Base Excision Repair Glycosylases Research Articles

Crystallization and preliminary crystallographic study of a ternary complex between the T4 phage beta-glucosyltransferase, uridine diphosphoglucose and a DNA fragment containing an abasic site.

A base-flipping phenomenon has been established for DNA methyltransferases and for DNA base-excision repair glycosylases and is likely to prove general for enzymes that need access to DNA bases to undergo chemical reaction. T4 phage beta-glucosyltransferase (BGT) is a good candidate for this novel mechanism. In order to confirm this, BGT was crystallized with an abasic site-containing DNA and uridine diphosphoglucose (UDP-glucose). The crystallization strategy is described. A complete data set was collected at 1.8 A resolution on a Cu Kalpha rotating-anode X-ray source. Molecular replacement was performed and the initial electron-density maps clearly show bound DNA.

Escherichia coli Apurinic-Apyrimidinic Endonucleases Enhance the Turnover of the Adenine Glycosylase MutY with G:A Substrates

The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG). MutY is a base excision repair (BER) glycosylase that removes misincorporated adenine residues from OG:A mispairs, as well as G:A and C:A mispairs. We have previously shown that, under conditions of low MutY concentrations relative to an OG:A or G:A substrate, the time course of the adenine glycosylase reaction exhibits biphasic kinetic behavior due to slow release of the DNA product by MutY. The dissociation of MutY from its product may require the recruitment of other proteins from the BER pathway, such as an apurinic-apyrimidinic (AP) endonuclease, as turnover-enhancing cofactors. The effect of the AP endonucleases endonuclease IV (Endo IV), exonuclease III (Exo III), and Ape1 on the reaction kinetics of MutY with G:A- and OG:A-containing substrates was investigated. The effect of the glycosylases UDG and MutM and the DNA polymerase pol I was also characterized. Endo IV and Exo III, unlike Ape1, UDG, and pol I, greatly enhance the rate of product release with a G:A substrate, whereas the rate constant for the adenine removal step remains unchanged. Furthermore, the turnover rate with a truncated form of MutY, Stop 225, which lacks 125 amino acids of the C terminus, is unaffected by the presence of Endo IV or Exo III. These results constitute the first evidence of an interaction between the MutY-product DNA complex and Endo IV or Exo III. Furthermore, they suggest a role for the C-terminal domain of MutY in mediating this interaction.

Mechanistic comparisons among base excision repair glycosylases

The mechanisms by which various DNA glycosylases initiate the base excision repair pathways are discussed. Fundamental distinctions are made between “simple glycosylases,” that do not form DNA single-strand breaks, and “glycosylases/abasic site lyases,” that do form single-strand breaks. Several groupings of BER substrate sites are defined and some interactions between these groupings and glycosylase mechanisms discussed. Two characteristics are proposed to be common among all BER glycosylases: a nucleotide flipping step that serves to expose the scissile glycosyl bond to catalysis, and a glycosylase transition state characterized by substantial tetrahedral character at the base glycosyl atom.

DNA mismatch repair and mutation avoidance pathways.

Unpaired and mispaired bases in DNA can arise by replication errors, spontaneous or induced base modifications, and during recombination. The major pathway for correction of mismatches arising during replication is the MutHLS pathway of Escherichia coli and related pathways in other organisms. MutS initiates repair by binding to the mismatch, and activates together with MutL the MutH endonuclease, which incises at hemimethylated dam sites and thereby mediates strand discrimination. Multiple MutS and MutL homologues exist in eukaryotes, which play different roles in the mismatch repair (MMR) pathway or in recombination. No MutH homologues have been identified in eukaryotes, suggesting that strand discrimination is different to E. coli. Repair can be initiated by the heterodimers MSH2-MSH6 (MutSalpha) and MSH2-MSH3 (MutSbeta). Interestingly, MSH3 (and thus MutSbeta) is missing in some genomes, as for example in Drosophila, or is present as in Schizosaccharomyces pombe but appears to play no role in MMR. MLH1-PMS1 (MutLalpha) is the major MutL homologous heterodimer. Again some, but not all, eukaryotes have additional MutL homologues, which all form a heterodimer with MLH1 and which play a minor role in MMR. Additional factors with a possible function in eukaryotic MMR are PCNA, EXO1, and the DNA polymerases delta and epsilon. MMR-independent pathways or factors that can process some types of mismatches in DNA are nucleotide-excision repair (NER), some base excision repair (BER) glycosylases, and the flap endonuclease FEN-1. A pathway has been identified in Saccharomyces cerevisiae and human that corrects loops with about 16 to several hundreds of unpaired nucleotides. Such large loops cannot be processed by MMR.

