth on glucose were documented as follows: (i) A. crystallopoietes, but not A. pyridinolis, possesses phosphoenolpyruvate carboxylase activity, and neither species contains pyruvate carboxylase; (ii) both A. pyridinolis and A. crystallopoietes possess glyoxylate pathways that are induced by acetate but not by hexoses; (iii) isocitrate lyase-deficient mutants of A. pyridinolis fail to grow on rhamnose and fructose as well as acetate; and (iv) mutants of A. crystallopoietes that require malate for growth on glucose are deficient in phosphoenolpyruvate carboxylase.     

Pathways of D-fructose transport in Arthrobacter pyridinolis

Previous work indicated that Arthrobacter pyridinolis can transport d-fructose by either a phosphoenolpyruvate: d-fructose phosphotransferase system or by a respiration-coupled system. The respiration-coupled transport system for d-fructose, which is stimulated by the addition of l-malate, has been characterized in membrane vesicles from d-fructose-grown cells. Such vesicles carry out malate-dependent uptake of d-fructose but not of d-glucose or l-rhamnose, indicating that there is a sugar-specific component to the respiration-coupled transport system. A mutant which is deficient in the d-fructose-specific component was isolated. Vesicles from fructose-glutamate-grown cells of a phosphotransferase-negative strain (AP100) exhibited malate-dependent d-fructose uptake, while phosphoenolpyruvate-dependent uptake was reduced to a small fraction of that seen with vesicles from wild-type cells. Inhibitors of electron transport, carbonyl cyanide m-chlorophenyl hydrazone, 2,4-dinitrophenol and N-ethylmaleimide caused marked inhibition of malate-dependent d-fructose uptake while exerting little or no effect on phosphoenolpyru-vate-dependent transport of the sugar in vesicles from wild-type cells. Activity of a flavin adenine dinucleotide-linked l-malic dehydrogenase was detected in membrane vesicles as well as in whole cells.     

D-Gluconate transport in Arthrobacter pyridinolis. Metabolic trapping of a protonated solute.

D-Gluconate uptake was studied in whole cells of Arthrobacter pyridinolis; the uptake activity was inducible, mutable and showed saturation kinetics (Km = 5 micrometer). Uptake of D-gluconate was not mediated by a phosphoenol-pyruvate : hexose phosphotransferase system, nor was it directly energized by ATP. A transmembrane pH gradient, delta pH, of --63 mV was generated by A. pyridinolis cells at pH 6.5, while at pH 7.5, delta pH = 0. Addition of 8 micrometer D-gluconate significantly reduced the delta pH. The transmembrane electrical potential, delta psi, which was --87 mV over a range of pH from 5.5 to 7.5, was unaffected by the presence of substrate. D-Gluconate accumulated at the same rate and as the protonated solute, at both pH 6.5 and 7.5. Experiments in which a diffusion potential was generated in cyanide-treated cells, indicated that the delta psi did not energize transport. Rather, the rate of D-gluconate uptake metabolism: (a) treatment of cells with valinomycin or nigericin, under conditions in which there was a loss of intracellular potassium, inhibited both D-gluconate uptake and the metabolism of pre-accumulated D-gluconate; (b) the effects of cyanide and azide on D-gluconate uptake were much more severe at pH 6.5 than pH 7.5, a pattern which paralleled the effects of these inhibitors on D-gluconate metabolism; (c) extraction and chromatography of intracellular label from D-gluconate uptake revealed that accumulation of unaltered D-gluconate was negligible; (d) a series of mutant strains with lower D-gluconate kinase activities also exhibited low rates of D-gluconate uptake; (e) spontaneous revertants of these mutant strains consistently regained both D-gluconate kinase activity and wild type levels of uptake.     

