Specificities of human, rat and E. coli O6-methylguanine-DNA methyltransferases towards the repair of O6-methyl and O6-ethylguanine in DNA

Cytotoxic and mutagenic properties of O6-alkyl-2'-deoxyguanosine lesions in Escherichia coli cells

Environmental exposure and cellular metabolism can give rise to DNA alkylation, which can occur on the nitrogen and oxygen atoms of nucleobases, as well as on the phosphate backbone. Although O6-alkyl-2'-deoxyguanosine (O6-alkyl-dG) lesions are known to be associated with cancer, not much is known about how the alkyl group structures in these lesions affect their repair and replicative bypass in vivo or how translesion synthesis DNA polymerases influence the latter process. To answer these questions, here we synthesized oligodeoxyribonucleotides harboring seven O6-alkyl-dG lesions, with the alkyl group being Me, Et, nPr, iPr, nBu, iBu, or sBu, and examined the impact of these lesions on DNA replication in Escherichia coli cells. We found that replication past all the O6-alkyl-dG lesions was highly efficient and that SOS-induced DNA polymerases play redundant roles in bypassing these lesions. Moreover, these lesions directed exclusively the G → A mutation, the frequency of which increased with the size of the alkyl group on the DNA. This could be attributed to the varied repair efficiencies of these lesions by O6-alkylguanine DNA alkyltransferase (MGMT) in cells, which involve the MGMT Ogt and, to a lesser extent, Ada. In conclusion, our study provides important new knowledge about the repair of the O6-alkyl-dG lesions and their recognition by the E. coli DNA replication machinery. Our results suggest that the lesions' carcinogenic potentials may be attributed, at least in part, to their strong mutagenic potential and their efficient bypass by the DNA replication machinery.

Integrated study on comparative transcriptome and skeletal muscle function in aged rats

The present study aimed to reveal aging-related changes in the skeletal muscle of SD rats by comparing transcriptome analysis, integrated with muscle physiological parameters. Ten rats aged 25 months were set as the old group (OG) and ten rats aged 6 months were set as the young group (YG). After 6 weeks of feeding, the body mass, grip strength, and gastrocnemius muscle mass were determined, and the differentially expressed genes were analyzed by transcriptome sequencing, followed by GO enrichment analysis and KEGG analysis. The results showed that the muscle index and the relative grip strength were lower in OG rats than YG rats. The expressions of AMPK, UCP3, IGF-1, several ion channel associated genes and collagen family genes were down-regulated in OG rats. MGMT, one of the strength determining genes and CHRNa1, a subunit of the acetylcholine receptor were up-regulated in OG rats. The present results supply the global transcriptomic information involved in aging related skeletal muscle dysfunction in rats. The reduced expressions of AMPK, IGF-1, and CASK can explain the losses of muscle mass and function in the aged rats. In addition, the up-regulation of MGMT and CHRNa1 also contribute to muscle wasting and weakness during aging.

Quaternary interactions and supercoiling modulate the cooperative DNA binding of AGT

Human O⁶-alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O⁶-alkylguanine and O⁴-alkylthymine adducts in single-stranded and duplex DNAs. The search for these lesions, through a vast excess of competing, unmodified genomic DNA, is a mechanistic challenge that may limit the repair rate in vivo. Here, we examine influences of DNA secondary structure and twist on protein–protein interactions in cooperative AGT complexes formed on lesion-free DNAs that model the unmodified parts of the genome. We used a new approach to resolve nearest neighbor (nn) and long-range (lr) components from the ensemble-average cooperativity, ω_ave. We found that while nearest-neighbor contacts were significant, long-range interactions dominated cooperativity and this pattern held true whether the DNA was single-stranded or duplex. Experiments with single plasmid topoisomers showed that the average cooperativity was sensitive to DNA twist, and was strongest when the DNA was slightly underwound. This suggests that AGT proteins are optimally juxtaposed when the DNA is near its torsionally-relaxed state. Most striking was the decline of binding stoichiometry with linking number. As stoichiometry and affinity differences were not correlated, we interpret this as evidence that supercoiling occludes AGT binding sites. These features suggest that AGT's lesion-search distributes preferentially to sites containing torsionally-relaxed DNA, in vivo.

