Radiosensitive and mitotic recombination phenotypes of the Saccharomyces cerevisiae dun1 mutant defective in DNA damage-inducible gene expression

DNA Damage Response Checkpoint Activation Drives KP1019 Dependent Pre-Anaphase Cell Cycle Delay in S. cerevisiae

Careful regulation of the cell cycle is required for proper replication, cell division, and DNA repair. DNA damage–including that induced by many anticancer drugs–results in cell cycle delay or arrest, which can allow time for repair of DNA lesions. Although its molecular mechanism of action remains a matter of debate, the anticancer ruthenium complex KP1019 has been shown to bind DNA in biophysical assays and to damage DNA of colorectal and ovarian cancer cells in vitro. KP1019 has also been shown to induce mutations and induce cell cycle arrest in Saccharomyces cerevisiae, suggesting that budding yeast can serve as an appropriate model for characterizing the cellular response to the drug. Here we use a transcriptomic approach to verify that KP1019 induces the DNA damage response (DDR) and find that KP1019 dependent expression of HUG1 requires the Dun1 checkpoint; both consistent with KP1019 DDR in budding yeast. We observe a robust KP1019 dependent delay in cell cycle progression as measured by increase in large budded cells, 2C DNA content, and accumulation of Pds1 which functions to inhibit anaphase. Importantly, we also find that deletion of RAD9, a gene required for the DDR, blocks drug-dependent changes in cell cycle progression, thereby establishing a causal link between the DDR and phenotypes induced by KP1019. Interestingly, yeast treated with KP1019 not only delay in G2/M, but also exhibit abnormal nuclear position, wherein the nucleus spans the bud neck. This morphology correlates with short, misaligned spindles and is dependent on the dynein heavy chain gene DYN1. We find that KP1019 creates an environment where cells respond to DNA damage through nuclear (transcriptional changes) and cytoplasmic (motor protein activity) events.

Regulation of ribosomal DNA amplification by the TOR pathway

Repeated regions are widespread in eukaryotic genomes, and key functional elements such as the ribosomal DNA tend to be formed of high copy repeated sequences organized in tandem arrays. In general, high copy repeats are remarkably stable, but a number of organisms display rapid ribosomal DNA amplification at specific times or under specific conditions. Here we demonstrate that target of rapamycin (TOR) signaling stimulates ribosomal DNA amplification in budding yeast, linking external nutrient availability to ribosomal DNA copy number. We show that ribosomal DNA amplification is regulated by three histone deacetylases: Sir2, Hst3, and Hst4. These enzymes control homologous recombination-dependent and nonhomologous recombination-dependent amplification pathways that act in concert to mediate rapid, directional ribosomal DNA copy number change. Amplification is completely repressed by rapamycin, an inhibitor of the nutrient-responsive TOR pathway; this effect is separable from growth rate and is mediated directly through Sir2, Hst3, and Hst4. Caloric restriction is known to up-regulate expression of nicotinamidase Pnc1, an enzyme that enhances Sir2, Hst3, and Hst4 activity. In contrast, normal glucose concentrations stretch the ribosome synthesis capacity of cells with low ribosomal DNA copy number, and we find that these cells show a previously unrecognized transcriptional response to caloric excess by reducing PNC1 expression. PNC1 down-regulation forms a key element in the control of ribosomal DNA amplification as overexpression of PNC1 substantially reduces ribosomal DNA amplification rate. Our results reveal how a signaling pathway can orchestrate specific genome changes and demonstrate that the copy number of repetitive DNA can be altered to suit environmental conditions.

