DNA polymerase delta is required for the replication feedback control of cell cycle progression in Schizosaccharomyces pombe

The kinase domain residue serine 173 of Schizosaccharomyces pombe Chk1 kinase is critical for the response to DNA replication stress

While mammalian Chk1 kinase regulates replication origins, safeguards fork integrity and promotes fork progression, yeast Chk1 acts only in G1 and G2. We report here that the mutation of serine 173 (S173A) in the kinase domain of fission yeast Chk1 abolishes the G1-M and S-M checkpoints with little impact on the G2-M arrest. This separation-of-function mutation strongly reduces the Rad3-dependent phosphorylation of Chk1 at serine 345 during logarithmic growth, but not when cells experience exogenous DNA damage. Loss of S173 lowers the restrictive temperature of a catalytic DNA polymerase epsilon mutant (cdc20.M10) and is epistatic with a mutation in DNA polymerase delta (cdc6.23) when DNA is alkylated by methyl-methanesulfate (MMS). The chk1-S173A allele is uniquely sensitive to high MMS concentrations where it displays a partial checkpoint defect. A complete checkpoint defect occurs only when DNA replication forks break in cells without the intra-S phase checkpoint kinase Cds1. Chk1-S173A is also unable to block mitosis when the G1 transcription factor Cdc10 (cdc10.V50) is impaired. We conclude that serine 173, which is equivalent to lysine 166 in the activation loop of human Chk1, is only critical in DNA polymerase mutants or when forks collapse in the absence of Cds1.

Summary: Mutation of serine-173 in the kinase domain of Chk1 increases genomic instability as it abolishes the response to DNA lesions that arise while chromosomes are being copied.

The Cid1 poly(U) polymerase

The Schizosaccharomyces pombe cytoplasmic protein Cid1 acts as a poly(U) polymerase (PUP). Polyadenylated actin mRNA, a target of this activity, is uridylated upon arrest in S phase and is likely to be one of many such Cid1 targets. This RNA uridylation pathway appears to be conserved, as Cid1 orthologs in Arabidopsis thaliana, Caenorhabditis elegans and humans display PUP activity either in vitro or in Xenopus laevis oocytes. Here, we review the literature on Cid1, other PUPs and uridylation, a conserved and previously under-appreciated mechanism of RNA regulation.

Fission yeast chk1 mutants show distinct responses to different types of DNA damaging treatments

BACKGROUND

Chk1 kinase is activated by phosphorylation at serine-345 by Rad3 checkpoint kinase and is required for DNA damage checkpoint in late S and G2 phase of S. pombe cell cycle. We studied the ability of two chk1 mutants, chk1-1 and chk1-2, to undergo phosphorylation and to delay cell cycle progression in response to different types of DNA lesions.

RESULTS

Both the Chk1-1 and Chk1-2 mutant proteins are phosphorylated to various extents when DNA is damaged in early G2 phase of cell cycle by either UV irradiation or gamma irradiation. However, chk1-2 mutant does not delay cell cycle progression in a dose dependent manner specifically upon gamma irradiations. This defect is not associated with an important loss of survival. Furthermore, both chk1 mutants survive to Camptothecin treatment despite undetectable Chk1-1 or Chk1-2 phosphorylated forms. We show that both mutant proteins are not phosphorylated in cds1 devoid cells treated with ribonucleotide reductase inhibitor hydroxyurea or when the replisome is affected by a thermosensitive mutation in DNA polymerase delta. This inability is associated with the loss of checkpoint function. We found that an increased level of Crb2/Rhp9 protein specifically complements the defect of the chk1-1 mutant allowing Chk1-1 phosphorylation upon treatment with hydroxyurea of dcds1 cells.

CONCLUSIONS

Mutants chk1-1 and chk1-2 behave differently according to the type of lesion generated on DNA.

Cell-cycle checkpoint kinases: checking in on the cell cycle

In response to DNA damage, cell-cycle checkpoints integrate cell-cycle control with DNA repair. The idea that checkpoint controls are an integral component of normal cell-cycle progression has arisen as a result of studies in Drosophila and mice. In addition, an appreciation that DNA damage arises as a natural consequence of cellular metabolism, including DNA replication itself, has influenced thinking regarding the nature of checkpoint pathways.