DNA Alkylating Agent Protects Against Spontaneous Hepatocellular Carcinoma Regardless of O6-Methylguanine-DNA Methyltransferase Status

Hepatocellular carcinoma is increasingly important in the United States as the incidence rate rose over the last 30 years. C3HeB/FeJ mice serve as a unique model to study hepatocellular carcinoma tumorigenesis because they mimic human hepatocellular carcinoma with delayed onset, male gender bias, approximately 50% incidence, and susceptibility to tumorigenesis is mediated through multiple genetic loci. Because a human O(6)-methylguanine-DNA methyltransferase (hMGMT) transgene reduces spontaneous tumorigenesis in this model, we hypothesized that hMGMT would also protect from methylation-induced hepatocarcinogenesis. To test this hypothesis, wild-type and hMGMT transgenic C3HeB/FeJ male mice were treated with two monofunctional alkylating agents: diethylnitrosamine (DEN; 0.025 μmol/g body weight) on day 12 of life with evaluation for glucose-6-phosphatase-deficient (G6PD) foci at 16, 24, and 32 weeks or N-methyl-N-nitrosurea (MNU; 25 mg MNU/kg body weight) once monthly for 7 months starting at 3 months of age with evaluation for liver tumors at 12 to 15 months of age. No difference in abundance or size of G6PD foci was measured with DEN treatment. In contrast, it was unexpectedly found that MNU reduces liver tumor prevalence in wild-type and hMGMT transgenic mice despite increased tumor prevalence in other tissues. hMGMT and MNU protections were additive, suggesting that MNU protects through a different mechanism, perhaps through the cytotoxic N7-alkylguanine and N3-alkyladenine lesions which have low mutagenic potential compared with O(6)-alkylguanine lesions. Together, these results suggest that targeting the repair of cytotoxic lesions may be a good preventative for patients at high risk of developing hepatocellular carcinoma.

Sequencing analysis of mutations induced by N-ethyl-N-nitrosourea at different sampling times in mouse bone marrow

In our previous study (Wang et al., 2004, Toxicol. Sci. 82: 124-128), we observed that the cII gene mutant frequency (MF) in the bone marrow of Big Blue mice showed significant increase as early as day 1, reached the maximum at day 3 and then decreased to a plateau by day 15 after a single dose of carcinogen N-ethyl-N-nitrosourea (ENU) treatment, which is different from the longer mutation manifestation time and the constancy of MFs after reaching their maximum in some other tissues. To determine the mechanism underlying the quick increase in MF and the peak formation in the mutant manifestation, we examined the mutation frequencies and spectra of the ENU-induced mutants collected from different sampling times in this study. The cII mutants from days 1, 3 and 120 after ENU treatment were randomly selected from different animals. The mutation frequencies were 33, 217, 305 and 144 x 10(-6) for control, days 1, 3, and 120, respectively. The mutation spectra at days 1 and 3 were significantly different from that at day 120. Considering that stem cells are responsible for the ultimate MF plateau (day 120) and transit cells are accountable for the earlier MF induction (days 1 or 3) in mouse bone marrow, we conclude that transit cells are much more sensitive to mutation induction than stem cells in mouse bone marrow, which resulted in the specific mutation manifestation induced by ENU.

Frequency and spectrum of lacI mutations in the liver of Big Blue mice following the administration of genotoxic carcinogens singly and in series

