Mismatch extension by Escherichia coli DNA polymerase III holoenzyme

Half-Intercalation Stabilizes Slipped Mispairing and Explains Genome Vulnerability to Frameshift Mutagenesis by Endogenous "Molecular Bookmarks"

Some 60 years ago chemicals that intercalate between base pairs of duplex DNA were found to amplify frameshift mutagenesis. Surprisingly, the robust induction of frameshifts by intercalators still lacks a mechanistic model, leaving this classic phenomenon annoyingly intractable. A promising idea of asymmetric half-intercalation-stabilizing frameshift intermediates during DNA synthesis has never been developed into a model. Instead, researchers of frameshift mutagenesis embraced the powerful slipped-mispairing concept that unexpectedly struggled with the role of intercalators in frameshifting. It is proposed that the slipped mispairing and the half-intercalation ideas are two sides of the same coin. Further, existing findings are reviewed to test predictions of the combined "half-intercalation into the slipped-mispairing intermediate" model against accumulated knowledge. The existence of potential endogenous intercalators and the phenomenon of "DNA bookmarks" reveal ample possibilities for natural frameshift mutagenisis in the cell. From this alarming perspective, it is discussed how the cell could prevent genome deterioration from frameshift mutagenesis.

High-accuracy lagging-strand DNA replication mediated by DNA polymerase dissociation

The fidelity of DNA replication is a critical factor in the rate at which cells incur mutations. Due to the antiparallel orientation of the two chromosomal DNA strands, one strand (leading strand) is replicated in a mostly processive manner, while the other (lagging strand) is synthesized in short sections called Okazaki fragments. A fundamental question that remains to be answered is whether the two strands are copied with the same intrinsic fidelity. In most experimental systems, this question is difficult to answer, as the replication complex contains a different DNA polymerase for each strand, such as, for example, DNA polymerases δ and ε in eukaryotes. Here we have investigated this question in the bacterium Escherichia coli, in which the replicase (DNA polymerase III holoenzyme) contains two copies of the same polymerase (Pol III, the dnaE gene product), and hence the two strands are copied by the same polymerase. Our in vivo mutagenesis data indicate that the two DNA strands are not copied with the same accuracy, and that, remarkably, the lagging strand has the highest fidelity. We postulate that this effect results from the greater dissociative character of the lagging-strand polymerase, which provides additional options for error removal. Our conclusion is strongly supported by results with dnaE antimutator polymerases characterized by increased dissociation rates.

Replisome-mediated translesion synthesis by a cellular replicase

Genome integrity relies on the ability of the replisome to navigate ubiquitous DNA damage during DNA replication. The Escherichia coli replisome transiently stalls at leading-strand template lesions and can either reinitiate replication downstream of the lesion or recruit specialized DNA polymerases that can bypass the lesion via translesion synthesis. Previous results had suggested that the E. coli replicase might play a role in lesion bypass, but this possibility has not been tested in reconstituted DNA replication systems. We report here that the DNA polymerase III holoenzyme in a stalled E. coli replisome can directly bypass a single cyclobutane pyrimidine dimer or abasic site by translesion synthesis in the absence of specialized translesion synthesis polymerases. Bypass efficiency was proportional to deoxynucleotide concentrations equivalent to those found in vivo and was dependent on the frequency of primer synthesis downstream of the lesion. Translesion synthesis came at the expense of lesion-skipping replication restart. Replication of a cyclobutane pyrimidine dimer was accurate, whereas replication of an abasic site resulted in mainly -1 frameshifts. Lesion bypass was accompanied by an increase in base substitution frequency for the base preceding the lesion. These findings suggest that DNA damage at the replication fork can be replicated directly by the replisome without the need to activate error-prone pathways.

