Yeast virus-like particles possess a capsid-associated single-stranded RNA polymerase.

Abstract

MANY species of fungi possess virus-like particles with double-stranded RNA (dsRNA) genomes1,2. In yeast the dsRNA genome has been shown to be the genetic determinant of the ‘killer’ character3–10. No cycle of lysis and reinfection has been demonstrated for ‘viruses’ of this group and replication is integrated with host growth1,2. Particles from several species of fungi have been shown to possess RNA polymerase activity and we have shown that yeast particles isolated from growing cells synthesise dsRNA (refs 11–14). We report here that particles isolated from non-growing cells synthesise both dsRNA and a single-stranded (ssRNA) species with properties suggesting that it is a complete transcript of the dsRNA genome. Our results with particles from growing cells suggested that the replicative cycle of yeast ‘virus’ resembles that of reovirus14 and does not occur through the ‘doubling cycle’ which was put forward by Buck13 to account for his results with Penicillium stoloniferum S. virus. The discovery of a ssRNA polymerase activity in yeast particles is consistent with the reovirus type of replicative cycle.