Nucleotide salvage deficiencies, DNA damage and neurodegeneration

Nucleotide balance is critically important not only in replicating cells but also in quiescent cells. This is especially true in the nervous system, where there is a high demand for adenosine triphosphate (ATP) produced from mitochondria. Mitochondria are particularly prone to oxidative stress-associated DNA damage because nucleotide imbalance can lead to mitochondrial depletion due to low replication fidelity. Failure to maintain nucleotide balance due to genetic defects can result in infantile death; however there is great variability in clinical presentation for particular diseases. This review compares genetic diseases that result from defects in specific nucleotide salvage enzymes and a signaling kinase that activates nucleotide salvage after DNA damage exposure. These diseases include Lesch-Nyhan syndrome, mitochondrial depletion syndromes, and ataxia telangiectasia. Although treatment options are available to palliate symptoms of these diseases, there is no cure. The conclusions drawn from this review include the critical role of guanine nucleotides in preventing neurodegeneration, the limitations of animals as disease models, and the need to further understand nucleotide imbalances in treatment regimens. Such knowledge will hopefully guide future studies into clinical therapies for genetic diseases.

Elevated dNTP levels suppress hyper-recombination in Saccharomyces cerevisiae S-phase checkpoint mutants

MEC1, the essential yeast homolog of the human ATR/ATM genes, controls the S-phase checkpoint and prevents replication fork collapse at slow zones of DNA replication. The viability of hypomorphic mec1-21 is reduced in the rad52 mutant, defective in homologous recombination, suggesting that replication generates recombinogenic lesions. We previously observed a 6-, 10- and 30-fold higher rate of spontaneous sister chromatid exchange (SCE), heteroallelic recombination and translocations, respectively, in mec1-21 mutants compared to wild-type. Here we report that the hyper-recombination phenotype correlates with lower deoxyribonucleoside triphosphate (dNTP) levels, compared to wild-type. By introducing a dun1 mutation, thus eliminating inducible expression of ribonucleotide reductase in mec1-21, rates of spontaneous SCE increased 15-fold above wild-type. All the hyper-recombination phenotypes were reduced by SML1 deletions, which increase dNTP levels. Measurements of dNTP pools indicated that, compared to wild-type, there was a significant decrease in dNTP levels in mec1-21, dun1 and mec1-21 dun1, while the dNTP levels of mec1-21 sml1, mec1-21 dun1 sml1 and sml1 mutants were approximately 2-fold higher. Interestingly, higher dNTP levels in mec1-21 dun1 sml1 correlate with approximately 2-fold higher rate of spontaneous mutagenesis, compared to mec1-21 dun1. We suggest that higher dNTP levels in specific checkpoint mutants suppress the formation of recombinogenic lesions.

Genetic analysis of CHK1 and CHK2 homologues revealed a unique cross talk between ATM and ATR pathways in Neurospora crassa

DNA damage checkpoint is an important mechanism for organisms to maintain genome integrity. In Neurospora crassa, mus-9 and mus-21 are homologues of ATR and ATM, respectively, which are pivotal factors of DNA damage checkpoint in mammals. A N. crassa clock gene prd-4 has been identified as a CHK2 homologue, but its role in DNA damage response had not been elucidated. In this study, we identified another CHK2 homologue and one CHK1 homologue from the N. crassa genome database. As disruption of these genes affected mutagen tolerance, we named them mus-59 and mus-58, respectively. The mus-58 mutant was sensitive to hydroxyurea (HU), but the mus-59 and prd-4 mutants showed the same HU sensitivity as that of the wild-type strain. This indicates the possibility that MUS-58 is involved in replication checkpoint and stabilization of stalled forks like mammalian CHK1. Phosphorylation of MUS-58 and MUS-59 was observed in the wild-type strain in response to mutagen treatments. Genetic relationships between those three genes and mus-9 or mus-21 indicated that the mus-9 mutation was epistatic to mus-58, and mus-21 was epistatic to prd-4. These relationships correspond to two signal pathways, ATR-CHK1 and ATM-CHK2 that have been established in mammalian cells. However, both the mus-9 mus-59 and mus-21 mus-58 double mutants showed an intermediate level between the two parental strains for CPT sensitivity. Furthermore, these double mutants showed severe growth defects. Our findings suggest that the DNA damage checkpoint of N. crassa is controlled by unique mechanisms.

