Detection of a yeast polyphosphate fraction localized outside the plasma membrane by the method of phosphorus-31 nuclear magnetic resonance

Phosphate Homeostasis in Conditions of Phosphate Proficiency and Limitation in the Wild Type and the phoP Mutant of Streptomyces lividans

Phosphate, as a constituent of the high energy molecules, ATP/GTP and polyphosphate, plays a crucial role in most of the metabolic processes of living organisms. Therefore, the adaptation to low Pi availability is a major challenge for bacteria. In Streptomyces, this adaptation is tightly controlled by the two component PhoR/PhoP system. In this study, the free intracellular Pi, ATP, ADP and polyP content of the wild type and the phoP mutant strain of S. lividans TK24 were analyzed at discrete time points throughout growth in Pi replete and limited media. PolyP length and content was shown to be directly related to the Pi content of the growth medium. In Pi repletion, ATP and high molecular weight (HMW) polyP contents were higher in the phoP mutant than in the WT strain. This supports the recently proposed repressive effect of PhoP on oxidative phosphorylation. High oxidative phosphorylation activity might also have a direct or indirect positive impact on HMW polyP synthesis. In Pi sufficiency as in Pi limitation, the degradation of these polymers was shown to be clearly delayed in the phoP mutant, indicating PhoP dependent expression of the enzymes involved in this degradation. The efficient storage of Pi as polyphosphate and/or its inefficient degradation in Pi in the phoP mutant resulted in low levels of free Pi and ATP that are likely to be, at least in part, responsible for the very poor growth of this mutant in Pi limitation. Furthermore, short polyP was shown to be present outside the cell, tightly bound to the mycelium via electrostatic interactions involving divalent cations. Less short polyP was found to be associated with the mycelium of the phoP mutant than with that of the WT strain, indicating that generation and externalization of these short polyP molecules was directly or indirectly dependent on PhoP.

Growing Saccharomyces cerevisiae in calcium-alginate beads induces cell alterations which accelerate glucose conversion to ethanol

Nongrowing Saccharomyces cerevisiae cells previously grown in alginate exhibit ethanol production rates 1.5 times greater than cells previously grown in suspension. Analysis of glucose, ethanol, and glycerol formation data using quasi-steady-state pathway stoichiometry shows that alginate-grown cells possess phosphofructokinase (PFK), ATPase, and polysaccharide synthesis maximum activities which are approximately two-, two-, and ninefold larger, respectively, than in suspension-grown cells. The estimated change in PFK maximum velocity is consistent with in vitro assays of PFK activity in extracts of suspension- and alginate-grown yeast. Estimation of ethanol production flux control coefficients using in vivo nuclear magnetic resonance (NMR) spectroscopy measurements of intracellular metabolite concentrations and a previously proposed detailed kinetic model of ethanol fermentation in yeast shows that glucose uptake dominates flux control in alginate-grown cells in suspension while earlier research revealed that PFK and ATPase exert significant flux control in suspension-grown cells. When placed in a calcium alginate matrix, alginate-grown cells produced ethanol 1.8 times more rapidly and accumulated substantially more polyphosphate than suspension-grown cells placed in alginate. Cells growing in alginate elicit responses at the genetic level which substantially alter pathway rates and flux control when these cells are used as either a suspended or an immobilized biocatalyst. These responses in gene expression to growth in alginate serve to reconfigure flux controls in alginate to a pattern which is similar to that obtained for suspended-grown cells in suspension.

Direct labeling of polyphosphate at the ultrastructural level in Saccharomyces cerevisiae by using the affinity of the polyphosphate binding domain of Escherichia coli exopolyphosphatase

