Studies on protein synthesis by protoplasts of Saccharomyces carlsbergensis. I. The effect of ribonuclease on protein synthesis

The Saccharomyces cerevisiae mitochondrial unselective channel behaves as a physiological uncoupling system regulated by Ca2+, Mg2+, phosphate and ATP

It is proposed that the Saccharomyces cerevisiae the Mitochondrial Unselective Channel ((Sc)MUC) is tightly regulated constituting a physiological uncoupling system that prevents overproduction of reactive oxygen species (ROS). Mg(2+), Ca(2+) or phosphate (Pi) close (Sc)MUC, while ATP or a high rate of oxygen consumption open it. We assessed (Sc)MUC activity by measuring in isolated mitochondria the respiratory control, transmembrane potential (ΔΨ), swelling and production of ROS. At increasing [Pi], less [Ca(2+)] and/or [Mg(2+)] were needed to close (Sc)MUC or increase ATP synthesis. The Ca(2+)-mediated closure of (Sc)MUC was prevented by high [ATP] while the Mg(2+) or Pi effect was not. When Ca(2+) and Mg(2+) were alternatively added or chelated, (Sc)MUC opened and closed reversibly. Different effects of Ca(2+) vs Mg(2+) effects were probably due to mitochondrial Mg(2+) uptake. Our results suggest that (Sc)MUC activity is dynamically controlled by both the ATP/Pi ratio and divalent cation fluctuations. It is proposed that the reversible opening/closing of (Sc)MUC leads to physiological uncoupling and a consequent decrease in ROS production.

In Saccharomyces cerevisiae, cations control the fate of the energy derived from oxidative metabolism through the opening and closing of the yeast mitochondrial unselective channel

The yeast mitochondrial unspecific channel (YMUC) sensitivity to inorganic (Ca2+ or Mg2+) or organic (hexyl or octyl-guanidine) cations was measured. The rate of oxygen consumption in State 3 and State 4, the transmembrane potential (deltapsi), mitochondrial swelling, and the polyethylene-glycol mediated recontraction were used to follow opening of the YMUC. Addition of 0.4 mM PO4 did not close the YMUC, although it did enhance the sensitivity to Ca2+ (I50 decreased from 50 to 0.3 mM) and Mg2+ (I50 decreased from 5 to 0.83 mM Mg2+). The Ca2+ concentration needed to close the YMUC was higher than the concentrations usually observed in the cell. Nonetheless, Mg2+, Ca2+, and PO4 exhibited additive effects. These cations did not inhibit contraction of preswollen mitochondria, suggesting that the YMUC/cation interaction was labile. Octyl-guanidine (OG-I50 7.5 microM) was the only cation which inhibited mitochondrial recontraction, probably as a result of membrane binding stabilization through its hydrophobic tail. The PO4-dependent, Ca(2+)/Mg(2+)-mediated closure of the YMUC may be a means to control the proportion of oxidative energy producing ATP or being lost as heat.

Closure of the yeast mitochondria unspecific channel (YMUC) unmasks a Mg2+ and quinine sensitive K+ uptake pathway in Saccharomyces cerevisiae

The K+ uptake pathways in yeast mitochondria are still undefined. Nonetheless, the K+-mediated mitochondrial swelling observed in the absence of phosphate (PO4) and in the presence of a respiratory substrate has led to propose that large K+ movements occur in yeast mitochondria. Thus, the uptake of K+ by isolated yeast mitochondria was evaluated. Two parallel experiments were conducted to evaluate K+ transport; these were mitochondrial swelling and the uptake of the radioactive K+ analog 86Rb+. The opening of the yeast mitochondrial unspecific channel (YMUC) was regulated by different PO4 concentrations. The high protein concentrations used to measure 86Rb+ uptake resulted in a slight stabilization of the transmembrane potential at 0.4 mM PO4 but not at 0 or 4 mM PO4. At 4 mM PO4 swelling was inhibited while, in contrast, 86Rb+ uptake was still observed. The results suggest that an energy-dependent K+ uptake mechanism was unmasked when the YMUC was closed. To further analyze the properties of this K+ uptake system, the Mg2+ and quinine sensitivity of both swelling and 86Rb+ uptake were evaluated. Under the conditions where the unspecific pore was closed, K+ transport sensitivity to Mg2+ and quinine increased. In addition, when Zn2+ was added as an antiport inhibitor, uptake of 86Rb+ increased. It is suggested that in yeast mitochondria, the K+ concentration is highly regulated by the equilibrium of uptake and exit of this cation through two specific transporters.

