Killer Systems in Saccharomyces cerevisiae

Abstract

INTRODUCTION
Killer strains of Saccharomyces cerevisiae are those secreting a protein toxin that is lethal to most nonkiller S. cerevisiae strains (Makower and Bevan 1963; Woods and Bevan 1968) (Fig. 1). The two distinct killer traits (K₁ and K₂) are carried by double-stranded (DS) RNA plasmids in intracellular particles. These particles are not infectious, but they have an RNA polymerase activity that produces a single-stranded (SS) product. The killer plasmids interact with each other and with other non-Mendelian genetic elements. These DS RNA genomes depend heavily on the host genome for their replication, expression, and regulation. The K₁ killer toxin is a small protein that binds to sensitive cells and kills them, apparently by inducing the release of cellular ATP and potassium into the medium.

The killer systems provide useful models for the study of the control of viral or plasmid replication, especially the role of the host, plasmid or viral interference, defective-interfering (DI) particles, protein processing and secretion, and toxin action and receptors. The facts, models, and problems of various aspects of these systems will be discussed in this paper. Several recent reviews have also appeared (Pietras and Bruenn 1976; Wickner 1976a, 1979a; Toh-e 1979; Vodkin and Alianell 1979; Bruenn 1980; Bussey 1981).

Killer strains have been identified in many fungal genera, including Saccharomyces, Ustilago, Torulopsis, Debaromyces, Hansenula, Kluyveromyces, Candida, Pichia, and Cryptococcus (Makower and Bevan 1963; Puhalla 1968; Hankin and Puhalla 1971; Maule and Thomas 1973; Naumov and Naumova 1973; Bussey and Skipper 1975; Philliskirk and Young 1975;...