Identification of a mutant DNA polymerase delta in Saccharomyces cerevisiae with an antimutator phenotype for frameshift mutations

Polγ coordinates DNA synthesis and proofreading to ensure mitochondrial genome integrity

Accurate replication of mitochondrial DNA (mtDNA) by DNA polymerase γ (Polγ) is essential for maintaining cellular energy supplies, metabolism, and cell cycle control. To illustrate the structural mechanism for Polγ coordinating polymerase (pol) and exonuclease (exo) activities to ensure rapid and accurate DNA synthesis, we determined four cryo-EM structures of Polγ captured after accurate or erroneous incorporation to a resolution of 2.4-3.0 Å. The structures show that Polγ employs a dual-checkpoint mechanism to sense nucleotide misincorporation and initiate proofreading. The transition from replication to error editing is accompanied by increased dynamics in both DNA and enzyme, in which the polymerase relaxes its processivity and the primer-template DNA unwinds, rotates, and backtracks to shuttle the mismatch-containing primer terminus 32 Å to the exo site for editing. Our structural and functional studies also provide a foundation for analyses of Polγ mutation-induced human diseases and aging.

Normally lethal amino acid substitutions suppress an ultramutator DNA Polymerase δ variant

In yeast, the pol3-01,L612M double mutant allele, which causes defects in DNA polymerase delta (Pol δ) proofreading (pol3-01) and nucleotide selectivity (pol3-L612M), confers an “ultramutator” phenotype that rapidly drives extinction of haploid and diploid MMR-proficient cells. Here, we investigate antimutator mutations that encode amino acid substitutions in Pol δ that suppress this lethal phenotype. We find that most of the antimutator mutations individually suppress the pol3-01 and pol3-L612M mutator phenotypes. The locations of many of the amino acid substitutions in Pol δ resemble those of previously identified antimutator substitutions; however, two novel mutations encode substitutions (R674G and Q697R) of amino acids in the fingers domain that coordinate the incoming dNTP. These mutations are lethal without pol3-L612M and markedly change the mutation spectra produced by the pol3-01,L612M mutator allele, suggesting that they alter nucleotide selection to offset the pol3-L612M mutator phenotype. Consistent with this hypothesis, mutations and drug treatments that perturb dNTP pool levels disproportionately influence the viability of pol3-L612M,R674G and pol3-L612M,Q697R cells. Taken together, our findings suggest that mutation rate can evolve through genetic changes that alter the balance of dNTP binding and dissociation from DNA polymerases.

5-methylcytosine-sensitive variants of Thermococcus kodakaraensis DNA polymerase

DNA methylation of cytosine in eukaryotic cells is a common epigenetic modification, which plays an important role in gene expression and thus affects various cellular processes like development and carcinogenesis. The occurrence of 5-methyl-2'-deoxycytosine (5mC) as well as the distribution pattern of this epigenetic marker were shown to be crucial for gene regulation and can serve as important biomarkers for diagnostics. DNA polymerases distinguish little, if any, between incorporation opposite C and 5mC, which is not surprising since the site of methylation is not involved in Watson-Crick recognition. Here, we describe the development of a DNA polymerase variant that incorporates the canonical 2'-deoxyguanosine 5'-monophosphate (dGMP) opposite C with higher efficiency compared to 5mC. The variant of Thermococcus kodakaraensis (KOD) exo- DNA polymerase was discovered by screening mutant libraries that were built by rational design. We discovered that an amino acid substitution at a single site that does not directly interact with the templating nucleobase, may alter the ability of the DNA polymerase in processing C in comparison to 5mC. Employing these findings in combination with a nucleotide, which is fluorescently labeled at the terminal phosphate, indicates the potential use of the mutant DNA polymerase in the detection of 5mC.

