T Cell Receptor (TCR) Mini-Gene mRNA Expression Regulated by Nonsense Codons: A Nuclear-associated Translation-like Mechanism

Premature termination codons (PTCs) are known to decrease mRNA levels. Here, we report our investigation of the mechanism for this downregulation using the TCR-β 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-β 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.