Impact of diabetes mellitus and poor glycemic control on the prevalence of left atrial low-voltage areas and rhythm outcome in patients with atrial fibrillation ablation

INTRODUCTION

Left atrial low-voltage areas (LVAs) are known to be correlated with atrial scarring and atrial fibrillation (AF) recurrence after ablation. However, the association between LVAs and glycemic status before ablation has not been fully clarified. The purpose of this study was to investigate associations among the prevalence of diabetes mellitus (DM), glycemic control, and the prevalence of LVAs in patients with AF ablation.

METHODS

In total, 912 (age, 68 ± 10 years; female, 299 [33%]; persistent AF, 513 [56%]) consecutive patients who underwent initial AF ablation were included. A preprocedure glycated hemoglobin A1c (HbA1c) ≥7% was set as the cutoff for poor glycemic control in patients with DM. LVAs were defined as areas with a bipolar voltage of <0.5 mV covering ≥5 cm2 of left atrium.

RESULTS

LVAs existed in 208 (23%) patients, and 168 (18%) patients had DM. LVAs were found more frequently in patients with DM and poor glycemic control. On multivariate analysis, DM with HbA1c ≥7% was an independent predictor of LVAs (odds ratio, 3.3; 95% confidence interval: 1.6-6.7; p = .001). In patients with LVAs, freedom from AF recurrence during the 24-month study period was significantly lower in patients who had DM with HbA1c ≥7% than in those without DM (37.9% vs. 54.7%, p = .02).

CONCLUSION

In patients with AF ablation, LVAs were found more frequently in patients with DM and poor glycemic control. DM with HbA1c ≥7% was an independent predictor of LVAs.

Prognostic impact of atrial cardiomyopathy: Long-term follow-up of patients with and without low-voltage areas following atrial fibrillation ablation

BACKGROUND

Atrial cardiomyopathy is known as an underlying pathophysiological factor in the majority of patients with atrial fibrillation (AF). Left atrial low-voltage areas (LVAs) are reported to coincide with fibrosis and likely represent atrial cardiomyopathy.

OBJECTIVE

The purpose of this study was to delineate differences in the long-term prognosis of patients stratified by the size of LVAs.

METHODS

This observational study included 1488 consecutive patients undergoing initial ablation for AF. LVAs were defined as regions with a bipolar peak-to-peak voltage <0.50 mV. The total study population was divided into 3 groups stratified by LVA size: patients with no LVAs (n = 1136); those with small (<20 cm2) LVAs (n = 250) LVAs; and those with extensive (≥20 cm2) LVAs (n = 102). Composite endpoints of death, heart failure, and stroke were followed for up to 5 years.

RESULTS

Composite endpoints developed in 105 of 1488 patients (7.1%), and AF recurrence occurred in 410 (27.6%). Composite endpoints developed more frequently in the order of patients with extensive LVAs (19.1%), small LVAs (10.8%), and no LVAs (5.1%) (P for trend <.0001). Multivariable analysis revealed that LVA presence was independently associated with higher incidence of composite endpoints, irrespective of AF recurrence (modified hazard ratio 1.73; 95% confidence interval 1.13-2.64; P = .011) CONCLUSION: LVA presence and its extent both were associated with poor long-term composite endpoints of death, heart failure, and stroke, irrespective of AF recurrence or other confounders. Underlying atrial cardiomyopathy seems to define a poor prognosis after AF ablation.

Clinical impact of left atrial remodeling pattern in patients with atrial fibrillation: Comparison of volumetric, electrical, and combined remodeling

INTRODUCTION

Atrial fibrillation (AF) is accompanied by various types of remodeling, including volumetric enlargement and histological degeneration. Electrical remodeling reportedly reflects histological degeneration.

PURPOSE

To clarify the differences in determinants and clinical impacts among types of remodeling.

METHODS

This observational study included 1118 consecutive patients undergoing initial ablation for AF. Patients were divided into four groups: minimal remodeling (left atrial volume index [LAVI] < mean value and no low-voltage area [LVA], n = 477); volumetric remodeling (LAVI ≥ mean value and no LVA, n = 361); electrical remodeling (LAVI < mean value and LVA presence, n = 96); and combined remodeling (LAVI ≥ mean value and LVA presence, n = 184). AF recurrence and other clinical outcomes were followed up for 2 and 5 years, respectively.

RESULTS

Major determinants of each remodeling pattern were high age for electrical (odds ratio = 2.32, 95% confidence interval = 1.68-3.25) and combined remodeling (2.57, 1.88-3.49); female for electrical (3.85, 2.21-6.71) and combined remodeling (4.92, 2.90-8.25); persistent AF for combined remodeling (7.09, 3.75-13.4); and heart failure for volumetric (1.71, 1.51-2.53) and combined remodeling (2.21, 1.30-3.75). Recurrence rate after initial ablation increased in the order of minimal remodeling (20.1%), volumetric (27.4%) or electrical remodeling (36.5%), and combined remodeling (50.0%, p < .0001). A composite endpoint of heart failure, stroke, and death occurred in the order of minimal (3.4%), volumetric (7.5%) or electrical (8.3%), and combined remodeling (15.2%, p < .0001).

CONCLUSION

Volumetric, electrical, and combined remodeling were each associated with a unique patient background, and defined rhythm and other clinical outcomes.

Gender Differences in Atrial Fibrosis and Cardiomyopathy Assessed by Left Atrial Low-Voltage Areas During Catheter Ablation of Atrial Fibrillation

Atrial myocardial degeneration predisposes to atrial fibrillation (AF), ischemic stroke, and heart failure. Studies suggest the presence of gender differences in atrial myocardial degeneration. This study aimed to delineate gender differences in the prevalence, predictors, and prognostic impact of left atrial low-voltage areas (LVAs). This observational study included 1,488 consecutive patients who underwent initial ablation for AF. Voltage mapping was performed after pulmonary vein isolation during sinus rhythm. LVAs were defined as regions where bipolar peak-to-peak voltage was <0.50 mV. LVA prevalence was higher in women (38.7%) than in men (16.0%). High age, persistent form of AF, diabetes mellitus, and a large left atrium were shown to be common predictors in both gender categories. Heart failure and history of stroke/thromboembolic events were men-specific predictors of LVA existence. Women experienced more AF recurrence than men (31.1% vs 25.7%, p = 0.027). LVA existence was significantly associated with increased AF recurrence in each gender category, with a respective hazard ratio, 95% confidence interval, and p value of 2.45, 1.87 to 3.22, and <0.0001 in men and 1.82, 1.33 to 2.49, and <0.0001 in women. In conclusion, LVA was more frequent in women than men, and predicted frequent AF recurrence irrespective of gender category.

