Binding of specific DNA base-pair mismatches by N-methylpurine-DNA glycosylase and its implication in initial damage recognition

Recognition of 1,N2-ethenoguanine by alkyladenine DNA glycosylase is restricted by a conserved active-site residue

The adenine, cytosine, and guanine bases of DNA are susceptible to alkylation by the aldehyde products of lipid peroxidation and by the metabolic byproducts of vinyl chloride pollutants. The resulting adducts spontaneously cyclize to form harmful etheno lesions. Cells employ a variety of DNA repair pathways to protect themselves from these pro-mutagenic modifications. Human alkyladenine DNA glycosylase (AAG) is thought to initiate base excision repair of both 1,N6-ethenoadenine (ϵA) and 1,N2-ethenoguanine (ϵG). However, it is not clear how AAG might accommodate ϵG in an active site that is complementary to ϵA. This prompted a thorough investigation of AAG-catalyzed excision of ϵG from several relevant contexts. Using single-turnover and multiple-turnover kinetic analyses, we found that ϵG in its natural ϵG·C context is very poorly recognized relative to ϵA·T. Bulged and mispaired ϵG contexts, which can form during DNA replication, were similarly poor substrates for AAG. Furthermore, AAG could not recognize an ϵG site in competition with excess undamaged DNA sites. Guided by previous structural studies, we hypothesized that Asn-169, a conserved residue in the AAG active-site pocket, contributes to discrimination against ϵG. Consistent with this model, the N169S variant of AAG was 7-fold more active for excision of ϵG compared with the wildtype (WT) enzyme. Taken together, these findings suggest that ϵG is not a primary substrate of AAG, and that current models for etheno lesion repair in humans should be revised. We propose that other repair and tolerance mechanisms operate in the case of ϵG lesions.

QM/MM Study of the Reaction Catalyzed by Alkyladenine DNA Glycosylase: Examination of the Substrate Specificity of a DNA Repair Enzyme

Human alkyladenine DNA glycosylase (AAG) functions as part of the base excision repair pathway to excise structurally diverse oxidized and alkylated DNA purines. Specifically, AAG uses a water molecule activated by a general base and a nonspecific active site lined with aromatic residues to cleave the N-glycosidic bond. Despite broad substrate specificity, AAG does not target the natural purines (adenine (A) and guanine (G)). Using the ONIOM(QM:MM) methodology, we provide fundamental atomic level details of AAG bound to DNA-containing a neutral substrate (hypoxanthine (Hx)), a nonsubstrate (G), or a cationic substrate (7-methylguanine (7MeG)) and probe changes in the reaction pathway that occur when AAG targets different nucleotides. We reveal that subtle differences in protein-DNA contacts upon binding different substrates within the flexible AAG active site can significantly affect the deglycosylation reaction. Notably, we predict that AAG excises Hx in a concerted mechanism that is facilitated through correct alignment of the (E125) general base due to hydrogen bonding with a neighboring aromatic amino acid (Y127). Hx departure is further stabilized by π-π interactions with aromatic amino acids and hydrogen bonds with active site water. Despite possessing a similar structure to Hx, G is not excised since the additional exocyclic amino group leads to misalignment of the general base due to disruption of the key E125-Y127 hydrogen bond, the catalytically unfavorable placement of water within the active site, and weakened π-contacts between aromatic amino acids and the nucleobase. In contrast, cationic 7MeG does not occupy the same position within the AAG active site as G due to steric clashes with the additional N7 methyl group, which results in the correct alignment of the general base and permits nucleobase excision as observed for neutral Hx. Overall, our structural data rationalizes the observed substrate specificity of AAG and contributes to our fundamental understanding of enzymes with flexible active sites and broad substrate specificities.

Guardians of the mycobacterial genome: A review on DNA repair systems in Mycobacterium tuberculosis

