Effects of yeast killer factor on sensitive cells

Yeast killer toxins: from ecological significance to application

Killer toxins are proteins that are often glycosylated and bind to specific receptors on the surface of their target microorganism, which is then killed through a target-specific mode of action. The killer phenotype is widespread among yeast and about 100 yeast killer species have been described to date. The spectrum of action of the killer toxins they produce targets spoilage and pathogenic microorganisms. Thus, they have potential as natural antimicrobials in food and for biological control of plant pathogens, as well as therapeutic agents against animal and human infections. In spite of this wide range of possible applications, their exploitation on the industrial level is still in its infancy. Here, we initially briefly report on the biodiversity of killer toxins and the ecological significance of their production. Their actual and possible applications in the agro-food industry are discussed, together with recent advances in their heterologous production and the manipulation for development of peptide-based therapeutic agents.

MTL genotypes, phenotypic switching, and susceptibility profiles of Candida parapsilosis species group compared to Lodderomyces elongisporus

Reference isolates of Candida parapsilosis (n = 8), Candida metapsilosis (n = 6), Candida orthopsilosis (n = 7), and Lodderomyces elongisporus (n = 11) were analyzed to gain insight into their pathobiology and virulence mechanisms. Initial evaluation using BBL Chromagar Candida medium misidentified L. elongisporus isolates as C. albicans. Polymerase chain reaction analysis of isolate MTL idiomorphs revealed that all C. parapsilosis isolates were MTLa homozygous and no MTL α1, α2, a1, or a2 gene was detected in L. elongisporus isolates. For C. orthopsilosis, two isolates were MTLa homozygous and five were MTL-heterozygous. Similarly, one C. metapsilosis isolate was MTLα homozygous whereas five were MTL-heterozygous. Isolate phenotypic switching analysis revealed potential phenotypic switching in the MTLα homozygous C. metapsilosis isolate, resulting in concomitant elongated cell formation. Minimum inhibitory concentrations of fluconazole (FLC) and FK506, alone or in combination, were determined by checkerboard assay, with data analyzed using the fractional inhibitory concentration index model. Synergistic or additive effects of these compounds were commonly observed in C. parapsilosis and L. elongisporus isolates. No killer activity was observed in the studied isolates, as determined phenotypically. No significant difference in virulence was seen for the four species in a Galleria mellonella model (P > 0.05). In conclusion, our results demonstrated phenotypic switching of C. metapsilosis CBS 2315 and that FLC and FK506 represent a promising drug combination against C. parapsilosis and L. elongisporus. The findings of the present study contribute to our understanding of the biology, diagnosis, and new possible treatments of the C. parapsilosis species group and L. elongisporus.

The atomic structure of the virally encoded antifungal protein, KP6

Killer toxins are produced by several genera of yeast and filamentous fungi. A small proportion of Ustilago maydis strains produce killer toxins, to which they are resistant, but sensitive strains are the majority in the wild populations. There are three killer types (P1, P4 and P6) that secrete KP1, KP4 and KP6 toxins, respectively, which are produced only by strains persistently infected with double-stranded RNA viruses (UmV) in the cell cytoplasm. Unlike nearly all other viruses, UmV are only transmitted through mitosis or meiosis. As shown here, KP6 is different from any other known cytotoxic protein. KP6 is neutral protein composed of two subunits: KP6α and KP6β. KP6α is responsible for targeting while KP6β is cytotoxic. Neither subunit is homologous in either sequence or structure to any other toxin, but they have highly similar structures to each other. The major difference between the two subunits is a hydrophobic helix at the N-terminus of KP6α and is likely key to target recognition. Unlike any other toxin, KP6 is translated as a single polypeptide with a 31-residue linker region in the middle of the protein. From structural prediction studies, this linker likely makes for a more compact KP6 structure that sequesters the hydrophobic helix of KP6α. A model whereby the protoxin undergoes a conformational activation process that exposes this helix immediately prior to secretion is presented.

