IDENTIFYING THE RIGHT STOP - DETERMINING HOW THE SURVEILLANCE COMPLEXRECOGNIZES AND DEGRADES AN ABERRANT MESSENGER-RNA

The nonsense-mediated mRNA decay (NMD) pathway functions by checking w hether translation termination has occurred prematurely and subsequent ly degrading the aberrant mRNAs, In Saccharomyces cerevisiae, it has b een proposed that a surveillance complex scans 3' of the premature ter mination codon and searches for the downstream element (DSE), whose re cognition by the complex identifies the transcript as aberrant and pro motes its rapid decay, The results presented here suggest that transla tion termination is important for assembly of the surveillance complex , Neither the activity of the initiation ternary complex after prematu re translation termination has occurred nor the elongation phase of tr anslation are essential for the activity of the NMD pathway, Once asse mbled, the surveillance complex is active for searching and recognizin g a DSE for similar to 200 nt 3' of the stop codon, We have also ident ified a stabilizer sequence (STE) in the GCN4 leader region that inact ivates the NMD pathway, Inactivation of the NMD pathway, as a conseque nce 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 ne w 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.