Translation is an important mechanism to monitor the quality of messenger RNAs (mRNAs), as exemplified by the translation-dependent recognition and degradation of transcripts harboring premature termination codons (PTCs) by the nonsense-mediated mRNA decay (NMD) pathway. We demonstrate in yeast that mRNAs lacking all termination codons are as labile as nonsense transcripts. Decay of "nonstop" transcripts in yeast requires translation but is mechanistically distinguished from NMD and the major mRNA turnover pathway that requires deadenylation, decapping, and 5′-to-3′ exonucleolytic decay. These data suggest that nonstop decay is initiated when the ribosome reaches the 3′ terminus of the message. We demonstrate multiple physiologic sources of nonstop transcripts and conservation of their accelerated decay in mammalian cells. This process regulates the stability and expression of mRNAs that fail to signal translational termination.
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