DNA polymerase 3'→5' exonuclease activity: Different roles of the beta hairpin structure in family-B DNA polymerases

Proofreading by the bacteriophage T4 and RB69 DNA polymerases requires a β hairpin structure that resides in the exonuclease domain. Genetic, biochemical and structural studies demonstrate that the phage β hairpin acts as a wedge to separate the primer-end from the template strand in exonuclease complexes. Single amino acid substitutions in the tip of the hairpin or deletion of the hairpin prevent proofreading and create "mutator" DNA polymerases. There is little known, however, about the function of similar hairpin structures in other family B DNA polymerases. We present mutational analysis of the yeast (Saccharomyces cerevisiae) DNA polymerase δ hairpin. Deletion of the DNA polymerase δ hairpin (hpΔ) did not significantly reduce DNA replication fidelity; thus, the β hairpin structure in yeast DNA polymerase δ is not essential for proofreading. However, replication efficiency was reduced as indicated by a slow growth phenotype. In contrast, the G447D amino acid substitution in the tip of the hairpin increased frameshift mutations and sensitivity to hydroxyurea (HU). A chimeric yeast DNA polymerase δ was constructed in which the T4 DNA polymerase hairpin (T4hp) replaced the yeast DNA polymerase δ hairpin; a strong increase in frameshift mutations was observed and the mutant strain was sensitive to HU and to the pyrophosphate analog, phosphonoacetic acid (PAA). But all phenotypes - slow growth, HU-sensitivity, PAA-sensitivity, and reduced fidelity, were observed only in the absence of mismatch repair (MMR), which implicates a role for MMR in mediating DNA polymerase δ replication problems. In comparison, another family B DNA polymerase, DNA polymerase ɛ, has only an atrophied hairpin with no apparent function. Thus, while family B DNA polymerases share conserved motifs and general structural features, the β hairpin has evolved to meet specific needs.

DNA polymerase delta in DNA replication and genome maintenance

The eukaryotic genome is in a constant state of modification and repair. Faithful transmission of the genomic information from parent to daughter cells depends upon an extensive system of surveillance, signaling, and DNA repair, as well as accurate synthesis of DNA during replication. Often, replicative synthesis occurs over regions of DNA that have not yet been repaired, presenting further challenges to genomic stability. DNA polymerase δ (pol δ) occupies a central role in all of these processes: catalyzing the accurate replication of a majority of the genome, participating in several DNA repair synthetic pathways, and contributing structurally to the accurate bypass of problematic lesions during translesion synthesis. The concerted actions of pol δ on the lagging strand, pol ϵ on the leading strand, associated replicative factors, and the mismatch repair (MMR) proteins results in a mutation rate of less than one misincorporation per genome per replication cycle. This low mutation rate provides a high level of protection against genetic defects during development and may prevent the initiation of malignancies in somatic cells. This review explores the role of pol δ in replication fidelity and genome maintenance.

Antimutator variants of DNA polymerases

Evolution balances DNA replication speed and accuracy to optimize replicative fitness and genetic stability. There is no selective pressure to improve DNA replication fidelity beyond the background mutation rate from other sources, such as DNA damage. However, DNA polymerases remain amenable to amino acid substitutions that lower intrinsic error rates. Here, we review these 'antimutagenic' changes in DNA polymerases and discuss what they reveal about mechanisms of replication fidelity. Pioneering studies with bacteriophage T4 DNA polymerase (T4 Pol) established the paradigm that antimutator amino acid substitutions reduce replication errors by increasing proofreading efficiency at the expense of polymerase processivity. The discoveries of antimutator substitutions in proofreading-deficient 'mutator' derivatives of bacterial Pols I and III and yeast Pol δ suggest there must be additional antimutagenic mechanisms. Remarkably, many of the affected amino acid positions from Pol I, Pol III, and Pol δ are similar to the original T4 Pol substitutions. The locations of antimutator substitutions within DNA polymerase structures suggest that they may increase nucleotide selectivity and/or promote dissociation of primer termini from polymerases poised for misincorporation, leading to expulsion of incorrect nucleotides. If misincorporation occurs, enhanced primer dissociation from polymerase domains may improve proofreading in cis by an intrinsic exonuclease or in trans by alternate cellular proofreading activities. Together, these studies reveal that natural selection can readily restore replication error rates to sustainable levels following an adaptive mutator phenotype.

