["Killer" plasmid mutants obtained by exposure to 5-fluorouracil].

Weak killers were tolerated from a killer strain of type k2 of Saccharomyces cerevisiae by treatment with 5-fluorouracil. They segregated sensitive clones during vegetative growth with a frequency from 2 to 20 per cent (nonstable killers, Knst mutants). Alteration of killer activity in these mutants is the result of a mutation in the cytoplasmic killer determinant (KIL-k). The mutant form of this determinant, KIL-nst, partially suppresses the wild type determinant. K-nst have the same dsRNA composition as the wild type killer strain. In some mutants the decrease in M dsRNA amount is possible. The segregation of sensitive mitotic clones of these mutants is probably connected with secondary alterations of the KIL-nst determinant.