Kinetic characterization of the rotenone-insensitive internal NADH: ubiquinone oxidoreductase of mitochondria from Saccharomyces cerevisiae

Saccharomyces cerevisiae mitochondria contain an NADH:Q6 oxidoreductase (internal NADH dehydrogenase) encoded by NDI1 gene in chromosome XIII. This enzyme catalyzes the transfer of electrons from NADH to ubiquinone without the translocation of protons across the membrane. From a structural point of view, the mature enzyme has a single subunit of 53 kDa with FAD as the only prosthetic group. Due to the fact that S. cerevisiae cells lack complex I, the expression of this protein is essential for cell growth under respiratory conditions. The results reported in this work show that the internal NADH dehydrogenase follows a ping-pong mechanism, with a Km for NADH of 9.4 microM and a Km for oxidized 2,6-dichorophenolindophenol (DCPIP) of 6.2 microM. NAD+, one of the products of the reaction, did not inhibit the enzyme while the other product, reduced DCPIP, inhibited the enzyme with a Ki of 11.5 microM. Two dead-end inhibitors, AMP and flavone, were used to further characterize the kinetic mechanism of the enzyme. AMP was a linear competitive inhibitor of NADH (Ki = 5.5 mM) and a linear uncompetitive inhibitor of oxidized DCPIP (Ki = 11.5 mM), in agreement with the ping-pong mechanism. On the other hand, flavone was a partial inhibitor displaying a hyperbolic uncompetitive inhibition regarding NADH, and a hyperbolic noncompetitive inhibition with respect to oxidized DCPIP. The apparent intercept inhibition constant (Kii = 5.4 microM) and the slope inhibition constant (Kis = 7.1 microM) were obtained by non linear regression analysis. The results indicate that the ternary complex F-DCPIPox-flavone catalyzes the reduction of DCPIP, although with lower efficiency. The effect of pH on Vmax was studied. The Vmax profile shows two groups with pKa values of 5.3 and 7.2 involved in the catalytic process.

Interactions of arsenate, sulfate and phosphate with yeast mitochondria

In the presence of K(+), addition of ATP or ethanol to yeast mitochondria triggers the depletion of the transmembrane potential (DeltaPsi) and this is prevented by millimolar concentrations of phosphate (PO(4)). Different monovalent and polyvalent anions were tested for their protective effects on mitochondria from Saccharomyces cerevisiae. Only arsenate (AsO(4)) and sulfate (SO(4)) were as efficient as PO(4) to protect mitochondria against the K(+) mediated swelling, depletion of the DeltaPsi, and decrease in the ratio of uncoupled state to state 4 respiration rates. Protection by PO(4), SO(4) or AsO(4) was inhibited by mersalyl, suggesting that these anions interact with a site located in the matrix side. In addition, the effects of SO(4) and AsO(4) on the F(1)F(0)-ATPase were tested: both SO(4) and AsO(4) inhibited the synthesis of ATP following competitive kinetics against PO(4) and non-competitive kinetics against ADP. The mersalyl sensitive uptake of (32)PO(4) was not inhibited by SO(4) or AsO(4), suggesting that the synthesis of ATP was inhibited at the F(1)F(0)-ATPase. The hydrolysis of ATP was not inhibited, only a stimulation was observed when AsO(4) or sulfite (SO(3)) were added. It is suggested that the structure and charge similarities of PO(4), AsO(4) and SO(4) result in undiscriminated binding to at least two sites located in the mitochondrial matrix: at one site, occupation by any of these three anions results in protection against uncoupling by K(+); at the second site, in the F(1)F(0)-ATPase, AsO(4) and SO(4) compete for binding against PO(4) leading to inhibition of the synthesis of ATP.

