RNA structural requirements for RNA binding, replication, and packaging in the yeast double-stranded RNA virus.

Several sites of interaction with viral proteins have been mapped in the plus strand of the Saccharomyces cerevisiae double-stranded RNA virus, ScV. These include a site necessary and sufficient for viral particle binding to plus strands (VBS) and a site necessary and sufficient for interference with replication of viral plus strands (INS). We show that the INS and VBS are identical and that they are necessary and sufficient for packaging. One of the viral RNAs has two adjacent VBS, which have additive INS activity. The second VBS has similar affinity for viral particles as the first but its complex with particles exhibits faster dissociation. Binding to the two sites in the viral RNA is independent but equivalent. This may mean two recognition sites per particle. This is what would be expected for two molecules of the protein thought to be responsible for sequence specific recognition, the cap-pol fusion polypeptide, per particle. Comparison of the secondary structures of several binding sites, as determined by site-directed mutagenesis and by ribonuclease mapping of single- and double-stranded regions, reveals a requirement for a specific loop sequence and two stem regions, in which there is no sequence specificity. Contrary to what was described in previous work, a bulge loop, but not a "bulged" A residue is necessary for binding of the second VBS. A minimal region of 30 bases is enough to cause both packaging and replication.