A kinase from fission yeast responsible for blocking mitosis in S phase

Two-step, extensive alterations in the transcriptome from G0 arrest to cell division in Schizosaccharomyces pombe

Body cells in multicellular organisms are in the G0 state, in which cells are arrested and terminally differentiated. To understand how the G0 state is maintained, the genes that are specifically expressed or repressed in G0 must be identified, as they control G0. In the fission yeast Schizosaccharomyces pombe, haploid cells are completely arrested under nitrogen source starvation with high viability. We examined the global transcriptome of G0 cells and cells on the course to resume vegetative growth. Approximately 20% of the transcripts of approximately 5000 genes increased or decreased more than fourfold in the two-step transitions that occur prior to replication. Of the top 30 abundant transcripts in G0, 23 were replaced by ribosome- and translation-related transcripts in the dividing vegetative state. Eight identified clusters with distinct alteration patterns of approximately 2700 transcripts were annotated by Gene Ontology. Disruption of 53 genes indicated that nine of them were necessary to support the proper G0 state. These nine genes included two C2H2 zinc finger transcription factors, a cyclin-like protein implicated in phosphorylation of RNA polymerase II, two putative autophagy regulators, a G-protein activating factor, and two CBS domain proteins, possibly involved in AMP-activated kinase.

Phosphorylation of HuR by Chk2 regulates SIRT1 expression

The RNA binding protein HuR regulates the stability of many target mRNAs. Here, we report that HuR associated with the 3' untranslated region of the mRNA encoding the longevity and stress-response protein SIRT1, stabilized the SIRT1 mRNA, and increased SIRT1 expression levels. Unexpectedly, oxidative stress triggered the dissociation of the [HuR-SIRT1 mRNA] complex, in turn promoting SIRT1 mRNA decay, reducing SIRT1 abundance, and lowering cell survival. The cell cycle checkpoint kinase Chk2 was activated by H(2)O(2), interacted with HuR, and was predicted to phosphorylate HuR at residues S88, S100, and T118. Mutation of these residues revealed a complex pattern of HuR binding, with S100 appearing to be important for [HuR-SIRT1 mRNA] dissociation after H(2)O(2). Our findings demonstrate that HuR regulates SIRT1 expression, underscore functional links between the two stress-response proteins, and implicate Chk2 in these processes.

Modulation of cell cycle-specific gene expressions at the onset of S phase arrest contributes to the robust DNA replication checkpoint response in fission yeast

Fission yeast replication checkpoint kinases Rad3p and Cds1p are essential for maintaining cell viability after transient treatment with hydroxyurea (HU), an agent that blocks DNA replication. Although current studies have focused on the cyclin-dependent protein kinase Cdc2p that is regulated by these checkpoint kinases, other aspects of their functions at the onset of S phase arrest have not been fully understood. In this study, we use genome-wide DNA microarray analyses to show that HU-induced change of expression profiles in synchronized G(2) cells occurs specifically at the onset of S phase arrest. Induction of many core environmental stress response genes and repression of ribosomal genes happen during S phase arrest. Significantly, peak expression level of the MluI-like cell cycle box (MCB)-cluster (G(1)) genes is maintained at the onset of S phase arrest in a Rad3p- and Cds1p-dependent manner. Expression level maintenance of the MCB-cluster is mediated through the accumulation of Rep2p, a putative transcriptional activator of the MBF complex. Conversely, the FKH-cluster (M) genes are repressed during the onset of S phase arrest in a Rad3p-dependent manner. Repression of the FKH-cluster genes is mediated through the decreased levels of one of the putative forkhead transcription factors, Sep1p, but not Fkh2p. Together, our results demonstrate that Rad3p and Cds1p modulate transcriptional response during the onset of S phase arrest.

Specific role of Chk1 phosphorylations in cell survival and checkpoint activation

Chk1 is a multifunctional protein kinase that plays essential roles in cell survival and cell cycle checkpoints. Chk1 is phosphorylated at multiple sites by several protein kinases, but the precise effects of these phosphorylations are largely unknown. Using a knockout-knockin system, we examined the abilities of Chk1 mutants to reverse the defects of Chk1-null cells. Wild-type Chk1 could rescue all the defects of Chk1-null cells. Like endogenous Chk1, wild-type Chk1 localized in both the cytoplasm and the nucleus, and its centrosomal association was enhanced by DNA damage. The mutation at S345 resulted in mitotic catastrophe, impaired checkpoints, and loss of the ability to localize in the cytoplasm, but the mutant retained the ability to be released from chromatin upon encountering genotoxic stressors. In contrast, the mutation at S317 resulted in impaired checkpoints and loss of chromatin release upon encountering genotoxic stressors, but its mutant retained the abilities to prevent mitotic catastrophes and to localize in the cytoplasm, suggesting the distinct effects of these phosphorylations. The forced immobilization of S317A/S345A in centrosomes resulted in the prevention of apoptosis in the presence or absence of DNA damage. Thus, two-step phosphorylation of Chk1 at S317 and S345 appeared to be required for proper localization of Chk1 to centrosomes.

Fission yeast Cut8 is required for the repair of DNA double-strand breaks, ribosomal DNA maintenance, and cell survival in the absence of Rqh1 helicase

Schizosaccharomyces pombe Rqh1 is a member of the RecQ DNA helicase family. Members of this protein family are mutated in cancer predisposition diseases, causing Bloom's, Werner, and Rothmund-Thomson syndromes. Rqh1 forms a complex with topoisomerase III and is proposed to process or disrupt aberrant recombination structures that arise during S phase to allow proper chromosome segregation during mitosis. Intriguingly, in the absence of Rqh1, processing of these structures appears to be dependent on Rad3 (human ATR) in a manner that is distinct from its role in checkpoint control. Here, we show that rad3 rqh1 mutants are normally committed to a lethal pathway of DNA repair requiring homologous recombination, but blocking this pathway by Rhp51 inactivation restores viability. Remarkably, viability is also restored by overexpression of Cut8, a nuclear envelope protein involved in tethering and proper function of the proteasome. In keeping with a recently described function of the proteasome in the repair of DNA double-strand breaks, we found that Cut8 is also required for DNA double-strand break repair and is essential for proper chromosome segregation in the absence of Rqh1, suggesting that these proteins might function in a common pathway in homologous recombination repair to ensure accurate nuclear division in S. pombe.

Production of reactive oxygen species in response to replication stress and inappropriate mitosis in fission yeast

Previous studies have indicated that replication stress can trigger apoptosis-like cell death, accompanied (where tested) by production of reactive oxygen species (ROS), in mammalian cells and budding yeast (Saccharomyces cerevisiae). In mammalian cells, inappropriate entry into mitosis also leads to cell death. Here, we report similar responses in fission yeast (Schizosaccharomyces pombe). We used ROS- and death-specific fluorescent stains to measure the effects of mutations in replication initiation and checkpoint genes in fission yeast on the frequencies of ROS production and cell death. We found that certain mutant alleles of each of the four tested replication initiation genes caused elevated ROS and cell death. Where tested, these effects were not enhanced by checkpoint-gene mutations. Instead, when cells competent for replication but defective in both the replication and damage checkpoints were treated with hydroxyurea, which slows replication fork movement, the frequencies of ROS production and cell death were greatly increased. This was a consequence of elevated CDK activity, which permitted inappropriate entry into mitosis. Thus, studies in fission yeast are likely to prove helpful in understanding the pathways that lead from replication stress and inappropriate mitosis to cell death in mammalian cells.