Characterization, ecological distribution, and population dynamics of Saccharomyces sensu stricto killer yeasts in the spontaneous grape must fermentations of southwestern Spain

Killer yeasts secrete protein toxins that are lethal to sensitive strains of the same or related yeast species. Among the four types of Saccharomyces killer yeasts already described (K1, K2, K28, and Klus), we found K2 and Klus killer yeasts in spontaneous wine fermentations from southwestern Spain. Both phenotypes were encoded by medium-size double-stranded RNA (dsRNA) viruses, Saccharomyces cerevisiae virus (ScV)-M2 and ScV-Mlus, whose genome sizes ranged from 1.3 to 1.75 kb and from 2.1 to 2.3 kb, respectively. The K2 yeasts were found in all the wine-producing subareas for all the vintages analyzed, while the Klus yeasts were found in the warmer subareas and mostly in the warmer ripening/harvest seasons. The middle-size isotypes of the M2 dsRNA were the most frequent among K2 yeasts, probably because they encoded the most intense K2 killer phenotype. However, the smallest isotype of the Mlus dsRNA was the most frequent for Klus yeasts, although it encoded the least intense Klus killer phenotype. The killer yeasts were present in most (59.5%) spontaneous fermentations. Most were K2, with Klus being the minority. The proportion of killer yeasts increased during fermentation, while the proportion of sensitive yeasts decreased. The fermentation speed, malic acid, and wine organoleptic quality decreased in those fermentations where the killer yeasts replaced at least 15% of a dominant population of sensitive yeasts, while volatile acidity and lactic acid increased, and the amount of bacteria in the tumultuous and the end fermentation stages also increased in an unusual way.

Transgenic maize plants expressing the Totivirus antifungal protein, KP4, are highly resistant to corn smut

The corn smut fungus, Ustilago maydis, is a global pathogen responsible for extensive agricultural losses. Control of corn smut using traditional breeding has met with limited success because natural resistance to U. maydis is organ specific and involves numerous maize genes. Here, we present a transgenic approach by constitutively expressing the Totivirus antifungal protein KP4, in maize. Transgenic maize plants expressed high levels of KP4 with no apparent negative impact on plant development and displayed robust resistance to U. maydis challenges to both the stem and ear tissues in the greenhouse. More broadly, these results demonstrate that a high level of organ independent fungal resistance can be afforded by transgenic expression of this family of antifungal proteins.

The virally encoded fungal toxin KP4 specifically blocks L-type voltage-gated calcium channels

KP4 is a virally encoded fungal toxin secreted by the P4 killer strain of Ustilago maydis. Previous studies demonstrated that this toxin inhibits growth of the target fungal cells by blocking calcium uptake rather than forming channels, as had been suggested previously. Unexpectedly, this toxin was also shown to inhibit voltage-gated calcium channel activity in mammalian cells. We used whole-cell patch-clamp techniques to further characterize this activity against mammalian cells. KP4 is shown to specifically block L-type calcium channels with weak voltage dependence to the block. Because KP4 activity is abrogated by calcium, KP4 probably binds competitively with calcium to the channel exterior. Finally, it is shown that chemical reagents that modify lysine residues reduce KP4 activity in both patch-clamp experiments on mammalian cells and in fungal killing assays. Because the only lysine residue is K42, this residue seems to be crucial for both mammalian and fungal channel activity. Our results defining the type of mammalian channel affected by this fungal toxin further support our contention that KP4 inhibits fungal growth by blocking transmembrane calcium flux through fungal calcium channels, and imply a high degree of structural homology between these fungal and mammalian calcium channels.

KP4 fungal toxin inhibits growth in Ustilago maydis by blocking calcium uptake

KP4 is a virally encoded fungal toxin secreted by the P4 killer strain of Ustilago maydis. From our previous structural studies, it seemed unlikely that KP4 acts by forming channels in the target cell membrane. Instead, KP4 was proposed to act by blocking fungal calcium channels, as KP4 was shown to inhibit voltage-gated calcium channels in rat neuronal cells, and its effects on fungal cells were abrogated by exogenously added calcium. Here, we extend these studies and demonstrate that KP4 acts in a reversible manner on the cell membrane and does not kill the cells, but rather inhibits cell division. This action is mimicked by EGTA and is abrogated specifically by low concentrations of calcium or non-specifically by high ionic strength buffers. We also demonstrate that KP4 affects (45)Ca uptake in U. maydis. Finally, we show that cAMP and a cAMP analogue, N 6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate, both abrogate KP4 effects. These results suggest that KP4 may inhibit cell growth and division by blocking calcium-regulated signal transduction pathways.

