Nonsense Mutations in the Yeast SUP35 Gene Affect the [PSI] Prion Propagation

The essential SUP35 gene encodes yeast translation termination factor eRF3. Previously, we isolated nonsense mutations sup35-n and proposed that the viability of such mutants can be explained by readthrough of the premature stop codon. Such mutations, as well as the prion [PSI⁺], can appear in natural yeast populations, and their combinations may have different effects on the cells. Here, we analyze the effects of the compatibility of sup35-n mutations with the [PSI⁺] prion in haploid and diploid cells. We demonstrated that sup35-n mutations are incompatible with the [PSI] prion, leading to lethality of sup35-n [PSI] haploid cells. In diploid cells the compatibility of [PSI] with sup35-n depends on how the corresponding diploid was obtained. Nonsense mutations sup35-21, sup35-74, and sup35-218 are compatible with the [PSI] prion in diploid strains, but affect [PSI] properties and lead to the formation of new prion variant. The only mutation that could replace the SUP35 wild-type allele in both haploid and diploid [PSI] strains, sup35-240, led to the prion loss. Possibly, short Sup35_1-55 protein, produced from the sup35-240 allele, is included in Sup35 aggregates and destabilize them. Alternatively, single molecules of Sup35_1-55 can stick to aggregate ends, and thus interrupt the fibril growth. Thus, we can conclude that sup35-240 mutation prevents [PSI] propagation and can be considered as a new pnm mutation.