Synthesis of bioactive PMMA bone cement via modification with methacryloxypropyltri-methoxysilane and calcium acetate

Bone cement consisting of polymethylmethacrylate (PMMA) powder and methylmethacrylate (MMA) liquid is clinically used for fixation of implants such as artificial hip joints. However, it does not show bone-bonding ability, i.e., bioactivity. The lack of bioactivity would be one of factors which cause loosening between the cement and the implant. The present authors recently showed the potential of bioactive PMMA-based bone cement through modification with gamma-methacryloxypropyltrimethoxysilane (MPS) and calcium acetate. In this study, the effects of the kinds of PMMA powder on setting time, apatite formation and compressive strength were investigated in a simulated body fluid (Kokubo solution). The cement modified with calcium acetate calcined at 220 degrees C could set within 15 min when the PMMA powder had an average molecular weight of 100,000 or less. The addition of calcium acetate calcined at 120 degrees C in the PMMA powder required a much longer period for setting. The modified cements formed an apatite layer after soaking in the Kokubo solution within 1 day for cement starting from PMMA powder with a molecular weight of 100,000 or less. Compressive strengths of the modified cements were more than 70 MPa for cements starting from 100,000 and 56,000 in molecular weight. After soaking in Kokubo solution for 7 days, the modified cement consisting of PMMA powder of 100,000 in molecular weight showed a smaller decrease in compressive strength than that consisting of 56,000 in molecular weight. These results indicate that bioactive PMMA cement can be produced with appropriate setting time and mechanical strength when PMMA powders with a suitable molecular weight are used. Such a type of design of bioactive PMMA bone cement leads to a novel development of bioactive material for bone substitutes.

Yeast replicative DNA polymerases and their role at the replication fork

The budding yeast, Saccharomyces cerevisiae, is an excellent model system for the study of DNA polymerases and their roles in DNA replication, repair, and recombination. Presently ten DNA polymerases have been purified and characterized from S. cerevisiae. Rapid advances in genome sequencing projects for yeast and other organisms have greatly facilitated and accelerated the identification of yeast enzymes and their homologues in other eukaryotic species. This article reviews current available research on yeast DNA polymerases and their functional roles in DNA metabolism. Relevant information about eukaryotic homologues of these enzymes will also be discussed.

DNA polymerase epsilon is required for coordinated and efficient chromosomal DNA replication in Xenopus egg extracts

DNA polymerase epsilon (Pol epsilon) is thought to be involved in DNA replication, repair, and cell-cycle checkpoint control in eukaryotic cells. Although the requirement of other replicative DNA polymerases, DNA polymerases alpha and delta (Pol alpha and delta), for chromosomal DNA replication has been well documented by genetic and biochemical studies, the precise role, if any, of Pol epsilon in chromosomal DNA replication is still obscure. Here we show, with the use of a cell-free replication system with Xenopus egg extracts, that Xenopus Pol epsilon is indeed required for chromosomal DNA replication. In Pol epsilon-depleted extracts, the elongation step of chromosomal DNA replication is markedly impaired, resulting in significant reduction of the overall DNA synthesis as well as accumulation of small replication intermediates. Moreover, despite the decreased DNA synthesis, excess amounts of Pol alpha are loaded onto the chromatin template in Pol epsilon-depleted extracts, indicative of the failure of proper assembly of DNA synthesis machinery at the fork. These findings strongly suggest that Pol epsilon, along with Pol alpha and Pol delta, is necessary for coordinated chromosomal DNA replication in eukaryotic cells.

Interactions between Mcm10p and other replication factors are required for proper initiation and elongation of chromosomal DNA replication in Saccharomyces cerevisiae

BACKGROUND

MCM10 is essential for the initiation of chromosomal DNA replication in Saccharomyces cerevisiae. Previous work showed that Mcm10p interacts with the Mcm2-7 protein complex that may be functioning as the replication-licensing factor. In addition, Mcm10p is required during origin activation and disassembly of the prereplicative complex, which allows smooth passage of replication forks.

RESULTS

We show that an mcm10 mutation causes a slow progression of DNA synthesis and a loss of chromosome integrity during the S phase and prevents entry into mitosis, despite apparent completion of chromosomal DNA replication at nonpermissive temperatures. Furthermore, Mcm10p interacts genetically with the origin recognition complex (ORC) and various replication elongation factors, including a subunit of DNA polymerases epsilon and delta. Mcm10p is an abundant protein (approximately 4 x 10(4) copies per haploid cell) that is almost exclusively localized in the chromatin and/or nuclear matrix fractions during all phases of the cell cycle. When it is visualized by the chromosome-spreading method followed by immunostaining, Mcm10p forms punctate foci on chromatin throughout the cell cycle and these foci mostly overlap with those of Orc1p, a component of ORC.

CONCLUSIONS

These results suggest that Mcm10p, like the Mcm2-7 proteins, is a critical component of the prereplication chromatin and acts together with ORC during the initiation of chromosomal DNA replication; in addition, Mcm10p plays an important role during the elongation of DNA replication.

Characterization of the yeast Cdc7p/Dbf4p complex purified from insect cells. Its protein kinase activity is regulated by Rad53p

The yeast Saccharomyces cerevisiae Cdc7p/Dbf4p protein kinase complex was purified to near homogeneity from insect cells. The complex efficiently phosphorylated yeast Mcm2p and less efficiently the remaining Mcm proteins or other replication proteins. Significantly, when pretreated with alkaline phosphatase, Mcm2p became completely inactive as a substrate, suggesting that it must be phosphorylated by other protein kinase(s) to be a substrate for the Cdc7p/Dbf4p complex. Mutant Cdc7p/Dbf4p complexes containing either Cdc7-1p or Dbf4-1 approximately 5p were also partially purified from insect cells and characterized in vitro. Furthermore, the autonomously replicating sequence binding activity of various dbf4 mutants was also analyzed. These studies suggest that the autonomously replicating sequence-binding and Cdc7p protein kinase activation domains of Dbf4p collaborate to form an active Cdc7p/Dbf4p complex and function during S phase in S. cerevisiae. It is shown that Rad53p phosphorylates the Cdc7p/Dbf4p complex in vitro and that this phosphorylation greatly inhibits the kinase activity of Cdc7p/Dbf4p. This result suggests that Rad53p controls the initiation of chromosomal DNA replication by regulating the protein kinase activity associated with the Cdc7p/Dbf4p complex.

