Identification of mutations that decrease the stability of a fragment of Saccharomyces cerevisiae chromosome III...

Area of Science:

• Cell Biology
• Genetics
• Molecular Biology

Background:

• Eukaryotic chromosome duplication during S phase ensures genetic stability.
• Replication initiation is typically controlled by cis-acting replicator sequences.
• Budding yeast Saccharomyces cerevisiae replicators are spaced approximately 40 kb apart.

Purpose of the Study:

• Investigate the maintenance mechanisms of yeast chromosome fragments lacking known replicators.
• Identify genes and pathways essential for the stable transmission of "originless" chromosome fragments.

Main Methods:

• Screened for "originless" fragment maintenance (ofm) mutants in Saccharomyces cerevisiae.
• Characterized ofm mutants defective in maintaining originless fragments but proficient with replicator-containing fragments.
• Analyzed mutations affecting DNA replication initiation, fork progression, and checkpoint activation.

Main Results:

• Yeast chromosome III derivatives lacking known replicators were replicated and segregated with high fidelity (>96%).
• Three ofm mutations (OFM1-1, ofm6, ofm14) were identified, disrupting distinct chromosome transmission processes.
• OFM1-1 mutants suggest an alternative replication initiation mechanism.
• ofm6 mutants indicate defects in replication fork progression.
• ofm14, an allele of RAD9, highlights the role of the DNA damage checkpoint in fragment maintenance.

Conclusions:

• "Originless" chromosome fragments are maintained through mechanisms distinct from canonical replication initiation.
• Alternative replication initiation, replication fork progression, and DNA damage checkpoints are crucial for "originless" fragment stability.
• The DNA damage checkpoint (via RAD9) plays a significant role in ensuring the faithful transmission of these fragments.