Mutator suppression and escape from replication error-induced extinction in yeast

Cells rely on a network of conserved pathways to govern DNA replication fidelity. Loss of polymerase proofreading or mismatch repair elevates spontaneous mutation and facilitates cellular adaptation. However, double mutants are inviable, suggesting that extreme mutation rates exceed an error threshold. Here we combine alleles that affect DNA polymerase δ (Pol δ) proofreading and mismatch repair to define the maximal error rate in haploid yeast and to characterize genetic suppressors of mutator phenotypes. We show that populations tolerate mutation rates 1,000-fold above wild-type levels but collapse when the rate exceeds 10⁻³ inactivating mutations per gene per cell division. Variants that escape this error-induced extinction (eex) rapidly emerge from mutator clones. One-third of the escape mutants result from second-site changes in Pol δ that suppress the proofreading-deficient phenotype, while two-thirds are extragenic. The structural locations of the Pol δ changes suggest multiple antimutator mechanisms. Our studies reveal the transient nature of eukaryotic mutators and show that mutator phenotypes are readily suppressed by genetic adaptation. This has implications for the role of mutator phenotypes in cancer.

Organisms strike a balance between genetic continuity and change. Most cells are well adapted to their niches and therefore invest heavily in mechanisms that maintain accurate DNA replication. When cell populations are confronted with changing environmental conditions, “mutator” clones with high mutation rates emerge and readily adapt to the new conditions by rapidly acquiring beneficial mutations. However, deleterious mutations also accumulate, raising the question: what level of mutational burden can cell populations sustain before collapsing? Here we experimentally determine the maximal mutation rate in haploid yeast. We observe that yeast can withstand a 1,000-fold increase in mutation rate without losing colony forming capacity. Yet no strains survive a 10,000-fold increase in mutation rate. Escape mutants with an “anti-mutator” phenotype frequently emerge from cell populations undergoing this error-induced extinction. The diversity of antimutator changes suggests that strong mutator phenotypes in nature may be inherently transient, ensuring that rapid adaptation is followed by genetic attenuation which preserves the beneficial, adaptive mutations. These observations are relevant to microbial populations during infection as well as the somatic evolution of cancer cells.

The p12 subunit of human polymerase delta modulates the rate and fidelity of DNA synthesis

This study examines the role of the p12 subunit in the function of the human DNA polymerase delta (Pol delta) holoenzyme by comparing the kinetics of DNA synthesis and degradation catalyzed by the four-subunit complex, the three-subunit complex lacking p12, and site-directed mutants of each lacking proofreading exonuclease activity. Results show that p12 modulates the rate and fidelity of DNA synthesis by Pol delta. All four complexes synthesize DNA in a rapid burst phase and a slower, more linear phase. In the presence of p12, the burst rates of DNA synthesis are approximately 5 times faster, while the affinity of the enzyme for its DNA and dNTP substrates appears unchanged. The p12 subunit alters Pol delta fidelity by modulating the proofreading 3' to 5' exonuclease activity. In the absence of p12, Pol delta is more likely to proofread DNA synthesis because it cleaves single-stranded DNA twice as fast and transfers mismatched DNA from the polymerase to the exonuclease sites 9 times faster. Pol delta also extends mismatched primers 3 times more slowly in the absence of p12. Taken together, the changes that p12 exerts on Pol delta are ones that can modulate its fidelity of DNA synthesis. The loss of p12, which occurs in cells upon exposure to DNA-damaging agents, converts Pol delta to a form that has an increased capacity for proofreading.