The genomic integrity of Mycobacterium tuberculosis is continuously threatened by the harsh survival conditions inside host macrophages, due to immune and antibiotic stresses. Faithful genome maintenance and repair must be accomplished under stress for the bacillus to survive in the host, necessitating a robust DNA repair system. The importance of DNA repair systems in pathogenesis is well established. Previous examination of the M. tuberculosis genome revealed homologues of almost all the major DNA repair systems, i.e. nucleotide excision repair (NER), base excision repair (BER), homologous recombination (HR) and non-homologous end joining (NHEJ). However, recent developments in the field have pointed to the presence of novel proteins and pathways in mycobacteria. Homologues of archeal mismatch repair proteins were recently reported in mycobacteria, a pathway previously thought to be absent. RecBCD, the major nuclease-helicase enzymes involved in HR in E. coli, were implicated in the single-strand annealing (SSA) pathway. Novel roles of archeo-eukaryotic primase (AEP) polymerases, previously thought to be exclusive to NHEJ, have been reported in BER. Many new proteins with a probable role in DNA repair have also been discovered. It is now realized that the DNA repair systems in M. tuberculosis are highly evolved and have redundant backup mechanisms to mend the damage. This review is an attempt to summarize our current understanding of the DNA repair systems in M. tuberculosis.

Thermodynamic signature of DNA damage: characterization of DNA with a 5-hydroxy-2'-deoxycytidine·2'-deoxyguanosine base pair

Oxidation of DNA due to exposure to reactive oxygen species is a major source of DNA damage. One of the oxidation lesions formed, 5-hydroxy-2'-deoxycytidine, has been shown to miscode by some replicative DNA polymerases but not by error prone polymerases capable of translesion synthesis. The 5-hydroxy-2'-deoxycytidine lesion is repaired by DNA glycosylases that require the 5-hydroxycytidine base to be extrahelical so it can enter into the enzyme's active site where it is excised off the DNA backbone to afford an abasic site. The thermodynamic and nuclear magnetic resonance results presented here describe the effect of a 5-hydroxy-2'-deoxycytidine·2'-deoxyguanosine base pair on the stability of two different DNA duplexes. The results demonstrate that the lesion is highly destabilizing and that the energy barrier for the unstacking of 5-hydroxy-2'-deoxycytidine from the DNA duplex may be low. This could provide a thermodynamic mode of adduct identification by DNA glycosylases that requires the lesion to be extrahelical.

Frameshift mutagenesis and microsatellite instability induced by human alkyladenine DNA glycosylase

Human alkyladenine DNA glycosylase (hAAG) excises alkylated purines, hypoxanthine, and etheno bases from DNA to form abasic (AP) sites. Surprisingly, elevated expression of hAAG increases spontaneous frameshift mutagenesis. By random mutagenesis of eight active site residues, we isolated hAAG-Y127I/H136L double mutant that induces even higher rates of frameshift mutation than does the wild-type hAAG; the Y127I mutation accounts for the majority of the hAAG-Y127I/H136L-induced mutator phenotype. The hAAG-Y127I/H136L and hAAG-Y127I mutants increased the rate of spontaneous frameshifts by up to 120-fold in S. cerevisiae and also induced high rates of microsatellite instability (MSI) in human cells. hAAG and its mutants bind DNA containing one and two base-pair loops with significant affinity, thus shielding them from mismatch repair; the strength of such binding correlates with their ability to induce the mutator phenotype. This study provides important insights into the mechanism of hAAG-induced genomic instability.

Kinetic mechanism for the flipping and excision of 1,N(6)-ethenoadenine by human alkyladenine DNA glycosylase

Human alkyladenine DNA glycosylase initiates the repair of a wide variety of alkylated and deaminated purine lesions in DNA. In this study, we take advantage of the natural fluorescence of the 1,N(6)-ethenoadenosine (epsilonA) lesion and report a kinetic analysis of binding, nucleotide flipping, base excision, and product release. The transient changes in the fluorescence of epsilonA revealed the existence of two distinct complexes that are formed prior to the hydrolysis step. An initial recognition complex forms rapidly and is characterized by partial disruption of the stacking interactions of the lesioned base. Subsequently, a very stable extrahelical complex is formed in which the epsilonA lesion is strongly quenched by interactions in the AAG active site pocket. Our results indicate that DNA binding and base flipping take place on the millisecond to second time scale. N-Glycosidic bond cleavage is much slower, taking place on the minute time scale. A pulse-chase experiment was used to demonstrate that even for the tightly bound epsilonA substrate, the extrahelical complex is not fully committed to excision. Nevertheless, flipping of epsilonA is highly favorable, and we calculate that the equilibrium constant for flipping is approximately 1300. This kinetic mechanism has important biological implications. First, two-step binding provides multiple opportunities to discriminate between damaged and undamaged nucleotides. Second, a rapid equilibrium flipping mechanism maximizes specificity for damaged versus undamaged bases, since undamaged bases generally form stronger base pairs than damaged bases. Finally, the highly favorable equilibrium for flipping of epsilonA ensures that epsilonA removal is independent of sequence context and highly efficient despite the relatively slow rate of N-glycosidic bond hydrolysis.

