The Effect of Cycloheximide on Membrane Transport in Euglena

Photo and Nutritional Regulation of Euglena Organelle Development

Euglena can use light and CO₂, photosynthesis, as well as a large variety of organic molecules as the sole source of carbon and energy for growth. Light induces the enzymes, in this case an entire organelle, the chloroplast, that is required to use CO₂ as the sole source of carbon and energy for growth. Ethanol, but not malate, inhibits the photoinduction of chloroplast enzymes and induces the synthesis of the glyoxylate cycle enzymes that comprise the unique metabolic pathway leading to two carbon, ethanol and acetate, assimilation. In resting, carbon starved cells, light mobilizes the degradation of the storage carbohydrate paramylum and transiently induces the mitochondrial proteins required for the aerobic metabolism of paramylum to provide the carbon and energy required for chloroplast development. Other mitochondrial proteins are degraded upon light exposure providing the amino acids required for the synthesis of light induced proteins. Changes in protein levels are due to increased and decreased rates of synthesis rather than changes in degradation rates. Changes in protein synthesis rates occur in the absence of a concomitant increase in the levels of mRNAs encoding these proteins indicative of photo and metabolic control at the translational rather than the transcriptional level. The fraction of mRNA encoding a light induced protein such as the light harvesting chlorophyll a/b binding protein of photosystem II, (LHCPII) associated with polysomes in the dark is similar to the fraction associated with polysomes in the light indicative of photoregulation at the level of translational elongation. Ethanol, a carbon source whose assimilation requires carbon source specific enzymes, the glyoxylate cycle enzymes, represses the synthesis of chloroplast enzymes uniquely required to use light and CO₂ as the sole source of carbon and energy for growth. The catabolite sensitivity of chloroplast development provides a mechanism to prioritize carbon source utilization. Euglena uses all of its resources to develop the metabolic capacity to utilize carbon sources such as ethanol which are rarely in the environment and delays until the rare carbon source is no longer available forming the chloroplast which is required to utilize the ubiquitous carbon source, light and CO₂.

Effects of acclimation temperature and cadmium exposure on cellular energy budgets in the marine mollusk Crassostrea virginica: linking cellular and mitochondrial responses

In order to understand the role of metabolic regulation in environmental stress tolerance, a comprehensive analysis of demand-side effects (i.e. changes in energy demands for basal maintenance) and supply-side effects (i.e. metabolic capacity to provide ATP to cover the energy demand) of environmental stressors is required. We have studied the effects of temperature (12, 20 and 28 degrees C) and exposure to a trace metal, cadmium (50 microg l(-1)), on the cellular energy budget of a model marine poikilotherm, Crassostrea virginica (eastern oysters), using oxygen demand for ATP turnover, protein synthesis, mitochondrial proton leak and non-mitochondrial respiration in isolated gill and hepatopancreas cells as demand-side endpoints and mitochondrial oxidation capacity, abundance and fractional volume as supply-side endpoints. Cadmium exposure and high acclimation temperatures resulted in a strong increase of oxygen demand in gill and hepatopancreas cells of oysters. Cd-induced increases in cellular energy demand were significant at 12 and 20 degrees C but not at 28 degrees C, possibly indicating a metabolic capacity limitation at the highest temperature. Elevated cellular demand in cells from Cd-exposed oysters was associated with a 2-6-fold increase in protein synthesis and, at cold acclimation temperatures, with a 1.5-fold elevated mitochondrial proton leak. Cellular aerobic capacity, as indicated by mitochondrial oxidation capacity, abundance and volume, did not increase in parallel to compensate for the elevated energy demand. Mitochondrial oxidation capacity was reduced in 28 degrees C-acclimated oysters, and mitochondrial abundance decreased in Cd-exposed oysters, with a stronger decrease (by 20-24%) in warm-acclimated oysters compared with cold-acclimated ones (by 8-13%). These data provide a mechanistic basis for synergism between temperature and cadmium stress on metabolism of marine poikilotherms. Exposure to combined temperature and cadmium stress may result in a strong energy deficiency due to the elevated energy demand on one hand and a reduced mitochondrial capacity to cover this demand on the other hand, which may have important implications for surviving seasonally and/or globally elevated temperatures in polluted estuaries.

Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Defining the physiological mechanisms that set metabolic rates and the`cost of living' is important for understanding the energy costs of development. Embryos and larvae of the sea urchin Lytechinus pictus(Verrill) were used to test hypotheses regarding differential costs of protein synthesis in animals differing in size, rates of protein synthesis, and physiological feeding states. For embryos, the rate of protein synthesis was 0.22±0.014 ng protein embryo-1 h-1 (mean ±s.e.m.) and decreased in unfed larvae to an average rate of 0.05±0.001 ng protein larva-1 h-1. Fed larvae had rates of synthesis that were up to 194 times faster than unfed larvae (9.7±0.81 ng protein larva-1 h-1). There was no significant difference, however, in the cost of protein synthesis between these larvae with very different physiological states. Furthermore, the cost of synthesis in the larval stages was also similar to costs measured for blastula and gastrula embryos of 8.4±0.99 J mg-1 protein synthesized. The cost of protein synthesis was obtained using both direct (`inhibitor') and indirect (`correlative') measurements; both methods gave essentially identical results. Protein synthesis accounted for up to 54±8% of metabolic rate in embryos. Percent of metabolism accounted for by protein synthesis in larvae was dependent on their physiological feeding state, with protein synthesis accounting for 16±4% in unfed larvae and 75±11% in fed larvae. This regulation of metabolic rate was due to differential rates of synthesis for a fixed energy cost per unit mass of protein synthesized. The cost of synthesizing a unit of protein did not change with increasing rates of protein synthesis. We conclude that the cost of protein synthesis is independent of the rate of synthesis, developmental stage, size and physiological feeding state during sea urchin development.

The protein synthesis machinery operates at the same expense in eurythermal and cold stenothermal pectinids

Translationally active cell-free systems from gills of the Antarctic scallop Adamussium colbecki and the European scallop Aequipecten opercularis were developed, characterised, and optimised for an analysis of translational capacity. The aim was to determine the energetic cost of protein synthesis in the in vitro cell-free system by directly measuring the required energy equivalents in the lysates. Protein synthesis rate in assays conducted with lysates of A. colbecki (1.029+/-0.061 micromol Phe min(-1) at 15 degrees C; Phe=phenylalanine) were higher compared with lysates of A. opercularis (0.087+/-0.013 micromol Phe min(-1) at 15 degrees C and 0.156+/-0.023 micromol Phe min(-1) at 25 degrees C). This can in part be attributed to the naturally occurring higher RNA content in lysates of A. colbecki (0.883+/-0.037 mg RNA mL(-1) lysate) compared with A. opercularis (0.468+/-0.013 mg RNA mL(-1) lysate). There was no significant difference in the energetic costs of protein synthesis in cell-free systems of gill lysates of the cold stenothermal A. colbecki with 4.3+/-0.7 energy equivalents per peptide bond formed and the eurythermal A. opercularis with 5.6+/-0.6 energy equivalents, indicating that there are no differences in the efficiency of the translation machinery. The energetic costs specified for protein synthesis correspond with the generally accepted theoretical value of four energy equivalents per peptide bond formed, especially in gill lysates of A. colbecki, whereas the value for gill lysates of A. opercularis was slightly higher.

