Cleavage of substrates with mismatched nucleotides by Flap endonuclease-1. Implications for mammalian Okazaki fragment processing

Modulation of mutagenesis in eukaryotes by DNA replication fork dynamics and quality of nucleotide pools

The rate of mutations in eukaryotes depends on a plethora of factors and is not immediately derived from the fidelity of DNA polymerases (Pols). Replication of chromosomes containing the anti-parallel strands of duplex DNA occurs through the copying of leading and lagging strand templates by a trio of Pols α, δ and ϵ, with the assistance of Pol ζ and Y-family Pols at difficult DNA template structures or sites of DNA damage. The parameters of the synthesis at a given location are dictated by the quality and quantity of nucleotides in the pools, replication fork architecture, transcription status, regulation of Pol switches, and structure of chromatin. The result of these transactions is a subject of survey and editing by DNA repair.

Methoxyamine sensitizes the resistant glioblastoma T98G cell line to the alkylating agent temozolomide

Chemoresistance represents a major obstacle to successful treatment for malignant glioma with temozolomide. N (7)-methyl-G and N (3)-methyl-A adducts comprise more than 80 % of DNA lesions induced by temozolomide and are processed by the base excision repair, suggesting that the cellular resistance could be caused, in part, by this efficient repair pathway, although few studies have focused on this subject. The aim of this study was to evaluate the cellular responses to temozolomide treatment associated with methoxyamine (blocker of base excision repair) in glioblastoma cell lines, in order to test the hypothesis that the blockage of base excision repair pathway might sensitize glioblastoma cells to temozolomide. For all the tested cell lines, only T98G showed significant differences between temozolomide and temozolomide plus methoxyamine treatment, observed by reduced survival rates, enhanced the levels of DNA damage, and induced an arrest at G2-phase. In addition, ~10 % of apoptotic cells (sub-G1 fraction) were observed at 48 h. Western blot analysis demonstrated that APE1 and FEN1 presented a slightly reduced expression levels under the combined treatment, probably due to AP sites blockade by methoxyamine, thus causing a minor requirement of base excision repair pathway downstream to the AP removal by APE1. On the other hand, PCNA expression in temozolomide plus methoxyamine-treated cells does not rule out the possibility that such alteration might be related to the blockage of cell cycle (G2-phase), as observed at 24 h of recovery time. The results obtained in the present study demonstrated the efficiency of methoxyamine to overcome glioblastoma resistance to temozolomide treatment.

Continuous and periodic expansion of CAG repeats in Huntington's disease R6/1 mice

Huntington's disease (HD) is one of several neurodegenerative disorders caused by expansion of CAG repeats in a coding gene. Somatic CAG expansion rates in HD vary between organs, and the greatest instability is observed in the brain, correlating with neuropathology. The fundamental mechanisms of somatic CAG repeat instability are poorly understood, but locally formed secondary DNA structures generated during replication and/or repair are believed to underlie triplet repeat expansion. Recent studies in HD mice have demonstrated that mismatch repair (MMR) and base excision repair (BER) proteins are expansion inducing components in brain tissues. This study was designed to simultaneously investigate the rates and modes of expansion in different tissues of HD R6/1 mice in order to further understand the expansion mechanisms in vivo. We demonstrate continuous small expansions in most somatic tissues (exemplified by tail), which bear the signature of many short, probably single-repeat expansions and contractions occurring over time. In contrast, striatum and cortex display a dramatic—and apparently irreversible—periodic expansion. Expansion profiles displaying this kind of periodicity in the expansion process have not previously been reported. These in vivo findings imply that mechanistically distinct expansion processes occur in different tissues.

Huntington's disease (HD) is a genetically determined neurodegenerative disorder identified by the presence of a mutation for a long series of CAG repeats (>36 repeats) in the Huntingtin (HTT) gene. Longer repeat sequences cause disease onset at a younger age. The mutation encodes an expanded glutamine tract within the huntingtin protein. This enlarged polyglutamine fragment in the protein leads to the formation of the huntingtin aggregates that are observed in HD brains. The stretch of CAG repeats expands with age in affected brain areas, increasing the length of the polyglutamine tract, and is believed to amplify the effect of the disease. Several HD mouse models display phenotypes relevant to the human disease. We have investigated the rate and modes of expansion in striatum, cortex, and tail in transgenic R6/1 mice. Tail was included as a stable tissue, however we observed a small continuous expansion of CAG repeats in tail tissues. In brain tissues, we identified a periodic expansion process consisting of predominantly seven repeat steps. Our findings point towards a very controlled molecular mechanism as the cause of expansion in the most severely affected tissues, which may provide useful targets that can be used to inhibit disease development.

