Yeast dsRNA viruses: replication and killer phenotypes

The cytoplasmic L-A dsRNA virus of Saccharomyces cerevisiae consists of a 4.5 kb dsRNA and the two gene products it encodes; the capsid (cap) and at least one copy of the capsid-polymerase (cap-pol) fusion protein. Virion cap-pol catalyses transcription of the plus (sense)-strand; this is extruded from the virus and serves as messenger for synthesis of cap and cap-pol. Nascent cap-pol binds to a specific domain in the plus strand to initiate encapsidation and then catalyses minus-strand synthesis to complete the replication cycle. Products of at least three host genes are required for replication, and virus copy number is kept at tolerable levels by the SKI antivirus system. S. cerevisiae killer viruses are satellite dsRNAs that use a similar encapsidation domain to parasitize the L-A replication machinery. They encode precursors of secreted polypeptide toxins and immunity (specific resistance) determinants and are self-selecting. Three unique killer types, K1, K2 and K28, are currently recognized. They are distinguished by an absence of cross-immunity and by toxin properties and lethal mechanisms; while K1 and K2 toxins bind to cell-wall glucan and disrupt membrane functions, K28 toxin binds to mannoprotein and causes inhibition of DNA synthesis.

Isolation and characterization of osmosensitive vacuolar mutants of Saccharomyces cerevisiae

The yeast vacuole plays an important role in nitrogen metabolism, storage and intracellular macromolecular degradation. Evidence suggests that it is also involved in osmohomeostasis of the cell. We have taken a mutational approach for the analysis of vacuolar function and biogenesis by the isolation of 97 mutants unable to grow if high concentrations of salt are present in the medium. Phenotypic analysis was able to demonstrate that apart from osmosensitivity the mutations also conferred other properties such as altered vacuolar morphology and secretion of the vacuolar enzymes carboxypeptidase Y, proteinase A, proteinase B and alpha-mannosidase. The mutants fall into at least 17 complementation groups, termed ssv for salt-sensitive vacuolar mutants, of which two are identical to complementation groups isolated by others. We conclude that in Saccharomyces cerevisiae correct vacuolar biogenesis and protein targeting is required for osmotolerance as well as other important cellular processes.

Amplification and substantial purification of cardiolipin synthase of Escherichia coli

A simple, specific, and sensitive assay procedure for cardiolipin synthase of Escherichia coli has been developed. This measures the radioactivity of glycerol formed from phosphatidyl [2-3H]glycerol and is mainly based on the findings that 400 mM phosphate and 0.015% Triton X-100 markedly activate the enzyme. Cardiolipin synthase was amplified 760-fold upon induction with isopropyl beta-D-thiogalactoside in cells harboring a pBR322 derivative in which the cls gene encoding this enzyme was preceded by the tac promoter. Under these conditions, cardiolipin content increased, membrane potential decreased, spheroplasts became fragile, cells lost viability, and inducer-resistant mutants appeared at a high frequency. The amplification enabled the isolation of an enzyme preparation with a specific activity approximately 10,000-times higher than that of wild-type whole cell lysate. This purification was simply achieved by extraction of the crude membrane fraction with Triton X-100 and a single phosphocellulose column chromatography. This preparation, together with the crude envelope fraction, was used to characterize the basic properties of E. coli cardiolipin synthase, some of which were utilized in setting up the assay conditions.

Yeast K1 killer toxin forms ion channels in sensitive yeast spheroplasts and in artificial liposomes

The patch-clamp technique was used to examine the plasma membranes of sensitive yeast spheroplasts exposed to partially purified killer toxin preparations. Asolectin liposomes in which the toxin was incorporated were also examined. Excised inside-out patches from these preparations often revealed at 118 pS conductance appearing in pairs. The current through this conductance flickered rapidly among three states: dwelling mostly at the unit-open state, less frequently at the two-unit-open state, and more rarely at the closed state. Membrane voltages from -80 to 80 mV had little influence on the opening probability. The current reversed near the equilibrium potential of K+ in asymmetric KCl solutions and also reversed near O mV at symmetric NaCl vs. KCl solutions. The two levels of the conductance were likely due to the toxin protein, as treatment of spheroplasts or liposomes with extracellular protein preparations from isogenic yeasts deleted for the toxin gene gave no such conductance levels. These results show that in vivo the killer-toxin fraction can form a cation channel that seldom closes regardless of membrane voltage. We suggest that this channel causes the death of sensitive yeast cells.