Different behaviors in vivo of mutations in the beta hairpin loop of the DNA polymerases of the closely related phages T4 and RB69

The T4 and RB69 DNA replicative polymerases are members of the B family and are highly similar. Both replicate DNA with high fidelity and employ the same mechanism that allows efficient switching of the primer terminus between the polymerase and exonuclease sites. Both polymerases have a beta hairpin loop (hereafter called the beta loop) in their exonuclease domains that plays an important role in active-site switching. The beta loop is involved in strand separation and is needed to stabilize partially strand-separated exonuclease complexes. In T4 DNA polymerase, modification of the beta-loop residue G255 to Ser confers a strong mutator phenotype in vivo due to a reduced ability to form editing complexes. Here, we describe the RB69 DNA polymerase mutant with the equivalent residue (G258) changed to Ser but showing only mild mutator activity in vivo. On the other hand, deletion of the tip of the RB69 beta loop confers a strong mutator phenotype in vivo. Based on detailed mutational spectral analyses, DNA binding activities, and coupled polymerase/exonuclease assays, we define the differences between the T4 and RB69 polymerases. We propose that their beta loops facilitate strand separation in both polymerases, while the residues that form the loop have low structural constraints.

Evolution can favor antagonistic epistasis

The accumulation of deleterious mutations plays a major role in evolution, and key to this are the interactions between their fitness effects, known as epistasis. Whether mutations tend to interact synergistically (with multiple mutations being more deleterious than would be expected from their individual fitness effects) or antagonistically is important for a variety of evolutionary questions, particularly the evolution of sex. Unfortunately, the experimental evidence on the prevalence and strength of epistasis is mixed and inconclusive. Here we study theoretically whether synergistic or antagonistic epistasis is likely to be favored by evolution and by how much. We find that in the presence of recombination, evolution favors less synergistic or more antagonistic epistasis whenever mutations that change the epistasis in this direction are possible. This is because evolution favors increased buffering against the effects of deleterious mutations. This suggests that we should not expect synergistic epistasis to be widespread in nature and hence that the mutational deterministic hypothesis for the advantage of sex may not apply widely.

A method to select for mutator DNA polymerase deltas in Saccharomyces cerevisiae

Proofreading DNA polymerases share common short peptide motifs that bind Mg(2+) in the exonuclease active center; however, hydrolysis rates are not the same for all of the enzymes, which indicates that there are functional and likely structural differences outside of the conserved residues. Since structural information is available for only a few proofreading DNA polymerases, we developed a genetic selection method to identify mutant alleles of the POL3 gene in Saccharomyces cerevisiae, which encode DNA polymerase delta mutants that replicate DNA with reduced fidelity. The selection procedure is based on genetic methods used to identify "mutator" DNA polymerases in bacteriophage T4. New yeast DNA polymerase delta mutants were identified, but some mutants expected from studies of the phage T4 DNA polymerase were not detected. This would indicate that there may be important differences in the proofreading pathways catalyzed by the two DNA polymerases.

DNA polymerase 4 of Saccharomyces cerevisiae is important for accurate repair of methyl-methanesulfonate-induced DNA damage

The DNA polymerase 4 protein (Pol4) of Saccharomyces cerevisiae is a member of the X family of DNA polymerases whose closest human relative appears to be DNA polymerase lambda. Results from previous genetic studies conflict over the role of Pol4 in vivo. Here we show that deletion of Pol4 in a diploid strain of the SK1 genetic background results in sensitivity to methyl methanesulfonate (MMS). However, deletion of Pol4 in other strain backgrounds and in haploid strains does not yield an observable phenotype. The MMS sensitivity of a Pol4-deficient strain can be rescued by deletion of YKu70. We also show that deletion of Pol4 results in a 6- to 14-fold increase in the MMS-induced mutation frequency and in a significant increase in AT-to-TA transversions. Our studies suggest that Pol4 is critical for accurate repair of DNA lesions induced by MMS.

