NMD (nonsense-mediated mRNA decay) is a cellular quality-control mechanism in which an otherwise stable mRNA is destabilized by the presence of a premature termination codon. We have defined the set of endogenous NMD substrates, demonstrated that they are available for NMD at every round of translation, and showed that premature termination and normal termination are not equivalent biochemical events. Premature termination is aberrant, and its NMD-stimulating defects can be reversed by the presence of tethered poly(A)-binding protein (Pab1p) or tethered eRF3 (eukaryotic release factor 3) (Sup35p). Thus NMD appears to be triggered by a ribosome's failure to terminate adjacent to a properly configured 3'-UTR (untranslated region), an event that may promote binding of the UPF/NMD factors to stimulate mRNA decapping.

A report on the Cold Spring Harbor Laboratory meeting 'Translational Control', Cold Spring Harbor, USA, 7-12 September 2004.

Nonsense-mediated messenger RNA decay (NMD) is triggered by premature translation termination, but the features distinguishing premature from normal termination are unknown. One model for NMD suggests that decay-inducing factors bound to mRNAs during early processing events are routinely removed by elongating ribosomes but remain associated with mRNAs when termination is premature, triggering rapid turnover. Recent experiments challenge this notion and suggest a model that posits that mRNA decay is activated by the intrinsically aberrant nature of premature termination. Here we use a primer extension inhibition (toeprinting) assay to delineate ribosome positioning and find that premature translation termination in yeast extracts is indeed aberrant. Ribosomes encountering premature UAA or UGA codons in the CAN1 mRNA fail to release and, instead, migrate to upstream AUGs. This anomaly depends on prior nonsense codon recognition and is eliminated in extracts derived from cells lacking the principal NMD factor, Upf1p, or by flanking the nonsense codon with a normal 3'-untranslated region (UTR). Tethered poly(A)-binding protein (Pab1p), used as a mimic of a normal 3'-UTR, recruits the termination factor Sup35p (eRF3) and stabilizes nonsense-containing mRNAs. These findings indicate that efficient termination and mRNA stability are dependent on a properly configured 3'-UTR.