Influence of monovalent cations on yeast cytoplasmic and vacuolar pH

The effects of monovalent cations on the internal pH of yeast were studied. Our former procedure was modified, inducing maximal alkalinization of the cells with 100 mM-NH4OH instead of Tris base. The pH values were lower than reported before (Peña et al., J. Baceteriol. 1995 177, 1017-1022). With glucose as substrate, the internal cytoplasmic pH reached higher values when incubating at an external pH of 6.0, as compared to pH 4.0. Monovalent cations added approximately 5 min after glucose produced a further increase in the internal pH, which was higher at a previous incubation pH of 4.0 than that observed at pH 6.0. The selectivity of the changes followed a similar order to that of the transport system for monovalent cations. When incubating cells with glucose for more than 30 min, the initial changes of the internal pH appeared to be regulated by the cell. However, under the fluorescence microscope, it was observed that pyranine, which was confined to the cytoplasm during the first 15 min, was progressively concentrated in the vacuole. By studying the fluorescence changes of cells electroporated and then incubated with glucose or glucose plus potassium, we could follow the internal pH of this organelle, obtaining values within the range reported by other authors. Also, in cells preincubated with glucose for 60 min, and electroporated afterwards, the fluorescence of pyranine, which only entered the cytoplasm, allowed us to measure the pH of this compartment, showing that it was more alkaline than the vacuole. Moreover, the cytoplasmic pH increased upon addition of glucose or potassium. The vacuolar pH, on the other hand, increased upon addition of potassium after glucose, but decreased upon addition of glucose. In addition, incubation of the cells with glucose with or without pyranine produced vesiculation of the vacuole.

Potassium collapses the deltaP in yeast mitochondria while the rate of ATP synthesis is inhibited only partially: modulation by phosphate

Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 microM phosphate and 200 microM Mg2+ led to uncoupled respiration and decreased protonmotive force (deltaP):at 0 K+ deltaP = 213 mV, negative inside, where deltapsi = 180 mV and deltapH = 33 mV, while at 20 mM K+ deltaP = 28 mV, where deltapsi = 16 mV and deltapH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for the Km and the Vmax in 400 microM Pi and increasing ADP: in 0 K+, Km = 18.6 microM and Vmax = 75.4 nmol (min x mg protein)-1, while in 20 mM K+, Km = 5.2 microM and Vmax = 46.0 nmol (min x mg protein)-1, i.e., when K+ depleted most of the deltaP, and at ADP concentrations below the Km, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+ was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+ had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+ on the deltaP of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM). At 4 mM phosphate, the deltaP was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ Km = 10.0 microM and Vmax = 88.3 nmol (min x mg protein)-1. At 20 mM K+, Km = 7.4 microM and Vmax = 133 nmol (min x mg protein)-1.

Purification of alcohol dehydrogenase from Entamoeba histolytica and Saccharomyces cerevisiae using zinc-affinity chromatography

We have developed a single-step method for the purification of NADP(+)-dependent alcohol dehydrogenase from Entamoeba histolytica and NAD(+)-dependent alcohol dehydrogenase from Saccharomyces cerevisiae. It is based on the affinity for zinc of both enzymes. The amebic enzyme was purified almost 800 times with a recovery of 54% and the yeast enzyme was purified 30 times with a recovery of 100%. The kinetic constants of the purified enzymes were similar to those reported for other purification methods. With mammalian alcohol dehydrogenase, we obtained a 40-kDa band suggestive of purified alcohol dehydrogenase, but we failed to retain enzymatic activity in this preparation. Our results suggest that the described method is more applicable to the purification of tetrameric alcohol dehydrogenases.

ACTION OF RIBONUCLEASE PREPARATIONS ON VIABLE YEAST CELLS AND SPHEROPLASTS

Schlenk, F. (Argonne National Laboratory, Argonne, Ill.), and J. L. Dainko. Action of ribonuclease preparations on viable yeast cells and spheroplasts. J. Bacteriol. 89:428-436. 1965.-Recrystallized pancreatic ribonuclease releases more than half of the ultraviolet-absorbing cellular constituents of Candida utilis and Saccharomyces cerevisiae. The presence of salts and buffer interferes with the action of the enzyme on the cytoplasmic membrane, and intracellular constituents show a similar inhibitory effect. Polymerized ribonuclease and conformational isomers cannot penetrate the pores of the yeast cell wall, but they are effective on the membrane of spheroplasts. All forms of the enzyme interfere strongly with the viability of the cells.