Inorganic polyphosphate (polyP) is a linear polymer of orthophosphate and has many biological functions in prokaryotic and eukaryotic organisms. To investigate polyP localization, we developed a novel technique using the affinity of the recombinant polyphosphate binding domain (PPBD) of Escherichia coli exopolyphosphatase to polyP. An epitope-tagged PPBD was expressed and purified from E. coli. Equilibrium binding assay of PPBD revealed its high affinity for long-chain polyP and its weak affinity for short-chain polyP and nucleic acids. To directly demonstrate polyP localization in Saccharomyces cerevisiae on resin sections prepared by rapid freezing and freeze-substitution, specimens were labeled with PPBD containing an epitope tag and then the epitope tag was detected by an indirect immunocytochemical method. A goat anti-mouse immunoglobulin G antibody conjugated with Alexa 488 for laser confocal microscopy or with colloidal gold for transmission electron microscopy was used. When the S. cerevisiae was cultured in yeast extract-peptone-dextrose medium (10 mM phosphate) for 10 h, polyP was distributed in a dispersed fashion in vacuoles in successfully cryofixed cells. A few polyP signals of the labeling were sometimes observed in cytosol around vacuoles with electron microscopy. Under our experimental conditions, polyP granules were not observed. Therefore, it remains unclear whether the method can detect the granule form. The method directly demonstrated the localization of polyP at the electron microscopic level for the first time and enabled the visualization of polyP localization with much higher specificity and resolution than with other conventional methods.

Regulation of phosphate acquisition in Saccharomyces cerevisiae

Membrane transport systems active in cellular inorganic phosphate (P(i)) acquisition play a key role in maintaining cellular P(i) homeostasis, independent of whether the cell is a unicellular microorganism or is contained in the tissue of a higher eukaryotic organism. Since unicellular eukaryotes such as yeast interact directly with the nutritious environment, regulation of P(i) transport is maintained solely by transduction of nutrient signals across the plasma membrane. The individual yeast cell thus recognizes nutrients that can act as both signals and sustenance. The present review provides an overview of P(i) acquisition via the plasma membrane P(i) transporters of Saccharomyces cerevisiae and the regulation of internal P(i) stores under the prevailing P(i) status.

Engineering polyphosphate metabolism in Escherichia coli: implications for bioremediation of inorganic contaminants

Polyphosphate metabolism plays an important role in the bioremediation of phosphate contamination in municipal wastewater, and may play a key role in heavy metal tolerance and bioremediation. However, little is known about the regulation of polyphosphate metabolism in microorganisms and its role in heavy metal toxicity. We have manipulated polyphosphate metabolism in Escherichia coli by overexpressing the genes for polyphosphate kinase (ppk) and for polyphosphatase (ppx) under control of their native promoters and inducible promoters. Overexpression of ppk results in high levels of intracellular polyphosphate, improved phosphate uptake, but no increase in tolerance to heavy metals. Overexpression of both ppk and ppx results in lower levels of intracellular polyphosphate, secretion of phosphate from the cell, and increased tolerance to heavy metals. Metabolic flux analysis indicates that the cell responds to increased flux through the PPK-PPX pathway by altering flux through the TCA cycle.

Regulation of intracellular toxic metals and other cations by hydrolysis of polyphosphate

Heavy metal tolerance in a number of microorganisms has been correlated with the presence of long-chain polymers of inorganic phosphate called polyphosphate. It has been proposed that the polyphosphate sequesters the metals, thereby reducing their effective intracellular concentration. However, recent evidence indicates that it is not only the amount of stored polyphosphate that is important for heavy metal tolerance but also the ability to degrade polyphosphate to orthophosphate. It is proposed that, in the presence of heavy metals, polyphosphate is degraded to orthophosphate by polyphosphatase and that the metal phosphates are transported out of the cell by the inorganic phosphate transport (PIT) system. Evidence supporting this hypothesis is presented.

Utilization by Escherichia coli of a high-molecular-weight, linear polyphosphate: roles of phosphatases and pore proteins