Dna2 on the road to Okazaki fragment processing and genome stability in eukaryotes

DNA replication is a primary mechanism for maintaining genome integrity, but it serves this purpose best by cooperating with other proteins involved in DNA repair and recombination. Unlike leading strand synthesis, lagging strand synthesis has a greater risk of faulty replication for several reasons: First, a significant part of DNA is synthesized by polymerase alpha, which lacks a proofreading function. Second, a great number of Okazaki fragments are synthesized, processed and ligated per cell division. Third, the principal mechanism of Okazaki fragment processing is via generation of flaps, which have the potential to form a variety of structures in their sequence context. Finally, many proteins for the lagging strand interact with factors involved in repair and recombination. Thus, lagging strand DNA synthesis could be the best example of a converging place of both replication and repair proteins. To achieve the risky task with extraordinary fidelity, Okazaki fragment processing may depend on multiple layers of redundant, but connected pathways. An essential Dna2 endonuclease/helicase plays a pivotal role in processing common structural intermediates that occur during diverse DNA metabolisms (e.g. lagging strand synthesis and telomere maintenance). Many roles of Dna2 suggest that the preemptive removal of long or structured flaps ultimately contributes to genome maintenance in eukaryotes. In this review, we describe the function of Dna2 in Okazaki fragment processing, and discuss its role in the maintenance of genome integrity with an emphasis on its functional interactions with other factors required for genome maintenance.

Functions of base selection step in human DNA polymerase alpha

Recent studies have revealed that the base selection step of DNA polymerases (pol) plays a role in prevention of DNA replication errors. We investigated whether base selection is required for the DNA replication fidelity of pol alpha and genomic stability in human cells. We introduced an Leu864 to Phe substitution (L864F) into human pol alpha and performed an in vitro LacZ alpha forward mutation assay. Our results showed that the overall mutation rate was increased by 180-fold as compared to that of the wild-type. Furthermore, steady state kinetics analyses consistently showed that L864F pol alpha had a decreased discrimination ability between correct and incorrect nucleotide incorporation, as well as between matched and mismatched primer termini. L864F pol alpha also exhibited increased translesion activity over the abasic, etheno-A, O(4)-methyl-T, and O(6)-methyl-G sites. In addition, our steady state kinetics analyses supported the finding of increased translesion activity of L864F pol alpha over O(6)-methyl-G. We also established stable clones transfected with pola1L864F utilizing the human cancer cell line HCT116. Using the HPRT gene as a reporter, the spontaneous mutation rate of pola1L864F cells was determined to be 2.4-fold greater than that of wild-type cells. Mutation assays were also carried out using cells transiently transfected with the wild-type or pola1L864F, and increased mutant frequencies were observed in pola1L864F cells under both spontaneous and methyl methanesulfonate-induced conditions. Together, our results indicate that the base selection step in human pol alpha functions to prevent DNA replication errors and maintain genomic integrity in HCT116 cells.

Early-onset lymphoma and extensive embryonic apoptosis in two domain-specific Fen1 mice mutants

Flap endonuclease 1 (FEN1) processes Okazaki fragments in lagging strand DNA synthesis, and FEN1 is involved in several DNA repair pathways. The interaction of FEN1 with the proliferating cell nuclear antigen (PCNA) processivity factor is central to the function of FEN1 in both DNA replication and repair. Here we present two gene-targeted mice with mutations in FEN1. The first mutant mouse carries a single amino acid point mutation in the active site of the nuclease domain of FEN1 (Fen1(E160D/E160D)), and the second mutant mouse contains two amino acid substitutions in the highly conserved PCNA interaction domain of FEN1 (Fen1(DeltaPCNA/DeltaPCNA)). Fen1(E160D/E160D) mice develop a considerably elevated incidence of B-cell lymphomas beginning at 6 months of age, particularly in females. By 16 months of age, more than 90% of the Fen1(E160D/E160D) females have tumors, primarily lymphomas. By contrast, Fen1(DeltaPCNA/DeltaPCNA) mouse embryos show extensive apoptosis in the forebrain and vertebrae area and die around stage E9.5 to E11.5.

