Selection and stability of yeast transformants expressing cDNA of an Ml killer toxin-immunity gene

Summary Transformants of sensitive yeast strains containing an expressed cDNA copy of the yeast killer toxin-immunity gene could be selected for by exposure to added killer toxin. For strain AH-22 the transformation frequency was approximately 10% that obtained by selection for leucine prototrophy. The procedure required time for expression of immunity prior to selection, and a screening step to remove non-transformed survivors. Under conditions where active toxin was produced, transformants containing the toxin-immunity gene were at a selective advantage, and cells losing the plasmid were killed. This resulted in self selection of transformants, and affords a way of maintaining plasmid stability in protrophic strains.