BINARY SPECIFICATION OF NONSENSE CODONS BY SPLICING AND CYTOPLASMIC TRANSLATION

Premature translation termination codons resulting from nonsense or fr ameshift mutations are common causes of genetic disorders. Complicatio ns arising from the synthesis of C-terminally truncated polypeptides c an be avoided by 'nonsense-mediated decay' of the mutant mRNAs. Premat ure termination codons in the beta-globin mRNA cause the common recess ive form of beta-thalassemia when the affected mRNA is degraded, but t he more severe dominant form when the mRNA escapes nonsense-mediated d ecay. We demonstrate that cells distinguish a premature termination co don within the beta-globin mRNA from the physiological translation ter mination codon by a two-step specification mechanism. According to the binary specification model proposed here, the positions of splice jun ctions are first tagged during splicing in the nucleus, defining a sto p codon operationally as a premature termination codon by the presence of a 3' splicing tag. In the second step, cytoplasmic translation is required to validate the 3' splicing tag for decay of the mRNA. This m odel explains nonsense-mediated decay on the basis of conventional mol ecular mechanisms and allows us to propose a common principle for nons ense-mediated decay from yeast to man.