A two-reservoir, hollow-fiber bioreactor for the study of mixed-population dynamics: design aspects and validation of the approach

A two-reservoir, membrane bioreactor for carrying out studies of mixed-population dynamics in batch fermentations is presented. Mixing requirements and design aspects for the validity of the approach are given and discussed. Equations describing mixing times between the reservoirs are presented and compared to the experimental results. The validity of the approach is demonstrated by the study of an amensalistic-type interaction, the protein-mediated killer phenomenon between two Saccharomyces cerevisiae strains. The validation consisted in the comparison between the results obtained in actual mixed culture and the results obtained by keeping the strains separated. A good agreement was found which demonstrates the viability of the designed bioreactor.

Killer factor interference in mixed opportunistic yeast cultures

The interaction of the killer yeast Pichia anomala UP 25F with the killer toxin-sensitive clinical isolate Candida albicans UCSC 10S and its natural toxin-resistant mutant derivative C. albicans UCSC 10R were studied under various conditions. A differential inhibition was shown to occur in vitro at pH and temperature values, which are not encountered in vivo, only by using preformed killer toxin, since antagonism due to yeast growth proved to be predominant on the killer effect. Under adverse growth conditions, the P. anomala killer yeast proved to be able to produce an anatoxin antigenically related to the active or heat inactivated killer toxin. These findings suggest that killer toxins may not function as potential virulence factors in the competition between the opportunistic killer yeast P. anomala and sensitive microorganisms for colonization in the course of natural human infections.

Structure and function of a virally encoded fungal toxin from Ustilago maydis: a fungal and mammalian Ca2+ channel inhibitor

BACKGROUND

The P4 strain of the corn smut fungus, Ustilago maydis, secretes a fungal toxin, KP4, encoded by a fungal virus (UMV4) that persistently infects its cells. UMV4, unlike most other (non-fungal) viruses, does not spread to uninfected cells by release into the extracellular milieu during its normal life cycle and is thus dependent upon host survival for replication. In symbiosis with the host fungus, UMV4 encodes KP4 to kill other competitive strains of U. maydis, thereby promoting both host and virus survival. KP4 belongs to a family of fungal toxins and determining its structure should lead to a better understanding of the function and evolutionary origins of these toxins. Elucidation of the mechanism of toxin action could lead to new anti-fungal agents against human pathogens.

RESULTS

We have determined the atomic structure of KP4 to 1.9 A resolution. KP4 belongs to the alpha/beta-sandwich family, and has a unique topology comprising a five-stranded antiparallel beta-sheet with two antiparallel alpha-helices lying at approximately 45 degrees to these strands. The structure has two left-handed beta alpha beta cross-overs and a basic protuberance extending from the beta-sheet. In vivo experiments demonstrated abrogation of toxin killing by Ca2+ and, to a lesser extent, Mg2+. These results led to experiments demonstrating that the toxin specifically inhibits voltage-gated Ca2+ channels in mammalian cells.

CONCLUSIONS

Similarities, although somewhat limited, between KP4 and scorpion toxins led us to investigate the possibility that the toxic effects of KP4 may be mediated by inhibition of cation channels. Our results suggest that certain properties of fungal Ca2+ channels are homologous to those in mammalian cells. KP4 may, therefore, be a new tool for studying mammalian Ca2+ channels and current mammalian Ca2+ channel inhibitors may be useful lead compounds for new anti-fungal agents.

