Most strains of S. cerevisiae harbor a symbiotic virus called 'Killer', a bipartite dsRNA viral system composed of the L-A helper virus and the M 1 satellite [reviewed in (37) ]. The 4.6-kb dsRNA L-A viral genome encodes the viral coat protein (Gag), and the Gag-pol replicase that is encoded by a programmed À1 ribosomal frameshift (À1 PRF) (38) . The M 1 satellite dsRNA is encapsidated and replicated inside of L-A encoded viral particles. The M 1 encoded preprotoxin is processed by the Kex1p and Kex2p cellular proteases into the mature secreted toxin. Cells infected by L-A and M 1 can kill uninfected cells, but are themselves immune, hence the name 'Killer'. The presence of Killer can be easily assayed by replica plating test cells onto a lawn of diploid uninfected (indicator) cells: Killer + cells will kill the nearby indicator cells, resulting in a ring of growth inhibition around the test cells. One of the first yeast mutants cloned, mak8-1, was first identified by its inability to maintain the Killer phenotype (Mak À phenotype) and encodes an allele of rpl3 (39) . Analyses of the L3 basic thumb mutants revealed that three of the single mutants (K236A, K237A and R247A) were completely unable to maintain the Killer virus, while R232A and R244A had weak Killer phenotypes. None of the double mutants were able to maintain the virus ( Figure 2B , summarized in Table 1 ).