PEPCase Activity Research Articles

Effect of the NH4/NO3ratio on GS and PEPCase activities and on dry matter production in wheat

Abstract Previous experiments have indicated that under greenhouse and hydroponic conditions, spring wheat (Triticum aestivum L.) produces higher yields, more tillers and dry matter when supplied with mixtures of NH4 and NO3 than when supplied with only one of them. The goal of this study was to evaluate the effect of selected ammonium and nitrate mixtures on dry matter yield, content of soluble protein, and phosphoenolpyruvate carboxylase (PEPCase) and glutamine synthetase (GS) enzymatic activities. Cultivar ‘Salamanca’ wheat plants, 21 days old, were grown in one of five solutions containing one of the following: 7/0, 5/2, 3.5/3.5, 2/5 or 0/7 meq l‐1 of NH4NO3. After two weeks of treatment applications, the highest dry matter production in both roots and shoots of the 35‐day‐old plants was observed in plants receiving the 2/5 NH4/NO3 ratio. The same response was observed on the accumulation of soluble protein and the potential activity of PEPCase. The specific activity of PEPCase was related to the plan...

Multiple forms of phosphoenolpyruvate carboxylase in mesophyll, epidermal and guard cells of Vicia faba

The distribution of phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31) in different leaf‐cell‐types and tissues of Vicia faba L. cv. 3‐fach Weiße was studied. The highest specific PEPCase activity was found in guard cell protoplasts (16.3 µmol mg−1 protein h−1) whereas for epidermal and mesophyll protoplasts remarkably lower specific activities were found (1.6 and 1.0 µmol mg−1 protein h−1, respectively). On chlorophyll and protoplast basis, a similar distribution of enzyme activity was observed. Compared with epidermal extracts, the specific PEPCase activity of mesophyll tissue was 17‐fold lower. Immunological studies with polyclonal antibodies to PEPCase indicated 3 immunoreactive proteins in epidermal tissue and guard cell protoplasts with molecular masses of 107 000, 110 000, and 112 000. Only the Mr 107 000 protein was found in extracts of mesophyll and epidermis protoplasts. Western immunoblots after native electrophoresis of epidermal and mesophyll proteins showed a significant difference in PEPCase mobility. It is assumed, that the immunostained proteins of Mr 110 000 and 112 000 represent isoforms or subunits of the PEPCase and that they are involved in stomatal movements.

Effect of Light and NO3− on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity

Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO(3) (-) leaves) or 40 millimolar KNO(3) (high-NO(3) (-) leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO(3) (-). In contrast, the lower rate of increase recorded for low-NO(3) (-) leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO(3) (-) effect increased linearly with the level of NO(3) (-) uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO(3) (-) leaves, illuminated high-NO(3) (-) leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, (32)P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO(3) (-) conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO(3) (-), an interpretation of the role of NO(3) (-) as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO(3) (-), the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO(3) (-) modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis.

Nitrogen metabolism inErica and soybean, two species differing by their sensitivity to inorganic N source

Growth of soybean was not altered, whatever the inorganic N-source (NO3, NH4 or a NO3/NH4 mixture); conversely, growth of Erica x darleyensis plants in vitro decreased more in. NH4 medium than in a NO3 medium, compared to a NO3/NH4 medium. The GS/GOGAT pathway (in NH4 medium), the nitrate and nitrite reductase activities (in NO3 medium), as the contents in free nitrogenous forms and total nitrogen (in NO3 and NH4 media) were not more altered in Erica than in soybean, compared to a NO3/NH4 medium. PEPCase activity was the highest in soybean irrespective of the N-treatments; the involvement of PEPCase in N-metabolism could be explained by its function in ionic and osmotic balances rather than its function in supplying carboxylates as acceptors for NH4-assimilation.

Environmental Control of Phosphoenolpyruvate Carboxylase Induction in Mature Mesembryanthemum crystallinum L.

