ASSESSMENT OF INACTIVATING STOP CODON MUTATIONS
IN FORTY SACCHAROMYCES CEREVISIAE STRAINS:
IMPLICATIONS FOR [PSI+] PRION- MEDIATED PHENOTYPES
David A. Fitzpatricka, Jennifer O’Brienb, Ciara Moranb, Naushaba Hasinb, and Gary
W. Jonesb
a
Genome Evolution Laboratory and bYeast Genetics Laboratory, Department of
Biology, National University of Ireland Maynooth, Maynooth, County Kildare,
Ireland. Email: gary.jones@nuim.ie
The yeast prion [PSI+] has been implicated in the generation of novel phenotypes by a
mechanism involving a reduction in translation fidelity causing read-through of both
naturally occurring stop codons and inactivating stop codon mutations (ISCMs). We
sequenced the genomes of two Saccharomyces cerevisiae strains that are commonly
used for the study of the yeast [PSI+] prion and identified approximately 26,000 and
6,500 single nucleotide polymorphisms (SNPs) in strains 74-D694 and G600
respectively, compared to strain S288C. In addition to SNPs that produce nonsynonymous amino acid changes we have also identified a number of SNPs that cause
potential ISCMs in these strains, one of which we show is associated with a [PSI+]dependent stress resistance phenotype in strain G600. Strain 74-D694 was found to
contain 22 potential ISCMs present in genes involved in a variety of cellular
processes, including nitrogen metabolism and oxidative stress response. The presence
of ISCMs in a subset of these genes provides a clear explanation for previously
identified [PSI+]- associated phenotypes in this strain. A comparison of ISCMs in
strain 74-D694 with S. cerevisiae strains sequenced as part of the Saccharomyces
Genome Re-sequencing Project shows a clear over representation of ISCMs in 74
D694 and suggests that the presence of [PSI+] has relaxed evolutionary constraints on
the generation of ISCMs. In addition a major difference in the abilities of these two
[PSI+] strains to deal with heat shock was identified, a difference that appears due to
the presence of amino acid differences in proteins controlling the heat shock response.
Given the well established role that chaperones play in yeast prion propagation the
extreme differences in the heat shock responses of these two well used laboratory
strains should be taken into consideration when interpreting data generated in these
diverse genetic backgrounds that eludes to the influence of chaperones on prion
propagation.