Neurospora crassa A mating-type region

The mating-type locus of the haploid filamentous fungus Neurospora crassa is a regulatory region that controls entry into the sexual cycle and prevents formation of mixed mating-type heterokaryons in the vegetative phase. The locus consists of alternative sequences called A and a. The A mating-type DNA sequence of Neurospora crassa is composed of a region of 5301 base pairs that has little similarity to the sequence present at the mating-type locus in an a mating-type strain. However, the sequences flanking the mating-type locus in the A haploid and a haploid genome are essentially identical. The region of the A mating-type sequence required for expression of the heterokaryon incompatibility and sexual functions has been localized to a single open reading frame (ORF) encoding a polypeptide of 288 amino acids. Sequence analysis of sterile, heterokaryon-compatible mutants reveals frameshift mutations in this same ORF. The putative 288-amino acid product has a region of similarity to the MAT alpha 1 polypeptide of Saccharomyces cerevisiae.

[Fusion of Erwinia chrysanthemi spheroplasts in an electric fields]

A new approach has been elaborated for electrofusion of Erwinia chrysanthemi spheroplasts. The new approach consists of superimposition of high voltage impulses on the pellet of tightly contacting cells in the course of centrifugation. The mixture of spheroplasts of two genetically marked strains was placed into the special centrifuge chambers and spinned for 15 min at 2500 g to get a compressed pellet between chamber electrodes. Three successive pulses of 6.6 kv/cm amplitude and 30 microseconds duration were applied to spheroplast pellet during centrifugation. Fusion products were viable and after plating on the surface of hypertonic medium regenerated to the rod forms. As a result, the hybrid clones carrying the markers of both parents were isolated.

Chromatin structure of transposon Tn903 cloned into a yeast plasmid

Transposon Tn903 contains the APH gene for kanamycin resistance, which is active in yeast [A. Jiménez and J. Davies (1980) Nature (London) 287, 869-871] and is flanked by two inverted repeats (IR) 1057 bp long. When plasmid pAJ50, carrying Tn903 and the 2-microns circle origin of replication, is cloned into Saccharomyces cerevisiae, nucleosomes are assembled in vivo on the prokaryotic DNA of the transposon. Indirect end labeling revealed that three nucleosomes are preferentially positioned on symmetrical sequences from both IRs. DNase I digestion also confirmed that the chromatin structure is symmetrical in both IRs. This suggests that sequence determinants are decisive for chromatin structure in these regions. We have calculated the rotational and translational fits [H. R. Drew and C. R. Calladine (1987) J. Mol. Biol. 195, 143-173] for the Tn903 sequence and the results indicate that the nucleosome positioning on the IRs is sequence-directed. Nucleosome deposition on the APH gene also occurs, but no clear positioning exists. Some sequence preference for positioning nucleosomes on the promoter can be predicted, especially from the translational fit. Experimental data indicate, however, that nucleosomes are absent from the promoter. Therefore, chromatin can be organized on prokaryotic DNA in a manner that resembles the typical eukaryotic chromatin structure.

Transformation of the archaebacterium Halobacterium volcanii with genomic DNA

We describe optimization of a transformation system for the halophilic archaebacterium Halobacterium volcanii. Transformation of spheroplasts in the presence of polyethylene glycol permits the uptake and expression of high-molecular-weight linear fragments of genomic DNA as well as plasmid or bacteriophage DNA. Transformations can be performed with either fresh or frozen cell preparations. Auxotrophic mutants were transformed to prototrophy with genomic DNA from wild-type cells with efficiencies of 5 x 10(4)/micrograms of DNA and frequencies of 8 x 10(-5) per regenerated spheroplast. The overall efficiency of transformation with genomic DNA implies that genetic recombination is an efficient process in H. volcanii.

