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.

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Sommer SS, Wickner RB

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