DOMINANT CHROMOSOMAL MUTATION BYPASSING CHROMOSOMAL GENES NEEDED FOR KILLER RNA PLASMID REPLICATION IN YEAST

Reed B. Wickner ¹ and Michael J. Leibowitz ¹

¹ Laboratory of Biochemical Pharmacology, National Institute of Arthritis, Metabolism, and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20014

Yeast strains carrying a double-stranded RNA plasmid of 1.4–1.7 x 10⁶ daltons encapsulated in virus-like particles secrete a toxin that kills strains lacking this plasmid. The plasmid requires at least 24 chromosomal genes (pets, and mak1 through mak23) for its replication or maintenance. We have detected dominant Mendelian mutations (called KRB1 for killer replication bypass) that bypass two chromosomal genes, mak7 and pets, normally needed for plasmid replication. Strains mutant in mak7 and carrying the bypass mutation (mak7–1 KRB1) are isolated as frequent K⁺R⁺ sectors of predominantly K^-R ^- segregants from crosses of mak7–1 with a wild-type killer. All KRB1 mutations isolated in this way are inherited as single dominant centromere-linked chromosomal changes. They define a new centromere. KRB1 is not a translational suppressor. KRB1 strains contain a genetically normal killer plasmid and ds RNA species approximately the same in size and amount as do wild-type killers. Bypass of both mak7 and pets by one mutation suggests that these two genes are functionally related.

Two properties of the inheritance of KRB1 indicate an unusually high reversion frequency: (1) Heat or cycloheximide (treatments known to cure strains of the wild-type killer plasmid) readily induce conversion of mak7–1 KRB1 strains from killers to nonkillers with concomitant disappearance of KRB1 as judged by further crosses, and (2) mating two strains of the type mak7–1 KRB1 with each other yields mostly 2 K⁺R⁺: 2 K^-R^- segregation, although the same KRB1 mutation and the same killer plasmid are present in both parents.