Among a large collection of nonsense (termination) suppressors of
Saccharomyces cerevisiae, a few remained obscure for their molecular nature. Of those, a group of weak and recessive suppressors,
sup111,
sup112 and
sup113, is of particular interest because of their dependency on [
PSI+], a yeast prion. From the facts that these suppressors map at positions quite similar to the
UPF2,
UPF3 and
UPF1 genes, respectively, and that some mutations in the
UPF genes confer termination suppressor activity, we suspected that
sup111,
sup112 and
sup113 would very well be mutant alleles of the
UPF genes. We tested our speculation and found that
sup113,
sup111 and
sup112 were in fact complemented with the wild-type alleles of
UPF1,
UPF2 and
UPF3, respectively. We further obtained evidence that the
UPF1,
UPF2 and
UPF3 loci of the strains carrying
sup113,
sup111 and
sup112, respectively, had point mutations. From these results, we conclude that
sup111,
sup112 and
sup113 are mutant alleles of
UPF2,
UPF3 and
UPF1, respectively, and thus attribute suppressor activity of these mutations to defects in the NMD (nonsense-mediated mRNA decay) machinery.
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