Suppression of termination mutations caused by defects of the NMD machinery in Saccharomyces cerevisiae

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.