Roles of RecA protein in spontaneous mutagenesis in Escherichia coli

To verify the extent of contribution of spontaneous DNA lesions to spontaneous mutagenesis, we have developed a new genetic system to examine simultaneously both forward mutations and recombination events occurring within about 600 base pairs of a transgenic rpsL target sequence located on Escherichia coli chromosome. In a wild-type strain, the recombination events were occurring at a frequency comparable to that of point mutations within the rpsL sequence. When the cells were UV-irradiated, the recombination events were induced much more sharply than point mutations. In a recA null mutant, no recombination event was observed. These data suggest that the blockage of DNA replication, probably caused by spontaneous DNA lesions, occurs often in normally growing E. coli cells and is mainly processed by cellular functions requiring the RecA protein. However, the recA mutant strain showed elevated frequencies of single-base frameshifts and large deletions, implying a novel mutator action of this strain. A similar mutator action of the recA mutant was also observed with a plasmid-based rpsL mutation assay. Therefore, if the recombinogenic problems in DNA replication are not properly processed by the RecA function, these would be a potential source for mutagenesis leading to single-base frameshift and large deletion in E. coli. Furthermore, the single-base frameshifts induced in the recA-deficient cells appeared to be efficiently suppressed by the mutS-dependent mismatch repair system. Thus, it seems likely that the single-base frameshifts are derived from slippage errors that are not directly caused by DNA lesions but made indirectly during some kind of error-prone DNA synthesis in the recA mutant cells.     

A Major Role for Bacteriophage T4 DNA Polymerase in Frameshift Mutagenesis

T4 DNA polymerase strongly influences the frequency and specificity of frameshift mutagenesis. Fifteen of 19 temperature-sensitive alleles of the DNA polymerase gene substantially influenced the reversion frequencies of frameshift mutations measured in the T4 rII genes. Most polymerase mutants increased frameshift frequencies, but a few alleles (previously noted as antimutators for base substitution mutations) decreased the frequencies of certain frameshifts while increasing the frequencies of others. The various patterns of enhanced or decreased frameshift mutation frequencies suggest that T4 DNA polymerase is likely to play a variety of roles in the metabolic events leading to frameshift mutation. A detailed genetic study of the specificity of the mutator properties of three DNA polymerase alleles (tsL56, tsL98 and tsL88) demonstrated that each produces a distinctive frameshift spectrum. Differences in frameshift frequencies at similar DNA sequences within the rII genes, the influence of mutant polymerase alleles on these frequencies, and the presence or absence of the dinucleotide sequence associated with initiation of Okazaki pieces at the frameshift site has led us to suggest that the discontinuities associated with discontinuous DNA replication may contribute to spontaneous frameshift mutation frequencies in T4.     

Sequence-dependent modulation of frameshift mutagenesis at NarI-derived mutation hot spots.

The NarI sequence is known to be the strongest mutation hot spot for induced frameshift mutagenesis. Indeed, a single N-2-acetylaminofluorene (AAF) adduct induces -2 frameshift mutations (5'-GGCGAAFCC--> 5'-GGCC) more than 10(7)-fold over background mutagenesis in Escherichia coli. The mechanism of induction of the frameshift mutation involves a two nucleotide primer-template misalignment event during replication of the adduct-containing sequence. The slipped mutagenic intermediate (SMI) that is thus formed is strongly stabilised by the AAF residue. In order to understand the origin of the extreme susceptibility of this sequence to frameshift mutagenesis, we analysed AAF-induced mutagenesis at sequences 5'-NaGCGAAFCNb-3' containing the core dinucleotide GCGC repeat present in the NarI sequence flanked by variable nucleotides Na and Nb. The nature of nucleotide Nb was found to strongly modulate the frequency of induced -2 frameshift mutagenesis (up to 30 to 50-fold), while little if any effect could be attributed to nucleotide Na. The induction of -2 frameshifts, regardless of nucleotides Na and Nb, was found to be SOS-inducible but umuDC-independent as previously found for the authentic NarI sequence. The NarI sequence (GGCGCC) and sequence TGCGCA (Na=T, Nb=A) were found to be equally "hot" for -2 frameshift mutation induction compared to the sequence AGCGCT where induced mutagenesis was 30 to 50-fold lower.The analysis of replication events using constructions containing a strand marker across from the adduct site allowed us to demonstrate that the large difference in -2 frameshift mutagenesis is due to an intrinsic difference in the propensity of these sequences to slip during replication. How the nature of the nucleotide flanking the adduct on its 3'-side (Nb) differentially stabilises the SMI will be discussed in the light of recent structural data and theoretical models.     

