Localization of sucrose and maltose fermenting systems in Saccharomyces cerevisiae

Optogenetic spatial patterning of cooperation in yeast populations

Microbial communities are shaped by complex metabolic interactions such as cooperation and competition for resources. Methods to control such interactions could lead to major advances in our ability to better engineer microbial consortia for synthetic biology applications. Here, we use optogenetics to control SUC2 invertase production in yeast, thereby shaping spatial assortment of cooperator and cheater cells. Yeast cells behave as cooperators (i.e., transform sucrose into hexose, a public good) upon blue light illumination or cheaters (i.e., consume hexose produced by cooperators to grow) in the dark. We show that cooperators benefit best from the hexoses they produce when their domain size is constrained between two cut-off length-scales. From an engineering point of view, the system behaves as a bandpass filter. The lower limit is the trace of cheaters' competition for hexoses, while the upper limit is defined by cooperators' competition for sucrose. Cooperation mostly occurs at the frontiers with cheater cells, which not only compete for hexoses but also cooperate passively by letting sucrose reach cooperators. We anticipate that this optogenetic method could be applied to shape metabolic interactions in a variety of microbial ecosystems.

Energy coupling in Saccharomyces cerevisiae: selected opportunities for metabolic engineering

Free-energy (ATP) conservation during product formation is crucial for the maximum product yield that can be obtained, but often overlooked in metabolic engineering strategies. Product pathways that do not yield ATP or even demand input of free energy (ATP) require an additional pathway to supply the ATP needed for product formation, cellular maintenance, and/or growth. On the other hand, product pathways with a high ATP yield may result in excess biomass formation at the expense of the product yield. This mini-review discusses the importance of the ATP yield for product formation and presents several opportunities for engineering free-energy (ATP) conservation, with a focus on sugar-based product formation by Saccharomyces cerevisiae. These engineering opportunities are not limited to the metabolic flexibility within S. cerevisiae itself, but also expression of heterologous reactions will be taken into account. As such, the diversity in microbial sugar uptake and phosphorylation mechanisms, carboxylation reactions, product export, and the flexibility of oxidative phosphorylation via the respiratory chain and H(+) -ATP synthase can be used to increase or decrease free-energy (ATP) conservation. For product pathways with a negative, zero or too high ATP yield, analysis and metabolic engineering of the ATP yield of product formation will provide a promising strategy to increase the product yield and simplify process conditions.

Engineering topology and kinetics of sucrose metabolism in Saccharomyces cerevisiae for improved ethanol yield

Sucrose is a major carbon source for industrial bioethanol production by Saccharomyces cerevisiae. In yeasts, two modes of sucrose metabolism occur: (i) extracellular hydrolysis by invertase, followed by uptake and metabolism of glucose and fructose, and (ii) uptake via sucrose-proton symport followed by intracellular hydrolysis and metabolism. Although alternative start codons in the SUC2 gene enable synthesis of extracellular and intracellular invertase isoforms, sucrose hydrolysis in S. cerevisiae predominantly occurs extracellularly. In anaerobic cultures, intracellular hydrolysis theoretically enables a 9% higher ethanol yield than extracellular hydrolysis, due to energy costs of sucrose-proton symport. This prediction was tested by engineering the promoter and 5' coding sequences of SUC2, resulting in predominant (94%) cytosolic localization of invertase. In anaerobic sucrose-limited chemostats, this iSUC2-strain showed an only 4% increased ethanol yield and high residual sucrose concentrations indicated suboptimal sucrose-transport kinetics. To improve sucrose-uptake affinity, it was subjected to 90 generations of laboratory evolution in anaerobic, sucrose-limited chemostat cultivation, resulting in a 20-fold decrease of residual sucrose concentrations and a 10-fold increase of the sucrose-transport capacity. A single-cell isolate showed an 11% higher ethanol yield on sucrose in chemostat cultures than an isogenic SUC2 reference strain, while transcriptome analysis revealed elevated expression of AGT1, encoding a disaccharide-proton symporter, and other maltose-related genes. After deletion of both copies of the duplicated AGT1, growth characteristics reverted to that of the unevolved SUC2 and iSUC2 strains. This study demonstrates that engineering the topology of sucrose metabolism is an attractive strategy to improve ethanol yields in industrial processes.