Anti-Candida activity of a novel killer toxin from the yeast Williopsis mrakii

A screening of putative killer yeast strains showed that spore-forming ascomycetous yeasts of the genera Pichia and Williopsis displayed the broadest range of activity against sensitive strains of Candida spp. and Saccharomyces cerevisiae. Williopsis mrakii (NCYC 500) showed extensive anti-Candida activity against strains isolated from clinical specimens. W. mrakii killer factor was produced in minimal media as a function of growth and its activity reached constant levels as cells entered stationary phase. The proteinaceous killer toxin was found to be unstable without a specific range of temperature and pH (above 30 degrees C and pH 4.0), and further analysis showed that the active toxin molecule was an acidic polypeptide with a relative molecular mass between 1.8-5.0 kDa. At critical concentrations the killer factor exerted a greater effect on stationary phase cells of Candida than cells from an exponential phase of growth. At low concentrations, the killer toxin produced a fungistatic effect on sensitive yeasts but at higher concentrations there was evidence to suggest that membrane damage accounted for the zymocidal effects of the killer factor. the cidal nature of the toxin was reflected in a rapid decrease in sensitive cell viability. Findings presented suggest that W. mrakii killer toxin has potential as a novel antimycotic agent in combatting medically important strains of Candida.

Interactions between killer yeasts and pathogenic fungi

A total of 17 presumptive killer yeast strains were tested in vitro for growth inhibitory and killing activity against a range of fungal pathogens of agronomic, environmental and clinical significance. Several yeasts were identified which displayed significant activity against important pathogenic fungi. For example, isolates of the opportunistic human pathogen, Candida albicans, were generally very sensitive to Williopsis mrakii killer yeast activity, whilst killer strains of Saccharomyces cerevisiae and Pichia anomala markedly inhibited the growth of certain wood decay basidiomycetes and plant pathogenic fungi. Results indicate that such yeasts, together with their killer toxins, may have potential as novel antimycotic biocontrol agents.

A rapid colorimetric assay of killer toxin activity in yeast

The pale yellow redox indicator 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) is reduced to a dark blue end-product, MTT-Formazan, by the mitochondrial dehydrogenases of living cells. MTT reduction can be measured spectrophotometrically at a wavelength of 570 nm and a method is described to assay the cidal activity of Williopsis mrakii killer toxin against sensitive cells of Candida glabrata. The MTT assay is rapid, quantitative and compares favourably with traditional plating techniques for the assessment of sensitive viability.

Fluorescent staining with bromocresol purple: a rapid method for determining yeast cell dead count developed as an assay of killer toxin activity

A method is described for detecting yeast cells with plasma membrane damage, based on cell staining with bromocresol purple (BCP) which has a convenient fluorescence after entering the cells at pH below 5.2. The method was used to determine the activity of Saccharomyces cerevisiae pore-forming killer toxin K1 in commonly used lethal units. The BCP test is rapid and as precise as the plating test.

Interfaces of the yeast killer phenomenon

A new prophylactic and therapeutic antimicrobial strategy based on a specific physiological target that is effectively used by killer yeasts in their natural ecological competition is theorized. The natural system exploited is the yeast killer phenomenon previously adopted as an epidemiological marker for intraspecific differentiation of opportunistic yeasts, hyphomycetes, and bacteria. Pathogenic microorganisms (Candida albicans) may be susceptible to the activity of yeast killer toxins due to the presence of specific cell wall receptors. On the basis of the idiotypic network, we report that antiidiotypic antibodies, produced against a monoclonal antibody bearing the receptor-like idiotype, are in vivo protecting animals immunized through idiotypic vaccination and in vitro mimicking the antimicrobial activity of yeast killer toxins, thus acting as antibiotics.