Transient suppression of MLH1 allows effective single-nucleotide substitution by single-stranded DNA oligonucleotides

Short synthetic single-stranded oligodeoxyribonucleotides (ssODNs) can be used to introduce subtle modifications into the genome of mouse embryonic stem cells (ESCs). We have previously shown that effective application of ssODN-mediated gene targeting in ESC requires (transient) suppression of DNA mismatch repair (MMR). However, whereas transient down-regulation of the mismatch recognition protein MSH2 allowed substitution of 3 or 4 nucleotides, 1 or 2 nucleotide substitutions were still suppressed. We now demonstrate that single- or dinucleotide substitution can effectively be achieved by transient down-regulation of the downstream MMR protein MLH1. By exploiting highly specific real-time PCR, we demonstrate the feasibility of substituting a single basepair in a non-selectable gene. However, disabling the MMR machinery may lead to inadvertent mutations. To obtain insight into the mutation rate associated with transient MMR suppression, we have compared the impact of transient and constitutive MMR deficiency on the repair of frameshift intermediates at mono- and dinucleotide repeats. Repair at these repeats relied on the substrate specificity and functional redundancy of the MSH2/MSH6 and MSH2/MSH3 MMR complexes. MLH1 knockdown increased the level of spontaneous mutagenesis, but modified ESCs remained germ line competent. Thus, transient MLH1 suppression provides a valuable extension of the MSH2 knockdown strategy, allowing rapid generation of mice carrying single basepair alterations in their genome.     

The mutational specificity of DNA polymerase-beta during in vitro DNA synthesis. Production of frameshift, base substitution, and deletion mutations

The frequency and specificity of mutations produced in vitro by eucaryotic DNA polymerase-beta have been determined in a forward mutation assay using a 250-base target sequence in M13mp2 DNA. Homogeneous DNA polymerase-beta, isolated from four different sources, produces mutations at a frequency of 4-6%/single round of gap-filling DNA synthesis. DNA sequence analyses of 460 independent mutants resulting from this error-prone DNA synthesis demonstrate a wide variety of mutational events. Frameshift and base substitutions are made at approximately equal frequency and together comprise about 90% of all mutations. Two mutational "hot spots" for frameshift and base substitution mutations were observed. The characteristics of the mutations at these sites suggest that certain base substitution errors result from dislocation of template bases rather than from direct mispair formation by DNA polymerase-beta. When considering the entire target sequence, single-base frameshift mutations occur primarily in runs of identical bases, usually pyrimidines. The loss of a single base occurs 20-80 times more frequently than single-base additions and much more frequently than the loss of two or more bases. Base substitutions occur at many sites throughout the target, representing a wide spectrum of mispair formations. Averaged over a large number of phenotypically detectable sites, the base substitution error frequency is greater than one mistake for every 5000 bases polymerized. Large deletion mutations are also observed, at a frequency more than 10-fold over background, indicating that purified DNA polymerases alone are capable of producing such deletions. These data are discussed in relation to the physical and kinetic properties of the purified enzymes and with respect to the proposed role for this DNA polymerase in vivo.     

