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Fate of mat1 DNA strands during mating-type switching in fission yeast
1Unité des Virus Oncogènes, URA 1644 du CNRS, Département des Biotechnologies, Institut Pasteur, Paris, France. barcan@pasteur.fr
Abstract:
The mating-type switching of the fission yeast, Schizosaccharomyces pombe, is highly regulated. Two consecutive asymmetric divisions are required to produce one mating-type switched cell among the four progeny. Using DNA density-gradient centrifugation we demonstrate that one-fourth of the mat1 DNA is not replicated by the conventional semi-conservative mode, but instead both DNA strands are synthesized de novo. Our data are consistent with a gene conversion event, initiated by a site- and strand-specific DNA break (SSB). We further demonstrate that the virgin switched mat1-containing chromatid no longer contained the nick, while it is reintroduced during the lagging strand synthesis of the mat1 locus on the sister chromatid. This finding establishes at the molecular level a firm experimental link between the phenotype and genotype in the process of asymmetric mating-type switching during mitotic divisions.
Insights
Fission yeast mating-type switching involves a unique DNA replication process where one-fourth of mat1 DNA is synthesized de novo, not semi-conservatively. This mechanism, involving gene conversion and DNA breaks, links molecular events to the switching phenotype.
Area of Science:
- Molecular Biology
- Genetics
- Cell Biology
Background:
- Mating-type switching in Schizosaccharomyces pombe is a precisely regulated process.
- This switching occurs over two asymmetric cell divisions, yielding one switched cell from four progeny.
Purpose of the Study:
- To elucidate the molecular mechanism of DNA replication during mating-type switching in fission yeast.
- To investigate the role of DNA breaks and gene conversion in the mat1 locus.
Main Methods:
- DNA density-gradient centrifugation was employed to analyze DNA replication patterns.
- Site- and strand-specific DNA breaks (SSBs) were investigated in the mat1 locus.
Main Results:
- One-fourth of mat1 DNA replication proceeds via de novo synthesis, deviating from the semi-conservative model.
- Evidence suggests a gene conversion event initiated by a site- and strand-specific DNA break.
- The nick present in the switched mat1 chromatid is reintroduced during lagging strand synthesis in the sister chromatid.
Conclusions:
- The study establishes a molecular link between DNA replication anomalies and the asymmetric mating-type switching phenotype.
- The findings provide a detailed molecular understanding of a unique DNA repair and replication process in fission yeast.