Conservative replication of double-stranded RNA in Saccharomyces cerevisiae by displacement of progeny single strands.

Saccharomyces cerevisiae contains two double-stranded RNA (dsRNA) molecules, L and M, encapsulated in virus-like particles. After cells are transferred from dense (13C 15N) to light (12C 14N) medium, only two density classes of dsRNA are found, fully light (LL) and fully dense (HH). Cells contain single-stranded copies of both dsRNAs and, at least for L dsRNA, greater than 99% of these single strands are the positive protein-encoding strand. Single-stranded copies of L and M dsRNA accumulate rapidly in cells arrested in the G1 phase. These results parallel previous observations on L dsRNA synthesis and are consistent with a role of the positive single strands as intermediates in dsRNA replication. We propose that new positive strands are displaced from parental molecules and subsequently copied to produce the completely new duplexes.