Mechanisms of ultraviolet-induced mutation. Mutational spectra in the Escherichia coli lacI gene for a wild-type and an excision-repair-deficient strain

We have analyzed the DNA sequence changes in a total of 409 ultraviolet light-induced mutations in the lacI gene of Escherichia coli: 227 in a Uvr+ and 182 in a UvrB- strain. Both differences and similarities were observed. In both strains the mutations were predominantly (60 to 75%) base substitutions, followed by smaller contributions of single-base frameshifts, deletions and frameshift hotspot mutations. The base substitutions proved largely similar in the two strains but differences were observed among the single-base frameshifts, the deletions and the hotspot mutations. Among the base substitutions, both transitions (72.5%) and transversions (27.5%) were observed. The largest single group was G.C----A.T (60% of all base substitutions). The sites where G.C----A.T changes occurred were strongly correlated (97.5%) with sequences of adjacent pyrimidines, indicating mutation targeted ultraviolet photoproducts. Comparable amounts of mutation occurred at cytosine/cytosine and (mixed) cytosine/thymine sites. From an analysis of the prevalence of mutation at either the 5' or 3' side of a dipyrimidine, we conclude that both cyclobutane dimers and (6-4) lesions may contribute to mutation. Despite the general similarity of the base-substitution spectra between the wild-type and excision-defective strains, a number of sites were uniquely mutable in the UvrB- strain. Analysis of their surrounding DNA sequences suggested that, in addition to damage directly at the site of mutation, the potential for nearby opposite-strand damage may be important in determining the mutability of a site. The ultraviolet light-induced frameshift mutations were largely single-base losses. Inspection of the DNA sequences at which the frameshifts occurred suggested that they resulted from targeted mutagenesis, probably at cyclobutane pyrimidine dimers. The prevalence of frameshift mutations at homodimers (TT or CC) suggests that their formation involves local misalignment (slippage) and that base-pairing properties are partially retained in cyclobutane dimers. While the frameshift mutations in the Uvr+ strain were distributed over many different sites, more than half in the UvrB- strain were concentrated at a single site. Ultraviolet light-induced deletions as well as frameshift hotspot mutations (+/- TGGC at positions 620 to 632) are considered to be examples of untargeted or semitargeted mutagenesis. Hotspot mutations in the Uvr+ strain showed an increased contribution by (-)TGGC relative to (+)TGGC, indicating that ultraviolet light may specifically promote the loss of the four bases.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of the dinB gene product in spontaneous mutation in Escherichia coli with an impaired replicative polymerase

We isolated several new mutator mutations of the Escherichia coli replicative polymerase dnaE subunit alpha and used them and a previously reported dnaE mutation to study spontaneous frameshift and base substitution mutations. Two of these dnaE strains produce many more mutants when grown on rich (Luria-Bertani) than on minimal medium. A differential effect of the medium was not observed when these dnaE mutations were combined with a mismatch repair mutation. The selection scheme for the dnaE mutations required that they be able to complement a temperature-sensitive strain. However, the ability to complement is not related to the mutator effect for at least one of the mutants. Comparison of the mutation rates for frameshift and base substitution mutations in mutS and dnaE mutS strains suggests that the mismatch repair proteins respond differently to the two types of change. Deletion of dinB from both chromosome and plasmid resulted in a four- to fivefold decrease in the rate of frameshift and base substitution mutations in a dnaE mutS double mutant background. This reduction indicates that most mistakes in replication occur as a result of the action of the auxiliary rather than the replicative polymerase in this dnaE mutant. Deletion of dinB from strains carrying a wild-type dnaE had a measurable effect, suggesting that a fraction of spontaneous mutations occur as a result of dinB polymerase action even in cells with a normal replicative polymerase.     