T4 endonuclease V: Use of NMR and borohydride trapping to provide evidence for covalent enzyme-substrate imine intermediate

The experimental methodologies used in earlier studies on DNA strand breakage characterized the nature of the ends of the product DNA strands, and the experimental data are consistent with the proposed β-elimination mechanism. These interpretations implied a Schiff base (imine) intermediate in the DNA strand breakage reaction and a mechanistic parallel with the aldolase reaction. In this class of enzyme mechanisms, the formation of the protonated imine labilizes the bond from the adjacent carbon to carbon or hydrogen. These studies constituted an early demonstration of the efficacy of amino group reductive methylation coupled with biochemical and NMR experiments in elucidating the involvement of specific residues in enzyme catalysis. The methodology could be more widely applied in the investigation of enzymes utilizing Schiff base intermediates. For example, [ 13 C] formaldehyde-labeled active site amino groups may be used as spectroscopic (NMR) probes to study the structural aspects of substrate binding. Similarly, the base excision repair glycosylases use a base flipping mechanism to achieve access to the scissile base-sugar bond. 13 C-Labeled active site amines may be used as probes of this process.

Backbone dynamics of DNA containing 8-oxoguanine: importance for substrate recognition by base excision repair glycosylases

Except for the functional groups sited within the major or minor grooves, the bases of B-DNA are quite protected from the external environment. Enzymes that modify the bases often "flip out" the target into an extrahelical position before the chemistry step is carried out. Examples of this mechanism are the base excision repair glycosylases and the restriction enzyme methylases. The question arises about the mechanism of substrate recognition for these enzymes and how closely it is linked to the base flipping step. Molecular dynamics simulations (AMBER, PME electrostatics) of fully solvated, cation neutralized, DNA sequences containing 8-oxoguanine (8OG) and of appropriate normal (control) DNAs have been carried out. The dynamics trajectories were analyzed to identify those properties of the DNA structure in the vicinity of the altered base, or its dynamics, that could contribute to molecular discrimination between substrate and non-substrate DNA sites. The results predict that the FPG enzyme should flip out the cytosine base paired with the scissile 8OG, not the target base itself.

A thermostable endonuclease III homolog from the archaeon Pyrobaculum aerophilum.

Pyrimidine adducts in cellular DNA arise from modification of the pyrimidine 5,6-double bond by oxidation, reduction or hydration. The biological outcome includes increased mutation rate and potential lethality. A major DNA N:-glycosylase responsible for the excision of modified pyrimidine bases is the base excision repair (BER) glycosylase endonuclease III, for which functional homologs have been identified and characterized in Escherichia coli, yeast and humans. So far, little is known about how hyperthermophilic Archaea cope with such pyrimidine damage. Here we report characterization of an endonuclease III homolog, PaNth, from the hyperthermophilic archaeon Pyrobaculum aerophilum, whose optimal growth temperature is 100 degrees C. The predicted product of 223 amino acids shares significant sequence homology with several [4Fe-4S]-containing DNA N:-glycosylases including E.coli endonuclease III (EcNth). The histidine-tagged recombinant protein was expressed in E.coli and purified. Under optimal conditions of 80-160 mM NaCl and 70 degrees C, PaNth displays DNA glycosylase/ss-lyase activity with the modified pyrimidine base 5,6-dihydrothymine (DHT). This activity is enhanced when DHT is paired with G. Our data, showing the structural and functional similarity between PaNth and EcNth, suggests that BER of modified pyrimidines may be a conserved repair mechanism in Archaea. Conserved amino acid residues are identified for five subfamilies of endonuclease III/UV endonuclease homologs clustered by phylogenetic analysis.

Efficient recognition of substrates and substrate analogs by the adenine glycosylase MutY requires the C-terminal domain.

