Infrequent translation of a nonsense codon is sufficient to decrease mRNA level

In many organisms nonsense mutations decrease the level of mRNA. In the cas e of mammalian cells, it is still controversial whether translation is requ ired for this nonsense-mediated RNA decrease (NMD). Although previous analy zes have shown that conditions that impede translation termination at nonse nse codons also prevent NMD, the residual level of termination was unknown in these experiments. Moreover, the conditions used to impede termination m ight also have interfered with NMD in other ways. Because of these uncertai nties, we have tested the effects of limiting translation of a nonsense cod on in a different way, using two mutations in the immunoglobulin mu heavy c hain gene. For this purpose we exploited an exceptional nonsense mutation a t codon 3, which efficiently terminates translation but nonetheless maintai ns a high level of mu mRNA. We have shown 1) that translation of Ter462 in the double mutant occurs at only similar to 4% the normal frequency, and 2) that Ter462 in cis with Ter3 can induce NMD. That is, translation of Ter46 2 at this low (4%) frequency is sufficient to induce NMD.