Human alkyladenine DNA glycosylase employs a processive search for DNA damage

DNA repair proteins conduct a genome-wide search to detect and repair sites of DNA damage wherever they occur. Human alkyladenine DNA glycosylase (AAG) is responsible for recognizing a variety of base lesions, including alkylated and deaminated purines, and initiating their repair via the base excision repair pathway. We have investigated the mechanism by which AAG locates sites of damage using an oligonucleotide substrate containing two sites of DNA damage. This substrate was designed so that AAG randomly binds to either of the two lesions. AAG-catalyzed base excision creates a repair intermediate, and the subsequent partitioning between dissociation and diffusion to the second site can be quantified from the rates of formation of the different products. Our results demonstrate that AAG has the ability to slide for short distances along DNA at physiological salt concentrations. The processivity of AAG decreases with increasing ionic strength to become fully distributive at high ionic strengths, suggesting that electrostatic interactions between the negatively charged DNA and the positively charged DNA binding surface are important for nonspecific DNA binding. Although the amino terminus of the protein is dispensable for glycosylase activity at a single site, we find that deletion of the 80 amino-terminal amino acids significantly decreases the processivity of AAG. These observations support the idea that diffusion on undamaged DNA contributes to the search for sites of DNA damage.

How PspGI, catalytic domain of EcoRII and Ecl18kI acquire specificities for different DNA targets

Restriction endonucleases Ecl18kI and PspGI/catalytic domain of EcoRII recognize CCNGG and CCWGG sequences (W stands for A or T), respectively. The enzymes are structurally similar, interact identically with the palindromic CC:GG parts of their recognition sequences and flip the nucleotides at their centers. Specificity for the central nucleotides could be influenced by the strength/stability of the base pair to be disrupted and/or by direct interactions of the enzymes with the flipped bases. Here, we address the importance of these contributions. We demonstrate that wt Ecl18kI cleaves oligoduplexes containing canonical, mismatched and abasic sites in the central position of its target sequence CCNGG with equal efficiencies. In contrast, substitutions in the binding pocket for the extrahelical base alter the Ecl18kI preference for the target site: the W61Y mutant prefers only certain mismatched substrates, and the W61A variant cuts exclusively at abasic sites, suggesting that pocket interactions play a major role in base discrimination. PspGI and catalytic domain of EcoRII probe the stability of the central base pair and the identity of the flipped bases in the pockets. This ‘double check’ mechanism explains their extraordinary specificity for an A/T pair in the flipping position.

Methylating agents and DNA repair responses: Methylated bases and sources of strand breaks

The chemical methylating agents methylmethane sulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have been used for decades as classical DNA damaging agents. These agents have been utilized to uncover and explore pathways of DNA repair, DNA damage response, and mutagenesis. MMS and MNNG modify DNA by adding methyl groups to a number of nucleophilic sites on the DNA bases, although MNNG produces a greater percentage of O-methyl adducts. There has been substantial progress elucidating direct reversal proteins that remove methyl groups and base excision repair (BER), which removes and replaces methylated bases. Direct reversal proteins and BER, thus, counteract the toxic, mutagenic, and clastogenic effects of methylating agents. Despite recent progress, the complexity of DNA damage responses to methylating agents is still being discovered. In particular, there is growing understanding of pathways such as homologous recombination, lesion bypass, and mismatch repair that react when the response of direct reversal proteins and BER is insufficient. Furthermore, the importance of proper balance within the steps in BER has been uncovered with the knowledge that DNA structural intermediates during BER are deleterious. A number of issues complicate the elucidation of the downstream responses when direct reversal is insufficient or BER is imbalanced. These include inter-species differences, cell-type-specific differences within mammals and between cancer cell lines, and the type of methyl damage or BER intermediate encountered. MMS also carries a misleading reputation of being a radiomimetic, that is, capable of directly producing strand breaks. This review focuses on the DNA methyl damage caused by MMS and MNNG for each site of potential methylation to summarize what is known about the repair of such damage and the downstream responses and consequences if the damage is not repaired.

