Spermidine or spermine requirement for killer double-stranded RNA plasmid replication in yeast.

The spe2 gene of Saccharomyces cerevisiae codes for S-adenosylmethionine decarboxylase, an enzyme in the biosynthetic pathway for the polyamines spermidine and spermine. We previously found that spe2 mutants completely lacking enzymic activity have no spermidine or spermine when grown in polyamine-free medium and require spermidine or spermine for meiotic sporulation and optimal growth. We now show that the killer plasmid, a 1.5 x 10” molecular weight doublestranded RNA (M) in intracellular virus-like particles, is lost from spe2 killer strains grown without spermidine or spermine. In contrast, the 3.0 x lo6 molecular weight double-stranded RNA (L), the mitochondrial genome (p), and the cytoplasmic element that potentiates weak ochre suppressors (4) are not lost under the same conditions. Once the killer plasmid is lost from a polyamine-deficient strain, it cannot be restored by polyamine addition, and such a strain is sensitive to the killer toxin excreted by strains containing the plasmid. Recessive mutations in any of four chromosomal genes (skil, ski2, ski3, and skil) that result in overproduction of killer toxin activity (A. Toh-e and R. B. Wickner, personal communication) bypass the polyamine requirement for killer plasmid maintenance, but not the spe2-related growth defect. Thus, spe2 skistrains grown on polyamine-free medium are killers. The polyamine requirement for killer plasmid maintenance is not bypassed by the absence of the mitochondrial genome or the chromosomal KRBl mutation, which do bypass other chromosomal genes needed for killer plasmid maintenance.