Single-strand-specific exonucleases prevent frameshift mutagenesis by suppressing SOS induction and the action of DinB/DNA polymerase IV in growing cells

Escherichia coli strains carrying null alleles of genes encoding single-strand-specific exonucleases ExoI and ExoVII display elevated frameshift mutation rates but not base substitution mutation rates. We characterized increased spontaneous frameshift mutation in ExoI- ExoVII- cells and report that some of this effect requires RecA, an inducible SOS DNA damage response, and the low-fidelity, SOS-induced DNA polymerase DinB/PolIV, which makes frameshift mutations preferentially. We also find that SOS is induced in ExoI- ExoVII- cells. The data imply a role for the single-stranded exonucleases in guarding the genome against mutagenesis by removing excess single-stranded DNA that, if left, leads to SOS induction and PolIV-dependent mutagenesis. Previous results implicated PolIV in E. coli mutagenesis specifically during starvation or antibiotic stresses. Our data imply that PolIV can also promote mutation in growing cells under genome stress due to excess single-stranded DNA.     

Base composition of mononucleotide runs affects DNA polymerase slippage and removal of frameshift intermediates by mismatch repair in Saccharomyces cerevisiae

The postreplicative mismatch repair (MMR) system is important for removing mutational intermediates that are generated during DNA replication, especially those that arise as a result of DNA polymerase slippage in simple repeats. Here, we use a forward mutation assay to systematically examine the accumulation of frameshift mutations within mononucleotide runs of variable composition in wild-type and MMR-defective yeast strains. These studies demonstrate that (i) DNA polymerase slippage occurs more often in 10-cytosine/10-guanine (10C/10G) runs than in 10-adenine/10-thymine (10A/10T) runs, (ii) the MMR system removes frameshift intermediates in 10A/10T runs more efficiently than in 10C/10G runs, (iii) the MMR system removes -1 frameshift intermediates more efficiently than +1 intermediates in all 10-nucleotide runs, and (iv) the repair specificities of the Msh2p-Msh3p and Msh2p-Msh6p mismatch recognition complexes with respect to 1-nucleotide insertion/deletion loops vary dramatically as a function of run composition. These observations are relevant to issues of genome stability, with both the rates and types of mutations that accumulate in mononucleotide runs being influenced by the primary sequence of the run as well as by the status of the MMR system.     

Spontaneous mutators of salmonella typhimurium LT2 generated by insertion of transposable elements.

Spontaneous mutators of Salmonella typhimurium LT2 were generated by inserting the transposable element Tn5 or Tn10 into the bacterial chromosome. Two mutators mapped at the position of the mutH and mutL loci of S. typhimurium, and two other mutators mapped at positions corresponding to the mutS and uvrD loci of Escherichia coli. A fifth mutator, mutB, did not map at a position corresponding to any of the known mutators of S. typhimurium or E. coli. The mutH,L,S and uvrD alleles increased the frequency of both spontaneous base substitution and frameshift mutations, whereas the mutB allele increased the frequency only of spontaneous base substitution mutations. The increased frequency of base substitution mutations was recA+ independent in the mutH, mutL, and uvrD strains and partially recA+ independent in the mutS strain. The uvrD mutation decreased the resistance of the cells to killing by ultraviolet irradiation. The mutH,L,S and uvrD strains showed an increased sensitivity to mutagenesis by the alkylating agents methyl methane sulfonate and ethyl methane sulfonate, but not to mutagenesis by 4-nitroquinoline-1-oxide.     

