RNA-DEPENDENT RNA-POLYMERASE ACTIVITY RELATED TO THE 20S RNA REPLICONOF SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae contains two double-stranded RNA (dsRNA) viru ses (L-A and L-BC) and two different single-stranded (ssRNA) replicons (20S RNA and 23S RNA). Replicase (dsRNA synthesis on a ssRNA template ) and transcriptase (ssRNA synthesis on a dsRNA template) activities h ave been described for L-A and L-BC viruses, but not for 20S or 23S RN A. We report the characterization of a new in vitro RNA replicase acti vity in S. cerevisiae. This activity is detected after partial purific ation of a particulate fraction in CsCl gradients where it migrates al the density of free protein. The activity does not require the presen ce of L-A or L-BC viruses or 23S RNA, and its presence or absence is c orrelated with the presence or absence of the 20S RNA replicon. Strain s lacking both this RNA polymerase activity and 20S RNA acquire this a ctivity when they acquire 20S RNA by cytoduction (cytoplasmic mixing). This polymerase activity converts added ssRNA to dsRNA by synthesis o f the complementary strand, but has no specificity for the 3' end or i nternal template sequence. Although it replicates all tested RNA templ ates, it has a template size requirement, being unable to replicate te mplates larger than 1 kb. The replicase makes dsRNA from a ssRNA templ ate, but many single-stranded products due to a terminal transferase a ctivity are also formed. These results suggest that, in contrast to th e L-A and L-BC RNA polymerases, dissociation of 20S RNA polymerase fro m its RNA (or perhaps some cellular factor) makes the enzyme change it s specificity.