Metabolism of D-fructose by Arthrobacter pyridinolis

Previous studies showed that Arthrobacter pyridinolis can transport and utilize d-glucose only after prior growth on certain Krebs cycle intermediates. In contrast, we found that d-fructose was taken up and metabolized by A. pyridinolis without special prior conditions of growth. d-Fructose was first converted to d-fructose-1-phosphate by a phosphoenolpyruvate (PEP):D-fructose phosphotransferase. This activity required both supernatant and pellet fractions from d-fructose-grown cells centrifuged at 150,000 x g. The d-fructose-1-phosphate formed was converted to d-fructose-1, 6-diphosphate. Mutants deficient in PEP:d-fructose phosphotransferase and d-fructose-1-phosphate kinase, or d-fructose-1, 6-diphosphatase (FDPase) were unable to grow on d-fructose but retained the normal ability to use d-glucose. Mutants forming reduced amounts of FDPase were completely unable to grow on d-fructose but were still capable of limited growth on Krebs cycle intermediates. A requirement for higher levels of FDPase for growth on d-fructose than for growth on Krebs cycle intermediates was also indicated by the higher specific activities of FDPase in d-fructose-grown cells than in cells grown on l-malate or amino acids.     

Inactivation of the Phosphoenolpyruvate-Dependent Phosphotransferase System in Various Species of Bacteria by Vinylglycolic Acid

The alkenoic hydroxyacid 2-hydroxy-3-butenoic acid (vinylglycolate) specifically inhibited the phosphotransferase system in a variety of bacteria while not affecting respiration-coupled transport systems.     

Substrate spectrum of L-rhamnulose kinase related to models derived from two ternary complex structures

The enzyme L-rhamnulose kinase from Escherichia coli participates in the degradation pathway of L-rhamnose, a common natural deoxy-hexose. The structure of the enzyme in a ternary complex with its substrates ADP and L-rhamnulose has been determined at 1.55A resolution and refined to R(cryst)/R(free) values of 0.179/0.209. The result was compared with the lower resolution structure of a corresponding complex containing L-fructose instead of L-rhamnulose. In light of the two established sugar positions and conformations, a number of rare sugars have been modeled into the active center of L-rhamnulose kinase and the model structures have been compared with the known enzymatic phosphorylation rates. Rare sugars are of rising interest for the synthesis of bioactive compounds.     

Amino acid transport in whole cells and membrane vesicles of Arthrobacter pyridinolis

Arginine, and several other amino acids, can only support growth of Arthrobacter pyridinolis if malate is also present in the medium. Arginine is transported by a high affinity lysine-arginine-ornithine-type transport system which is stimulated by malate in both whole cells and vesicles, is respiration-coupled, and appears to depend upon a respiration-generated membrane potential but not on a ΔpH. Arginine is also transported by a low-affinity system which transports canavanine. Studies of an arginine auxotroph suggest that the lysine-arginine-ornithine system may be the system of major physiological significance for arginine transport. Phenylalanine is one of a few amino acids which can act as sole source of carbon for A. pyridinolis. Transport of phenylalanine occurs by two kinetically distinct systems. Both of these transport systems are respiration-coupled, are not appreciably stimulated by malate either in cells or vesicles, but are markedly stimulated by ascorbate-phenazine methosulfate. Studies with inhibitors indicate that the transport systems for phenylalanine utilize both a ΔpH and a membrane potential.     

Inhibition of isocitrate lyase: the basis for inhibition of growth of two Arthrobacter species by pyruvate