Purification and characterization of the anti-Candida toxin of Pichia anomala WC 65

Pichia anomala WC 65 secretes a toxin that is inhibitory to a variety of yeasts, including strains of the animal pathogen Candida albicans. The toxin was purified to homogeneity by ultrafiltration, ethanol precipitation, ion-exchange chromatography with a Mono Q column, and gel permeation chromatography with a Superose 12 column. The toxin had a molecular weight of 83,300 as determined by electrophoresis on sodium dodecyl sulfate-polyacrylamide gradient gels and a molecular weight of 85,290 as determined by gel permeation chromatography. The isoelectric point of the toxin was pH 5.0. The toxin was stable between pH 2.0 and 5.0. Chemical analysis of the purified toxin indicated that the toxin was a glycoprotein composed of about 86% protein and 14% carbohydrate. At high concentrations, the toxin showed a tendency to aggregate, with loss of biological activity against C. albicans, Pichia bimundalis, and Saccharomycodes ludwigii. Purified toxin expressed killing activity against C. albicans in contrast to the static activity of the crude toxin.

Potential therapeutic effect of yeast killer toxin

Experimental infections were produced in guinea pigs, rabbits and dogs with lesions similar to those seen in human seborrheic dermatitis and otitis externa by cutaneous application of cultures of Malassezia furfur and M. pachydermatis. Infected animals were treated by topical application of a concentrated yeast killer toxin (Hansenula anomala UCSC 25F). Clinical recovery as well as negative mycological test cultures of infected animals proved to the clearly associated with the treatment by the killer toxin.

Comparison of the killer toxin of several yeasts and the purification of a toxin of type K2

A total of 13 killer toxin producing strains belonging to the genera Saccharomyces, Candida and Pichia were tested against each other and against a sensitive yeast strain. Based on the activity of the toxins 4 different toxins of Saccharomyces cerevisiae, 2 different toxins of Pichia and one toxin of Candida were recognized. The culture filtrate of Pichia and Candida showed a much smaller activity than the strains of Saccharomyces. Extracellular killer toxins of 3 types of Saccharomyces were concentrated and partially purified. The pH optimum and the isoelectric point were determined. The killer toxins of S. cerevisiae strain NCYC 738, strain 399 and strain 28 were glycoproteins and had a molecular weight of Mr = 16,000. The amino acid composition of the toxin type K2 of S. cerevisiae strain 399 was determined and compared with the composition of two other toxins.

Killer system: a simple method for differentiating Candida albicans strains

The killer effect of 37 species of Candida, Cryptococcus, Hansenula, Pichia, Rhodotorula, Saccharomyces, and Trichosporon on 100 Candida albicans isolates of human and animal origin was studied. All of the C. albicans cultures were sensitive to one or more killer yeasts. The factors affecting the killer phenomenon on C. albicans were investigated for realizing a simple system for the differentiation of the 100 C. albicans isolates. By using this system, it was possible to differentiate up to 512 isolates of C. albicans according to their susceptibility to the killer effect of nine selected killer yeasts. The use of this method as an epidemiological marker in the case of presumptive nosocomial infections due to C. albicans is also reported.

Physiological conditions affecting the sensitivity of Saccharomyces cerevisiae to a Pichia kluyveri killer toxin and energy requirement for toxin action

The interaction between the killer toxin of Pichia kluyveri 1002 and cells of Saccharomyces cerevisiae SCF 1717 is strongly affected by the physiological state of sensitive cells. The killing effect is maximal for cells in the lag and early exponential phase of growth, whereas stationary cells are completely resistant. Furthermore, sensitivity is markedly enhanced by a rise of the pH (from 3.2 to 6.8) at which cells are cultured. Three successive stages can be distinguished in the killing process: (I) binding of the toxin to the primary binding site; (II) transmission of the toxin to its reactive site in the plasma membrane; (III) occurrence of functional damage (K+-leakage; decrease of intracellular pH). The transition from stage I to II is prevented in the absence of metabolic energy or at low temperature (below 10 degrees C). Sensitive cells in stage I can be rescued from toxin-induced killing by a short incubation at pH 7.0, which treatment is not effective for cells in stage II. Cells in stage II are able to resume growth when plated in a rich medium containing suitable concentrations of potassium and hydrogen ions. Rescue was not observed for cells in stage III of the killing process.

