Spk1/Rad53 is regulated by Mec1-dependent protein phosphorylation in DNA replication and damage checkpoint pathways

A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

DRC1, DNA replication and checkpoint protein 1, functions with DPB11 to control DNA replication and the S-phase checkpoint in Saccharomyces cerevisiae

In addition to DNA polymerase complexes, DNA replication requires the coordinate action of a series of proteins, including regulators Cdc28/Clb and Dbf4/Cdc7 kinases, Orcs, Mcms, Cdc6, Cdc45, and Dpb11. Of these, Dpb11, an essential BRCT repeat protein, has remained particularly enigmatic. The Schizosaccharomyces pombe homolog of DPB11, cut5, has been implicated in the DNA replication checkpoint as has the POL2 gene with which DPB11 genetically interacts. Here we describe a gene, DRC1, isolated as a dosage suppressor of dpb11-1. DRC1 is an essential cell cycle-regulated gene required for DNA replication. We show that both Dpb11 and Drc1 are required for the S-phase checkpoint, including the proper activation of the Rad53 kinase in response to DNA damage and replication blocks. Dpb11 is the second BRCT-repeat protein shown to control Rad53 function, possibly indicating a general function for this class of proteins. DRC1 and DPB11 show synthetic lethality and reciprocal dosage suppression. The Drc1 and Dpb11 proteins physically associate and function together to coordinate DNA replication and the cell cycle.

RAD53 regulates DBF4 independently of checkpoint function in Saccharomyces cerevisiae

The Cdc7p and Dbf4p proteins form an active kinase complex in Saccharomyces cerevisiae that is essential for the initiation of DNA replication. A genetic screen for mutations that are lethal in combination with cdc7-1 led to the isolation of seven lsd (lethal with seven defect) complementation groups. The lsd7 complementation group contained two temperature-sensitive dbf4 alleles. The lsd1 complementation group contained a new allele of RAD53, which was designated rad53-31. RAD53 encodes an essential protein kinase that is required for the activation of DNA damage and DNA replication checkpoint pathways, and that is implicated as a positive regulator of S phase. Unlike other RAD53 alleles, we demonstrate that the rad53-31 allele retains an intact checkpoint function. Thus, the checkpoint function and the DNA replication function of RAD53 can be functionally separated. The activation of DNA replication through RAD53 most likely occurs through DBF4. Two-hybrid analysis indicates that the Rad53p protein binds to Dbf4p. Furthermore, the steady-state level of DBF4 message and Dbf4p protein is reduced in several rad53 mutant strains, indicating that RAD53 positively regulates DBF4. These results suggest that two different functions of the cell cycle, initiation of DNA replication and the checkpoint function, can be coordinately regulated through the common intermediate RAD53.

Human homologue of the Drosophila melanogaster lats tumour suppressor modulates CDC2 activity

We have previously used mosaic flies to screen for tumour suppressors or negative regulators of cell proliferation. The cellular composition of these flies resembles that of cancer patients who are chimaeric individuals carrying a small number of mutated somatic cells. One of the genes we identified is the large tumour suppressor gene, lats (also known as wts), which encodes a putative serine/threonine kinase. Somatic cells mutant for lats undergo extensive proliferation and form large tumours in many tissues in mosaic adults. Homozygous mutants for various lats alleles display a range of developmental defects including embryonic lethality. Although many tumour suppressors have been identified in Drosophila melanogaster, it is not clear whether these fly genes are directly relevant to tumorigenesis in mammals. Here, we have isolated mammalian homologues of Drosophila lats. Human LATS1 suppresses tumour growth and rescues all developmental defects, including embryonic lethality in flies. In mammalian cells, LATS1 is phosphorylated in a cell-cycle-dependent manner and complexes with CDC2 in early mitosis. LATS1-associated CDC2 has no mitotic cyclin partner and no kinase activity for histone H1. Furthermore, lats mutant cells in Drosophila abnormally accumulate cyclin A. These biochemical observations indicate that LATS is a novel negative regulator of CDC2/cyclin A, a finding supported by genetic data in Drosophila demonstrating that lats specifically interacts with cdc2 and cyclin A.

Interaction between Set1p and checkpoint protein Mec3p in DNA repair and telomere functions

The yeast protein Set1p, inactivation of which alleviates telomeric position effect (TPE), contains a conserved SET domain present in chromosomal proteins involved in epigenetic control of transcription. Mec3p is required for efficient DNA-damage-dependent checkpoints at G1/S, intra-S and G2/M (refs 3-7). We show here that the SET domain of Set1p interacts with Mec3p. Deletion of SET1 increases the viability of mec3delta mutants after DNA damage (in a process that is mostly independent of Rad53p kinase, which has a central role in checkpoint control) but does not significantly affect cell-cycle progression. Deletion of MEC3 enhances TPE and attenuates the Set1delta-induced silencing defect. Furthermore, restoration of TPE in a Set1delta mutant by overexpression of the isolated SET domain requires Mec3p. Finally, deletion of MEC3 results in telomere elongation, whereas cells with deletions of both SET1 and MEC3 do not have elongated telomeres. Our findings indicate that interactions between SET1 and MEC3 have a role in DNA repair and telomere function.