Growth of Arthrobacter atrocyaneus and A. pyridinolis on certain growth substrates was found to be inhibited by pyruvate and compounds which can be converted to pyruvate. Growth of A. atrocyaneus on acetate, for example, was completely inhibited by 5 mm pyruvate; growth of this organism on glucose was less sensitive and growth on succinate was insensitive to inhibition by pyruvate. Growth of a third Arthrobacter species, A. crystallopoietes, on acetate and other substrates was not inhibited by pyruvate. The site of pyruvate inhibition was shown to be the isocitrate lyase reaction. Glyoxylate, which affords a bypass of this reaction, restored the ability of A. atrocyaneus to evolve (14)CO(2) from acetate in the presence of pyruvate. The isocitrate lyases from A. atrocyaneus and A. pyridinolis were competitively inhibited by concentrations of pyruvate as low as 1 mm, whereas the enzyme from A. crystallopoietes was unaffected by this concentration of pyruvate. Comparable levels of phosphoenolpyruvate did not inhibit the isocitrate lyases from any of the species. A mutant strain of A. atrocyaneus, PW11, which is deficient in isocitrate lyase activity, grew on glucose at a reduced rate that was comparable to the rate of growth of the wild-type strain on glucose plus lactate. Addition of lactate to PW11 did not further reduce its rate of growth on glucose. Thus, the glyoxylate pathway appears to be used as an anaplerotic pathway during growth of A. atrocyaneus on glucose. Two other considerations suggest that A. atrocyaneus and A. pyridinolis, but not A. crystallopoietes, may be deficient in the ability to convert pyruvate to 4-carbon acids. First, the former two species accumulate intracellular pyruvate from exogenous l-alanine to a much greater extent than does A. crystallopoietes. Moreover, A. atrocyaneus and A. pyridinolis are incapable of growth on lactate as sole source of carbon whereas A. crystallopoietes can grow on lactate.     

Abolition of crypticity of Arthrobacter pyridinolis toward glucose and alpha-glucosides by tricarboxylic acid cycle intermediates

Arthrobacter pyridinolis cannot grow on glucose as sole carbon source, although the cells possess catabolic enzymes of the Embden-Meyerhof and pentose phosphate pathways as well as a complete tricarboxylic acid cycle. Crypticity toward glucose is abolished by a period of growth in a medium containing malate, succinate, citrate, or fumarate in addition to glucose. Other carbon sources, which support as rapid growth as does malate (e.g. asparagine), do not enable the cells to use glucose. Malate, succinate, citrate, and fumarate abolish crypticity toward glucose only in the second phase of diauxic growth after the tricarboxylic acid cycle intermediate has been depleted. This sequence of events, first observed in growth curves, has been verified by experiments in which the incorporation of radioactive substrates into trichloroacetic acid-insoluble cellular material was followed. The tricarboxylic acid cycle intermediates which confer the ability to utilize glucose also enhance the utilization of the alphaglucosides sucrose and maltose. The mechanism whereby growth on certain tricarboxylic acid cycle intermediates confers the subsequent ability to grow on glucose is related to a transport system for glucose and alpha-glucosides. This transport system has been assayed by measuring the uptake of [1-(14)C]-2-deoxyglucose. Cells grown for varying periods of time in asparagine, asparagine plus glucose, or malate do not transport 2-deoxyglucose. Cells from malate-glucose cultures that are in the exponential phase of growth on glucose can transport 2-deoxyglucose. Transport of 2-deoxyglucose shows Michaelis-Menten kinetics with a K(m) of 2.9 x 10(-4) M. It is competitively inhibited by glucose, alpha-methylglucopyranoside, and maltose. The transport of 2-deoxyglucose is inhibited by cyanide, dinitrophenol, azide, and N-ethylmaleimide, but not by malonate or fluoride. No phosphoenolpyruvate: d-glucose phosphotransferase activity has been detected, and the 2-deoxyglucose transported into the cell is not phosphorylated.     

Growth inhibition of Streptococcus mutans and Leuconostoc mesenteroides by sodium fluoride and ionic tin.

Sodium fluoride caused inhibition of growth rate and growth levels of Streptococcus mutans with glucose as the primary energy and carbon source. Stannous fluoride increased growth lag nad caused a much greater inhibition of growth rate than did sodium fluoride. Neither compound was found to be bactericidal when culture viability was measured after 6 days of incubation. Leuconostoc mesenteroides, which lacks a phosphotransferase system for sugar transport, showed less inhibition of growth rate with both inhibitors than did S. mutans, which possesses a phosphotransferase system. Metabolism of glucose or lactose which requires enolase activity shoed sodium fluoride inhibition, whereas metabolism of arginine or pyruvate does not involve enolase activity and showed no inhibition of growth.     

