ISOLATION AND NUCLEOTIDE-SEQUENCES OF THE GENES ENCODING KILLER TOXINS FROM HANSENULA-MRAKII AND H-SATURNUS

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. satur mus nus strain IFO 0117, were cloned and sequenced. The HMK and HSK ge nes 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 h omology 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 plac ed, separately, downstream from the yeast GAL10 promoter and introduce d into a mutant of Saccharomyces cerevisiae that was resistant to the HMK. The transformants were capable of killing sensitive yeasts in med ium 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 a nd killed sensitive yeasts, perhaps by the same mechanism as that asso ciated with the donor strains and, moreover, that the difference in pr imary structure between the two toxins is responsible for the differen ce in their killing spectra.