A cryptic RNA-binding domain in the Pol region of the L-A double-stranded RNA virus Gag-Pol fusion protein.

The Pol region of the Gag-Pol fusion protein of the L-A double-stranded (ds) RNA virus of Saccharomyces cerevisiae has (i) a domain essential for packaging viral positive strands, (ii) consensus amino acid sequence patterns typical of RNA-dependent RNA polymerases, and (iii) two single-stranded RNA binding domains. We describe here a third single-stranded RNA binding domain (Pol residues 374 to 432), which is unique in being cryptic. Its activity is revealed only after deletion of an inhibitory region C terminal to the binding domain itself. This cryptic RNA binding domain is necessary for propagation of M1 satellite dsRNA, but it is not necessary for viral particle assembly or for packaging of viral positive-strand single-stranded RNA. The cryptic RNA binding domain includes a sequence pattern common among positive-strand single-stranded RNA and dsRNA viral RNA-dependent RNA polymerases, suggesting that it has a role in RNA polymerase activity.