Flap endonuclease 1 is overexpressed in prostate cancer and is associated with a high Gleason score

OBJECTIVE

To investigate the expression and potential clinical usefulness of structure-specific flap endonuclease 1 (FEN-1) in human primary prostate cancer using tissue microarray technology, as FEN-1 was recently identified to be overexpressed in CL1.1, the most aggressive clone generated from the hormone-refractory prostate cancer cell line CL1.

MATERIALS AND METHODS

Immunohistochemistry was performed on tissue microarrays constructed from paraffin-embedded specimens of primary prostate cancer from 246 patients who had had a radical prostatectomy. Prostatic intraepithelial neoplasia (PIN), benign prostatic hyperplasia (BPH) and normal prostate epithelium were represented on the array. FEN-1 nuclear expression was scored based on the percentage of target cells staining positively, and correlated with Gleason score, preoperative prostate-specific antigen (PSA) level and pathological stage. The time to PSA recurrence was also analysed.

RESULTS

The mean expression of FEN-1 was significantly higher in cancer (36.7%) than in normal (13.2%), BPH (4.5%) and PIN (15.4%) specimens (P < 0.001). FEN-1 expression was significantly correlated with Gleason score (ó = 0.23, P = 0.002). A higher preoperative serum PSA level (P = 0.015), Gleason score > or = 7 (P < 0.001), seminal vesicle invasion (P < 0.001) and capsular involvement (P = 0.004) were associated with PSA recurrence, whereas FEN-1 expression was not. In a multivariate analysis, only Gleason score > or = 7 (P < 0.001), seminal vesicle invasion (P = 0.005) and capsular involvement (P = 0.009) were retained as independent predictors for PSA recurrence.

CONCLUSIONS

FEN-1 is overexpressed in prostate cancer compared with matched normal prostate, and its expression increases with tumour dedifferentiation, as shown by increasing Gleason score. These results suggest that FEN-1 might be a potential marker for selecting patients at high risk, and a potential target for prostate cancer diagnosis and therapy.

Fen1 is induced p53 dependently and involved in the recovery from UV-light-induced replication inhibition

Mouse embryonic fibroblasts (MEFs) that lack p53 are hypersensitive to the cytotoxic and genotoxic effect of ultraviolet (UV-C) light. They also display a defect in the recovery from UV-C-induced DNA replication inhibition. An enzyme involved in processing stalled DNA replication forks is flap endonuclease 1 (Fen1). Gene expression profiling of UV-C-irradiated MEFs revealed fen1 to be upregulated, which was confirmed by RT-PCR and Western blot experiments. Increased Fen1 levels upon UV-C exposure are due to transcriptional activation, as revealed by inhibitor studies. Fen1 induction was dose- and time-dependent; it occurred on protein level already 3 h after irradiation. Induction of Fen1 by UV-C requires p53 since it was observed in p53 wild-type (wt) but not in p53 null (p53-/-) fibroblasts. Fen1 upregulation paralleled the increase in p53 protein level in replicating wt cells, whereas in nonreplicating cells both Fen1 and p53 were not induced by UV-C. The mouse fen1 promoter was cloned and shown to harbor a p53 consensus sequence to which p53 binds. In cotransfection experiments, p53 stimulated the expression of a fen1 promoter-reporter construct. Transgenic expression of Fen1 in p53 null cells attenuated UV-C light-induced DNA replication inhibition, supporting the hypothesis that Fen1 induction is involved in the recovery of cells from DNA damage.

Roles of DNA Polymerases in Replication, Repair, and Recombination in Eukaryotes

The functioning of the eukaryotic genome depends on efficient and accurate DNA replication and repair. The process of replication is complicated by the ongoing decomposition of DNA and damage of the genome by endogenous and exogenous factors. DNA damage can alter base coding potential resulting in mutations, or block DNA replication, which can lead to double-strand breaks (DSB) and to subsequent chromosome loss. Replication is coordinated with DNA repair systems that operate in cells to remove or tolerate DNA lesions. DNA polymerases can serve as sensors in the cell cycle checkpoint pathways that delay cell division until damaged DNA is repaired and replication is completed. Eukaryotic DNA template-dependent DNA polymerases have different properties adapted to perform an amazingly wide spectrum of DNA transactions. In this review, we discuss the structure, the mechanism, and the evolutionary relationships of DNA polymerases and their possible functions in the replication of intact and damaged chromosomes, DNA damage repair, and recombination.