L-lyxose metabolism employs the L-rhamnose pathway in mutant cells of Escherichia coli adapted to grow on L-lyxose.

Escherichia coli cannot grow on L-lyxose, a pentose analog of the 6-deoxyhexose L-rhamnose, which supports the growth of this and other enteric bacteria. L-Rhamnose is metabolized in E. coli by a system that consists of a rhamnose permease, rhamnose isomerase, rhamnulose kinase, and rhamnulose-1-phosphate aldolase, which yields the degradation products dihydroxyacetone phosphate and L-lactaldehyde. This aldehyde is oxidized to L-lactate by lactaldehyde dehydrogenase. All enzymes of the rhamnose system were found to be inducible not only by L-rhamnose but also by L-lyxose. L-Lyxose competed with L-rhamnose for the rhamnose transport system, and purified rhamnose isomerase catalyzed the conversion of L-lyxose into L-xylulose. However, rhamnulose kinase did not phosphorylate L-xylulose sufficiently to support the growth of wild-type E. coli on L-lyxose. Mutants able to grow on L-lyxose were analyzed and found to have a mutated rhamnulose kinase which phosphorylated L-xylulose as efficiently as the wild-type enzyme phosphorylated L-rhamnulose. Thus, the mutated kinase, mapped in the rha locus, enabled the growth of the mutant cells on L-lyxose. The glycolaldehyde generated in the cleavage of L-xylulose 1-phosphate by the rhamnulose-1-phosphate aldolase was oxidized by lactaldehyde dehydrogenase to glycolate, a compound normally utilized by E. coli.     

In vivo selection for the directed evolution of L-rhamnulose aldolase from L-rhamnulose-1-phosphate aldolase (RhaD)

Dihydroxyacetone phosphate (DHAP)-dependent aldolases have been widely used for organic synthesis. The major drawback of DHAP-dependent aldolases is their strict donor substrate specificity toward DHAP, which is expensive and unstable. Here we report the development of an in vivo selection system for the directed evolution of the DHAP-dependent aldolase, L-rhamnulose-1-phosphate aldolase (RhaD), to alter its donor substrate specificity from DHAP to dihydroxyacetone (DHA). We also report preliminary results on mutants that were discovered with this screen. A strain deficient in the L-rhamnose metabolic pathway in Escherichia coli (DeltarhaDAB, DE3) was constructed and used as a selection host strain. Co-expression of L-rhamnose isomerase (rhaA) and rhaD in the selection host did not restore its growth on minimal plate supplemented with L-rhamnose as a sole carbon source, because of the lack of L-rhamnulose kinase (RhaB) activity and the inability of WT RhaD aldolase to use unphosphorylated L-rhamnulose as a substrate. Use of this selection host and co-expression vector system gives us an in vivo selection for the desired mutant RhaD which can cleave unphosphorylated L-rhamnulose and allow the mutant to grow in the minimal media. An error-prone PCR (ep-PCR) library of rhaD gene on the co-expression vector was constructed and introduced into the rha-mutant, and survivors were selected in minimal media with l-rhamnose (MMRha media). An initial round of screening gave mutants allowing the selection strain to grow on MMRha plates. This in vivo selection system allows rapid screening of mutated aldolases that can utilize dihydroxyacetone as a donor substrate.     

