Regulation of CTR2 mRNA by the nonsense-mediated mRNA decay pathway.

The nonsense-mediated mRNA decay (NMD) pathway was originally identified as a pathway that degrades mRNAs with premature termination codons; however, NMD is now known to regulate natural mRNAs as well. Natural mRNAs are degraded by NMD due to the presence of specific NMD targeting features. An atypically long 3'-UTR is one of the features that has been shown to induce the rapid degradation of mRNAs by NMD in Saccharomyces cerevisiae and other organisms. S. cerevisiae CTR2 mRNAs have long 3'-UTRs and are sensitive to NMD, although the extent by which these long 3'-UTRs target the CTR2 mRNAs to the pathway is unknown. Here, we investigated the sequence elements that induce NMD of the CTR2 mRNAs and determined that the long CTR2 3'-UTR is sufficient to target an NMD-insensitive mRNA to the pathway. We also found that, although the CTR2 3'-UTR contributes to NMD-induced degradation, CTR2 mRNAs contain additional NMD-inducing features that function cooperatively with the atypically long 3'-UTR to trigger mRNA degradation. Lengthening the CTR2 ORF abrogates NMD and renders the mRNAs immune to the NMD pathway. Moreover, we found that transcription of CTR2 driven by the GPD promoter, which is not identical to the CTR2 promoter, affects degradation of the transcripts by NMD.