DNA replication and damage checkpoints and meiotic cell cycle controls in the fission and budding yeasts

The cell cycle checkpoint mechanisms ensure the order of cell cycle events to preserve genomic integrity. Among these, the DNA-replication and DNA-damage checkpoints prevent chromosome segregation when DNA replication is inhibited or DNA is damaged. Recent studies have identified an outline of the regulatory networks for both of these controls, which apparently operate in all eukaryotes. In addition, it appears that these checkpoints have two arrest points, one is just before entry into mitosis and the other is prior to chromosome separation. The former point requires the central cell-cycle regulator Cdc2 kinase, whereas the latter involves several key regulators and substrates of the ubiquitin ligase called the anaphase promoting complex. Linkages between these cell-cycle regulators and several key checkpoint proteins are beginning to emerge. Recent findings on post-translational modifications and protein-protein interactions of the checkpoint proteins provide new insights into the checkpoint responses, although the functional significance of these biochemical properties often remains unclear. We have reviewed the molecular mechanisms acting at the DNA-replication and DNA-damage checkpoints in the fission yeast Schizosaccharomyces pombe, and the modifications of these controls during the meiotic cell cycle. We have made comparisons with the controls in fission yeast and other organisms, mainly the distantly related budding yeast.

Cid1, a fission yeast protein required for S-M checkpoint control when DNA polymerase delta or epsilon is inactivated

The S-M checkpoint is an intracellular signaling pathway that ensures that mitosis is not initiated in cells undergoing DNA replication. We identified cid1, a novel fission yeast gene, through its ability when overexpressed to confer specific resistance to a combination of hydroxyurea, which inhibits DNA replication, and caffeine, which overrides the S-M checkpoint. Cid1 overexpression also partially suppressed the hydroxyurea sensitivity characteristic of DNA polymerase delta mutants and mutants defective in the "checkpoint Rad" pathway. Cid1 is a member of a family of putative nucleotidyltransferases including budding yeast Trf4 and Trf5, and mutation of amino acid residues predicted to be essential for this activity resulted in loss of Cid1 function in vivo. Two additional Cid1-like proteins play similar but nonredundant checkpoint-signaling roles in fission yeast. Cells lacking Cid1 were found to be viable but specifically sensitive to the combination of hydroxyurea and caffeine and to be S-M checkpoint defective in the absence of Cds1. Genetic data suggest that Cid1 acts in association with Crb2/Rhp9 and through the checkpoint-signaling kinase Chk1 to inhibit unscheduled mitosis specifically when DNA polymerase delta or epsilon is inhibited.

The Drosophila ATM homologue Mei-41 has an essential checkpoint function at the midblastula transition

BACKGROUND

Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required.

RESULTS

We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT.

CONCLUSIONS

The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.

Reduced dosage of a single fission yeast MCM protein causes genetic instability and S phase delay

MCM proteins are a conserved family of eukaryotic replication factors implicated in the initiation of DNA replication and in the discrimination between replicated and unreplicated chromatin. However, most mcm mutants in yeast arrest the cell cycle after bulk DNA synthesis has occurred. We investigated the basis for this late S phase arrest by analyzing the effects of a temperature-sensitive mutation in fission yeast cdc19(+)(mcm2(+)). cdc19-P1 cells show a dramatic loss of viability at the restrictive temperature, which is not typical of all S phase mutants. The cdc19-P1 cell cycle arrest requires an intact damage-response checkpoint and is accompanied by increased rates of chromosome loss and mitotic recombination. Chromosomes from cdc19-P1 cells migrate aberrantly in pulsed-field gels, typical of strains arrested with unresolved replication intermediates. The cdc19-P1 mutation reduces the level of the Cdc19 protein at all temperatures. We compared the effects of disruptions of cdc19(+)(mcm2(+)), cdc21(+)(mcm4(+)), nda4(+)(mcm5(+)) and mis5(+)(mcm6(+)); in all cases, the null mutants underwent delayed S phase but were unable to proceed through the cell cycle. Examination of protein levels suggests that this delayed S phase reflects limiting, but not absent, MCM proteins. Thus, reduced dosage of MCM proteins allows replication initiation, but is insufficient for completion of S phase and cell cycle progression.