Intrinsic Kinase Activity and SQ/TQ Domain of Chk2 Kinase as Well as N-terminal Domain of Wip1 Phosphatase Are Required for Regulation of Chk2 by Wip1

The anti-oncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell cycle checkpoint regulation. Recently, we have shown that Chk2 associates with the oncogenic Wip1 (PPM1D) phosphatase and that Wip1 acts as a negative regulator of Chk2 during DNA damage response by dephosphorylating phosphorylated Thr-68 in activated Chk2 (Fujimoto, H., Onishi, N., Kato, N., Takekawa, M., Xu, X. Z., Kosugi, A., Kondo, T., Imamura, M., Oishi, I., Yoda, A., and Minami, Y. (2006) Cell Death Differ. 13, 1170-1180). Here, we performed structure-function analyses of Chk2 and Wip1 by using a series of deletion or amino acid-substituted mutant proteins of Chk2 and Wip1. We show that nuclear localization of both Chk2 and Wip1 is required for their association in cultured cells and that the serine-glutamine (SQ)/threonine-glutamine (TQ) domain of Chk2, containing Thr-68, and the N-terminal domain of Wip1, comprising about 100 amino acids, are necessary and sufficient for the association of both molecules. However, it was found that an intrinsic kinase activity of Chk2, but not phosphatase activity of Wip1, is required for the association of fulllength Chk2 and Wip1. Interestingly, we also show that the mutant Wip1 proteins, bearing the N-terminal domain of Wip1 alone or lacking an intrinsic phosphatase activity, exhibit dominant negative effects on the functions of the wild-type Wip1, i.e. ectopic expression of either of these Wip1 mutants inhibits dephosphorylation of Thr-68 in Chk2 by Wip1 and anti-apoptotic function of Wip1. These results provide a molecular basis for developing novel anti-cancer drugs, targeting oncogenic Wip1 phosphatase.

Regulation of DNA replication machinery by Mrc1 in fission yeast

Faithful replication of chromosomes is crucial to genome integrity. In yeast, the ORC binds replication origins throughout the cell cycle. However, Cdc45 binds these before S-phase, and, during replication, it moves along the DNA with MCM helicase. When replication progression is inhibited, checkpoint regulation is believed to stabilize the replication fork; the detailed mechanism, however, remains unclear. To examine the relationship between replication initiation and elongation defects and the response to replication elongation block, we used fission yeast mutants of Orc1 and Cdc45--orp1-4 and sna41-928, respectively--at their respective semipermissive temperatures with regard to BrdU incorporation. Both orp1 and sna41 cells exhibited HU hypersensitivity in the absence of Chk1, a DNA damage checkpoint kinase, and were defective in full activation of Cds1, a replication checkpoint kinase, indicating that normal replication is required for Cds1 activation. Mrc1 is required to activate Cds1 and prevent the replication machinery from uncoupling from DNA synthesis. We observed that, while either the orp1 or the sna41 mutation partially suppressed HU sensitivity of cds1 cells, sna41 specifically suppressed that of mrc1 cells. Interestingly, sna41 alleviated the defect in recovery from HU arrest without increasing Cds1 activity. In addition to sna41, specific mutations of MCM suppressed the HU sensitivity of mrc1 cells. Thus, during elongation, Mrc1 may negatively regulate Cdc45 and MCM helicase to render stalled forks capable of resuming replication.

Postreplication repair and PCNA modification in Schizosaccharomyces pombe

Ubiquitination of proliferating cell nuclear antigen (PCNA) plays a crucial role in regulating replication past DNA damage in eukaryotes, but the detailed mechanisms appear to vary in different organisms. We have examined the modification of PCNA in Schizosaccharomyces pombe. We find that, in response to UV irradiation, PCNA is mono- and poly-ubiquitinated in a manner similar to that in Saccharomyces cerevisiae. However in undamaged Schizosaccharomyces pombe cells, PCNA is ubiquitinated in S phase, whereas in S. cerevisiae it is sumoylated. Furthermore we find that, unlike in S. cerevisiae, mutants defective in ubiquitination of PCNA are also sensitive to ionizing radiation, and PCNA is ubiquitinated after exposure of cells to ionizing radiation, in a manner similar to the response to UV-irradiation. We show that PCNA modification and cell cycle checkpoints represent two independent signals in response to DNA damage. Finally, we unexpectedly find that PCNA is ubiquitinated in response to DNA damage when cells are arrested in G2.

Genomic mapping of single-stranded DNA in hydroxyurea-challenged yeasts identifies origins of replication

We report a genome-wide analysis of single-stranded DNA formation during DNA replication in wild type and checkpoint-deficient rad53 yeast cells in the presence of hydroxyurea. In wild type cells, ssDNA first appears at a subset of replication origins and later “migrates” bi-directionally, suggesting that ssDNA formation is associated with continuously moving replication forks. In rad53 cells, ssDNA appears at virtually every known origin, but remains there over time, suggesting that replication forks stall. Telomeric regions appear to be especially sensitive to the loss of Rad53 checkpoint function. We also mapped replication origins in Schizosaccharomyces pombe using our method.

DNA damage checkpoints in mammals

DNA damage is a common event and probably leads to mutation or deletion within chromosomal DNA, which may cause cancer or premature aging. DNA damage induces several cellular responses including DNA repair, checkpoint activity and the triggering of apoptotic pathways. DNA damage checkpoints are associated with biochemical pathways that end delay or arrest of cell-cycle progression. These checkpoints engage damage sensor proteins, such as the Rad9-Rad1-Hus1 (9-1-1) complex, and the Rad17-RFC complex, in the detection of DNA damage and transduction of signals to ATM, ATR, Chk1 and Chk2 kinases. Chk1 and Chk2 kinases regulate Cdc25, Wee1 and p53 that ultimately inactivate cyclin-dependent kinases (Cdks) which inhibit cell-cycle progression. In this review, we discuss the molecular mechanisms by which DNA damage is recognized by sensor proteins and signals are transmitted to Cdks. We classify the genes involved in checkpoint signaling into four categories, namely sensors, mediators, transducers and effectors, although their proteins have the broad activity, and thus this classification is for convenience and is not definitive.

Regulation of the antioncogenic Chk2 kinase by the oncogenic Wip1 phosphatase

The antioncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell-cycle checkpoint regulation. Here we show that Chk2 associates with the oncogenic protein Wip1 (wild-type p53-inducible phosphatase 1) (PPM1D), a p53-inducible protein phosphatase. Phosphorylation of Chk2 at threonine68 (Thr68), a critical event for Chk2 activation, which is normally induced by DNA damage or overexpression of Chk2, is inhibited by expression of wild-type (WT), but not a phosphatase-deficient mutant (D314A) of Wip1 in cultured cells. Furthermore, an in vitro phosphatase assay revealed that Wip1 (WT), but not Wip1 (D314A), dephosphorylates Thr68 on phosphorylated Chk2 in vitro, resulting in the inhibition of Chk2 kinase activity toward glutathione S-transferase-Cdc25C. Moreover, inhibition of Wip1 expression by RNA interference results in abnormally sustained Thr68 phosphorylation of Chk2 and increased susceptibility of cells in response to DNA damage, indicating that Wip1 acts as a negative regulator of Chk2 in response to DNA damage.