The origins of high-affinity enzyme binding to an extrahelical DNA base

Base flipping is a highly conserved strategy used by enzymes to gain catalytic access to DNA bases that would otherwise be sequestered in the duplex structure. A classic example is the DNA repair enzyme uracil DNA glycosylase (UDG) which recognizes and excises unwanted uracil bases from DNA using a flipping mechanism. Previous work has suggested that enzymatic base flipping begins with dynamic breathing motions of the enzyme-bound DNA substrate, and then, only very late during the reaction trajectory do strong specific interactions with the extrahelical uracil occur. Here we report that UDG kinetically and thermodynamically prefers substrate sites where the uracil is paired with an unnatural adenine analogue that lacks any Watson-Crick hydrogen-bonding groups. The magnitude of the preference is a striking 43000-fold as compared to an adenine analogue that forms three H-bonds. Transient kinetic and fluorescence measurements suggest that preferential recognition of uracil in the context of a series of incrementally destabilized base pairs arises from two distinct effects: weak or absent hydrogen bonding, which thermodynamically assists extrusion, and, most importantly, increased flexibility of the site which facilitates DNA bending during base flipping. A coupled, stepwise reaction coordinate is implicated in which DNA bending precedes base pair rupture and flipping.

Human 3-methyladenine-DNA glycosylase: effect of sequence context on excision, association with PCNA, and stimulation by AP endonuclease

Human 3-methyladenine-DNA glycosylase (MPG protein) is involved in the base excision repair (BER) pathway responsible mainly for the repair of small DNA base modifications. It initiates BER by recognizing DNA adducts and cleaving the glycosylic bond leaving an abasic site. Here, we explore several of the factors that could influence excision of adducts recognized by MPG, including sequence context, effect of APE1, and interaction with other proteins. To investigate sequence context, we used 13 different 25 bp oligodeoxyribonucleotides containing a unique hypoxanthine residue (Hx) and show that the steady-state specificity of Hx excision by MPG varied by 17-fold. If APE1 protein is used in the reaction for Hx removal by MPG, the steady-state kinetic parameters increase by between fivefold and 27-fold, depending on the oligodeoxyribonucleotide. Since MPG has a role in removing adducts such as 3-methyladenine that block DNA synthesis and there is a potential sequence for proliferating cell nuclear antigen (PCNA) interaction, we hypothesized that MPG protein could interact with PCNA, a protein involved in repair and replication. We demonstrate that PCNA associates with MPG using immunoprecipitation with either purified proteins or whole cell extracts. Moreover, PCNA binds to both APE1 and MPG at different sites, and loading PCNA onto a nicked, closed circular substrate with a unique Hx residue enhances MPG catalyzed excision. These data are consistent with an interaction that facilitates repair by MPG or APE1 by association with PCNA. Thus, PCNA could have a role in short-patch BER as well as in long-patch BER. Overall, the data reported here show how multiple factors contribute to the activity of MPG in cells.

Short-patch correction of C/C mismatches in human cells

We examined whether the human nucleotide excision repair complex, which is specialized on the removal of bulky DNA adducts, also displays a correcting activity on base mismatches. The cytosine/cytosine (C/C) lesion was used as a model substrate to monitor the correction of base mismatches in human cells. Fibroblasts with different repair capabilities were transfected with shuttle vectors that contain a site-directed C/C mismatch in the replication origin, accompanied by an additional C/C mismatch in one of the flanking sequences that are not essential for replication. Analysis of the vector progeny obtained from these doubly modified substrates revealed that C/C mismatches were eliminated before DNA synthesis not only in the repair-proficient background, but also when the target cells carried a genetic defect in long-patch mismatch repair, in nucleotide excision repair, or when both pathways were deleted. Furthermore, cells deficient for long-patch mismatch repair as well as a cell line that combines mismatch and nucleotide excision repair defects were able to correct multiple C/C mispairs, placed at distances of 21-44 nt, in an independent manner, such that the removal of each lesion led to individual repair patches. These results support the existence of a concurrent short-patch mechanism that rectifies C/C mismatches.

Linear free energy correlations for enzymatic base flipping: how do damaged base pairs facilitate specific recognition?

