DECOYING THE CAP(-) MESSENGER-RNA DEGRADATION SYSTEM BY A DOUBLE-STRANDED-RNA VIRUS AND POLY(A)- MESSENGER-RNA SURVEILLANCE BY A YEAST ANTIVIRAL SYSTEM

The major coat protein of the L-A double-stranded RNA virus of Sacchar omyces cerevisiae covalently binds m(7)GMP from 5' capped mRNAs in vit ro. We show that this cap binding also occurs in vivo and that, while this activity is required for expression of viral information (killer toxin mRNA level and toxin production) in a wild-type strain, this req uirement is suppressed by deletion of SKI1/XRN1/SEP1. We propose that the virus creates decapped cellular mRNAs to decoy the 5' --> 3' exori bonuclease specific for cap(-) RNA encoded by XRN1. The SKI2 antiviral gene represses the copy numbers of the L-A and L-BC viruses and the 2 0S RNA replicon, apparently by specifically blocking translation of vi ral RNA. We show that SKI2, SKI3, and SKI8 inhibit translation of elec troporated luciferase and beta-glucuronidase mRNAs in vivo, but only i f they lack the 3' poly(A) structure. Thus, L-A decoys the SKI1/XRN1/S EP1 exonuclease directed at 5' uncapped ends, but translation of the L -A poly(A)(-) mRNA is repressed by Ski2,3,8p. The SKI2-SKI3-SKI8 syste m is more effective against cap(+) poly(A)(-) mRNA, suggesting a (none ssential) role in blocking translation of fragmented cellular mRNAs.