Killer of Saccharomyces cerevisiae: a double stranded ribonucleic acid plasmid

Abstract

Certain strains of the yeast S. cerevisiae (called killers) secrete a substance (the killer toxin) which is lethal to other strains of the same species (called sensitives). Current evidence indicates that the yeasts secreting the toxin(s) are those carrying a double stranded ribonucleic acid (dsRNA) species encapsulated in intracellular virus like particles. The maintenance or replication of the killer plasmid (dsRNA in virus like particles) requires at least 10 chromosomal genes (mak), whereas expression of killing and resistance requires 3 other chromosomal genes (kex and rex). Defective interfering plasmid mutants ('suppressives') and plasmid mutants dependent on chromosomal diploidy for expression and maintenance have been described. The killer character of yeast is a non Mendelian trait which has been correlated with the presence of a dsRNA species of molecular weight about 1.4 x 106 to 1.7 x 106 encapsulated in isometric virus like particles of about 40 nm diameter. The fact that this RNA species is double stranded and is encapsulated suggests that it is indeed the cytoplasmic killer genome rather than a transcript of a DNA genome, but this conclusion will not be established until hybridization experiments show whether or not a DNA genome is present in killer strains. Infection of cells with the isolated virus like particles or transformation with the isolated dsRNA would be convincing positive evidence of the nature of the cytoplasmic killer genome. There is at present no information concerning how the killer dsRNA replicates; the process might resemble that described for reovirus, but there are other possibilities, including DNA involvement, as in RNA tumor viruses. RNA tumor is reasonable to expect that such a small genome, able to code for only about 100,000 daltons of protein would depend heavily on host proteins for its replication and for expression of its information. Although the cytoplasmic killer genome is necessary for toxin production and imparts resistance to a normal host, it is not yet clear that the toxin is coded for by the cytoplasmic genome.