Structure and function of the fourth subunit (Dpb4p) of DNA polymerase epsilon in Saccharomyces cerevisiae

DNA polymerase epsilon (Polepsilon) of Saccharomyces cerevisiae is purified as a complex of four polypeptides with molecular masses of >250, 80, 34 (and 31) and 29 kDa as determined by SDS-PAGE. The genes POL2, DPB2 and DPB3, encoding the catalytic Pol2p, the second (Dpb2p) and the third largest subunits (Dpb3p) of the complex, respectively, were previously cloned and characterised. This paper reports the partial amino acid sequence of the fourth subunit (Dpb4p) of Polepsilon. This protein sequence matches parts of the predicted amino acid sequence from the YDR121w open reading frame on S.cerevisiae chromosome IV. Thus, YDR121w was renamed DPB4. A deletion mutant of DPB4 (Deltadpb4) is not lethal, but chromosomal DNA replication is slightly disturbed in this mutant. A double mutant haploid strain carrying the Deltadpb4 deletion and either pol2-11 or dpb11-1 is lethal at all temperatures tested. Furthermore, the restrictive temperature of double mutants carrying Deltadpb4 and dpb2-1, rad53-1 or rad53-21 is lower than in the corresponding single mutants. These results strongly suggest that Dpb4p plays an important role in maintaining the complex structure of Polepsilon in S.cerevisiae, even if it is not essential for cell growth. Structural homologues of DPB4 are present in other eukaryotic genomes, suggesting that the complex structure of S. cerevisiae Polepsilon is conserved in eukaryotes.

Transcription of the catalytic 180-kDa subunit gene of mouse DNA polymerase alpha is controlled by E2F, an Ets-related transcription factor, and Sp1

We have isolated a genomic DNA fragment spanning the 5'-end of the gene encoding the catalytic subunit of mouse DNA polymerase alpha. The nucleotide sequence of the upstream region was G/C-rich and lacked a TATA box. Transient expression assays in cycling NIH 3T3 cells demonstrated that the GC box of 20 bp (at nucleotides -112/-93 with respect to the transcription initiation site) and the palindromic sequence of 14 bp (at nucleotides -71/-58) were essential for basal promoter activity. Electrophoretic mobility shift assays showed that Sp1 binds to the GC box. We also purified a protein capable of binding to the palindrome and identified it as GA-binding protein (GABP), an Ets- and Notch-related transcription factor. Transient expression assays in synchronized NIH 3T3 cells revealed that three variant E2F sites near the transcription initiation site (at nucleotides -23/-16, -1/+7 and +17/+29) had no basal promoter activity by themselves, but were essential for growth-dependent stimulation of the gene expression. These data indicate that E2F, GABP and Sp1 regulate the gene expression of this principal replication enzyme.

A Cdc7p-Dbf4p protein kinase activity is conserved from yeast to humans

DBF4 and CDC7 were identified as budding yeast cell cycle mutants that arrest immediately before S phase. The Dbf4p and Cdc7p proteins interact to form a protein kinase, Cdc7p being the catalytic subunit and Dbf4p is a cyclin-like molecule that activates the kinase in late G1. Dbf4p also targets Cdc7p to origins of replication where likely substrates include the Mcm proteins. Dbf4p and Cdc7p related proteins occur in the fission yeast and in metazoans. These also phosphorylate Mcm proteins and preliminary evidence indicates a similar function to Dbf4p/Cdc7p in budding yeast. The Dbf4p/Cdc7p activity will therefore very likely be conserved in all eukaryotes.

Dpb11 controls the association between DNA polymerases alpha and epsilon and the autonomously replicating sequence region of budding yeast

Dpb11 is required for chromosomal DNA replication and the S-phase checkpoint in Saccharomyces cerevisiae. Here, we report detection of a physical complex containing Dpb11 and DNA polymerase epsilon (Dpb11-Polepsilon complex). During the S phase of the cell cycle, Dpb11 associated preferentially with DNA fragments containing autonomously replicating sequences (ARSs), at the same time as Polepsilon associated with these fragments. Association of Dpb11 and Polepsilon with these fragments was mutually dependent, suggesting that the Dpb11-Polepsilon complex associates with the ARS. Moreover, Dpb11 was required for the association of Polalpha-primase with the fragments. Thus, it seems likely that association of the Dpb11-Polepsilon complex with the ARS fragments is required for the association of the Polalpha-primase complex. Hydroxyurea inhibits late-origin firing in S. cerevisiae, and the checkpoint genes, RAD53 and MEC1, are involved in this inhibition. In the presence of hydroxyurea at temperatures permissive for cell growth, Polepsilon in dpb11-1 cells associated with early- and late-origin fragments. In wild-type cells, however, it associated only with early-origin fragments. This indicates that Dpb11 may also be involved in the regulation of late-origin firing. Overall, these results suggest that Dpb11 controls the association between DNA polymerases alpha and epsilon and the ARS.

First the CDKs, now the DDKs

In budding yeast, Dbf4p and Cdc7p control initiation of DNA synthesis. They form a protein kinase - Cdc7p being the catalytic subunit and Dbf4p a cyclin-like molecule that activates the kinase in late G1 phase. Dbf4p also targets Cdc7p to origins of replication, where probable substrates include certain Mcm proteins. Recent studies have identified Dbf4p- and Cdc7p-related proteins in fission yeast and metazoans. These homologues also phosphorylate Mcm proteins and could have a similar function to that of Dbf4p-Cdc7p in budding yeast. Thus, it seems likely that, like the cyclin-dependent kinases (CDKs), the Dbf4p-Cdc7p activity is conserved in all eukaryotes.

DNA helicase III of Saccharomyces cerevisiae, encoded by YER176w (HEL1), highly unwinds covalently closed, circular DNA in the presence of a DNA topoisomerase and yRF-A

Previously, we have purified and characterized DNA helicase III from the yeast Saccharomyces cerevisiae [Shimizu, K. and Sugino, A. (1993) J. Biol. Chem. 268, 9578-9584]. Here, we have further characterized DNA helicase III activity. It was found that the combined action of the helicase III, yeast DNA topoisomerase I (yTop I), and yeast RPA protein on a covalently closed, circular DNA generates a highly underwound DNA species that has been called form I* or form U. Furthermore, these underwound structures can be accessed by yeast DNA polymerase I (alpha)-primase to initiate DNA synthesis. These reactions mimic in vivo initiation of chromosomal DNA replication. In order to clone the gene encoding DNA helicase III, a partial amino acid sequence of the purified DNA helicase III polypeptide was determined. Using a mix oligonucleotides synthesized based on the amino acid sequence of the helicase, we cloned the gene encoding the helicase III and found it to be identical to YER176W (HEL1) on chromosome V. The amino acid sequence predicted from the nucleotide sequence of the gene has conserved DNA helicase domains that are highly homologous to those of DNA helicases required for DNA replication. However, complete deletion of the gene from the chromosome did not result in any growth defect, suggesting that the gene product is not required for DNA synthesis or that it is functionally substituted by other helicase(s). Furthermore, the deletion strain does not exhibit sensitivity to any DNA-damaging reagents, although it is hypersensitive to calcofluor white, hygromycin, and papulacandin.

