DNA replication stress restricts ribosomal DNA copy number

Area of Science:

• Cell Biology
• Genetics
• Molecular Biology

Background:

• Ribosomal RNAs (rRNAs) are encoded by ribosomal DNA (rDNA) repeats, crucial for ribosome biogenesis.
• Eukaryotic cells maintain rDNA copy numbers far exceeding immediate needs, but the mechanisms for maintaining this
• normal
• copy number are unknown.

Purpose of the Study:

• To identify essential genes involved in maintaining rDNA copy number.
• To investigate the relationship between DNA replication stress and rDNA copy number.
• To explore the functional significance of rDNA copy number variations.

Main Methods:

• Developed a droplet digital PCR assay to quantify rDNA copy number in yeast.
• Screened a collection of essential gene mutants under conditional temperature-sensitive conditions.
• Applied DNA replication stress to yeast cultures and monitored rDNA array size.

Main Results:

• Low rDNA copy number was linked to impaired DNA replication.
• Yeast cultures subjected to replication stress showed a contraction of the rDNA array, independent of Fob1.
• Cells with contracted rDNA arrays exhibited enhanced growth under replication stress conditions.

Conclusions:

• DNA replication stress selects for a reduced rDNA array size.
• A smaller rDNA array may optimize replication factor allocation for the rest of the genome.
• Reduced rDNA copy number could serve as a biomarker for historical DNA replication stress, as observed in MCM2-deficient mouse tumors.