Transgenic mouse models offer a unique opportunity to study in vivo mutagenicity in any tissue of interest. In this study, the authors have determined the liver mutant frequency (MF) and mutational spectra (MS) of 12 week-old male Big Blue B6C3F1 transgenic mice exposed to the genotoxic carcinogens benzo[a]pyrene (B[a]P; 250 mg/kg/day), N-nitrosodimethylamine (NDMA; 7 mg/kg/day), or N-ethyl-1-nitrosourea (ENU; 50 mg/kg/day) singly (3 daily oral doses) or in series (B[a]P on day 1, NDMA on day 2, and ENU on day 3). All genotoxic agents, alone or in series, increased MF in the liver (three- to sixfold). MS analyses of liver DNA revealed a high percentage of G:C --> A:T transitions in the control (88%) and the NDMA (64%) groups. In contrast, B[a]P, ENU, and the series treatment induced a high percentage (> or = 50%) of transversions. Significantly, 46% (19 out of 41) of the mutations in the series treatment group occurred at CpG dinucleotides, compared to less than 22% in the other treatment groups. The MS from the series exposure was most similar to B[a]P with a high percentage of transversion mutations occurring at guanine nucleotides (36%). These preliminary data suggest that genotoxic carcinogens, when exposed in series, produce a unique MS profile characterized not only by shifts in mutation class but also sequence context.

The L84F and the I143V polymorphisms in the O6-methylguanine-DNA-methyltransferase (MGMT) gene increase human sensitivity to the genotoxic effects of the tobacco-specific nitrosamine carcinogen NNK

O-Methylguanine-DNA-methyltransferase (MGMT) is a direct-reversal DNA repair protein that removes DNA adducts formed by alkylating mutagens found in tobacco smoke. Several coding single nucleotide polymorphisms (cSNPs) in the MGMT gene have been reported. However, their effect on the levels and types of genetic damage induced by specific environmental carcinogens remains to be fully elucidated. We developed two novel genotyping techniques and used them, in conjunction with the mutagen-sensitivity assay, to test the hypothesis that the L84F and I143V cSNPs in the MGMT gene confer increased sensitivity to genetic damage induced by the alkylating tobacco-specific nitrosamine carcinogen NNK. Lymphocytes from 114 healthy volunteers were exposed in vitro to NNK, and the genotoxic response was assessed by measuring chromosome aberration (CA) frequencies. A significant (P<0.02) increase in NNK-induced CA was observed in cells from individuals with the 84F polymorphism compared to cells from individuals homozygous for the referent L84 allele. A significant positive interaction between this cSNP and smoking, gender and age was observed (P<0.03). In subjects with the variant 143V allele, significantly higher levels of NNK-induced CA were observed in males and in young subjects (<43 years old) compared to subjects homozygous for the referent I143 allele (P<0.02). Individuals who inherited two cSNPs had significantly higher levels of NNK-induced CA compared to individuals with none or with one cSNP (P<0.002). These new data suggest that the 84F and 143V cSNPs may alter the function characteristics of the MGMT protein, resulting in suboptimal repair of genetic damage induced by NNK.

Age-dependent sensitivity of Big Blue transgenic mice to the mutagenicity of N-ethyl-N-nitrosourea (ENU) in liver

The incidence of childhood cancer is increasing and recent evidence suggests an association between childhood cancer and environmental exposure to genotoxins. In the present study, the Big Blue transgenic mouse model was used to determine whether specific periods in early life represent windows of vulnerability to mutation induction by genotoxins in mouse liver. Groups of mice were treated with single doses of 120 mg N-ethyl-N-nitrosourea (ENU)/kg body weight or the vehicle either transplacentally to the 18-day-old fetus or at postnatal days (PNDs) 1, 8, 15, 42 or 126; the animals were sacrificed 6 weeks after their treatment. The cII mutation assay was performed to determine the mutant frequencies (MFs) in the livers of the mice. Liver cII MFs for both sexes were dependent on the age at which the animals were treated. Perinatal treatment with ENU (either transplacental treatment to the 18-day-old fetus or i.p. injection at PND 1) induced relatively high MFs. However, ENU treatment at PNDs 8 and 15 resulted in the highest mutation induction. The lowest mutation induction occurred in those animals treated as adults (PND 126). For instance, the cII MF for the PND 8 female group was 646 x 10(-6) while the MF for female adults was only 145 x 10(-6), a more than 4-fold difference. Molecular analysis of the mutants found that A:T-->T:A transversions and A:T-->G:C transitions characterized the pattern of mutations induced by ENU in both the neonate and adult mice, while the predominate type of mutation in the controls was G:C-->A:T. The results indicate that mouse liver is most sensitive to ENU-induced mutation during infancy. This period correlates well with the age-dependent sensitivity to carcinogenicity in mouse liver, suggesting that mutation is an important rate-limiting factor for age-related carcinogenesis.

