The topoisomerase III gene ( top3 (+)) from Schizosaccharomyces pombe was isolated and a targeted gene disruption ( top3 :: kan (R)) was used to make a diploid strain heterozygous for top3 (+). The diploid was sporulated and the top3 :: kan (R)spores went through four to eight cell divisions before arresting as elongated, predominantly binucleated cells with incompletely segregated chromosomes. This demonstrates that top3 (+)is essential for vegetative growth in fission yeast. The aberrant chromosomal segregation seen in top3 :: kan (R)cells is unlike the 'cut' phenotype seen in mitosis-defective mutants and so we refer to this phenotype as 'torn'. A deletion mutant, rad12-hd ( rad12 is a homolog of Saccharomyces cerevisiae SGS1), partially suppressed the lethality of top3 mutants. A point mutant, rad12-K547I, which presumably eliminates helicase activity, also suppresses the lethality of top3 mutants, demonstrating that the lethality seen in top3 (-)cells is most likely caused by the helicase activity of Rad12. This double mutant grows very slowly and has much lower viability compared to rad12-hd top3 :: kan (R)cells, implying that the helicase activity of Rad12 is not the only cause of top3 (-)lethality. The low viability of rad12 (-) top3 (-)mutants compared with rad12 single mutants suggests that Top3 also functions independently of Rad12.

The human BLM gene is a member of the Escherichia coli recQ helicase family, which includes the Saccharomyces cerevisiae SGS1 and human WRN genes. Defects in BLM are responsible for the human disease Bloom's syndrome, which is characterized in part by genomic instability and a high incidence of cancer. Here we describe the cloning of rad12 ؉ , which is the fission yeast homolog of BLM and is identical to the recently reported rhq1؉ gene. We showed that rad12 null cells are sensitive to DNA damage induced by UV light and ␥ radiation, as well as to the DNA synthesis inhibitor hydroxyurea. Overexpression of the wild-type rad12 ؉ gene also leads to sensitivity to these agents and to defects associated with the loss of the S-phase and G 2 -phase checkpoint control. We showed genetically and biochemically that rad12 ؉ acts upstream from rad9 ؉ , one of the fission yeast G 2 checkpoint control genes, in regulating exit from the S-phase checkpoint. The physical chromosome segregation defects seen in rad12 null cells combined with the checkpoint regulation defect seen in the rad12 ؉ overproducer implicate rad12 ؉ as a key coupler of chromosomal integrity with cell cycle progression.The fission yeast Schizosaccharomyces pombe undergoes a dose-dependent G 2 delay in response to DNA damage caused by radiation (1, 27). Cells remain arrested at this G 2 checkpoint while DNA damage is repaired, then enter mitosis and resume progression through the cell cycle. Six checkpoint rad genes have been identified in S. pombe: rad1 ϩ , rad3ϩ , rad26 ϩ , and hus1 ϩ (1, 2, 9, 27). Mutations in any of these genes result in almost identical phenotypes. The mutant strains are all sensitive to radiation and to agents which transiently inhibit DNA replication, and all lack the G 2 checkpoint, preventing mitotic entry in the presence of such damage (1, 2, 9, 27). While little is known about G 2 checkpoint genes in humans, homologs of S. pombe rad3 ϩ (4, 5) and rad9 ϩ (19) have recently been isolated. Recent studies have implicated cell cycle gene mutations in causing cancer. It is likely that mutations in genes regulating the G 1 and G 2 checkpoints also have the potential to increase cancer susceptibility.The phenomenon of genomic instability has been firmly established as a crucial step in the genesis of cancer (14,31,33). While genomic instability is seen as a step in the progression of cancer cells, there are also several genetic diseases that lead to genomic instability and, ultimately, to cancer. Among these diseases is the rare autosomal recessive disorder Bloom's syndrome (12). The defective gene in Bloom's syndrome, BLM, encodes a putative DNA helicase that is homologous to the Escherichia coli RecQ and Saccharomyces cerevisiae Sgs1 helicases (11,34). While the BLM and SGS1 proteins contain centrally located helicase domains that are homologous to RecQ, they are much larger than RecQ and show homology, both 5Ј and 3Ј, to the helicase core (26, 35, 37).We describe here the cloning of the fission yeast rad12 ϩ gene, which encodes a helic...

Our interest in the Schizosaccharomyces pombe RecQ helicase, rqh1+, led us to investigate the function of a related putative DNA helicase, srs2+. We identified the srs2+ homolog in S.pombe, and found that srs2+ is not essential for cell viability. A Deltasrs2 Deltarqh1 double mutant grows extremely slowly with aberrant shaped cells and low viability. This slow growth does not appear to be related to stalled replication, as Deltasrs2 Deltarqh1 cells showed higher survival rates, compared with Deltarqh1, when stalled forks were increased by UV irradiation or hydroxy urea treatment. Consistent with this result, we found that Deltasrs2 Deltarqh1 cells progress through S-phase with a slight delay, but undergo a checkpoint-dependent arrest presumably at G2/M. Further, we found that Deltasrs2 Deltarqh1 slow growth is related to recombination, as loss of either the rhp51+ or rhp57+ recombination genes improves cell growth in the double mutant. Deltasrs2 is also synthetic lethal with Deltarhp54, another homologous recombination gene. This lethality is suppressed in a Deltarhp51 background. Together, these results demonstrate a clear genetic interaction between rqh1+, srs2+ and the genes of the homologous recombination pathway.

Compound A increases sister chromatid exchanges at concentrations (27 ppm) obtained in low-flow systems when sevoflurane is used at concentrations approximating minimum alveolar concentration.

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