To efficiently maintain their genomic integrity, DNA repair glycosylases must exhibit high catalytic specificity for their cognate damaged bases using an extrahelical recognition mechanism. One possible contribution to specificity is the weak base pairing and inherent instability of damaged sites which may lead to increased extrahelicity of the damaged base and enhanced recognition of these sites. This model predicts that the binding affinity of the enzyme should increase as the thermodynamic stability of the lesion base pair decreases, because less work is required to extrude the base into its active site. We have tested this hypothesis with uracil DNA glycosylase (UDG) by constructing a series of DNA duplexes containing a single uracil (U) opposite a variety of bases (X) that formed from zero to three hydrogen bonds with U. Linear free energy (LFE) relationships were observed that correlated UDG binding affinity with the entropy and enthalpy of duplex melting, and the dynamic accessibility of the damaged site to chemical oxidation. These LFEs indicate that the increased conformational freedom of the damaged site brought about by enthalpic destabilization of the base pair promotes the formation of extrahelical states that enhance specific recognition by as much as 3000-fold. However, given the small stability differences between normal base pairs and U.A or U.G base pairs, relative base pair stability contributes little to the >10(6)-fold discrimination of UDG for uracil sites in cellular DNA. In contrast, the intrinsic instability of other more egregious DNA lesions may contribute significantly to the specificity of other DNA repair enzymes that bind to extrahelical bases.

The Escherichia coli 3-methyladenine DNA glycosylase AlkA has a remarkably versatile active site

3-Methyladenine DNA glycosylase II (AlkA) from Escherichia coli is induced in response to DNA alkylation, and it protects cells from alkylated nucleobases by catalyzing their excision. In contrast to the highly specific 3-methyladenine DNA glycosylase I (E. coli TAG) that catalyzes the excision of 3-methyl adducts of adenosine and guanosine from DNA, AlkA catalyzes the excision of a wide variety of alkylated bases including N-3 and N-7 adducts of adenosine and guanosine and O(2) adducts of thymidine and cytidine. We have investigated how AlkA can recognize a diverse set of damaged bases by characterizing its discrimination between oligonucleotide substrates in vitro. Similar rate enhancements are observed for the excision of a structurally diverse set of substituted purine bases and of the normal purines adenine and guanine. These results are consistent with a remarkably indiscriminate active site and suggest that the rate of AlkA-catalyzed excision is dictated not by the catalytic recognition of a specific substrate but instead by the reactivity of the N-glycosidic bond of each substrate. Damaged bases with altered base pairing have a modest advantage, as mismatches are processed up to 400-fold faster than stable Watson-Crick base pairs. Nevertheless, AlkA does not effectively exclude undamaged DNA from its active site. The resulting deleterious excision of normal bases is expected to have a substantial cost associated with the expression of AlkA.

Repairing DNA-methylation damage

Methylating agents modify DNA at many different sites, thereby producing lethal and mutagenic lesions. To remove all the main harmful base lesions, at least three types of DNA-repair activities can be used, each of which involves a different reaction mechanism. These activities include DNA-glycosylases, DNA-methyltransferases and the recently characterized DNA-dioxygenases. The Escherichia coli AlkB dioxygenase and the two human homologues, ABH2 and ABH3, represent a novel mechanism of DNA repair. They use iron-oxo intermediates to oxidize stable methylated bases in DNA and directly revert them to the unmodified form.

Dissecting the broad substrate specificity of human 3-methyladenine-DNA glycosylase

Human alkyladenine-DNA glycosylase (AAG) catalyzes the excision of a broad range of modified bases, protecting the genome from many types of alkylative and oxidative DNA damage. We have investigated how AAG discriminates against normal DNA bases, while accommodating a structurally diverse set of lesioned bases, by measuring the rates of AAG-catalyzed (k(st)) and spontaneous N-glycosidic bond hydrolysis (k(non)) for damaged and undamaged DNA oligonucleotides. The rate enhancements for excision of different bases reveal that AAG is most adept at excising the deaminated lesion hypoxanthine (k(st)/k(non) = 10(8)), suggesting that enzymatic activity may have evolved in response to this lesion. Comparisons of the rate enhancements for excision of normal and modified purine nucleobases provide evidence that AAG excludes the normal purines via steric clashes with the exocyclic amino groups of adenine and guanine. However, methylated purines are more chemically labile, and only modest rate enhancements are required for their efficient excision. Base flipping also contributes to specificity as destabilized mismatched base pairs are better substrates than stable Watson-Crick pairs, and many of the lesions recognized by AAG are compromised in their ability to base pair. These findings suggest that AAG reconciles a broad substrate tolerance with the biological imperative to avoid normal DNA by excluding normal bases from the active site rather than by specifically recognizing each lesion.

