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The chk1 pathway is required to prevent mitosis following cell-cycle arrest at 'start'
A M Carr1, M Moudjou, N J Bentley
1MRC Cell Mutation Unit, Sussex University, Falmer, UK.
Background:
The G2-M-phase transition is controlled by cell-cycle checkpoint pathways which inhibit mitosis if previous events are incomplete or if the DNA is damaged. Genetic analyses in yeast have defined two related, but distinct, pathways which prevent mitosis--one which acts when S phase is inhibited, and one which acts when the DNA is damaged. In the fission yeast Schizosaccharomyces pombe, many of the gene products involved have been identified. Six 'radiation checkpoint' (rad) gene products are required for both the S-M and DNA-damage checkpoints, whereas Chk1, a putative protein kinase, is required only for the DNA-damage checkpoint and not for the S-M checkpoint following the inhibition of DNA synthesis.
Results:
We have genetically defined a third mitotic control checkpoint pathway in fission yeast which prevents mitosis when passage through 'start' (the commitment point in G1) is compromized. In cycling cells arrested at start, mitosis is prevented by a Chk1-dependent pathway. In the absence of Chk1, G1 cells attempt an abortive mitosis with a 1C DNA content without entering S phase. Similar results are seen in the absence of Rad17, a typical example of a rad gene product.
Conclusions:
Genetic dissection of checkpoints in logarithmically growing fission yeast has identified a pathway that couples mitosis to correct passage through start. This pathway is related to the DNA-structure check-points which ensure that mitosis is dependent on the completion of replication and the integrity of the DNA. We propose that all three mitotic control checkpoints monitor distinct DNA or protein structures at different stages in the cell cycle.
Insights
Researchers identified a third cell-cycle checkpoint in fission yeast that links mitosis to proper passage through the G1/S transition (start). This pathway, dependent on Chk1 and Rad17, prevents mitosis if G1 progression is compromised.
Area of Science:
- Cell Biology
- Genetics
- Molecular Biology
Background:
- Cell-cycle checkpoints regulate the G2-M transition, inhibiting mitosis if DNA is damaged or S phase is incomplete.
- Two known pathways in yeast prevent mitosis: one for inhibited S phase and one for DNA damage.
- In Schizosaccharomyces pombe, six 'radiation checkpoint' (rad) gene products are crucial for both S-M and DNA-damage checkpoints, while Chk1 is specific to the DNA-damage checkpoint.
Purpose of the Study:
- To genetically define a third mitotic control checkpoint pathway in fission yeast.
- To investigate the role of Chk1 and Rad17 in coupling mitosis to passage through the G1/S transition ('start').
Main Methods:
- Genetic analysis in fission yeast (Schizosaccharomyces pombe).
- Observation of cell-cycle progression and DNA content in mutant strains.
- Analysis of checkpoint activation in response to compromised passage through 'start'.
Main Results:
- A third mitotic control checkpoint pathway was identified, preventing mitosis when passage through 'start' is compromised.
- In cycling cells arrested at 'start', mitosis is inhibited via a Chk1-dependent pathway.
- Absence of Chk1 or Rad17 leads to abortive mitosis in G1 cells without S phase entry.
Conclusions:
- A pathway coupling mitosis to correct passage through 'start' has been identified in fission yeast.
- This pathway is related to DNA-structure checkpoints ensuring replication completion and DNA integrity.
- All three mitotic control checkpoints likely monitor distinct DNA or protein structures at different cell-cycle stages.