Proper functioning of the GINS complex is important for the fidelity of DNA replication in yeast

The role of replicative DNA polymerases in ensuring genome stability is intensively studied, but the role of other components of the replisome is still not fully understood. One of such component is the GINS complex (comprising the Psf1, Psf2, Psf3 and Sld5 subunits), which participates in both initiation and elongation of DNA replication. Until now, the understanding of the physiological role of GINS mostly originated from biochemical studies. In this article, we present genetic evidence for an essential role of GINS in the maintenance of replication fidelity in Saccharomyces cerevisiae. In our studies we employed the psf1-1 allele (Takayama et al., 2003) and a novel psf1-100 allele isolated in our laboratory. Analysis of the levels and specificity of mutations in the psf1 strains indicates that the destabilization of the GINS complex or its impaired interaction with DNA polymerase epsilon increases the level of spontaneous mutagenesis and the participation of the error-prone DNA polymerase zeta. Additionally, a synergistic mutator effect was found for the defects in Psf1p and in the proofreading activity of Pol epsilon, suggesting that proper functioning of GINS is crucial for facilitating error-free processing of terminal mismatches created by Pol epsilon.

DNA replication fidelity in Escherichia coli: a multi-DNA polymerase affair

High accuracy (fidelity) of DNA replication is important for cells to preserve the genetic identity and to prevent the accumulation of deleterious mutations. The error rate during DNA replication is as low as 10(-9) to 10(-11) errors per base pair. How this low level is achieved is an issue of major interest. This review is concerned with the mechanisms underlying the fidelity of the chromosomal replication in the model system Escherichia coli by DNA polymerase III holoenzyme, with further emphasis on participation of the other, accessory DNA polymerases, of which E. coli contains four (Pols I, II, IV, and V). Detailed genetic analysis of mutation rates revealed that (1) Pol II has an important role as a back-up proofreader for Pol III, (2) Pols IV and V do not normally contribute significantly to replication fidelity, but can readily do so under conditions of elevated expression, (3) participation of Pols IV and V, in contrast to that of Pol II, is specific to the lagging strand, and (4) Pol I also makes a lagging-strand-specific fidelity contribution, limited, however, to the faithful filling of the Okazaki fragment gaps. The fidelity role of the Pol III τ subunit is also reviewed.

Increase in dNTP pool size during the DNA damage response plays a key role in spontaneous and induced-mutagenesis in Escherichia coli

Exposure of Escherichia coli to UV light increases expression of NrdAB, the major ribonucleotide reductase leading to a moderate increase in dNTP levels. The role of elevated dNTP levels during translesion synthesis (TLS) across specific replication-blocking lesions was investigated. Here we show that although the specialized DNA polymerase PolV is necessary for replication across UV-lesions, such as cyclobutane pyrimidine dimers or pyrimidine(6-4)pyrimidone photoproduct, Pol V per se is not sufficient. Indeed, efficient TLS additionally requires elevated dNTP levels. Similarly, for the bypass of an N-2-acetylaminofluorene-guanine adduct that requires Pol II instead of PolV, efficient TLS is only observed under conditions of high dNTP levels. We suggest that increased dNTP levels transiently modify the activity balance of Pol III (i.e., increasing the polymerase and reducing the proofreading functions). Indeed, we show that the stimulation of TLS by elevated dNTP levels can be mimicked by genetic inactivation of the proofreading function (mutD5 allele). We also show that spontaneous mutagenesis increases proportionally to dNTP pool levels, thus defining a unique spontaneous mutator phenotype. The so-called "dNTP mutator" phenotype does not depend upon any of the specialized DNA polymerases, and is thus likely to reflect an increase in Pol III's own replication errors because of the modified activity balance of Pol III. As up-regulation of the dNTP pool size represents a common physiological response to DNA damage, the present model is likely to represent a general and unique paradigm for TLS pathways in many organisms.

dnaX36 Mutator of Escherichia coli: effects of the {tau} subunit of the DNA polymerase III holoenzyme on chromosomal DNA replication fidelity

The Escherichia coli dnaX36 mutant displays a mutator effect, reflecting a fidelity function of the dnaX-encoded τ subunit of the DNA polymerase III (Pol III) holoenzyme. We have shown that this fidelity function (i) applies to both leading- and lagging-strand synthesis, (ii) is independent of Pol IV, and (iii) is limited by Pol II.

