Feedback controls and G2 checkpoints: fission yeast as a model system.

Dependency relationships within the cell cycle allow cells to arrest the cycle reversibly in response to agents or conditions that interfere with specific aspects of its normal progression. In addition, overlapping pathways exist which also arrest the cell cycle in response to DNA damage. Collectively, these control mechanisms have become known as checkpoints. Analysis of checkpoints is facilitated by the fact that dependency relationships within the cell cycle, such as the dependency of mitosis on the completion of DNA synthesis, and the DNA damage checkpoint can be separated genetically. In fission yeast, Schizosaccharomyces pombe, the dependency of mitosis on prior completion of DNA synthesis is mediated through tyrosine-15 phosphorylation of the ubiquitous mitotic regulator p34cdc2. In contrast, the arrest of mitosis caused by DNA damage acts through a separate mechanism that appears to be independent of tyrosine-15 phosphorylation. Despite these distinct interactions with the mitotic machinery, the majority of fission yeast mutants that are deficient in mitotic arrest after DNA damage are also unable to respond to inhibition of DNA synthesis. In this essay we survey the current knowledge concerning feedback controls and checkpoints within fission yeast and relate this to information derived from other systems.