Sld2, which interacts with Dpb11 in Saccharomyces cerevisiae, is required for chromosomal DNA replication

The DPB11 gene, which genetically interacts with DNA polymerase II (epsilon), one of three replicative DNA polymerases, is required for DNA replication and the S phase checkpoint in Saccharomyces cerevisiae. To identify factors interacting with Dbp11, we have isolated sld (synthetically lethal with dpb11-1) mutations which fall into six complementation groups (sld1 to -6). In this study, we characterized SLD2, encoding an essential 52-kDa protein. High-copy SLD2 suppressed the thermosensitive growth defect caused by dpb11-1. Conversely, high-copy DPB11 suppressed the temperature-sensitive growth defect caused by sld2-6. The interaction between Sld2 and Dpb11 was detected in a two-hybrid assay. This interaction was evident at 25 degreesC but not at 34 degreesC when Sld2-6 or Dpb11-1 replaced its wild-type protein. No interaction between Sld2-6 and Dpb11-1 could be detected even at 25 degreesC. Immunoprecipitation experiments confirmed that Dpb11 physically interacts with Sld2. sld2-6 cells were defective in DNA replication at the restrictive temperature, as were dpb11-1 cells. Further, in dpb11-1 and sld2-6 cells, the bubble-shaped replication intermediates formed in the region of the autonomously replicating sequence reduced quickly after a temperature shift. These results strongly suggest the involvement of the Dpb11-Sld2 complex in a step close to the initiation of DNA replication.

DNA polymerase II (epsilon) of Saccharomyces cerevisiae dissociates from the DNA template by sensing single-stranded DNA

Two forms of DNA polymerase II (epsilon) of Saccharomyces cerevisiae, Pol II* and Pol II, were purified to near homogeneity from yeast cells. Pol II* is a four-subunit complex containing a 256-kDa catalytic polypeptide, whereas Pol II consists solely of a 145-kDa polypeptide derived from the N-terminal half of the 256-kDa polypeptide of Pol II*. We show that Pol II* and Pol II are indistinguishable with respect to the processivity and rate of DNA-chain elongation. The equilibrium dissociation constants of the complexes of Pol II* and Pol II with the DNA template showed that the stability of these complexes is almost the same. However, when the rates of dissociation of the Pol II* and Pol II from the DNA template were measured using single-stranded DNA as a trap for the dissociated polymerase, Pol II* dissociated 75-fold faster than Pol II. Furthermore, the rate of dissociation of Pol II* from the DNA template became faster as the concentration of the single-stranded DNA was increased. These results indicate that the rapid dissociation of Pol II* from the DNA template is actively promoted by single-stranded DNA. The dissociation of Pol II from the DNA template was also shown to be promoted by single-stranded DNA, although at a much slower rate. These results suggest that the site for sensing single-stranded DNA resides within the 145-kDa N-terminal portion of the catalytic subunit and that the efficiency for sensing single-stranded DNA by this site is positively modulated by either the C-terminal half of the catalytic subunit and/or the other subunits.

The RFC2 gene, encoding the third-largest subunit of the replication factor C complex, is required for an S-phase checkpoint in Saccharomyces cerevisiae

Replication factor C (RF-C), an auxiliary factor for DNA polymerases delta and epsilon, is a multiprotein complex consisting of five different polypeptides. It recognizes a primer on a template DNA, binds to a primer terminus, and helps load proliferating cell nuclear antigen onto the DNA template. The RFC2 gene encodes the third-largest subunit of the RF-C complex. To elucidate the role of this subunit in DNA metabolism, we isolated a thermosensitive mutation (rfc2-1) in the RFC2 gene. It was shown that mutant cells having the rfc2-1 mutation exhibit (i) temperature-sensitive cell growth; (ii) defects in the integrity of chromosomal DNA at restrictive temperatures; (iii) progression through cell cycle without definitive terminal morphology and rapid loss of cell viability at restrictive temperatures; (iv) sensitivity to hydroxyurea, methyl methanesulfonate, and UV light; and (v) increased rate of spontaneous mitotic recombination and chromosome loss. These phenotypes of the mutant suggest that the RFC2 gene product is required not only for chromosomal DNA replication but also for a cell cycle checkpoint. It was also shown that the rfc2-1 mutation is synthetically lethal with either the cdc44-1 or rfc5-1 mutation and that the restrictive temperature of rfc2-1 mutant cells can be lowered by combining either with the cdc2-2 or pol2-11 mutation. Finally, it was shown that the temperature-sensitive cell growth phenotype and checkpoint defect of the rfc2-1 mutation can be suppressed by a multicopy plasmid containing the RFC5 gene. These results suggest that the RFC2 gene product interacts with the CDC44/RFC1 and RFC5 gene products in the RF-C complex and with both DNA polymerases delta and epsilon during chromosomal DNA replication.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

The second subunit of DNA polymerase III (delta) is encoded by the HYS2 gene in Saccharomyces cerevisiae

DNA polymerase III (delta) of Saccharomyces cerevisiae is purified as a complex of at least two polypeptides with molecular masses of 125 and 55 kDa as judged by SDS-PAGE. In this paper we determine partial amino acid sequences of the 125 and 55 kDa polypeptides and find that they match parts of the amino acid sequences predicted from the nucleotide sequence of the CDC2 and HYS2 genes respectively. We also show by Western blotting that Hys2 protein co-purifies with DNA polymerase III activity as well as Cdc2 polypeptide. The complex form of DNA polymerase III activity could not be detected in thermosensitive hys2 mutant cell extracts, although another form of DNA polymerase III was found. This form of DNA polymerase III, which could also be detected in wild-type extracts, was not associated with Hys2 protein and was not stimulated by addition of proliferating cell nuclear antigen (PCNA), replication factor A (RF-A) or replication factor C (RF-C). The temperature-sensitive growth phenotype of hys2-1 and hys2-2 mutations could be suppressed by the CDC2 gene on a multicopy plasmid. These data suggest that the 55 kDa polypeptide encoded by the HYS2 gene is one of the subunits of DNA polymerase III complex in S.cerevisiae and is required for highly processive DNA synthesis catalyzed by DNA polymerase III in the presence of PCNA, RF-A and RF-C.

Mcm2 is a target of regulation by Cdc7-Dbf4 during the initiation of DNA synthesis

The initiation of DNA synthesis is an important cell cycle event that defines the beginning of S phase. This critical event involves the participation of proteins whose functions are regulated by cyclin dependent protein kinases (Cdks). The Mcm2-7 proteins are a family of six conserved proteins that are essential for the initiation of DNA synthesis in all eukaryotes. In Saccharomyces cerevisiae, members of the Mcm2-7 family undergo cell cycle-specific phosphorylation. Phosphorylation of Mcm proteins at the beginning of S phase coincides with the removal of these proteins from chromatin and the onset of DNA synthesis. In this study, we identified DBF4, which encodes the regulatory subunit of a Cdk-like protein kinase Cdc7-Dbf4, in a screen for second site suppressors of mcm2-1. The dbf4 suppressor mutation restores competence to initiate DNA synthesis to the mcm2-1 mutant. Cdc7-Dbf4 interacts physically with Mcm2 and phosphorylates Mcm2 and three other members of the Mcm2-7 family in vitro. Blocking the kinase activity of Cdc7-Dbf4 at the G1-to-S phase transition also blocks the phosphorylation of Mcm2 at this defined point of the cell cycle. Taken together, our data suggest that phosphorylation of Mcm2 and probably other members of the Mcm2-7 proteins by Cdc7-Dbf4 at the G1-to-S phase transition is a critical step in the initiation of DNA synthesis at replication origins.