DNA damage and replication checkpoints in the fission yeast, Schizosaccharomyces pombe

Eukaryotic organisms have developed an array of mechanisms for minimizing the consequences of damage to their DNA molecules and the consequences of interference with their DNA replication. Among these mechanisms are the DNA damage and replication checkpoints, which inhibit passage from one cell cycle stage to the next when DNA is damaged or replication is incomplete. Studies of these checkpoints in the fission yeast, Schizosaccharomyces pombe, complement studies in other organisms and provide valuable insight into the nature of the proteins responsible for these checkpoints and how such proteins may function.

Mutant DNA polymerase delta from thermosensitive Schizosaccharomyces pombe strains display reduced stimulation by proliferating cell nuclear antigen

We have isolated and characterized DNA polymerase delta (pol delta) from two thermosensitive Schizosaccharomyces pombe strains, poldeltats1 and poldeltats3, mutated in two different evolutionarily conserved domains of the catalytic subunit. At the restrictive temperature of 37 degreesC poldeltats1 and poldeltats3 mutant strains arrest growth in the S phase of the cell cycle. We show that at low levels of primer ends, in vitro stimulation by proliferating cell nuclear antigen (PCNA) of mutant enzymes is lower than stimulation of wild-type pol delta. Affinity for primer (3'-OH) ends and processivity of mutant enzymes do not appear different from wild-type pol delta. In contrast, Vmax values are lower than the wild-type value. The major in vitro defect appears to be decreased stimulation of mutant enzymes by PCNA, resulting in reduced velocity of DNA synthesis. In addition, ts1 pol delta is not stimulated by low PCNA concentration at 37 degreesC, although low concentrations stimulate activity at 25 degreesC, suggesting that this thermolability at low levels of primer ends could be its critical defect in vivo. Thus, both ts1 and ts3 pol delta mutations are located in regions of the catalytic subunit that seem necessary, directly or indirectly, for its efficient interaction with PCNA.

The DNA replication fork in eukaryotic cells

Replication of the two template strands at eukaryotic cell DNA replication forks is a highly coordinated process that ensures accurate and efficient genome duplication. Biochemical studies, principally of plasmid DNAs containing the Simian Virus 40 origin of DNA replication, and yeast genetic studies have uncovered the fundamental mechanisms of replication fork progression. At least two different DNA polymerases, a single-stranded DNA-binding protein, a clamp-loading complex, and a polymerase clamp combine to replicate DNA. Okazaki fragment synthesis involves a DNA polymerase-switching mechanism, and maturation occurs by the recruitment of specific nucleases, a helicase, and a ligase. The process of DNA replication is also coupled to cell-cycle progression and to DNA repair to maintain genome integrity.

Mutational effect of fission yeast polalpha on cell cycle events

Polalpha is the principal DNA polymerase for initiation of DNA replication and also functions in postinitiation DNA synthesis. In this study, we investigated the cell cycle responses induced by mutations in polalpha+. Germinating spores carrying either a deletion of polalpha+ (polalphaDelta) or a structurally intact but catalytically dead polalpha mutation proceed to inappropriate mitosis with no DNA synthesis. This suggests that the catalytic function, and not the physical presence of Polalpha, is required to generate the signal that prevents the cells from entering mitosis prematurely. Cells with a polalphats allele arrest the cell cycle near the hydroxyurea arrest point, but, surprisingly, polalphats in cdc20 (polepsilon mutant) background arrested with a cdc phenoytpe, not a polalphats-like phenotype. At 25 degrees C, replication perturbation caused by polalphats alleles induces Cds1 kinase activity and requires the checkpoint Rads, Cds1, and Rqh1, but not Chk1, to maintain cell viability. At 36 degrees C, replication disruption caused by polalphats alleles induces the phosphorylation of Chk1; however, mutant cells arrest with heterogeneous cell sizes with a population of the cells entering aberrant mitosis. Together, our results indicate that the initiation DNA structure synthesized by Polalpha is required to bring about the S phase to mitosis checkpoint, whereas replication defects of different severity caused by polalphats mutations induce differential downstream kinase responses.