Proton pumping and the internal pH of yeast cells, measured with pyranine introduced by electroporation

The internal pH of yeast cells was determined by measuring the fluorescence changes of pyranine (8-hydroxy-1,3,6-pyrene-trisulfonic acid), which was introduced into the cells by electroporation. This may be a suitable procedure for the following reasons. (i) Only minor changes in the physiological status of the cells seemed to be produced. (ii) The dye did not seem to leak at a significant rate from the cells. (iii) Different incubation conditions produced large fluorescence changes in the dye, which in general agree with present knowledge of the proton movements of the yeast cell under different conditions. (iv) Pyranine introduced by electroporation seemed to be located in the cytoplasm and to avoid the vacuole, and therefore it probably measured actual cytoplasmic pH. (v) Correction factors to obtain a more precise estimation of the internal pH are not difficult to apply, and the procedure may be useful for other yeasts and microorganisms, as well as for the introduction of other substances into cells. Values for the cytoplasmic pHs of yeast cells that were higher than those reported previously were obtained, probably because this fluorescent indicator did not seem to penetrate into the cell vacuole.

Interactions of calcium and magnesium with the mitochondrial inorganic pyrophosphatase from Saccharomyces cerevisiae

The activity of the mitochondrial inorganic pyrophosphatase from Saccharomyces cerevisiae was measured in the presence of increasing concentrations of magnesium and calcium. Calcium pyrophosphate (dissociation constant Kd = 1.9 microM) inhibited pyrophosphatase by competition with magnesium pyrophosphate (Kd = 50 microM). The small movements of calcium detected in mitochondria from yeast may be physiologically significant for the control of inorganic pyrophosphatase activity and the concentration of pyrophosphate in the matrix of yeast mitochondria.

An energy-dependent efflux system for potassium ions in yeast

An efflux of potassium ions was demonstrated in mutants of yeast cells lacking a functional high affinity carrier system for monovalent cations. This efflux showed the following characteristics: (a) It was stimulated by the presence of a substrate, either glucose or ethanol. (b) It was stimulated by several cationic organic molecules, such as ethidium bromide, dihydrostreptomycin, diethylaminoethyldextran, and also by trivalent cations, such as Al3+ and lanthanides; this stimulation also depended on the presence of a substrate. (c) K+ efflux was decreased in yeast mutants with decreased ATPase activity, which generated a lower membrane potential. (d) Although the efflux appeared to be of an electrogenic nature, producing hyperpolarization of cells, it was accompanied by the efflux of phosphate, probably as an anion partially compensating for the large amount of cations leaving the cell. (e) K+ efflux was also accompanied by an uptake of protons. (f) The efflux appeared more clearly in cells grown in YPD medium, and not in more complex media nor in the same YPD medium if supplemented with Ca2+ or Mg2+. Efflux of monovalent cations produced by Tb3+ and organic cationic agents was also demonstrated in wild type strains. This efflux system appears to be, at least partially, electrogenic, but seems to be also an exchange system for protons and to function as a symport with phosphate; it may be involved in the regulation of the internal pH of the cell, and appears to be regulated by its link to the energetic status of the cell, probably through the membrane potential.

Determination of the physicochemical constants and spectrophotometric characteristics of the metallochromic Zincon and its potential use in biological systems

In this work we report a detailed characterization of the metallochromic Zincon. Zincon forms complexes with Zn2+ and Cu2+, producing change in colour; the complexes with Fe2+, Mn2+ and Ca2+ cause the bleaching of the Zincon solutions. Mg2+ does not interact with Zincon nor does it change its spectral characteristics. The presence of Ca2+ and Mg2+ does not interfere with the spectral characteristics of the Zn-Zincon complex. The Kd, Ks and delta epsilon values for the complexes were determined. The delta epsilon values were very high, making this spectrophotometric method very sensitive. The complex Zn-Zincon is fully reversible; however, the complex Cu-Zincon is only partially reversible. The free Zincon, and the complexes Zn-Zincon and Cu-Zincon, does not partition into organic solvents, does not permeate liposome membrane, and neither does it interact with biological membranes. All these characteristics make the metallochromic Zincon useful in biological systems.

