Suppressors of Cdc25p Overexpression Identify Two Pathways That Influence the G2/M Checkpoint in Fission Yeast

Corresponding author: Kristi Chrispell Forbes, Department of Genetics, Harvard Medical School, 200 Longwood Ave., Boston, MA 02115..

Communicating editor: M. D. ROSE

Checkpoints maintain the order of cell-cycle events. At G2/M, a checkpoint blocks mitosis in response to damaged or unreplicated DNA. There are significant differences in the checkpoint responses to damaged DNA and unreplicated DNA, although many of the same genes are involved in both responses. To identify new genes that function specifically in the DNA replication checkpoint pathway, we searched for high-copy suppressors of overproducer of Cdc25p (OPcdc25⁺), which lacks a DNA replication checkpoint. Two classes of suppressors were isolated. One class includes a new gene encoding a putative DEAD box helicase, suppressor of uncontrolled mitosis (sum3⁺). This gene negatively regulates the cell-cycle response to stress when overexpressed and restores the checkpoint response by a mechanism that is independent of Cdc2p tyrosine phosphorylation. The second class includes chk1⁺ and the two Schizosaccharomyces pombe 14-3-3 genes, rad24⁺ and rad25⁺, which appear to suppress the checkpoint defect by inhibiting Cdc25p. We show that rad24 mutants are defective in the checkpoint response to the DNA replication inhibitor hydroxyurea at 37° and that cds1 rad24 mutants, like cds1 chk1 mutants, are entirely checkpoint deficient at 29°. These results suggest that chk1⁺ and rad24⁺ may function redundantly with cds1⁺ in the checkpoint response to unreplicated DNA.

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