Translational analysis of the killer-associated virus-like particle dsRNA genome of S. cerevisiae: M dsRNA encodes toxin

The M species (medium sized) dsRNA (1.1-1.4 x 10(6) daltons) isolated from a toxin-producing yeast killer strain (K+R+) and three related, defective interfering (suppressive) S species dsRNAs of the yeast killer-associated cytoplasmic multicomponent viral-like particle system were analyzed by in vitro translation in a wheat germ cell-free protein synthesis system. Heat-denatured M species dsRNA programmed the synthesis of two major polypeptides, M-P1 (32,000 daltons) and M-P2 (30,000 daltons). M-P1 has been shown by the criteria of proteolytic peptide mapping and cross-antigenicity to contain ihe 12,000 dalton polypeptide corresponding to the in vivo produced killer toxin, thus establishing thiat it is the M species dsRNA which carries the toxin gene. An M species dsRNA obtained from a neutral strain (K-R+) also programmed the in vitro synthesis of a polypeptide identical in molecular weight to M-P1, thus indicating that the cytoplasmic determinant of the mutant neutral phenotype is either a simple point mutation in the dsRNA toxin gene or a mutation in a dsRNA gene which is required for functional toxin production. In vitro translation of each of the three different suppressive S dsRNAs resulted in the production of a polypeptide (S-P1) of approximately 8000 daltons instead of the 32,000 dalton M-P1 polypeptide programmed by M dsRNA. This result is consistent with the heteroduplex analysis of these dsRNAs by Fried and Fink (1978), which shows retention of M dsRNA ends, accompanied by large internal deletions in each of the S dsRNAs translated.

"Superkiller" mutations suppress chromosomal mutations affecting double-stranded RNA killer plasmid replication in saccharomyces cerevisiae

Saccharomyces cerevisiae strains carrying a 1.5 x 10(6)-dalton double-stranded RNA genome in virus-like particles (killer plasmid) secrete a protein toxin that kills strains not carrying this plasmid. At least 28 chromosomal genes (mak genes) are required to maintain or replicate this plasmid. Recessive mutations in any of four other chromosomal genes (ski for superkiller) result in enhanced toxin production. We report that many ski- mak- double mutants are able to maintain the killer plasmid, indicating that the SKI products have an effect on plasmid replication. The ski1-1 mutation suppresses (bypasses) all mak mutations tested except mak16-1. A variant killer plasmid is described that confers the superkiller phenotype and, like chromosomal ski mutations, makes several mak genes dispensable for plasmid replication.

Effects of Pichia kluyveri killer toxin on sensitive cells

The killer toxin produced by Pichia kluyveri 1002 kills yeast strains of the genera Candida, Saccharomyces and Torulopsis, including several S. cerevisiae killer strains. Binding of a lethal amount of the toxin to cells of S. cerevisiae SCF 1717 occurs rapidly after toxin addition. After treatment with the toxin for 10 min sensitive cells partially recovered when incubated under conditions that favor protein synthesis. Only after a lag time of 50--90 min sensitive cells changed physiologically. Killing of sensitive cells was characterized by leakage of potassium and adenosine 5'-triphosphate, decrease of intracellular pH, and inhibition of the active uptake of amino acids. These effects coincided with cell shrinkage and varied with incubation conditions. Uptake of the amino acid leucine in sensitive cells involved two apparently distinct transport systems (Km1 = 0.04 mM; Km2 = 0.46 mM). The toxin showed different effects on these transport systems.

Killer phenomenon in pathogenic yeast

Potentially pathogenic yeast strains from four genera, Candida, Cryptococcus, Torulopsis, and Trichosporon, were examined for killer activity and sensitivity using Saccharomyces and Torulopsis strains of known killer status. Tests were performed by using a streak method and by concentration of culture fluid by lyophilization. Of 236 strains examined, killers were found with low prevalence among Torulopsis and Cryptococcus strains; Candida and Trichosporon isolates showed no killing activity. Different specificities of killing activity were observed among strains of a single species. Sensitive strains were found with varying frequencies in all genera but Trichosporon.

