A New Non-Mendelian Genetic Element of Yeast That Increases Cytopathology Produced by M₁ Double-Stranded RNA in ski Strains

Rosa Esteban ¹ and Reed B. Wickner ¹

¹ Section on Genetics of Simple Eukaryotes, Laboratory of Biochemical Pharmacology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892

The Saccharomyces cerevisiae SKI (superkiller) genes are repressors of replication of M, L-A, and L-BC double-stranded (ds) RNAs; ski strains have an increased M dsRNA copy number and, as a result, are cold-sensitive for growth at 8°. Growth is normal, however, at higher temperatures. We have found a new cytoplasmic genetic element [D] (for disease) that makes M₁ dsRNA-containing superkiller strains grow slowly at 30°, not at all at 37°, and only very poorly at 20°. These growth defects require three factors: a chromosomal ski mutation, the presence of M₁ dsRNA, and the presence of the new cytoplasmic factor, [D]. We have isolated mutants unable to ma intain [D] (mad), at least one of which is due to mutation of a single chromosomal locus. Further, [D] can be cured by growth at 37–39°. We present evidence that [D] is not M, L-A, L-BC or W dsRNAs or mitochondrial DNA, 2µ DNA, or [psi], but [D] depends on L-A for its maintenance. We also show that [D] is distinct from [B], a cytoplasmic element that allows M ₁ dsRNA to be stably replicated and maintained in spite of defects in certain chromosomal MAK genes that would otherwise be necessary. [D] activity is blocked by the presence of another extrachromosomal element, called [DIN] (for [D] interference). [D] and [DIN] may be different natural variants of the same molecule.

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