Sensitivity to phosphonoacetic acid: a new phenotype to probe DNA polymerase delta in Saccharomyces cerevisiae

A mutant allele (pol3-L612M) of the DNA polymerase delta gene in Saccharomyces cerevisiae that confers sensitivity to the antiviral drug phosphonoacetic acid (PAA) was constructed. We report that PAA-sensitivity tagging DNA polymerases is a useful method for selectively and reversibly inhibiting one type of DNA polymerase. Our initial studies reveal that replication by the L612M-DNA pol delta requires Rad27 flap endonuclease activity since the pol3-L612M strain is not viable in the absence of RAD27 function. The L612M-DNA pol delta also strongly depends on mismatch repair (MMR). Reduced viability is observed in the absence of any of the core MMR proteins-Msh2, Mlh1, or Pms1-and severe sensitivity to PAA is observed in the absence of the core proteins Msh6 or Exo1, but not Msh3. We propose that pol3-L612M cells need the Rad27 flap endonuclease and MMR complexes composed of Msh2/Msh6, Mlh1/Pms1, and Exo1 for correct processing of Okazaki fragments.

The multiple biological roles of the 3'-->5' exonuclease of Saccharomyces cerevisiae DNA polymerase delta require switching between the polymerase and exonuclease domains

Until recently, the only biological function attributed to the 3'-->5' exonuclease activity of DNA polymerases was proofreading of replication errors. Based on genetic and biochemical analysis of the 3'-->5' exonuclease of yeast DNA polymerase delta (Pol delta) we have discerned additional biological roles for this exonuclease in Okazaki fragment maturation and mismatch repair. We asked whether Pol delta exonuclease performs all these biological functions in association with the replicative complex or as an exonuclease separate from the replicating holoenzyme. We have identified yeast Pol delta mutants at Leu523 that are defective in processive DNA synthesis when the rate of misincorporation is high because of a deoxynucleoside triphosphate (dNTP) imbalance. Yet the mutants retain robust 3'-->5' exonuclease activity. Based on biochemical studies, the mutant enzymes appear to be impaired in switching of the nascent 3' end between the polymerase and the exonuclease sites, resulting in severely impaired biological functions. Mutation rates and spectra and synergistic interactions of the pol3-L523X mutations with msh2, exo1, and rad27/fen1 defects were indistinguishable from those observed with previously studied exonuclease-defective mutants of the Pol delta. We conclude that the three biological functions of the 3'-->5' exonuclease addressed in this study are performed intramolecularly within the replicating holoenzyme.

Natural selection and the emergence of a mutation phenotype: an update of the evolutionary synthesis considering mechanisms that affect genome variation

Most descriptions of evolution assume that all mutations are completely random with respect to their potential effects on survival. However, much like other phenotypic variations that affect the survival of the descendants, intrinsic variations in the probability, type, and location of genetic change can feel the pressure of natural selection. From site-specific recombination to changes in polymerase fidelity and repair of DNA damage, an organism's gene products affect what genetic changes occur in its genome. Through the action of natural selection on these gene products, potentially favorable mutations can become more probable than random. With examples from variation in bacterial surface proteins to the vertebrate immune response, it is clear that a great deal of genetic change is better than "random" with respect to its potential effect on survival. Indeed, some potentially useful mutations are so probable that they can be viewed as being encoded implicitly in the genome. An updated evolutionary theory includes emergence, under selective pressure, of genomic information that affects the probability of different classes of mutation, with consequences for genome survival.

Origins of spontaneous mutations: specificity and directionality of base-substitution, frameshift, and sequence-substitution mutageneses

Spontaneous mutations are derived from various sources, including errors made during replication of undamaged template DNA, mutagenic nucleotide substrates, and endogenous DNA lesions. These sources vary in their frequencies and resultant mutations, and are differently affected by the DNA sequence, DNA transactions, and cellular metabolism. Organisms possess a variety of cellular functions to suppress spontaneous mutagenesis, and the specificity and effectiveness of each function strongly affect the pattern of spontaneous mutations. Base substitutions and single-base frameshifts, two major classes of spontaneous mutations, occur non-randomly throughout the genome. Within target DNA sequences there are hotspots for particular types of spontaneous mutations; outside of the hotspots, spontaneous mutations occur more randomly and much less frequently. Hotspot mutations are attributable more to endogenous DNA lesions than to replication errors. Recently, a novel class of mutagenic pathway that depends on short inverted repeats was identified as another important source of hotspot mutagenesis.

