Figure 1
Specification of premature (versus normal) stop codons in mammalian cells. (a) In the nucleus, a normal intron-containing transcript is subjected to multiple modifications, such as splicing, addition of a 7mGpppN cap and polyadenylation, to yield a mature mRNA. This mRNA can be translated after export to the cytoplasm. The exon-exon junction complex (EJC) is associated with the mRNA just upstream of each splice site. (b) If an mRNA carries a premature termination codon (PTC), however, it can be degraded by the NMD system. Note that the physiological stop codon has no EJC positioned downstream of it, whereas NMD-sensitive PTCs are usually followed by at least one EJC (curved arrows in (a,b)). The EJC is therefore proposed to play an important role in mRNA surveillance.