Isolation and nucleotide sequences of the genes encoding killer toxins from Hansenula mrakii and H. saturnus

Abstract The HMK gene, encoding a killer toxin (HMK) of Hansenula mrakii strain IFO 0895, and the HSK gene, encoding a killer toxin (HSK) of H. saturnus strain IFO 0117, were cloned and sequenced. The HMK and HSK genes encode precursors to killer toxins of 125 amino acids (aa) and 124 aa, respectively. Both precursors have an N-terminal signal sequence of 37 aa which may be removed by a signal peptidase, and a propeptide which may be cleaved off by a KEX2-like protease. There is extensive homology between the aa sequences of HMK and HSK with the exception of the addition of one aa residue in HMK. The HMK and HSK genes were placed, separately, downstream from the yeast GAL10 promoter and introduced into a mutant of Saccharomyces cerevisiae that was resistant to the HMK. The transformants were capable of killing sensitive yeasts in medium that contained galactose with killing spectra similar to those of the donor strains of the toxins. These observations suggest that both killer toxins were synthesized and secreted from S. cerevisiae cells and killed sensitive yeasts, perhaps by the same mechanism as that associated with the donor strains and, moreover, that the difference in primary structure between the two toxins is responsible for the difference in their killing spectra.