Chapter XXIV Production and Use of Fungal Protoplasts

Regeneration and molecular characterization of an intergeneric hybrid between Graphium putredinis and Trichoderma harzianum by protoplasmic fusion

The fungal strains Graphium putredinis and Trichoderma harzianum were selected as parents for fusant development. Protoplasts were isolated using the combination of lysing enzymes Novozym 234 and cellulase with 0.6M KCl as osmotic stabilizer. The optimum conditions for release of viable protoplasts from the fungal mycelium viz. age of the mycelium, lytic enzymes, osmotic stabilizers, pH, incubation period and regeneration medium were determined. Intergeneric protoplast fusion was carried out using 50% polyethylene glycol with calcium chloride (CaCl(2)) and glycine buffer and the conditions for effective protoplast fusion, viz. fusogen, osmotic stabilizer, pH, incubation period and regeneration medium were optimized. At optimum conditions, the regeneration frequency of the fused protoplasts on potato dextrose agar (PDA) medium and fusion frequency were calculated. The regeneration frequency on non-selective (PDA) and selective media (PDA amended with starch) was determined for the parental and fusant strains in which, fusant showed a higher rate of regeneration. Fusant formation was confirmed by morphological markers (colony morphology and spore size and shape) and genetical markers like, mycelial protein pattern, restriction digestion pattern and random amplified polymorphic DNA (RAPD) analysis. The efficiency of these parental strains and their intergeneric fusant in the production of hydrolytic enzymes - amylases (treatment plant for sago factory effluent), cellulases (bioethanol), xylanases (bleaching agents for waste paper pulp) and proteases (additives in commercial detergents) - have probable applications in various industrial processes.

Mutagenesis of Protoplasts and Regeneration of Mycelium in the Mushroom Volvariella volvacea

Regenerating protoplasts were obtained from mycelial culture of the mushroom Volvariella volvacea by the action of the lytic enzyme Novozym 234 in the presence of 0.01 M phosphate buffer (pH 6.0) containing 0.6 M NaCl. Regeneration was found to be poor in liquid medium, but more than 50% regeneration was achieved on solid 2% agar medium overlaid with 0.5% agar. Protoplasts of V. volvacea were found to be highly sensitive to the killing action of both UV irradiation and N -methyl- N ′-nitro- N -nitrosoguanidine. However, no morphological or auxotrophic mutants could be obtained from protoplasts by chemical mutagenesis. Four types of morphological mutants and one auxotrophic (adenine-negative) mutant were obtained from UV-irradiated protoplasts. The adenine-negative mutant of V. volvacea was found to be stable, not losing auxotrophy on repeated subculture.

Protoplasts of filamentous fungi in genetics and metabolite production

Fungal protoplasts are interesting structures for genetic studies. They can be fused and their fusion products can revert to osmotic stable cells. The ability to fuse protoplasts of diverse origin makes them versatile tools in fundamental and applied genetics. Intraspecies protoplast fusion provides an efficient method to induce the parasexual cycle, making genetic analysis as well as strain breeding through mitotic recombination feasible in all kinds of species, regardless the occurrence of parasexuality by conventional means. Interspecies protoplast fusion allows the generation of different types of hybrids, depending on somatic and/or nuclear compatibility. Crosses between closely related species resulted in the formation of stable haploid recombinants as the consequence of genetic processes similar as occurring in the intraspecies parasexual cycle. From fusions between less related species hybrid progeny with differences in morphology and stability arose. Unstable hybrids segregated to other hybrid progeny or to one of the parental species, without recovery of the other parent. The exact genetic background of these hybrids remained obscure. Several studies demonstrated that the novel genetic combination arisen after interspecies protoplast fusion could result in changed gene expression and in the synthesis of novel or hybrid molecules.

Distribution of beta-glucanases within the genus Bacillus

Representative strains (368) from 36 species in the genus Bacillus were screened for the secretion of beta-glucanases. (1 leads to 6)-beta-glucanases active on pustulan were produced by a minority of the organisms studied (4%), but (1 leads to 3)-beta-glucanases which hydrolyzed laminarin and pachyman were more widespread and were secreted by 56 and 44% of the strains, respectively.

Isolation and characterization of protoplasts from Saccharomyces rouxii

Cells of the osmotolerant yeast Saccharomyces rouxii were transformed to protoplasts in good yield (85%) by digesting cell walls with snail-gut enzyme in the presence of 10 mM dithioerythritol, 0.1 M sodium phosphate buffer (pH 6.8), and 2.0 M KCl. The requirement for 2.0 M KCl compares with that for S. bisporus var. mellis (another osmotolerant species) and contrasts with the 0.3 to 0.8 M KCl concentrations used in the preparation of most yeast protoplasts. Short digestions (60 min or less) produced mostly spheroplasts; longer incubations (90 min or more) yielded mostly protoplasts as judged by electron micrographs. These protoplasts could be transferred to 1.0 M KCl or 2.0 M sorbitol without lysing, but lysis was pronounced in 0.5 M KCl or 1.0 M mannitol and complete in 0.02 M KCl. Protoplasts were separated from isolated cell wall remnants and debris by centrifugation on a linear gradient of Ficoll 400 (35 to 17.5%, wt/vol) containing 2.0 M KCl. Both crude and fractionated protoplast preparations contained vesicles which were identified with the periplasmic bodies of whole cells. Some of the periplasmic bodies were connected to protoplasts by fine pedicels; others appeared free. Independent degeneracy of periplasmic bodies was occasionally observed. beta-Fructofuranosidase (EC 3.2.1.26) activity is cryptic (physically) in cells of S. rouxii in contrast to the expressed enzyme (periplasmic space) of other Saccharomyces species. This enzyme remains cryptic in protoplast preparations of S. rouxii but is expressed upon lysis. The same specific activities were found per unit cell or protoplast. The possible association of the cryptic enzyme with periplasmic bodies is discussed.