Effects of cyclohexane, an industrial solvent, on the yeast Saccharomyces cerevisiae and on isolated yeast mitochondria

Little information on the effects of cyclohexane at the cellular or subcellular level is available. In Saccharomyces cerevisiae, cyclohexane inhibited respiration and diverse energy-dependent processes. In mitochondria isolated from S. cerevisiae, oxygen uptake and ATP synthesis were inhibited, although ATPase activity was not affected. Cyclohexane effects were similar to those reported for beta-pinene and limonene, suggesting that the cyclohexane ring in these monoterpenes may be a determinant for their biological activities.

Electrochemical potential and ion transport in vesicles of yeast plasma membrane

Vesicles from yeast plasma membrane were prepared according to Franzusoff and Cirillo [1983) J. Biol. Chem. 258, 3608), with slight modifications. When Mg-ATP was added, this preparation was able to generate a membrane potential, that was sensitive to inhibitors of the yeast H+-ATPase and uncouplers, and could be decreased by the addition of permeant anions, as measured by the fluorescence changes of the dye oxonol V. The addition of ATP could also generate a pH gradient, detectable by the fluorescence changes of the monitor aminochloromethoxyacridine. This gradient was sensitive to inhibitors of ATPase and uncouplers, and could be increased by the addition of permeant anions to the incubation mixture. When the vesicles were loaded with KCl, an increased rate of K+ efflux was produced upon the addition of ATP. Cytochrome oxidase from bovine heart could be reconstituted into the vesicles and was shown to generate a membrane potential difference, negative inside, evidenced by the fluorescence quenching of the cyanide dipropylthiacarbocyanine and the uptake of tetraphenylphosphonium. Besides, in these vesicles, K+ and Rb+, but not Na+ or NH+4 could decrease the quenching of fluorescence and the uptake of tetraphenylphosphonium produced when the electron-donor system was present. In the vesicles in which cytochrome oxidase was incorporated, upon the addition of cytochrome c and ascorbate, the uptake of 86Rb+ could be demonstrated also. This uptake was found to be saturable and inhibited by K+, and to a lesser degree by Na+. The results obtained indicate that these vesicles are reasonably sealed and capable of generating and maintaining a membrane potential. The membrane potential could be used to drive ions across the membrane of the vesicles, indicating the presence and functionality of the monovalent cation carrier. The vesicles, in general terms seem to be suitable for studying transport of ions and metabolites in yeast.

Early metabolic effects and mechanism of ammonium transport in yeast

Studies were performed to define the effects and mechanism of NH+4 transport in yeast. The following results were obtained. Glucose was a better facilitator than ethanol-H2O2 for ammonium transport; low concentrations of uncouplers or respiratory inhibitors could inhibit the transport with ethanol as the substrate. With glucose, respiratory inhibitors showed only small inhibitory effects, and only high concentrations of azide or trifluoromethoxy carbonylcyanide phenylhydrazone could inhibit ammonium transport. Ammonium in the free state could be concentrated approximately 200-fold by the cells. Also, the addition of ammonium produced stimulation of both respiration and fermentation; an increased rate of H+ extrusion and an alkalinization of the interior of the cell; a decrease of the membrane potential, as monitored by fluorescent cyanine; an immediate decrease of the levels of ATP and an increase of ADP, which may account for the stimulation of both fermentation and respiration; and an increase of the levels of inorganic phosphate. Ammonium was found to inhibit 86Rb+ transport much less than K+. Also, while K+ produced a competitive type of inhibition, that produced by NH4+ was of the noncompetitive type. From the distribution ratio of ammonium and the pH gradient, an electrochemical potential gradient of around -180 mV was calculated. The results indicate that ammonium is transported in yeast by a mechanism similar to that of monovalent alkaline cations, driven by a membrane potential. The immediate metabolic effects of this cation seem to be due to an increased [H+]ATPase, to which its transport is coupled. However, the carriers seem to be different. The transport system studied in this work was that of low affinity.