On the roles of Saccharomyces cerevisiae Dna2p and Flap endonuclease 1 in Okazaki fragment processing

Short DNA segments designated Okazaki fragments are intermediates in eukaryotic DNA replication. Each contains an initiator RNA/DNA primer (iRNA/DNA), which is converted into a 5'-flap and then removed prior to fragment joining. In one model for this process, the flap endonuclease 1 (FEN1) removes the iRNA. In the other, the single-stranded binding protein, replication protein A (RPA), coats the flap, inhibits FEN1, but stimulates cleavage by the Dna2p helicase/nuclease. RPA dissociates from the resultant short flap, allowing FEN1 cleavage. To determine the most likely process, we analyzed cleavage of short and long 5'-flaps. FEN1 cleaves 10-nucleotide fixed or equilibrating flaps in an efficient reaction, insensitive to even high levels of RPA or Dna2p. On 30-nucleotide fixed or equilibrating flaps, RPA partially inhibits FEN1. CTG flaps can form foldback structures and were inhibitory to both nucleases, however, addition of a dT(12) to the 5'-end of a CTG flap allowed Dna2p cleavage. The presence of high Dna2p activity, under reaction conditions favoring helicase activity, substantially stimulated FEN1 cleavage of tailed-foldback flaps and also 30-nucleotide unstructured flaps. Our results suggest Dna2p is not used for processing of most flaps. However, Dna2p has a role in a pathway for processing structured flaps, in which it aids FEN1 using both its nuclease and helicase activities.

Failure to produce mitochondrial DNA results in embryonic lethality in Rnaseh1 null mice

Although ribonucleases H (RNases H) have long been implicated in DNA metabolism, they are not required for viability in prokaryotes or unicellular eukaryotes. We generated Rnaseh1(-/-) mice to investigate the role of RNase H1 in mammals and observed developmental arrest at E8.5 in null embryos. A fraction of the mainly nuclear RNase H1 was targeted to mitochondria, and its absence in embryos resulted in a significant decrease in mitochondrial DNA content, leading to apoptotic cell death. This report links RNase H1 to generation of mitochondrial DNA, providing direct support for the strand-coupled mechanism of mitochondrial DNA replication. These findings also have important implications for therapy of mitochondrial dysfunctions and drug development for the structurally related RNase H of HIV.

DNA mismatch repair and mutation avoidance pathways

Unpaired and mispaired bases in DNA can arise by replication errors, spontaneous or induced base modifications, and during recombination. The major pathway for correction of mismatches arising during replication is the MutHLS pathway of Escherichia coli and related pathways in other organisms. MutS initiates repair by binding to the mismatch, and activates together with MutL the MutH endonuclease, which incises at hemimethylated dam sites and thereby mediates strand discrimination. Multiple MutS and MutL homologues exist in eukaryotes, which play different roles in the mismatch repair (MMR) pathway or in recombination. No MutH homologues have been identified in eukaryotes, suggesting that strand discrimination is different to E. coli. Repair can be initiated by the heterodimers MSH2-MSH6 (MutSalpha) and MSH2-MSH3 (MutSbeta). Interestingly, MSH3 (and thus MutSbeta) is missing in some genomes, as for example in Drosophila, or is present as in Schizosaccharomyces pombe but appears to play no role in MMR. MLH1-PMS1 (MutLalpha) is the major MutL homologous heterodimer. Again some, but not all, eukaryotes have additional MutL homologues, which all form a heterodimer with MLH1 and which play a minor role in MMR. Additional factors with a possible function in eukaryotic MMR are PCNA, EXO1, and the DNA polymerases delta and epsilon. MMR-independent pathways or factors that can process some types of mismatches in DNA are nucleotide-excision repair (NER), some base excision repair (BER) glycosylases, and the flap endonuclease FEN-1. A pathway has been identified in Saccharomyces cerevisiae and human that corrects loops with about 16 to several hundreds of unpaired nucleotides. Such large loops cannot be processed by MMR.

Okazaki fragment processing: modulation of the strand displacement activity of DNA polymerase delta by the concerted action of replication protein A, proliferating cell nuclear antigen, and flap endonuclease-1

DNA polymerase (pol) delta is essential for both leading and lagging strand DNA synthesis during chromosomal replication in eukaryotes. Pol delta has been implicated in the Okazaki fragment maturation process for the extension of the newly synthesized fragment and for the displacement of the RNA/DNA segment of the preexisting downstream fragment generating an intermediate flap structure that is the target for the Dna2 and flap endonuclease-1 (Fen 1) endonucleases. Using a single-stranded minicircular template with an annealed RNA/DNA primer, we could measure strand displacement by pol delta coupled to DNA synthesis. Our results suggested that pol delta alone can displace up to 72 nucleotides while synthesizing through a double-stranded DNA region in a distributive manner. Proliferating cell nuclear antigen (PCNA) reduced the template dissociation rate of pol delta, thus increasing the processivity of both synthesis and strand displacement, whereas replication protein A (RP-A) limited the size of the displaced fragment down to 20-30 nucleotides, by generating a "locked" flap DNA structure, which was a substrate for processing of the displaced fragment by Fen 1 into a ligatable product. Our data support a model for Okazaki fragment processing where the strand displacement activity of DNA polymerase delta is modulated by the concerted action of PCNA, RP-A and Fen 1.