Distinct modes of DNA damage response in S. pombe G0 and vegetative cells

Upon nitrogen-starvation, mostly G2 vegetative (VE) fission yeast cells promote two rounds of division and enter the G0 state with 1C DNA via an uncommitted G1. Whilst G0 cells are permanently arrested, they keep viability through recycling the intracellular nitrogen. We here show that, whilst the DNA damages are efficiently repaired in G0 cells, neither Chk1 activation nor Cdc2 implication for Crb2 (53BP1 like) do not occur. ATR-like Rad3 and non-hyperphosphorylated Crb2 participate the repair processes in G0 cells that are more sensitive to UV and gamma-ray than in VE cells. The sensitivity like in VE cells is restored after replication in the nitrogen-replenished medium, suggesting that the damage hyper-sensitive nature of G0 cells is due to the error-prone repair for single DNA duplex chromosome. The double-strand break (DSB) repair in G0 cells required Pku80, one of non-homologous end joining (NHEJ) proteins. S. pombe G0 cells upon DNA damages thus respond distinctively from VE cells in regard with regulation of checkpoint proteins and the mode of repair that is dependent upon the use of NHEJ.

Natural meiotic recombination hot spots in the Schizosaccharomyces pombe genome successfully predicted from the simple sequence motif M26

The M26 hot spot of meiotic recombination in Schizosaccharomyces pombe is the eukaryotic hot spot most thoroughly investigated at the nucleotide level. The minimum sequence required for M26 activity was previously determined to be 5'-ATGACGT-3'. Originally identified by a mutant allele, ade6-M26, the M26 heptamer sequence occurs in the wild-type S. pombe genome approximately 300 times, but it has been unclear whether any of these are active hot spots. Recently, we showed that the M26 heptamer forms part of a larger consensus sequence, which is significantly more active than the heptamer alone. We used this expanded sequence as a guide to identify a smaller number of sites most likely to be active hot spots. Ten of the 15 sites tested showed meiotic DNA breaks, a hallmark of recombination hot spots, within 1 kb of the M26 sequence. Among those 10 sites, one occurred within a gene, cds1(+), and hot spot activity of this site was confirmed genetically. These results are, to our knowledge, the first demonstration in any organism of a simple, defined nucleotide sequence accurately predicting the locations of natural meiotic recombination hot spots. M26 may be the first example among a diverse group of simple sequences that determine the distribution, and hence predictability, of meiotic recombination hot spots in eukaryotic genomes.

Grp/DChk1 is required for G2-M checkpoint activation in Drosophila S2 cells, whereas Dmnk/DChk2 is dispensable

Cell-cycle checkpoints are signal-transduction pathways required to maintain genomic stability in dividing cells. Previously, it was reported that two kinases essential for checkpoint signalling, Chk1 and Chk2 are structurally conserved. In contrast to yeast, Xenopus and mammals, the Chk1- and Chk2-dependent pathways in Drosophila are not understood in detail. Here, we report the function of these checkpoint kinases, referred to as Grp/DChk1 and Dmnk/DChk2 in Drosophila Schneider's cells, and identify an upstream regulator as well as downstream targets of Grp/DChk1. First, we demonstrate that S2 cells are a suitable model for G(2)/M checkpoint studies. S2 cells display Grp/DChk1-dependent and Dmnk/DChk2-independent cell-cycle-checkpoint activation in response to hydroxyurea and ionizing radiation. S2 cells depleted for Grp/DChk1 using RNA interference enter mitosis in the presence of impaired DNA integrity, resulting in prolonged mitosis and mitotic catastrophe. Grp/DChk1 is phosphorylated in a Mei-41/DATR-dependent manner in response to hydroxyurea and ionizing radiation, indicating that Mei-41/ATR is an upstream component in the Grp/DChk1 DNA replication and DNA-damage-response pathways. The level of Cdc25(Stg) and phosphorylation status of Cdc2 are modulated in a Grp/DChk1-dependent manner in response to hydroxyurea and irradiation, indicating that these cell-cycle regulators are downstream targets of the Grp/DChk1-dependent DNA replication and DNA-damage responses. By contrast, depletion of Dmnk/DChk2 by RNA interference had little effect on checkpoint responses to hydroxyurea and irradiation. We conclude that Grp/DChk1, and not Dmnk/DChk2, is the main effector kinase involved in G(2)/M checkpoint control in Drosophila cells.

Regulation of Cdc2p and Cdc13p Is Required for Cell Cycle Arrest Induced by Defective RNA Splicing in Fission Yeast

Screening of cdc mutants of fission yeast for those whose cell cycle arrest is independent of the DNA damage checkpoint identified the RNA splicing-deficient cdc28 mutant. A search for mutants of cdc28 cells that enter mitosis with unspliced RNA resulted in the identification of an orb5 point mutant. The orb5+ gene, which encodes a catalytic subunit of casein kinase II, was found to be required for cell cycle arrest in other mutants with defective RNA metabolism but not for operation of the DNA replication or DNA damage checkpoints. Loss of function of wee1+ or rad24+ also suppressed the arrest of several splicing mutants. Overexpression of the major B-type cyclin Cdc13p induced cdc28 cells to enter mitosis. The abundance of Cdc13p was reduced, and the phosphorylation of Cdc2p on tyrosine 15 was maintained in splicing-defective cells. These results suggest that regulation of Cdc13p and Cdc2p is required for G2 arrest in splicing mutants.

The fission yeast Crb2/Chk1 pathway coordinates the DNA damage and spindle checkpoint in response to replication stress induced by topoisomerase I inhibitor

Living organisms experience constant threats that challenge their genome stability. The DNA damage checkpoint pathway coordinates cell cycle progression with DNA repair when DNA is damaged, thus ensuring faithful transmission of the genome. The spindle assembly checkpoint inhibits chromosome segregation until all chromosomes are properly attached to the spindle, ensuring accurate partition of the genetic material. Both the DNA damage and spindle checkpoint pathways participate in genome integrity. However, no clear connection between these two pathways has been described. Here, we analyze mutants in the BRCT domains of fission yeast Crb2, which mediates Chk1 activation, and provide evidence for a novel function of the Chk1 pathway. When the Crb2 mutants experience damaged replication forks upon inhibition of the religation activity of topoisomerase I, the Chk1 DNA damage pathway induces sustained activation of the spindle checkpoint, which in turn delays metaphase-to-anaphase transition in a Mad2-dependent fashion. This new pathway enhances cell survival and genome stability when cells undergo replicative stress in the absence of a proficient G(2)/M DNA damage checkpoint.

Systematic deletion analysis of fission yeast protein kinases

Eukaryotic protein kinases are key molecules mediating signal transduction that play a pivotal role in the regulation of various biological processes, including cell cycle progression, cellular morphogenesis, development, and cellular response to environmental changes. A total of 106 eukaryotic protein kinase catalytic-domain-containing proteins have been found in the entire fission yeast genome, 44% (or 64%) of which possess orthologues (or nearest homologues) in humans, based on sequence similarity within catalytic domains. Systematic deletion analysis of all putative protein kinase-encoding genes have revealed that 17 out of 106 were essential for viability, including three previously uncharacterized putative protein kinases. Although the remaining 89 protein kinase mutants were able to form colonies under optimal growth conditions, 46% of the mutants exhibited hypersensitivity to at least 1 of the 17 different stress factors tested. Phenotypic assessment of these mutants allowed us to arrange kinases into functional groups. Based on the results of this assay, we propose also the existence of four major signaling pathways that are involved in the response to 17 stresses tested. Microarray analysis demonstrated a significant correlation between the expression signature and growth phenotype of kinase mutants tested. Our complete microarray data sets are available at http://giscompute.gis.a-star.edu.sg/~gisljh/kinome.