Analysis of gene expression in yeast protoplasts using DNA microarrays and their application for efficient production of invertase and α-glucosidase

The global gene expression of cultured Saccharomyces cerevisiae protoplasts was compared with that of cells using DNA microarray. Quantitative and qualitative analyses revealed that after 6 h of cultivation, 416 gene transcript levels (about 7.1% in all) in the cultured protoplasts were different from those in the cells. Various characteristics and functions of the protoplasts were predicted from the analysis of the gene functions. The cultured protoplasts were more sensitive to oxidative stress than the cultured cells. Their cell cycles were arrested at the G1 phase and cell wall synthesis was promoted. Carbohydrate metabolism was activated in cultured protoplasts, while amino acid biosynthesis was inhibited. Furthermore, some genes associated with the secretory pathway of metabolites were activated, leading to active secretion of these metabolites into the broth. As an example of the application of DNA microarray analysis, we developed two novel methods for the production of useful enzymes based on the characteristics of protoplasts. One was the production of invertase based on the activated secretory pathway, while the other was the production of alpha-glucosidase based on the activated carbohydrate metabolism. The secretion of invertase and alpha-glucosidase was promoted in cultured protoplasts. The invertase and alpha-glucosidase productivities in the cultured protoplasts were 657 U and 218 U, respectively. On the other hand, only 227 U of invertase was produced, while alpha-glucosidase was not detected, in the cultured cells. The fragile protoplasts were immobilized in agarose gel to protect them from hydrodynamic stress. Four repeated-batch cultures with the immobilized protoplasts were performed, leading to the production of 1574 U of invertase and 739 U of alpha-glucosidase. The same productivities were obtained when this system was scaled up by 10-fold (invertase: 13304 U; alpha-glucosidase: 7688 U).

Inhibition by antibiotics of the growth of bacterial and yeast protoplasts

Shockman, Gerald D. (Temple University School of Medicine, Philadelphia, Pa.) and J. Oliver Lampen. Inhibition by antibiotics of the growth of bacterial and yeast protoplasts. J. Bacteriol. 84:508-512. 1962.-The characteristics and requirements for growth of bacterial (Streptococcus faecalis) and yeast (Saccharomyces cerevisiae) protoplasts were established and the effect of a variety of antibacterial and antifungal antibiotics determined. A clear differentiation was obtained between such inhibitors of bacterial cell wall synthesis as penicillin and cycloserine, which did not prevent protoplast growth, and all others, antibacterial and antifungal, which inhibited protoplasts and intact organisms at the same range of concentration. Novobiocin, previously reported to inhibit bacterial wall synthesis, was also effective against a reaction(s) essential to the growth of S. faecalis protoplasts. The antibacterial action of streptomycin, neomycin, and kanamycin was essentially eliminated by the high salt concentration needed to maintain the protoplasts. Removal of the cell wall did not significantly increase antibiotic susceptibility of a resistant species. Protoplasts of Bacillus megaterium were insensitive to the antifungal agent, nystatin, and did not bind it to any detectable degree. Thus, the yeast or bacterial cell wall does not appear to play a major role in determining relative antibiotic susceptibility by masking internal sensitive target sites. A variety of antifungal antibiotics tested on the growth of log-phase yeast cells failed to produce osmotically fragile forms.