Mutant frequency and mutational spectra in the Tk and Hprt genes of N-ethyl-N-nitrosourea-treated mouse lymphoma cellsdagger

The mouse lymphoma assay (MLA) utilizing the Tk gene is widely used to identify chemical mutagens. The autosomal location of the Tk gene allows for the detection of a wide range of mutational events, from point mutations to chromosome alterations. However, chemically induced point mutation spectra in the Tk gene of mouse lymphoma cells have not been characterized. In this study, we determined and compared the mutagenicity and mutational spectra of N-ethyl-N-nitrosourea (ENU) in the Tk and Hprt genes of mouse lymphoma cells. Treatment of L5178Y mouse lymphoma cells with 100 microg/ml ENU induced a Tk mutant frequency of 756 x 10(-6) and an Hprt mutant frequency of 311 x 10(-6). Sequence analysis of Tk and Hprt mutant cDNAs showed a similar overall mutation pattern in the two genes with base-pair substitutions accounting for 83% of non-loss of heterozygosity mutations in the Tk gene and 75% of all mutations in the Hprt gene. The most common point mutation induced by ENU was G:C --> A:T transition (36 and 28% of independent mutations detected in the Tk and Hprt genes, respectively). The mutation spectra induced by ENU in both the Tk and Hprt genes were different from the respective patterns produced in mutants from untreated cells. About 9% of Tk and 7% of Hprt mutations from control cells were in-frame deletions, whereas no such mutations were found among the ENU-induced Tk and Hprt mutations. Our results indicate that ENU produces a chemical-specific point mutational profile in the Tk gene of mouse lymphoma cells that is remarkably similar to that found in the X-linked Hprt gene. This study provides evidence that the MLA can be used not only to detect point mutagens but also for analysis of mutational spectra.

Repair of O(6)-alkylguanine by alkyltransferases

The predominant pathway for the repair of O(6)-methylguanine in DNA is via the activity of an alkyltransferase protein that transfers the methyl group to a cysteine acceptor site on the protein itself. This review article describes recent studies on this alkyltransferase. The protein repairs not only methyl groups but also 2-chloroethyl-, benzyl- and pyridyloxobutyl-adducts. It acts on double-stranded DNA by flipping the O(6)-guanine adduct out of the DNA helix and into a binding pocket. The free base, O(6)-benzylguanine, is able to bind in this pocket and react with the cysteine, rendering it an effective inactivator of mammalian alkyltransferases. The alkylated form of the protein is rapidly degraded by the ubiquitin/proteasomal system. Some tumor cells do not express alkyltransferase despite having an intact gene. Methylation of key sites in CpG-rich islands in the promoter region are involved in this silencing and a change in the nuclear localization of an enhancer binding protein may also contribute. The alkyltransferase promoter contains Sp1, GRE and AP-1 sites and is slightly inducible by glucocorticoids and protein kinase C activators. There is a complex relationship between p53 and alkyltransferase expression with p53 mediating a rise in alkyltransferase in response to ionizing radiation but having no clear effect on basal levels. DNA adducts at the O(6)-position of guanine are a major factor in the carcinogenic, mutagenic, apoptopic and clastogenic actions of methylating agents and chloroethylating agents. Studies with transgenic mice in which alkyltransferase levels are increased or decreased confirm the importance of this repair pathway in protecting against carcinogenesis. Alkyltransferase activity in tumors protects them from therapeutic agents such as temozolomide and BCNU. This resistance is abolished by O(6)-benzylguanine and this drug is currently in clinical trials to enhance cancer chemotherapy by these agents. Studies are in progress to reduce the toxicity of such therapy towards the bone marrow by gene therapy to express alkyltransferases with mutations imparting resistance to O(6)-benzylguanine at high levels in marrow stem cells. Several polymorphisms in the human alkyltransferase gene have been identified but the significance of these in terms of alkyltransferase action is currently unknown.