Role of Escherichia coli DNA polymerase I in chromosomal DNA replication fidelity

We have investigated the possible role of Escherichia coli DNA polymerase (Pol) I in chromosomal replication fidelity. This was done by substituting the chromosomal polA gene by the polAexo variant containing an inactivated 3'-->5' exonuclease, which serves as a proofreader for this enzyme's misinsertion errors. Using this strain, activities of Pol I during DNA replication might be detectable as increases in the bacterial mutation rate. Using a series of defined lacZ reversion alleles in two orientations on the chromosome as markers for mutagenesis, 1.5- to 4-fold increases in mutant frequencies were observed. In general, these increases were largest for lac orientations favouring events during lagging strand DNA replication. Further analysis of these effects in strains affected in other E. coli DNA replication functions indicated that this polAexo mutator effect is best explained by an effect that is additive compared with other error-producing events at the replication fork. No evidence was found that Pol I participates in the polymerase switching between Pol II, III and IV at the fork. Instead, our data suggest that the additional errors produced by polAexo are created during the maturation of Okazaki fragments in the lagging strand.

Dpb2p, a noncatalytic subunit of DNA polymerase epsilon, contributes to the fidelity of DNA replication in Saccharomyces cerevisiae

Most replicases are multi-subunit complexes. DNA polymerase epsilon from Saccharomyces cerevisiae is composed of four subunits: Pol2p, Dpb2p, Dpb3p, and Dpb4p. Pol2p and Dpb2p are essential. To investigate a possible role for the Dpb2p subunit in maintaining the fidelity of DNA replication, we isolated temperature-sensitive mutants in the DPB2 gene. Several of the newly isolated dpb2 alleles are strong mutators, exhibiting mutation rates equivalent to pol2 mutants defective in the 3' --> 5' proofreading exonuclease (pol2-4) or to mutants defective in mismatch repair (msh6). The dpb2 pol2-4 and dpb2 msh6 double mutants show a synergistic increase in mutation rate, indicating that the mutations arising in the dpb2 mutants are due to DNA replication errors normally corrected by mismatch repair. The dpb2 mutations decrease the affinity of Dpb2p for the Pol2p subunit as measured by two-hybrid analysis, providing a possible mechanistic explanation for the loss of high-fidelity synthesis. Our results show that DNA polymerase subunits other than those housing the DNA polymerase and 3' --> 5' exonuclease are essential in controlling the level of spontaneous mutagenesis and genetic stability in yeast cells.

Role of accessory DNA polymerases in DNA replication in Escherichia coli: analysis of the dnaX36 mutator mutant

The dnaX36(TS) mutant of Escherichia coli confers a distinct mutator phenotype characterized by enhancement of transversion base substitutions and certain (-1) frameshift mutations. Here, we have further investigated the possible mechanism(s) underlying this mutator effect, focusing in particular on the role of the various E. coli DNA polymerases. The dnaX gene encodes the tau subunit of DNA polymerase III (Pol III) holoenzyme, the enzyme responsible for replication of the bacterial chromosome. The dnaX36 defect resides in the C-terminal domain V of tau, essential for interaction of tau with the alpha (polymerase) subunit, suggesting that the mutator phenotype is caused by an impaired or altered alpha-tau interaction. We previously proposed that the mutator activity results from aberrant processing of terminal mismatches created by Pol III insertion errors. The present results, including lack of interaction of dnaX36 with mutM, mutY, and recA defects, support our assumption that dnaX36-mediated mutations originate as errors of replication rather than DNA damage-related events. Second, an important role is described for DNA Pol II and Pol IV in preventing and producing, respectively, the mutations. In the system used, a high fraction of the mutations is dependent on the action of Pol IV in a (dinB) gene dosage-dependent manner. However, an even larger but opposing role is deduced for Pol II, revealing Pol II to be a major editor of Pol III mediated replication errors. Overall, the results provide insight into the interplay of the various DNA polymerases, and of tau subunit, in securing a high fidelity of replication.

