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By
From the Department of Immunology, The University of Texas M. D. Anderson Cancer Center,
Houston, Texas 77030
Premature termination codons (PTCs) are known to decrease mRNA levels. Here, we report
our investigation of the mechanism for this downregulation using the TCR-
gene, which acquires PTCs as a result of programmed rearrangements that occur during normal thymic development. We found that a mini-gene version of this gene, which contains only three TCR-
exons, exhibited efficient downregulation in response to PTCs. This demonstrates that the full
coding sequence is not necessary for appropriate regulation. Mutation of the translation start
AUG and a downstream in-frame AUG that displayed similarity to the Kozak consensus sequence reversed the downregulatory response to PTCs. Thus, an AUG start codon is required
to define the reading frame of a PTC. Specific suppressor tRNAs also reversed the downregulatory response, strongly implicating the involvement of a translation-like process. Remarkably, the addition of suppressor tRNAs or the inactivation of the start AUGs caused a dramatic rise
in the levels of PTC-bearing transcripts in the nuclear fraction prepared by two independent
methods. Collectively, our results provide evidence for a codon-based surveillance mechanism associated with the nucleus that downregulates aberrant transcripts encoding potentially toxic
polypeptides from nonproductively rearranged genes.
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