Ethylnitrosourea-induced mutation and molecular analysis of transgenic mice containing the gpt shuttle vector

Novel transgenic mice were developed in order to study the in vivo mutagenesis. The transgenic mice carried pCGK shuttle vector, which contained the Escherichia coli gpt gene as a mutational target, the kanamycin-resistant gene (Kanr) and cos region derived from bacteriophage lambda. The shuttle vector can be recovered from the transgenic mouse genome into the gpt-deficient E. coli by an in vitro packaging method and is selectable as a Kanr phenotype. Mutations induced at the gpt gene can be easily detected with a selective agent, 6-thioguanine (6-TG). In the previous study, the pCGK shuttle vector was incorporated into Chinese hamster CHL/IU cells and the resultant transgenic cell line was shown to be a useful system to study in vitro mutagenesis at the gpt gene. Therefore, an advantage of the shuttle vector is that in vivo mutational data obtained from the transgenic mouse can be compared with those of transgenic cell line in vitro. A transgenic CD-1 mouse line, designated as #128, that carried approximately 50 copies of pCGK shuttle vectors, was selected among 4 transgenic mouse lines. To investigate the sensitivity of the #128 line, the transgenic mice were treated with a single intraperitoneal injection of 250 mg/kg of N-ethyl-N-nitrosourea (ENU) or with 50 mg kg-1 day-1 of ENU for 5 consecutive days, and bone marrow, spleen and liver were dissected to investigate their mutational responses. The background mutant frequency was between 18x10(-6) and 75x10(-6) among all tissues tested. ENU induced significant increases in the mutant frequency above the background level in all three tissues at 14 days after single or 5-day treatment with the chemical. The increases in the mutant frequencies in bone marrow, spleen and liver were 6.4- to 6.8-fold, 3.0- to 5.6-fold and 3.0- to 3.3-fold, respectively. The shuttle vector DNA was recovered from the bone marrow of both spontaneous and ENU-treated mice and the gpt gene was amplified by polymerase chain reaction. The amplified DNA was subject to DNA sequence analysis. Out of 79 spontaneous and 52 ENU-induced mutants, the gpt gene could be amplified from 28 spontaneous and 46 ENU-induced mutants. DNA sequence analysis showed that predominant mutations were identified as A:T to T:A transversions (22 out of 46 sequenced mutants) and G:C to A:T transitions (9/46) in ENU-induced mutants, whereas G:C to T:A transversions (7 out of 28 sequenced mutants) were predominant in spontaneous mutants. These results demonstrate that this transgenic mouse, in combination with the transgenic CHL/IU cell line, is a useful system to study in vivo and in vitro mutational events at the same target gene.

Identification and characterisation of the Drosophila melanogaster O6-alkylguanine-DNA alkyltransferase cDNA

The protein O 6-alkylguanine-DNA alkyltransferase(alkyltransferase) is involved in the repair of O 6-alkylguanine and O 4-alkylthymine in DNA and plays an important role in most organisms in attenuating the cytotoxic and mutagenic effects of certain classes of alkylating agents. A genomic clone encompassing the Drosophila melanogaster alkyltransferase gene ( DmAGT ) was identified on the basis of sequence homology with corresponding genes in Saccharomyces cerevisiae and man. The DmAGT gene is located at position 84A on the third chromosome. The nucleotide sequence of DmAGT cDNA revealed an open reading frame encoding 194 amino acids. The MNNG-hypersensitive phenotype of alkyltransferase-deficient bacteria was rescued by expression of the DmAGT cDNA. Furthermore, alkyltransferase activity was identified in crude extracts of Escherichia coli harbouring DmAGT cDNA and this activity was inhibited by preincubation of the extract with an oligonucleotide containing a single O6-methylguanine lesion. Similar to E.coli Ogt and yeast alkyltransferase but in contrast to the human alkyltransferase, the Drosophila alkyltransferase is resistant to inactivation by O 6-benzylguanine. In an E.coli lac Z reversion assay, expression of DmAGT efficiently suppressed MNNG-induced G:C-->A:T as well as A:T-->G:C transition mutations in vivo. These results demonstrate the presence of an alkyltransferase specific for the repair of O 6-methylguanine and O 4-methylthymine in Drosophila.