A molecular characterization of spontaneous frameshift mutagenesis within the trpA gene of Escherichia coli

Spontaneous frameshift mutations are an important source of genetic variation in all species and cause a large number of genetic disorders in humans. To enhance our understanding of the molecular mechanisms of frameshift mutagenesis, 583 spontaneous Trp+ revertants of two trpA frameshift alleles in Escherichia coli were isolated and DNA sequenced. In order to measure the contribution of methyl-directed mismatch repair to frameshift production, mutational spectra were constructed for both mismatch repair-proficient and repair-defective strains. The molecular origins of practically all of the frameshifts analyzed could be explained by one of six simple models based upon misalignment of the template or nascent DNA strands with or without misincorporation of primer nucleotides during DNA replication. Most frameshifts occurred within mononucleotide runs as has been shown often in previous studies but the location of the 76 frameshift sites was usually outside of runs. Mismatch repair generally was most effective in preventing the occurrence of frameshifts within runs but there was much variation from site to site. Most frameshift sites outside of runs appear to be refractory to mismatch repair although the small number of occurrences at most of these sites make firm conclusions impossible. There was a dense pattern of reversion sites within the trpA DNA region where reversion events could occur, suggesting that, in general, most DNA sequences are capable of undergoing spontaneous mutational events during replication that can lead to small deletions and insertions. Many of these errors are likely to occur at low frequencies and be tolerated as events too costly to prevent or repair. These studies also revealed an unpredicted flexibility in the primary amino acid sequence of the trpA product, the alpha subunit of tryptophan synthase.

Role of DNA polymerase IV in Escherichia coli SOS mutator activity

Constitutive expression of the SOS regulon in Escherichia coli recA730 strains leads to a mutator phenotype (SOS mutator) that is dependent on DNA polymerase V (umuDC gene product). Here we show that a significant fraction of this effect also requires DNA polymerase IV (dinB gene product).

Mutator mutants of Escherichia coli carrying a defect in the DNA polymerase III tau subunit

To investigate the possible role of accessory subunits of Escherichia coli DNA polymerase III holoenzyme (HE) in determining chromosomal replication fidelity, we have investigated the role of the dnaX gene. This gene encodes both the tau and gamma subunits of HE, which play a central role in the organization and functioning of HE at the replication fork. We find that a classical, temperature-sensitive dnaX allele, dnaX36, displays a pronounced mutator effect, characterized by an unusual specificity: preferential enhancement of transversions and -1 frameshifts. The latter occur predominantly at non-run sequences. The dnaX36 defect does not affect the gamma subunit, but produces a tau subunit carrying a missense substitution (E601K) in its C-terminal domain (domain V) that is involved in interaction with the Pol III alpha subunit. A search for new mutators in the dnaX region of the chromosome yielded six additional dnaX mutators, all carrying a specific tau subunit defect. The new mutators displayed phenotypes similar to dnaX36: strong enhancement of transversions and frameshifts and only weak enhancement for transitions. The combined findings suggest that the tau subunit of HE plays an important role in determining the fidelity of the chromosomal replication, specifically in the avoidance of transversions and frameshift mutations.

Mutator phenotype resulting from DNA polymerase IV overproduction in Escherichia coli: preferential mutagenesis on the lagging strand

We investigated the mutator effect resulting from overproduction of Escherichia coli DNA polymerase IV. Using lac mutational targets in the two possible orientations on the chromosome, we observed preferential mutagenesis during lagging strand synthesis. The mutator activity likely results from extension of mismatches produced by polymerase III holoenzyme.

DNA polymerase II as a fidelity factor in chromosomal DNA synthesis in Escherichia coli

Escherichia coli DNA polymerase III holoenzyme (HE) is the main replicase responsible for replication of the bacterial chromosome. E. coli contains four additional polymerases, and it is a relevant question whether these might also contribute to chromosomal replication and its fidelity. Here, we have investigated the role of DNA polymerase II (Pol II) (polB gene product). Mismatch repair-defective strains containing the polBex1 allele--encoding a polymerase-proficient but exonucleolytically defective Pol II--displayed a mutator activity for four different chromosomal lac mutational markers. The mutator effect was dependent on the chromosomal orientation of the lacZ gene. The results indicate that Pol II plays a role in chromosomal replication and that its role is not equal in leading- versus lagging-strand replication. In particular, the role of Pol II appeared larger in the lagging strand. When combined with dnaQ or dnaE mutator alleles, polBex1 showed strong, near multiplicative effects. The results fit a model in which Pol II acts as proofreader for HE-produced misinsertion errors. A second role of Pol II is to protect mismatched 3' termini against the mutagenic action of polymerase IV (dinB product). Overall, Pol II may be considered a main player in the polymerase trafficking at the replication fork.