Fission yeast cdc24(+) encodes a novel replication factor required for chromosome integrity

A mutation within the Schizosaccharomyces pombe cdc24(+) gene was identified previously in a screen for cell division cycle mutants and the cdc24(+) gene was determined to be essential for S phase in this yeast. We have isolated the cdc24(+) gene by complementation of a new temperature-sensitive allele of the gene, cdc24-G1. The DNA sequence predicts the presence of an open reading frame punctuated by six introns which encodes a pioneer protein of 58 kD. A cdc24 null mutant was generated by homologous recombination. Haploid cells lacking cdc24(+) are inviable, indicating that cdc24(+) is an essential gene. The transcript of cdc24(+) is present at constant levels throughout the cell cycle. Cells lacking cdc24(+) function show a checkpoint-dependent arrest with a 2N DNA content, indicating a block late in S phase. Arrest is accompanied by a rapid loss of viability and chromosome breakage. An S. pombe homolog of the replicative DNA helicase DNA2 of S. cerevisiae suppresses cdc24. These results suggest that Cdc24p plays a role in the progression of normal DNA replication and is required to maintain genomic integrity.

Regulation of telomere length by checkpoint genes in Schizosaccharomyces pombe

We have studied telomere length in Schizosaccharomyces pombe strains carrying mutations affecting cell cycle checkpoints, DNA repair, and regulation of the Cdc2 protein kinase. Telomere shortening was found in rad1, rad3, rad17, and rad26 mutants. Telomere lengths in previously characterized rad1 mutants paralleled the replication checkpoint proficiency of those mutants. In contrast, rad9, chk1, hus1, and cds1 mutants had intact telomeres. No difference in telomere length was seen in mutants affected in the regulation of Cdc2, whereas some of the DNA repair mutants examined had slightly longer telomeres than did the wild type. Overexpression of the rad1(+) gene caused telomeres to elongate slightly. The kinetics of telomere shortening was monitored by following telomere length after disruption of the rad1(+) gene; the rate was approximately 1 nucleotide per generation. Wild-type telomere length could be restored by reintroduction of the wild-type rad1(+) gene. Expression of the Saccharomyces cerevisiae RCK1 protein kinase gene, which suppresses the radiation and hydroxyurea sensitivity of Sz. pombe checkpoint mutants, was able to attenuate telomere shortening in rad1 mutant cells and to increase telomere length in a wild-type background. The functional effects of telomere shortening in rad1 mutants were assayed by measuring loss of a linear and a circular minichromosome. A minor increase in loss rate was seen with the linear minichromosome, and an even smaller difference compared with wild-type was detected with the circular plasmid.

DNA polymerase epsilon encoded by cdc20+ is required for chromosomal DNA replication in the fission yeast Schizosaccharomyces pombe

BACKGROUND

DNA polymerase II (PolII), the homologue of mammalian DNA polymerase epsilon, is essential for chromosomal DNA replication in the budding yeast Saccharomyces cerevisiae and also participates in S-phase checkpoint control. An important issue is whether chromosomal DNA replication in other eukaryotes, including the fission yeast Schizosaccharomyces pombe--in which the characteristics of replication origins are poorly defined--also requires DNA polymerase epsilon. It has been shown that DNA polymerase epsilon is not required for the in vitro replication of SV40 DNA by human cell extracts.

RESULTS

We have cloned and sequenced S. pombe pol2+, which is identical to the cell-cycle gene cdc20+, encoding the catalytic polypeptide of DNA polymerase epsilon (Pol epsilon). The predicted amino acid sequence of Pol epsilon is highly homologous to that of S. cerevisiae PolII and human Pol epsilon. Consistent with this, the Pol epsilon polypeptide was recognized by polyclonal antibodies against S. cerevisiae PolII holoenzyme (PolII*). The terminal morphology of cells containing the disrupted pol2 gene was similar to that of DNA replication mutant cells and cdc20 mutant cells. Furthermore, the Pol epsilon activity from temperature-sensitive S. pombe cdc20 mutant cells was temperature-sensitive, and chromosomal DNA replication in the mutant cells was inhibited at the restrictive temperatures.

CONCLUSION

These data strongly suggest that Pol epsilon is required for normal chromosomal DNA replication in S. pombe, as is PolII in S. cerevisiae. Thus, eukaryotic chromosomal DNA is replicated differently from that of viral SV40 DNA.