The use of fungal protoplasts in the study of aflatoxin biosynthesis

Protoplasts derived from Aspergillus flavus are shown to be capable of synthesizing aflatoxins when incubated in a chemically defined medium. 14C-Acetate and 14C-Versicolorin A, added to protoplasts from 3-day-old mycelium, are incorporated into aflatoxin B1.

Permeability of the cell envelope and osmotic behavior in Saccharomyces cerevisiae

Bakers' yeast (Saccharomyces cerevisiae) was equilibrated with distilled water and then packed into standardized pellets by centrifugation. The fractional space (S value) that was accessible to passive permeation was probed with a variety of mono- and divalent salts, mono- and disaccharides, polyols, substrates and products of beta-fructofuranosidase (EC 3.2.1.26) and acid phosphatase (EC 3.1.3.2), and a cross-linked polymer of sucrose (Ficoll 400). A simple but very reproducible method was developed to measure pellet volume. At the limit of zero osmolality for bathing medium, the interstitial space was 0.223 ml/ml of pellet, and the aqueous volume of cell envelopes was 0.117 ml/ml of pellet. Thus the cell envelope for this yeast, under these conditions, was approximately 15% of the total cell volume. At a finite osmolality, the space in a yeast pellet that was accessible to salt was accounted for by the sum of initial interstitial space, the volume of the cell envelopes, and the volume of water abstracted from the cells by osmosis. Plots of S value versus osmolality were linear for uncharged probes and curvilinear for all salts. When Ficoll and potassium thiocyanate were presented to the yeast in admixture, the S values for the salt increased continuously over the range of osmolality studied. However, the S values for Ficoll 400 (which did not penetrate the cell wall) were lower by an amount equilivalent to the cell envelopes; they increased in parallel with the S curve for salt up to 1.15 osmol/kg and then plateaued. The results support the concept of incipient plasmolysis at 1.15 osmol/kg, and the separation of protoplasm from the cell wall is indicated with more concentrated solutions. Such cells were still viable if slowly diluted in distilled water, but they were injured by the shock of rapid dilution. However, shocking the cells did not release beta-fructofuranosidase into the medium. The complete accessibility of salts toward killed cells was demonstrated with yeast that had been pretreated with heat, organic solvents, or glutaraldehyde.

Liberation of protoplasts from mycelium of Neurospora crassa by means of enzymes obtained from autolyzed cultures of this fungus

Spherical protoplast-like structures were released from Neuropsora crassa mycelia under the influence of cell-wall-lytic enzymes obtained from autolyzed cultures of this fungus, in 50 mM borate-citrate-phosphate buffer, pH 5.5 with 0.5-0.8 M mannitol for appropriate osmotic pressure. The protoplasts retain their morphological aspect for several days at 4 C, or for a longer time when they are washed by centrifugation and incubated with 0.8 M mannitol. The free protoplasts are sensitive to osmotic shock and lyse immediately when placed in distilled water; the vacuoles and other cytoplasmatic material remain intact for some time.

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.

Lysis of yeast cell walls: glucanases from Bacillus circulans WL-12

Endo-beta-(1 --> 3)- and endo-beta-(1 --> 6)-glucanases are produced in high concentration in the culture fluid of Bacillus circulans WL-12 when grown in a mineral medium with bakers' yeast cell walls as the sole carbon source. Much lower enzyme levels were found when laminarin, pustulan, or mannitol was the substrate. The two enzyme activities were well separated during Sephadex G-100 chromatography. The endo-beta-(1 --> 3)-glucanase was further purified by diethylaminoethyl-cellulose and hydroxyapatite chromatography, whereas the endo-beta-(1 --> 6)-glucanase could be purified further by diethylamino-ethyl-cellulose and carboxymethyl cellulose chromatography. The endo-beta-(1 --> 3)-glucanase was specific for the beta-(1 --> 3)-glucosidic bond, but it did not hydrolyze laminaribiose; laminaritriose was split very slowly. beta-(1 --> 4)-Bonds in oat glucan in which the glucosyl moiety is substituted in the 3-position were also cleaved. The kinetics of laminarin hydrolysis (optimum pH 5.0) were complex but appeared to follow Michaelis-Menten theory, especially at the lower substrate concentrations. Glucono-delta-lactone was a noncompetitive inhibitor and Hg(2+) inhibited strongly. The enzyme has no metal ion requirements or essential sulfhydryl groups. The purified beta-(1 --> 6)-glucanase has an optimum pH of 5.5, and its properties were studied in less detail. In contrast to the crude culture fluid, the two purified beta-glucanases have only a very limited hydrolytic action on cell wall of either bakers' yeast or of Schizosaccharomyces pombe. Although our previous work had assumed that the two glucanases studied here are responsible for cell wall lysis, it now appears that the culture fluid contains in addition a specific lytic enzyme which is eliminated during the extensive purification process.

Stability of Candida utilis cells and spheroplasts toward gravitational forces

Candida utilis cells and spheroplasts containing uric acid crystals in their vacuoles were not damaged by the mechanical stress of centrifugation at 20,000 x g for 10 min, as judged by plating, microscopy, and spectrophotometry.