Suppressors of cdc25p overexpression identify two pathways that influence the G2/M checkpoint in fission yeast

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Cell-cycle checkpoints are essential for maintaining genomic integrity by preventing cell division with damaged or unreplicated DNA.
• The G2/M checkpoint specifically halts mitosis in response to DNA damage or replication issues.
• While some genes are involved in both DNA damage and replication checkpoints, distinct mechanisms and genes likely exist for each.

Purpose of the Study:

• To identify novel genes that specifically function in the DNA replication checkpoint pathway.
• To understand the genetic basis of the cell-cycle response to unreplicated DNA.

Main Methods:

• Screening for high-copy suppressors of OPcdc25(+), a strain lacking a DNA replication checkpoint.
• Genetic analysis of suppressor genes, including gene deletion studies and characterization of mutant phenotypes.
• Investigating the mechanism of suppression, particularly concerning Cdc2p activity and tyrosine phosphorylation.

Main Results:

• Two classes of suppressors were identified: one containing a novel DEAD box helicase gene, sum3(+), and another containing known genes chk1(+), rad24(+), and rad25(+).
• SUM3 negatively regulates cell-cycle response to stress when overexpressed and restores the checkpoint independently of Cdc2p tyrosine phosphorylation.
• RAD24 and CHK1 appear to suppress the checkpoint defect by inhibiting Cdc25p; rad24 mutants show defects in hydroxyurea response, and double mutants (cds1Δ rad24Δ) are checkpoint deficient.

Conclusions:

• SUM3 is a newly identified gene involved in the DNA replication checkpoint.
• CHK1 and RAD24 may function redundantly with CDS1 in the DNA replication checkpoint pathway.
• The findings highlight the complexity of cell-cycle checkpoints and identify new players in DNA replication surveillance.