Acetylaminofluorene bound to different guanines of the sequence -GGCGCC- is excised with different efficiencies by the UvrABC excision nuclease in a pattern not correlated to the potency of mutation induction

Dissociation Dynamics of XPC-RAD23B from Damaged DNA Is a Determining Factor of NER Efficiency

XPC-RAD23B (XPC) plays a critical role in human nucleotide excision repair (hNER) as this complex recognizes DNA adducts to initiate NER. To determine the mutagenic potential of structurally different bulky DNA damages, various studies have been conducted to define the correlation of XPC-DNA damage equilibrium binding affinity with NER efficiency. However, little is known about the effects of XPC-DNA damage recognition kinetics on hNER. Although association of XPC is important, our current work shows that the XPC-DNA dissociation rate also plays a pivotal role in achieving NER efficiency. We characterized for the first time the binding of XPC to mono- versus di-AAF-modified sequences by using the real time monitoring surface plasmon resonance technique. Strikingly, the half-life (t1/2 or the retention time of XPC in association with damaged DNA) shares an inverse relationship with NER efficiency. This is particularly true when XPC remained bound to clustered adducts for a much longer period of time as compared to mono-adducts. Our results suggest that XPC dissociation from the damage site could become a rate-limiting step in NER of certain types of DNA adducts, leading to repression of NER.

Unusual sequence effects on nucleotide excision repair of arylamine lesions: DNA bending/distortion as a primary recognition factor

The environmental arylamine mutagens are implicated in the etiology of various sporadic human cancers. Arylamine-modified dG lesions were studied in two fully paired 11-mer duplexes with a -G*CN- sequence context, in which G* is a C8-substituted dG adduct derived from fluorinated analogs of 4-aminobiphenyl (FABP), 2-aminofluorene (FAF) or 2-acetylaminofluorene (FAAF), and N is either dA or dT. The FABP and FAF lesions exist in a simple mixture of 'stacked' (S) and 'B-type' (B) conformers, whereas the N-acetylated FAAF also samples a 'wedge' (W) conformer. FAAF is repaired three to four times more efficiently than FABP and FAF. A simple A- to -T polarity swap in the G*CA/G*CT transition produced a dramatic increase in syn-conformation and resulted in 2- to 3-fold lower nucleotide excision repair (NER) efficiencies in Escherichia coli. These results indicate that lesion-induced DNA bending/thermodynamic destabilization is an important DNA damage recognition factor, more so than the local S/B-conformational heterogeneity that was observed previously for FAF and FAAF in certain sequence contexts. This work represents a novel 3'-next flanking sequence effect as a unique NER factor for bulky arylamine lesions in E. coli.

The efficiencies of damage recognition and excision correlate with duplex destabilization induced by acetylaminofluorene adducts in human nucleotide excision repair

Nucleotide excision repair (NER) removes lesions caused by environmental mutagens or UV light from DNA. A hallmark of NER is the extraordinarily wide substrate specificity, raising the question of how one set of proteins is able to recognize structurally diverse lesions. Two key features of good NER substrates are that they are bulky and thermodynamically destabilize DNA duplexes. To understand what the limiting step in damage recognition in NER is, we set out to test the hypothesis that there is a correlation of the degree of thermodynamic destabilization induced by a lesion, binding affinity to the damage recognition protein XPC-RAD23B, and overall NER efficiency. We chose to use acetylaminofluorene (AAF) and aminofluorene (AF) adducts at the C8 position of guanine in different positions within the NarI (GGCGCC) sequence, as it is known that the structures of the duplexes depend on the position of the lesion in this context. We found that the efficiency of NER and the binding affinity of the damage recognition factor XPC-RAD23B correlated with the thermodynamic destabilization induced by the lesion. Our study is the first systematic analysis correlating these three parameters and supports the idea that initial damage recognition by XPC-RAD23B is a key rate-limiting step in NER.