Energy cost of whole-body protein synthesis measured in vivo in chicks

1. Energy cost of whole-body protein synthesis was measured in vivo in chicks by comparing the changes in protein synthesis and heat production after the administration of cycloheximide, an inhibitor of protein synthesis. 2. Incorporation of phenylalanine into whole-body protein fraction was promptly inhibited after the intravenous injection of cycloheximide, and the effect was sustained for at least 3 hr. 3. Both whole-body protein synthesis and total heat production were significantly reduced by the cycloheximide administration. 4. The energy cost of whole-body protein synthesis was calculated to be 5.35 kJ per g protein synthesis, and hence on a molar basis 7.52 ATPs are required per peptide bond synthesis.

Dissociation of RNA synthesis from the calcium requirement for serum-increased ornithine decarboxylase activity in rat glioma cells

When C6-2B rat glioma cells were stimulated with calf serum in the presence of calcium, ornithine decarboxylase activity increased maximally in 6-8 h after an initial 2-3 h lag period wherein RNA synthesis occurred. The increase of ornithine decarboxylase activity in serum-stimulated C6-2B cells was prevented by the calcium chelator EGTA, but EGTA had no effect upon RNA synthesis as judged by [3H]uridine incorporation into RNA. In addition, the calcium requirement for increased ornithine decarboxylase activity was temporally distal to the lag period. EGTA appeared to inhibit the synthesis of ornithine decarboxylase, because the half-life values of ornithine decarboxylase activity were similar (37-47 min) in the presence of EGTA or protein synthesis inhibitors such as cycloheximide or emetine. Also, calcium readdition rapidly reversed EGTA inhibition of ornithine decarboxylase activity by a mechanism which could be blocked by cycloheximide.

Dark-induced chloroplast dedifferentiation in Euglena gracilis

Transfer of light-grown autotrophic Euglena gracilis cells to darkness and carbon (glucose) containing heterotrophic media causes structural and functional decomposition of the photosynthetic apparatus. The process can be ascribed to a strict diluting-out mechanism of stroma constituents among the progeny, as shown for ribulose-1,5-bisphosphate carboxylase (RuBPCase, EC 4.1.1.39), and aminoacyl-tRNA synthetases (Aa-RS; especially Leu-RS, EC 6.1.1.4) activities. The diluting-out effect of thylakoid membranes and chlorophyll seems to be superimposed by additional degradations, beginning soon after the transfer of cells to darkness. Cultivation of cells in darkness in 0.03 M KCl or without utilizable organic carbon (resting media) preserves chloroplast structure and function over a long period, indicating negligible turnover in these cells. Thus, under both growing and resting conditions, darkness induces the arrest of synthesis of plastid constituents. Experiments with the inhibitors cycloheximide, chloramphenicol, and nalidixic acid demonstrate that chloroplast dedifferentiation does not require organelle gene expression, but it is more strictly dependent on biosynthetic events in the nucleo-cytoplasmic compartment than the reverse process, light-induced chloroplast formation. Since cycloheximide at low concentrations in growth medium causes a marked suppression of precursor uptake or re-utilization similar to that in cells of resting media, intracellular precursor deficiency is suggested to control the observed blockade in cytoplasmic synthesis of plastid proteins. On the other hand, darkness might signalize the stop of gene expression in the organelles.

Muliple sites of action of cycloheximide in addition to inhibition of protein synthesis in Physarum polycephalum

The specificity action of cycloheximide was tested using a cycloheximide resistant mutant of Physarum polycephalum. This resistance has previously been shown to reside with the ribosomes, making cytoplasmic protein synthesis refractile to the action of the drug. We show here that cycloheximide in the mutant strain causes specific alterations in metabolism without influencing the growth rate. These are: 1. lowered specific activity of glutamate dehydrogenase during starvation, 2. alteration of the molecular weight of glutamate dehydrogenase, 3. inhibition of uptake of amino acids from the medium into the internal pools. Possible explanations for these effects of cycloheximide outside of protein synthesis per se are considered. We conclude that cycloheximide may not be considered a specific inhibitor of protein synthesis, and that a causal relationship between protein synthesis and any biological process cannot be claimed unless such specificity is demonstrated in each case, preferably by use of mutants.

