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Infrequent translation of a nonsense codon is sufficient to decrease mRNA level
1Departments of Immunology and Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada M5S 1A8.
Abstract:
In many organisms nonsense mutations decrease the level of mRNA. In the case of mammalian cells, it is still controversial whether translation is required for this nonsense-mediated RNA decrease (NMD). Although previous analyzes have shown that conditions that impede translation termination at nonsense codons also prevent NMD, the residual level of termination was unknown in these experiments. Moreover, the conditions used to impede termination might also have interfered with NMD in other ways. Because of these uncertainties, we have tested the effects of limiting translation of a nonsense codon in a different way, using two mutations in the immunoglobulin mu heavy chain gene. For this purpose we exploited an exceptional nonsense mutation at codon 3, which efficiently terminates translation but nonetheless maintains a high level of mu mRNA. We have shown 1) that translation of Ter462 in the double mutant occurs at only approximately 4% the normal frequency, and 2) that Ter462 in cis with Ter3 can induce NMD. That is, translation of Ter462 at this low (4%) frequency is sufficient to induce NMD.
Insights
Nonsense-mediated RNA decay (NMD) in mammalian cells requires translation. Even low-level translation (4%) of a nonsense mutation is sufficient to trigger NMD, clarifying its role in mRNA regulation.
Area of Science:
- Molecular Biology
- Genetics
- Cell Biology
Background:
- Nonsense mutations typically reduce mRNA levels in many organisms.
- The requirement of translation for nonsense-mediated RNA decay (NMD) in mammalian cells remains controversial.
- Previous studies faced uncertainties regarding residual translation levels and potential interference with NMD.
Purpose of the Study:
- To investigate whether translation is necessary for nonsense-mediated RNA decay (NMD) in mammalian cells.
- To clarify the role of translation efficiency in NMD induction.
- To address uncertainties from previous experimental conditions that impeded translation termination.
Main Methods:
- Utilized two mutations in the immunoglobulin mu heavy chain gene.
- Exploited an exceptional nonsense mutation at codon 3 that allows high mu mRNA levels.
- Quantified translation frequency at the nonsense codon (Ter462) in a double mutant.
Main Results:
- Translation of the Ter462 nonsense mutation occurred at approximately 4% of the normal frequency.
- This low-frequency translation (4%) of Ter462 was sufficient to induce NMD when cis-linked with Ter3.
- Demonstrated that NMD can be triggered by minimal translation of a premature termination codon.
Conclusions:
- Translation is required for nonsense-mediated RNA decay (NMD) in mammalian cells.
- Even a low level of translation (approximately 4%) of a nonsense codon is sufficient to initiate the NMD pathway.
- Provides a clearer understanding of the molecular mechanisms underlying NMD in mammals.