Brc1-mediated DNA repair and damage tolerance

The structural maintenance of chromosome (SMC) proteins are key elements in controlling chromosome dynamics. In eukaryotic cells, three essential SMC complexes have been defined: cohesin, condensin, and the Smc5/6 complex. The latter is essential for DNA damage responses; in its absence both repair and checkpoint responses fail. In fission yeast, the UV-C and ionizing radiation (IR) sensitivity of a specific hypomorphic allele encoding the Smc6 subunit, rad18-74 (renamed smc6-74), is suppressed by mild overexpression of a six-BRCT-domain protein, Brc1. Deletion of brc1 does not result in a hypersensitivity to UV-C or IR, and thus the function of Brc1 relative to the Smc5/6 complex has remained unclear. Here we show that brc1Delta cells are hypersensitive to a range of radiomimetic drugs that share the feature of creating lesions that are an impediment to the completion of DNA replication. Through a genetic analysis of brc1Delta epistasis and by defining genes required for Brc1 to suppress smc6-74, we find that Brc1 functions to promote recombination through a novel postreplication repair pathway and the structure-specific nucleases Slx1 and Mus81. Activation of this pathway through overproduction of Brc1 bypasses a repair defect in smc6-74, reestablishing resolution of lesions by recombination.

Genetic and physical interactions between Schizosaccharomyces pombe Mcl1 and Rad2, Dna2 and DNA polymerase alpha: evidence for a multifunctional role of Mcl1 in DNA replication and repair

Schizosaccharomyces pombe rad2 is involved in Okazaki fragments processing during lagging-strand DNA replication. Previous studies identified several slr mutants that are co-lethal with rad2Delta and sensitive to methyl methanesulfonate as single mutants. One of these mutants, slr3-1, is characterized here. Complementation and sequence analyses show that slr3-1 (mcl1-101) is allelic to mcl1(+), which is required for chromosome replication, cohesion and segregation. mcl1-101 is temperature-sensitive for growth and is highly sensitive to DNA damage. mcl1 cells arrest with 2C DNA content and chromosomal DNA double-strand breaks accumulate at the restrictive temperature. Mcl1p, which belongs to the Ctf4p/SepBp family, interacts both genetically and physically with DNA polymerase alpha. Mutations in rhp51 and dna2 enhance the growth defect of the mcl1-101 mutant. These results strongly suggest that Mcl1p is a functional homologue of Saccharomyces cerevisiae Ctf4p and plays a role in lagging-strand synthesis and Okazaki fragment processing, in addition to DNA repair.

Schizosaccharomyces pombe Swi1, Swi3, and Hsk1 are components of a novel S-phase response pathway to alkylation damage

The Swi1 and Swi3 proteins are required for mat1 imprinting and mating-type switching in Schizosaccharomyces pombe, where they mediate a pause of leading-strand replication in response to a lagging-strand signal. In addition, Swi1 has been demonstrated to be involved in the checkpoint response to stalled replication forks, as was described for the Saccharomyces cerevisiae homologue Tof1. This study addresses the roles of Swi1 and Swi3 during a replication process perturbed by the presence of template bases alkylated by methyl methanesulfonate (MMS). Both the swi1 and swi3 mutations have additive effects on MMS sensitivity and on the MMS-induced damage checkpoint response when combined with chk1 and cds1, but they are nonadditive with hsk1. Cells with swi1, swi3, or hsk1 mutations are also defective in slowing progression through S phase in response to MMS damage. Moreover, swi1 and swi3 strains show increased levels of genomic instability even in the absence of exogenously induced DNA damage. Chromosome fragmentation, increased levels of single-stranded DNA, increased recombination, and instability of replication forks stalled in the presence of hydroxyurea are observed, consistent with the possibility that the replication process is affected in these mutants. In conclusion, Swi1, Swi3, and Hsk1 act in a novel S-phase checkpoint pathway that contributes to replication fork maintenance and to survival of alkylation damage.

Replication checkpoint kinase Cds1 regulates Mus81 to preserve genome integrity during replication stress

The replication checkpoint kinase Cds1 preserves genome integrity by stabilizing stalled replication forks. Cds1 targets substrates through its FHA domain. The Cds1 FHA domain interacts with Mus81, a subunit of the Mus81-Eme1 structure-specific endonuclease. We report here that Mus81 and Rhp51 are required for generating deletion mutations in fission yeast replication mutants that experience replication stress. A mutation in the Mus81 FHA-binding motif eliminates its Cds1-binding and Cds1-dependent phosphorylation. Furthermore, this mutation exacerbates the deletion mutator phenotype of a replication mutant, and induces a hyper-recombination phenotype in hydroxyurea-treated cells. In unperturbed cells, Mus81 associates with chromatin throughout S phase. In replication mutants grown at semipermissive temperature, Mus81 undergoes minor Cds1-dependent phosphorylation, remains chromatin-associated, generates deletion mutations, and maintains cell growth. Upon S-phase arrest by acute hydroxyurea treatment, Mus81 is not required for cell viability but is essential for recovery from replication fork collapse. Moreover, Mus81 undergoes extensive Cds1-dependent phosphorylation and dissociates from chromatin in hydroxyurea-arrested cells, thereby preventing it from cleaving stalled replication forks that could lead to fork breakage and chromosomal rearrangement. These results provide novel insights into how Cds1 regulates Mus81 accordingly when cells experience different replication stress to preserve genome integrity.

A checkpoint control linking meiotic S phase and recombination initiation in fission yeast

During meiosis, high levels of recombination initiated by DNA double-strand breaks (DSBs) occur only after DNA replication. However, how DSB formation is coupled to DNA replication is unknown. We examined several DNA replication proteins for a role in this coupling in Schizosaccharomyces pombe, and we show that ribonucleotide reductase, the rate-limiting enzyme of deoxyribonucleotide synthesis and the target of the DNA synthesis inhibitor hydroxyurea (HU) is indirectly required for DSB formation linked to DNA replication. However, in cells in which the function of the DNA-replication-checkpoint proteins Rad1p, Rad3p, Rad9p, Rad17p, Rad26p, Hus1p, or Cds1p was compromised, DSB formation occurred at similar frequencies in the absence or presence of HU. The DSBs in the HU-treated mutant cells occurred at normal sites and were associated with recombination. In addition, Cdc2p is apparently not involved in this process. We propose that the sequence of meiotic S phase and initiation of recombination is coordinated by DNA-replication-checkpoint proteins.

Chk1-dependent S-M checkpoint delay in vertebrate cells is linked to maintenance of viable replication structures

We investigated mitotic delay during replication arrest (the S-M checkpoint) in DT40 B-lymphoma cells deficient in the Chk1 or Chk2 kinase. We show here that cells lacking Chk1, but not those lacking Chk2, enter mitosis with incompletely replicated DNA when DNA synthesis is blocked, but only after an initial delay. This initial delay persists when S-M checkpoint failure is induced in Chk2-/- cells with the Chk1 inhibitor UCN-01, indicating that it does not depend on Chk1 or Chk2 activity. Surprisingly, dephosphorylation of tyrosine 15 did not accompany Cdc2 activation during premature entry to mitosis in Chk1-/- cells, although mitotic phosphorylation of cyclin B2 did occur. Previous studies have shown that Chk1 is required to stabilize stalled replication forks during replication arrest, and strikingly, premature mitosis occurs only in Chk1-deficient cells which have lost the capacity to synthesize DNA as a result of progressive replication fork inactivation. These results suggest that Chk1 maintains the S-M checkpoint indirectly by preserving the viability of replication structures and that it is the continued presence of such structures, rather than the activation of Chk1 per se, which delays mitosis until DNA replication is complete.