Nutritional regulation of organelle biogenesis inEuglena: Photo- and metabolite induction of mitochondria

Exposure of dark-grown restingEuglena gracilis Klebs var.bacillaris Cori to light, ethanol, or malate produced an increase in the specific activity of fumarase (EC. 4.2.1.2) and succinate dehydrogenase (EC. 1.3.99.1) during the first 8-12 h of exposure to inducer, followed by a decrease in the specific activity of both mitochondrial enzymes between 12 and 72 h. The increased specific activity represented a net increase in the level of active enzyme, and it was dependent upon cytoplasmic protein synthesis. The photoinduction of fumarase required continuous illumination while the subsequent decrease in fumarase specific activity was independent of light. Light had little effect on the ethanol and malate induction of fumarase and succinate dehydrogenase. In the mutant W3BUL, which has no detectable protochlorophyll(ide) and chloroplast DNA, light induced both mitochondrial enzymes and the kinetics of enzyme induction were similar to the induction kinetics in wild-type cells. The induction of mitochondrial enzymes appears to be controlled by a non-chloroplast photoreceptor. Dark-grown resting cells of the plastidless mutant W10SmL have lost the ability to regulate fumarase levels. In this mutant, the specific activity of fumarase fluctuated and light had little effect on these fluctuations, indicating that fumarase synthesis was uncoupled from the nonchloroplast photoreceptor. Ethanol addition produced transient changes in fumarase specific activity in W10SmL indicating that in this mutant, mitochondrial enzymes are still inductible by metabolites. Fumarase synthesis in wild-type cells was not induced in the dark by levulinic acid, a chemical inducer of the breakdown ofEuglena storage carbohydrates. Taken together, our results indicate that the photoinduction of mitochondrial enzyme synthesis is not a result of the photoinduction of carbohydrate breakdown. The mechanisms by which light and organic carbon induce the synthesis ofEuglena mitochondria may differ.

Inhibition of DNA synthesis by cycloheximide and blasticidin-S is independent of their effect on protein synthesis

The effects of cycloheximide and related glutarimide antibiotics on DNA synthesis in Achlya bisexualis Coker and A. Couch were compared to those of other protein synthesis inhibitors, puromycin, p-fluorophenylalanine and blasticidin-S. The inhibitors had no significant effects on intrahyphal pool sizes of dTTP, dCTP, ATP, UTP and CTP, nor on the specific activity of the dTTP pool labelled by [3H] thymidine. DNA was the sole acid-insoluble product of [3H]-thymidine incorporation. Cycloheximide, isocycloheximide, streptimidone and blasticidin-S inhibited DNA synthesis rapidly and completely and anhydrocycloheximide was less effective. Cycloheximide acetate, puromycin and p-fluorophenylalanine did not inhibit DNA synthesis. It is concluded that the effects of the several glutarimide antibiotics and of blasticidin-S on DNA synthesis were independent of their effects on protein synthesis.

Evidence for cycloheximide acting as a glutamine analogue in plant tissue

In growing maize root tissue [14C]asparagine formation in inhibited and [14C]glutamine accumulation stimulated by treatment with cycloheximide or glutamine analogs such as azaserine. In contrast, puromycin enhances the accumulation of [14C]asparagine but not [14C]glutamine. Cycloheximide and puromycin alone inhibit protein synthesis. This is interpreted to mean that the alteration in amide metabolism following cycloheximide treatment is a direct result of the antibiotic acting as a glutamine analog. While cycloheximide is often the cytoplasmic protein synthesis inhibitor of choice due to its potency and rapid action, its assumed specificity of action of eukaryotes is doubtful.