Mesembryanthemum crystallinum L. plants shift the mode of carbon assimilation from C(3) to Crassulacean acid metabolism when stressed by high salinity. A prerequisite for Crassulacean acid metabolism induction is the synthesis of phosphoenolpyruvate carboxylase (PEPCase). A moderate increase in the abundance of PEPCase transcripts and activity is observed in 7-week-old, well-watered plants. This increase in PEPCase coincides in time with a decrease in the growth rate of the shoots. The steady-state level of PEPCase activity is uniform along the leaves of well-watered plants, as can be shown by comparing leaves of different age from individual 7-week-old plants. In contrast, the rate of induction in response to salt stress varies with the age of plants and to a lesser extent with the age of the leaves. Two-week-old seedlings induce PEPCase slowly under a moderate salt stress regimen, whereas older plants induce faster. When individual leaves from a seven-week-old plant are compared with respect to induction velocity, no clear-cut correlation with leaf age is apparent. The highest induction rate is observed in leaves from node five that are about 2 weeks old at the beginning of the experiment. PEPCase transcripts are readily down-regulated to minute levels when detached leaves are hydrated. The levels reached after 8 hours of rehydration are very similar, regardless of whether the leaves were cut from young or old plants or whether the plants were previously salt-stressed or well-watered. It is concluded that environmental rather than developmental factors are predominant in determining abundance of PEPCase activity and transcripts in leaves of mature M. crystallinum plants.

Phosphoenolpyruvate Carboxylase Activity In Norway Spruce Needles: Effects Of Air Pollutants Under Controlled Conditions

The effect of mixtures of air pollutants (SO2, NO2, O3) on phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activities from needles of four-year-old Norway spruce (Picea abies [L.] Karst.) in closed fumigation chambers were analyzed. Three months exposure to high concentrations of O3 (1030 µg/m3) together with SO2 (1030 µg/m3) or NO2 (615 µg/m3) resulted in a dramatic increase in PEPCase activities

Changes in activities and levels of pyruvate, orthophosphate dikinase with induction of Crassulacean acid metabolism in mesembryanthemum crystallinum L.

In Mesembryanthetnum crystallinum L.leaves exhibiting C3 photosynthesis, Crassulacean acid metabolism (CAM), as characterized by CO2 uptake during the dark period, was induced by exposing the plant roots to high NaCl concentration over a week. The increase in malate content in the leaves at the end of the dark period correlated with the increases in the activities of phosphoenolpyruvate carboxylase (PEPCase), NADP malic enzyme (NADP-ME) and pyruvate, orthophosphate dikinase (PPDK) which exhibited a maximum about a week after NaCl treatment. It was found that PPDK existed in C3-M. ctystallinum and the increases in the activities of PEPCase and PPDK were accompanied by the de novo synthesis of these proteins

Diurnal changes in phosphoenolpyruvate carboxylase and pyruvate, orthophosphate dikinase properties in the natural environment: interplay of light and temperature in a C4 thermophile

Activities of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured in leaf extracts of field grown Amaranthus paniculatus L. (C4) during a natural diurnal irradiance and temperature pattern. Enzyme assays were run at both fixed (30°C) and the corresponding leaf temperature at the time of harvest. Light activation of PEP carboxylase (PEPCase) at fixed assay temperatures was expressed as a decrease in S0–5 (PEP) after a threshold (> 330 μmol m–2 s–1) photon fluence rate was surpassed at noon. Earlier in the morning, increase in apparent enzyme affinity for PEP was observed when the assay was run at leaf temperature, indicating a physiologically meaningfull effect of temperature on S0.5 (PEP). The 3.3‐fold increase in PEPCase activity at low PEP and fixed assay temperature between the minimal and maximal irradiance and temperature hours of the day, became 12.8‐, 11.5‐ and 7.4‐fold when assays were run at the corresponding leaf temperature during three diurnal cycles with respective temperature differences (max minus min) of 9.0, 8.3 and 7.4°C. The extent of malate inhibition was the same for both day and night forms of PEPCase assayed at 35°C, but increased considerably with night enzyme at 25°C. The results indicate that light increases the apparent affinity of PEPCase for PEP and that at lower temperatures malate becomes more inhibitory. Pyruvate orthophosphate dikinase activity started to increase immediately after sunrise and the 10‐fold increase at fixed temperature became 14.8‐, 14.2‐ and 13.1‐fold when assays were run at the above leaf temperatures. This indicates that the light effect predominates with pyruvate, orthophosphate dikinase, while with phosphoenolpyravate carboxylase, light and temperature co‐operate to increase the day enzyme activities.

