A mutant killer plasmid whose replication depends on a chromosomal "superkiller" mutation.

Yeast strains carrying a 1.5 x 10(6) molecular weight linear double-stranded RNA in virus-like particles (M dsRNA, the killer plasmid or virus) secrete a toxin that is lethal to strains not carrying this plasmid. Recessive mutations in any of four chromosomal genes (called ski1-ski4) result in increased production of toxin activity. We report here a mutation of the killer plasmid (called [KIL-sd] for ski-dependent) that makes the killer plasmid dependent for its replication on the presence of a chromosomal mutation in any ski gene. Thus, the [KIL-sd] plasmid is lost from SKI(+) strains. When the wild-type killer plasmid, [KIL-k], is introduced into a ski2-2 [KIL-o] strain, the killer plasmid changes to a [KIL-sd] plasmid. This may represent a specific form of mutagenesis or selective replication in the ski2-2 strain of [KIL-sd] variants (mutants) in the normal [KIL-k] population. The ski2-1 and ski2-3 mutations do not convert [KIL-k] to [KIL-sd], but ski2-3 does allow maintenance of the [KIL-sd] plasmid. The [KIL-sd] plasmid thus lacks a plasmid site or product needed for replication in wild-type cells.