Rfc5, a small subunit of replication factor C complex, couples DNA replication and mitosis in budding yeast

The inhibition of DNA synthesis prevents mitotic entry through the action of the S phase checkpoint. In the yeast Saccharomyces cerevisiae, an essential protein kinase, Spk1/Mec2/Rad53/Sad1, controls the coupling of S phase to mitosis. In an attempt to identify genes that genetically interact with Spk1, we have isolated a temperature-sensitive mutation, rfc5-1, that can be suppressed by overexpression of SPK1. The RFC5 gene encodes a small subunit of replication factor C complex. At the restrictive temperature, rfc5-1 mutant cells entered mitosis with unevenly separated or fragmented chromosomes, resulting in loss of viability. Thus, the rfc5 mutation defective for DNA replication is also impaired in the S phase checkpoint. Overexpression of POL30, which encodes the proliferating cell nuclear antigen, suppressed the replication defect of the rfc5 mutant but not its checkpoint defect. Taken together, these results suggested that replication factor C has a direct role in sensing the state of DNA replication and transmitting the signal to the checkpoint machinery.

Dpb11, which interacts with DNA polymerase II(epsilon) in Saccharomyces cerevisiae, has a dual role in S-phase progression and at a cell cycle checkpoint

DPB11, a gene that suppresses mutations in two essential subunits of Saccharomyces cerevisiae DNA polymerase II(epsilon) encoded by POL2 and DPB2, was isolated on a multicopy plasmid. The nucleotide sequence of the DPB11 gene revealed an open reading frame predicting an 87-kDa protein. This protein is homologous to the Schizosaccharomyces pombe rad4+/cut5+ gene product that has a cell cycle checkpoint function. Disruption of DPB11 is lethal, indicating that DPB11 is essential for cell proliferation. In thermosensitive dpb11-1 mutant cells, S-phase progression is defective at the nonpermissive temperature, followed by cell division with unequal chromosomal segregation accompanied by loss of viability.dpb11-1 is synthetic lethal with any one of the dpb2-1, pol2-11, and pol2-18 mutations at all temperatures. Moreover, dpb11 cells are sensitive to hydroxyurea, methyl methanesulfonate, and UV irradiation. These results strongly suggest that Dpb11 is a part of the DNA polymerase II complex during chromosomal DNA replication and also acts in a checkpoint pathway during the S phase of the cell cycle to sense stalled DNA replication.

Replication slippage between distant short repeats in Saccharomyces cerevisiae depends on the direction of replication and the RAD50 and RAD52 genes

Small direct repeats, which are frequent in all genomes, are a potential source of genome instability. To study the occurrence and genetic control of repeat-associated deletions, we developed a system in the yeast Saccharomyces cerevisiae that was based on small direct repeats separated by either random sequences or inverted repeats. Deletions were examined in the LYS2 gene, using a set of 31- to 156-bp inserts that included inserts with no apparent potential for secondary structure as well as two quasipalindromes. All inserts were flanked by 6- to 9-bp direct repeats of LYS2 sequence, providing an opportunity for Lys+ reversion via precise excision. Reversions could arise by extended deletions involving either direct repeats or random sequences and by -1-or +2-bp frameshift mutations. The deletion breakpoints were always associated with short (3- to 9-bp) perfect or imperfect direct repeats. Compared with the POL+ strain, deletions between small direct repeats were increased as much as 100-fold, and the spectrum was changed in a temperature-sensitive DNA polymerase delta pol3-t mutant, suggesting a role for replication. The type of deletion depended on orientation relative to the origin of replication. On the basis of these results, we propose (i) that extended deletions between small repeats arise by replication slippage and (ii) that the deletions occur primarily in either the leading or lagging strand. The RAD50 and RAD52 genes, which are required for the recombinational repair of many kinds of DNA double-strand breaks, appeared to be required also for the production of up to 90% of the deletions arising between separated repeats in the pol3-t mutant, suggesting a newly identified role for these genes in genome stability and possibly replication.

Yeast DNA polymerases and their role at the replication fork

The DNA polymerases of the yeast Saccharomyces cerevisiae serve as a model system for the study of the replication fork during DNA replication. To date, six S. cerevisiae DNA polymerases have been at least partially characterized (compared with four in mammals so far), with further candidates being identified as open reading frames in the yeast genome sequencing project. Here, we review the current state of knowledge of the yeast polymerases, and discuss, where possible, their biological role during DNA replication.

The yeast Saccharomyces cerevisiae DNA polymerase IV: possible involvement in double strand break DNA repair

We identified and purified a new DNA polymerase (DNA polymerase IV), which is similar to mammalian DNA polymerase beta, from Saccharomyces cerevisiae and suggested that it is encoded by YCR14C (POLX) on chromosome III. Here, we provided a direct evidence that the purified DNA polymerase IV is indeed encoded by POLX. Strains harboring a pol4 deletion mutation exhibit neither mitotic growth defect nor a meiosis defect, suggesting that DNA polymerase IV participates in nonessential functions in DNA metabolism. The deletion strains did not exhibit UV-sensitivity. However, they did show weak sensitivity to MMS-treatment and exhibited a hyper-recombination phenotype when intragenic recombination was measured during meiosis. Furthermore, MAT alpha pol4 delta segregants had a higher frequency of illegitimate mating with a MAT alpha tester strain than that of wild-type cells. These results suggest that DNA polymerase IV participates in a double-strand break repair pathway. A 3.2kb of the POL4 transcript was weakly expressed in mitotically growing cells. During meiosis, a 2.2 kb POL4 transcript was greatly induced, while the 3.2 kb transcript stayed at constant levels. This induction was delayed in a swi4 delta strain during meiosis, while no effect was observed in a swi6 delta strain.