Effects of soil phosphorus and Glomus intraradices on growth, nonstructural carbohydrates, and photosynthetic activity of Citrus aurantium

Sour orange (Citrus aurantium L.) grown in low-P (9–12 ppm) and high-P (420 ppm) soil inoculated with or without Glomus intraradices (G.i.), were evaluated for biomass, carbohydrates, ribulose bisphosphate carboxylase (RuBPCase), phosphoenolpyruvate carboxylase (PEPCase) activity, leaf 14CO2 incorporation, and other physiological parameters. Growth of plants in the low-P, noninoculated soil was lowest, with total dry biomass reduced up to half of the low-P, inoculum treatment. Total nonstructural carbohydrates were 40% lower in leaves of plants in the low-P, noninoculated soil, compared with the other treatments. Inoculation of the low-P soil enhanced leaf 14CO2 incorporation by 67%, total chlorophyll content by 28%, and RuBPCase activity by 42%, compared with low-P, noninoculated treatment. Improved P-use efficiency by G.i. in low-P soil was comparable to high-P nutrition in improving leaf 14CO2 incorporation and concentration of major leaf photosynthetic products that include starch and sucrose. Leaf PEPCase activity in the low-P, noninoculated treatment, however, was at least threefold higher than the other treatments, suggesting a possible alteration in organic acid metabolism in sour orange leaves as a result of P deficiency.

Proteins, non-structural carbohydrates and organic acids in the flowers of sweet orange ( Citrus sinensis (L.) osbeck)

The concentration of soluble proteins in open flowers of sweet orange ( Citrus sinensis (L.) Osbeck) was about 35% of the protein content found in green leaves, while ovary and stigma, with attached style, contained up to 85%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicates that flowers and leaves contained ribulose bisphosphate carboxylase-oxygenase (Rubisco). At least 12 polypeptides, ranging from 22 to 100 kDa, were more distinct in flowers than in leaves. The polypeptide band at about 100 kDa in the flower extract suggests the presence of phosphoenolpyruvate carboxylase (PEPCase) in citrus flowers. Activity of Rubisco and PEPCase, expressed in μmol g tissue fresh weight −1 hr −1, averaged 14.2 and 64.6 for intact open flowers, and 547.9 and 63.3 for the leaves, respectively. Starch and reducing sugars were 1.2- and 8.4-fold higher, respectively, and dark respiration rates were about five-fold greater for open flowers than fully expanded, sun leaves. Quinic and malic acids made up about 96% of the organic acids found in flowers and leaves. Ascorbic, citric, fumaric, and shikimic acids were in small or trace amounts. The large accumulation of starch and soluble sugars, in addition to the presence and activity of Rubisco and PEPCase, indicates that the flowers of sweet orange would have some capability to perform photosynthetic CO 2 assimilation, the metabolites of which might play some important role in flower development and fruit setting.

A requirement for phosphoenolpyruvate carboxylase in the cyanobacterium Synechococcus PCC 7942

The gene encoding phosphoenolpyruvate carboxylase (PEPCase) in the cyanobacterium Synechococcus PCC 7942 has been isolated and characterized. As a first step in determining the role of this enzyme in cyanobacterial carbon metabolism we have attempted to generate PEPCase deficient mutants by insertional inactivation of the PEPCase gene (ppc) and recombination into the wild-type genome. Transformants generated by these constructs appear to be merodiploids in which some copies of ppc remain intact and PEPCase activity is present. Successful insertional inactivation of regions of the genome on either side of ppc suggest that the merodiploid state is a result of a requirement for PEPCase activity by the cyanobacteria. Attempts to select for ppc mutants by nutritional complementation during segregation are also described.