The essential C family DnaE polymerase is error-prone and efficient at lesion bypass

DnaE-type DNA polymerases belong to the C family of DNA polymerases and are responsible for chromosomal replication in prokaryotes. Like most closely related Gram-positive cells, Streptococcus pyogenes has two DnaE homologs Pol C and DnaE; both are essential to cell viability. Pol C is an established replicative polymerase, and DnaE has been proposed to serve a replicative role. In this report, we characterize S. pyogenes DnaE polymerase and find that it is highly error-prone. DnaE can bypass coding and noncoding lesions with high efficiency. Error-prone extension is accomplished by either of two pathways, template-primer misalignment or direct primer extension. The bypass of abasic sites is accomplished mainly through "dNTP-stabilized" misalignment of template, thereby generating (-1) deletions in the newly synthesized strand. This mechanism may be similar to the dNTP-stabilized misalignment mechanism used by the Y family of DNA polymerases and is the first example of lesion bypass and error-prone synthesis catalyzed by a C family polymerase. Thus, DnaE may function in an error-prone capacity that may be essential in Gram-positive cells but not Gram-negative cells, suggesting a fundamental difference in DNA metabolism between these two classes of bacteria.

Involvement of DnaE, the second replicative DNA polymerase from Bacillus subtilis, in DNA mutagenesis

In a large group of organisms including low G + C bacteria and eukaryotic cells, DNA synthesis at the replication fork strictly requires two distinct replicative DNA polymerases. These are designated pol C and DnaE in Bacillus subtilis. We recently proposed that DnaE might be preferentially involved in lagging strand synthesis, whereas pol C would mainly carry out leading strand synthesis. The biochemical analysis of DnaE reported here is consistent with its postulated function, as it is a highly potent enzyme, replicating as fast as 240 nucleotides/s, and stalling for more than 30 s when encountering annealed 5'-DNA end. DnaE is devoid of 3' --> 5'-proofreading exonuclease activity and has a low processivity (1-75 nucleotides), suggesting that it requires additional factors to fulfill its role in replication. Interestingly, we found that (i) DnaE is SOS-inducible; (ii) variation in DnaE or pol C concentration has no effect on spontaneous mutagenesis; (iii) depletion of pol C or DnaE prevents UV-induced mutagenesis; and (iv) purified DnaE has a rather relaxed active site as it can bypass lesions that generally block other replicative polymerases. These results suggest that DnaE and possibly pol C have a function in DNA repair/mutagenesis, in addition to their role in DNA replication.

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.

Fidelity of replication of repetitive DNA in mutS and repair proficient Escherichia coli

Replication fidelity is not constant among strains within a species or at all genetic loci within a genome. Altered fidelity of replication may affect patterns of pathogenesis and the evolution of these strains. We have been studying replication fidelity in Escherichia coli, both in laboratory attenuated strains and in food-borne pathogens. To understand the altered patterns of mutagenesis at the molecular level, we used a shuttle vector plasmid with a tRNA mutational marker gene which had been altered to include homopolymeric runs of five, seven and nine [G:C] pairs, as well as non-repetitive DNA. Replication of the plasmid in mutS strains resulted in a 20-fold increase in mutant progeny plasmids. The mutations were almost all (>90%) frameshift mutations, while base substitution mutations were rare. Most mutations were insertions or deletions of one or two [G:C] pairs in the longest homopolymeric runs. Larger deletions (5 to >70bp), also targeted to the repetitive sequence, were likewise common. Mutations increased exponentially with the length of the homopolymeric run. These patterns of mutation, including unexpectedly high levels in repair proficient strains, led to an examination of the E. coli K-12 genome for homopolymeric DNA. This sequence motif was found to be rare, particularly in genes and open reading frames. Amino acid homotrimers were found to avoid usage of homopolymeric codons, even when they are preferred among synonymous codons in E. coli. There appears to be active selection against tandem direct nucleotide repeats in the E. coli genome, correlated with the inability of the organism to accurately replicate such sequence.