We observed that wild-type Escherichia coli utilized a linear polyphosphate with a chain length of 100 phosphate residues (poly-P100) as the sole source of phosphate in growth medium. A mutation in the gene phoA of alkaline phosphatase or phoB, the positive regulatory gene, prevented growth in this medium. Since no alkaline phosphatase activity was detected outside the wild-type cells, the periplasmic presence of the enzyme was necessary for the degradation of polyphosphate. A 90% reduction in the activity of periplasmic acid phosphatase with a pH optimum of 2.5 (delta appA mutants) did not affect polyphosphate utilization. Of the porins analyzed (OmpC, OmpF, and PhoE), the phoB-inducible porin PhoE was not essential since its absence did not prevent growth. To study how poly-P100 diffused into the cells, we used high-resolution 31P nuclear magnetic resonance (31P NMR) spectroscopy. The results suggest that poly-P100 entered the periplasm and remained in equilibrium between the periplasm and the medium. When present individually, porins PhoE and OmpF facilitated a higher permeability for poly-P100 than porin OmpC did. The degradation of polyphosphate by intact cells of E. coli observed by 31P NMR showed a time-dependent increase in cellular phosphate and a decrease in polyphosphate concentration.

Genotoxicity of excess thymidylate in thymidylate low-requiring Saccharomyces cerevisiae is associated with changes in phosphate metabolism

When dTMP in concentrations greater than 100 microM is offered to growing cells of thymidylate low-requiring yeast strains it is both mutagenic and toxic. At exposure concentrations greater than 1 mM dTMP interferes significantly with the low-affinity phosphate permease even in the presence of exogenous phosphate concentrations of 6 mM. Chemical analysis and 31P NMR spectroscopy reveal that excess dTMP disturbs phosphate metabolism in thymidylate low-requiring strains but not in the wild type. The most prominent changes in phosphorus-containing molecules are found in polyphosphates of which up to 20% are broken down within a 20-min time span with a concomitant increase in orthophosphate pools.

A 31P-NMR study of Propionibacterium acnes, including effects caused by near-ultraviolet irradiation

161.8 MHz 31P-NMR spectra were recorded from the light sensitive skin bacterium Propionibacterium acnes. The cells were grown anaerobically on synthetic phosphate-buffered Eagle's medium or on a complex yeast extract medium. The spectra showed a large accumulation of polyphosphates when grown on Eagles medium. A splitting of the inorganic phosphate peak indicated a difference between internal and external pH of the cells. Addition of glucose to the cell suspension gave rise to a change in the pH gradient across the cell membrane, as reported for other Gram-positive bacteria. A decrease in the polyphosphate peak was observed after addition of glucose. A lethal dose of broad-band near-ultraviolet light (corresponding to a 10% survival in a survival test), increased the amount of polyphosphates visible in the NMR-spectra. The addition of glucose to irradiated cells decreased the pH in the external solution, but no splitting of the inorganic phosphate peak could however be observed. 31P-NMR can, therefore, be used to study immediate near-ultraviolet-induced effects at the cellular level, at least in the case of P. acnes.

Control of yeast neutral trehalase by distinct polyphosphates and ribonucleic acid

The activity of yeast trehalase when assayed at pH 7 in a crude extract was found to increase 2- to 3-fold upon incubation with 0.1% (v/v) polyethyleneimine or other polycations such as polylysine (0.075-mMol) and calf thymus histones (0.08 mMol). Incubation with 3 mM-Mn2+ and 5 mM-Ca2+ also led to 3- and 1.6-fold increases in trehalase activity, respectively. The activities of 11 other enzymes assayed in the crude yeast extract did not increase after addition of polyethylene imine. At concentrations of polyethyleneimine that maximally stimulated trehalase activity, 97% of the total RNA present in the crude extract, 40% of total protein, and 60% of the polyphosphate (assayed as inorganic phosphate liberated during 7 min incubation at 95 degrees C and pH O) were found to be precipitated. A similar finding was made with trehalase-stimulating concentrations of Mn2+. Activation of trehalase by polyethylene imine rendered this enzyme susceptible to inhibition by a preparation of total yeast RNA, inorganic polyphosphates, and related polyanions. We present further evidence that the removal of a distinct RNA and/or polyphosphate is the basic principle of polyethyleneimine-induced activation of trehalase. A more pronounced stimulation of trehalase activity (4-fold) could be obtained by enzymatic phosphorylation with ATP in the presence of cyclic AMP and Mg2+ as described by van Solingen and van der Plaat (1975) [9].(ABSTRACT TRUNCATED AT 250 WORDS)