Meiotic S-phase damage activates recombination without checkpoint arrest

Checkpoints operate during meiosis to ensure the completion of DNA synthesis and programmed recombination before the initiation of meiotic divisions. Studies in the fission yeast Schizosaccharomyces pombe suggest that the meiotic response to DNA damage due to a failed replication checkpoint response differs substantially from the vegetative response, and may be influenced by the presence of homologous chromosomes. The checkpoint responses to DNA damage during fission yeast meiosis are not well characterized. Here we report that DNA damage induced during meiotic S-phase does not activate checkpoint arrest. We also find that in wild-type cells, markers for DNA breaks can persist at least to the first meiotic division. We also observe increased spontaneous S-phase damage in checkpoint mutants, which is repaired by recombination without activating checkpoint arrest. Our results suggest that fission yeast meiosis is exceptionally tolerant of DNA damage, and that some forms of spontaneous S-phase damage can be repaired by recombination without activating checkpoint arrest.

TTK/hMps1 participates in the regulation of DNA damage checkpoint response by phosphorylating CHK2 on threonine 68

CHK2/hCds1 plays important roles in the DNA damage-induced cell cycle checkpoint by phosphorylating several important targets, such as Cdc25 and p53. To obtain a better understanding of the CHK2 signaling pathway, we have carried out a yeast two-hybrid screen to search for potential CHK2-interacting proteins. Here, we report the identification of the mitotic checkpoint kinase, TTK/hMps1, as a novel CHK2-interacting protein. TTK/hMps1 directly phosphorylates CHK2 on Thr-68 in vitro. Expression of a TTK kinase-dead mutant, TTK(D647A), interferes with the G(2)/M arrest induced by either ionizing radiation or UV light. Interestingly, induction of CHK2 Thr-68 phosphorylation and of several downstream events, such as cyclin B1 accumulation and Cdc2 Tyr-15 phosphorylation, is also affected. Furthermore, ablation of TTK expression using small interfering RNA results not only in reduced CHK2 Thr-68 phosphorylation, but also in impaired growth arrest. Our results are consistent with a model in which TTK functions upstream from CHK2 in response to DNA damage and suggest possible cross-talk between the spindle assembly checkpoint and the DNA damage checkpoint.

Mapping and mutation of the conserved DNA polymerase interaction motif (DPIM) located in the C-terminal domain of fission yeast DNA polymerase delta subunit Cdc27

Background

DNA polymerases α and δ play essential roles in the replication of chromosomal DNA in eukaryotic cells. DNA polymerase α (Pol α)-primase is required to prime synthesis of the leading strand and each Okazaki fragment on the lagging strand, whereas DNA polymerase δ (Pol δ) is required for the elongation stages of replication, a function it appears capable of performing on both leading and lagging strands, at least in the absence of DNA polymerase ε (Pol ε).

Results

Here it is shown that the catalytic subunit of Pol α, Pol1, interacts with Cdc27, one of three non-catalytic subunits of fission yeast Pol δ, both in vivo and in vitro. Pol1 interacts with the C-terminal domain of Cdc27, at a site distinct from the previously identified binding sites for Cdc1 and PCNA. Comparative protein sequence analysis identifies a protein sequence motif, called the DNA polymerase interaction motif (DPIM), in Cdc27 orthologues from a wide variety of eukaryotic species, including mammals. Mutational analysis shows that the DPIM in fission yeast Cdc27 is not required for effective DNA replication, repair or checkpoint function.

Conclusions

The absence of any detectable phenotypic consequences arising from mutation of the DPIM suggests that despite its evolutionary conservation, the interaction between the two polymerases mediated by this motif is a non-essential one.

DNA Replication Checkpoint Prevents Precocious Chromosome Segregation by Regulating Spindle Behavior

The DNA replication checkpoint maintains replication fork integrity and prevents chromosome segregation during replication stresses. Mec1 and Rad53 (human ATM/ATR- and Chk2-like kinases, respectively) are critical effectors of this pathway in yeast. When treated with replication inhibitors, checkpoint-deficient mec1 or rad53 mutant fails to maintain replication fork integrity and proceeds to partition unreplicated chromosomes. We show that this unnatural chromosome segregation requires neither the onset of mitosis nor APC activation, cohesin cleavage, or biorientation of kinetochores. Instead, the checkpoint deficiency leads to deregulation of microtubule-associated proteins Cin8 and Stu2, which, in the absence of both chromosome cohesion and bipolar attachment of kinetochores to microtubules, induce untimely spindle elongation, causing premature chromosome separation. The checkpoint's ability to prevent nuclear division is abolished by combined deficiency of microtubule-destabilizing motor Kip3 and Mad2 functions. Thus, the DNA replication checkpoint prevents precocious chromosome segregation, not by inhibiting entry into mitosis as widely believed, but by directly regulating spindle dynamics.

Germinating fission yeast spores delay in G1 in response to UV irradiation

Background

Checkpoint mechanisms prevent cell cycle transitions until previous events have been completed or damaged DNA has been repaired. In fission yeast, checkpoint mechanisms are known to regulate entry into mitosis, but so far no checkpoint inhibiting S phase entry has been identified.

Results

We have studied the response of germinating Schizosaccharomyces pombe spores to UV irradiation in G1. When germinating spores are irradiated in early G1 phase, entry into S phase is delayed. We argue that the observed delay is caused by two separate mechanisms. The first takes place before entry into S phase, does not depend on the checkpoint proteins Rad3, Cds1 and Chk1 and is independent of Cdc2 phosphorylation. Furthermore, it is not dependent upon inhibiting the Cdc10-dependent transcription required for S phase entry, unlike a G1/S checkpoint described in budding yeast. We show that expression of Cdt1, a protein essential for initiation of DNA replication, is delayed upon UV irradiation. The second part of the delay occurs after entry into S phase and depends on Rad3 and Cds1 and is probably due to the intra-S checkpoint. If the germinating spores are irradiated in late G1, they enter S phase without delay and arrest in S phase, suggesting that the delay we observe upon UV irradiation in early G1 is not caused by nonspecific effects of UV irradiation.

Conclusions

We have studied the response of germinating S. pombe spores to UV irradiation in G1 and shown that S phase entry is delayed by a mechanism that is different from classical checkpoint responses. Our results point to a mechanism delaying expression of proteins required for S phase entry.

On the Slowing of S Phase in Response to DNA Damage in Fission Yeast

Eukaryotic cells slow their progression through S phase upon DNA damage. The mechanism that leads to this slowing is called the intra-S-phase checkpoint. Previous studies demonstrated that in the fission yeast Schizosaccharomyces pombe this checkpoint is mediated by a pathway that includes Rad3 (similar to human ATR and ATM) and Cds1 (similar to human Chk1 and Chk2). Here we present evidence that a major downstream target of this pathway is the cyclin-dependent kinase, Cdc2. We also present evidence suggesting that the intra-S-phase checkpoint makes a relatively minor contribution to the survival of cells with damaged DNA.

DNA replication checkpoint control mediated by the spindle checkpoint protein Mad2p in fission yeast

The relationship between the DNA replication and spindle checkpoints of the cell cycle is unclear, given that in most eukaryotes, spindle formation occurs only after DNA replication is complete. Fission yeast rad3 mutant cells, which are deficient in DNA replication checkpoint function, enter, progress through, and exit mitosis even when DNA replication is blocked. In contrast, the entry of cds1 mutant cells into mitosis is delayed by several hours when DNA replication is inhibited. We show here that this delay in mitotic entry in cds1 cells is due in part to activation of the spindle checkpoint protein Mad2p. In the presence of the DNA replication inhibitor hydroxyurea (HU), cds1 mad2 cells entered and progressed through mitosis earlier than did cds1 cells. Overexpression of Mad2p or inactivation of Slp1p, a regulator of the anaphase-promoting complex, also rescued the checkpoint defect of HU-treated rad3 cells. Rad3p was shown to be involved in the physical interaction between Mad2p and Slp1p in the presence of HU. These results suggested that Mad2p and Slp1p act downstream of Rad3p in the DNA replication checkpoint and that Mad2p is required for the DNA replication checkpoint when Cds1p is compromised.