Fast repair of O6-ethylguanine, but not O6-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea

Differential repair of structurally distinct mutagenic lesions in critical genes may influence the cellular risk of malignant conversion. We have investigated rat mammary tumorigenesis induced by N-ethyl-N-nitrosourea (EtNU) versus N-methyl-N-nitrosourea (MeNU) with respect to tumor incidence, ras gene mutation, and gene-specific repair. Both carcinogens induced mammary adenocarcinomas at high yield. In mammary epithelia (very low expression of O6-alkylguanine-DNA alkyltransferase, MGMT), O6-methylguanine (O6-MeGua) was eliminated from transcribed (H-ras and beta-actin) and inactive genes (IgE heavy chain) at the same slow rate as determined for bulk genomic DNA. The persistence of O6-MeGua in DNA correlated with a high frequency of G:C --> A:T transition mutations at codon 12 of the H-ras gene in MeNU-induced tumors. Repair of O6-ethylguanine (O6-EtGua), too, was slow in the IgE heavy chain gene as in bulk DNA. Contrasting with O6-MeGua, however, O6-EtGua was removed approximately 20 times faster from the active H-ras and beta-actin genes via MGMT-independent mechanism(s). Accordingly, no H-ras codon 12 mutations were found in EtNU-induced tumors, and 5- to 8-fold surplus alkyltransferase activity of the mammary epithelia-via a bacterial ada transgene-did not significantly counteract tumorigenesis in EtNU-exposed contrary to MeNU-treated animals. Neither MeNU- nor EtNU-induced tumors exhibited mutations at codons 13 and 61 of H-ras or codons 12, 13, and 61 of K-ras. Fast repair of O6-EtGua, but not O6-MeGua, in transcribed genes thus prevents mutational activation of H-ras when rat mammary carcinogenesis is initiated by EtNU in place of MeNU.

A comparison of the genotoxicity of ethylnitrosourea and ethyl methanesulfonate in lacZ transgenic mice (Muta Mouse)

We compared the induction of gene mutations and chromosomal aberrations by ethylating agents in lacZ transgenic mice (Muta Mouse). Chromosomal aberrations were detected by the peripheral blood micronucleus assay. Gene mutations were detected in the lacZ transgene. A small amount of blood was sampled from a tail vessel during the expression time for fixation of gene mutations in vivo; this enabled us to detect and compare clastogenicity and gene mutations in the identical mouse. Single intraperitoneal injections of ENU (50-200 mg/kg) and EMS (100-400 mg/kg) strongly induced micronucleated reticulocytes (MN) detectable in peripheral blood 48 h after treatment. The maximum MN frequencies induced were 6.6% and 3.3% for ENU (100 mg/kg) and EMS (400 mg/kg), respectively (the control value was 0.3%). lacZ mutant frequency (MF) was analyzed in bone marrow and liver 7 days after treatment. Spontaneous MFs were 2.0-4.6 x 10(-6). MF in bone marrow was increased by ENU to 3.4 x 10(-5) at 200 mg/kg and induced by EMS to 1.8 x 10(-5) at 400 mg/kg. In liver, however, both chemicals at their highest doses induced only slight increases in MF. The induction of both micronuclei and lacZ mutations in bone marrow by both ENU and EMS correlated better with O6-ethylguanine adducts than with N7-ethylguanine adducts. The mutants (19 for ENU and 12 for EMS) were subjected to DNA sequence analysis. Among EMS-induced mutations, 75% were GC to AT transitions, which were probably caused by O6-ethylguanine. Among ENU-induced mutations, in contrast, 40% occurred as AT base pair substitutions (6 AT to TA transversions and 2 AT to GC transitions) (no such mutations were induced by EMS). These results, together with the known reactivity of ENU to thymine suggest that thymine adducts play a significant role in the ENU mutagenesis.