EVIDENCE FOR AN EXOCELLULAR SITE FOR THE ACID PHOSPHATASE OF SACCHAROMYCES MELLIS

Weimberg, Ralph (Northern Regional Research Laboratory, Peoria, Ill.), and William L. Orton. Evidence for an exocellular site for the acid phosphatase of Saccharomyces mellis. J. Bacteriol. 88:1743-1754. 1964.-Evidence is presented which demonstrates an exocellular location for acid phosphatase in Saccharomyces mellis. Derepressed intact cells exhibit acid phosphatase activity. The properties of the system are similar to those shown by the enzyme in cell-free extracts. There is no increase in total activity when cell-free extracts are prepared. Enzymatically active cell walls were prepared by leaching acetone-dried cells of this yeast in dilute acetate buffer (pH 6.5) plus beta-mercaptoethanol. The insoluble residue, consisting mainly of cell-wall material and containing the phosphatase, was treated with a variety of hydrolytic enzymes and other chemicals. Only papain and crude snail gut extracts dissociated the enzyme from the particulate fraction in nearly quantitative amounts. The mechanism of release by these two enzymes probably differs. Of all enzymes tested, only the snail gut extract digested the cell walls. By dividing the procedure for making protoplasts of S. mellis into two steps, acid phosphatase may be dissociated from resting cells and recovered as an active soluble enzyme. The first step is to pretreat the cells with a thiol reagent. The second step is to digest the cell wall by enzymes present in crude snail gut extracts. Arsenite must be included in the second step to protect the phosphatase from inactivation. The phosphatase is quantitatively released before the cell becomes osmotically fragile.

Location of the beta-galactosidase of the yeast Kluyveromyces marxianus var. marxianus ATCC 10022

During the growth of Kluyveromyces marxianus var. marxianus ATCC 10022 on lactose, peaks of glucose, but not beta-galactosidase activity, were detected in culture medium. Harvested and washed whole cells produced glucose and galactose from lactose, or ortho-nitro-phenol from the chromogenic substrate ortho-nitro-phenyl-beta-D-galactopyranoside (ONPG), indicating that beta-galactosidase is physically associated with cells. ONPG hydrolysis by whole cells presented a monophasic kinetics (Km 36.6 mM) in lactose exponential growth phase cells, but a biphasic kinetics (Km 0.2 and 36.6 mM) in stationary growth phase cells. Permeabilization with digitonin or disruption of cells from both growth phases led to monosite ONPG hydrolysis (Km 2.2 to 2.5 mM), indicating that beta-galactosidase is not located in the periplasm. In addition, the energy inhibitors fluoride or arsenate, as well as the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) prevented ONPG hydrolysis by whole cells. These findings indicate that energy coupled transmembrane transport is the rate-limiting step for intracellular ONPG cleavage. The taxonomic and physiologic implications of the exclusive intracellular location of beta-galactosidase of K. marxianus var. marxianus ATCC 10022 are discussed.

Purification and characterisation of an Aspergillus niger invertase and its DNA sequence

A secreted invertase was purified 23-fold by ultrafiltration, ion-exchange, and gel filtration chromatography from the culture supernatant of 18 h sucrose-grown cultures of Aspergillus niger. The purified enzyme hydrolysed sucrose and raffinose but there was no detectable hydrolysis of inulin, melezitose or PNPG. Invertase activity was optimal at pH 5.5 and 50 degrees C. The molecular mass of reduced invertase was 115 kDa, as determined by SDS gel electrophoresis. The native molecular weight of between 225 kDa and 250 kDa, estimated by electrophoresis under non-denaturing conditions, suggests that the protein is a dimer of identical subunits. The suc1 gene encoding this protein was completely-sequenced. The translated sequence yields a protein of 566 amino acids with a calculated molecular mass of 61 kDa, suggesting that carbohydrates represent about 50% of the mass of the protein.

Extracellular Enzymes Produced by the Cultivated Mushroom Lentinus edodes during Degradation of a Lignocellulosic Medium

Although the commercially important mushroom Lentinus (= Lentinula ) edodes (Berk.) Sing. can be rapidly cultivated on supplemented wood particles, fruiting is not reliable. This study addressed the problem by developing more information about growth and development on a practical oakwood-oatmeal medium. The study determined (i) the components degraded during a 150-day incubation at 22�C, (ii) the apparent vegetative growth pattern, (iii) the likely growth-limiting nutrient, and (iv) assays that can be used to study key extracellular enzymes. All major components of the medium were degraded, lignin selectively so. The vegetative growth rate was most rapid during the initial 90 days, during which weight loss correlated with glucosamine accumulation (assayed after acid hydrolysis). The rate then slowed; in apparent preparation for fruiting, the cultures rapidly accumulated glucosamine (or its oligomer or polymer). Nitrogen was growth limiting. Certain enzyme activities were associated with the pattern of medium degradation, with growth, or with development. They included cellulolytic system enzymes, hemicellulases, the ligninolytic system, (gluco-)amylase, pectinase, acid protease, cell wall lytic enzymes (laminarinase, 1,4-β- d -glucosidase, β- N -acetyl- d -glucosaminidase, α- d -galactosidase, β- d -mannosidase), acid phosphatase, and laccase. Enzyme activities over the 150-day incubation period with and without a fruiting stimulus are reported. These results provide a basis for future investigations into the physiology and biochemistry of growth and fruiting.