The Escherichia coli DNA repair enzyme MutY plays an important role in the prevention of DNA mutations by removing misincorporated adenine residues from 7, 8-dihydro-8-oxo-2'-deoxyguanosine:2'-deoxyadenosine (OG:A) mispairs. The N-terminal domain of MutY (Stop 225, Met1-Lys225) has a sequence and structure that is characteristic of a superfamily of base excision repair glycosylases; however, MutY and its homologs contain a unique C-terminal domain. Previous studies have shown that the C-terminal domain confers specificity for OG:A substrates over G:A substrates and exhibits homology to the d(OG)TPase MutT, suggesting a role in OG recognition. In order to provide additional information on the importance of the C-terminal domain in damage recognition, we have investigated the kinetic properties of a form lacking this domain (Stop 225) under multiple- and single-turnover conditions. In addition, the interaction of Stop 225 with a series of non-cleavable substrate and product analogs was evaluated using gel retardation assays and footprinting experiments. Under multiple-turnover conditions Stop 225 exhibits biphasic kinetic behavior with both OG:A and G:A substrates, likely due to rate-limiting DNA product release. However, the rate of turnover of Stop 225 was increased 2-fold with OG:A substrates compared to the wild-type enzyme. In contrast, the intrinsic rate for adenine removal by Stop 225 from both G:A and OG:A substrates is significantly reduced (10- to 25-fold) compared to the wild-type. The affinity of Stop 225 for substrate analogs was dramatically reduced, as was the ability to discriminate between substrate analogs paired with OG over G. Interestingly, similar hydroxyl radical and DMS footprinting patterns are observed for Stop 225 and wild-type MutY bound to DNA duplexes containing OG opposite an abasic site mimic or a non-hydrogen bonding A analog, suggesting that similar regions of the DNA are contacted by both enzyme forms. Importantly, Stop 225 has a reduced ability to prevent DNA mutations in vivo. This implies that the reduced adenine glycosylase activity translates to a reduced capacity of Stop 225 to prevent DNA mutations in vivo.

Removal of hydantoin products of 8-oxoguanine oxidation by the Escherichia coli DNA repair enzyme, FPG.

An intriguing feature of 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG) is that it is highly reactive toward further oxidation. Indeed, OG has been shown to be a "hot spot" for oxidative damage and susceptible to oxidation by a variety of cellular oxidants. Recent work has identified two new DNA lesions, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), resulting from one-electron oxidation of OG. The presence of Gh and Sp lesions in DNA templates has been shown to result in misinsertion of G and A by DNA polymerases, and therefore, both are potentially mutagenic DNA lesions. The base excision repair (BER) glycosylases Fpg and MutY serve to prevent mutations associated with OG in Escherichia coli, and therefore, we have investigated the ability of these two enzymes to process DNA duplex substrates containing the further oxidized OG lesions, Gh and Sp. The Fpg protein, which removes OG and a variety of other oxidized purine base lesions, was found to remove Gh and Sp efficiently opposite all four of the natural DNA bases. The intrinsic rate of damaged base excision by Fpg was measured under single-turnover conditions and was found to be highly dependent upon the identity of the base opposite the OG, Gh, or Sp lesion; as expected, OG is removed more readily from an OG:C- than an OG:A-containing substrate. However, when adenine is paired with Gh or Sp, the rate of removal of these damaged lesions by Fpg was significantly increased relative to the rate of removal of OG from an OG:A mismatch. The adenine glycosylase MutY, which removes misincorporated A residues from OG:A mismatches, is unable to remove A paired with Gh or Sp. Thus, the activity of Fpg on Gh and Sp lesions may dramatically influence their mutagenic potential. This work suggests that, in addition to OG, oxidative products resulting from further oxidation of OG should be considered when evaluating oxidative DNA damage and its associated effects on DNA mutagenesis.

Mapping Oxidative DNA Damage Using Ligation-Mediated Polymerase Chain Reaction Technology

Reactive oxygen species induce a pharmacopoeia of oxidized bases in DNA. DNA can be cleaved at most of the sites of these modified bases by digestion with a combination of two base excision repair glycosylases from Escherichia coli, Fpg glycosylase, and endonuclease III. The frequency of the resulting glycosylase-dependent 5′-phosphoryl ends can be mapped at nucleotide resolution along a sequencing gel autoradiogram by a genomic sequencing technique, ligation-mediated polymerase chain reaction (LMPCR). In cultured rat cells, the frequency of endogenous oxidized bases in mitochondrial DNA is sufficiently high, about one oxidized base per 100 kb, to be directly mapped from 0.1 μg of total cellular DNA preparations by LMPCR. Nuclear DNA has a lower frequency of endogenous oxidative base damage which cannot be mapped from 1-μg preparations of total cellular DNA. Preparative gel electrophoresis of the PGK1 and p53 genes from 300 μg of restriction endonuclease-digested genomic DNA showed a 25-fold enrichment for the genes and, after endonuclease digestion followed by LMPCR, gave sufficient signal to map the frequency of oxidized bases from human cells treated with 50 μM H2O2.

