Killer toxin producing strains of the yeasts Hanseniaspora uvarum and Pichia kluyveri

The Prevalence of Killer Yeasts in the Gardens of Fungus-Growing Ants and the Discovery of Novel Killer Toxin named Ksino

Killer toxins are proteinaceous antifungal molecules produced by yeasts, with activity against a wide range of human and plant pathogenic fungi. Fungus gardens of attine ants in Brazil were surveyed to determine the presence of killer toxin-producing yeasts and to define their antifungal activities and ecological importance. Our results indicate that up to 46% of yeasts isolated from specific fungal gardens can be killer yeasts, with an overall prevalence of 17% across all strains tested. Killer yeasts were less likely to inhibit the growth of yeasts isolated from the same environment but more effective at inhibiting yeast isolated from other environments, supporting a role for killer yeasts in shaping community composition. All killer yeasts harbored genome-encoded killer toxins due to the lack of cytoplasmic toxin-encoding elements (i.e., double-stranded RNA satellites and linear double-stranded DNAs). Of all the killer yeasts identified, an isolate of Candida sinolaborantium showed a broad spectrum of antifungal activities against 57% of yeast strains tested for toxin susceptibility. The complete genome sequence of C. sinolaborantium identified a new killer toxin, Ksino, with primary and tertiary structure homology to the Saccharomyces cerevisiae killer toxin named Klus. Genome-encoded homologs of Ksino were found in yeast strains of Saccharomycetes and Pichiomycetes, as well as other species of Ascomycota and Basidiomycota filamentous fungi. This demonstrates that killer yeasts can be widespread in attine ant fungus gardens, possibly influencing fungal community composition and the importance of these complex microbial communities for discovering novel antifungal molecules.

Exploring future applications of the apiculate yeast Hanseniaspora

As a metaphor, lemons get a bad rap; however the proverb 'if life gives you lemons, make lemonade' is often used in a motivational context. The same could be said of Hanseniaspora in winemaking. Despite its predominance in vineyards and grape must, this lemon-shaped yeast is underappreciated in terms of its contribution to the overall sensory profile of fine wine. Species belonging to this apiculate yeast are known for being common isolates not just on grape berries, but on many other fruits. They play a critical role in the early stages of a fermentation and can influence the quality of the final product. Their deliberate addition within mixed-culture fermentations shows promise in adding to the complexity of a wine and thus provide sensorial benefits. Hanseniaspora species are also key participants in the fermentations of a variety of other foodstuffs ranging from chocolate to apple cider. Outside of their role in fermentation, Hanseniaspora species have attractive biotechnological possibilities as revealed through studies on biocontrol potential, use as a whole-cell biocatalyst and important interactions with Drosophila flies. The growing amount of 'omics data on Hanseniaspora is revealing interesting features of the genus that sets it apart from the other Ascomycetes. This review collates the fields of research conducted on this apiculate yeast genus.

The Species-Specific Acquisition and Diversification of a K1-like Family of Killer Toxins in Budding Yeasts of the Saccharomycotina

Killer toxins are extracellular antifungal proteins that are produced by a wide variety of fungi, including Saccharomyces yeasts. Although many Saccharomyces killer toxins have been previously identified, their evolutionary origins remain uncertain given that many of these genes have been mobilized by double-stranded RNA (dsRNA) viruses. A survey of yeasts from the Saccharomyces genus has identified a novel killer toxin with a unique spectrum of activity produced by Saccharomyces paradoxus. The expression of this killer toxin is associated with the presence of a dsRNA totivirus and a satellite dsRNA. Genetic sequencing of the satellite dsRNA confirmed that it encodes a killer toxin with homology to the canonical ionophoric K1 toxin from Saccharomyces cerevisiae and has been named K1-like (K1L). Genomic homologs of K1L were identified in six non-Saccharomyces yeast species of the Saccharomycotina subphylum, predominantly in subtelomeric regions of the genome. When ectopically expressed in S. cerevisiae from cloned cDNAs, both K1L and its homologs can inhibit the growth of competing yeast species, confirming the discovery of a family of biologically active K1-like killer toxins. The sporadic distribution of these genes supports their acquisition by horizontal gene transfer followed by diversification. The phylogenetic relationship between K1L and its genomic homologs suggests a common ancestry and gene flow via dsRNAs and DNAs across taxonomic divisions. This appears to enable the acquisition of a diverse arsenal of killer toxins by different yeast species for potential use in niche competition.

Anti-infective nitazoxanide disrupts transcription of ribosome biogenesis-related genes in yeast

BACKGROUND

Nitazoxanide is a broad-spectrum, anti-parasitic, anti-protozoal, anti-viral drug, whose mechanisms of action have remained elusive.

OBJECTIVE

In this study, we aimed to provide insight into the mechanisms of action of nitazoxanide and the related eukaryotic host responses by characterizing transcriptome profiles of Saccharomyces cerevisiae exposed to nitazoxanide.