The RFC2 gene encoding a subunit of replication factor C of Saccharomyces cerevisiae

Replication Factor C (RF-C) of Saccharomyces cerevisiae is a complex that consists of several different polypeptides ranging from 120- to 37 kDa (Yoder and Burgers, 1991; Fien and Stillman, 1992), similar to human RF-C. We have isolated a gene, RFC2, that appears to be a component of the yeast RF-C. The RFC2 gene is located on chromosome X of S. cerevisiae and is essential for cell growth. Disruption of the RFC2 gene led to a dumbbell-shaped terminal morphology, common to mutants having a defect in chromosomal DNA replication. The steady-state levels of RFC2 mRNA fluctuated less during the cell cycle than other genes involved in DNA replication. Nucleotide sequence of the gene revealed an open reading frame corresponding to a polypeptide with a calculated Mr of 39,716 and a high degree of amino acid sequence homology to the 37-kDa subunit of human RF-C. Polyclonal antibodies against bacterially expressed Rfc2 protein specifically reduced RF-C activity in the RF-C-dependent reaction catalyzed by yeast DNA polymerase III. Furthermore, the Rfc2 protein was copurified with RF-C activity throughout RF-C purification. These results strongly suggest that the RFC2 gene product is a component of yeast RF-C. The bacterially expressed Rfc2 protein preferentially bound to primed single-strand DNA and weakly to ATP.

Molecular analysis of the dhp1+ gene of Schizosaccharomyces pombe: an essential gene that has homology to the DST2 and RAT1 genes of Saccharomyces cerevisiae

The DST2 gene of Saccharomyces cerevisiae encodes a DNA strand exchange protein, STP beta, which is required for homologous recombination in both mitotic or meiotic cells. We have cloned a DST2-related gene from the fission yeast Schizosaccharomyces pombe and designated it dhp1+. The nucleotide sequence of dhp1+ revealed an open reading frame encoding a protein composed of 991 amino acids. The predicted amino acid sequence was significantly homologous to the S. cerevisiae STP beta, but lacked the carboxy-terminal sequence present in STP beta. Furthermore, dhp1+ shows greater homology to RAT1/HKE1, a gene which is involved in RNA trafficking and processing. Genetic experiments showed that dhp1+ on an S. cerevisiae expression vector could rescue both the defects of the S. cerevisiae DST2 disruptant, slow growth rate and a sporulation defect, and the lethality of the S. cerevisiae rat1ts mutation. This implies the functional similarity of dhp1+ to both DST2 and RAT1. However unlike DST2, dhp1+ is an essential gene for cell growth in S. pombe, suggesting that dhp1+ is not the true homologue of DST2 but rather of RAT1 in S. pombe. The possible roles of dhp1+ in recombination and cell growth in S. pombe are discussed.

The 3'-->5' exonucleases of both DNA polymerases delta and epsilon participate in correcting errors of DNA replication in Saccharomyces cerevisiae

DNA polymerases II (epsilon) and III (delta) are the only nuclear DNA polymerases known to possess an intrinsic 3'-->5' exonuclease in Saccharomyces cerevisiae. We have investigated the spontaneous mutator phenotypes of DNA polymerase delta and epsilon 3'-->5' exonuclease-deficient mutants, pol3-01 and pol2-4, respectively. pol3-01 and pol2-4 increased spontaneous mutation rates by factors of the order of 10(2) and 10(1), respectively, measured as URA3 forward mutation and his7-2 reversion. Surprisingly, a double mutant pol2-4 pol3-01 haploid was inviable. This was probably due to accumulation of unedited errors, since a pol2-4/pol2-4 pol3-01/pol3-01 diploid was viable, with the spontaneous his7-2 reversion rate increased by about 2 x 10(3)-fold. Analysis of mutation rates of double mutants indicated that the 3'-->5' exonucleases of DNA polymerases delta and epsilon can act competitively and that, like the 3'-->5' exonuclease of DNA polymerase delta, the 3'-->5' exonuclease of DNA polymerase epsilon acts in series with the PMS1 mismatch correction system. Mutational spectra at a URA3 gene placed in both orientations near to a defined replication origin provided evidence that the 3'-->5' exonucleases of DNA polymerases delta and epsilon act on opposite DNA strands, but were in sufficient to distinguish conclusively between different models of DNA replication.

Purification and characterization of a new DNA polymerase from budding yeast Saccharomyces cerevisiae. A probable homolog of mammalian DNA polymerase beta

A new DNA polymerase activity was identified and purified to near homogeneity from extracts of mitotic and meiotic cells of the yeast Saccharomyces cerevisiae. This activity increased at least 5-fold during meiosis, and it was shown to be associated with a 68-kDa polypeptide as determined by SDS-polyacrylamide gel electrophoresis. This new DNA polymerase did not have any detectable 3'-->5' exonuclease activity and preferred small gapped DNA as a template-primer. The activity was inhibited by dideoxyribonucleoside 5'-triphosphates and N-ethylmaleimide but not by concentrations of aphidicolin which completely inhibit either DNA polymerases I (alpha), II (epsilon), or III (delta). Since no polypeptide(s) in the extensively purified DNA polymerase fractions cross-reacted with antibodies raised against yeast DNA polymerases I, II, and III, we called this enzyme DNA polymerase IV. The DNA polymerase IV activity increased at least 10-fold in a yeast strain overexpressing the gene product predicted from the YCR14C open-reading frame (identified on S. cerevisiae chromosome III and provisionally called POLX), while no activity was detected in a strain where POLX was deleted. These results strongly suggest that DNA polymerase IV is encoded by the POLX gene and is a probable homolog of mammalian DNA polymerase beta.

A multicopy suppressor gene of the Saccharomyces cerevisiae G1 cell cycle mutant gene dbf4 encodes a protein kinase and is identified as CDC5

We have isolated a multicopy suppressor of the temperature-sensitive growth phenotype of organisms carrying mutations of DBF4, a gene that is required for the initiation of chromosomal DNA replication in Saccharomyces cerevisiae and that interacts with the CDC7 protein kinase. Nucleotide sequence analysis of the suppressor gene, provisionally named MSD2, revealed an open reading frame encoding a protein with a calculated M(r) of 81,024, with amino acid sequence similarity to the catalytic domains of protein kinases. Both genetic linkage and complementation analyses indicated that MSD2 is identical to the cell division cycle gene CDC5. An activity that phosphorylated exogenously added casein was immunoprecipitated by antiserum against a TrpE-Cdc5 fusion protein from lysates of wild-type cells containing CDC5 on a multicopy plasmid but not of cells bearing a small deletion in the predicted protein kinase domain of CDC5 on the plasmid. Deletion of CDC5 was lethal and resulted in a dumbbell-shaped terminal morphology, with the nuclei almost divided but still connected. Consistent with the function at the G2/M boundary, the CDC5 transcript accumulated periodically during the cell cycle, peaking at the G2/M boundary. CDC5 on a multicopy plasmid also suppresses temperature-sensitive cdc15, cdc20, and dbf2 mutations which affect mitosis during the cell cycle.