Role of a complex containing Rad17, Mec3, and Ddc1 in the yeast DNA damage checkpoint pathway

Genetic analysis has suggested that RAD17, RAD24, MEC3, and DDC1 play similar roles in the DNA damage checkpoint control in budding yeast. These genes are required for DNA damage-induced Rad53 phosphorylation and considered to function upstream of RAD53 in the DNA damage checkpoint pathway. Here we identify Mec3 as a protein that associates with Rad17 in a two-hybrid screen and demonstrate that Rad17 and Mec3 interact physically in vivo. The amino terminus of Rad17 is required for its interaction with Mec3, and the protein encoded by the rad17-1 allele, containing a missense mutation at the amino terminus, is defective for its interaction with Mec3 in vivo. Ddc1 interacts physically and cosediments with both Rad17 and Mec3, indicating that these three proteins form a complex. On the other hand, Rad24 is not found to associate with Rad17, Mec3, and Ddc1. DDC1 overexpression can partially suppress the phenotypes of the rad24Delta mutation: sensitivity to DNA damage, defect in the DNA damage checkpoint and decrease in DNA damage-induced phosphorylation of Rad53. Taken together, our results suggest that Rad17, Mec3, and Ddc1 form a complex which functions downstream of Rad24 in the DNA damage checkpoint pathway.

Interaction between the MEC1-dependent DNA synthesis checkpoint and G1 cyclin function in Saccharomyces cerevisiae

The completion of DNA synthesis in yeast is monitored by a checkpoint that requires MEC1 and RAD53. Here we show that deletion of the Saccharomyces cerevisiae G1 cyclins CLN1 and CLN2 suppressed the essential requirement for MEC1 function. Wild-type levels of CLN1 and CLN2, or overexpression of CLN1, CLN2, or CLB5, but not CLN3, killed mec1 strains. We identified RNR1, which encodes a subunit of ribonucleotide reductase, as a high-copy suppressor of the lethality of mec1 GAL1-CLN1. Northern analysis demonstrated that RNR1 expression is reduced by CLN1 or CLN2 overexpression. Because limiting RNR1 expression would be expected to decrease dNTP pools, CLN1 and CLN2 may cause lethality in mec1 strains by causing initiation of DNA replication with inadequate dNTPs. In contrast to mec1 mutants, MEC1 strains with low dNTPs would be able to delay S phase and thereby remain viable. We propose that the essential function for MEC1 may be the same as its checkpoint function during hydroxyurea treatment, namely, to slow S phase when nucleotides are limiting. In a cln1 cln2 background, a prolonged period of expression of genes turned on at the G1-S border, such as RNR1, has been observed. Thus deletion of CLN1 and CLN2 could function similarly to overexpression of RNR1 in suppressing mec1 lethality.

A human homologue of the checkpoint kinase Cds1 directly inhibits Cdc25 phosphatase

BACKGROUND

In human cells, the mitosis-inducing kinase Cdc2 is inhibited by phosphorylation on Thr14 and Tyr15. Disruption of these phosphorylation sites abrogates checkpoint-mediated regulation of Cdc2 and renders cells highly sensitive to agents that damage DNA. Phosphorylation of these sites is controlled by the opposing activities of the Wee1/Myt1 kinases and the Cdc25 phosphatase. The regulation of these enzymes is therefore likely to be crucial for the operation of the G2-M DNA-damage checkpoint.

RESULTS

Here, we show that the activity of Cdc25 decreased following exposure to ionizing radiation. The irradiation-induced decrease in Cdc25 activity was suppressed by wortmannin, an inhibitor of phosphatidylinositol (PI) 3-kinases, and was dependent on the function of the gene that is mutated in ataxia telangiectasia. We also identified two human kinases that phosphorylate and inactivate Cdc25 in vitro. One is the previously characterized Chk1 kinase. The second is novel and is homologous to the Cds1/Rad53 family of checkpoint kinases in yeast. Human Cds1 was found to be activated in response to DNA damage.

CONCLUSIONS

These results suggest that, in human cells, the DNA-damage checkpoint involves direct inactivation of Cdc25 catalyzed by Cds1 and/or Chk1.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

Mitotic DNA damage and replication checkpoints in yeast

Studies of the genetics of G2/M checkpoints in budding and fission yeasts have produced many of the defining concepts of checkpoint biology. Recent progress in the biochemistry of the checkpoint gene products is adding a mechanistic understanding to our models and identifying the components of the normal cell cycle machinery that are targeted by checkpoints.

Differential regulation of two closely clustered yeast genes, MAG1 and DDI1, by cell-cycle checkpoints

Eukaryotic DNA-damage checkpoint genes have been shown to not only arrest cells at certain stages, but are also involved in the transcriptional response to DNA damage. However, while the signal transduction for cell-cycle checkpoint is well characterized, it is not clear whether the same signal transduction pathway is responsible for the regulation of all DNA damage-inducible genes. In order to understand how different checkpoint genes are involved in gene regulation, the effects of various checkpoint mutations on the expression of a unique yeast MAG1 - DDI1 dual promoter were examined in this study. MAG1 and DDI1 are transcribed from a common promoter region and co-induced by a variety of DNA damaging agents. However, gene-specific cis -acting elements were also identified, and the two genes are indeed differentially expressed under certain conditions. We found that DDI1 induction was not affected in any of the checkpoint mutants. In contrast, MAG1 induction was completely abolished in the pol2 and rad53 mutants. However, in the mec1-1 or any of the G1/S and G2/M checkpoint mutants, including rad9, rad17 and rad24, DNA damage-induced MAG1 expression was not significantly affected, and a rad9 rad17 double mutation only slightly reduced MAG1 induction. Based on this and previous studies, we present two models for the role of checkpoint genes in transcriptional regulation in response to DNA damage.

