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Cytoplasmic mRNA for human triosephosphate isomerase is immune to nonsense-mediated decay despite forming polysomes
1Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
Abstract:
Nonsense codons between position 14 within the first exon and position 193 within the penultimate exon of the human gene for triosephosphate isomerase reduce mRNA abundance to 25% of normal. The reduction in abundance is due to the decay of newly synthesized mRNA that copurifies with nuclei. TPI mRNA that copurifies with cytoplasm is immune to decay. We show here that immunity is not due to the failure of nonsense-containing mRNA to form polysomes. This finding indicates that cytoplasmic mRNA, in contrast to nucleus-associated mRNA, may have lost one or more factors that are required for nonsense-mediated decay or gained one or more factors that confer immunity to nonsense-mediated decay.
Insights
Nonsense codons in the triosephosphate isomerase (TPI) gene drastically reduce mRNA levels by triggering decay in the nucleus. Cytoplasmic TPI mRNA, however, evades this decay, suggesting altered regulatory factors.
Area of Science:
- Molecular Biology
- Genetics
- Gene Expression Regulation
Background:
- Nonsense codons prematurely terminate protein synthesis, often leading to mRNA degradation.
- The human triosephosphate isomerase (TPI) gene is crucial for glycolysis.
- Mechanisms controlling mRNA stability in response to premature stop codons are complex.
Purpose of the Study:
- To investigate the impact of nonsense codons on TPI mRNA abundance and localization.
- To determine the factors influencing nonsense-mediated mRNA decay (NMD) in different cellular compartments.
Main Methods:
- Analysis of TPI mRNA levels in cells containing nonsense mutations.
- Copurification of mRNA with nuclear and cytoplasmic fractions.
- Assessment of polysome association for nuclear and cytoplasmic mRNA.
Main Results:
- Nonsense codons between exon 1 and the penultimate exon reduced TPI mRNA to 25% of normal levels.
- mRNA decay occurred primarily in the nucleus, affecting newly synthesized transcripts.
- Cytoplasmic TPI mRNA, even with nonsense codons, was resistant to decay and formed polysomes.
Conclusions:
- Nuclear-associated mRNA is susceptible to nonsense-mediated decay, while cytoplasmic mRNA is protected.
- Cytoplasmic mRNA may possess mechanisms conferring immunity to NMD or lack NMD-promoting factors present in the nucleus.