The (+) strand of the L-A dsRNA virus of Saccharomyces cerevisiae has two large open reading frames, ORF1, encoding the major coat protein, and ORF2 that encodes a single-stranded RNA binding protein having a sequence diagnostic of viral RNA-dependent RNA polymerases. ORF2 is expressed only as a """"""""gagpol"""""""" type fusion protein with ORF1. We have constructed a plasmid expressing these proteins and show that it can support the replication of the killer toxin-encoding Ml satellite virus in the absence of an L-A dsRNA helper virus. ORF1 expression makes the strain a host ski-phenocopy, suppressing M1's requirement for MAK11, MAK18, and MAK27 genes, allowing a defective L-A (L-A-E) to support M1 replication and making a wild-type killer a superkiller. We have defined the packaging signal present on L-A (+) strands and on the (+) strands of satellite viruses of L-A. into When this signal is inserted foreign RNAs, those RNAs are packaged in L-A virus particles. have we demonstrated that ORF1 and ORF2 are fused to make the """"""""gag-pol"""""""" fusion protein by -1 ribosomal frameshifting, the same mechanism used by retroviruses for this purpose. Our studies of this process in the case of L-A show that the strength of mRNA-tRNA binding at the ribosomal A-site is an important factor determining the efficiency of frameshifting. we propose that the fusion proteins of L-A and retroviruses use their pol domain to hold onto genomic RNA while their gag domain associates with gag molecules to prime coat formation.