Damage and replication checkpoint control in fission yeast is ensured by interactions of Crb2, a protein with BRCT motif, with Cut5 and Chk1

Fission yeast Cut5/Rad4 plays a unique role in the genome maintenance as it is required for replication, replication checkpoint, and normal UV sensitivity. It is unknown, however, how Cut5 protein is linked to other checkpoint proteins, and what part it plays in replication and UV sensitivity. Here we report that Cut5 interacts with a novel checkpoint protein Crb2 and that this interaction is needed for normal genome maintenance. The carboxyl terminus of Crb2 resembles yeast Rad9 and human 53BP1 and BRCA1. Crb2 is required for checkpoint arrests induced by irradiation and polymerase mutations, but not for those induced by inhibited nucleotide supply. Upon UV damage, Crb2 is transiently modified, probably phosphorylated, with a similar timing of phosphorylation in Chk1 kinase, which is reported to restrain Cdc2 activation. Crb2 modification requires other damage-sensing checkpoint proteins but not Chk1, suggesting that Crb2 acts at the upstream of Chk1. The modified Crb2 exists as a slowly sedimenting form, whereas Crb2 in undamaged cells is in a rapidly sedimenting structure. Cut5 and Crb2 interact with Chk1 in a two-hybrid system. Moreover, moderate overexpression of Chk1 suppresses the phenotypes of cut5 and crb2 mutants. Cut5, Crb2, and Chk1 thus may form a checkpoint sensor-transmitter pathway to arrest the cell cycle.

Role of Schizosaccharomyces pombe RecQ homolog, recombination, and checkpoint genes in UV damage tolerance

The cellular responses to DNA damage are complex and include direct DNA repair pathways that remove the damage and indirect damage responses which allow cells to survive DNA damage that has not been, or cannot be, removed. We have identified the gene mutated in the rad12.502 strain as a Schizosaccharomyces pombe recQ homolog. The same gene (designated rqh1) is also mutated in the hus2.22 mutant. We show that Rqhl is involved in a DNA damage survival mechanism which prevents cell death when UV-induced DNA damage cannot be removed. This pathway also requires the correct functioning of the recombination machinery and the six checkpoint rad gene products plus the Cdsl kinase. Our data suggest that Rqh1 operates during S phase as part of a mechanism which prevents DNA damage causing cell lethality. This process may involve the bypass of DNA damage sites by the replication fork. Finally, in contrast with the reported literature, we do not find that rqh1 (rad12) mutant cells are defective in UV dimer endonuclease activity.

Nuclear and non-nuclear targets of genotoxic agents in the induction of gene expression. Shared principles in yeast, rodents, man and plants

The interplay between environmental cues and the genetic response is decisive for the development, health and well-being of an organism. For some environmental factors a narrow margin separates beneficial and toxic impacts. With the increasing exposure to UV-B this dichotomy has reached public attention. This review will be concerned with the mechanisms that mediate a cellular genetic response to noxious agents. The toxic stimuli find access to the regulatory network inside cells by interacting at several points with cellular molecules - a process that converts the 'outside information' into 'cellular language'. As a consequence of such interactions, many adverse agents cause massive signal transduction and changes of gene expression. There is an interesting conservation of the mechanisms from yeast to man. An understanding of the genetic programs and of their phenotypic consequences is lagging behind.

DNA polymerase delta isolated from Schizosaccharomyces pombe contains five subunits

DNA polymerase delta (pol delta) plays an essential role in DNA replication, repair, and recombination. We have purified pol delta from Schizosaccharomyces pombe more than 10(3)-fold and demonstrated that the polymerase activity of purified S. pombe pol delta is completely dependent on proliferating cell nuclear antigen and replication factor C. SDS/PAGE analysis of the purified fraction indicated that the pol delta complex consists of five subunits that migrate with apparent molecular masses of 125, 55, 54, 42, and 22 kDa. Western blot analysis indicated that the 125, 55, and 54 kDa proteins are the large catalytic subunit (Pol3), Cdc1, and Cdc27, respectively. The identity of the other two subunits, p42 and p22, was determined following proteolytic digestion and sequence analysis of the resulting peptides. The peptide sequences derived from the p22 subunit indicated that this subunit is identical to Cdm1, previously identified as a multicopy suppressor of the temperature-sensitive cdc1-P13 mutant, whereas peptide sequences derived from the p42 subunit were identical to a previously uncharacterized ORF located on S. pombe chromosome 1.

