The expression of cDNA clones of yeast M1 double-stranded RNA in yeast confers both killer and immunity phenotypes.

Two cDNA clones of the segment of Saccharomyces cerevisiae M1 double-stranded RNA, which codes for the yeast killer toxin, have been expressed in yeast using the expression vector pYT760. Toxin expression and secretion depended upon the presence of a yeast promoter. Transformants not only contain an authentic preprotoxin precursor, as determined by precipitation of intracellular proteins with antitoxin antisera, but also display an immunity phenotype. The evidence is that the immunity protein is part of the preprotoxin and may act by masking toxin binding sites. Neither cDNA clone had a complete 5' terminus and the preprotoxin translational start was missing. The promoter and the initiator ATG were supplied by the expression vector. One clone with a full-length preprotoxin but altered N-terminal amino acids gave a normal glycosylated intracellular precursor. A clone with an N-terminal nine amino acid deletion gave a precursor which was not glycosylated but toxin was still secreted.