Figure 1. Premature termination codons (PTCs) are distinguished from genuine termination codons by the presence of downstream cis elements which normally do not occur 3¢ of a terminating ribosome. In yeast, the downstream destabilizing element (DSE), which binds Hrp1p, serves this purpose. In mammals, exon–exon junctions, marked by proteins which are deposited concurrent with splicing (postsplicing complexes), provide this function.

Figure 2. (a) Normal mRNA decay initiates when the poly(A) tail reaches a critical length (~10 nucleotides) that can no longer support interaction with poly(A)-binding protein (PABP). This is thought to destabilize the cap-binding complex consisting of eukaryotic initiation factor (eIF) 4G, eIF4E and eIF4A. Further rearrangements, requiring the like-Sm (Lsm) protein complex, ultimately expose the 5¢ cap to the decapping enzyme (Dcp1p in yeast). Following removal of the cap, the body of the transcript is rapidly degraded by the exonuclease Xrn1p. (b) Nonsense transcripts are decapped prior to poly(A) tail shortening. The surveillance complex, consisting of Upf1p–3p, is believed directly to destabilize the cap-binding complex upon encountering a premature termination codon, leading to Dcp1p- and Xrn1p-mediated decay.