Repair of O6-benzylguanine by the Escherichia coli Ada and Ogt and the human O6-alkylguanine-DNA alkyltransferases

O6-Methylguanine is removed from DNA via the transfer of the methyl group to a cysteine acceptor site present in the DNA repair protein O6-alkylguanine-DNA alkyltransferase. The human alkyltransferase is inactivated by the free base O6-benzylguanine, raising the possibility that substantially larger alkyl groups could also be accepted as substrates. However, the Escherichia coli alkyltransferase, Ada-C, is not inactivated by O6-benzylguanine. The Ada-C protein was rendered capable of reaction by the incorporation of two site-directed mutations converting Ala316 to a proline (A316P) and Trp336 to alanine (W336A) or glycine (W336G). These changes increase the space at the active site of the protein where Cys321 is buried and thus permit access of the O6-benzylguanine inhibitor. Reaction of the mutant A316P/W336A-Ada-C with O6-benzylguanine was greatly stimulated by the presence of DNA, providing strong support for the concept that binding of DNA to the Ada-C protein activates the protein. The Ada-C protein was able to repair O6-benzylguanine in a 16-mer oligodeoxyribonucleotide. However, the rate of repair was very slow, whereas the E. coli Ogt, the human alkyltransferase, and the mutant A316P/W336A-Ada-C alkyltransferases reacted very rapidly with this 16-mer substrate and preferentially repaired it when incubated with a mixture of the methylated and benzylated 16-mers. These results show that benzyl groups are better substrates than methyl groups for alkyltransferases provided that steric factors do not prevent binding of the substrate in the correct orientation for alkyl group transfer.

Binding and repair of O6-ethylguanine in double-stranded oligodeoxynucleotides by recombinant human O6-alkylguanine-DNA alkyltransferase do not exhibit significant dependence on sequence context

Double-stranded (ds) oligodeoxynucleotides (29mers) containing an O6-ethylguanine (O6-EtGua) flanked 5' and 3' by different bases (5'..TGT..3'; 5'..CGG..3', 5'..GGT..3'; 5'..GGG..3'; 5'..GGA..3') were synthesized to investigate the binding and repair characteristics of recombinant human O6-alkylguanine-DNA alkyltransferase (AT) in vitro. The apparent association constant (KA(app)) of AT to the oligomers and the repair rate constant for O6-EtGua (k) respectively, were determined by gel retardation and a monoclonal antibody-based filter binding assay. When ds- or single-stranded (ss) oligomers with or without O6-EtGua were used, no major differences in KA(app) values were observed with either substrate: KA(app) values for native AT were 7.1 and 8.4 x 10(5) M(-1) respectively, for unmodified and [O6-EtGua]-containing ds-oligomers. The corresponding values for ss-oligomers were 1.0 and 4.9 x 10(5) M(-1). The N-terminal first 56 amino acids of AT only exert a limited influence on DNA binding; the KA(app) values for an N-terminally truncated AT protein (1.1 x 10(5) M(-1)) and native AT were of the same order. Moreover, KA(app) was hardly affected by Cys(145)-methylated AT (2.0 x 10(5) M(-1)). The k-values (6.5-11.5 x 10(6) M(-1)s(-1)) were not significantly dependent on nucleotide sequence. k-values of 5.3 and 4.0 x 10(6) M(-1)s(-1) respectively, were obtained with the N-terminally truncated AT protein and for repair of the postreplicative mispair [O6-EtGua]: T by native AT. The low KA(app), the negligible influence on O6 of ethylation, and the minor modulation KA(app) and k by varying the bases flanking O6-EtGua, all indicate that the binding of AT to DNA is non-specific and mediated mainly by ionic interactions [reduced KA(app) and k-values at increased ionic strength]. Surplus DNA reduces the rate of O6-EtGua repair in ds-oligomers by competitive binding of AT molecules. The reaction mechanism of AT with DNA in vivo requires further investigation.