The SFP1 gene product of Saccharomyces cerevisiae regulates G2/M transitions during the mitotic cell cycle and DNA-damage response

In eukaryotic cells, checkpoint pathways arrest cell-cycle progression if a particular event has failed to complete appropriately or if an important intracellular structure is defective or damaged. Saccharomyces cerevisiae strains that lack the SFP1 gene fail to arrest at the G2 DNA-damage checkpoint in response to genomic injury, but maintain their ability to arrest at the replication and spindle-assembly checkpoints. sfp1Delta mutants are characterized by a premature entrance into mitosis during a normal (undamaged) cell cycle, while strains that overexpress Sfp1p exhibit delays in G2. Sfp1p therefore acts as a repressor of the G2/M transition, both in the normal cell cycle and in the G2 checkpoint pathway. Sfp1 is a nuclear protein with two Cys2His2 zinc-finger domains commonly found in transcription factors. We propose that Sfp1p regulates the expression of gene products involved in the G2/M transition during the mitotic cell cycle and the DNA-damage response. In support of this model, overexpression of Sfp1p induces the expression of the PDS1 gene, which is known to encode a protein that regulates the G2 checkpoint.

Synaptonemal complex morphogenesis and sister-chromatid cohesion require Mek1-dependent phosphorylation of a meiotic chromosomal protein

Development of yeast meiotic chromosome cores into full-length synaptonemal complexes requires the MEK1 gene product, a meiosis-specific protein kinase homolog. The Mek1 protein associates with meiotic chromosomes and colocalizes with the Red1 protein, which is a component of meiotic chromosome cores. Mek1 and Red1 interact physically in meiotic cells, as demonstrated by coimmunoprecipitation and the two-hybrid protein system. Hop1, another protein associated with meiotic chromosome cores, also interacts with Mek1 but only in the presence of Red1. Red1 displays Mek1-dependent phosphorylation, both in vitro and in vivo, and Mek1 kinase activity is necessary for Mek1 function in vivo. Fluorescent in situ hybridization analysis indicates that Mek1-mediated phosphorylation of Red1 is required for meiotic sister-chromatid cohesion, raising the possibility that cohesion is regulated by protein phosphorylation.

DNA damage checkpoint in budding yeast

Eukaryotic cells have evolved a network of control mechanisms, known as checkpoints, which coordinate cell-cycle progression in response to internal and external cues. The yeast Saccharomyces cerevisiae has been invaluable in dissecting genetically the DNA damage checkpoint pathway. Recent results on posttranslational modifications and protein-protein interactions of some key factors provide new insights into the architecture of checkpoint protein complexes and their order of function.

The budding yeast Rad9 checkpoint protein is subjected to Mec1/Tel1-dependent hyperphosphorylation and interacts with Rad53 after DNA damage

The Saccharomyces cerevisiae RAD9 checkpoint gene is required for transient cell-cycle arrests and transcriptional induction of DNA repair genes in response to DNA damage. Polyclonal antibodies raised against the Rad9 protein recognized several polypeptides in asynchronous cultures, and in cells arrested in S or G2/M phases while a single form was observed in G1-arrested cells. Treatment with various DNA damaging agents, i.e. UV, ionizing radiation or methyl methane sulfonate, resulted in the appearance of hypermodified forms of the protein. All modifications detected during a normal cell cycle and after DNA damage were sensitive to phosphatase treatment, indicating that they resulted from phosphorylation. Damage-induced hyperphosphorylation of Rad9 correlated with checkpoint functions (cell-cycle arrest and transcriptional induction) and was cell-cycle stage- and progression-independent. In asynchronous cultures, Rad9 hyperphosphorylation was dependent on MEC1 and TEL1, homologues of the ATR and ATM genes. In G1-arrested cells, damage-dependent hyperphosphorylation required functional MEC1 in addition to RAD17, RAD24, MEC3 and DDC1, demonstrating cell-cycle stage specificity of the checkpoint genes in this response to DNA damage. Analysis of checkpoint protein interactions after DNA damage revealed that Rad9 physically associates with Rad53.

Telomere structure, replication and length maintenance

Telomeres are the termini of linear eukaryotic chromosomes consisting of tandem repeats of DNA and proteins that bind to these repeat sequences. Telomeres ensure the complete replication of chromosome ends, impart protection to ends from nucleolytic degradation, end-to-end fusion, and guide the localization of chromosomes within the nucleus. In addition, a combination of genetic, biochemical, and molecular biological approaches have implicated key roles for telomeres in diverse cellular processes such as regulation of gene expression, cell division, cell senescence, and cancer. This review focuses on recent advances in our understanding of the organization of telomeres, telomere replication, proteins that bind telomeric DNA, and the establishment of telomere length equilibrium.

The Xenopus Chk1 protein kinase mediates a caffeine-sensitive pathway of checkpoint control in cell-free extracts

We have analyzed the role of the protein kinase Chk1 in checkpoint control by using cell-free extracts from Xenopus eggs. Recombinant Xenopus Chk1 (Xchk1) phosphorylates the mitotic inducer Cdc25 in vitro on multiple sites including Ser-287. The Xchk1-catalyzed phosphorylation of Cdc25 on Ser-287 is sufficient to confer the binding of 14-3-3 proteins. Egg extracts from which Xchk1 has been removed by immunodepletion are strongly but not totally compromised in their ability to undergo a cell cycle delay in response to the presence of unreplicated DNA. Cdc25 in Xchk1-depleted extracts remains bound to 14-3-3 due to the action of a distinct Ser-287-specific kinase in addition to Xchk1. Xchk1 is highly phosphorylated in the presence of unreplicated or damaged DNA, and this phosphorylation is abolished by caffeine, an agent which attenuates checkpoint control. The checkpoint response to unreplicated DNA in this system involves both caffeine-sensitive and caffeine-insensitive steps. Our results indicate that caffeine disrupts the checkpoint pathway containing Xchk1.

