Mutual antagonism among killer yeasts: competition between K1 and K2 killers and a novel cDNA-based K1-K2 killer strain of Saccharomyces cerevisiae.

Mutually antagonistic K1 and K2 killer strains compete when mixed and serially subcultured. At pH 4.6, where the K1 killer toxin is more stable in vitro, the K1 strain outcompeted the K2 strains at both 18 and 30 degrees C. At pH 4.0, closer to the in vitro pH optimum of the K2 killer toxin, the K1 strain again predominated at 18 degrees C, but at 30 degrees C the K2 strains became the sole cell type on subculture. To show more clearly that these results were dependent upon the respective killer toxins, control experiments were conducted with isogenic, nonkiller strains cured of the dsRNA-based killer virions. Such nonkiller strains were unable to compete with antagonistic killers under conditions where their isogenic killer parents could, strongly suggesting that the killer phenotype was important in these competitions. Double K1-K2 killer strains cannot stably exist, as their dsRNA genomes compete at a replicative level. Using recombinant DNA methodology, a stable K1-K2 killer strain was constructed. This strain outcompeted both K1 and K2 killers when serially subcultured under conditions where either the K1 or the K2 strains would normally predominate in mixed cultures. Such a double killer may be useful in commercial fermentations, where there is a risk of contamination by killer yeasts.