Fungal protoplasts: isolation, reversion, and fusion

Genetic engineering of filamentous fungi

Filamentous fungi are important in medicine, industry, agriculture, and basic biological research. For example, some fungal species are pathogenic to humans, whereas others produce beta-lactam antibiotics (penicillin and cephalosporin). Industrial strains produce large amounts of enzymes, such as glucoamylase and proteases, and low molecular weight compounds, such as citric acid. The largest and most economically important group of plant pathogens are fungi. Several fungal species have biological properties and genetic systems that make them ideally suited for basic biological research. Recently developed techniques for genetic engineering of filamentous fungi make it possible to alter their detrimental and beneficial activities in novel ways.

Antibiotic tetaine--a selective inhibitor of chitin and mannoprotein biosynthesis in Candida albicans

The antibiotic tetaine inhibits in Candida albicans the biosynthesis of two important cell wall constituents, chitin and mannoprotein. This effect is a consequence of inactivation of the enzyme glucosamine-6-phosphate synthetase. Due to the lack of glucosamine-6-phosphate the effective secretion of mannoprotein enzymes, acid phosphatase and invertase, by Candida albicans spheroplasts is inhibited. In the presence of tetaine, probably a modified mannoprotein, lacking a branched polymannan, is synthesized. The antibiotic action decreases the viability of Candida albicans cells, especially that of mycelial forms of this fungus.

Intergeneric hybrids of Saccharomyces cerevisiae and Zygosaccharomyces fermentati obtained by protoplast fusion

To obtain strains that are able to efficiently produce ethanol from different carbohydrates, mainly cellulose hydrolysates, several species of the genus Candida and a Zygosaccharomyces fermentati strain were examined for their ability to utilize cellobiose and produce ethanol, as well as for their thermotolerance and the possibility of genetic manipulation. Candida obtusa and Zygosaccharomyces fermentati tolerated the maximal temperature for growth, possessed the highest cellobiase activity, and offered the possibility of genetic manipulation, although neither of them proved to be a good producer of ethanol. Intergeneric hybrids of Saccharomyces cerevisiae and Z. fermentati were obtained after protoplast fusion. They were selected as prototrophic strains, after isolation of auxotrophic mutants from Z. fermentati and fusion with an S. cerevisiae strain which was also auxotrophic. The hybrids, which appeared at a frequency of 2 X 10(-7), presented characteristics of both parents, such as resistance to certain drugs and the ability to grow with either cellobiose or lactic acid as the sole carbon source; they were very stable, even under nonselective conditions. These hybrids may have important industrial applications as good fermenting strains.

Effects of mitotic and tubulin mutations on microtubule architecture in actively growing protoplasts of Aspergillus nidulans

We used immunofluorescent microscopy to characterize microtubule (MT) architecture in wild-type and mutant protoplasts of Aspergillus nidulans at interphase and at mitosis. Because the visualization of MTs by immunofluorescence is technically difficult in intact hyphae of A. nidulans, we developed a method for removing the cell wall under conditions that do not perturb cell physiology, as evidenced by the fact that the resulting protoplasts undergo nuclear division at a normal rate and that cell cycle mutant phenotypes are expressed at restrictive temperature. Interphase cells exhibited an extensive network of cytoplasmic MTs. During mitosis the cytoplasmic MTs mostly disappeared and an intranuclear mitotic spindle appeared. We have previously shown that the benA 33 beta-tubulin mutation causes hyperstabilization of the mitotic spindle, and we have presented additional indirect evidence that suggested that the tubA1 and tubA4 alpha-tubulin mutations destabilize spindle MTs. In this paper, we show that the benA33 mutation increases the stability of cytoplasmic MTs as well as spindle MTs and that the tubA1 and tubA4 mutations destabilize both spindle and cytoplasmic MTs.