Human alkyladenine DNA glycosylase uses acid-base catalysis for selective excision of damaged purines

Human alkyladenine DNA glycosylase (AAG) protects against alkylative and oxidative DNA damage, flipping damaged nucleotides out of double-stranded DNA and catalyzing the hydrolytic cleavage of the N-glycosidic bond to release the damaged nucleobase. The crystal structure of AAG bound to a DNA substrate reveals features of the active site that could discriminate between oxidatively damaged or alkylated purines and normal purines. A water molecule bound in the active site adjacent to the anomeric carbon of the N-glycosidic bond is suggestive of direct attack by water, with Glu125 acting as a general base. However, biochemical evidence for this proposed mechanism has been lacking. The structure also fails to explain why smaller pyrimidine nucleosides are excluded as substrates from this relatively permissive active site that catalyzes the excision of a structurally diverse group of damaged purine bases. We have used pH dependencies, site-directed mutagenesis, and a variety of substrates to investigate the catalytic mechanism of AAG. Single-turnover excision of hypoxanthine and 1,N(6)-ethenoadenine follows bell-shaped pH-rate profiles, indicating that AAG-catalyzed excision of these neutral lesions requires the action of both a general acid and a general base. In contrast, the pH-rate profile for the reaction of 7-methylguanine, a positively charged substrate, shows only a single ionization corresponding to a general base. These results suggest that AAG activates neutral lesions by protonation of the nucleobase leaving group. Glu125 must be deprotonated in the active form of the enzyme, consistent with a role as a general base that activates and positions a water nucleophile. Acid-base catalysis can account for much of the 10(8)-fold rate enhancement that is achieved by AAG in the excision of hypoxanthine. The prominent role of nucleobase protonation in catalysis by AAG provides a rationale for its specialization toward damaged purines while effectively excluding pyrimidines.

Influence of local duplex stability and N6-methyladenine on uracil recognition by mismatch-specific uracil-DNA glycosylase (Mug)

To maintain genomic integrity, DNA repair enzymes continually remove damaged bases and lesions resulting from endogenous and exogenous processes. These repair enzymes must distinguish damaged bases from normal bases to prevent the inadvertent removal of normal bases, which would promote genomic instability. The mechanisms by which this high level of specificity is accomplished are as yet unresolved. One member of the uracil-DNA glycosylase family of repair enzymes, Escherichia coli mismatch-specific uracil-DNA glycosylase (Mug), is reported to distinguish U:G mispairs from U:A base pairs based upon specific contacts with the mispaired guanine after flipping the target uracil out of the duplex. However, recent studies suggest other mechanisms for base selection, including local duplex stability. In this study, we used the modified base N6-methyladenine to probe the effect of local helix perturbation on Mug recognition of uracil. N6-Methyladenine is found in E. coli as part of both the mismatch repair and restriction-modification systems. In its cis isomer, N6-methyladenine destabilizes hydrogen bonding by interfering with pseudo-Watson-Crick base pairing. It is observed that the selection of uracil by Mug is sequence dependent and that uracil residues in sequences of reduced thermostability are preferentially removed. The replacement of adenine by N6-methyladenine increases the frequency of removal of the uracil residue paired opposite the modified adenine. These results are in accord with suggestions that local helix stability is an important determinant of base recognition by some DNA repair enzymes and provide a potential strategy for identifying the sequence location of modified bases in DNA.

Active-site clashes prevent the human 3-methyladenine DNA glycosylase from improperly removing bases

The human 3-methyladenine DNA glycosylase (AAG, MPG) removes a diverse array of damaged purines via a nucleotide-flipping mechanism. In the crystal structure of AAG bound to DNA containing 1,N(6) ethenoadenine, an asparagine (N169) occupies the active-site floor, in close proximity to the C-2 position of the flipped-out 1,N(6) ethenoadenine. We engineered site-specific AAG mutants to determine whether N169 prevents normal bases from mistakenly entering the active site. Substituting alanine or serine resulted in mutants that excised substrates at a faster rate than wild-type. Furthermore, these mutants acquired the ability to excise normal guanine within mispairs but not opposite cytosine. The results suggest that AAG can recognize helical deformations, such as mispairs. However, the active site then prevents the mistaken excision of bases, which prevents AAG from acquiring a mutator activity.