There are at least two yeast viral double-stranded RNAs of the same size: an explanation for viral exclusion.

The yeast virus ScV is similar to other double-stranded RNA fungal viruses, which persist indefinitely without harm to their host cells. A single large viral double-stranded RNA (L) of 4.8 kilobase pairs codes for the major viral capsid polypeptide. Some strains have a second, satellite virus, with a genomic RNA (M) of 1.9 kilobase pairs, that codes for an extracellular toxin (killer toxin) responsible for killing sensitive cells. Sensitive cells lack a functional resistance gene on M. There are a number of different varieties of this satellite virus with different toxin and resistance specificities. Two of these, k1 and k2, are well characterized. A cytoplasmic genetic element causes the loss of the k2 virus. We show that this element is actually one of the k1 viruses; there are two k1 viruses with L genomic RNAs of the same size but different sequence, which account for some of the previously observed 3'-end heterogeneity of L.