RIBOSOMAL FRAMESHIFTING EFFICIENCY AND GAG GAG-POL RATIO ARE CRITICAL FOR YEAST M(1) DOUBLE-STRANDED-RNA VIRUS PROPAGATION

About 1.9% of ribosomes translating the gag open reading frame of the yeast L-A double-stranded RNA virus positive strand undergo a - 1 frameshift and continue translating in the pol open reading frame to make a 170-kDa gag-pol fusion protein. The importance of frameshifting efficiency for viral propagation was tested in a system where the M1 (killer toxin-encoding) satellite RNA is supported by a full-length L-A cDNA clone. Either increasing or decreasing the frameshift efficiency more than twofold by alterations in the slippery site disrupted viral propagation. A threefold increase caused by a chromosomal mutation, hsh1 (high shifter), had the same effect. Substituting a +1 ribosomal frameshift site from Tyl with the correct efficiency also allowed support of M, propagation. The normal -1 frameshift efficiency is similar to the observed molar ratio in viral particles of the 170-kDa gag-pol protein to the 70-kDa gag gene product, the major coat protein. The results are interpreted in terms of a packaging model for L-A.