Specificity of polyamine requirements for the replication and maintenance of different double-stranded RNA plasmids in Saccharomyces cerevisiae.

We have shown previously that the M1 double-stranded (ds) RNA (i.e., the killer plasmid [KIL-k1]) that codes for a protein toxin requires spermidine or spermine for its replication. We now report that replication of two other ds RNA plasmids of yeast also requires polyamines: (i) M2 ds RNA [( KIL-k2]) and (ii) L-A-E, a ds RNA plasmid carrying the non-Mendelian genetic element [EXL]. Putrescine alone is sufficient to maintain L-A-E but is not sufficient to maintain either M1 ds RNA or M2 ds RNA, which require either spermidine or spermine. Once M1 or M2 or L-A-E is lost, it cannot be restored by the addition of polyamines. In contrast, L-A-HN, a ds RNA molecule that carries the cytoplasmic genes [HOK] and [NEX], is not lost during polyamine deprivation. It is striking that polyamine deprivation differentially affects L-A-E and L-A-HN, even though these two ds RNA molecules have more than 99% homology. L-C, which is the same size as L-A but very different in sequence, is also not lost on polyamine starvation.