Identifying the right stop: determining how the surveillance complex recognizes and degrades an aberrant mRNA.

The nonsense-mediated mRNA decay (NMD) pathway functions by checking whether translation termination has occurred prematurely and subsequently degrading the aberrant mRNAs. In Saccharomyces cerevisiae, it has been proposed that a surveillance complex scans 3' of the premature termination codon and searches for the downstream element (DSE), whose recognition by the complex identifies the transcript as aberrant and promotes its rapid decay. The results presented here suggest that translation termination is important for assembly of the surveillance complex. Neither the activity of the initiation ternary complex after premature translation termination has occurred nor the elongation phase of translation are essential for the activity of the NMD pathway. Once assembled, the surveillance complex is active for searching and recognizing a DSE for approximately 200 nt 3' of the stop codon. We have also identified a stabilizer sequence (STE) in the GCN4 leader region that inactivates the NMD pathway. Inactivation of the NMD pathway, as a consequence of either the DSE being too far from a stop codon or the presence of the STE, can be circumvented by inserting sequences containing a new translation initiation/termination cycle immediately 5' of the DSE. Further, the results indicate that the STE functions in the context of the GCN4 transcript to inactivate the NMD pathway.