Recovery from DNA replicational stress is the essential function of the S-phase checkpoint pathway

RAD53 and MEC1 are essential genes required for the transcriptional and cell cycle responses to DNA damage and DNA replication blocks. We have examined the essential function of these genes and found that their lethality but not their checkpoint defects can be suppressed by increased expression of genes encoding ribonucleotide reductase. Analysis of viable null alleles revealed that Mec1 plays a greater role in response to inhibition of DNA synthesis than Rad53. The loss of survival in mec1 and rad53 null or point mutants in response to transient inhibition of DNA synthesis is not a result of inappropriate anaphase entry but primarily to an inability to complete chromosome replication. We propose that this checkpoint pathway plays an important role in the maintenance of DNA synthetic capabilities when DNA replication is stressed.

The DNA replication and damage checkpoint pathways induce transcription by inhibition of the Crt1 repressor

We have identified the yeast CRT1 gene as an effector of the DNA damage and replication checkpoint pathway. CRT1 encodes a DNA-binding protein that recruits the general repressors Ssn6 and Tup1 to the promoters of damage-inducible genes. Derepression of the Crt1 regulon suppresses the lethality of mec1 and rad53 null alleles and is essential for cell viability during replicative stress. In response to DNA damage and replication blocks, Crt1 becomes hyperphosphorylated and no longer binds DNA, resulting in transcriptional induction. CRT1 is autoregulated and is itself induced by DNA damage, indicating the existence of a negative feedback pathway that facilitates return to the repressed state after elimination of damage. The inhibition of an autoregulatory repressor in response to DNA damage is a strategy conserved throughout prokaryotic and eukaryotic evolution.

Functional and physical interaction between Rad24 and Rfc5 in the yeast checkpoint pathways

The RFC5 gene encodes a small subunit of replication factor C (RFC) complex in Saccharomyces cerevisiae and has been shown to be required for the checkpoints which respond to replication block and DNA damage. Here we describe the isolation of RAD24, known to play a role in the DNA damage checkpoint, as a dosage-dependent suppressor of rfc5-1. RAD24 overexpression suppresses the sensitivity of rfc5-1 cells to DNA-damaging agents and the defect in DNA damage-induced Rad53 phosphorylation. Rad24, like Rfc5, is required for the regulation of Rad53 phosphorylation in response to DNA damage. The Rad24 protein, which is structurally related to the RFC subunits, interacts physically with RFC subunits Rfc2 and Rfc5 and cosediments with Rfc5. Although the rad24Delta mutation alone does not cause a defect in the replication block checkpoint, it does enhance the defect in rfc5-1 mutants. Furthermore, overexpression of RAD24 suppresses the rfc5-1 defect in the replication block checkpoint. Taken together, our results demonstrate a physical and functional interaction between Rad24 and Rfc5 in the checkpoint pathways.

A suppressor of two essential checkpoint genes identifies a novel protein that negatively affects dNTP pools

In Saccharomyces cerevisiae, MEC1 and RAD53 are essential for cell growth and checkpoint function. Their essential role in growth can be bypassed by deletion of a novel gene, SML1, which functions after several genes whose overexpression also suppresses mec1 inviability. In addition, sml1 affects various cellular processes analogous to overproducing the large subunit of ribonucleotide reductase, RNR1. These include effects on mitochondrial biogenesis, on the DNA damage response, and on cell growth. Consistent with these observations, the levels of dNTP pools in sml1 delta strains are increased compared to wild-type. This effect is not due to an increase in RNR transcription. Finally, both in vivo and in vitro experiments show that Sml1 binds to Rnr1. We propose that Sml1 inhibits dNTP synthesis posttranslationally by binding directly to Rnr1 and that Mec1 and Rad53 are required to relieve this inhibition.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

MEC1-dependent phosphorylation of Rad9p in response to DNA damage

In budding yeast, DNA damage can activate a checkpoint surveillance system controlled by the RAD9, RAD53, and MEC1 genes, resulting in a delay in cell cycle progression. Here, I report that DNA damage induces rapid and extensive phosphorylation of Rad9p in a manner that correlates directly with checkpoint activation. This response is dependent on MEC1, which encodes a member of the evolutionarily conserved ATM family of protein kinases, and on gene products of the RAD24 epistasis group, which have been implicated in the recognition and processing of DNA lesions. Since the phosphorylated form of Rad9p appears capable of interacting stably with Rad53p in vivo, this phosphorylation response likely controls checkpoint signaling by Rad9p.

Mec1p is essential for phosphorylation of the yeast DNA damage checkpoint protein Ddc1p, which physically interacts with Mec3p

Checkpoints prevent DNA replication or nuclear division when chromosomes are damaged. The Saccharomyces cerevisiae DDC1 gene belongs to the RAD17, MEC3 and RAD24 epistasis group which, together with RAD9, is proposed to act at the beginning of the DNA damage checkpoint pathway. Ddc1p is periodically phosphorylated during unperturbed cell cycle and hyperphosphorylated in response to DNA damage. We demonstrate that Ddc1p interacts physically in vivo with Mec3p, and this interaction requires Rad17p. We also show that phosphorylation of Ddc1p depends on the key checkpoint protein Mec1p and also on Rad24p, Rad17p and Mec3p. This suggests that Mec1p might act together with the Rad24 group of proteins at an early step of the DNA damage checkpoint response. On the other hand, Ddc1p phosphorylation is independent of Rad53p and Rad9p. Moreover, while Ddc1p is required for Rad53p phosphorylation, it does not play any major role in the phosphorylation of the anaphase inhibitor Pds1p, which requires RAD9 and MEC1. We suggest that Rad9p and Ddc1p might function in separated branches of the DNA damage checkpoint pathway, playing different roles in determining Mec1p activity and/or substrate specificity.