Yeast killer toxin: purification and characterisation of the protein toxin from Saccharomyces cerevisiae

Killer toxin from killer strains of Saccharomyces cerevisiae was isolated from concentrates of extracellular medium by precipitation in poly(ethylene glycol) and chromatography through glyceryl-controlled-pore glass. The toxin migrated as a single protein band on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A molecular weight of 11470 was determined for the toxin protein from its electrophoretic mobility and amino acid composition. Gel filtration of the active toxin indicated that the 11,470-Mr monomer was the active unit. Electrophoretic comparison of extracellular concentrates from a killer strain and an isogenic non-killer showed the presence of the toxin protein only in the killer-derived material. The activity of the toxin was most stable between pH 4.2 and 4.6. At 30 degrees C toxin from a superkiller strain was more stable than that from a normal killer.

Production, purification and properties of a Pichia kluyveri killer toxin

Production of the killer toxin of Pichia kluyveri 1002 was stimulated in the presence of yeast extract. In a minimal medium production was optimal at pH 3.8-4.0 and 22--25 degrees C. Addition of gelatin and nonionic detergents, like Brij-58 (polyoxyethylene 20 cetyl ether) and Triton-X-100, to this medium enhanced production significantly. The killer toxin was purified 140-fold by use of a stepwise ethanol precipitation and butyl Sepharose column chromatography. The purified killer toxin, which still contained some carbohydrates, appeared to be glycoprotein with a mol wt of about 19 000 and an isoelectric point of 4.3. It was stable between pH 2.5 and 4.7 and up to 40 degrees C.

Chromosomal superkiller mutants of Saccharomyces cerevisiae

Yeast strains carrying a 1.5 X 10(6)-dalton double-stranded RNA in virus-like particles secrete a protein toxin which is lethal to strains not carrying this species of double-stranded RNA. We find that recessive mutations in any of four chromosomal genes result in the superkiller phenotype, i.e., increased secretion of killer toxin activity by strains carrying the killer genome. These genes are designated ski1 through ski4 (for superkiller), ski3 and ski4 are located on chromosome XIV, and ski1 is on chromosome VII. A ski1 mutation results in a decreased rate of cell growth. The kex1 and kex2 mutations are epistatic to each ski mutation.

Pet18: a chromosomal gene required for cell growth and for the maintenance of mitochondrial DNA and the killer plasmid of yeast

Mutations in the pet18 gene of Saccharomyces cerevisiae (formerly denoted pets) confer three phenotypes on mutant strains: (i) inability to respire (petite), (ii) inability to maintain the double-stranded RNA killer plasmid (sensitive), and (iii) temperature sensitivity for growth. We find that pet18 mutants lack mitochondrial DNA. However, despite their inability to maintain the killer RNA plasmid and mitochondrial DNA, pet18 mutants still can carry the other yeast plasmids, [URE3--1], [PSI], and 2-micron DNA. The temperature sensitivity of the pet18 mutants is not expressed as a selective defect in total DNA, RNA, or protein synthesis.

A comparison of the killer character in different yeasts and its classification

The interactions between 20 killer yeasts of various genera and species were examined. Ten distinct groups were recognised with respect to killer activity and 10 distinct groups with respect to resistance to killer action. Using both killing and resistance phenotypes, 13 classes of killer yeast were found. With the exception of Torulopsis glabrata NCYC 388, non-Saccharomyces strains of yeast were not killed by a member of the genus Saccharomyes. The killer character of the 3 killing groups of Saccharomyces identified could be cured by treatment with cycloheximide or incubation at elevated temperature and the effectiveness of these procedures was indicative of the category of killer yeast examined. Killer yeasts not belonging to the genus Saccharomyces could not be cured of their activity. Double-stranded ribonucleic acids were extracted only from Saccharomyces spp. and the molecular weights of the species present were a function of the killer class to which a strain belonged. By an analysis of the effects of proteolytic enzymes, temperature and pH on killer activity and by gel chromatography of crude preparations of killer factors, the toxins of different killer classes were shown to be biochemically distinct. However all toxins had certain properties in common consistent with there being a protein component essential to killer action.