Stop codon-mediated suppression of splicing is a novel nuclear scanning mechanism not affected by elements of protein synthesis and NMD.

The pre-mRNA splicing machine must frequently discriminate between normal and many potential 5'splice sites that match the consensus sequence but remain latent. Suppression of splicing (SOS) at such latent 5'splice sites is required for the maintenance of an open reading frame, and to ensure that only RNAs that encode for functional proteins will be formed. In this study we show that SOS is a novel mechanism distinct from the known RNA surveillance mechanisms. First, SOS is distinct from nonsense-mediated mRNA decay (NMD) because it is not dependent on translation and is not affected by RNAi-mediated down-regulation of hUpf1 and hUpf2--two key components of the NMD pathway. Second, SOS is distinct from nonsense-associated alternative splicing (NAS), because a mutant of hUpf1, which was shown to abrogate NAS, does not activate latent splicing. Elucidating the mechanism of SOS is pertinent to human disease in view of the large number of human genes that harbor latent splice sites.