Role of cell wall in Saccharomyces cerevisiae mutants resistant to Hg2+

Hg2+-resistant mutants were isolated from Saccharomyces cerevisiae. Although they were very much like the parental strains in terms of colony-forming ability, they grew faster than the parental strains in the presence of sublethal doses of Hg2+. The Hg2+-resistant mutations were dominant. They were centromere linked and were divided into two groups by means of recombination; one of the mutations, designated HGR1-1, was mapped on chromosome IV because of its linkage to the TRP1 locus. The Hg2+-resistant mutants took up Hg2+ as much as, or slightly more than, the parental strains did. The mutants and parental strains retained only about 5 and 15%, respectively, of the cell-associated Hg2+ after removal of the cell wall; therefore, the mutants had less spheroplast-associated Hg2+ than did the parental strains. These results indicate that the cell wall plays an important role in protection against Hg2+ by acting as an adsorption filter and that the mutations described confer Hg2+ resistance by increasing the Hg2+-binding capacity of the cell wall.

Progress in culture and subculture of spheroplasts and fastidious acid-fast bacilli isolated from intestinal tissues

The efficiency of culture media was compared for the culture and subculture of very slowly growing acid-fast bacilli and spheroplast forms obtained from intestinal tissues of patients with Crohn's disease and ulcerative colitis and from controls without inflammatory bowel disease. Media were developed by modifying a nutrient broth medium based on veal infusion broth and yeast extract. We evaluated the effects of pH and the addition of Tween 80, Dubo oleic albumin complex, an extract from intestinal tissue from a patient with Crohn's disease, horse serum, sucrose, magnesium sulfate, ferrous ammonium sulfate, and sodium citrate. All media contained mycobactin J (2 micrograms/ml). We developed a medium (MG3) which was highly successful in promoting the growth of very fastidious organisms and promoted reversion of spheroplasts to acid-fast rods. MG3 contained veal infusion broth, 1% yeast extract, 10% horse serum, 0.3 M sucrose, 0.2% MgSO4, 0.1% ferrous ammonium sulfate, 0.1% sodium citrate, and 2 mg of mycobactin J per liter. We were able to obtain quantities of organisms sufficient for examination of the organisms by molecular techniques. Successful cultivation of all isolates and reversion of spheroplasts to acid-fast forms encourage further studies of the possibility of a complex association of mycobacteria and Crohn's disease.

Formation and regeneration of protoplasts and spheroplasts of gastrointestinal strains of lactobacilli

Methods were developed for the formation of protoplasts and spheroplasts of gastrointestinal strains of Lactobacillus reuteri, Lactobacillus gasseri, and Lactobacillus salivarius. Attempts to regenerate vegetative cells from protoplasts were not successful, but spheroplasts could be regenerated consistently for five of six strains.

Helix hand fidelity in Bacillus subtilis macrofibers after spheroplast regeneration

Left- and right-handed Bacillus subtilis macrofibers produced by strains FJ7 and C6D were converted to spheroplasts. Intact cells were regenerated and macrofibers were produced under conditions conducive for production of left- and right-handed structures. The resulting helix hand phenotypes always corresponded to those expected on the basis of the parental genotype.

Parasexual genetics of Torulopsis glabrata

Prototrophic hybrids were generated in the asexual yeast Torulopsis glabrata by the fusion of spheroplasts derived from parent strains which bore complementing auxotrophic markers. The DNA content (per cell) of two hybrids was essentially that predicted by summing the corresponding parental values. UV irradiation of these two hybrids resulted in the formation of sectored colonies with genetic properties consistent with their origin by either mitotic recombination or chromosomal nondisjunction.