Purification and characterization of DNA helicase III from the yeast Saccharomyces cerevisiae

Two forms of DNA helicase activity, Rad3 and ATPase III, were previously purified from the yeast Saccharomyces cerevisiae and characterized. Here, we have identified and purified an additional DNA helicase activity from S. cerevisiae to near homogeneity. This helicase differs from those described previously in its chromatographic behavior, molecular weight, enzymatic properties, and genetic properties. Thus, we named it DNA helicase III. Its apparent molecular mass is about 120 kDa as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. DNA helicase III requires a divalent cation Mg2+ or Mn2+, either ATP or dATP, and a single-stranded portion on the duplex substrate. Helicase III moves in the 5'-->3' direction on single-stranded portions of the substrate and unwinds the strand of DNA in the 3'-->5' direction. It also has an intrinsic DNA-dependent ATPase (dATPase) activity that hydrolyzes either ATP or dATP to ADP or dADP and orthophosphate in the presence of DNA. DNA helicase III activity was not affected by either rad3 or radH mutations, suggesting that it is encoded by a gene different from RAD3 and RADH.

Pathway correcting DNA replication errors in Saccharomyces cerevisiae

Mutation of predicted 3'-->5' exonuclease active site residues of Saccharomyces cerevisiae POL3 DNA polymerase (delta) or deletion of the PMS1 mismatch repair gene lead to relative (to wild type) spontaneous mutation rates of approximately 130 and 41, respectively, measured at a URA3 reporter gene inserted near to a defined replication origin. The POL3 exonuclease-deficient mutant pol3-01 generated most classes of single base mutation in URA3, indicating a broad specificity that generally corresponds to that of the PMS1 system. pol3-01 pms1 haploid cells ceased growth after a few divisions with no unique terminal cell morphology. A pol3-01/pol3-01 pms1/pms1 diploid was viable and displayed an estimated URA3 relative mutation rate of 2 x 10(4), which we calculate to be catastrophically high in a haploid. The relationship between the relative mutation rates of pol3-01 and pms1 was multiplicative, indicating action in series. The PMS1 transcript showed the same cell cycle periodicity as those of a set of DNA replication genes that includes POL3, suggesting PMS1 is co-regulated with these genes. We propose that the POL3 3'-->5' exonuclease and the PMS1 mismatch repair system act on a common pathway analogous to the dnaQ-->mutHLS pathway of DNA replication error correction in Escherichia coli.

Reduction and inactivation of the sarcoplasmic Ca(2+)-ATPase by 2-mercaptoethanol--contrast to the (Na+,K+) ATPase

The sarcoplasmic Ca(2+)-ATPase was reduced with 300 mM 2-mercaptoethanol at elevated temperatures (40-45 degrees C) with a concomitant loss of ATPase activity. The reduction and inactivation of the Ca(2+)-ATPase proceeded rapidly in the absence of Ca2+. The Ca(2+)-ATPase was also inactivated with 2-mercaptoethanol in the presence of diluted SDS (0.4 mg/ml) even at 20 degrees C. In contrast to the (Na+, K+) ATPase, the inactivated Ca(2+)-ATPase in the presence of diluted SDS was sedimented by the centrifugation at 100,000 x g for 30 min.

Temperature-sensitive cdc7 mutations of Saccharomyces cerevisiae are suppressed by the DBF4 gene, which is required for the G1/S cell cycle transition

When present on a multicopy plasmid, a gene from a Saccharomyces cerevisiae genomic library suppresses the temperature-sensitive cdc7-1 mutation. The gene was identified as DBF4, which was previously isolated by complementation in dbf4-1 mutant cells and is required for the G1----S phase progression of the cell cycle. DBF4 has an open reading frame encoding 695 amino acid residues and the predicted molecular mass of the gene product is 80 kD. The suppression is allele-specific because a CDC7 deletion is not suppressed by DBF4. Suppression is mitosis-specific and the sporulation defect of cdc7 mutations is not suppressed by DBF4. Conversely, CDC7 on a multicopy plasmid suppresses the dbf4-1, -2, -3 and -4 mutations but not dbf4-5 and DBF4 deletion mutations. Furthermore, cdc7 mutations are incompatible with the temperature-sensitive dbf4 mutations. These results suggest that the CDC7 and DBF4 polypeptides interact directly or indirectly to permit initiation of yeast chromosome replication.

DNA polymerase II, the probable homolog of mammalian DNA polymerase epsilon, replicates chromosomal DNA in the yeast Saccharomyces cerevisiae

Two temperature-sensitive DNA polymerase II mutants (pol2-9 and pol2-18) of the yeast Saccharomyces cerevisiae were isolated by the plasmid shuffling method. DNA polymerase II activity partially purified from both mutants was thermolabile, while DNA polymerase I and III activities remained thermotolerant. At the restrictive temperature, the pol2 mutants were defective in chromosomal DNA replication and exhibited the dumbbell terminal morphology typical of DNA replication mutants. The POL2 transcript accumulated periodically during the cell cycle, peaking at the G1/S boundary in the same manner as the transcripts of more than 10 other DNA replication genes. These results indicate that DNA polymerase II participates in nuclear DNA replication. The similarities in structure and activities between the DNA polymerases of yeast and mammals make it likely that mammalian DNA polymerase epsilon too is required for chromosomal DNA replication.

Eukaryotic DNA polymerase amino acid sequence required for 3'----5' exonuclease activity

We have identified an amino-proximal sequence motif, Phe-Asp-Ile-Glu-Thr, in Saccharomyces cerevisiae DNA polymerase II that is almost identical to a sequence comprising part of the 3'----5' exonuclease active site of Escherichia coli DNA polymerase I. Similar motifs were identified by amino acid sequence alignment in related, aphidicolin-sensitive DNA polymerases possessing 3'----5' proofreading exonuclease activity. Substitution of Ala for the Asp and Glu residues in the motif reduced the exonuclease activity of partially purified DNA polymerase II at least 100-fold while preserving the polymerase activity. Yeast strains expressing the exonuclease-deficient DNA polymerase II had on average about a 22-fold increase in spontaneous mutation rate, consistent with a presumed proofreading role in vivo. In multiple amino acid sequence alignments of this and two other conserved motifs described previously, five residues of the 3'----5' exonuclease active site of E. coli DNA polymerase I appeared to be invariant in aphidicolin-sensitive DNA polymerases known to possess 3'----5' proofreading exonuclease activity. None of these residues, however, appeared to be identifiable in the catalytic subunits of human, yeast, or Drosophila alpha DNA polymerases.