UV but not X rays stimulate homologous recombination between sister chromatids and homologs in a Saccharomyces cerevisiae mec1 (ATR) hypomorphic mutant

MEC1, the essential yeast ATM/ATR homolog, prevents replication fork collapse and is required for the cellular response to DNA damage. We had previously observed higher rates of spontaneous SCE, heteroallelic recombination and translocations in mec1-21 mutants, which still retain some G2 checkpoint function, compared to mec1 null mutants, which are completely defective in checkpoint function, and wild type. However, the types of DNA lesions that are more recombinogenic in mec1-21, compared to wild type, are unknown. Here, we measured DNA damage-associated SCE, homolog (heteroallelic) recombination, and homology-directed translocations in mec1-21, and characterized types of DNA damage-associated chromosomal rearrangements that occur in mec1-21. Although frequencies of UV-associated recombination were higher in mec1-21, the mutant was defective in double-strand break-associated SCE and heteroallelic recombination. Over-expression of Rad53 in mec1-21 reduced UV-associated recombination but did not suppress the defect in X-ray-associated recombination. Both X ray and UV exposure increased translocation frequencies in mec1-21, but the majority of the UV-associated products were non-reciprocal translocations. We suggest that although recombinational repair of double-stand breaks is less efficient in mec1 mutants, recombinants may be generated by other mechanisms, such as break-induced replication.

Stimulation of sister chromatid exchanges and mutation by aflatoxin B1-DNA adducts in Saccharomyces cerevisiae requires MEC1 (ATR), RAD53, and DUN1

The hepatocarcinogen aflatoxin B(1) (AFB(1)) is a potent recombinagen but weak mutagen in the yeast Saccharomyces cerevisiae. AFB(1) exposure induces DNA damage-inducible genes, such as RAD51 and those encoding ribonucleotide reductase (RNR), through a MEC1 (ATR homolog)-dependent pathway. Previous studies have indicated that MEC1 is required for both AFB(1)-associated recombination and mutation, and suggested that AFB(1)-DNA adducts are common substrates for recombination and mutagenesis. However, little is known about the downstream effectors of MEC1 required for genotoxic events associated with AFB(1) exposure. Here we show that AFB(1) exposure increases frequencies of RAD51-dependent unequal sister chromatid exchange (SCE) and activates Rad53 (CHK2). We found that MEC1, RAD53, and DUN1 are required for both AFB(1)-associated mutation and SCE. Deletion of SML1, which encodes an inhibitor of RNR, did not suppress the DUN1-dependent requirement for AFB(1)-associated genetic events, indicating that higher dNTP levels could not suppress the dun1 phenotype. We identified AFB(1)-DNA adducts and show that approximately the same number of adducts are obtained in both wild type and rad53 mutants. Since DUN1 is not required for UV-associated mutation and recombination, these studies define a distinct role for DUN1 in AFB(1)-associated mutagenesis and recombination. We speculate that AFB(1)-associated DNA adducts stall DNA replication, a consequence of which can either be mutation or recombination.

Mechanisms of Rad52-independent spontaneous and UV-induced mitotic recombination in Saccharomyces cerevisiae

In wild-type diploid cells, heteroallelic recombination between his4A and his4C alleles leads mostly to His+ gene conversions that have a parental configuration of flanking markers, but approximately 22% of recombinants have associated reciprocal crossovers. In rad52 strains, gene conversion is reduced 75-fold and the majority of His+ recombinants are crossover associated, with the largest class being half-crossovers in which the other participating chromatid is lost. We report that UV irradiating rad52 cells results in an increase in overall recombination frequency, comparable to increases induced in wild-type (WT) cells, and surprisingly results in a pattern of recombination products quite similar to RAD52 cells: gene conversion without exchange is favored, and the number of 2n - 1 events is markedly reduced. Both spontaneous and UV-induced RAD52-independent recombination depends strongly on Rad50, whereas rad50 has no effect in cells restored to RAD52. The high level of noncrossover gene conversion outcomes in UV-induced rad52 cells depends on Rad51, but not on Rad59. Those outcomes also rely on the UV-inducible kinase Dun1 and Dun1's target, the repressor Crt1, whereas gene conversion events arising spontaneously depend on Rad59 and Crt1. Thus, there are at least two Rad52-independent recombination pathways in budding yeast.

