spo12 is a multicopy suppressor of mcs3 that is periodically expressed in fission yeast mitosis
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ORIGINAL PAPER
J. M. Samuel á N. Fournier á V. Simanis J. B. A. Millar
spo12 is a multicopy suppressor of mcs3 that is periodically expressed in ®ssion yeast mitosis Received: 10 January 2000 / Accepted: 22 June 2000 / Published online: 9 August 2000 Ó Springer-Verlag 2000
Abstract Hyperactivation of Cdc2 in ®ssion yeast causes cells to undergo a lethal premature mitosis, a phenomenon called mitotic catastrophe. This phenotype is observed in cdc2-3w wee1-50 cells at high temperature and is suppressed by a single recessive mutant, mcs3-12. Mcs3 acts independently of the Wee1 kinase and Cdc25 phosphatase, two major regulators of Cdc2. We have isolated multicopy suppressors of the cell cycle arrest phenotype of mcs3-12 wee1-50 cdc25-22 cells, but did not identify the mcs3 gene itself. Instead several known mitotic regulators were isolated, including the Cdc25 phosphatase, Wis2 cyclophilin, Cek1 kinase, and an Hsp90 homologue, Swo1. We also isolated clones encoding non-functional, truncated forms of the Wee1 kinase and Dis2 type 1 phosphatase. In addition we identi®ed a multicopy suppressor that encodes a structural homologue of the budding yeast SPO12 gene. We ®nd that overexpression of ®ssion yeast spo12 not only suppresses the phenotype of the mcs3-12 wee1-50 cdc2522 strain, but also that of a win1-1 wee1-50 cdc25-22 strain at high temperature, indicating that the function of spo12 is not directly related to mcs3. We show that spo12 mRNA is periodically expressed during the ®ssion yeast cell cycle, peaking at the G2/M transition coincidently with cdc15. Deletion of spo12, however, has no overt eect on either the mitotic or meiotic cell cycles, except when the function of the major B type cyclin, Cdc13, is compromised. Key words Cell cycle á spo12 á Mitosis á cdc2 á Schizosaccharomyces pombe Communicated by C. P. Hollenberg J. M. Samuel á J. B. A. Millar (&) Division of Yeast Genetics, National Institute of Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK Tel.: +44-181-9593666; Fax: +44-181-9138536 N. Fournier á V. Simanis Cell Cycle Control Laboratory, ISREC, Chemin des Boveresses 155, 1066 Epalinges, Switzerland
Introduction In eukaryotic cells, both the onset of DNA replication (S phase) and initiation of mitosis (M phase) are triggered by members of the cyclin-dependent kinase (Cdk) family in association with a regulatory cyclin subunit. Considerable eorts have been made in the past 10 years to understand exactly how Cdk/cyclin complexes are regulated. In the ®ssion yeast both major transitions are catalysed by a single gene product, Cdc2 (Nurse and Bissett 1981). Although Cdc2 associates with four distinct cyclins, encoded by the cdc13, cig1, cig2 and puc1 genes, only one of these, Cdc13, is indispensable for cycle progression and is sucient to trigger both S phase and the initiation of mitosis in the absence of the other cyclins (Fisher and Nurse 1996; Stern and Nurse 1996). Although the activity of the Cdc2/Cdc13 complex oscillates periodically through the cell cycle, our understanding of how
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