Viruses in fungi: infection of yeast with the K1 and K2 killer viruses

We demonstrate here that yeast killer viruses, previously thought to be transmitted only by cytoplasmic mixing during division, mating, or other induced forms of cell fusion, are capable of extracellular transmission. Viral particles from standard K1 and K2 killer strains were used to inoculate sensitive cells of Saccharomyces cerevisiae, rendered competent by spheroplasting, lithium acetate treatment, or by natural mating. Extracellular transmission of the killer viruses was judged by the following criteria and controls. Filter-sterilized virus inocula were shown to be free of viable yeast cells, and host cells treated in the absence of added virus did not yield killer progeny. Infected clones originating from spheroplasts or lithium acetate-treated cells were shown to possess the genotype of the host strain and the killer phenotype of the infecting virus. Infected clones derived from complementary mating pairs were found to be wild-type diploids, whose meiotic segregants exhibited 2:2 segregation for unlinked nutritional markers and 4:0 segregation for the killer phenotype. This technique is generally applicable to the study of interactions between yeast viruses and different hosts and suggests that extracellular transmission may be a natural route for the inheritance and dissemination of mycoviruses.

Pressure-sensitive ion channel in Escherichia coli

We have used the patch-clamp electrical recording technique on giant spheroplasts of Escherichia coli and have discovered pressure-activated ion channels. The channels have the following properties: activation by slight positive or negative pressure; voltage dependence; large conductance; selectivity for anions over cations; dependence of activity on the species of permeant ions. We believe that these channels may be involved in bacterial osmoregulation and osmotaxis.

Efficient transfection of the archaebacterium Halobacterium halobium

We developed an efficient polyethylene glycol-mediated spheroplast transfection method for the extremely halophilic archaebacterium Halobacterium halobium. The 59-kilobase-pair linear phage phi H DNA molecule routinely produced between 5 X 10(6) and 2 X 10(7) transfectants per microgram of DNA. Between 0.5 and 1% of spheroplasts were transfected per microgram of luminal diameter H DNA. Under our conditions, survival and regeneration of H. halobium spheroplasts were also quite efficient, suggesting that this method will be useful for introducing other DNAs into these bacteria.

Effects of microwave irradiation on some membrane-related processes in bacteria

In a series of experiments performed on intact cells or spheroplasts of E. coli and Bac. subtilis a possibility of non-thermal effects induced by continuous microwave irradiation of a low power density (at wave length range from 0.0 to 7.8 mm) was studied. Thymidine and thymine uptake, leakage of potassium and hydrogen ions as well as the uptake of the transforming DNA by the component cells of Bac. subtilis were shown to be affected in a way typical of that due to heating of a sample. No specific dependence of the effects observed on wavelength was found.

Saccharomyces cerevisiae nuclear fusion requires prior activation by alpha factor

We have developed a protocol for efficient fusion of spheroplasts of the same mating type. Nuclear fusion in this whole-cell system is also efficient and closely parallels nuclear fusion in heterosexual mating of intact cells. In the spheroplast fusion system, nuclear fusion is dependent on both the KAR1 gene and prior exposure to alpha factor. The major products of nuclear fusion in the spheroplast fusion assay were true diploids that were homozygous at the mating-type locus. An additional 10% of the products were cells of ploidy greater than diploid. The dependence of nuclear fusion on alpha factor treatment could not be replaced by synchronization in G1 by mutations in CDC28 and CDC35 or by prior arrest in stationary phase. These data suggest that nuclear fusion is not a constitutive function of the nucleus, but rather is specifically induced by mating hormone.

Correlation between suppressed meiotic recombination and the lack of DNA strand-breaks in the rRNA genes of Saccharomyces cerevisiae

We have examined whether the suppressed homologous meiotic recombination within the rDNA of S. cerevisiae is reflected by a lack of possibly recombination-initiating strand-breaks in this part of the genome. Our findings indicate that bulk DNA in the ds-break repair deficient mutant rad52/rad52 accumulates a limited number of both ss- and ds-breaks during meiosis as compared to a RAD+/rad52 heterozygote. The rDNA-containing chromosome is however protected against these breaks, and thus this may be an explanation for the suppression of recombination in the rDNA. The fact that ds-breaks seem to be involved gives indirect support to the ds-break-repair model for recombination.

Phospholipids and glycerides composition during spheroplasts formation of Mycobacterium smegmatis ATCC 14468

The phospholipids and glycerides composition of spheroplasts of Mycobacterium smegmatis ATCC 14468 was examined. The percent total phospholipids in total lipids as well as cardiolipin were found to be higher in spheroplasts as compared to their parent forms. Increase in cardiolipin and free fatty acids content and decrease in triglycerides levels were observed during spheroplasts formation. The results suggest that increase in cardiolipin content in spheroplasts is an adaptational change concomitant with the loss of cell walls.