Effect of inhibitors of DNA replication on early zebrafish embryos: evidence for coordinate activation of multiple intrinsic cell-cycle checkpoints at the mid-blastula transition

We address the developmental activation, in the zebrafish embryo, of intrinsic cell-cycle checkpoints which monitor the DNA replication process and progression through the cell cycle. Eukaryotic DNA replication is probably carried out by a multiprotein complex containing numerous enzymes and accessory factors that act in concert to effect processive DNA synthesis (Applegren, N. et al. (1995) J. Cell. Biochem. 59, 91-107). We have exposed early zebrafish embryos to three chemical agents which are predicted to specifically inhibit the DNA polymerase alpha, topoisomerase I and topoisomerase II components of the DNA replication complex. We present four findings: (1) Before mid-blastula transition (MBT) an inhibition of DNA synthesis does not block cells from attempting to proceed through mitosis, implying the lack of functional checkpoints. (2) After MBT, the embryo displays two distinct modes of intrinsic checkpoint operation. One mode is a rapid and complete stop of cell division, and the other is an 'adaptive' response in which the cell cycle continues to operate, perhaps in a 'repair' mode, to generate daughter nuclei with few visible defects. (3) The embryo does not display a maximal capability for the 'adaptive' response until several hours after MBT, which is consistent with a slow transcriptional control mechanism for checkpoint activation. (4) The slow activation of checkpoints at MBT provides a window of time during which inhibitors of DNA synthesis will induce cytogenetic lesions without killing the embryo. This could be useful in the design of a deletion-mutagenesis strategy.

p56(chk1) protein kinase is required for the DNA replication checkpoint at 37 degrees C in fission yeast

Fission yeast p56(chk1) kinase is known to be involved in the DNA damage checkpoint but not to be required for cell cycle arrest following exposure to the DNA replication inhibitor hydroxyurea (HU). For this reason, p56(chk1) is considered not to be necessary for the DNA replication checkpoint which acts through the inhibitory phosphorylation of p34(cdc2) kinase activity. In a search for Schizosaccharomyces pombe mutants that abolish the S phase cell cycle arrest of a thermosensitive DNA polymerase delta strain at 37 degrees C, we isolated two chk1 alleles. These alleles are proficient for the DNA damage checkpoint, but induce mitotic catastrophe in several S phase thermosensitive mutants. We show that the mitotic catastrophe correlates with a decreased level of tyrosine phosphorylation of p34(cdc2). In addition, we found that the deletion of chk1 and the chk1 alleles abolish the cell cycle arrest and induce mitotic catastrophe in cells exposed to HU, if the cells are grown at 37 degrees C. These findings suggest that chk1 is important for the maintenance of the DNA replication checkpoint in S phase thermosensitive mutants and that the p56(chk1) kinase must possess a novel function that prevents premature activation of p34(cdc2) kinase under conditions of impaired DNA replication at 37 degrees C.

A role for DNA primase in coupling DNA replication to DNA damage response

The temperature-sensitive yeast DNA primase mutant pri1-M4 fails to execute an early step of DNA replication and exhibits a dominant, allele-specific sensitivity to DNA-damaging agents. pri1-M4 is defective in slowing down the rate of S phase progression and partially delaying the G1-S transition in response to DNA damage. Conversely, the G2 DNA damage response and the S-M checkpoint coupling completion of DNA replication to mitosis are unaffected. The signal transduction pathway leading to Rad53p phosphorylation induced by DNA damage is proficient in pri1-M4, and cell cycle delay caused by Rad53p overexpression is counteracted by the pri1-M4 mutation. Altogether, our results suggest that DNA primase plays an essential role in a subset of the Rad53p-dependent checkpoint pathways controlling cell cycle progression in response to DNA damage.