Saccharomyces cerevisiae RAD53 (CHK2) but not CHK1 is required for double-strand break-initiated SCE and DNA damage-associated SCE after exposure to X rays and chemical agents

Saccharomyces cerevisiae RAD53 (CHK2) and CHK1 control two parallel branches of the RAD9-mediated pathway for DNA damage-induced G(2) arrest. Previous studies indicate that RAD9 is required for X-ray-associated sister chromatid exchange (SCE), suppresses homology-directed translocations, and is involved in pathways for double-strand break repair (DSB) repair that are different than those controlled by PDS1. We measured DNA damage-associated SCE in strains containing two tandem fragments of his3, his3-Delta5' and his3-Delta3'::HOcs, and rates of spontaneous translocations in diploids containing GAL1::his3-Delta5' and trp1::his3-Delta3'::HOcs. DNA damage-associated SCE was measured after log phase cells were exposed to methyl methanesulfonate (MMS), 4-nitroquinoline 1-oxide (4-NQO), UV, X rays and HO-induced DSBs. We observed that rad53 mutants were defective in MMS-, 4-NQO, X-ray-associated and HO-induced SCE but not in UV-associated SCE. Similar to rad9 pds1 double mutants, rad53 pds1 double mutants exhibited more X-ray sensitivity than the single mutants. rad53 sml1 diploid mutants exhibited a 10-fold higher rate of spontaneous translocations compared to the sml1 diploid mutants. chk1 mutants were not deficient in DNA damage-associated SCE after exposure to DNA damaging agents or after DSBs were generated at trp1::his3-Delta5'his3-Delta3'::HOcs. These data indicate that RAD53, not CHK1, is required for DSB-initiated SCE, and DNA damage-associated SCE after exposure to X-ray-mimetic and UV-mimetic chemicals.

Activation of the DNA damage checkpoint in yeast lacking the histone chaperone anti-silencing function 1

The packaging of the eukaryotic genome into chromatin is likely to be important for the maintenance of genomic integrity. Chromatin structures are assembled onto newly synthesized DNA by the action of chromatin assembly factors, including anti-silencing function 1 (ASF1). To investigate the role of chromatin structure in the maintenance of genomic integrity, we examined budding yeast lacking the histone chaperone Asf1p. We found that yeast lacking Asf1p accumulate in metaphase of the cell cycle due to activation of the DNA damage checkpoint. Furthermore, yeast lacking Asf1p are highly sensitive to mutations in DNA polymerase alpha and to DNA replicational stresses. Although yeast lacking Asf1p do complete DNA replication, they have greatly elevated rates of DNA damage occurring during DNA replication, as indicated by spontaneous Ddc2p-green fluorescent protein foci. The presence of elevated levels of spontaneous DNA damage in asf1 mutants is due to increased DNA damage, rather than the failure to repair double-strand DNA breaks, because asf1 mutants are fully functional for double-strand DNA repair. Our data indicate that the altered chromatin structure in asf1 mutants leads to elevated rates of spontaneous recombination, mutation, and DNA damage foci formation arising during DNA replication, which in turn activates cell cycle checkpoints that respond to DNA damage.

SCH9, a putative protein kinase from Saccharomyces cerevisiae, affects HOT1 -stimulated recombination

HOT1 is a mitotic recombination hotspot derived from yeast rDNA. To further study HOT1 function, trans-acting H OT1 recombination mutants (hrm) that alter hotspot activity were isolated. hrm2-1 mutants have decreased HOT1 activity and grow slowly. The HRM2 gene was cloned and found to be identical to SCH9, a gene that affects a growth-control mechanism that is partially redundant with the cAMP-dependent protein kinase A (PKA) pathway. Deletion of SCH9 decreases HOT1 and rDNA recombination but not other mitotic exchange. Although high levels of RNA polymerase I transcription initiated at HOT1 are required for its recombination-stimulating activity, sch9 mutations do not affect transcription initiated within HOT1. Thus, transcription is necessary but not sufficient for HOT1 activity. TPK1, which encodes a catalytic subunit of PKA, is a multicopy suppressor of the recombination and growth defects of sch9 mutants, suggesting that increased PKA activity compensates for SCH9 loss. RAS2( val19), which codes for a hyperactive RAS protein and increases PKA activity, suppresses both phenotypic defects of sch9 mutants. In contrast to TPK1 and RAS2(val19), the gene for split zinc finger protein 1 (SFP1) on a multicopy vector suppresses only the growth defects of sch9 mutants, indicating that growth and HOT1 functions of Sch9p are separable. Sch9p may affect signal transduction pathways which regulate proteins that are specifically required for HOT1-stimulated exchange.

