Checkpoints that respond to DNA structure changes were originally defined by the inability of yeast mutants to prevent mitosis following DNA damage or S-phase arrest. Genetic analysis has subsequently identified subpathways of the DNA structure checkpoints, including the reversible arrest of DNA synthesis. Here, we show that the Cds1 kinase is required to slow S phase in the presence of DNA-damaging agents. Cds1 is phosphorylated and activated by S-phase arrest and activated by DNA damage during S phase, but not during G 1 or G 2 . Activation of Cds1 during S phase is dependent on all six checkpoint Rad proteins, and Cds1 interacts both genetically and physically with Rad26. Unlike its Saccharomyces cerevisiae counterpart Rad53, Cds1 is not required for the mitotic arrest checkpoints and, thus, defines an S-phase specific subpathway of the checkpoint response. We propose a model for the DNA structure checkpoints that offers a new perspective on the function of the DNA structure checkpoint proteins. This model suggests that an intrinsic mechanism linking S phase and mitosis may function independently of the known checkpoint proteins.[Key Words: Cds1 kinase; S-phase arrest; DNA structure checkpoints; S. pombe] Received August 11, 1997; revised version accepted November 24, 1997.Checkpoint pathways that respond to changes in DNA structure ensure the integrity of the DNA. After detection of specific DNA or DNA-protein structures, a signal is transduced to effector molecules that implement checkpoint-dependent responses such as cell-cycle arrest (Elledge 1996). Many components of the DNA-structure checkpoint pathways have been identified (Carr and Hoekstra 1995). In the fission yeast Schizosaccharomyces pombe, a group of six checkpoint Rad proteins (Rad1, Rad3, Rad9, Rad17, Rad26, and Hus1) are thought to participate in the monitoring and signaling processes that detect both DNA damage and incomplete DNA replication (Al-Khodairy and Carr 1992;Enoch et al. 1992;Rowley et al. 1992;Al-Khodairy et al. 1994). Central to this group is the Rad3 protein, which shares homology with both budding yeast and human checkpoint proteins (Savitsky et al. 1995;Bentley et al. 1996;Cimprich et al. 1996). Rad3 is a member of a larger subfamily of protein kinases that share structural similarities. This subfamily consists of large proteins with a lipid kinase-related domain at the carboxyl terminus. One member, DNA-PKcs, is well characterized as a protein kinase that is activated by association with DNA-binding subunits (Jeggo et al. 1995). By analogy with DNA-PK, we have proposed that Rad3 is activated by the other checkpoint Rad proteins, which may interact with the specific DNA or DNA-protein structures generated by DNA damage and DNA synthesis (Carr 1997).DNA structure checkpoints respond to several distinct signals. The best characterized are DNA damage caused by UV or ␥-irradiation and S-phase arrest resulting from hydroxyurea (HU) exposure. In response to DNA damage, but not S-phase arrest, Chk1 kinase becomes phosphorylated in a m...

To investigate the mechanisms that ensure the dependency relationships between cell cycle events and to investigate the checkpoints that prevent progression through the cell cycle after DNA damage, we have isolated mutants defective in the checkpoint and feedback control pathways. We report the isolation and characterization of 11 new loci that define distinct classes of mutants defective in one or more of the checkpoint and feedback control pathways. Two mutants, rad26.T12 and rad27.T15, were selected for molecular analysis. The null allele of the rad26 gene (rad26.d) shares the phenotype reported for the "checkpoint rad" mutants rad1, rad3, rad9, rad17, and hus1, which are defective in the radiation checkpoint and in the feedback controls that ensure the order of cell cycle events. The null allele of the rad27 gene (rad27.d) defines a new class of Schizosaccharomyces pombe mutant. The rad27 complementing gene codes for a putative protein kinase that is required for cell cycle arrest after DNA damage but not for the feedback control that links mitosis to the completion of prior DNA synthesis (the same gene has recently been described by Walworth et al. (1993) as chk1). These properties are similar to those of the rad9 gene of Saccharomyces cerevisiae. A comparative analysis of the radiation responses in rad26.d, rad26.T12, and rad27.d cells has revealed the existence of two separable responses to DNA damage controlled by the "checkpoint rad" genes. The first, G2 arrest, is defective in rad27.d and rad26.d but is unaffected in rad26.T12 cells. The second response is not associated with G2 arrest after DNA damage and is defective in rad26.d and rad26.T12 but not rad27.d cells. A study of the radiation sensitivity of these mutants through the cell cycle suggests that this second response is associated with S phase and that the checkpoint rad mutants, in addition to an inability to arrest mitosis after radiation, are defective in an S phase radiation checkpoint.