Requirement for Schizosaccharomyces pombe Top3 in the maintenance of chromosome integrity

In Schizosaccharomyces pombe, topoisomerase III is encoded by a single gene, top3(+), which is essential for cell viability and proper chromosome segregation. Deletion of rqh1(+), which encodes the sole RecQ family helicase in S. pombe, suppresses the lethality caused by loss of top3. Here, we provide evidence suggesting that the lethality in top3 mutants is due to accumulation of aberrant DNA structures that arise during S phase, as judged by pulsed-field gel electrophoresis. Using a top3 shut-off strain, we show here that depletion of Top3 activates the DNA damage checkpoint associated with phosphorylation of the checkpoint kinase Chk1. Despite activation of this checkpoint, top3 cells exit the arrest but fail to undergo faithful chromosome segregation. However, these mitotic defects are secondary to chromosomal abnormalities that lead to the lethality, because advance into mitosis did not adversely affect cell survival. Furthermore, top3 function is required for maintenance of nucleolar structure, possibly due to its ability to prevent recombination at the rDNA loci. Our data are consistent with the notion that Top3 has a key function in homologous recombinational repair during S phase that is essential for ensuring subsequent fidelity of chromosome segregation.

Separase-mediated cleavage of cohesin at interphase is required for DNA repair

Sister chromatids are held together by cohesins. At anaphase, separase is activated by degradation of its inhibitory partner, securin. Separase then cleaves cohesins, thus allowing sister chromatid separation. Fission yeast securin (Cut2) has destruction boxes and a separase (Cut1) interaction site in the amino and carboxyl terminus, respectively. Here we show that securin is essential for separase stability and also for proper repair of DNA damaged by ultraviolet, X-ray and gamma-ray irradiation. The cut2(EA2) mutant is defective in the repair of ultraviolet damage lesions, although the DNA damage checkpoint is activated normally. In double mutant analysis of ultraviolet sensitivity, checkpoint kinase chk1 (ref. 9) and excision repair rad13 (ref. 10) mutants were additive with cut2(EA2), whereas recombination repair rhp51 (ref. 11) and cohesin subunit rad21 (ref. 12) mutants were not. Cohesin was hyper-modified on ultraviolet irradiation in a Rad3 kinase-dependent way. Experiments using either mutant cohesin that cannot be cleaved by separase or a protease-dead separase provide evidence that this DNA repair function of securin-separase acts through the cleavage of cohesin. We propose that the securin-separase complex might aid DNA repair by removing local cohesin in interphase cells.

Schizosaccharomyces pombe replication protein Cdc45/Sna41 requires Hsk1/Cdc7 and Rad4/Cut5 for chromatin binding

Cdc45 is a conserved protein required for firing of replication origins and processive DNA replication. We used an in situ chromatin-binding assay to determine factors required for fission yeast Cdc45p chromatin binding. Assembly of the pre-replicative complex is essential for Cdc45p chromatin binding, but pre-replicative complex assembly occurs independently of Cdc45p. Fission yeast Cdc45p associates with MCM proteins in asynchronously growing cells and cells arrested in S phase by hydroxyurea, but not in cells arrested at the G2/M transition. Both hsk1+ (the fission yeast CDC7 homologue) and rad4+/ cut5+ (the fission yeast DPB11 homologue) are required for Cdc45p chromatin binding. Cdc45p also remains chromatin-bound in mutants that fail to recover from replication arrest. In summary, Cdc45p chromatin binding requires an intact pre-replicative complex as well as signaling from both the Dbf4-dependent kinase and cyclin-dependent kinases.

Cds1 Phosphorylation by Rad3-Rad26 Kinase Is Mediated by Forkhead-associated Domain Interaction with Mrc1

The protein kinase Cds1 is an effector of the replication checkpoint in the fission yeast Schizosaccharomyces pombe. Cds1 is required to stabilize stalled replication forks, and it helps to prevent the onset of mitosis until the genome is fully replicated. Mrc1 (mediator of the replication checkpoint-1) and Rad3-Rad26 kinase are required for Cds1 activation, but exactly how Mrc1 mediates Cds1 activation is unknown. Here we show that Mrc1 is required for the initial threonine 11 phosphorylation of Cds1 by Rad3-Rad26. Mrc1 specifically interacts with the forkhead-associated (FHA) domain of Cds1 in yeast two-hybrid assays. Mutations in the FHA domain that abolish this interaction also eliminate Thr-11 phosphorylation of Cds1. Weak Thr-11 phosphorylation of a "kinase-dead" mutant of Cds1 is rescued by co-expression of wild type Cds1. The requirement for Mrc1 in the replication checkpoint can be partially eliminated by expression of a Rad26-Cds1 fusion protein. These findings suggest that recognition of Mrc1 by the FHA domain of Cds1 serves to recruit Cds1 to Rad3-Rad26. This interaction mediates the initial Thr-11 phosphorylation of Cds1 by Rad3-Rad26 with subsequent intermolecular phosphorylation events leading to full activation of Cds1.

Human SAD1 kinase is involved in UV-induced DNA damage checkpoint function

Checkpoint activation by DNA damage during G(2) prevents activation of cyclin B/Cdc2 complexes, and as a consequence, mitotic entry is blocked. Although initiation and maintenance of G(2) arrest are known to be regulated by at least two distinct signaling pathways, including those of p38MAPK and ataxia-telangiectasia-mutated (ATM)- and Rad3-related (ATR)-Chk1 in higher eukaryotes, the actual number of signaling pathways involved in this regulation is still elusive. In the present study, we identified human SAD1 (hsSAD1) by searching a sequence data base. The predicted hsSAD1 protein comprises 778 amino acids and shares significant homology with the fission yeast Cdr2, a mitosis-regulatory kinase, and Caenorhabditis elegans SAD1, a neuronal cell polarity regulator. HsSAD1 transcript was expressed ubiquitously with the highest levels of expression in brain and testis. HsSAD1 specifically phosphorylated Wee1A, Cdc25-C, and -B on Ser-642, Ser-216, and Ser-361 in vitro, respectively. Overexpression of hsSAD1 resulted in an increased phosphorylation of Cdc25C on Ser-216 in vivo. DNA damage induced by UV or methyl methane sulfonate but not by IR enhanced endogenous hsSAD1 kinase activity in a caffeine-sensitive manner and caused translocation of its protein from cytoplasm to nucleus. Overexpression of wild-type hsSAD1 induced G(2)/M arrest in HeLa S2 cells. Furthermore, UV-induced G(2)/M arrest was partially abrogated by the reduced expression of hsSAD1 using small interfering RNA. These results suggest that hsSAD1 acts as checkpoint kinase upon DNA damage induced by UV or methyl methane sulfonate. The identification of this new kinase suggests the existence of an alternative checkpoint pathway other than those of ATR-Chk1 and p38MAPK.