A role for Saccharomyces cerevisiae Chk1p in the response to replication blocks

Replication blocks and DNA damage incurred during S phase activate the S-phase and intra-S-phase checkpoint responses, respectively, regulated by the Atrp and Chk1p checkpoint kinases in metazoans. In Saccharomyces cerevisiae, these checkpoints are regulated by the Atrp homologue Mec1p and the kinase Rad53p. A conserved role of these checkpoints is to block mitotic progression until DNA replication and repair are completed. In S. cerevisiae, these checkpoints include a transcriptional response regulated by the kinase Dun1p; however, dun1Delta cells are proficient for the S-phase-checkpoint-induced anaphase block. Yeast Chk1p kinase regulates the metaphase-to-anaphase transition in the DNA-damage checkpoint pathway via securin (Pds1p) phosphorylation. However, like Dun1p, yeast Chk1p is not required for the S-phase-checkpoint-induced anaphase block. Here we report that Chk1p has a role in the intra-S-phase checkpoint activated when yeast cells replicate their DNA in the presence of low concentrations of hydroxyurea (HU). Chk1p was modified and Pds1p was transiently phosphorylated in this response. Cells lacking Dun1p were dependent on Chk1p for survival in HU, and chk1Delta dun1Delta cells were defective in the recovery from replication interference caused by transient HU exposure. These studies establish a relationship between the S-phase and DNA-damage checkpoint pathways in S. cerevisiae and suggest that at least in some genetic backgrounds, the Chk1p/securin pathway is required for the recovery from stalled or collapsed replication forks.

Mutation induction in haploid yeast after split-dose radiation exposure. II. Combination of UV-irradiation and X-rays

Split-dose protocols can be used to investigate the kinetics of recovery from radiation damage and to elucidate the mechanisms of cell inactivation and mutation induction. In this study, a haploid strain of the yeast, Saccharomyces cerevisiae, wild-type with regard to radiation sensitivity, was irradiated with 254-nm ultraviolet (UV) light and then exposed to X-rays after incubation for 0-6 hr. The cells were incubated either on nutrient medium or salt agar between the treatments. Loss of reproductive ability and mutation to canavanine resistance were measured. When the X-ray exposure immediately followed UV-irradiation, the X-ray survival curves had the same slope irrespective of the pretreatment, while the X-ray mutation induction curves were changed from linear to linear quadratic with increasing UV fluence. Incubations up to about 3 hr on nutrient medium between the treatments led to synergism with respect to cell inactivation and antagonism with respect to mutation, but after 4-6 hr the two treatments acted independently. Incubation on salt agar did not cause any change in the survival curves, but there was a strong suppression of X-ray-induced mutation with increasing UV fluence. On the basis of these results, we suggest that mutation after combined UV and X-ray exposure is affected not only by the induction and suppression of DNA repair processes, but also by radiation-induced modifications of cell-cycle progression and changes in the expression of the mutant phenotype.

Amino acid changes in Xrs2p, Dun1p, and Rfa2p that remove the preferred targets of the ATM family of protein kinases do not affect DNA repair or telomere length in Saccharomyces cerevisiae