Metabolic fate of L-lactaldehyde derived from an alternative L-rhamnose pathway

Fungal Pichia stipitis and bacterial Azotobacter vinelandii possess an alternative pathway of L-rhamnose metabolism, which is different from the known bacterial pathway. In a previous study (Watanabe S, Saimura M & Makino K (2008) Eukaryotic and bacterial gene clusters related to an alternative pathway of non-phosphorylated L-rhamnose metabolism. J Biol Chem283, 20372-20382), we identified and characterized the gene clusters encoding the four metabolic enzymes [L-rhamnose 1-dehydrogenase (LRA1), L-rhamnono-gamma-lactonase (LRA2), L-rhamnonate dehydratase (LRA3) and l-2-keto-3-deoxyrhamnonate aldolase (LRA4)]. In the known and alternative L-rhamnose pathways, L-lactaldehyde is commonly produced from l-2-keto-3-deoxyrhamnonate and L-rhamnulose 1-phosphate by each specific aldolase, respectively. To estimate the metabolic fate of L-lactaldehyde in fungi, we purified L-lactaldehyde dehydrogenase (LADH) from P. stipitis cells L-rhamnose-grown to homogeneity, and identified the gene encoding this enzyme (PsLADH) by matrix-assisted laser desorption ionization-quadruple ion trap-time of flight mass spectrometry. In contrast, LADH of A. vinelandii (AvLADH) was clustered with the LRA1-4 gene on the genome. Physiological characterization using recombinant enzymes revealed that, of the tested aldehyde substrates, L-lactaldehyde is the best substrate for both PsLADH and AvLADH, and that PsLADH shows broad substrate specificity and relaxed coenzyme specificity compared with AvLADH. In the phylogenetic tree of the aldehyde dehydrogenase superfamily, PsLADH is poorly related to the known bacterial LADHs, including that of Escherichia coli (EcLADH). However, despite its involvement in different L-rhamnose metabolism, AvLADH belongs to the same subfamily as EcLADH. This suggests that the substrate specificities for L-lactaldehyde between fungal and bacterial LADHs have been acquired independently.     

High-cell-density fermentation for production of L-N-carbamoylase using an expression system based on the Escherichia coli rhaBAD promoter

A high-cell-density fed-batch fermentation for the production of heterologous proteins in Escherichia coli was developed using the positively regulated Escherichia coli rhaBAD promoter. The expression system was improved by reducing of the amount of expensive L-rhamnose necessary for induction of the rhamnose promoter and by increasing the vector stability. Consumption of the inducer L-rhamnose was inhibited by inactivation of L-rhamnulose kinase encoding gene rhaB of Escherichia coli W3110, responsible for the first irreversible step in rhamnose catabolism. Plasmid instability caused by multimerization of the expression vector in the recombination-proficient W3110 was prevented by insertion of the multimer resolution site cer from the ColE1 plasmid into the vector. Fermentation experiments with the optimized system resulted in the production of 100 g x L(-1) cell dry weight and 3.8 g x L(-1) of recombinant L-N-carbamoylase, an enzyme, which is needed for the production of enantiomeric pure amino acids in a two-step reaction from hydantoins.     

L-Rhamnose utilisation in Salmonella typhimurium

A khy , M.T., B rown , C.M. & O ld , D.C. 1984. L-Rhamnose utilisation in Salmonella typhimurium. Journal of Applied Bacteriology 56 , 269–274.
L-Rhamnose is degraded by strains of Salmonella typhimurium by isomerisation to L-rhamnulose, phosphorylation to L-rhamnulose-1-phosphate and cleavage to lac-taldehyde and dihydroxyacetone phosphate. The enzymes involved are, respectively, rhamnose isomerase (Rhal), rhamnulokinase (RhuK) and an aldolase (Ald). Strains able to grow rapidly on L-rhamnose contained a high-affinity uptake system for ³H-L-rhamnose that was induced by L-rhamnose and repressed by D-glucose. The synthesis of Rhal and RhuK was also induced by L-rhamnose but was not repressed by D-glucose. The synthesis of Ald was constitutive. Data are presented on some strains which grow very slowly on L-rhamnose and on others which do not utilise it.     

