Fig. 1. Mechanisms of mRNA turnover. (A) Normal mRNAs are degraded by different pathways. Following deadenylation, mRNAs can either be degraded from the 3' end by the exosome or decapped and degraded from the 5' end (shown in purple). Alternatively, mRNAs can be cleaved by endonucleases and those fragments degraded from both ends (green). (B) A premature stop codon triggers rapid decapping and 5' to 3' exonucleolytic digestion. (C) Transcripts lacking a termination codon are degraded by the exosome in a deadenylase-independent manner.