Influence of DNA repair on mutation spectra in Salmonella

This paper reviews the influence of DNA repair on spontaneous and mutagen-induced mutation spectra at the base-substitution (hisG46) and -1 frameshift (hisD3052) alleles present in strains of the Salmonella (Ames) mutagenicity assay. At the frameshift allele (mostly a CGCGCGCG target), ΔuvrB influences the frequency of spontaneous hotspot mutations (−CG), duplications, and deletions, and it also shifts the sites of deletions and duplications. Cells with pKM101+ΔuvrB spontaneously produce complex frameshifts (frameshifts with an adjacent base substitution). The spontaneous frequency of 1-base insertions or concerted (templated) mutations is unaffected by DNA repair, and neither mutation is inducible by mutagens. Glu-P-1, 1-nitropyrene (1NP), and 2-acetylaminofluorene (2AAF) induce only hotspot mutations and are unaffected by pKM101, whereas benzo(a)pyrene and 4-aminobiphenyl induce only hotspot in pKM101⁻, and hotspot plus complex in pKM101⁺. At the base-substitution allele (mostly a CC/GG target), the ΔuvrB allele increases spontaneous transitions in the absence of pKM101 and increases transversions in its presence. The frequency of suppressor mutations is decreased 4× by ΔuvrB, but increased 7.5× by pKM101. Both repair factors cause a shift in the proportion of mutations to the second position of the CC/GG target. With UV light and γ-rays, the ΔuvrB allele increases the proportion of transitions relative to transversions. pKM101 is required for mutagenesis by Glu-P-1 and 4-AB, and the types and positions of the substitutions are not altered by the addition of the ΔuvrB allele. Changes in DNA repair appear to cause more changes in spontaneous than in mutagen-induced mutation spectra at both alleles. There is a high correlation (r²=0.8) between a mutagen's ability to induce complex frameshifts and its relative base-substitution/frameshift mutagenic potency. A mutagen induces the same primary class of base substitution in TA100 (ΔuvrB, pKM101) as it does in Escherichia coli, mammalian cells, or rodents as well as in the p53 gene of human tumors associated with exposure to that mutagen. Thus, a mutagen induces the same primary class of base substitution in most organisms, reflecting the conserved nature of DNA replication and repair processes.     

Mechanisms of frameshift mutagenesis by aflatoxin B1-2,3-dichloride

In order to characterize frameshift mutagenesis by aflatoxin B1-2,3-dichloride (AFB1Cl2), we have introduced a +1 (BK8) or a -1 (HS8) frameshift within the lacZ alpha gene segment contained in the phage M13mp8 to obtain lacZ alpha- derivatives. BK8 or HS8 replicative form DNA was modified with AFB1Cl2 in vitro, transfected into appropriate Escherichia coli hosts and lacZ alpha+ revertants scored and defined by DNA sequencing. The -1 frameshift (BK8) results suggest the following. (1) The E. coli recA gene is not absolutely required for AFB1Cl2-induced frameshift mutagenesis; however, in recA+ cells, ultraviolet light (SOS) induction enhances AFB1Cl2 mutagenesis, but such ultraviolet induction is not required. The plasmid pGW270 (mucAB+) significantly enhances the AFB1Cl2-induced frameshift mutagenesis. The uvrABC+ excision system plays a major role in the repair of AFB1Cl2-induced damage. (2) Sequence analysis reveals that AFB1Cl2 induces two classes of -1 frameshift mutations: the simple class in which the frameshift is due to the loss of one base-pair, and the complex class in which the loss of a base-pair is coupled to a vicinal base substitution. Both types of mutations occur predominantly at G.C runs, which are hotspots for AFB1Cl2 damage. The complex mutations appear to be concerted events targeted by a single AFB1Cl2 adduct. The frequency of these complex mutations is significantly enhanced by mucAB activity. In this system, recA activity is required for generation of significant levels of complex mutations. An analysis of the +1 frameshifts (HS8) reveals that AFB1Cl2 induces +1 frameshifts with an efficiency comparable to that for -1 frameshifts. Most +1 frameshifts occur by the addition of a base, and a third of the additions are complex mutations because they are accompanied by at least one base substitution. All simple additions occur at G.C runs; however, in a striking contrast to spontaneous insertions, a majority of the induced events introduce an A.T pair at these sites. Our data suggest a model for the generation of base substitution as well as simple and complex frameshift mutations induced by AFB1Cl2. To the extent determined, the frameshift specificity of aflatoxin B1 activated by metabolic enzymes is similar to that of AFB1Cl2.     

