Chromosomal and nonchromosomal mutations affecting the "killer character" of Saccharomyces cerevisiae

The "killer character" of Saccharomyces cerevisiae is a nonchromosomal genetic element which imparts to cells carrying it (a) the ability to kill cells which lack it, by secreting the soluble macromolecular killer substance and (b) the ability to resist the killing effects of the killer substance.-Mutants have been isolated from S. cerevisiae carrying the "killer character". Mutants were found in two nuclear genes (mak1 and mak2) involved in maintenance of the genetic element of the killer character. Mutants in three other nuclear genes could maintain the cytoplasmic genetic element normally; two of these were able to express resistance to killing, but not the ability to kill (kex1 and kex2), while the third could express the killing function, but was sensitive to killing and is thus a "suicide" strain (rex1). These five nuclear genes were all distinct.-Several mutants were detected which showed non-mendelian segregation indicating mutation of the "cytoplasmic" genetic element. Some such mutants had normal resistance to killing but were either unable to kill at all or showed marked reduction in the ability to kill. Others had lost both killing ability and resistance. None were suppressive.-Two nonkilling mutants, both of which showed cytoplasmic inheritance, were mated. A large, but variable, proportion of diploid colonies, plated after several generations of growth, showed normal killing which has then stably inherited on further subcloning.