Rad53 FHA domain associated with phosphorylated Rad9 in the DNA damage checkpoint

The Rad53 protein kinase of Saccharomyces cerevisiae is required for checkpoints that prevent cell division in cells with damaged or incompletely replicated DNA. The Rad9 protein was phosphorylated in response to DNA damage, and phosphorylated Rad9 interacted with the COOH-terminal forkhead homology-associated (FHA) domain of Rad53. Inactivation of this domain abolished DNA damage-dependent Rad53 phosphorylation, G2/M cell cycle phase arrest, and increase of RNR3 transcription but did not affect replication inhibition-dependent Rad53 phosphorylation. Thus, Rad53 integrates DNA damage signals by coupling with phosphorylated Rad9. The hitherto uncharacterized FHA domain appears to be a modular protein-binding domain.

RAD9 and RAD24 define two additive, interacting branches of the DNA damage checkpoint pathway in budding yeast normally required for Rad53 modification and activation

In budding yeast, RAD9 and RAD24/RAD17/MEC3 are believed to function upstream of MEC1 and RAD53 in signalling the presence of DNA damage. Deletion of any one of these genes reduces the normal G1/S and G2/M checkpoint delays after UV irradiation, whereas in rad9Delta-rad24Delta cells the G1/S checkpoint is undetectable, although there is a residual G2/M checkpoint. We have shown previously that RAD9 also controls the transcriptional induction of a DNA damage regulon (DDR). We now report that efficient DDR induction requires all the above-mentioned checkpoint genes. Residual induction of the DDR after UV irradiation observed in all single mutants is not detectable in rad9Delta-rad24Delta. We have examined the G2/M checkpoint and UV sensitivity of single mutants after overexpression of the checkpoint proteins. This analysis indicates that RAD9 and the RAD24 epistasis group can be placed onto two separate, additive branches that converge on MEC1 and RAD53. Furthermore, MEC3 appears to function downstream of RAD24/RAD17. The transcriptional response to DNA damage revealed unexpected and specific antagonism between RAD9 and RAD24. Further support for genetic interaction between RAD9 and RAD24 comes from study of the modification and activation of Rad53 after damage. Evidence for bypass of RAD53 function under some conditions is also presented.

"Isogaba Maware": quality control of genome DNA by checkpoints

Checkpoints maintain the interdependency of cell cycle events by permitting the onset of an event only after the completion of the preceding event. The DNA replication checkpoint induces a cell cycle arrest until the completion of the DNA replication. Similarly, the DNA damage checkpoint arrests cell cycle progression if DNA repair is incomplete. A number of genes that play a role in the two checkpoints have been identified through genetic studies in yeasts, and their homologues have been found in fly, mouse, and human. They form signaling cascades activated by a DNA replication block or DNA damage and subsequently generate the negative constraints on cell cycle regulators. The failure of these signaling cascades results in producing offspring that carry mutations or that lack a portion of the genome. In humans, defects in the checkpoints are often associated with cancer-prone diseases. Focusing mainly on the studies in budding and fission yeasts, we summarize the recent progress.

Cellular responses to DNA damage

The exposure of cells to DNA damage inducers triggers a wide range of cellular responses including an alteration in gene expression, a delay in cell-cycle progression and the stimulation of DNA repair. In multicellular organisms, DNA damage can also activate programmed cell death. Recently, several signaling pathways that link DNA damage to gene expression and to the cell-cycle checkpoints have been identified. These pathways establish a framework for future studies of DNA damage responses.

Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1+ gene is one of six fission yeast genes, termed the checkpoint rad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1+, we have demonstrated a genetic interaction between hus1+ and another checkpoint rad gene, rad1+. Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9+, suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3+ and each of the other checkpoint rad genes. Although there is no gene related to hus1+ in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.

DNA damage checkpoints update: getting molecular

Eukaryotic checkpoint controls impose delays in the cell cycle in response to DNA damage or defects in DNA replication. Genetic and physiological studies in budding yeast have identified key genes and defined genetic pathways involved in checkpoint-mediated responses. Recent studies now lead to biochemical models that explain at least in part the arrest in G1 and delays during DNA replication after damage. Though progress in checkpoint controls has indeed been rapid, several observations identify puzzling aspects of checkpoint controls with few plausible explanations.

Components of the Ku-dependent non-homologous end-joining pathway are involved in telomeric length maintenance and telomeric silencing

In the budding yeast, Saccharomyces cerevisiae, genes in close proximity to telomeres are subject to transcriptional silencing through the process of telomere position effect (TPE). Here, we show that the protein Ku, previously implicated in DNA double-strand break (DSB) repair and in telomeric length maintenance, is also essential for telomeric silencing. Furthermore, using an in vivo plasmid rejoining assay, we demonstrate that SIR2, SIR3 and SIR4, three genes shown previously to function in TPE, are essential for Ku-dependent DSB repair. As is the case for Ku-deficient strains, residual repair operating in the absence of the SIR gene products ensues through an error-prone DNA repair pathway that results in terminal deletions. To identify novel components of the Ku-associated DSB repair pathway, we have tested several other candidate genes for their involvement in DNA DSB repair, telomeric maintenance and TPE. We show that TEL1, a gene required for telomeric length maintenance, is not required for either DNA DSB repair or TPE. However, RAD50, MRE11 and XRS2 function both in Ku-dependent DNA DSB repair and in telomeric length maintenance, although they have no major effects on TPE. These data provide important insights into DNA DSB repair and the linkage of this process to telomere length homeostasis and transcriptional silencing.