Ultrastructure of protoplasts from mycelium and microconidia of Trichophyton mentagrophytes

Protoplast formation from mycelium and microconidia of Trichophyton mentagrophytes was achieved with Novozym 234. Pretreatment procedures with dithiothreitol or urea mercaptoethanol sodium lauryl sulphate before digestion with Novozym 234 greatly reduced protoplast yield from mycelium. Snail gut enzyme did not protoplasts in good yield. Scanning electron microscopy of mycelium protoplasts showed the acquired spherical shape. The plasma membrane appeared finely granular although remnants of cell wall could sometimes be observed. Transmission electron microscopy showed the cell interior of these protoplasts was plasmolysed. Microconidia treated with Novozym 234 displayed a range of cell wall digestion, with intact protoplasts showing distinct cytoplasmic organelles.

Efficient synthesis of enzymatically active calf chymosin in Saccharomyces cerevisiae

We have constructed a high-efficiency expression vector to direct the synthesis of heterologous polypeptides in yeast. The vector is termed a sandwich expression vector as the heterologous gene is inserted between the 5' and 3' control regions of the efficiently expressed yeast PGK gene. We have used this vector to direct the expression of three derivatives of the calf chymosin cDNA gene; preprochymosin, prochymosin and chymosin. Prochymosin is synthesised to at least 5% of total yeast-cell protein and furthermore, it can be readily activated to produce an enzyme which has milk-clotting activity.

Extracellular protein release and its response to pH level in Saccharomyces cerevisiae

Saccharomyces cerevisiae grown in batch culture at pH 5.5 releases 0.1 to 0.2 pg protein per cell to the external medium over a period of four to five days, final concentration 20-40 micrograms/ml. Cells grown at pH 3.0 release 10-fold this quantity (1-2 pg/cell, final concentration 100-200 micrograms/ml). A kinetic model based on published behavior of periplasmic protein gave a good fit to the observed kinetics of exoprotein yield. The electrophoretic pattern of exoprotein differed from that of cell lysate protein, and exoprotein synthesis was apparently limited to early stages of the life cycle. These results are consistent with the identification of exoprotein as periplasmic protein released to the external medium through the cell wall. Analysis of the observed kinetics of exoprotein yield, utilizing the kinetic model suggests that the greater exoprotein production of cells grown at pH 3.0 was due entirely to greater synthesis of periplasmic proteins while the fraction of periplasmic protein released per unit time was greater for cells grown at pH 5.5. The latter conclusion is supported by thicker cell walls of cells grown at pH 3.0 as observed by electron microscopy. At an applied level the apparent limitation of exoprotein synthesis to the first few hours of cell life, the slow leakage of exoprotein through the cell wall, and the dilute nature of a yeast suspension do not favor the utilization of yeast cells for direct conversion of substrate into protein released to the external medium.

Mutant defective in processing of an enzyme located in the lysosome-like vacuole of Saccharomyces cerevisiae

Carboxypeptidase Y, a vacuolar enzyme in Saccharomyces cerevisiae, is synthesized as a larger precursor whose apparent molecular mass is approximately 67,000 daltons. We have characterized a recessive mutation, pep4-3, that prevents maturation of this precursor. The accumulated precursor does not possess enzymatic activity. We have shown that the precursor accumulating in the pep4-3 mutant is not produced in a doubly mutant strain that also bears a mutation in the carboxypeptidase Y structural gene that eliminates production of carboxypeptidase Y. We have also shown that a nonsense fragment of carboxypeptidase Y is processed. Although there is evidence that proteinase B can catalyze the conversion of the precursor to a mature form in vitro, nonsense mutations in the structural gene for proteinase B, PRB1, do not affect the levels of carboxypeptidase Y activity, and strains bearing these mutations produce a carboxypeptidase Y of apparently normal size. Hence, proteinase B is not essential for the maturation of carboxypeptidase Y precursor in vivo. The pep4-3 mutation affects at least five vacuolar enzymes. This suggests that there is a processing event common to all of these enzymes.