Negative regulation of Chk2 expression by p53 is dependent on the CCAAT-binding transcription factor NF-Y

The kinase Chk2 and tumor suppressor p53 participate in an ill defined regulatory interaction in mammalian cells. The abundance of Chk2 mRNA and protein has now been shown to be decreased by the induction of p53 in Saos2 cells. Ionizing radiation also triggered the phosphorylation and subsequent down-regulation of Chk2 in human colorectal HCT116 (p53(+/+)) cancer cells; irradiation of its isogenic mutant HCT116 (p53(-/-)) cells, which lack functional p53, induced Chk2 phosphorylation but not its down-regulation. In addition, HCT116 (p53(+/+)) cells constitutively expressing a dominant negative p53 (V143A) failed to suppress Chk2 expression after irradiation. Reporter gene assays in HCT116 (p53(+/+)) cells revealed that wild-type p53 repressed, whereas a dominant negative p53 mutant increased, the activity of the human Chk2 gene promoter. Mutational analysis showed that a CCAAT box located between nucleotides -152 and -138 of the promoter was responsible for its negative regulation by p53. Electrophoretic mobility shift assays demonstrated that the transcription factor NF-Y binds to this CCAAT sequence. A dominant negative mutant of NF-YA abolished the effect of p53 on Chk2 promoter activity. These results suggest that p53 negatively regulates Chk2 gene transcription through modulation of NF-Y function and that this regulation may be important for reentry of cells into the cell cycle after DNA damage is repaired.

Global gene expression responses of fission yeast to ionizing radiation

A coordinated transcriptional response to DNA-damaging agents is required to maintain genome stability. We have examined the global gene expression responses of the fission yeast Schizosaccharomyces pombe to ionizing radiation (IR) by using DNA microarrays. We identified approximately 200 genes whose transcript levels were significantly altered at least twofold in response to 500 Gy of gamma IR in a temporally defined manner. The majority of induced genes were core environmental stress response genes, whereas the remaining genes define a transcriptional response to DNA damage in fission yeast. Surprisingly, few DNA repair and checkpoint genes were transcriptionally modulated in response to IR. We define a role for the stress-activated mitogen-activated protein kinase Sty1/Spc1 and the DNA damage checkpoint kinase Rad3 in regulating core environmental stress response genes and IR-specific response genes, both independently and in concert. These findings suggest a complex network of regulatory pathways coordinate gene expression responses to IR in eukaryotes.

p73alpha regulation by Chk1 in response to DNA damage

The checkpoint kinase 1 (Chk1) is an essential component of the DNA damage checkpoint. Previous studies have demonstrated an indispensable role for the p53-related transcription factor p73alpha in DNA damage-induced apoptosis. Here, we provide evidence that p73alpha is a target of Chk1. We found that endogenous p73alpha is serine phosphorylated by endogenous Chk1 upon DNA damage, which is a mechanism required for the apoptotic-inducing function of p73alpha. Consistent with this, we discovered that endogenous p73alpha interacts with Chk1 and is phosphorylated by Chk1 at serine 47 in vitro and in vivo. In contrast, Chk2 does not phosphorylate p73alpha in vitro. Moreover, mutation of serine 47 abolishes both Chk1-dependent phosphorylation of p73alpha upon DNA damage in vivo and the ability of Chk1 to upregulate the transactivation capacity of p73alpha. Our data indicate a novel biochemical pathway through which the p73alpha proapoptotic function requires DNA damage-triggered p73alpha phosphorylation by Chk1.

Swi1 prevents replication fork collapse and controls checkpoint kinase Cds1

The replication checkpoint is a dedicated sensor-response system activated by impeded replication forks. It stabilizes stalled forks and arrests division, thereby preserving genome integrity and promoting cell survival. In budding yeast, Tof1 is thought to act as a specific mediator of the replication checkpoint signal that activates the effector kinase Rad53. Here we report studies of fission yeast Swi1, a Tof1-related protein required for a programmed fork-pausing event necessary for mating type switching. Our studies have shown that Swi1 is vital for proficient activation of the Rad53-like checkpoint kinase Cds1. Together they are required to prevent fork collapse in the ribosomal DNA repeats, and they also prevent irreversible fork arrest at a newly identified hydroxyurea pause site. Swi1 also has Cds1-independent functions. Rad22 DNA repair foci form during S phase in swi1 mutants and to a lesser extent in cds1 mutants, indicative of fork collapse. Mus81, a DNA endonuclease required for recovery from collapsed forks, is vital in swi1 but not cds1 mutants. Swi1 is recruited to chromatin during S phase. We propose that Swi1 stabilizes replication forks in a configuration that is recognized by replication checkpoint sensors.

Tissue-specific regulation of checkpoint kinase 2 expression by p53

Chk2 (Checkpoint kinase 2) is emerging as a critical mediator of genotoxic stress and diverse cellular responses. Upon ionizing radiation, Chk2 is activated to phosphorylate Cdc25C, leading to G2 phase arrest. p53 has been reported as another substrate of Chk2. Chk2 phosphorylates and stabilizes p53 in response to ionizing radiation. Previous studies found that p53 regulates the Chk2 homologue Chk1 expression both in vitro and in vivo. Using the p53-deficient mouse model, here we demonstrate by immunohistochemistry, Western blot analysis, and RT-PCR that mChk2 expression is reduced in the heart, kidney, lung, and liver of p53(-/-) mice compared to p53(+/+) controls. Similar Chk2 expression was observed in the brain, skin, spleen, and testis in p53(+/+) and p53(-/-) mice. These data indicate that p53 regulates Chk2 expression in a tissue-specific manner.

Schizosaccharomyces pombe Ddb1 is functionally linked to the replication checkpoint pathway

Schizosaccharomyces pombe Ddb1 is homologous to the mammalian DDB1 protein, which has been implicated in damaged-DNA recognition and global genomic repair. However, a recent study suggested that the S. pombe Ddb1 is involved in cell division and chromosomal segregation. Here, we provide evidence that the S. pombe Ddb1 is functionally linked to the replication checkpoint control gene cds1. We show that the S. pombe strain lacking ddb1 has slow growth due to delayed replication progression. Flow cytometric analysis shows an extensive heterogeneity in DNA content. Furthermore, the Deltaddb1 strain is hypersensitive to UV irradiation in S phase and is unable to tolerate a prolonged replication block imposed by hydroxyurea. Interestingly, the Deltaddb1 strain exhibits a high level of the Cds1 kinase activity during passage through S phase. Moreover, mutation of the cds1 gene relieves the defects observed in Deltaddb1 strain. The results suggest that many of the defects observed in Deltaddb1 cells are linked to an aberrant activation of Cds1, and that Ddb1 is functionally linked to Cds1.

Intra-G1 arrest in response to UV irradiation in fission yeast

G1 is a crucial phase of cell growth because the decision to begin another mitotic cycle is made during this period. Occurrence of DNA damage in G1 poses a particular challenge, because replication of damaged DNA can be deleterious and because no sister chromatid is present to provide a template for recombinational repair. We therefore have studied the response of Schizosaccharomyces pombe cells to UV irradiation in early G1 phase. We find that irradiation results in delayed progression through G1, as manifested most critically in the delayed formation of the pre-replication complex. This delay does not have the molecular hallmarks of known checkpoint responses: it is independent of the checkpoint proteins Rad3, Cds1, and Chk1 and does not elicit inhibitory phosphorylation of Cdc2. Irradiated cells eventually progress into S phase and arrest in early S by a rad3- and cds1-dependent mechanism, most likely the intra-S checkpoint. Caffeine alleviates both the intra-G1- and intra-S-phase delays. We suggest that intra-G1 delay may be widely conserved and discuss significance and possible mechanisms.

