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Vol. 17, Issue 6, 2746-2756, June 2006

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The Function and Regulation of Budding Yeast Swe1 in Response to Interrupted DNA Synthesis

Hong Liu^*, and Yanchang Wang^*,

Department of Biomedical Sciences, College of Medicine and *Department of Biology, Florida State University, Tallahassee, FL 32306

Submitted December 1, 2005; Revised February 13, 2006; Accepted March 8, 2006
Monitoring Editor: Mark Solomon

Periodically regulated cyclin-dependent kinase (Cdk) is required for DNA synthesis and mitosis. Hydroxyurea (HU) inhibits DNA synthesis by depleting dNTPs, the basic unit for DNA synthesis. HU treatment triggers the S-phase checkpoint, which arrests cells at S-phase, inhibits late origin firing and stabilizes replication forks. Using budding yeast as a model system, we found that Swe1, a negative regulator of Cdk, appears at S-phase and accumulates in HU treatment cells. Interestingly, this accumulation is not dependent on S-phase checkpoint. hsl1, hsl7, and cdc5-2 mutants, which have defects in Swe1 degradation, show HU sensitivity because of high Swe1 protein levels. We further demonstrated that their HU sensitivity is not a result of DNA damage accumulation or incomplete DNA synthesis; instead the sensitivity is due to their dramatically delayed recovery from HU-induced S-phase arrest. Strikingly, our in vivo data indicate that Swe1 inhibits the kinase activity of Clb2-Cdk1, but not that of Clb5-Cdk1. Therefore, S-phase accumulated Swe1 prevents Clb2-Cdk1–mediated mitotic activities, but has little effects on Clb5-Cdk1–associated S-phase progression.

Address correspondence to: Yanchang Wang ( yanchang.wang{at}med.fsu.edu)

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