Effects of beta-pinene on yeast membrane functions

The effects of beta-pinene on yeast cells were studied. This terpene inhibited respiration with glucose or ethanol as the substrate. The inhibition depended on the ratio of the terpene to the amount of yeast cells; for a fixed concentration of pinene, inhibition decreased as the amount of yeast cells increased. Pinene also inhibited the pumping of protons and K+ transport, but this inhibition was more marked with with ethanol than with glucose as the substrate, indicating the mitochondrial localization of the inhibition. The studies on isolated mitochondria showed a series of effects, starting with the disappearance of the respiratory control and deenergization of the organelles and followed by an inhibition of respiration at higher concentrations of the terpene. The effect on respiration could be localized to the cytochrome b region of the electron transport chain. No effect could be detected on the activity of ATPase. The effects can be ascribed to a localization of pinene on membranes which was also accompanied by a decrease in the fluorescence polarization of diphenyl hexatriene, probably meaning an increase in the fluidity of the membrane, localized preferentially to the mitochondria.

Reverse transcription of yeast double-stranded RNA and ribosomal RNA using synthetic oligonucleotide primers

The ability of the four oligodeoxyribonucleotide primers oligo(dT)12-18, oligo(dA)12-18, oligo(dG)12-18 and oligo(dC)12-18 to act as primers for avian myeloblastosis virus reverse transcriptase on denatured yeast double-stranded (ds) RNA templates was investigated. Oligo(dT) and oligo(dA) were found to prime the synthesis of 1.1 and 1.0 kb reverse transcripts, respectively, using denatured M dsRNA as a template. The oligo(dT)- and oligo(dA)-primed cDNAs of M dsRNA hybridized to the region of the M dsRNA that encoded the killer toxin and to each other. Addition of oligo(dT) to reverse transcription reactions of denatured L dsRNA produced a 4.3 kb cDNA. During the course of this investigation oligo(dC) was observed to be a highly efficient primer for reverse transcription of yeast 18 S ribosomal RNA. Oligo(dC) primed the synthesis of a 1.0 kb transcript of 18 S rRNA which hybridized to the large Eco RI fragment of the 18 S rRNA gene. Reverse transcription of double-stranded RNA and 25 S ribosomal RNA was found to occur to some extent in the absence of added oligonucleotide primer.

Methylated nucleosides in polyadenylate-containing yeast messenger ribonucleic acid

Yeast messenger RNA was found to be methylated. A calculation of the specific methylation showed that the average yeast messenger RNA molecule contains only two methylated nucleosides which occur in one alkali stable oligonucleotide. Similar to other eukaryotic messengers, the 5' terminus of yeast messenger RNA is blocked by 7-methylguanosine, linked through a di- or triphosphate bridge to a ribosemethylated nucleoside. Contrary to the messengers of high eucaryotic organisms, no additional base methylated constituents were found.

Distribution of ribosomal ribonucleic acid cistrons among yeast chromosomes

High-molecular-weight deoxyribonucleic acid (DNA) of Saccharomyces carls bergensis has been fractionated by sucrose density gradient centrifugation. The main DNA fraction has an average molecular weight of about 500 x 10(6). A major fraction of the DNA molecules containing sequences homologous to ribosomal ribonucleic acid (RNA) sediments as material of this molecular weight. The remainder sediments as material of a molecular weight of about 250 x 10(6). The latter fraction contains relatively more ribosomal RNA cistrons than the former. Studies on the buoyant density of high-molecular-weight DNA homologous to ribosomal RNA have led to the conclusion that the ribosomal RNA cistrons occur in groups attached to a relatively large amount of nonribosomal RNA and suggest that ribosomal RNA cistrons are distributed over a number of yeast chromosomes.

Differential sensitivities of the two malate dehydrogenases and the maltose permease to the effect of glucose in Saccharomyces carlsbergensis

In Saccharomyces carlsbergensis the two malate dehydrogenase activities, which are localized in different compartments of the cell, were found to differ in their response to glucose. The cytoplasmic malate dehydrogenase activity appears to be sensitive to inactivation by very low concentrations of glucose. The mitochondrial malate dehydrogenase activity is only repressed at a higher glucose concentration. Maltose permease is also sensitive to inactivation by glucose. Conditions were found such that the maltose permease was present while the cytoplasmic malate dehydrogenase was inactivated. The different sensitivities of the two malate dehydrogenases and maltose permease to the effect of glucose may explain the preferential use of glucose, maltose, and products of glucose metabolism (2- and 3-carbon skeletons) as carbon sources for growth in the order as mentioned.

