A -1 ribosomal frameshift in a double-stranded RNA virus of yeast forms a gag-pol fusion protein

The L-A double-stranded RNA (dsRNA) virus of Saccharomyces cerevisiae has two open reading frames (ORFs). ORF1 encodes the 80-kDa major coat protein (gag). ORF2, which is expressed only as a 180-kDa fusion protein with ORF1, encodes a single-stranded RNA-binding domain and has the consensus sequence for RNA-dependent RNA polymerases of (+)-strand and double-stranded RNA viruses (pol). We show that the 180-kDa protein is formed by -1 ribosomal frame-shifting by a mechanism indistinguishable from that of retro-viruses. Analysis of the "slippery site" suggests that a low probability of unpairing of the aminoacyl-tRNA from the 0-frame codon at the ribosomal A site reduces the efficiency of frameshifting more than the reluctance of a given tRNA to have its wobble base mispaired. Frameshifting of L-A requires a pseudoknot structure just downstream of the shift site. The efficiency of the L-A frameshift site is 1.8%, similar to the observed molar ratio in viral particles of the 180-kDa fusion protein to the major coat protein.