Mutational effect of fission yeast Polα on cell cycle events

Polar is the principal DNA polymerase for initiation of DNA replication and also functions in postinitiation DNA synthesis. Ln this study, we investigated the cell cycle responses induced by mutations in pol alpha(+). Germinating spores carrying either a deletion of pol alpha(+) (pol alpha Delta) or a structurally intact but catalytically dead pol alpha mutation proceed to inappropriate mitosis with no DNA synthesis. This suggests that the catalytic function, and not the physical presence of Pol alpha, is required to generate the signal that prevents the cells from entering mitosis prematurely. Cells with a pal alpha ts allele arrest the cell cycle near the hydroxyurea arrest point, but, surprisingly, pol alpha ts in cdc20 (pol epsilon mutant) background arrested with a cdc phenoytpe, not a pol alpha ts-like phenotype. At 25 degrees C, replication perturbation caused by pol alpha ts alleles induces Cds1 kinase activity and requires the checkpoint Rads, Cds1, and Rqh1, but not Chk1, to maintain cell viability. At 36 degrees C, replication disruption caused by pol alpha ts alleles induces the phosphorylation of Chk1; however, mutant cells arrest with heterogeneous cell sizes with a population of the cells entering aberrant mitosis. Together, our results indicate that the initiation DNA structure synthesized by Pol alpha is required to bring about the S phase to mitosis checkpoint, whereas replication defects of different severity caused by pol alpha ts mutations induce differential downstream kinase responses.