Polynucleotide Phosphorylase Plays an Important Role in the Generation of Spontaneous Mutations in Escherichia coli

Polynucleotide phosphorylase (PNP) plays a central role in RNA degradation, generating a pool of ribonucleoside diphosphates (rNDPs) that can be converted to deoxyribonucleoside diphosphates (dNDPs) by ribonucleotide reductase. We report here that spontaneous mutations resulting from replication errors, which are normally repaired by the mismatch repair (MMR) system, are sharply reduced in a PNP-deficient Escherichia coli strain. This is true for base substitution mutations that occur in the rpoB gene leading to Rif^r and the gyrB gene leading to Nal^r and for base substitution and frameshift mutations that occur in the lacZ gene. These results suggest that the increase in the rNDP pools generated by polynucleotide phosphorylase (PNP) degradation of RNA is responsible for the spontaneous mutations observed in an MMR-deficient background. The PNP-derived pool also appears responsible for the observed mutations in the mutT mutator background and those that occur after treatment with 5-bromodeoxyuridine, as these mutations are also drastically reduced in a PNP-deficient strain. However, mutation frequencies are not reduced in a mutY mutator background or after treatment with 2-aminopurine. These results highlight the central role in mutagenesis played by the rNDP pools (and the subsequent dNTP pools) derived from RNA degradation.     

UV-induced base substitution mutations in a shuttle vector plasmid propagated in group C xeroderma pigmentosum cells.

To assess the contribution to mutagenesis of human DNA repair defects, the UV-irradiated shuttle vector plasmid pZ189 was propagated in fibroblasts derived from a xeroderma pigmentosum (XP) patient in DNA repair complementation group C. In comparison to results with DNA repair-proficient human cells (WI-38 VA13), UV-irradiated pZ189 propagated in the XP-C (XP4PA(SV)) cells showed fewer surviving plasmids and a higher frequency of mutated plasmids. Base sequence analysis of 67 mutated plasmids recovered from the XP-C cells revealed similar classes of point mutations and mutation spectrum, and a higher frequency of G:C to A:T transitions along with a lower frequency of transversions among plasmids with single or tandem mutations compared to plasmids recovered from the normal line. Most single-base substitution mutations (83%) occurred at G:C base pairs in which the 5'-adjacent base of the cytosine was thymine or cytosine. These results indicate that the DNA repair defects in XP-C, in comparison to data previously reported for XP-A, XP-D and XP-F, result in different UV survival and mutation frequency but in similar types of base substitution mutations.     

Frameshift mutagenesis in Escherichia coli by reversible DNA intercalators: sequence specificity

The mutagenic potency of the simple reversible intercalators isopropyl-OPC (iPr-OPC) and 9-aminoacridine (9-AA) is assessed in E. coli using reversion assays based on plasmids derived from pBR322 carrying various frameshift mutations within the tetracycline resistance gene in repetitive sequences: +/- 2 frameshift mutations within alternating GC sequences; +/- 1 frameshift mutation at runs of guanines. The results obtained show that iPr-OPC and 9-AA have a sequence specificity for mutagenesis: they revert +1 and -1 frameshift mutations within runs of monotonous G:C base pairs. The precise determination of the size of a small restriction fragment which contains the mutation allowed us to demonstrate that reversion occurred by -1 deletions for the +1 frameshift mutations and by +1 additions for the -1 frameshift mutations. The possible relations of this specific reversion with the base sequence specificity of the mutagenesis are briefly discussed.     

Mutational specificity of the dnaE173 mutator associated with a defect in the catalytic subunit of DNA polymerase III of Escherichia coli.