In eukaryotes, mutations in a number of genes that affect DNA damage checkpoints or DNA replication also affect telomere length [Curr. Opin. Cell Biol. 13 (2001) 281]. Saccharomyces cerevisae strains with mutations in the TEL1 gene (encoding an ATM-like protein kinase) have very short telomeres, as do strains with mutations in XRS2, RAD50, or MRE11 (encoding members of a trimeric complex). Xrs2p and Mre11p are phosphorylated in a Tel1p-dependent manner in response to DNA damage [Genes Dev. 15 (2001) 2238; Mol. Cell 7 (2001) 1255]. We found that Xrs2p, but not Mre11p or Rad50p, is efficiently phosphorylated in vitro by immunopreciptated Tel1p. Strains with mutations eliminating all SQ and TQ motifs in Xrs2p (preferred targets of the ATM kinase family) had wild-type length telomeres and wild-type sensitivity to DNA damaging agents. We also showed that Rfa2p (a subunit of RPA) and the Dun1p checkpoint kinase, which are required for DNA damage repair and which are phosphorylated in response to DNA damage in vivo, are in vitro substrates of the Tel1p and Mec1p kinases. In addition, Dun1p substrates with no SQ or TQ motifs are phosphorylated by Mec1p in vitro very inefficiently, but retain most of their ability to be phosphorylated by Tel1p. We demonstrated that null alleles of DUN1 and certain mutant alleles of RFA2 result in short telomeres. As observed with Xrs2p, however, strains with mutations of DUN1 or RFA2 that eliminate SQ motifs have no effect on telomere length or DNA damage sensitivity.

Saccharomyces cerevisiae Dap1p, a novel DNA damage response protein related to the mammalian membrane-associated progesterone receptor

The response to damage is crucial for cellular survival, and eukaryotic cells require a broad array of proteins for an intact damage response. We have found that the YPL170W (DAP1 [for damage response protein related to membrane-associated progesterone receptors]) gene is required for growth in the presence of the methylating agent methyl methanesulfonate (MMS). The DAP1 open reading frame shares homology with a broadly conserved family of membrane-associated progesterone receptors (MAPRs). Deletion of DAP1 leads to sensitivity to MMS, elongated telomeres, loss of mitochondrial function, and partial arrest in sterol synthesis. Sensitivity of dap1 strains to MMS is not due to loss of damage checkpoints. Instead, dap1 cells are arrested as unbudded cells after MMS treatment, suggesting that Dap1p is required for cell cycle progression following damage. Dap1p also directs resistance to itraconazole and fluconazole, inhibitors of sterol synthesis. We have found that dap1 cells have slightly decreased levels of ergosterol but increased levels of the ergosterol intermediates squalene and lanosterol, indicating that dap1 cells have a partial defect in sterol synthesis. This is the first evidence linking a MAPR family member to sterol regulation or the response to damage, and these functions are probably conserved in a variety of eukaryotes.

A genome-wide screen for methyl methanesulfonate-sensitive mutants reveals genes required for S phase progression in the presence of DNA damage

We performed a systematic screen of the set of approximately 5,000 viable Saccharomyces cerevisiae haploid gene deletion mutants and have identified 103 genes whose deletion causes sensitivity to the DNA-damaging agent methyl methanesulfonate (MMS). In total, 40 previously uncharacterized alkylation damage response genes were identified. Comparison with the set of genes known to be transcriptionally induced in response to MMS revealed surprisingly little overlap with those required for MMS resistance, indicating that transcriptional regulation plays little, if any, role in the response to MMS damage. Clustering of the MMS response genes on the basis of their cross-sensitivities to hydroxyurea, UV radiation, and ionizing radiation revealed a DNA damage core of genes required for responses to a broad range of DNA-damaging agents. Of particular significance, we identified a subset of genes that show a specific MMS response, displaying defects in S phase progression only in the presence of MMS. These genes may promote replication fork stability or processivity during encounters between replication forks and DNA damage.

Posttranscriptional regulation of the RAD5 DNA repair gene by the Dun1 kinase and the Pan2-Pan3 poly(A)-nuclease complex contributes to survival of replication blocks