Reduction of nucleic acid content in Candida yeast cells by bovine pancreatic ribonuclease A treatment

Yeast as a source of protein for human consumption is limited by its relatively high nucleic acid content. In this study, we developed an enzymatic method of decreasing the nucleic acid content. Candida utilis cells, heat-shocked at 80 C for 30 sec, were treated with bovine pancreatic ribonuclease A. Maximum leakage of nucleic acid was observed when the incubation temperature was between 55 and 65 C, the pH of the system from 6.75 to 8.0, and the enzyme-to-cell ratio 1:10,000 on a weight-by-weight basis. Other factors, such as yeast strain, age of cells, and method of propagation, did not influence the susceptibility of the yeast cells to the action of ribonuclease. Buffers and monovalent cations had no inhibiting effects. Magnesium and calcium ions at concentrations greater than 0.001 m showed marked inhibition on the rate of nucleic acid leakage. This enzymatic method reduced the nucleic acid content of yeast cells from 7.5 to 9.0% to 1.5 to 2.0% with no significant concomitant loss of protein.

Osmotic regulation of invertase formation and secretion by protoplasts of Saccharomyces

Osmotic regulation of invertase formation and secretion by protoplasts of Saccharomyces was examined using sorbitol, KCl, NaCl, or magnesium sulfate as the osmotic support. The synthesis and secretion of the enzyme is remarkably sensitive to the osmolarity of the supporting medium irrespective of the particular support employed. Invertase formation was inhibited at high osmolarity and was maximal at 0.65 to 0.75 osmolal, even though some leakage of the intracellular enzyme alpha-glucosidase and of ultraviolet (UV)-absorbing materials occurred under these conditions. The reduction of invertase formation and secretion due to high osmolarity was eliminated promptly when protoplasts were transferred into a medium of lower osmolarity. The rate of fructose uptake and of threonine incorporation into protein was decreased by high osmolarity; also reduction of invertase formation could be partially reversed by increasing the level of sugar supplied as energy source. Thus changes in the permeability of the plasma membrane (and presumably also in its structure) are important factors in the response of protoplasts to high osmolarity, though certainly not the complete explanation. Protoplasts suspended in 0.8 m sorbitol, with 10mm fructose as the energy source, increased their invertase level 5- to 10-fold during a 2-hr incubation without substantial release of alpha-glucosidase or UV-absorbing materials. Both the large and small forms of invertase were present in the protoplasts, but only the large form was released into the medium when enzyme was being actively synthesized. Formation and secretion of newly formed invertase and the release of enzyme initially present were inhibited by cycloheximide.

Effects of cycloheximide and 5-fluorouracil on formation of low-molecular-weight ribonucleic acid in yeast

The effects of cycloheximide and 5-fluorouracil on the formation of low-molecular-weight ribonucleic acid (RNA) in yeast were investigated. Both compounds were found to affect the synthesis of low-molecular-weight RNA of ribosomal origin more than transfer RNA but less than the high-molecular-weight ribosomal RNA. 5-Fluorouracil-containing transfer RNA was separated from normal transfer RNA by chromatography on diethylaminoethyl cellulose at 80 C in the presence of 7 m urea.

Effects of 5-fluorouracil and 6-azauracil on the synthesis of ribonucleic acid and protein in Saccharomyces carlsbergensis

1. Some effects of 6-azauracil and 5-fluorouracil on protein and RNA synthesis in Saccharomyces carlsbergensis were studied. 2. Both analogues caused a severe inhibition of RNA formation, whereas protein synthesis was much less affected. 3. Induced alpha-glucosidase formation was only slightly impaired. 4. Both analogues caused an inhibition of ribosome formation, although 5-fluorouracil was far more effective. 5. In the presence of the latter analogue abnormal RNA of high molecular weight and of more DNA-like base composition accumulated. On reincubation in medium free of analogue but containing uracil the abnormal RNA disappeared and was replaced by the normally sedimenting high-molecular-weight RNA species.