We developed a system to examine forward mutations that occurred in the rpsL gene of Escherichia coli placed on a multicopy plasmid. Using this system we determined the mutational specificity for a dnaE173 mutator strain in which the editing function of DNA polymerase III is impeded. The frequency of rpsL- mutations increased 32,000-fold, due to the dnaE173 mutator, and 87 independent rpsL- mutations in the mutator strain were analyzed by DNA sequencing, together with 100 mutants recovered from dnaE+ strain, as the control. While half the number of mutations that occurred in the wild-type strain were caused by insertion elements, no such mutations were recovered from the mutator strain. A novel class of mutation, named "sequence substitution" was present in mutants raised in the dnaE173 strain; seven sequence substitutions induced in the mutator strain occurred at six sites, and all were located in quasipalindromic sequences, carrying the GTG or CAC sequence at one or both endpoints. While other types of mutation were found in both strains, single-base frameshifts were the most frequent events in the mutator strain. Thus, the mutator effect on this class of mutation was 175,000-fold. A total of 95% of the single-base frameshifts in the mutator strain were additions, most of which occurred at runs of A or C bases so as to increase the number of identical residues. Base substitutions, the frequency of which was enhanced 25,000-fold by the mutator effect, occurred primarily at several hotspots in the mutator strain, whereas those induced in the wild-type strain were more randomly distributed throughout the rpsL sequence. The dnaE173 mutator also increased the frequency of duplications 28,000-fold. Of the three duplications recovered from the mutator strain, one was a simple duplication, the region of which was flanked by direct repeats. The other duplications were complex, one half part of which was in the inverted orientation of a region containing two sets of inverted repeats. The same duplications were also recovered from the wild-type strain. The present data suggest that dnaE173 is a novel class of mutator that sharply induces sequence-directed mutagenesis, yielding high frequencies of single base frameshifts, duplications with inversions, sequence substitutions and base substitutions at hotspots.     

Comparison of the Frequencies of Spontaneous and Chemically-Induced 5-Bromodeoxyuridine-Resistance Mutations in Wild-Type and Revertant Bhk-21/13 Cells

5-bromodeoxyuridine resistance mutations induced by mutagenesis were studied. The average expression time for induced mutations varied with the concentration of the mutagen ethyl methanesulfonate (EMS). However, a constant number of two generation times was necessary for half maximal expression of induced mutations. Also, induced mutation rates were compared under optimal expression conditions for bromodeoxyuridine, fluorodeoxyuridine and azaguanine resistance markers. Ten independent bromodeoxy-uridine-resistant clones were tested for reversion. Two clones reverted-one spontaneously and the other after mutagenesis. The spontaneous rate of mutation to bromodeoxyuridine resistance, estimated by the fluctuation test, was high in revertant clones (4 x 10(-6) / cell / generation) and low in the wild-type cells (< 3.5 x 10(-8) / cell / generation). A comparison of induced mutation frequencies at variable EMS concentrations showed a single-hit curve for revertant clones and a multihit curve for the wild-type cells. Thymidine kinase activities of resistant clones were usually less than 2% of that of the wild-type clone. Inducibility, thermal stability and intracellular localization of the thymidine kinases of the wild-type cells and of a revertant clone were identical. A low, but significant (P < 0.10), Km discrepancy was observed between enzyme extracts of these lines. The genetic implications of these results are discussed.     

Errors in DNA synthesis: a source of spontaneous mutations

Spontaneous mutations in somatic cells may engender several pathologic processes, including cancer. The sources of these mutations remain to be established. We present a conceptual framework in which to analyze the sources of spontaneous mutations and focus here on 3 endogenous processes that have the potential to generate spontaneous sequence alterations in DNA. These are: replication errors, depurination of DNA, and damage to DNA by the generation of active-oxygen species. Each of these processes occurs more frequently than the rate of mutagenesis in somatic cells, but are repaired by different and overlapping mechanisms. Model systems are being developed to determine the spectrum of mutations produced by each of these processes in vitro. A comparison of these spectra with the overall spectrum of spontaneous mutations in somatic cells may help to determine the contribution of each of these processes to spontaneous mutation.