DNA replication fidelity

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

Minor groove interactions at the DNA polymerase beta active site modulate single-base deletion error rates

The structures of open and closed conformations of DNA polymerase beta (pol beta) suggests that the rate of single-nucleotide deletions during synthesis may be modulated by interactions in the DNA minor groove that align the templating base with the incoming dNTP. To test this hypothesis, we measured the single-base deletion error rates of wild-type pol beta and lysine and alanine mutants of Arg(283), whose side chain interacts with the minor groove edge of the templating nucleotide at the active site. The error rates of both mutant enzymes are increased >100-fold relative to wild-type pol beta. Template engineering experiments performed to distinguish among three possible models for deletion formation suggest that most deletions in repetitive sequences by pol beta initiate by strand slippage. However, pol beta also generates deletions by a different mechanism that is strongly enhanced by the substitutions at Arg(283). Analysis of error specificity suggests that this mechanism involves nucleotide misinsertion followed by primer relocation, creating a misaligned intermediate. The structure of pol beta bound to non-gapped DNA also indicates that the templating nucleotide and its downstream neighbor are out of register in the open conformation and this could facilitate misalignment (dNTP or primer terminus) with the next template base.

Escherichia coli DNA polymerase IV mutator activity: genetic requirements and mutational specificity

The dinB gene of Escherichia coli is known to be involved in the untargeted mutagenesis of lambda phage. Recently, we have demonstrated that this damage-inducible and SOS-controlled gene encodes a novel DNA polymerase, DNA Pol IV, which is able to dramatically increase the untargeted mutagenesis of F' plasmid. At the amino acid level, DNA Pol IV shares sequence homologies with E. coli UmuC (DNA Pol V), Rev1p, and Rad30p (DNA polymerase eta) of Saccharomyces cerevisiae and human Rad30A (XPV) proteins, all of which are involved in translesion DNA synthesis. To better characterize the Pol IV-dependent untargeted mutagenesis, i.e., the DNA Pol IV mutator activity, we analyzed the genetic requirements of this activity and determined the forward mutation spectrum generated by this protein within the cII gene of lambda phage. The results indicated that the DNA Pol IV mutator activity is independent of polA, polB, recA, umuDC, uvrA, and mutS functions. The analysis of more than 300 independent mutations obtained in the wild-type or mutS background revealed that the mutator activity clearly promotes single-nucleotide substitutions as well as one-base deletions in the ratio of about 1:2. The base changes were strikingly biased for substitutions toward G:C base pairs, and about 70% of them occurred in 5'-GX-3' sequences, where X represents the base (T, A, or C) that is mutated to G. These results are discussed with respect to the recently described biochemical characteristics of DNA Pol IV.

Strand asymmetry of +1 frameshift mutagenesis at a homopolymeric run by DNA polymerase III holoenzyme of Escherichia coli

We have recently shown that single-base frameshifts were predominant among mutations induced within the rpsL target sequence upon oriC plasmid DNA replication in vitro. We found that the occurrence of +1 frameshifts at a run of 6 residues of dA/dT could be increased proportionally by increasing the concentration of dATP present in the in vitro replication. Using single-stranded circular DNA containing either the coding sequence of the rpsL gene or its complementary sequence, the +1 frameshift mutagenesis by DNA polymerase III holoenzyme of Escherichia coli was extensively examined. A(6) --> A(7) frameshifts occurred 30 to 90 times more frequently during DNA synthesis with the noncoding sequence (dT tract) template than with the coding sequence (dA tract). Excess dATP enhanced the occurrence of +1 frameshifts during DNA synthesis with the dT tract template, but no other dNTPs showed such an effect. In the presence of 0.1 mM dATP, the A(6) --> A(7) mutagenesis with the dT tract template was not inhibited by 1.5 mM dCTP, which is complementary to the residue immediately upstream of the dT tract. These results strongly suggested that the A(6) --> A(7) frameshift mutagenesis possesses an asymmetric strand nature and that slippage errors leading to the +1 frameshift are made during chain elongation within the tract rather than by misincorporation of nucleotides opposite residues next to the tract.