S-phase-specific activation of Cds1 kinase defines a subpathway of the checkpoint response in Schizosaccharomyces pombe

Checkpoints that respond to DNA structure changes were originally defined by the inability of yeast mutants to prevent mitosis following DNA damage or S-phase arrest. Genetic analysis has subsequently identified subpathways of the DNA structure checkpoints, including the reversible arrest of DNA synthesis. Here, we show that the Cds1 kinase is required to slow S phase in the presence of DNA-damaging agents. Cds1 is phosphorylated and activated by S-phase arrest and activated by DNA damage during S phase, but not during G1 or G2. Activation of Cds1 during S phase is dependent on all six checkpoint Rad proteins, and Cds1 interacts both genetically and physically with Rad26. Unlike its Saccharomyces cerevisiae counterpart Rad53, Cds1 is not required for the mitotic arrest checkpoints and, thus, defines an S-phase specific subpathway of the checkpoint response. We propose a model for the DNA structure checkpoints that offers a new perspective on the function of the DNA structure checkpoint proteins. This model suggests that an intrinsic mechanism linking S phase and mitosis may function independently of the known checkpoint proteins.

Involvement of Brca2 in DNA repair

Abnormalities precipitated by a targeted truncation in the murine gene Brca2 define its involvement in DNA repair. In culture, cells harboring truncated Brca2 exhibit a proliferative impediment that worsens with successive passages. Arrest in the G1 and G2/M phases is accompanied by elevated p53 and p21 expression. Increased sensitivity to genotoxic agents, particularly ultraviolet light and methylmethanesulfonate, shows that Brca2 function is essential for the ability to survive DNA damage. But checkpoint activation and apoptotic mechanisms are largely unaffected, thereby implicating Brca2 in repair. This is substantiated by the spontaneous accumulation of chromosomal abnormalities, including breaks and aberrant chromatid exchanges. These findings define a function of Brca2 in DNA repair, whose loss precipitates replicative failure, mutagen sensitivity, and genetic instability reminiscent of Bloom syndrome and Fanconi anemia.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

The anaphase inhibitor of Saccharomyces cerevisiae Pds1p is a target of the DNA damage checkpoint pathway

Inhibition of DNA replication and physical DNA damage induce checkpoint responses that arrest cell cycle progression at two different stages. In Saccharomyces cerevisiae, the execution of both checkpoint responses requires the Mec1 and Rad53 proteins. This observation led to the suggestion that these checkpoint responses are mediated through a common signal transduction pathway. However, because the checkpoint-induced arrests occur at different cell cycle stages, the downstream effectors mediating these arrests are likely to be distinct. We have previously shown that the S. cerevisiae protein Pds1p is an anaphase inhibitor and is essential for cell cycle arrest in mitosis in the presence DNA damage. Herein we show that DNA damage, but not inhibition of DNA replication, induces the phosphorylation of Pds1p. Analyses of Pds1p phosphorylation in different checkpoint mutants reveal that in the presence of DNA damage, Pds1p is phosphorylated in a Mec1p- and Rad9p-dependent but Rad53p-independent manner. Our data place Pds1p and Rad53p on parallel branches of the DNA damage checkpoint pathway. We suggest that Pds1p is a downstream target of the DNA damage checkpoint pathway and that it is involved in implementing the DNA damage checkpoint arrest specifically in mitosis.

Green fluorescent protein as a reporter for the DNA damage-induced gene RAD54 in Saccharomyces cerevisiae

The green fluorescent protein (GFP) of Aequorea victoria is now an established marker for gene expression and subcellular localization in budding yeast. Relatively high expression (greater than 2500 copies per cell) of GFP is required for direct microscopic visualization. This report provides a method for studying the expression of less highly expressed genes by the analysis of crude cell extracts--a simple and cheap alternative to the fluorescent activated cell sorter (FACS). The utility of this marker is demonstrated in a study of the expression of the RAD54 gene. It is shown that the induction of the RAD54 promoter leads to the accumulation of Rad54p and of GFP and that the fluorescence induction is correctly regulated. This method should allow the screening of large numbers of novel gene disrupters for their effects on RAD54 expression and so identify trans-acting factors involved in the cellular response to DNA damage.

Rad53-dependent phosphorylation of Swi6 and down-regulation of CLN1 and CLN2 transcription occur in response to DNA damage in Saccharomyces cerevisiae

Budding yeast possesses a checkpoint-dependent mechanism of delaying G1 progression in response to UV and ionizing radiation DNA damage. We have shown that after a pulse of DNA damage in G1 with the alkylating agent MMS, there is also a MEC1-, RAD53-, and RAD9-dependent delay in G1. This delay occurs at or before Start, as the MMS-treated cells do not bud, remain sensitive to alpha-factor, and have low CLN1 and CLN2 transcript levels for a longer time than untreated cells. We further show that MMS directly and reversibly down-regulates CLN1 and CLN2 transcript levels. The initial drop in CLN transcript levels in MMS is not RAD53 dependent, but the kinetics of reaccumulation of CLN messages as cells recover from the damage is faster in rad53-11 cells than in wild type cells. This is not an indirect effect of faster progression through G1, because CLN transcripts reaccumulate faster in rad53-11 mutants arrested in G1 as well. In addition, the recovery of CLN mRNA levels can be also hastened by a SWI6 deletion or by overexpression of the truncated Swi4 (Swi4-t) that lacks the carboxy-terminal domain through which Swi4 associates with Swi6. This indicates that both Rad53 and Swi6 are negative regulators of CLN expression after DNA damage. Finally, Swi6 undergoes an MMS-inducible, RAD53-dependent phosphorylation in G1 cells, and Rad53, immunoprecipitated from MMS-treated cells, phosphorylates Swi6 in vitro. On the basis of these observations, we suggest that the Rad53-dependent phosphorylation of Swi6 may delay the transition to S phase by inhibiting CLN transcription.