The rad18 mutant of Schizosaccharomyces pombe is very sensitive to killing by both UV and ␥ radiation. We have cloned and sequenced the rad18 gene and isolated and sequenced its homolog from Saccharomyces cerevisiae, designated RHC18. The predicted Rad18 protein has all the structural properties characteristic of the SMC family of proteins, suggesting a motor function-the first implicated in DNA repair. Gene deletion shows that both rad18 and RHC18 are essential for proliferation. Genetic and biochemical analyses suggest that the product of the rad18 gene acts in a DNA repair pathway for removal of UV-induced DNA damage that is distinct from classical nucleotide excision repair. This second repair pathway involves the products of the rhp51 gene (the homolog of the RAD51 gene of S. cerevisiae) and the rad2 gene.Cells of all organisms have evolved an intricate series of DNA repair pathways to counteract the deleterious effects of all types of DNA damage. In Escherichia coli, nucleotide excision repair (NER) of UV damage requires the products of six genes. A complex of the UvrA and UvrB proteins binds to DNA and translocates to the site of the damage. The UvrC product then attaches to the complex, displacing UvrA, and the damaged DNA strand is nicked on both sides of the damaged site. The helicase activity of the UvrD product releases the oligonucleotide containing the damage, and DNA polymerase I and ligase complete the repair process (24). In eukaryotes, NER requires considerably more gene products, most of which are highly conserved (20). In Saccharomyces cerevisiae, the products of the RAD1, -2, -3, -4, -10, -14, and -25 genes are absolutely required for excision repair of UV damage, whereas there is only a partial requirement for RAD7, -16, and -23. There is evidence that the products of many of these genes form a multisubunit complex (e.g., see reference 53). In Schizosaccharomyces pombe, genes encoding highly homologous proteins have been identified (9,10,30,39), demonstrating the conservation of the classical NER pathway in this yeast.Null mutations in the S. cerevisiae NER genes RAD1, -2, -3, -10, and -14 result in a total deficiency in excision repair of UV-induced cyclobutane dimers and 6-4 photoproducts (28). Null mutants of the S. pombe homologs of RAD1 and RAD2 (rad16 and rad13, respectively), while showing many properties expected of excision repair-deficient mutants, are still able to excise UV-induced cyclobutane dimers and 6-4 photoproducts at a significant rate (7, 27). These results suggest that, in contrast to S. cerevisiae, there is a second pathway in S. pombe for removal of UV photoproducts.Most of the rad mutants of S. pombe are sensitive to both UV and ␥ irradiation. Some of these are involved in checkpoint control of the cell cycle to radiation (2, 3, 41). Others, which are particularly sensitive to ionizing radiation, are deficient in recombination repair (6,29,33,54). No mutant which is sensitive to ionizing but not to UV irradiation has yet been identified. The S. pombe rad18-X mutant ...

During the cell cycle, DNA is replicated and segregated equally into two daughter cells. The DNA damage checkpoint ensures that DNA damage is repaired before mitosis is attempted. Genetic studies of the fission yeast Schizosaccharomyces pombe have identified two genes, rad24 and rad25, that are required for this checkpoint. These genes encode 14-3-3 protein homologs that together provide a function that is essential for cell proliferation. In addition, S. pombe rad24 null mutants, and to a lesser extent rad25 null mutants, enter mitosis prematurely, which indicates that 14-3-3 proteins have a role in determining the timing of mitosis.

No abstract

contributed equally to this work 4Corresponding author Following DNA damage or a block to DNA synthesis, checkpoint pathways act to arrest mitosis and prevent the attempted segregation of damaged or unreplicated DNA. The radl7 locus of Schizosaccharomyces pombe is one of seven known radiation-sensitive (rad) loci which are absolutely required to prevent mitosis following DNA damage in fission yeast. Six of these (radi, rad3, rad9, radl 7, rad26 and husi) are also required for the checkpoint which prevents mitosis from occurring before DNA replication is complete. We report here that the predicted radl7 gene product is a basic hydrophilic protein of 606 amino acids which contains five domains with sequence homology to replication factor C (RF-C)/activator 1 subunits. Western analysis and fusion with Green Fluorescent Protein indicate that the abundance and electrophoretic mobility of Radl7 is not significantly modified following a block to DNA synthesis or following DNA damage, and that Radl7 is localized in the nucleus. Radl7 function is not essential for growth, but is required for the function of the DNA structure-dependent checkpoints. Sitedirected mutagenesis has been used to demonstrate the biological significance of the RF-C/activator 1-related domains. These studies have also defined an element of the radiation sensitivity caused by loss of Radl7 function which is not associated with the radiationinduced G2 arrest defect seen in the radl7.d null mutant cells.