Modulation of the activity of phosphoenolypyruvate carboxylase during potassium-induced swelling of guard-cell protoplasts of Vicia faba L. after light and dark treatments

Phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity was found to be modulated by light and darkness when measured in the presence of K(+), which had been added to induce swelling of guard-cell protoplasts (GCPs) from Vicia faba L., whereas no modulation was detected in the absence of K(+) (PEPcase activity remained constant at 1.5±0.15 pmol PEP metabolized · GCP(-1) ·h(-1); subsequently, pmol GCP(-1) ·h(-1) will be used). The activity of PEPCase increased by 100% (from 1.5 to 3 pmol·protoplast(-1)·h(-1)) in darkness and by 200% (from 1.7 to 5 pmol·protoplast(-1)· h(-1)) in light and oscillations in activity of these magnitudes were repeated at intervals of 2 min (dark) and 2.5 min (light) for a period of 10 min during K(+)-induced increase in the volume of GCPs. The oscillations were reflected in changes in malate-pool sizes determined in plastids, mitochondria and the supernatant fraction (consisting of the cytosol and the vacuole). Malate probably functioned as a mitochondrial substrate, thus supplying ATP for K(+) uptake and the swelling of the protoplasts. On the basis of the present paper and previous results (H. Schnabl and B. Michalke 1988, Life Sci. Adv. Plant Physiol. 7, 203-207) involving adenine nucleotidepool sizes in fractionated GCPs, a model is proposed to explain the cause-effect relationship between K(+), PEPCase, the cytosolic and mitochondrial malate levels and ATP levels during the K(+)-induced increase of GCP volume.

Rubia peregrina L.: A stress resistant weed

Rubia peregrina L. is a weed able to survive under extreme hydric and thermal stress conditions. Morphological and physiological studies were conducted in an attempt to understand the survival strategies used by this plant. Its main morphological characteristic is the presence of underground stems, partially red and partially yellow, which form a closely branching underground network. They can be considered to be stolons because of a lack of a thickened endodermis, a typical structure found in rhizomes. The yellow and red dyes are glycosil-alizarines. Ultrastructural and pigment studies showed that the yellow segments have starch-filled plastids and carotenoids, while the red segments have no starch reserves. The yellow structures have a higher water content and a correspondingly higher PEP-case (phosphoenolpyruvate carboxylase) activity than the other organs. The survival of the plant after hydric and thermal stress is due to several factors: low transpiration with good photosynthesis in the aerial part and the capacity to conserve water and to have large starch reserves in the subterranean portion. The reserves provide energy and substrates and probably also function as gravity receptors (statoliths). The PEP-case activity helps maintain turgor by means of the malate-cation relationship, and the alizarinic pigments perform an antiparasitic role. Under favorable conditions new buds emerge from the underground stems which then cover the infested area with branches and leaves.

Period and phase control by temperature in the circadian rhythm of carbon dioxide fixation in illuminated leaves of Bryophyllum fedtschenkoi