The 70 kDa subunit of replication protein A is required for the G1/S and intra-S DNA damage checkpoints in budding yeast

The rfa1-M2 and rfa1-M4 Saccharomyces cerevisiae mutants, which are altered in the 70 kDa subunit of replication protein A (RPA) and sensitive to UV and methyl methane sulfonate (MMS), have been analyzed for possible checkpoint defects. The G1/S and intra-S DNA damage checkpoints are defective in the rfa1-M2 mutant, since rfa1-M2 cells fail to properly delay cell cycle progression in response to UV irradiation in G1 and MMS treatment during S phase. Conversely, the G2/M DNA damage checkpoint and the S/M checkpoint are proficient in rfa1-M2 cells and all the checkpoints tested are functional in the rfa1-M4 mutant. Preventing S phase entry by alpha-factor treatment after UV irradiation in G1 does not change rfa1-M4 cell lethality, while it allows partial recovery of rfa1-M2 cell viability. Therefore, the hypersensitivity to UV and MMS treatments observed in the rfa1-M4 mutant might only be due to impairment of RPA function in DNA repair, while the rfa1-M2 mutation seems to affect both the DNA repair and checkpoint functions of Rpa70.

Yeast pip3/mec3 mutants fail to delay entry into S phase and to slow DNA replication in response to DNA damage, and they define a functional link between Mec3 and DNA primase

The catalytic DNA primase subunit of the DNA polymerase alpha-primase complex is encoded by the essential PRI1 gene in Saccharomyces cerevisiae. To identify factors that functionally interact with yeast DNA primase in living cells, we developed a genetic screen for mutants that are lethal at the permissive temperature in a cold-sensitive pril-2 genetic background. Twenty-four recessive mutations belonging to seven complementation groups were identified. Some mutants showed additional phenotypes, such as increased sensitivity to UV irradiation, methyl methanesulfonate, and hydroxyurea, that were suggestive of defects in DNA repair and/or checkpoint mechanisms. We have cloned and characterized the gene of one complementation group, PIP3, whose product is necessary both for delaying entry into S phase or mitosis when cells are UV irradiated in G1 or G2 phase and for lowering the rate of ongoing DNA synthesis in the presence of methyl methanesulfonate. PIP3 turned out to be the MEC3 gene, previously identified as a component of the G2 DNA damage checkpoint. The finding that Mec3 is also required for the G1- and S-phase DNA damage checkpoints, together with the analysis of genetic interactions between a mec3 null allele and several conditional DNA replication mutations at the permissive temperature, suggests that Mec3 could be part of a mechanism coupling DNA replication with repair of DNA damage, and DNA primase might be involved in this process.

Separation of phenotypes in mutant alleles of the Schizosaccharomyces pombe cell-cycle checkpoint gene rad1+

The Schizosaccharomyces pombe rad1+ gene is involved in the G2 DNA damage cell-cycle checkpoint and in coupling mitosis to completed DNA replication. It is also required for viability when the cdc17 (DNA ligase) or wee1 proteins are inactivated. We have introduced mutations into the coding regions of rad1+ by site-directed mutagenesis. The effects of these mutations on the DNA damage and DNA replication checkpoints have been analyzed, as well as their associated phenotypes in a cdc17-K42 or a wee1-50 background. For all alleles, the resistance to radiation or hydroxyurea correlates well with the degree of functioning of checkpoint pathways activated by these treatments. One mutation, rad1-S3, completely abolishes the DNA replication checkpoint while partially retaining the DNA damage checkpoint. As single mutants, the rad1-S1, rad1-S2, rad1-S5, and rad1-S6 alleles have a wild-type phenotype with respect to radiation sensitivity and checkpoint functions; however, like the rad1 null allele, the rad1-S1 and rad1-S2 alleles exhibit synthetic lethality at the restrictive temperature with the cdc17-K42 or the wee1-50 mutation. The rad1-S5 and rad1-S6 alleles allow growth at higher temperatures in a cdc17-K42 or wee1-50 background than does wild-type rad1+, and thus behave like "superalleles." In most cases both chromosomal and multi-copy episomal mutant alleles have been investigated, and the agreement between these two states is very good. We provide evidence that the functions of rad1 can be dissociated into three groups by specific mutations. Models for the action of these rad1 alleles are discussed. In addition, a putative negative regulatory domain of rad1 is identified.