The L3 basic thumb mutants affect cell growth, programmed À1 ribosomal frameshifting and yeast Killer virus maintenance rpd3Dcells harboring wild-type pRPL3-Ura were transformed with mutant rpl3 alleles expressed from a low copy TRP1 vector, and the viability of mutants was assessed by their ability to grow in the presence of 5-FOA. All of the L3 basic thumb single mutants (R232A, K236A, K237A, K241A, R244A, R247A and R248A) were viable as the sole forms of L3 with the exception of R240, which was lethal (Figure 2A , summarized in Table 1 ). Among the viable single mutants, only R232A and R247A conferred noticeable growth defects. A series of double mutants were constructed based on the viable mutants and their physical locations relative to one another. This analysis revealed that the K236A/R247A and K241A/R244A mutants significantly affected cell growth, while the R247A/ R248A mutant was inviable. The K236A/K237A double mutant did not grossly affect cell growth.