Xylitol-mediated transient inhibition of ribitol utilization by Lactobacillus casei

The growth of Lactobacillus casei strain Cl-16 at the expense or ribitol was inhibited if the non-metabolizable substrate xylitol was included in the medium at concentrations of 6 mM or greater. At these concentrations, xylitol, did not competitively inhibit ribitol transport. The cessation of growth was caused by the intracellular accumulation of xylitol-5-phosphate, which occurred because growth on ribitol had gratuitously induced a functional xylitol-specific phosphotransferase system but not the enzymes necessary for the further metabolism of xylitol-5-phosphate. Eventually, the cells overcame the xylitol-mediated inhibition by repressing the synthesis of enzyme II of the xylitol phosphotransferase system so that xylitol-5-phosphate would no longer be accumulated within the cell.     

A model for accelerated uptake and accumulation of sugars arising from phosphorylation at the inner surface of the cell membrane

A model transport system for cellular accumulation of sugar coupled to phosphorylation is described. Sugar permeates the cell membrane via a passive facilitated transport system. On the inside surface of the membrane the bound sugar is either phosphorylated to form impermeable hexose phosphate, which is released into the intracellular solution, or released directly into the cytosol. Sugar may be regenerated from hexose phosphate in the cytosol via a phosphatase reaction. The reduction of the proportion of sites on the inner membrane surface occupied by permeable sugar, caused by the kinase reaction, increases both net and unidirectional passive inflow and reduces both net and unidirectional exit of sugar, thereby permitting large stationary state gradients of free sugar to be maintained between the cytosol and bathing solution. In cells where there is a high passive membrane permeability to free sugar, steady-state accumulation of free sugar within the cytosol, linked to metabolism is inexplicable in terms of conventional transport kinetics based on equilibrium thermodynamic assumptions. This phenomenon is analysed in terms of non-equilibrium stationary state flows of ligands via a probability network. The effects of metabolism on exchange transport are also examined. The model provides a framework to explain how sugar transport is loosely coupled to phosphorylation in mammalian epithelial cells, adipocytes, yeasts and bacteria, so that a high rate of substrate accumulation is maintained without requiring a reduction in the intracellular concentration of permeable substrate below that in the external solution.     

Growth and pigment production by Arthrobacter pyridinolis n. sp.

A new bacterium capable of growing on 2-hydroxypyridine as sole source of carbon and nitrogen was isolated from soil. During its growth on solid medium, approximately 50% of this substrate was converted to a brilliant blue crystalline pigment which was deposited extracellularly in the colony mass. The pigment was identical to that produced by Arthrobacter crystallopoietes during its growth on 2-hydroxypyridine. The new isolate exhibited the typical cycle of morphogenesis characteristic of the genus Arthrobacter. The organism is different from all other reported species of Arthrobacter. It is proposed that the organism be named Arthorbacter pyridinolis n. sp.     

Effects of colicins E1 and K on transport systems

The effect of colicins E1 and K on active transport of beta-d-galactosides and of alpha-methyl-d-glucoside (alphaMG) by Escherichia coli was studied. These colicins strongly inhibited the accumulation of thio-methyl-galactoside (TMG) by bacteria and caused rapid exit of previously accumulated TMG. The inhibition effect was limited to the accumulation phase of galactoside transport; the rate of hydrolysis of o-nitrophenyl galactoside, which is dependent on transport of the substrate by the lactose-permease system, was only slightly affected. The accumulation of alphaMG was highly resistant to inhibition by these colicins under conditions which caused complete suppression of TMG accumulation. These effects of the colicins on transport resemble qualitatively those of sodium azide. The findings were interpreted by assuming that colicins E1 and K inhibit the energy-dependent steps in the accumulation of TMG but do not affect facilitated diffusion of galactosides mediated by the specific transport mechanism. The continued accumulation of alphaMG was attributed to the fact that this compound is stored by E. coli cells as a phosphorylated compound by a phosphoenolpyruvate-dependent transport system rather than by an adenosine triphosphate-linked accumulation mechanism.