Role of UME6 in transcriptional regulation of a DNA repair gene in Saccharomyces cerevisiae

In Saccharomyces cerevisiae UV radiation and a variety of chemical DNA-damaging agents induce the transcription of specific genes, including several involved in DNA repair. One of the best characterized of these genes is PHR1, which encodes the apoenzyme for DNA photolyase. Basal-level and damage-induced expression of PHR1 require an upstream activation sequence, UAS(PHR1), which has homology with DRC elements found upstream of at least 19 other DNA repair and DNA metabolism genes in yeast. Here we report the identification of the UME6 gene of S. cerevisiae as a regulator of UAS(PHR1) activity. Multiple copies of UME6 stimulate expression from UAS(PHR1) and the intact PHR1 gene. Surprisingly, the effect of deletion of UME6 is growth phase dependent. In wild-type cells PHR1 is induced in late exponential phase, concomitant with the initiation of glycogen accumulation that precedes the diauxic shift. Deletion of UME6 abolishes this induction, decreases the steady-state concentration of photolyase molecules and PHR1 mRNA, and increases the UV sensitivity of a rad2 mutant. Despite the fact that UAS(PHR1) does not contain the URS1 sequence, which has been previously implicated in UME6-mediated transcriptional regulation, we find that Ume6p binds to UAS(PHR1) with an affinity and a specificity similar to those seen for a URS1 site. Similar binding is also seen for DRC elements from RAD2, RAD7, and RAD53, suggesting that UME6 contributes to the regulated expression of a subset of damage-responsive genes in yeast.

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.

The DNA polymerase alpha-primase complex couples DNA replication, cell-cycle progression and DNA-damage response

The highly conserved DNA polymerase alpha-primase complex (pol-prism) is the only eukaryotic DNA polymerase that can initiate DNA synthesis de novo. It is required both for the initiation of DNA replication at chromosomal origins and for the discontinuous synthesis of Okazaki fragments on the lagging strand of the replication fork. The dual role of pol-prim makes it a likely target for mechanisms that control cell-cycle S-phase entry and progression.

Rfc5, a replication factor C component, is required for regulation of Rad53 protein kinase in the yeast checkpoint pathway

The RFC5 gene encodes a small subunit of replication factor C (RFC) complex in Saccharomyces cerevisiae. We have previously shown that a temperature-sensitive (ts) rfc5-1 mutation is impaired in the S-phase checkpoint. In this report, we show that the rfc5-1 mutation is sensitive to DNA-damaging agents. RFC5 is necessary for slowing the S-phase progression in response to DNA damage. The phosphorylation of the essential central transducer, Rad53 protein kinase, is reduced in response to DNA damage in rfc5-1 mutants during the S phase. Furthermore, the inducibility of RNR3 transcription in response to DNA damage is dependent on RFC5. It has been shown that phosphorylation of Rad53 is controlled by Mec1 and Tel1, members of the subfamily of ataxia-telangiectasia mutated (ATM) kinases. We also demonstrate that overexpression of TEL1 suppresses the ts growth defect and DNA damage sensitivity of rfc5-1 mutants and restores phosphorylation of Rad53 and RNR3 induction in response to DNA damage in rfc5-1. Our results, together with the observation that overexpression of RAD53 suppresses the defects of the rfc5-1 mutation, suggest that Rfc5 is part of a mechanism transducing the DNA damage signal to the activation of the central transducer Rad53.

Identification of RNR4, encoding a second essential small subunit of ribonucleotide reductase in Saccharomyces cerevisiae

Ribonucleotide reductase (RNR), which catalyzes the rate-limiting step for deoxyribonucleotide production required for DNA synthesis, is an alpha2beta2 tetramer consisting of two large and two small subunits. RNR2 encodes a small subunit and is essential for mitotic viability in Saccharomyces cerevisiae. We have cloned a second essential gene encoding a homologous small subunit, RNR4. RNR4 and RNR2 appear to have nonoverlapping functions and cannot substitute for each other even when overproduced. The lethality of RNR4 deletion mutations can be suppressed by overexpression of RNR1 and RNR3, two genes encoding the large subunit of the RNR enzyme, indicating genetic interactions among the RNR genes. RNR2 and RNR4 may be present in the same reductase complex in vivo, since they coimmunoprecipitate from cell extracts. Like the other RNR genes, RNR4 is inducible by DNA-damaging agents through the same signal transduction pathway involving MEC1, RAD53, and DUN1 kinase genes. Analysis of DNA damage inducibility of RNR2 and RNR4 revealed partial inducibility in dun1 mutants, indicating a DUN1-independent branch of the transcriptional response to DNA damage.

The novel DNA damage checkpoint protein ddc1p is phosphorylated periodically during the cell cycle and in response to DNA damage in budding yeast

The DDC1 gene was identified, together with MEC3 and other checkpoint genes, during a screening for mutations causing synthetic lethality when combined with a conditional allele altering DNA primase. Deletion of DDC1 causes sensitivity to UV radiation, methyl methanesulfonate (MMS) and hydroxyurea (HU). ddc1Delta mutants are defective in delaying G1-S and G2-M transition and in slowing down the rate of DNA synthesis when DNA is damaged during G1, G2 or S phase, respectively. Therefore, DDC1 is involved in all the known DNA damage checkpoints. Conversely, Ddc1p is not required for delaying entry into mitosis when DNA synthesis is inhibited. ddc1 and mec3 mutants belong to the same epistasis group, and DDC1 overexpression can partially suppress MMS and HU sensitivity of mec3Delta strains, as well as their checkpoint defects. Moreover, Ddc1p is phosphorylated periodically during a normal cell cycle and becomes hyperphosphorylated in response to DNA damage. Both phosphorylation events are at least partially dependent on a functional MEC3 gene.

