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doi: 10.1242/10.1242/jcs.00004

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A novel chk1-dependent G1/M checkpoint in fission yeast

Department of Cell Biology, Institute for Cancer Research, Montebello, 0310 Oslo, Norway

^* Author for correspondence (e-mail: beata.grallert{at}labmed.uio.no)

Accepted 8 June 2002

Fission yeast cells with a temperature-sensitive Orp1 protein, a component of the origin recognition complex, cannot perform DNA replication at the restrictive temperature. Seventy percent of orp1-4 cells arrest with a 1C DNA content, whereas 30% proceed to mitosis (`cut'). The arrest depends upon the checkpoint Rad proteins and, surprisingly, the Chk1 protein, which is thought to act only from late S phase. The arrested cells maintain a 1C DNA content, as judged by flow cytometry, and the early origin ars3001 has not been initiated, as judged by 2D gel analysis. We show that in G1-arrested orp1-4 cells, Wee1 phosphorylates and inactivates Cdc2. Activation of Chk1 occurs earlier than Cdc2 phosphorylation, indicating a novel role for Chk1, namely to induce and/or maintain Cdc2 phosphorylation upon checkpoint activation in G1. We also show that commitment to cutting occurs already in early G1 phase.

Key words: Checkpoint, S. pombe, orp1, DNA replication, Mitosis