External and internal forms of yeast aminopeptidase II

1. Intact cells of Saccharomyces cerevisiae catalyze the hydrolysis of various aminopeptidase substrates. This activity is not due to permeation of substrates and products but exerted by an external enzyme. 2. From its substrate specificity and the effects of pH and inhibitors the enzyme was identified as aminopeptidase II. 3. About 40% of total aminopeptidase II activity is detectable with untreated exponentially growing cells. Up to two thirds of the external enzyme is released into the medium during enzymic digestion of the cell wall, while little enzyme is liberated by osmotic shock. Membrane preparations contained only small amounts of aminopeptidase II; thus, the localization of the external enzyme appears to be similar to that of the so-called 'periplasmic' yeast hydrolases. 4. By cytochemical methods the presence of aminopeptidase II in the cell envelope was visualized. 5. In contrast to aminopeptidase II, yeast dipeptidase is an entirely intracellular enzyme.

An evaluation of D-glucosamine as a gratuitous catabolite repressor of Saccharomyces carlsbergensis

Glucose represses mitochondrial biogenesis and the fermentation of maltose, galactose and sucrose in yeast. We have analyzed the effect of D-glucosamine on these functions in order to determine if it can produce a similar repression. It was found that glucosamine represses the respiration rate (QO2) but more rapidly than glucose and to a final level slightly higher than in glucose-treated cells. Derepression of the respiration rate following either glucose or glucosamine repression was similar. A two hour lag was followed by a linear increase in QO2 to the derepressed level. Both glucose and glucosamine repressed the level of cytochrome oxidase to the same level. Glucosamine was also found to repress maltose and galactose fermentation but not sucrose fermentation. The derepression of maltase synthesis was inhibited by glucosamine. The constitutive synthesis of maltase was repressed by the addition of glucosamine. Glucosamine was judged to produce a repressed state similar to glucose repression in many respects.

Large and small invertases and the yeast cell cycle. Pattern of synthesis and sensitivity to tunicamycin

We have examined the pattern of synthesis of the glycoprotein form of invertase and of the smaller carbohydratefree from in synchronous culture to obtain further infromation concerning their biosynthetic relationship. Saccharomyces mutant 1710 was chosen since its invertase production is almost completely derepressed during growth in 0.1 M mannose medium. The large enzyme, unlike the small form, binds to concanavalin A-Sepharose, and on this basis the two types can conveniently be separated for analysis. Large invertase was produced throughout the cell cycle. Synthesis of the small invertase was periodic; the single burst occurred at or close to the budding stage. Tunicamycin, which inhibits the sypthesis of external glycoproteins, halted formation of the large enzyme but not of the small form, and there was no accumulation of invertase activity with the properties of the small enzyme. Hence, it is unlikely that the small form is a precursor of the large one. Despite marked differences in their amino acid compositions, the two enzymes have many similarities. They are probably, in part, the products of the same gene(s), and the differences between them may largely reflect differences in post-translational processing.

Use of snail enzyme for the selection of glucose-and mannose-negative mutants in Saccharomyces cerevisiae

The relationship between growth and sensitivity of Saccharomyces cerevisiae cells to snail enzyme led us to work out the conditions for using this enzyme to isolate mannose-negative and glucose-negative mutants. The technique is based on a prior growth on a glycerol medium in which any cell becomes resistant to snail enzyme, followed by a period of growth in a medium in which only the wild-type cells can grow, thereby becoming sensitive to the enzyme. This technique was very effective in both tested cases. This result suggests that the method described might also be applied to isolate other kinds of mutant.