Role of the DNA repair nucleases Rad13, Rad2 and Uve1 of Schizosaccharomyces pombe in mismatch correction

Repair of mismatched DNA occurs mainly by the long-patch mismatch repair (MMR) pathway, requiring Msh2 and Pms1. In Schizosaccharomyces pombe mismatches can be repaired by a short-patch repair system, containing nucleotide excision repair (NER) factors. We studied mismatch correction efficiency in cells with inactivated DNA repair nucleases Rad13, Rad2 or Uve1 in MMR proficient and deficient background. Rad13 incises 3' of damaged DNA during NER. Rad2 has a function in the Uve1-dependent repair of DNA damages and in replication. Loss of Rad13 caused a strong reduction of short-patch processing of mismatches formed during meiotic recombination. Mitotic mutation rates were increased, but not to the same extent as in the NER mutant swi10, which is defective in 5' incision. The difference might be caused by an additional role of Rad13 in base excision repair or due to partial redundancy with other 3' endonucleases. Meiotic mismatch repair was not or only slightly affected in rad2 and uve1 mutants. In addition, inactivation of uve1 caused only weak effects on mutation avoidance. Mutation rates were elevated when rad2 was mutated, but not further increased in swi10 rad2 and rad13 rad2 double mutants, indicating an epistatic relationship. However, the mutation spectra of rad2 were different from that of swi10 and rad13. Thus, the function of Rad2 in mutation avoidance is rather independent of NER. rad13, swi10 and rad2, but not uve1 mutants were sensitive to the DNA-damaging agent methyl methane sulphonate. Cell survival was further reduced in the double mutants swi10 rad2, rad13 rad2 and, surprisingly, swi10 rad13. These data confirm that NER and Rad2 act in distinct damage repair pathways and further indicate that the function of Rad13 in repair of alkylated bases is partially independent of NER.

DNA replication fidelity

DNA replication fidelity is a key determinant of genome stability and is central to the evolution of species and to the origins of human diseases. Here we review our current understanding of replication fidelity, with emphasis on structural and biochemical studies of DNA polymerases that provide new insights into the importance of hydrogen bonding, base pair geometry, and substrate-induced conformational changes to fidelity. These studies also reveal polymerase interactions with the DNA minor groove at and upstream of the active site that influence nucleotide selectivity, the efficiency of exonucleolytic proofreading, and the rate of forming errors via strand misalignments. We highlight common features that are relevant to the fidelity of any DNA synthesis reaction, and consider why fidelity varies depending on the enzymes, the error, and the local sequence environment.

A coordinated interplay: proteins with multiple functions in DNA replication, DNA repair, cell cycle/checkpoint control, and transcription

In eukaryotic cells, DNA transactions such as replication, repair, and transcription require a large set of proteins. In all of these events, complexes of more than 30 polypetides appear to function in highly organized and structurally well-defined machines. We have learned in the past few years that the three essential macromolecular events, replication, repair, and transcription, have common functional entities and are coordinated by complex regulatory mechanisms. This can be documented for replication and repair, for replication and checkpoint control, and for replication and cell cycle control, as well as for replication and transcription. In this review we cover the three different protein classes: DNA polymerases, DNA polymerase accessory proteins, and selected transcription factors. The "common enzyme-different pathway strategy" is fascinating from several points of view: first, it might guarantee that these events are coordinated; second, it can be viewed from an evolutionary angle; and third, this strategy might provide cells with backup mechanisms for essential physiological tasks.

