Nonsense-mediated decay (NMD) was originally known as a surveillance mechanism in eukaryotes that selectively removes mRNAs containing erroneously generated premature termination codons (PTCs). However, several recent studies have suggested that NMD may be another normal mechanism of post-transcriptional gene expression regulation [343555]. Consistently, a recent study has shown that the levels of the expressions of genes important for plant development are regulated by NMD [36]. The question is how NMD recognizes the PTC-containing transcripts, i.e., what the molecular characteristics of the NMD target transcripts are. Generally, NMD recognizes the transcript on which an exon-exon junction complex (EJC) resides more than 50~55 base-pairs downstream of an authentic termination codon as the premature transcripts, i.e., its target mRNAs, implicating that introns somehow play a role in recognizing the premature mRNA targets. Kalyna et al. [36] have shown that introns located in 5' or 3' untranslated regions (UTRs) play important roles in controlling NMD-sensitivity of transcripts.