The relationship between enzyme activity, cell geometry, and fitness in Saccharomyces cerevisiae

The relationship between enzyme activity, cell geometry, and the ploidy levels has been investigated in Saccharomyces cerevisiae. Diploid cells have 1.57 times the volume of haploid cells under nonlimiting growth conditions (minimal medium). However, when diploid cells are grown under conditions of carbon limitation, they have the same volume as haploid cells. Thus, by altering the environmental conditions, cell size can be varied independently of the degree of ploidy. The results indicate that the basic biochemical parameters of the cell are primarily determined by cell geometry rather than ploidy level. RNA content, protein content, and ornithine transcarbamylase (carbamoylphosphate: L-ornithine carbamoyltransferase, EC 2.1.3.3), tryptophan synthetase [L-serine hydro-lyase (adding indole), EC 4.2.1.20], and invertase (alpha-D-glucoside glucohydrolase, Ec 3.2.1.20) activity are related to cell volume, whereas acid phosphatase (orthophosphoric-monoester phosphohydrolase, EC 3.1.3.2) activity, a cell surface enzyme, is related to the surface area of the cells. Fitness is determined by the activity of certain cell surface enzymes, such as acid phosphatase, diploids would be expected to have a lower fitness than haploids because of the lower surface area/volume ratio. However, when fitness is determined by the activity of an internal enzyme, diploids would be expected to have the same fitness as haploids. Results from competition experiments between haploids and diploids are consistent with these predictions. The significance of these results to the evolution of diploidy as the predominant phase of the life cycle of higher plants and animals is discussed.

Molecular forms of yeast invertase

The molecular forms of yeast invertase have been studied. It is shown that by gel filtration on Sephadex G-200 it is possible to demonstrate the presence not only of a light, carbohydrate-free, invertase, and a heavy invertase containing 50% carbohydrate, but also of a continuous spectrum of molecular forms that probably represent the sequential addition of mannose to the light form during the secretion process, which culminates in the formation on the heavy enzyme that is found outside the cytoplasmic membrane. The elution volume-void volume ratio in Sephadex G-200 varies from 1.75 of the light to 1.05 of the heavy invertase. The separation of invertase has also been achieved by ion-exchange chromatography and by isoelectric focusing and is facilitated by removal of the heavy form by ammonium sulphate precipitation. During the protoplasting process the removal of the cell wall is accompanied by the loss of most of the heavy form. Thintermediate forms are exclusively detected inside the protoplast, together with the light invertase and a small amount of heavy invertase. The effect of 2-deoxy-D-glucose and cycloheximide on the biosynthesis and distribution of molecular forms of yeast invertase has also been studied. In the presence of 10 mM glucose Saccharomyces 303-67 repressed cells readily synthesize invertase during the two-hour incubation period. Upon the addition of 2-deoxy-D-glucose, at a concentration of 75 mu g/ml, the observed inhibition in the cells is 60%, but if the activity is measured after breaking the cells, only a 31% inhibition is found, revealing an accumulation of invertase inside the protoplast. 2-Deoxy-D-glucose originates a pile-up of the light and intermediate forms at the expense of the formation of the heavy enzyme, showing that the inhibition of the glycosilation and, therefore, the secretion process, has taken place. In the absence of de novo invertase synthesis originated by cycloheximide, the glycosilation process still takes place as indicated by the accumulation of the heavy form at the expense of the light, carbohydrate-free, enzyme.

The absorption of protons with specific amino acids and carbohydrates by yeast

1. Proton uptake in the presence of various amino acids was studied in washed yeast suspensions containing deoxyglucose and antimycin to inhibit energy metabolism. A series of mutant strains of Saccharomyces cerevisiae with defective amino acid permeases was used. The fast absorption of glycine, l-citrulline and l-methionine through the general amino acid permease was associated with the uptake of about 2 extra equivalents of protons per mol of amino acid absorbed, whereas the slower absorption of l-methionine, l-proline and, possibly, l-arginine through their specific permeases was associated with about 1 proton equivalent. l-Canavanine and l-lysine were also absorbed with 1-2 equivalents of protons. 2. A strain of Saccharomyces carlsbergensis behaved similarly with these amino acids. 3. Preparations of the latter yeast grown with maltose subsequently absorbed it with 2-3 equivalents of protons. The accelerated rate of proton uptake increased up to a maximum value with the maltose concentration (K(m)=1.6mm). The uptake of protons was also faster in the presence of alpha-methylglucoside and sucrose, but not in the presence of glucose, galactose or 2-deoxyglucose. All of these compounds except the last could cause acid formation. The uptake of protons induced by maltose, alpha-methylglucoside and sucrose was not observed when the yeast was grown with glucose, although acid was then formed both from sucrose and glucose. 4. A strain of Saccharomyces fragilis that both fermented and formed acid from lactose absorbed extra protons in the presence of lactose. 5. The observations show that protons were co-substrates in the systems transporting the amino acids and certain of the carbohydrates.

