Genetic interactions between [ PSI ⁺ ] and nonstop mRNA decay affect phenotypic variation

Abstract

The functions of the eRF3 protein in translation termination and prion propagation are easily separated. The N-terminal domain of eRF3 is required for prion propagation (7,8). Point mutations in, or deletion of, the N-terminal domain disrupt the prion propagation function of yeast eRF3, and the N-terminal domain by itself is sufficient for prion propagation (3,7,9). The translation termination function of eRF3 requires the well conserved C-terminal domain. Deletion of the C-terminal domain is lethal, presumably because of defects in translation termination, and point mutations in the C-terminal domain reduce translation termination efficiency (9). Thus, the prion property of eRF3 is a property of the N-terminal domain, whereas the C-terminal domain is required for translation termination.The ability of eRF3 to exist in [PSI ϩ ] and [psi Ϫ ] states has been conserved during evolution and is present in the eRF3 proteins of several species and genera of yeast (10)(11)(12)(13)(14) ] induces phenotypic variation by suppressing premature stop codons, it has been reported that in seven cases the effect of [PSI ϩ ] was similar to the effect of upf1⌬, which also suppresses premature stop codons (see below) (16). However, it was also reported that in four other cases the effect of [PSI ϩ ] was not mimicked by upf1⌬, suggesting that there may be additional mechanisms by which [PSI ϩ ] affects phenotypes (16).Two lines of evidence suggest that [PSI ϩ ] affects translation termination not only at premature stop codons, but also at wild-type stop codons naturally found at the 3Ј end of coding regions. First, the frequency of extra stop codons just 3Ј of the normal stop codon of yeast genes is higher than expected, suggesting that read-through of the normal stop codons occurs at a frequency that is a significant factor over evolutionary time (19). Second, Namy et al. (20) identified eight genes that had a poor stop codon context. In an artificial reporter gene, translation termination at these stop codon contexts was affected by [PSI ϩ ]. Thus, [PSI ϩ ] might affect phenotypic variation by promoting read-through of normal stop codons (16)(17)(18)20).In addition to PSI directly affecting translation termination of premature and͞or normal stop codons, there may be indirect effects on mRNA stability because translation termination and mRNA stability are intimately linked (16). Two aspects of this link are nonsense-mediated mRNA decay and nonstop mRNA decay. Nonsense-mediated mRNA decay is the process of rapidly degrading mRNAs that contain a premature stop codon (reviewed in ref. 21). It is thought that nonsense-containing mRNAs are recognized as a consequence of premature translation termination. Because [PSI ϩ ] affects the efficiency of translation termination, it may also affect nonsense-mediated mRNA decay. Nonstop mRNA decay is the process of degrading mRNAs that do not contain stop codons (22,23). Nonstop mRNAs are

The Paf1 Complex Subunit Rtf1 Buffers Cells Against the Toxic Effects of [PSI+] and Defects in Rkr1-Dependent Protein Quality Control in Saccharomyces cerevisiae

“…Further investigation into the suppression mechanism of an hsp104 mutation showed that rtf1D rkr1D strains are invia- Wilson et al 2005). In addition, because degradation of many aberrant mRNAs depends on proper translation termination, the presence of [PSI + ] also affects the degradation of these transcripts (Wilson et al 2005). Therefore, in combination, the presence of [PSI + ] and the absence of RKR1 likely results in increased levels of nonstop proteins that cannot be efficiently recognized and degraded, as well as in an increased burden on mRNA quality control.…”

“…The overexpression, deletion, or GuHCl-mediated inactivation of HSP104, as well as the overexpression of the prion-coding genes URE2 and LSM4, all rescue rtf1D rkr1D synthetic lethality and clear cells of [PSI + ]. [PSI + ], the prion aggregate of Sup35, a translation termination factor necessary for proper stop codon recognition, results in readthrough of normal stop codons (Paushkin et al 1996;Wilson et al 2005). In turn, Rkr1 is required for the efficient ubiquitylation and degradation of nonstop proteins by recognition of a polylysine tract resulting from translation through the poly(A) tail (Bengtson and Joazeiro 2010).…”

“…As the prion form of Sup35, [PSI + ] results in suppression of nonsense codons in nonsense mRNAs as well as readthrough of normal stop codons (Paushkin et al 1996;Wilson et al 2005). Rkr1 has been reported to interact with ribosomes and be localized in the cytoplasm, where it is then necessary for nonstop protein degradation (Fleischer et al 2006;Bengtson and Joazeiro 2010).…”

“…A transposon mutagenesis screen for genetic suppressors of rtf1D rkr1D synthetic lethality identified a mutation in the gene encoding Hsp104. Further investigation into the suppression mechanism of an hsp104 mutation showed that rtf1D rkr1D strains are invia- Wilson et al 2005). In addition, because degradation of many aberrant mRNAs depends on proper translation termination, the presence of [PSI + ] also affects the degradation of these transcripts (Wilson et al 2005).…”

An engineered nonsense URA3 allele provides a versatile system to detect the presence, absence and appearance of the [PSI+] prion in Saccharomyces cerevisiae

“…L. Manogaran, K. T. Kirkland and S. W. Liebman (Liebman et al, 1975;Cox et al, 1980;Wilson et al, 2005 …”