The killer systems of Saccharomyces Cerevisiae and other yeasts

One common strategy of viruses is to establish a carrier state in which the host cell survives unharmed. Some bacteriophages, for example, may adopt the disguise of plasmids. Other strategies vary from the simple nonlethal infection of the filamentous single-stranded DNA bacteriophages to the complicated integrative pathways pursued by lambda and the retroviruses. Frequently, among prokaryotes, viruses or plasmids provide the host with a selective advantage: synthesis of restriction enzymes, colicins, antibiotic degradative enzymes, or provision of immunity to infection. In one group of simple eukaryotes, the fungi, two such associations of plasmids or viruses conferring a selective advantage on the host are known. These are the killer systems of the yeasts and of Ustilago maydis, a corn smut that adopts a yeast-like mode of growth on laboratory media. Cells with a double-stranded RNA (dsRNA) virus, in several yeasts and in Ustilago, or with a linear double-stranded DNA (dsDNA) plasmid, in the yeast Kluyveromyces lactis, synthesize a secreted toxin that kills cells of the same (or in some cases different) species lacking the virus or plasmid. The easily recognizable phenotype in the killer systems has made it possible to define their necessary elements. There are both nuclear and viral genes.