The rhythm of CO2 assimilation exhibited by leaves of Bryophyllum fedtschenkoi maintained in light and normal air occurs only at constant ambient temperatures between 10°C and 30°C. Over this range the period increases linearly with increasing temperature from the extremely low value of 15.7 h to 23.3 h, but shows a considerable degree of temperature compensation. Outside the range 10°C-30°C the rhythm is inhibited but re-starts on changing the temperature to 15°C. Prolonged exposure of leaves to high (40°C) and low (2°C) temperature inhibits the rhythm by driving the basic oscillator to fixed phase points in the cycle which differ by 180°, and which have been characterised in terms of the malate status of the leaf cells. At both temperatures loss of the circadian rhythm of CO2 assimilation is due to the inhibition of phosphoenolpyruvate carboxylase (PEPCase) activity, but the inhibition is apparently achieved in different ways at 40°C and 2°C. High temperature appears to inhibit directly PEPCase activity, but not the activity of the enzymes responsible for the breakdown of malate, with the result that the leaf acquires a low malate status. In contrast, low temperature does not directly inhibit PEPCase activity, but does inhibit enzymes responsible for malate breakdown, so that the malate level in the leaf increases to a high value and PEPCase is eventually allosterically inhibited. The different malate status of leaves held at these two temperatures accounts for the phases of the rhythms being reversed on returning the leaves to 15°C. After exposure to high temperature, CO2 fixation by PEPCase activity can begin immediately, whereas after exposure to low temperature, the large amount of malate accumulated in the leaves has to be decarboxylated before CO2 fixation can begin.

Regulatory seryl-phosphorylation of C 4 phospho enolpyruvate carboxylase by a soluble protein kinase from maize leaves

A reconstituted system composed of purified phospho enolpyruvate carboxylase (PEP-Case) and a soluble protein kinase (PK) from green maize leaves was developed to critically assess the effects of in vitro protein phosphorylation on the catalytic and regulatory (malate sensitivity) properties of the target enzyme. The PK was partially purified from light-adapted leaf tissue by ammonium sulfate fractionation (0–60% saturation fraction) of a crude extract and blue dextran-agarose affinity chromatography. The resulting preparation was free of PEPCase. This partially purified protein kinase activated PEP-Case from dark-adapted green maize leaves in an ATP-, Mg 2+-, time-, and temperature-dependent fashion. Concomitant with these changes in PEPCase activity was a marked decrease in the target enzyme's sensitivity to feedback inhibition by l-malate. The PK-mediated incorporation of 32P from [γ- 32P]ATP into the protein substrate was directly correlated with these changes in PEPCase activity and malate sensitivity. The maximal molar 32P-incorporation value was about 0.25 per 100-kDa PEPCase subunit (i.e., 1 per holoenzyme). Phosphoamino acid analysis of the 32P-labeled target enzyme by two-dimensional thin-layer electrophoresis revealed the exclusive presence of phosphoserine. These in vitro results, together with our recent studies on the light-induced changes in phosphorylation status of green maize leaf PEPCase in vivo ( J.-A. Jiao and R. Chollet (1988) Arch. Biochem. Biophys. 261, 409–417 ), collectively provide the first unequivocal evidence that the seryl-phosphorylation of the dark-form enzyme by a soluble protein kinase is responsible for the changes in catalytic activity and malate sensitivity of C 4 PEPCase observed in vivo during dark/light transitions of the parent leaf tissue.

Phosphoenolpyruvate carboxylase level in soybean seed highly correlates to its contents of protein and lipid.

In this communication, we discuss the role of PEPCase in soybean seeds depending upon the results on PEPCase activity and contents of protein and lipid in matured seeds of various Japanese domestic soybean cultivars

A Possible Induction of CAM by NaCl-Stress in Heterotrophic Cell Suspension Cultures of Mesembryantbemum crystallinum

Summary NaCl-induced stress in cell suspension cultures of the facultative CAM plant M. crystallinum caused an increase of PEPcase activity which yet was distinctly less than the activation of the enzyme in the whole plants under salt stress. The activation was obtained after both long-term and shock treatments with salt. Immunotitration revealed that more PEPcase protein was present in the stressed cells compared with the non-stressed controls. This demonstrates that the observed increase of PEPcase activity was due to a de novo synthesis of the enzyme. Moreover, the identical appearance of the immunotitration curves suggests that the ratio of PEPcase protein to PEPcase activity was identical and that the serological characteristics of PEPcase protein were virtually the same in stressed and non-stressed cells. The label distribution among metabolites after 14 CO 2 fixation showed that the degree of labelling did not depend on NaCl pretreatments of the cells. Malate was always the main labelled product. Total 14 C incorporation rates were higher in the unstressed cells. The absolute malate content was slightly increased in the salt treated cells. The present results show that also in cell cultures of M. crystallinum salt stress leads to an increase in the potential activity of PEPcase and in the malate levels. Therefore it can be concluded that salt stress can directly stimulate the biochemical pathway of PEP-carboxylation involved in CAM, though the whole CAM syndrome cannot be expressed in the case of heterotrophic suspension cells, due to the culture conditions.

