All organisms require a reliable mechanism to turn genes on and off. This regulation of gene expression underlies cellular processes ranging from the response to environmental signals to the development of multi-cellular organisms and cell-cell communication. Understandably, the cell tightly controls gene expression at every step from DNA to protein. Recent work has given new insights into these control mechanisms and revealed dedicated pathways that target mRNA for degradation, thereby efficiently turning genes off.
After export to the cytoplasm, mRNA is protected from degradation by a 5’ cap structure and a 3’ poly adenine tail. In the deadenylation dependent mRNA decay pathway, the polyA tail is gradually shortened by exonucleases. This ultimately attracts the degradation machinery that rapidly degrades the mRNA in both in the 5’ to 3’ direction and in the 3’ to 5’ direction. Additional mechanisms, including the nonsense mediated decay pathway, bypass the need for deadenylation and can remove the mRNA from the transcriptional pool independently. Interestingly, the same enzymes are responsible for the actual degradation of the mRNA independent of the pathway taken (see figure).
Figure 1: Simplified mechanism of mRNA decay. Dotted arrows indicate (indirect) interactions. Details of the pathways vary between different eukaryotes. PAB: poly-A binding protein.