IDENTIFICATION AND CHARACTERIZATION OF GENES THAT ARE REQUIRED FOR THE ACCELERATED DEGRADATION OF MESSENGER-RNAS CONTAINING A PREMATURE TRANSLATIONAL TERMINATION CODON

In both prokaryotes and eukaryotes nonsense mutations in a gene can en hance the decay rate of the mRNA transcribed from that gene, a phenome non described as nonsense-mediated mRNA decay. In yeast, the products of the UPF1 and UPF3 genes are required for this decay pathway, and in this report we focus on the identification and characterization of ad ditional factors required for rapid decay of nonsense-containing mRNAs . We present evidence that the product of the UPF2 gene is a new facto r involved in this decay pathway. Mutation of the UPF2 gene or deletio n of it from the chromosome resulted in stabilization of nonsense-cont aining mRNAs, whereas the decay of wild-type transcripts was not affec ted, The UPF2 gene was isolated, and its transcript was characterized, Our results demonstrate that the UPF2 gene encodes a putative 126.7-k D protein with an acidic region at its carboxyl terminus (-D-E)(n) fou nd in many nucleolar and transcriptional activator proteins. The UPF2 transcript is 3600 nucleotides in length and contains an intron near i ts 5' end. The UPF2 gene is dispensable for vegetative growth, but upf 2 Delta strains were found to be more sensitive to the translational e longation inhibitor cycloheximide than UPF2(+). A genetic analysis of other alleles proposed to be involved in nonsense-mediated mRNA decay revealed that the UPF2 gene is allelic to the previously identified su a1 allele, a suppressor of an out-of-frame ATG insertion shown previou sly to reduce translational initiation from the normal ATG of the CYC1 gene. In addition, we demonstrate that another suppressor of this cyc 1 mutation, sua6, is allelic to upf3, a previously identified lesion i nvolved in nonsense-mediated mRNA decay.