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Maturation-specific deadenylation in Xenopus oocytes requires nuclear and cytoplasmic factors
S M Varnum1, C A Hurney, W M Wormington
1Department of Biology, Brandeis University, Waltham, Massachusetts 02254.
Abstract:
During the meiotic maturation of Xenopus oocytes, maternal mRNAs that lack a cytoplasmic polyadenylation element are deadenylated and translationally inactivated. In this report, we have characterized the regulation of poly(A) removal during maturation. Deadenylation in vivo is detected only after germinal vesicle breakdown and does not require de novo protein synthesis. Enucleated oocytes do not deadenylate either endogenous or microinjected RNAs upon maturation, indicating that a nuclear component is required for poly(A) removal. Whole cell extracts prepared from both immature and mature oocytes deadenylate exogenous RNA substrates in vitro. Deadenylation activity is not detected in isolated nuclear or cytoplasmic extracts obtained from immature oocytes, but is reconstituted when these fractions are combined in vitro. These results indicate that the factors required for deadenylation activity are present in immature oocytes, but that poly(A) removal is prevented by the sequestration of one or more of these components within the nucleus. Maturation-specific deadenylation of maternal mRNAs occurs upon the release of nuclear factors into the cytoplasm at germinal vesicle breakdown.
Insights
Maternal mRNA deadenylation in Xenopus oocytes requires a nuclear factor released during maturation. This process inactivates translation of specific RNAs after germinal vesicle breakdown, without needing new protein synthesis.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- During Xenopus oocyte maturation, maternal mRNAs lacking a cytoplasmic polyadenylation element are deadenylated and translationally silenced.
- Polyadenylation is a key regulator of mRNA stability and translation.
- Understanding the regulation of poly(A) removal is crucial for comprehending gene expression control during development.
Purpose of the Study:
- To investigate the regulation of poly(A) removal (deadenylation) during Xenopus oocyte maturation.
- To identify the cellular compartment and factors involved in maturation-specific deadenylation.
Main Methods:
- In vivo studies using Xenopus oocytes, including enucleation experiments.
- In vitro deadenylation assays using whole cell, nuclear, and cytoplasmic extracts from immature and mature oocytes.
- Analysis of both endogenous and microinjected RNA substrates.
Main Results:
- Deadenylation occurs in vivo only after germinal vesicle breakdown and does not require de novo protein synthesis.
- A nuclear component is essential for poly(A) removal, as enucleated oocytes fail to deadenylate RNAs.
- Deadenylation activity is present in immature oocytes but requires the combination of nuclear and cytoplasmic fractions, suggesting nuclear sequestration of factors.
- Maturation involves the release of nuclear factors, enabling cytoplasmic deadenylation.
Conclusions:
- Maturation-specific deadenylation of maternal mRNAs in Xenopus oocytes is regulated by the release of nuclear factors into the cytoplasm at germinal vesicle breakdown.
- The deadenylation machinery is present in immature oocytes but its activity is repressed by nuclear sequestration.
- This mechanism ensures timely translational inactivation of specific maternal mRNAs during oocyte maturation.