Generally, mRNAs that prematurely terminate translation are abnormally low in abundance. In the case of mammalian cells, nonsense codons most often mediate a reduction in the abundance of newly synthesized, nucleus-associated mRNA by a mechanism that is not well understood. With the aim of defining cis-acting sequences that are important to the reduction process, the effects of particular β-globin gene rearrangements on the metabolism of β-globin mRNAs harboring one of a series of nonsense codons have been assessed. Results indicate that nonsense codons located 54 bp or more upstream of the 3′-most intron, intron 2, reduce the abundance of nucleus-associated mRNA to 10–15% of normal without altering the level of either of the two introns within pre-mRNA. The level of cytoplasmic mRNA is also reduced to 10–15% of normal, indicating that decay does not take place once the mRNA is released from an association with nuclei into the cytoplasm. A nonsense codon within exon 2 that does not reduce mRNA abundance can be converted to the type that does by (1) inserting a sufficiently large in-frame sequence immediately upstream of intron 2 or (2) deleting and reinserting intron 2 a sufficient distance downstream of its usual position. These findings indicate that only those nonsense codons located more than 54 bp upstream of the 3′-most intron reduce β-globin mRNA abundance, which is remarkably consistent with which nonsense codons within the triosephosphate isomerase (TPI) gene reduce TPI mRNA abundance. We propose that the 3′-most exon–exon junction of β-globin mRNA and, possibly, most mRNAs is marked by the removal of the 3′-most intron during pre-mRNA splicing and that the “mark” accompanies mRNA during transport to the cytoplasm. When cytoplasmic ribosomes terminate translation more than 54 nt upstream of the mark during or immediately after transport, the mRNA is subjected to nonsense-mediated decay. The finding that deletion of β-globin intron 2 does not appreciably alter the effect of any nonsense codon on β-globin mRNA abundance suggests that another cis-acting sequence functions in nonsense-mediated decay comparably to intron 2, at least in the absence of intron 2, possibly as a fail-safe mechanism. The analysis of deletions and insertions indicates that this sequence resides within the coding region and can be functionally substituted by intron 2.
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