Dynamic changes of BRCA1 subnuclear location and phosphorylation state are initiated by DNA damage

BRCA1 localizes to discrete nuclear foci (dots) during S phase. Hydroxyurea-mediated DNA synthesis arrest of S phase MCF7 cells led to a loss of BRCA1 from these structures. Ultraviolet light, mitomycin C, or gamma irradiation produced a similar effect but with no concurrent arrest of DNA synthesis. BARD1 and Rad51, two proteins associated with the BRCA1 dots, behaved similarly. Loss of the BRCA1 foci was accompanied by a specific, dose-dependent change(s) in the state of BRCA1 phosphorylation. Three distinct DNA damaging agents preferentially induced this change in S phase. The S phase BRCA1 phosphorylation response to DNA damage occurred in cells lacking, respectively, two DNA damage-sensing protein kinases, DNA-PK and Atm, implying that neither plays a prime role in this process. Finally, after BRCA1 dot dispersal, BRCA1, BARD1, and Rad51 accumulated, focally, on PCNA+ replication structures, implying an interaction of BRCA1/BARD1/Rad51 containing complexes with damaged, replicating DNA. Taken together, the data imply that the BRCA1 S phase foci are dynamic physiological elements, responsive to DNA damage, and that BRCA1-containing multiprotein complexes participate in a replication checkpoint response.

Characterization of the checkpoint gene RAD53/MEC2 in Saccharomyces cerevisiae

Saccharomyces cerevisiae cells carrying mutations in RAD53/MEC2 fail to arrest in the S phase when DNA replication is blocked (the S/M checkpoint) or in the G2 phase when DNA is damaged (the G2/M checkpoint). We isolated and determined the DNA sequence of RAD53 and found that it is identical to the SPK1 gene previously identified by Stern et al. (1991). In addition to its checkpoint functions, we show here that RAD53 is essential for cell viability because null mutants are inviable. Weak genomic suppressors of the essential function do arise frequently, though they do not suppress the checkpoint defects of the null mutant. This genetically separates the essential and checkpoint functions. We show genetically that the protein kinase domain is essential for all RAD53-dependent functions tested because a site-specific mutation that inactivates the protein kinase activity results in a mutant phenotype indistinguishable from that of a null mutant. Overexpression of RAD53, or its kinase domain alone, resulted in a delay in cell-cycle progression that required the intact kinase function. The cell-cycle delay did not require any of the checkpoint genes tested (e.g. rad9 or mecl), indicating that the cell-cycle delay is either unrelated to the checkpoint responses, or that it occurs constitutively because RAD53 acts further downstream of the checkpoint genes tested. Finally, elimination of sequences in the promoter region of RAD53 revealed complex regulatory elements.

Casein kinase II catalyzes tyrosine phosphorylation of the yeast nucleolar immunophilin Fpr3

In the yeast Saccharomyces cerevisiae, the nucleolar immunophilin, Fpr3, is phosphorylated at tyrosine and dephosphorylated by the phosphotyrosine-specific phosphoprotein phosphatase, Ptp1. In Ptp1-deficient cells, Fpr3 contains phospho-Tyr at a single site (Tyr184), but also contains phospho-Ser and phospho-Thr. Ser186 (adjacent to Tyr184) is situated within a canonical site for phosphorylation by casein kinase II (CKII). Yeast cell lysates contain an activity that binds to Fpr3 in vitro and phosphorylates Fpr3 at Ser, Thr, and Tyr; this activity was found to be dependent on expression of functional yeast CKII. Moreover, purified Fpr3 was phosphorylated on Tyr184 in vitro by either purified yeast CKII or purified, bacterially-expressed human CKII. Likewise, phosphorylation of Fpr3 at tyrosine in vivo was markedly enhanced in yeast cells overexpressing a heterologous (Drosophila) CKII, but was undetectable in yeast cells carrying only a temperature-sensitive allele of the endogenous CKII, even when the cells were grown at a permissive temperature. Phosphorylation of Fpr3 at Tyr184 by CKII in vitro lagged behind phosphorylation of Fpr3 at Ser, and was accelerated by pre-phosphorylation of Fpr3 at Ser using CKII. Furthermore, synthetic peptides corresponding to the sequence surrounding Tyr184 that contained P-Ser (or Glu) at position 186 were much more efficient substrates for CKII phosphorylation of Tyr184 than a synthetic peptide containing Ala at position 186. These findings indicate that CKII phosphorylates Fpr3 at tyrosine and serine both in vivo and in vitro and thus possesses dual specificity. These results also indicate that Tyr184 is phosphorylated by CKII via a two-step process, in which phosphorylation at the +2 position provides a negatively-charged specificity determinant that allows subsequent phosphorylation of Tyr184.

Fragments of ATM which have dominant-negative or complementing activity

The ATM protein has been implicated in pathways controlling cell cycle checkpoints, radiosensitivity, genetic instability, and aging. Expression of ATM fragments containing a leucine zipper motif in a human tumor cell line abrogated the S-phase checkpoint after ionizing irradiation and enhanced radiosensitivity and chromosomal breakage. These fragments did not abrogate irradiation-induced G1 or G2 checkpoints, suggesting that cell cycle checkpoint defects alone cannot account for chromosomal instability in ataxia telangiectasia (AT) cells. Expression of the carboxy-terminal portion of ATM, which contains the PI-3 kinase domain, complemented radiosensitivity and the S-phase checkpoint and reduced chromosomal breakage after irradiation in AT cells. These observations suggest that ATM function is dependent on interactions with itself or other proteins through the leucine zipper region and that the PI-3 kinase domain contains much of the significant activity of ATM.

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.