117 Translation termination in eukaryotic cells is deter mined by proteins Sup35 (eRF3) and Sup45 (eRF1) [1], which interact with a large number of partners [2]. In yeast Saccharomyces cerevisiae, protein Sup35 can form an aggregating epigenetically inherited con former (prion) [PSI+] [3]. This prion is carried through the cytoplasm and causes disturbances in translation termination, which are phenotypically identified as the dominant omnipotent nonsense sup pression. [PSI+] variants with different properties (nonsense suppression efficiency and transmission stability in mitosis) can be obtained in the same yeast strain. The presence of prion [PSI+] leads to lethality in the haploid yeast strain carrying mutations in the gene encoding another termination factor, Sup45 [4]. We have shown that the combination in the diploid strain of some mutant alleles of the SUP45 gene in the heterozygous state with prion [PSI+] entails the death of the hybrid [5]. The synthetic lethality of prion [PSI+] and mutant allele of the sup45 gene depends both on the type of mutant allele and the prion variant. Variant [PSI+], which is a strong suppressor (“strong” [PSI+], or [PSI+]S), causes synthetic lethality with all nonsense mutations and some missense mutations sup45 in the heterozygote. Our data indicate that the lethality of hybrids is correlated with a decreased activity of the Sup45 protein in the cell in case of sup45 mutations. This paper describes a test system that allows identification of proteins that affect the stability of prion [PSI+] and/or the efficiency of translation ter mination by their effect on the synthetic lethality of the prion conformer Sup35 and mutant alleles of SUP45. This test system is suitable to search for pro teins that affect the translation termination efficiency and/or prion maintenance in yeast cells. Gene library screening using this test system allowed us to identify the CUR1 gene, whose influence on another prion, [URE3], was shown earlier but the effect on transla tion termination factors was not known. This test system is based on qualitative assessment of the viability of a diploid strain containing simulta neously (1) prion [PSI+], (2) sup45 mutation in the heterozygote, and (3) test gene(s) in a multicopy plas mid (Fig. 1). Synthetic lethality is manifested as the absence of growth of diploids with genotype SUP45/sup45 [PSI+] (Fig. 2, mutations 101–107, 111, and 116 are incompatible with the “strong” [PSI+]S). If the efficiency of translation termination increases due to the presence of additional copies of the test gene (Х), formation of viable hybrids with genotype +/sup45 [PSI+] [gene Х]n is observed. Conversely, a decrease in the translation termination efficiency in Identification of Genes Influencing Synthetic Lethality of Genetic and Epigenetic Alterations in Translation Termination Factors in Yeast