Activity of phosphoenolpyruvate carboxylase of an anthracycline-producing streptomycete.

During fermantation studies on the production of anthracycline antibiotics by Streptomyces C5, it was observed that among the intermediate metabolism enzymes tested, only phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) increased significantly in specific activity during stationary phase. The specific activity of the Streptomyces C5 PEPCase increased ca. 3-fold during antibiotic production phase from the logarithmic phase levels. To characterize the regulation of the enzyme further, the Streptomyces C5 PEPCase was purified 150-fold from crude extracts. Acetyl-CoA and Mg2+ were shown to be required for PEPCase activity. The activity of the partially purified PEPCase was stimulated slightly by fructose 1,6-bisphosphate and AMP, and was inhibited severely by oxaloacetate, aspartate, malate, succinate, ATP, citrate, and CoASH.

Initiation and characterization of a cotton (Gossypium hirsutum L.) photoautotrophic cell suspension culture

A heterotrophic cotton (Gossypium hirsutum L. cv. Stoneville 825) cell suspension culture was adapted to grow photoautotrophically. After two years in continuous photoautotrophic culture at 5% CO2 (balance air), the maximum growth rate of the photoautotrophic cell line was a 400% fresh weight increase in eight days. The Chl concentration was approximately 500 μg per g fresh weight.Elevated CO2 (1%-5%) was required for culture growth, while the ambient air of the culture room (600 to 700 ul CO2 1(-1)) or darkness were lethal. The cell line had no net photosynthesis at 350 ul 1(-1) CO2, 2% O2, and dark respiration ranged from 29 to 44 μmol CO2 mg(-1) Chl h(-1). Photosynthesis was inhibited by O2. The approximate 1:1 ratio of ribulose 1,5-bisphosphate carboxylase (RuBPcase) to phosphoenolpyruvate carboxylase (PEPcase) (normally about 6:1 in mature leaves of C3 plants) was due to low RuBPcase activity relative to that of C3 leaves, not to high PEPcase activity. The PEPcase activity per unit Chl in the cell line was identical to that of spinach leaves, while the RuBPcase activity was only 15% of the spinach leaf RuBPcase activity. RuBPcase activity in the photoautotrophic cells was not limited by a lack of activation in vivo, since the enzyme in a rapidly prepared cell extract was 73% activated. No evidence of enzyme inactivation by secondary compounds in the cells was found as can be found with cotton leaves. Low RuBPcase activity and high respiration rates are most likely important factors in the low photosynthetic efficiency of the cells at ambient CO2.

PEPCase Transcript Levels in Mesembryanthemum crystallinum Decline Rapidly upon Relief from Salt Stress

Mesembryanthemum crystallinum plants respond to water stress by changing their pathway of carbon assimilation from C(3) to Crassulacean acid metabolism (CAM). Stressed plants are characterized by elevated levels of phosphoenolpyruvate carboxylase (PEPCase) mRNA, protein, and enzyme activity. We wanted to determine whether CAM is a reversible response to environmental conditions or a developmentally programmed adaptation that is irreversibly expressed once induced. Plants were osmotically stressed by irrigation with 500 millimolar NaCl for 12 days to elicit CAM. Salt was then thoroughly flushed from the soil and PEPCase protein and transcript levels were monitored. PEPCase mRNA levels dropped by 77% within 2.5 hours after salt removal. PEPCase activity and polypeptide levels declined more slowly, with a half-life of 2 to 3 days. These results show that PEPCase expression in M. crystallinum is a reversible response to stress that is regulated at the level of transcription or stability of the PEPCase mRNA.