Saccharomyces mutants with invertase formation resistant to repression by hexoses

Production of invertase by many strains of yeast is repressed in the presence of hexoses. This phenomenon interferes with studies on the secretion of invertase and with the preparation of large quantities of the enzyme for examination of its chemical and physical characteristics. Saccharomyces strain 303-67, a diploid carrying the single gene SUC-2 for (hexose repressible) invertase production, was subjected to ultraviolet irradiation. No single-step mutations to high level resistance were detected. By a two-step irradiation process mutants were obtained with differing degrees of resistance. The biochemical and genetic characteristics of these mutants are summarized with particular emphasis on FH4C (the most resistant). Although the steady state level of cyclic 3', 5'-adenosine monophosphate (cyclic AMP) was usually slightly higher in cells grown in low- rather than in high-glucose media, the level of cyclic AMP was not correlated with the sensitivity of invertase synthesis to glucose repression. In mutant FH4C, 1 to 2% of the total cell protein is present as invertase; synthesis of alpha-glucosidase is also resistant to repression by hexoses. This mutant does not sporulate and is probably a haploid of a-mating type with low frequency of conjugation and poor viability of conjugants. Mutants 1016 and 1710 are substantially resistant to hexose repression and still sporulate well. They may be useful for genetic analysis of hexose resistance.

Location of acid phosphatase and -fructofuranosidase within yeast cell envelopes

After 16 hr of incubation in a low-phosphate, aerated medium, bakers' yeast was obtained with a high titer of acid phosphatase (EC 3.1.3.2) and beta-fructofuranosidase (EC 3.2.1.26). All of the beta-fructofuranosidase and 75% of the acid phosphatase were easily released by mechanical disruption in a French pressure cell. The cell wall suffered a limited number of cracks, but this was sufficient for the co-release of these enzymes. Both enzymes were subject to autolytic release, although correlation was inconclusive because of the relative instability of acid phosphatase. The data are consistent with the bulk of the two enzymes being located in the periplasmic space. Ethylacetate treatments yielded ghosts with high beta-fructofuranosidase but low acid phosphatase activities. The surviving acid phosphatase was not representative of that in live cells. It was resistant to release by mechanical disruption and showed a high susceptibility to heat inactivation. The beta-fructofuranosidase in live cells and in ethylacetatetreated cells exhibited polydispersity in heat inactivation susceptibility; but the kinetics were indistinguishable, and facile release by mechanical disruption was shown in both cases.

Inhibition by 2-deoxy-D-glucose of synthesis of glycoprotein enzymes by protoplasts of Saccharomyces: relation to inhibition of sugar uptake and metabolism

The synthesis of the glycoprotein enzymes, invertase and acid phosphatase, by protoplasts of Saccharomyces mutant 1016, is inhibited by 2-deoxy-d-glucose (2-dG) after a 20- to 30-min lag period under conditions (external sugar to 2-dG ratio of 40:1) which cause only a slight decrease in total protein synthesis. Formation of one intracellular enzyme, alpha-glucosidase, is also sensitive, but production of another, alkaline phosphatase, is unaffected. A nonmetabolized glucose analogue, 6-deoxy-d-glucose, had no inhibitory effect. The total uptake of external fructose and maltose was decreased by 2-dG after a lag period of about the same duration as that before the inhibition of synthesis of enzymes or of mannan and glucan; during this time 2-dG was taken up by the protoplasts and accumulated primarily as 2-dG-6-phosphate (2-dG-6-P). Studies in vitro showed that 2-dG-6-P inhibits both yeast phosphoglucose isomerase and phosphomannose isomerase. The intracellular levels of the 6-phosphates of glucose, fructose, and mannose did not increase in the presence of 2-dG. We suggest that the high internal level of 2-dG-6-P blocks synthesis of the cell wall polysaccharides and glycoproteins in two ways. It directly inhibits the conversion of fructose-6-P to glucose-6-P and to mannose-6-P. At the same time, it restricts the transport of fructose and maltose into the cell; however, the continuing limited uptake of the sugars still provides sufficient energy for protein synthesis. The cessation of alpha-glucosidase synthesis is probably a result of depletion of the internal pool of maltose (the inducer). Our findings support the suggestion that restriction of synthesis of the carbohydrate moiety of glycoproteins reduces formation of the active enzyme.

