Plasmids controlling exclusion of the K2 killer double-stranded RNA plasmid of yeast

Saccharomyces strains of two types (K₁⁺R₁⁺ and K₂⁺R₂⁺) kill each other and K^?R^?-sensitive strains by secreting protein toxins. K₁ killer strains carry a 1.25 × 10⁶ dalton double-stranded RNA plasmid, KIL-k₁], while K₂ killers have a 1.0 × 10⁶ dalton double-stranded RNA plasmid, KIL-k₁]. Mating KIL-k₁] haploids with KIL-k₂] haploids yields only KIL-k₁] diploids, that is, KIL-k₁] excludes KIL-k₂]. EXL], a new non-Mendellan genetic element from a nonkiller strain, excludes KIL-k₂] but does not exclude KIL-k₂]. A second new non-Mendelian genetic element, called NEX], when present prevents EXL] from excluding KIL-k₂]. NEX] does not prevent KIL-k₁] or KIL-s₁] (a suppressive mutant of KIL-k₁]) from excluding KIL-k₂]. A chromosomal gene, called MKT1, is needed for maintenance of KIL-k₂] if NEX] is present. In the absence of NEX], KIL-k₂] does not need MKT1. KIL-k₁] does not need MKT1 even if NEX] is present. EXL] replication depends on at least the products of MAK1, MAK3, MAK10and PET18. NEX]replication depends on MAK3 but is independent of MAK4, MAKE, MAK27 and MKT1.