METHODS

RNA-Seq was used to investigate the transcriptome profiles of three strains of S. cerevisiae with dsRNA virus-like elements, including a strain that hosts M28 encoding the toxic protein K28. From the strain with M28, an additional sub-strain was prepared by excluding M28 using a nitazoxanide treatment.

RESULTS

Our transcriptome analysis revealed the effects of nitazoxanide on ribosome biogenesis. Many genes related to the UTP A, UTP B, Mpp10-Imp3-Imp4, and Box C/D snoRNP complexes were differentially regulated by nitazoxanide exposure in all of the four tested strains/sub-strains. Examples of the differentially regulated genes included UTP14, UTP4, NOP4, UTP21, UTP6, and IMP3. The comparison between the M28-laden and non-M28-laden sub-strains showed that the mitotic cell cycle was more significantly affected by nitazoxanide exposure in the non-M28-laden sub-strain.

CONCLUSIONS

Overall, our study reveals that nitazoxanide disrupts regulation of ribosome biogenesis-related genes in yeast.

Yeast viral killer toxins: lethality and self-protection

Since the discovery of toxin-secreting killer yeasts more than 40 years ago, research into this phenomenon has provided insights into eukaryotic cell biology and virus-host-cell interactions. This review focuses on the most recent advances in our understanding of the basic biology of virus-carrying killer yeasts, in particular the toxin-encoding killer viruses, and the intracellular processing, maturation and toxicity of the viral protein toxins. The strategy of using eukaryotic viral toxins to effectively penetrate and eventually kill a eukaryotic target cell will be discussed, and the cellular mechanisms of self-defence and protective immunity will also be addressed.

The effects of yeasts involved in the fermentation of Coffea arabica in East Africa on growth and ochratoxin A (OTA) production by Aspergillus ochraceus

The effects of Pichia anomala, Pichia kluyveri and Hanseniaspora uvarum predominant during coffee processing on growth of Aspergillus ochraceus and production of ochratoxin A (OTA) on malt extract agar (MEA) and on coffee agar (CA) were studied. The three yeasts were able to inhibit growth of A. ochraceus when co-cultured in MEA and CA. Growth inhibition was significantly higher on MEA than on CA. Furthermore, P. anomala and P. kluyveri were found to have a stronger effect on growth of A. ochraceus than H. uvarum. The three yeasts were able to prevent spore germination of A. ochraceus in yeast glucose peptone (MYGP) broth. In yeast-free supernatant of MYGP broth after an incubation period of 72 h, spores of A. ochraceus were able to germinate with very short germ tubes, but further development of the germ tubes was inhibited. The three yeasts decreased the pH of MYGP broth from 5.6 to a range of 4.4-4.7, which was found to have no effect on spore germination of A. ochraceus. P. anomala, P. kluyveri and H. uvarum were able to prevent production of OTA by A. ochraceus when co-cultured on MEA. On CA medium, P. anomala and P. kluyveri prevented A. ochraceus from producing OTA. H. uvarum did not affect production of OTA by A. ochraceus on CA medium.

Biocontrol of the food-borne pathogens Listeria monocytogenes and Salmonella enterica serovar Poona on fresh-cut apples with naturally occurring bacterial and yeast antagonists

Fresh-cut apples contaminated with either Listeria monocytogenes or Salmonella enterica serovar Poona, using strains implicated in outbreaks, were treated with one of 17 antagonists originally selected for their ability to inhibit fungal postharvest decay on fruit. While most of the antagonists increased the growth of the food-borne pathogens, four of them, including Gluconobacter asaii (T1-D1), a Candida sp. (T4-E4), Discosphaerina fagi (ST1-C9), and Metschnikowia pulcherrima (T1-E2), proved effective in preventing the growth or survival of food-borne human pathogens on fresh-cut apple tissue. The contaminated apple tissue plugs were stored for up to 7 days at two different temperatures. The four antagonists survived or grew on the apple tissue at 10 or 25 degrees C. These four antagonists reduced the Listeria monocytogenes populations and except for the Candida sp. (T4-E4), also reduced the S. enterica serovar Poona populations. The reduction was higher at 25 degrees C than at 10 degrees C, and the growth of the antagonists, as well as pathogens, increased at the higher temperature.

The viral killer system in yeast: from molecular biology to application

Since the initial discovery of the yeast killer system almost 40 years ago, intensive studies have substantially strengthened our knowledge in many areas of biology and provided deeper insights into basic aspects of eukaryotic cell biology as well as into virus-host cell interactions and general yeast virology. Analysis of killer toxin structure, synthesis and secretion has fostered understanding of essential cellular mechanisms such as post-translational prepro-protein processing in the secretory pathway. Furthermore, investigation of the receptor-mediated mode of toxin action proved to be an effective means for dissecting the molecular structure and in vivo assembly of yeast and fungal cell walls, providing important insights relevant to combating infections by human pathogenic yeasts. Besides their general importance in understanding eukaryotic cell biology, killer yeasts, killer toxins and killer viruses are also becoming increasingly interesting with respect to possible applications in biomedicine and gene technology. This review will try to address all these aspects.