The yeast Dun1 kinase has complex checkpoint functions including DNA damage-dependent cell cycle arrest in G(2)/M, transcriptional induction of repair genes, and regulation of postreplicative DNA repair pathways. Here we report that the Dun1 forkhead-associated domain interacts with the Pan3 subunit of the poly(A)-nuclease complex and that dun1pan2 and dun1pan3 double mutants are dramatically hypersensitive to replicational stress. This phenotype was independent of the function of Dun1 in regulating deoxyribonucleotide levels as it was also observed in strains lacking the ribonucleotide reductase inhibitor Sml1. dun1pan2 mutants initially arrested normally in response to replication blocks but died in the presence of persistent replication blocks with considerably delayed kinetics compared with mutants lacking the Rad53 kinase, indicating that the double mutation does not compromise the intra-S phase checkpoint. Interestingly, the RAD5 gene involved in error-free postreplication repair pathways was specifically up-regulated in dun1pan2 double mutants. Moreover, inducible overexpression of RAD5 mimicked the double mutant phenotype by hypersensitizing dun1 mutants to replication blocks. The data indicate that Dun1 and Pan2-Pan3 cooperate to regulate the stoichiometry and thereby the activity of postreplication repair complexes, suggesting that posttranscriptional mechanisms complement the transcriptional response in the regulation of gene expression by checkpoint signaling pathways in Saccharomyces cerevisiae.

UV irradiation causes the loss of viable mitotic recombinants in Schizosaccharomyces pombe cells lacking the G(2)/M DNA damage checkpoint

Elevated mitotic recombination and cell cycle delays are two of the cellular responses to UV-induced DNA damage. Cell cycle delays in response to DNA damage are mediated via checkpoint proteins. Two distinct DNA damage checkpoints have been characterized in Schizosaccharomyces pombe: an intra-S-phase checkpoint slows replication and a G(2)/M checkpoint stops cells passing from G(2) into mitosis. In this study we have sought to determine whether UV damage-induced mitotic intrachromosomal recombination relies on damage-induced cell cycle delays. The spontaneous and UV-induced recombination phenotypes were determined for checkpoint mutants lacking the intra-S and/or the G(2)/M checkpoint. Spontaneous mitotic recombinants are thought to arise due to endogenous DNA damage and/or intrinsic stalling of replication forks. Cells lacking only the intra-S checkpoint exhibited no UV-induced increase in the frequency of recombinants above spontaneous levels. Mutants lacking the G(2)/M checkpoint exhibited a novel phenotype; following UV irradiation the recombinant frequency fell below the frequency of spontaneous recombinants. This implies that, as well as UV-induced recombinants, spontaneous recombinants are also lost in G(2)/M mutants after UV irradiation. Therefore, as well as lack of time for DNA repair, loss of spontaneous and damage-induced recombinants also contributes to cell death in UV-irradiated G(2)/M checkpoint mutants.

Activation of Ty transposition by mutagens

The induction of Ty1 transposition by mutagens (MMS and 4NQO) in asynchronous cultures and cells blocked in G1 and G2/M suggested G1 dependence of activation of Ty1 element by DNA damage. Northern blot analysis revealed immediate five-fold increase in levels of Ty1 transcript after 20min incubation of cells with 1 microg/ml 4NQO and four-fold increase in Ty1 RNA after treatment the cells with 0.1% MMS. Western blot analysis showed no difference in TyA protein in treated and untreated with mutagen cells. Quantitative mutagenicity assay and Northern blot analysis demonstrated dependence of induction of Ty1 element by DNA-damaging agents on the function of RAD9 gene and independence on DUN1 gene.

DNA repair protein Rad55 is a terminal substrate of the DNA damage checkpoints

Checkpoints, which are integral to the cellular response to DNA damage, coordinate transient cell cycle arrest and the induced expression of DNA repair genes after genotoxic stress. DNA repair ensures cellular survival and genomic stability, utilizing a multipathway network. Here we report evidence that the two systems, DNA damage checkpoint control and DNA repair, are directly connected by demonstrating that the Rad55 double-strand break repair protein of the recombinational repair pathway is a terminal substrate of DNA damage and replication block checkpoints. Rad55p was specifically phosphorylated in response to DNA damage induced by the alkylating agent methyl methanesulfonate, dependent on an active DNA damage checkpoint. Rad55p modification was also observed after gamma ray and UV radiation. The rapid time course of phosphorylation and the recombination defects identified in checkpoint-deficient cells are consistent with a role of the DNA damage checkpoint in activating recombinational repair. Rad55p phosphorylation possibly affects the balance between different competing DNA repair pathways.