The prevalence of killer yeasts and double-str anded RNAs in the budding yeast Saccharomyces cerevisiae

Abstract

Killer toxins are antifungal proteins produced by many species of "killer"yeasts, including the brewer's and baker's yeast Saccharomyces cerevisiae . Screening 1270 strains of S. cerevisiae for killer toxin production found that 50% are killer yeasts, with a higher prevalence of yeasts isolated from human clinical samples and winemaking processes. Since many killer toxins are encoded by satellite double- str anded RN As (dsRN As) associated with m ycoviruses, S . cer evisiae str ains w ere also assay ed for the presence of dsRN As. This screen identified that 51% of strains contained dsRNAs from the mycovirus families Totiviridae and Partitiviridae , as well as satellite dsRNAs. Killer toxin production was correlated with the presence of satellite dsRNAs but not myco viruses. Ho wever, in most killer yeasts, whole genome analysis identified the killer toxin gene KHS1 as significantly associated with killer toxin production. Most killer yeasts had unique spectrums of antifungal activities compared to canonical killer toxins, and sequence analysis identified mutations that altered their antifungal activities. The prevalence of mycoviruses and killer toxins in S . cer evisiae is important because of their known impact on yeast fitness, with implications for academic research and industrial application of this yeast species.