Production and characteristics of Debaryomyces hansenii killer toxin

The optimal conditions for the production of the killer toxin of Debaryomyces hansenii CYC 1021 have been studied. The lethal activity of the killer toxin increased with the presence of NaCl in the medium used for testing the killing action. Production of the killer toxin was stimulated in the presence of proteins of complex culture media. Addition of nonionic detergents and other additives, such as dimethylsulfoxide enhanced killer toxin production significantly. Killer toxin secretion pattern followed the growth curve and reached its maximum activity at the early stationary phase. Optimal stability was observed at pH 4.5 and temperatures up to 20 degrees C. Above pH 4.5 a steep decrease of the stability was noted. The activity was hardly detectable at pH 5.1.

Isolation, purification, and characterization of a killer protein from Schwanniomyces occidentalis

The yeast Schwanniomyces occidentalis produces a killer toxin lethal to sensitive strains of Saccharomyces cerevisiae. Killer activity is lost after pepsin and papain treatment, suggesting that the toxin is a protein. We purified the killer protein and found that it was composed of two subunits with molecular masses of approximately 7.4 and 4.9 kDa, respectively, but was not detectable with periodic acid-Schiff staining. A BLAST search revealed that residues 3 to 14 of the 4.9-kDa subunit had 75% identity and 83% similarity with killer toxin K2 from S. cerevisiae at positions 271 to 283. Maximum killer activity was between pH 4.2 and 4.8. The protein was stable between pH 2.0 and 5.0 and inactivated at temperatures above 40 degrees C. The killer protein was chromosomally encoded. Mannan, but not beta-glucan or laminarin, prevented sensitive yeast cells from being killed by the killer protein, suggesting that mannan may bind to the killer protein. Identification and characterization of a killer strain of S. occidentalis may help reduce the risk of contamination by undesirable yeast strains during commercial fermentations.

Genetics of heat-curability of killer virus of yeast

The cytoplasmically inherited M double-stranded (ds) RNA genome segment of killer virus of Saccharomyces cerevisiae is heat-curable in some yeast strains but not in others. Temperature sensitivity is conferred on both M1 and M2 dsRNA satellite virus segments by the L-A-HN allele of the killer helper virus genome, but not by the L-A-H allele. Both diploidy and mating type heterozygosity of the host cell are also correlated with increased virus curability.

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.

Killer toxin production in Pichia acaciae is associated with linear DNA plasmids

We have identified a strain of the yeast Pichia acaciae which produces a "killer" toxin active against the yeast Debaryomyces tamarii. The killer phenotype was associated with the presence of two DNA plasmids, pPacl-1 (13.6 kilobase pairs) and pPacl-2 (7.3 kilobase pairs). P. acaciae strains, cured of these plasmids by irradiation with ultraviolet light, lacked killer activity and were sensitive to toxin produced by the parental strain. A partially cured strain, GS-1215, missing only the smaller plasmid, pPacl-2, also exhibited loss of both toxin activity and immunity. Exonuclease studies revealed that both plasmids were linear double-stranded DNA molecules with 5' protected ends. The P. acaciae system differs from that of the well-studied Kluyveromyces lactis "killer" system both in the range of susceptible strains and in the sizes of the plasmids involved. Our studies contradict previous reports that Pichia killer systems are invariably chromosomal.

Killer toxin of Hanseniaspora uvarum

The yeast Hanseniaspora uvarum liberates a killer toxin lethal to sensitive strains of the species Saccharomyces cerevisiae. Secretion of this killer toxin was inhibited by tunicamycin, an inhibitor of N-glycosylation, although the mature killer protein did not show any detectable carbohydrate structures. Culture supernatants of the killer strain were concentrated by ultrafiltration and the extracellular killer toxin was precipitated with ethanol and purified by ion exchange chromatography. SDS-PAGE of the electrophoretically homogenous killer protein indicated an apparent molecular mass of 18,000. Additional investigations of the primary toxin binding sites within the cell wall of sensitive yeast strains showed that the killer toxin of Hanseniaspora uvarum is bound by beta-1, 6-D-glucans.

A K2 neutral Saccharomyces cerevisiae strain contains a variant K2 M genome

K2 neutral strain Saccharomyces cerevisiae USM12 was identified and characterized. This strain carried an M double-stranded RNA (dsRNA) genome encoding for resistance to K2 toxin. The M dsRNA was larger than the K2 killer yeast M dsRNA and homoduplex analysis of denatured and reannealed K2 neurtal M dsRNA revealed an inverted duplication. Heteroduplex analysis showed that two thirds of the K2 M genome had homology with the K2 neutral M genome. Hybridization showed that the USM12 M dsRNA had significant homology with the K2 M dsRNA. Protein profiles of extracellular proteins from USM12 and a cured strain indicated that USM12 did not secrete any toxin. This is the first time that a K2 neutral yeast strain has been characterized.