PRIONS AND RNA VIRUSES OF SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae is host to the dsRNA viruses L-A (including i ts killer toxin-encoding satellite, M) and L-BC, the 20S and 23S ssRNA replicons, and the putative prions, [URE3] and [PSI]. review the gene tic and biochemical evidence indicating that [URE3] and [PSI] are prio n forms of Ure2p and Sup35p, respectively. Each has an N-terminal doma in involved in propagation or generation of the prion state and a C-te rminal domain responsible for the protein's normal function, nitrogen regulation, or translation termination, respectively. The L-A dsRNA vi rus expression, replication, and RNA packaging are reviewed. L-A uses a -1 ribosomal frameshift to produce a Gag-Pol fusion protein. The hos t SKI2, SKI3, and SKI8 proteins block translation of nonpoly(A) mRNAs (such as viral mRNA). Mutants deficient in 60S ribosomal subunits repl icate L-A poorly, but not if cells are also ski(-). Interaction of 60S subunits with the 3' polyA is suggested. SKI1/XRN1 is a 5' --> 3' exo ribonuclease that degrades uncapped mRNAs. The viral Gag protein decap itates cellular mRNAs apparently to decoy this enzyme from working on viral mRNA.