Structural studies of human alkyladenine glycosylase and E. coli 3-methyladenine glycosylase

Human alkyladenine glycosylase (AAG) and Escherichia coli 3-methyladenine glycosylase (AlkA) are base excision repair glycosylases that recognize and excise a variety of alkylated bases from DNA. The crystal structures of these enzymes have provided insight into their substrate specificity and mechanisms of catalysis. Both enzymes utilize DNA bending and base-flipping mechanisms to expose and bind substrate bases. Crystal structures of AAG complexed to DNA suggest that the enzyme selects substrate bases through a combination of hydrogen bonding and the steric constraints of the active site, and that the enzyme activates a water molecule for an in-line backside attack of the N-glycosylic bond. In contrast to AAG, the structure of the AlkA–DNA complex suggests that AlkA substrate recognition and catalytic specificity are intimately integrated in a SN1 type mechanism in which the catalytic Asp238 directly promotes the release of modified bases.

A Novel 3-Methyladenine DNA Glycosylase from Helicobacter pylori Defines a New Class within the Endonuclease III Family of Base Excision Repair Glycosylases

The cloning, purification, and characterization of MagIII, a 3-methyladenine DNA glycosylase from Helicobacter pylori, is presented in this paper. Sequence analysis of the genome of this pathogen failed to identify open reading frames potentially coding for proteins with a 3-methyladenine DNA glycosylase activity. The putative product of the HP602 open reading frame, reported as an endonuclease III, shares extensive amino acid sequence homology with some bacterial members of this family and has the canonic active site helix-hairpin-helix-GPD motif. Surprisingly, this predicted H. pylori endonuclease III encodes a 25,220-Da protein able to release 3-methyladenine, but not oxidized bases, from modified DNA. MagIII has no abasic site lyase activity and displays the substrate specificity of the 3-methyladenine-DNA glycosylase type I of Escherichia coli (Tag) because it is not able to recognize 7-methylguanine or hypoxanthine as substrates. The expression of the magIII open reading frame in null 3-methyladenine glycosylase E. coli (tag alkA) restores to this mutant partial resistance to alkylating agents. MagIII-deficient H. pylori cells show an alkylation-sensitive phenotype. H. pylori wild type cells exposed to alkylating agents present an adaptive response by inducing the expression of magIII. MagIII is thus a novel bacterial member of the endonuclease III family, which displays biochemical properties not described for any of the members of this group until now.

DNA bending and a flip-out mechanism for base excision by the helix-hairpin-helix DNA glycosylase, Escherichia coli AlkA.

The Escherichia coli AlkA protein is a base excision repair glycosylase that removes a variety of alkylated bases from DNA. The 2.5 A crystal structure of AlkA complexed to DNA shows a large distortion in the bound DNA. The enzyme flips a 1-azaribose abasic nucleotide out of DNA and induces a 66 degrees bend in the DNA with a marked widening of the minor groove. The position of the 1-azaribose in the enzyme active site suggests an S(N)1-type mechanism for the glycosylase reaction, in which the essential catalytic Asp238 provides direct assistance for base removal. Catalytic selectivity might result from the enhanced stacking of positively charged, alkylated bases against the aromatic side chain of Trp272 in conjunction with the relative ease of cleaving the weakened glycosylic bond of these modified nucleotides. The structure of the AlkA-DNA complex offers the first glimpse of a helix-hairpin-helix (HhH) glycosylase complexed to DNA. Modeling studies suggest that other HhH glycosylases can bind to DNA in a similar manner.

Single-turnover and pre-steady-state kinetics of the reaction of the adenine glycosylase MutY with mismatch-containing DNA substrates.

The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG) in DNA by the removal of misincorporated adenine residues in OG:A mispairs. MutY also exhibits adenine glycosylase activity toward adenine in G:A and C:A mismatches, although the importance of this activity in vivo has not been established. We have investigated the kinetic properties of MutY's glycosylase activity with OG:A and G:A containing DNA duplexes. Our results indicate that MutY's processing of these two substrates is distinctly different. By using single-turnover experiments, the intrinsic rate for adenine removal by MutY from an OG:A substrate was found to be at least 6-fold faster than that from the corresponding G:A substrate. However, under conditions where [MutY] << [DNA], OG:A substrates are not quantitatively converted to product due to the inefficient turnover resulting from slow product release. In contrast, with a G:A substrate MutY's dissociation from the corresponding product is more facile, such that complete conversion of the substrate to product can be achieved under similar conditions. The kinetic results illustrate that the glycosylase reaction catalyzed by MutY has significant differences depending on the characteristics of the substrate. The lingering of MutY with the product of its reaction with OG:A mispairs may be biologically significant to prevent premature removal of OG. Thus, this approach is providing insight into factors that may be influencing the repair of damaged and mismatched DNA in vivo by base-excision repair glycosylases.