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The protein-coding region of c-myc mRNA contains a sequence that specifies rapid mRNA turnover and induction by
1Molecular Biology and Virology Laboratory, Salk Institute, San Diego, California 92138.
Abstract:
The stability of certain mRNAs is known to be affected by translation. Some mRNAs appear to be protected from rapid degradation by translation, whereas degradation is coupled to translation for other mRNAs. The molecular determinants of this selective effect of translation are unknown. One example of this effect is the induction of early-response gene mRNAs in the presence of translation inhibitors. To define the molecular basis of induction of early-response gene mRNA expression by inhibitors of protein synthesis, we have performed a mutational analysis of one member of the early response gene family, the c-myc gene. We find that induction by cycloheximide is due to stabilization of c-myc transcripts. The requirements for increased expression of c-myc mRNA by cycloheximide are the presence of the sequence encoding c-myc amino acids 335-439 on a mRNA that can be translated; all other portions of the c-myc gene are dispensable, and this sequence can confer induction of mRNA expression by protein synthesis inhibitors on a heterologous gene. By direct measurement of mRNA turnover in the absence of transcription-blocking drugs, we show that this sequence can function as a selective mRNA destabilizing element, that turnover mediated by this element is translation dependent, and turnover mediated by this element is inhibited by actinomycin D. Our results support the hypothesis that degradation of c-myc mRNA is coupled to translation, that the sequences specifying this form of degradation are contained in the protein-coding sequence, and that translation inhibitors induce expression of c-myc mRNA by blocking turnover mediated by this element.
Insights
Translation impacts messenger RNA (mRNA) stability. Protein synthesis inhibitors stabilize c-myc mRNA by blocking translation-dependent degradation, revealing a specific protein-coding sequence element that controls mRNA turnover.
Area of Science:
- Molecular Biology
- Gene Regulation
- mRNA Stability
Background:
- Translation influences messenger RNA (mRNA) stability, with some mRNAs protected and others degraded during translation.
- The molecular mechanisms governing translation-dependent mRNA stability remain largely unknown.
- Early-response gene mRNAs, like c-myc, are induced by protein synthesis inhibitors.
Purpose of the Study:
- To elucidate the molecular basis for the induction of early-response gene mRNA expression by protein synthesis inhibitors.
- To identify the specific sequences within the c-myc gene responsible for this phenomenon.
Main Methods:
- Conducted a mutational analysis of the c-myc gene.
- Assessed mRNA turnover rates directly, without transcription inhibitors.
- Utilized a heterologous gene system to test the function of identified sequences.
Main Results:
- Induction of c-myc mRNA by cycloheximide results from transcript stabilization.
- A specific region (amino acids 335-439) within the c-myc coding sequence is necessary and sufficient for this induction.
- This sequence acts as a translation-dependent mRNA destabilizing element, inhibited by actinomycin D.
Conclusions:
- c-myc mRNA degradation is coupled to translation.
- The protein-coding sequence contains elements that direct translation-dependent degradation.
- Protein synthesis inhibitors induce c-myc mRNA expression by inhibiting this degradation pathway.