Characterization of L-asparagine transport systems in Stemphylium botryosum

L-Asparagine uptake by Stemphylium botryosum is mediated by two distinct energy- and temperature-dependent transport systems. One permease is relatively specific for L-asparagine and L-glutamine and is present in nutrient-sufficient mycelium. The specific permease shows an optimum pH at 5.2, saturation kinetics (Km = 4.4 x 10(-4) M, Vmax = 1.1 mumol/g per min), competitive gradient of L-asparagine, and higher affinity towards the L-isomer of asparagine. Amide derivatives of L-asparagine (5-diazo-4-oxo-L-norvaline or L-aspartyl hydroxamate) are the most effective competitors, alpha-amino derivative (N-acetyl asparagine) is a moderate competitor, and alpha-carboxyl derivative (L-asparagine-t-butylester) shows only slight inhibition of the specific permease. Derivatives of L-glutamine are significantly less effective competitors than those of L-asparatine. The level of the specific permease is affected by nitrogen sources and increases approximately threefold upon starvation. The nonspecific permease possesses an optimum pH at 6.8, saturation kinetics (Km = 7 x 10(-5) M, Vmax = 5 mumol/g per min, Kt = 7.4 x 10(-5) M for L-leucine), and high affinity towards various types of amino acids.

Uptake and incorporation of pyrimidines in Euglena gracilis

In photoorganotrophically grown cells of Euglena gracilis the uptake and incorporation degree of 12 different pyrimidines were tested. The rate of uptake of pyrimidines has distinct maxima in the late log phase and in the stationary phase of cell multiplication. The kinetics of uptake are linear in the first 2 h, do not show saturation at various concentrations and increase with the concetrations. No accumulation of the pyrimidines at various concentrations could be observed in the first 2 h of incubation. Membrane inhibitors as uranyl acetate inhibit the uptake of the reference substance alpha-AIB, which is wellknown transported by an active transport mechanism, but have no effect on uptake rate of uracil and cytosine. It could not be observed an energy requirement tested in temperature dependence and with electron transport inhibitors. Uptake of uridine, uracil, barbituric acid and alpha-AIB is inhibited by cycloheximide in a different manner after 5 - 10 min.

Effect of weak acids on amino acid transport by Penicillium chrysogenum: evidence for a proton or charge gradient as the driving force

A variety of weak acids at and below their pK(a) are potent inhibitors of transport in Penicillium chrysogenum. The effective compounds include sorbate, benzoate, and propionate (common antifungal agents), indoleacetate (a plant hormone), acetylsalicylate (aspirin), hexachlorophene, and a yellow pigment produced by the mycelia under nutrient-deficient conditions, as well as the classical uncouplers 2,4-dinitrophenol, p-nitrophenol, and azide. The results suggest that a proton gradient or charge gradient is involved in energizing membrane transport in P. chrysogenum. The unionized form of the weak acids could discharge the gradient by diffusing through the membrane and ionizing when they reach an interior compartment of higher pH. Experiments with 2,4-dinitrophenol and p-nitrophenol established that the ionized species are not absorbed by the mycelium to any great extent. The transport inhibitors also caused a decrease in cellular adenosine 5'-triphosphate (ATP) levels, but there was no constant correlation between inhibition of transport and suppression of cellular ATP. A decrease in aeration of the mycelial suspension had the same effect on transport and ATP levels as the addition of a weak organic acid. The effects on transport rates and ATP levels were reversible. The instantaneous inhibition of [(14)C]l-leucine transport by NH(4) (-) (and vice-versa) in nitrogen-starved mycelia at pH values of 7 or below can be explained by competition for a common energy-coupling system. The inhibition is not observed in carbon-starved mycelia in which the NH(4) (+) transport system is absent or inactive (but the general amino acid transport is fully active), or in iodoacetate-treated mycelia in which the NH(4) (+) transport system has been differentially inactivated. At pH values greater than 7.0, NH(3) and HPO(4) (2-) inhibit transport, presumably by discharging the membrane proton or charge gradient. Aniline counteracts the inhibitory effect of NH(3) and HPO(4) (2-) possibly by acting as a proton reservoir or buffer within the membrane.