Figure 5.

Ribosome reinitiation after a PTC can prevent NMD.A, when the ribosome terminates at a PTC, it is recognized by downstream UPF1, with the aid of the EJC, and the transcript undergoes NMD. See Figure 2 for more details. B, when ribosome recycling occurs poorly at a premature termination codon, for example, because of limiting recycling factors like ABCE1, this can lead to the ribosome reinitiation at a nearby downstream start codon. Reinitiation causes displacement of NMD activating UPF and EJC proteins, escape from NMD and production of an N-terminally truncated or a unique polypeptide. EJC, exon junction complex; NMD, nonsense-mediated mRNA decay; PTC, premature termination codon.