Killer DNA Plasmids of the Yeast Kluyveromyces lactis : I. Mutations Affecting the Killer Phenotype.

The killer character of Kluyveromyces lactis is associated with a cytoplasmic genetic element which confers on the host cells: a) the capacity to kill sensitive cells of various yeast by producing a diffusible toxin, and b) the immunity to that toxin. The killing activity is associated with the presence of two linear DNA plasmids that we call kl and k2. Both plasmids seem to be required for the expression of the killing activity. Mutants defective for the killing activity have been isolated by mutagenesis and are either cytoplasmic or nuclear. Four classes of cytoplasmic mutations have been found: 1) loss of kl ; 2) loss of both k1 and k2; 3) internal deletion in kl and 4) probable point mutations in the plasmids. Among the nuclear mutants, many conserved the normal plasmids, but expression of the killer phenotype was blocked. In some respects, the organisation of this plasmid system resembles a DNA version of the double-stranded RNA killer system of Saccharomyces cerevisiae.