Meiotic karyotype of the yeast Saccharomyces cerevisiae

A cytogenetic study of the meiotic chromosomes of the budding yeast Saccharomyces cerevisiae was undertaken by high resolution epifluorescence microscopy. Condensation of chromatin into separate chromosomes takes place during prophase I. At metaphase I, there are 16 separate and distinct bivalents which are roughly classified into three groups by morphological differences and DNA content.

Relationship between cell surface composition, adherence, and virulence of Candida albicans

A comparison was made of the adherence to acrylic and to human buccal epithelial cells of seven strains of Candida albicans isolated from active infections (I strains) and two strains obtained from asymptomatic carriers (C strains). After growth in defined medium containing a relatively low concentration (50 mM) of glucose as the carbon source, the adherence of I and C strains to either surface was similar and all strains were sensitive to spheroplast formation with Zymolyase 5000. Growth in medium containing a high concentration (500 mM) of sucrose or galactose enhanced the adherence of I strains up to 5- and 11-fold, respectively, and there were corresponding increases in resistance to spheroplast formation. Sucrose- or galactose-grown C strains showed only small increases in adherence and remained relatively sensitive to spheroplast formation. When inoculated intravenously into mice, I strains grown in 500 mM sucrose were up to five times more virulent than organisms grown in 50 mM glucose, while I strains grown in 500 mM galactose showed a 5- to 24-fold increase in virulence. Fifty percent lethal doses obtained for C strains were similar after growth on all three carbon sources. We conclude that I strains are able to modify their surface composition in response to high concentrations of certain sugars in the growth environment. Such modification can enhance both their ability to adhere to surfaces and their virulence. C strains lack this capability, or possess it to a lower degree, and may therefore have a lower pathogenic potential.

Isolation and structure of glucan from regenerating spheroplasts of Candida albicans

Regenerating spheroplasts of Candida albicans formed organized glucan nets in liquid culture. The nets consisted of interwoven microfibrils about 50 nm wide, but of an undetermined length. Partial acid hydrolysis of the polysaccharide showed the presence of chains of beta(1----3)- and beta(1----6)-linked glucose residues, but no intrachain beta(1----3) and beta(1----6) linkages. Periodate oxidation and GLC of the methylated glucan indicated a highly branched polymer (9.5% branch points). Sequential enzymic degradation of the isolated nets confirmed the presence of chains of beta(1----3)- and beta(1----6)-linked glucose residues. Degradation by (1----3)-beta- and (1----6)-beta-glucanase released 23% (w/w) and 30% (w/w) respectively of the carbohydrate as glucose equivalents. The residual material was degraded by chitinase. Equal amounts of N-acetylglucosamine and glucose equivalents were detected in the chitinase hydrolysate, suggesting a possible linkage between glucan and chitin. Our data indicate that the cell wall of C. albicans contains at least two highly branched glucans with predominantly beta(1----3) or beta(1----6) linkages.

Assay of cytotoxicity and mutagenicity of alkylating agents by using Neurospora spheroplasts

A system relying on the use of Neurospora crassa spheroplasts has been developed for the assay of cytotoxicity and mutagenicity of chemical compounds. Mutagenicity was assayed by using reversion of alleles in the am gene selected to recognize certain specified transitions and also undefined point mutations. Cytotoxicity was quantified by measuring a 'cytotoxicity parameter', m, which appears in the exponential function that fits the survival/dose curve for each compound (under standard incubation conditions). Of the compounds tested, nitrogen mustard (Cl(CH2)2 NMe(CH2)2Cl) was cytotoxic and non-mutagenic, and ethyl nitrosourea was highly mutagenic but not cytotoxic. Of the remaining compounds tested, methyl nitrosourea, butadiene diepoxide, and cis platin (cis diammonia platinum II chloride) all showed comparable mutagenicity per survivor, although the values of m covered a wide range. Differences were found between the different compounds in the effects of the uvs-2 allele on survival and on the preponderance of G to A transitions.