Strains of Saccharomyces cerevisiae carrying a small double-stranded RNA species (the killer plasmid) secrete a toxin which is lethal only to strains not carrying this plasmid. We have isolated mutants in eight chromosomal genes essential for replication or maintenance of the killer plasmid, called mak1 through mak8. Seven of these genes have been mapped. mak4 and mak5 are on chromosome II; mak1 and mak8 are on chromosome XV; mak3 and mak6 are on chromosome XVI; and mak7 is on chromosome VIII. We have not yet located mak2. Two other chromosomal genes, m and pets, have been shown to be required for replication or maintenance of the killer plasmid. One allele of mak1 results in temperature sensitivity for host growth. Two independent pets isolates also result in the petite phenotype, as well as temperature sensitivity for growth. Wild-type killer strains have been reported to carry two species of doublestranded RNA of 2.5 × 106 and 1.4 × 106 molecular weight (designated L and M, respectively); wild-type non-killers carried only L. We estimate the size of the L and M species at 3.0 × 106 and 1.7 × 106 daltons, respectively. We have also detected a third species of double-stranded RNA of molecular weight 3.8 × 106 (XL) present in all killer and non-killer strains examined. Mutation of any of mak1 through mak8 results in loss of the killer-associated species of double-stranded RNA (M; 1.7 × 106). These mutants retain both the L species (3.0 × 106) and the XL species (3.8 × 106) of double-stranded RNA, and have acquired two new minor RNA species.
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