Use of a temperature-sensitive, protoplast-forming Neurospora crassa strain for the detection of antifungal antibiotics

Protoplasts of the temperature-sensitive osmotic-1 mutant of Neurospora crassa grew and divided as cell wall-less cells when incubated under certain conditions at 37 degrees C. Each protoplast regenerated cell wall and formed a mycelium when the temperature was shifted to 22 degrees C. Cell wall regeneration, but not cell growth, was prevented by the inhibition of cell wall assembly functions. Thus, the inhibition of cell wall regeneration could serve as an indicator of the mode of action of antibiotic drugs. A method for detecting cell wall-inhibiting antifungal compounds with osmotic-1 protoplasts is described.

Cell wall assembly of Neurospora crassa: lack of evidence for preexisting cell wall acting as primer or template

Cell wall formation of Neurospora crassa and other filamentous fungi involves the apical extension of preexisting cell wall in a complex assembly sequence; however, it is not known if preexisting wall participates in the formation of new cell wall. It was found that temperature-sensitive protoplasts which lack detectable preexisting wall form cell wall upon a shift to a permissive temperature. Similarly, temperature-sensitive colonial mutants form morphologically normal cell wall directly from preexisting abnormal hyphae after a shift to a permissive temperature. These results are consistent with the idea that cell wall assembly occurs without the participation of preexisting cell wall as either primer or template for new cell wall assembly.

[Protoplast liberation and regeneration in the ascomycete Hypomyces ochraceus (Pers.) Tul]

A rapid and convenient method for producing protoplasts from 3 d old mycelium of the ascomycete Hypomyces ochraceus is described. The procedure involves a Helix pomatia enzyme preparation and sucrose (20%) for stabilization. Pretreatment with disulfide bond reducing agents reduced the amount of viable protoplasts. Formation of protoplasts and different stages of regeneration were observed by phase contrast microscopy. There was only one type of true regeneration from protoplasts to hyphae in 15-30% gelatine medium by direct forming a germ tube from the original protoplast. Cytological events and physiological conditions are discussed.

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.

Pullulan Elaboration by Aureobasidium pullulans Protoplasts

Protoplasts of Aureobasidium pullulans are capable of producing pullulan. Biosynthesis of the polymer pullulan required induction with kinetics similar to those of whole cells. The protoplasts also produced a heteropolysaccharide component containing mannose, glucose, and galactose. The relative proportions of the pullulan and heteropolysaccharide fractions were a function of glucose concentration, with the pullulan content of the total polysaccharide rising from 20% at 2.5 mM glucose to 45% at 20 mM glucose. Elaboration of pullulan by both cells and protoplasts was sensitive to 0.6 M KCl, which was present as the osmotic stabilizer in protoplast experiments. The presence of KCl resulted in a shift in the pH optimum to a more acidic value. The molecular weight of the protoplast-derived pullulan was sharply reduced from the molecular weight of the whole-cell-derived product. Exposure of the protoplasts to proteolytic enzymes had no effect on polysaccharide elaboration.

Plasmid transfer and genetic recombination by protoplast fusion in staphylococci

The experimental conditions for plasmid transfer and genetic recombination in Staphylococcus aureus and some coagulase-negative staphylococci by protoplast fusion are described. Protoplasts were prepared by treatment with lysostaphin and lysozyme in a buffered medium with 0.7 to 0.8 M sucrose. Regeneration of cell walls was accomplished on a hypertonic agar medium containing succinate and bovine serum albumin. Transfer of plasmids occurred after treatment of the protoplast mixtures with polyethylene glycol (molecular weight, 6,000) not only between strains of the same species but also between parents of different species, although at approximately 100 times lower frequency in the latter case. Recombination of the chromosomal genes in fused protoplasts required simultaneous treatment of the mixed protoplasts with polyethylene glycol and CaCl2. A method was developed for isolation of recombinants after fusion between mutants of S. areus carrying unselectable markers. Antibiotic resistance plasmids were introduced into the parental strains and used as primary markers to detect protoplast fusion. Chromosomal recombinants were found among the clones with both parental plasmids at a high frequency. The method appears to have simple applications in the construction of strains with multiple mutant characters.