The 14‐3‐3 proteins comprise a family of highly conserved acidic proteins. Several activities have been ascribed to these proteins, including activation of tyrosine and tryptophan hydroxylases in the presence of calcium/calmodulin‐dependent protein kinase II, regulation of protein kinase C, phospholipase A2 activity, stimulation of exocytosis and activation of bacterial exoenzyme S (ExoS) during ADP‐ribosy‐lation of host proteins. In addition, a plant 14‐3‐3 protein is present in a G‐box DNA/protein‐binding complex. Previously, we isolated the BMH1 gene from Saccharomyces cerevisiae encoding a putative 14‐3‐3 protein. Using the polymerase chain reaction method, we have isolated a second yeast gene encoding a 14‐3‐3 protein (BMH2). While disruption of either BMH1 or BMH2 alone had little effect, it was impossible to obtain viable cells with both genes disrupted. The cDNA encoding a plant 14‐3‐3 protein under the control of the inducible GAL1 promoter complemented the double disruption. Transfer of the complemented double disruptant to a medium with glucose resulted in the appearance of a high percentage of large budded cells. After prolonged incubation, these cells became enlarged with irregular buds and chains of cells defective in cell‐cell separation became visible. These results suggest an essential role of the 14‐3‐3 proteins, possibly at a later stage of the yeast cell cycle.

The radiO, radl6, rad2O, and swi9 mutants of the fission yeast Schizosaccharomyces pombe, isolated by their radiation sensitivity or abnormal mating-type switching, have been shown previously to be allelic. We have cloned DNA correcting the UV sensitivity or mating-type switching phenotype of these mutants and shown that the correcting DNA is encompassed in a single open reading frame. The gene, which we will refer to as radi6, is approximately 3 kb in length, contains seven introns, and encodes a protein of 892 amino acids. It is not essential for viability of S. pombe. The predicted protein is the homolog of the Saccharomyces cerevisiae RADI protein, which is involved in an early step in excision-repair of UV damage from DNA. The approximately 30% sequence identity between the predicted proteins from the two yeasts is distributed throughout the protein. Two-hybrid experiments indicate a strong protein-protein interaction between the products of the radl6 and swilO genes of S. pombe, which mirrors that reported for RADJ and RADIO in S. cerevisiae. We have identified the mutations in the four alleles of radl6. They mapped to the N-terminal (radiO), central (rad2O), and C-terminal (radi6 and swi9) regions. The radiO and rad2O mutations are in the splice donor sequences of introns 2 and 4, respectively. The plasmid correcting the UV sensitivity of the rad2O mutation was missing the sequence corresponding to the 335 N-terminal amino acids of the predicted protein. Neither smaller nor larger truncations were, however, able to correct its UV sensitivity.DNA repair is a process of fundamental importance in maintaining the integrity of the genome. In humans, deficiencies in DNA repair result in severe multisystem genetic disorders such as xeroderma pigmentosum and ataxia telangiectasia. In order to counteract the deleterious effects of DNA damage, cells have evolved a series of complex pathways whereby damage of different types can be reversed, removed, or tolerated. Many genes involved in these repair pathways have been cloned from humans and from both budding and fission yeasts. Genes involved in nucleotide excision repair show a striking level of cross-species sequence conservation (18,19). Thus, the cloned human ERCCJ, ERCC2 (XPD), ERCC3 (XPB), ERCC5 (XPG), XPA and XPC genes all share homologies with yeast excision repair genes (3,19,(21)(22)(23)37).Although DNA repair processes have been most extensively studied in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe is also proving to be an excellent model for higher eukaryotes. Early work on radiation-sensitive mutants of S. pombe suggested the existence of at least two phenotypic groups (29). Mutants in the first group, thought to be involved in excision-repair, are characterized by sensitivity to UV but not y-radiation, caffeine sensitization to UV damage, and pronounced UV-hypermutability. Mutants in the second group are sensitive to both UV and y-radiation and have lost the caffeine sensitization to UV damage. Their mutabil...