A DNA polymerase epsilon mutant that specifically causes +1 frameshift mutations within homonucleotide runs in yeast

The DNA polymerases delta and epsilon are the major replicative polymerases in the yeast Saccharomyces cerevisiae that possess 3' --> 5' exonuclease proofreading activity. Many errors arising during replication are corrected by these exonuclease activities. We have investigated the contributions of regions of Polepsilon other than the proofreading motifs to replication accuracy. An allele, pol2-C1089Y, was identified in a screen of Polepsilon mutants that in combination with an exonuclease I (exo1) mutation could cause a synergistic increase in mutations within homonucleotide runs. In contrast to other polymerase mutators, this allele specifically results in insertion frameshifts. When pol2-C1089Y was combined with deletions of EXO1 or RAD27 (homologue of human FEN1), mutation rates were increased for +1 frameshifts while there was almost no effect on -1 frameshifts. On the basis of genetic analysis, the pol2-C1089Y mutation did not cause a defect in proofreading. In combination with a deletion of the mismatch repair gene MSH2, the +1 frameshift mutation rate for a short homonucleotide run was increased nearly 100-fold whereas the -1 frameshift rate was unchanged. This suggests that the Pol2-C1089Y protein makes +1 frameshift errors during replication of homonucleotide runs and that these errors can be corrected by either mismatch repair (MMR) or proofreading (in short runs). This is the first report of a +1-specific mutator for homonucleotide runs in vivo. The pol2-C1089Y mutation defines a functionally important residue in Polepsilon.

The nature of the 5'-terminus is a major determinant for DNA processing by Schizosaccharomyces pombe Rad2p, a FEN-1 family nuclease

The nuclease activity of FEN-1 is essential for both DNA replication and repair. Intermediate DNA products formed during these processes possess a variety of structures and termini. We have previously demonstrated that the 5'-->3' exonuclease activity of the Schizosaccharomyces pombe FEN-1 protein Rad2p requires a 5'-phosphoryl moiety to efficiently degrade a nick-containing substrate in a reconstituted alternative excision repair system. Here we report the effect of different 5'-terminal moieties of a variety of DNA substrates on Rad2p activity. We also show that Rad2p possesses a 5'-->3' single-stranded exonuclease activity, similar to Saccharomyces cerevisiae Rad27p and phage T5 5'-->3' exonuclease (also a FEN-1 homolog). FEN-1 nucleases have been associated with the base excision repair pathway, specifically processing cleaved abasic sites. Because several enzymes cleave abasic sites through different mechanisms resulting in different 5'-termini, we investigated the ability of Rad2p to process several different types of cleaved abasic sites. With varying efficiency, Rad2p degrades the products of an abasic site cleaved by Escherichia coli endonuclease III and endonuclease IV (prototype AP endonucleases) and S.POMBE: Uve1p. These results provide important insights into the roles of Rad2p in DNA repair processes in S.POMBE:

Quality control by DNA repair

Faithful maintenance of the genome is crucial to the individual and to species. DNA damage arises from both endogenous sources such as water and oxygen and exogenous sources such as sunlight and tobacco smoke. In human cells, base alterations are generally removed by excision repair pathways that counteract the mutagenic effects of DNA lesions. This serves to maintain the integrity of the genetic information, although not all of the pathways are absolutely error-free. In some cases, DNA damage is not repaired but is instead bypassed by specialized DNA polymerases.

Functional analysis of human FEN1 in Saccharomyces cerevisiae and its role in genome stability

The flap endonuclease, FEN1, is an evolutionarily conserved component of DNA replication from archaebacteria to humans. Based on in vitro results, it processes Okazaki fragments during replication and is involved in base excision repair. FEN1 removes the last primer ribonucleotide on the lagging strand and it cleaves a 5' flap that may result from strand displacement during replication or during base excision repair. Its biological importance has been revealed largely through studies in the yeast Saccharomyces cerevisiae where deletion of the homologous gene RAD27 results in genome instability and mutagen sensitivity. While the in vivo function of Rad27 has been well characterized through genetic and biochemical approaches, little is understood about the in vivo functions of human FEN1. Guided by our recent results with yeast RAD27, we explored the function of human FEN1 in yeast. We found that the human FEN1 protein complements a yeast rad27 null mutant for a variety of defects including mutagen sensitivity, genetic instability and the synthetic lethal interactions of a rad27 rad51 and a rad27 pol3-01 mutant. Furthermore, a mutant form of FEN1 lacking nuclease function exhibits dominant-negative effects on cell growth and genome instability similar to those seen with the homologous yeast rad27 mutation. This genetic impact is stronger when the human and yeast PCNA-binding domains are exchanged. These data indicate that the human FEN1 and yeast Rad27 proteins act on the same substrate in vivo. Our study defines a sensitive yeast system for the identification and characterization of mutations in FEN1.