Cloning DPB3, the gene encoding the third subunit of DNA polymerase II of Saccharomyces cerevisiae

DNA polymerase II purified from Saccharomyces cerevisiae contains polypeptides with apparent molecular masses of greater than 200, 80, 34, 30 and 29 kDa, the two largest of which (subunits A and B) are encoded by the essential genes POL2 and DPB2. By probing a lambda gt11 expression library of yeast DNA with antiserum against DNA polymerase II, we isolated a single gene, DPB3, that encodes both the 34- and 30-kDa polypeptides (subunit C and C'). The nucleotide sequence of DPB3 contained an open reading frame encoding a 23-kDa protein, significantly smaller than the observed molecular masses, 34- or 30-kDa, which might represent post-translationally modified forms of the DPB3 product. The predicted amino acid sequence contained a possible NTP-binding motif and a glutamate-rich region. NTP-binding motif and a glutamate-rich region. A dpb3 deletion mutant (dpb3 delta) was viable and yielded a DNA polymerase II lacking the 34- and 30-kDa polypeptides. dpb3 delta strains exhibited an increased spontaneous mutation rate, suggesting that the DPB3 product is required to maintain fidelity of chromosomal replication. Since a fifth, 29-kDa polypeptide was present in DNA polymerase II preparations from wild-type cell extracts throughout purification, the subunit composition appears to be A, B, C (or C and C') and D. The 5' nontranscribed region of DPB3 contained the MulI-related sequence ACGCGA, while the 0.9-kb DPB3 transcript accumulated periodically during the cell cycle and peaked at the G1/S boundary. The level of DPB3 transcript thus appears to be under the same cell cycle control as those of POL2, DPB2 and other DNA replication genes. DPB3 was mapped to chromosome II, 30 cM distal to his7.

The fidelity of DNA synthesis by the catalytic subunit of yeast DNA polymerase alpha alone and with accessory proteins

The fidelity of DNA synthesis catalyzed by the 180-kDa catalytic subunit (p180) of DNA polymerase alpha from Saccharomyces cerevisiae has been determined. Despite the presence of a 3'----5' exonuclease activity (Brooke et al., 1991, J. Biol. Chem., 266, 3005-3015), its accuracy is similar to several exonuclease-deficient DNA polymerases and much lower than other DNA polymerases that have associated exonucleolytic proofreading activity. Average error rates are 1/9900 and 1/12,000, respectively, for single base-substitution and minus-one nucleotide frameshift errors; the polymerase generates deletions as well. Similar error rates are observed with reactions containing the 180-kDa subunit plus an 86-kDa subunit (p86), or with these two polypeptides plus two additional subunits (p58 and p49) comprising the DNA primase activity required for DNA replication. Finally, addition of yeast replication factor-A (RF-A), a protein preparation that stimulates DNA synthesis and has single-stranded DNA-binding activity, yields a polymerization reaction with 7 polypeptides required for replication, yet fidelity remains low relative to error rates for semiconservative replication. The data suggest that neither exonucleolytic proofreading activity, the beta subunit, the DNA primase subunits nor RF-A contributes substantially to base substitution or frameshift error discrimination by the DNA polymerase alpha catalytic subunit.

The cell-cycle-regulated budding yeast gene DBF2, encoding a putative protein kinase, has a homologue that is not under cell-cycle control

The budding yeast cell-cycle gene, DBF2, encoding a putative protein kinase, was shown to have a homologue, designated DBF20. This gene was cloned, sequenced, and confirmed to be highly homologous to DBF2, with over 80% identity in the 490 most C-terminal amino acid residues. Either gene could be deleted by itself, but deletion of both genes simultaneously was lethal, indicating that they are redundant for at least one vital function in yeast. In contrast to the DBF2 mRNA, which is expressed under cell-cycle control at or near START [Johnston et al., Mol. Cell. Biol. 10 (1990) 1358-1366], the DBF20 mRNA is expressed at a low level and not under cell-cycle control. Assuming there is no translational control, the differential expression of the mRNAs would result in a cell-cycle fluctuation of the relative levels of the gene products, which may constitute a novel form of regulation.

DPB2, the gene encoding DNA polymerase II subunit B, is required for chromosome replication in Saccharomyces cerevisiae

The Saccharomyces cerevisiae DNA polymerase II holoenzyme consists of five polypeptides. The largest is the catalytic subunit, whose gene (POL2) has been cloned and sequenced. Herein we describe the cloning and sequencing of DPB2, the gene for the second largest subunit of DNA polymerase II, and the isolation of temperature-sensitive dpb2 mutations. The DNA sequence revealed an open reading frame encoding a protein of Mr 79,461 and lacking significant sequence similarity to any protein in data bases. Disruption of DPB2 was lethal for the cell and the temperature-sensitive dpb2-1 mutant was partially defective in DNA synthesis at the restrictive temperature, indicating that the DPB2 protein is required for normal yeast chromosomal replication. Furthermore, the DNA polymerase II complex was difficult to obtain from dpb2-1 mutant cells, suggesting that a stable DNA polymerase II complex requires DPB2 and is essential for chromosomal replication. The DPB2 transcript periodically fluctuated during the cell cycle and, like those of other genes encoding DNA replication proteins, peaked at the G1/S phase boundary.

Cloning and characterization of DST2, the gene for DNA strand transfer protein beta from Saccharomyces cerevisiae

The gene encoding the 180-kDa DNA strand transfer protein beta from the yeast Saccharomyces cerevisiae was identified and sequenced. This gene, DST2 (DNA strand transferase 2), was located on chromosome VII. dst2 gene disruption mutants exhibited temperature-sensitive sporulation and a 50% longer generation time during vegetative growth than did the wild type. Spontaneous mitotic recombination in the mutants was reduced severalfold for both intrachromosomal recombination and intragenic gene conversion. The mutants also had reduced levels of the intragenic recombination that is induced during meiosis. Meiotic recombinants were, however, somewhat unstable in the mutants, with a decrease in recombinants and survival upon prolonged incubation in sporulation media. spo13 or spo13 rad50 mutations did not relieve the sporulation defect of dst2 mutations. A dst1 dst2 double mutant has the same phenotype as a dst2 single mutant. All phenotypes associated with the dst2 mutations could be complemented by a plasmid containing DST2.

Isolation, DNA sequence, and regulation of a Saccharomyces cerevisiae gene that encodes DNA strand transfer protein alpha

DNA strand transfer protein alpha (STP alpha) from meiotic Saccharomyces cerevisiae cells promotes homologous pairing of DNA without any nucleotide cofactor in the presence of yeast single-stranded DNA binding protein. This gene (DNA strand transferase 1, DST1) encodes a 309-amino-acid protein with a predicted molecular mass of 34,800 Da. The STP alpha protein level is constant in both mitotic and meiotic cells, but during meiosis the polypeptide is activated by an unknown mechanism, resulting in a large increase in its specific activity. A dst1::URA3/dst1::URA3 mutant grows normally in mitotic media; however, meiotic cells exhibit a greatly reduced induction of both DNA strand transfer activity and intragenic recombination between his1 heteroalleles. Spore viability is normal. These results suggest that DST1 is required for much of the observed induction of homologous recombination in S. cerevisiae during meiosis but not for normal sporulation.