Occurrence of killer yeast strains in industrial and clinical yeast isolates

Marcelo E Baeza,Mario A Sanhueza,Víctor H Cifuentes

doi:10.4067/s0716-97602008000200007

Abstract

The secretion of proteinaceous toxins is a widespread characteristic in environmental and laboratory yeast isolates, a phenomenon called "killer system". The killer phenotype (K+) can be encoded by extrachromosomal genetic elements (EGEs) as double stranded DNA or RNA molecules (dsDNA, dsRNA) or in nuclear genes. The spectrum of action and the activity of killer toxins are influenced by temperature, salinity and pH of media. In the present work we determined the existence of K+ in a collection of S. cerevisiae and P. anomala yeasts isolated from environmental, industrial and clinical sources. The assays were performed in strains belonging to three yeast genera used as sensitive cells and under a wide range of pH and temperatures. Approximately 51 % of isolates tested showed toxicity against at least one sensitive yeast strain under the conditions tested. The K+ P. anomala isolates showed a wide spectrum of action and two of them had toxic activity against strains of the three yeast genera assayed, including C. albicans strains. In all S. cerevisiae K+ isolates an extrachromosomal dsRNA molecule (4.2 Kb) was observed, contrary to P. anomala K+ isolates, which do not possess any EGEs. The K+ phenotype is produced by an exported protein factor and the kinetics of killer activity production was similar in all isolates with high activity in the log phase of growth, decaying in the stationary phase.

Highlights

Many yeast synthesize and export proteins or glycoproteins with toxic effects against sensitive yeasts, a phenomenon called “killer system” (Young and Yagiu, 1978; Tipper and Bostian, 1984; Magliani et al, 1997; Marquina et al, 2002)
The determinations of the optimal pH for killer activity were performed at 22 oC for S. cerevisiae and R. sloffiae, and at 30 oC for C. albicans lawns
Differences in the spectrum of activity among S. cerevisiae isolated from wine fermentation suggest the existence of different strains that can be grouped in: i) active only against S. cerevisiae AH22 (10 isolates); ii) active against S. cerevisiae AH22 and R. sloffiae (2 isolates); and iii) no killer strains

Summary

Introduction

Many yeast synthesize and export proteins or glycoproteins with toxic effects against sensitive yeasts, a phenomenon called “killer system” (Young and Yagiu, 1978; Tipper and Bostian, 1984; Magliani et al, 1997; Marquina et al, 2002). The killer activity of yeast is detectable only when it is assayed against proper yeast as sensitive, and is dependent on several factors, such as pH, salinity and temperature. The killer toxins described are active at pH values from 3 to 5.5 (Golubev and Shabalin, 1994; Marquina et al, 2002). The genetic elements that encode for a killer phenotype may be double stranded RNA molecules (dsRNA) encapsulated in viruslike particles (VLPs), linear double stranded DNA plasmids (dsDNA) or nuclear genes (Schmitt and Breinig, 2002). The killer system of S. cerevisiae is the best studied model, corresponding to a genetically complex phenomenon because it depends both on cytoplasmic factors and approximately forty cellular genes

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Conclusion