Figure 1.

Recognition of premature termination codons in humans is splicing dependent. (a) During pre-mRNA processing, introns are removed and a set of proteins called the exon-junction complex is deposited. According to the current model for mammalian NMD, these complexes serve to facilitate transport from the nucleus and to remember the gene structure. During the first, pioneering, round of translation, the ribosome will displace all exon-junction complexes in its path until it reaches a stop codon. If the termination codon is on or near the final exon, as is the case for most genes, the ribosome will have displaced all exon-junction complexes. The mRNA will then undergo multiple rounds of translation. (b) If the termination codon is sufficiently far upstream of the final intron position, exon-junction complexes will remain. Interactions ensue that result in the degradation of the mRNA by NMD.