Co-curing of plasmids affecting killer double-stranded RNAs of Saccharomyces cerevisiae: [HOK], [NEX], and the abundance of L are related and further evidence that M1 requires L.

We describe two sets of plasmid-plasmid interactions in the yeast Saccharomyces cerevisiae. HOK], EXL], NEX], and KIL-k1] are genetically defined plasmids, and M1 and L are biochemically defined double-stranded RNA plasmids. We show that (i) HOK], NEX], and the abundance of L are related, and (ii) under submaximal curing conditions, all colonies retaining M1 also retain L. There are three pieces of evidence that either NEX] required HOK] for replication or NEX] and HOK] are on the same plasmid. The evidence is as follows. (i) The great majority of strains containing HOK] also contain NEX]. However, two HOK] NEX-o] strains do exist. (ii) Growth at 39 degrees C or growth at 34 degrees C with 3% ethanol or 2-propanol cures HOK] and NEX]. In a HOK] NEX] strain, the two plasmids are always co-cured. (iii) HOK] and NEX] are both maintained in mak4, mak6, and mak27 strains (mak = maintenance of KIL-k1]), but not in mak3, mak10, and pet18 strains. Strains containing HOK] and NEX] have about fourfold more L double-stranded RNA than their isochromosomal, cured derivatives. In addition, a cytoductant which has acquired HOK] and NEX] has fourfold more L than its parent. These results are consistent with either HOK] being a form of L or HOK] increasing the copy number of L. Using a K1 killer strain in which L, as well as M1, could be cured by growth at 38 degrees C, we examined the distribution of loss of M1 and L under conditions giving 98% M-o colonies and at least 50% L-o colonies. No M1L-o colonies were observed, supporting the previous suggestion by others that M1 requires L.