Bacteriophage T4 genome

Phage T4 has provided countless contributions to the paradigms of genetics and biochemistry. Its complete genome sequence of 168,903 bp encodes about 300 gene products. T4 biology and its genomic sequence provide the best-understood model for modern functional genomics and proteomics. Variations on gene expression, including overlapping genes, internal translation initiation, spliced genes, translational bypassing, and RNA processing, alert us to the caveats of purely computational methods. The T4 transcriptional pattern reflects its dependence on the host RNA polymerase and the use of phage-encoded proteins that sequentially modify RNA polymerase; transcriptional activator proteins, a phage sigma factor, anti-sigma, and sigma decoy proteins also act to specify early, middle, and late promoter recognition. Posttranscriptional controls by T4 provide excellent systems for the study of RNA-dependent processes, particularly at the structural level. The redundancy of DNA replication and recombination systems of T4 reveals how phage and other genomes are stably replicated and repaired in different environments, providing insight into genome evolution and adaptations to new hosts and growth environments. Moreover, genomic sequence analysis has provided new insights into tail fiber variation, lysis, gene duplications, and membrane localization of proteins, while high-resolution structural determination of the "cell-puncturing device," combined with the three-dimensional image reconstruction of the baseplate, has revealed the mechanism of penetration during infection. Despite these advances, nearly 130 potential T4 genes remain uncharacterized. Current phage-sequencing initiatives are now revealing the similarities and differences among members of the T4 family, including those that infect bacteria other than Escherichia coli. T4 functional genomics will aid in the interpretation of these newly sequenced T4-related genomes and in broadening our understanding of the complex evolution and ecology of phages-the most abundant and among the most ancient biological entities on Earth.

The roles of REV3 and RAD57 in double-strand-break-repair-induced mutagenesis of Saccharomyces cerevisiae

The DNA synthesis associated with recombinational repair of chromosomal double-strand breaks (DSBs) has a lower fidelity than normal replicative DNA synthesis. Here, we use an inverted-repeat substrate to monitor the fidelity of repair of a site-specific DSB. DSB induction made by the HO endonuclease stimulates recombination >5000-fold and is associated with a >1000-fold increase in mutagenesis of an adjacent gene. We demonstrate that most break-repair-induced mutations (BRIMs) are point mutations and have a higher proportion of frameshifts than do spontaneous mutations of the same substrate. Although the REV3 translesion DNA polymerase is not required for recombination, it introduces approximately 75% of the BRIMs and approximately 90% of the base substitution mutations. Recombinational repair of the DSB is strongly dependent upon genes of the RAD52 epistasis group; however, the residual recombinants present in rad57 mutants are associated with a 5- to 20-fold increase in BRIMs. The spectrum of mutations in rad57 mutants is similar to that seen in the wild-type strain and is similarly affected by REV3. We also find that REV3 is required for the repair of MMS-induced lesions when recombinational repair is compromised. Our data suggest that Rad55p/Rad57p help limit the generation of substrates that require pol zeta during recombination.

Caenorhabditis elegans DNA mismatch repair gene msh-2 is required for microsatellite stability and maintenance of genome integrity

Mismatch repair genes are important in maintaining the fidelity of DNA replication. To determine the function of the Caenorhabditis elegans homologue of the MSH2 mismatch repair gene (msh-2), we isolated a strain of C. elegans with an insertion of the transposable element Tc1 within msh-2. Early-passage msh-2 mutants were similar to wild-type worms with regard to lifespan and meiotic chromosome segregation but had slightly reduced fertility. The mutant worms had reduced DNA damage-induced germ-line apoptosis after genotoxic stress. The msh-2 mutants also had elevated levels of microsatellite instability and increased rates of reversion of the dominant unc-58(e665) mutation. In addition, serially passaged cultures of msh-2 worms died out much more quickly than those of wild-type worms. These results demonstrate that msh-2 function in C. elegans is important in regulating both short- and long-term genomic stability.