Cold osmotic shock in Saccharomyces cerevisiae

Saccharomyces cerevisiae NCYC 366 is susceptible to cold osmotic shock. Exponentially growing cells from batch cultures grown in defined medium at 30 C, after being suspended in 0.8 m mannitol containing 10 mm ethylenedia-minetetraacetic acid and then resuspended in ice-cold 0.5 mm MgCl(2), accumulated the nonmetabolizable solutes d-glucosamine-hydrochloride and 2-aminoisobutyrate at slower rates than unshocked cells; shocked cells retained their viability. Storage of unshocked batch-grown cells in buffer at 10 C led to an increase in ability to accumulate glucosamine, and further experiments were confined to cells grown in a chemostat under conditions of glucose limitation, thereby obviating the need for storing cells before use. A study was made of the effect of the different stages in the cold osmotic shock procedure, including the osmotic stress, the chelating agent, and the cold Mg(2+)-containing diluent, on viability and solute-accumulating ability. Growth of shocked cells in defined medium resembled that of unshocked cells; however, in malt extract-yeast extract-glucose-peptone medium, the shocked cells had a longer lag phase of growth and initially grew at a slower rate. Cold osmotic shock caused the release of low-molecular-weight compounds and about 6 to 8% of the cell protein. Neither the cell envelope enzymes, invertase, acid phosphatase and l-leucine-beta-naphthylamidase, nor the cytoplasmic enzyme, alkaline phosphatase, were released when yeast cells were subjected to cold osmotic shock.

Secretion of cell-wall glycoproteins by yeast protoplasts. Effect of 2-deoxy-D-glucose and cycloheximide

The effect of 2-deoxy-d-glucose and cycloheximide on the synthesis and secretion of the cell-wall constituents protein and mannan in yeast protoplasts was examined in detail. Although the 2-deoxy-d-glucose hardly influenced protein synthesis, a significant parallel inhibition of carbohydrate and protein secretion into the medium was observed. The mechanism of this inhibition is considered as an interference of metabolites of 2-deoxy-d-glucose with the synthesis of yeast mannan. Cycloheximide, which is an effective inhibitor of protein synthesis in yeast (Kerridge, 1958), inhibited the secretion of non-diffusible carbohydrate in yeast protoplasts, but on the other hand had no effect on the activity of particulate yeast mannan synthetase. Our results clearly show that blocking the synthesis of either part of the mannan-protein complex prevents the extracellular appearance of the other component. The nature of this phenomenon is discussed.

The lipolytic activities of the isolated cell envelope fracttions of Baker's yeast

1. The existence of phospholipase and lipase activities in the isolated cell envelopes of baker's yeast was demonstrated. 2. The content of phospholipase was found to be markedly higher than that of lipase. 3. After partial enzymic digestion of the isolated cell envelopes, the bulk of the lipolytic activities was recovered in the sedimentable preparations, which consisted of the fragments of the plasma membrane. 4. During repeated washings, the lipase was completely released from the cell envelopes, as were also the bulk of the lipid components and most of the Mg(2+)-dependent adenosine triphosphatase, an enzyme connected with the plasma membrane. The phospholipase was more firmly bound to the preparation but not so firmly as the external saccharase. 5. These results indicate that the lipolytic enzymes found in the cell envelopes are mostly located in the plasma membrane.