Delineating the position of rad4+/cut5+ within the DNA-structure checkpoint pathways in Schizosaccharomyces pombe

The fission yeast BRCT domain protein Rad4/Cut5 is required for genome integrity checkpoint responses and DNA replication. Here we address the position at which Rad4/Cut5 acts within the checkpoint response pathways. Rad4 is shown to act upstream of the effector kinases Chk1 and Cds1, as both Chk1 phosphorylation and Cds1 kinase activity require functional Rad4. Phosphorylation of Rad9, Rad26 and Hus1 in response to either DNA damage or inhibition of DNA replication are independent of Rad4/Cut5 checkpoint function. Further we show that a novel, epitope-tagged allele of rad4+/cut5+ acts as a dominant suppressor of the checkpoint deficiencies of rad3-, rad26- and rad17- mutants. Suppression results in the restoration of mitotic arrest and is dependent upon the remaining checkpoint Rad proteins and the two effector kinases. High-level expression of the rad4+/cut5+ allele in rad17 mutant cells restores the nuclear localization of Rad9, but this does not fully account for the observed suppression. We conclude from these data that Rad4/Cut5 acts with Rad3, Rad26 and Rad17 to effect the checkpoint response, and a model for its function is discussed.

XRad17 is required for the activation of XChk1 but not XCds1 during checkpoint signaling in Xenopus

The DNA damage/replication checkpoints act by sensing the presence of damaged DNA or stalled replication forks and initiate signaling pathways that arrest cell cycle progression. Here we report the cloning and characterization of Xenopus orthologues of the RFCand PCNA-related checkpoint proteins. XRad17 shares regions of homology with the five subunits of Replication factor C. XRad9, XRad1, and XHus1 (components of the 9-1-1 complex) all show homology to the DNA polymerase processivity factor PCNA. We demonstrate that these proteins associate with chromatin and are phosphorylated when replication is inhibited by aphidicolin. Phosphorylation of X9-1-1 is caffeine sensitive, but the chromatin association of XRad17 and the X9-1-1 complex after replication block is unaffected by caffeine. This suggests that the X9-1-1 complex can associate with chromatin independently of XAtm/XAtr activity. We further demonstrate that XRad17 is essential for the chromatin binding and checkpoint-dependent phosphorylation of X9-1-1 and for the activation of XChk1 when the replication checkpoint is induced by aphidicolin. XRad17 is not, however, required for the activation of XCds1 in response to dsDNA ends.

Ibp1p, a novel Cdc25-related phosphatase, suppresses Schizosaccharomyces pombe hsk1 ( cdc7)

We report the identification of a novel Cdc25-like protein phosphatase, Ibp1, in the fission yeast Schizosaccharomyces pombe. Ibp1 is closely related to the catalytic subunit of the Cdc25 dual-specificity phosphatases and has phosphatase activity in vitro. Over-production of catalytically active Ibp1 robustly suppresses a mutation in the replication initiation kinase Hsk1p, a member of the Cdc7 family of protein kinases and weakly suppresses mutation of Rad4/Cut5, a DNA polymerase epsilon-associated factor. Ibp1 is not required for viability, suggesting it may be a non-essential regulator of DNA replication or chromosome structure during S phase.

Schizosaccharomyces pombe checkpoint response to DNA interstrand cross-links

Drugs that produce covalent interstrand cross-links (ICLs) in DNA remain central to the treatment of cancer, but the cell cycle checkpoints activated by ICLs have received little attention. We have used the fission yeast, Schizosaccharomyces pombe, to elucidate the checkpoint responses to the ICL-inducing anticancer drugs nitrogen mustard and mitomycin C. First we confirmed that the repair pathways acting on ICLs in this yeast are similar to those in the main organisms studied to date (Escherichia coli, budding yeast, and mammalian cells), principally nucleotide excision repair and homologous recombination. We also identified and disrupted the S. pombe homologue of the Saccharomyces cerevisiae SNM1/PSO2 ICL repair gene and found that this activity is required for normal resistance to cross-linking agents, but not other forms of DNA damage. Survival and biochemical analysis indicated a key role for the "checkpoint Rad" family acting through the chk1-dependent DNA damage checkpoint in the ICL response. Rhp9-dependent phosphorylation of Chk1 correlates with G(2) arrest following ICL induction. In cells able to bypass the G(2) block, a second-cycle (S-phase) arrest was observed. Only a transient activation of the Cds1 DNA replication checkpoint factor occurs following ICL formation in wild-type cells, but this is increased and persists in G(2) arrest-deficient mutants. This likely reflects the fraction of cells escaping the G(2) damage checkpoint and arresting in the subsequent S phase due to ICL replication blocks. Disruption of cds1 confers increased resistance to ICLs, suggesting that this second-cycle S-phase arrest might be a lethal event.

Identification and characterization of human MUS81-MMS4 structure-specific endonuclease

Replication forks may stall when they reach a block on the DNA template such as DNA damage, and the recovery of such stalled replication forks plays a crucial role in the maintenance of genomic stability. Holliday junctions, which are X-shaped DNA structures, are formed at the stalled replication forks and can accumulate if they are not cleaved by structure-specific endonucleases. Recently, a novel nuclease involved in resolving Holliday junction-like structures, Mus81, has been reported in yeast and humans. MUS81 has sequence homology to another DNA nuclease, XPF, which, with its partner ERCC1, makes the 5' incision during nucleotide excision repair. MUS81 also has a binding partner named Mms4 in Saccharomyces cerevisiae and Eme1 in Schizosaccharomyces pombe, but no such partner was identified in human cells. Here, we report identification of the binding partner of human MUS81, which we designate hMMS4. Using immunoaffinity purification we show that hMUS81 or hMMS4 alone have no detectable nuclease activity, but that the hMUS81.hMMS4 complex is a structure-specific nuclease that is capable of resolving fork structures.

Cell-cycle responses to DNA damage in G2

Cellular reproduction, at its basic level, is simply the passing of genetic information from a single parent cell into two daughter cells. As the cellular genome encodes all the information that defines a cell, it is crucial that the genome be accurately replicated. Furthermore, the duplicated genome must be properly segregated so that each daughter cell contains the exact same information as the parent cell. The processes by which this occurs is known as the cell cycle. The failure of either duplication or segregation of the genome can have disastrous consequences for an organism, including cancer and death. This article discusses what is known about checkpoints, the surveillance mechanisms that monitor both the fidelity and accuracy of DNA replication and segregation. Specifically, we will focus on the G2 checkpoint that is responsible for ensuring proper segregation of the duplicated genome into the daughter cells and how this checkpoint functions to arrest entry into mitosis in response to DNA damage.

Chk1-deficient tumour cells are viable but exhibit multiple checkpoint and survival defects

The conserved protein kinase Chk1 is believed to play an important role in checkpoint responses to aberrant DNA structures; however, genetic analysis of Chk1 functions in metazoans is complicated by lethality of Chk1-deficient embryonic cells. We have used gene targeting to eliminate Chk1 function in somatic DT40 B-lymphoma cells. We find that Chk1-deficient DT40 cells are viable, but fail to arrest in G(2)/M in response to and are hypersensitive to killing by ionizing radiation. Chk1-deficient cells also fail to maintain viable replication forks or suppress futile origin firing when DNA polymerase is inhibited, leading to incomplete genome duplication and diminished cell survival after release from replication arrest. In contrast to embryonic cells, however, Chk1 is not required to delay mitosis when DNA synthesis is inhibited. Thus, Chk1 is dispensable for normal cell division in somatic DT40 cells but is essential for DNA damage-induced G(2)/M arrest and a subset of replication checkpoint responses. Furthermore, Chk1-dependent processes promote tumour cell survival after perturbations of DNA structure or metabolism.