Nucleotide excision repair of 2-acetylaminofluorene- and 2-aminofluorene-(C8)-guanine adducts: molecular dynamics simulations elucidate how lesion structure and base sequence context impact repair efficiencies

Nucleotide excision repair (NER) efficiencies of DNA lesions can vary by orders of magnitude, for reasons that remain unclear. An example is the pair of N-(2′-deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) and N-(2′-deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-C8-AAF) adducts that differ by a single acetyl group. The NER efficiencies in human HeLa cell extracts of these lesions are significantly different when placed at G₁, G₂ or G₃ in the duplex sequence (5′-CTCG₁G₂CG₃CCATC-3′) containing the NarI mutational hot spot. Furthermore, the dG-C8-AAF adduct is a better substrate of NER than dG-C8-AF in all three NarI sequence contexts. The conformations of each of these adducts were investigated by Molecular dynamics (MD) simulation methods. In the base-displaced conformational family, the greater repair susceptibility of dG-C8-AAF in all sequences stems from steric hindrance effects of the acetyl group which significantly diminish the adduct-base stabilizing van der Waals stacking interactions relative to the dG-C8-AF case. Base sequence context effects for each adduct are caused by differences in helix untwisting and minor groove opening that are derived from the differences in stacking patterns. Overall, the greater NER efficiencies are correlated with greater extents of base sequence-dependent local untwisting and minor groove opening together with weaker stacking interactions.

Conformational and thermodynamic properties modulate the nucleotide excision repair of 2-aminofluorene and 2-acetylaminofluorene dG adducts in the NarI sequence

Nucleotide excision repair (NER) is a major repair pathway that recognizes and corrects various lesions in cellular DNA. We hypothesize that damage recognition is an initial step in NER that senses conformational anomalies in the DNA caused by lesions. We prepared three DNA duplexes containing the carcinogen adduct N-(2'-deoxyguanosin-8-yl)-7-fluoro-2-acetylaminofluorene (FAAF) at G(1), G(2) or G(3) of NarI sequence (5'-CCG(1)G(2)CG(3)CC-3'). Our (19)F-NMR/ICD results showed that FAAF at G(1) and G(3) prefer syn S- and W-conformers, whereas anti B-conformer was predominant for G(2). We found that the repair of FAAF occurs in a conformation-specific manner, i.e. the highly S/W-conformeric G(3) and -G(1) duplexes incised more efficiently than the B-type G(2) duplex (G(3)∼G(1)> G(2)). The melting and thermodynamic data indicate that the S- and W-conformers produce greater DNA distortion and thermodynamic destabilization. The N-deacetylated N-(2'-deoxyguanosin-8-yl)-7-fluoro-2-aminofluorene (FAF) adducts in the same NarI sequence are repaired 2- to 3-fold less than FAAF: however, the incision efficiency was in order of G(2)∼G(1)> G(3), a reverse trend of the FAAF case. We have envisioned the so-called N-acetyl factor as it could raise conformational barriers of FAAF versus FAF. The present results provide valuable conformational insight into the sequence-dependent UvrABC incisions of the bulky aminofluorene DNA adducts.

Can mutagenicity information be useful in an Integrated Testing Strategy (ITS) for skin sensitization?

Our previous work has investigated the utility of mutagenicity data in the development and application of Integrated Testing Strategies (ITS) for skin sensitization by focusing on the chemical mechanisms at play and substantiating these with experimental data where available. The hybrid expert system TIMES (Tissue Metabolism Simulator) was applied in the identification of the chemical mechanisms since it encodes a comprehensive set of established structure-activity relationships for both skin sensitization and mutagenicity. Based on the evaluation, the experimental determination of mutagenicity was thought to be potentially helpful in the evaluation of skin sensitization potential. This study has evaluated the dataset reported by Wolfreys and Basketter (Cutan. Ocul. Toxicol. 23 (2004), pp. 197-205). Upon an update of the experimental data, the original reported concordance of 68% was found to increase to 88%. There were several compounds that were 'outliers' in the two experimental evaluations which are discussed from a mechanistic basis. The discrepancies were found to be mainly associated with the differences between skin and liver metabolism. Mutagenicity information can play a significant role in evaluating sensitization potential as part of an ITS though careful attention needs to be made to ensure that any information is interpreted in the appropriate context.

Translesion synthesis past the C8- and N2-deoxyguanosine adducts of the dietary mutagen 2-Amino-3-methylimidazo[4,5-f]quinoline in the NarI recognition sequence by prokaryotic DNA polymerases

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) is found in cooked meats and forms DNA adducts at the C8- and N2-positions of dGuo after appropriate activation. IQ is a potent inducer of frameshift mutations in bacteria and is carcinogenic in laboratory animals. We have incorporated both IQ-adducts into the G1- and G3-positions of the NarI recognition sequence (5'-G1G2CG3CC-3'), which is a hotspot for arylamine modification. The in vitro replication of the oligonucleotides was examined with Escherichia coli pol I Klenow fragment exo-, E. coli pol II exo-, and Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4), and the extension products were sequenced by tandem mass spectrometry. Replication of the C8-adduct at the G3-position resulted in two-base deletions with all three polymerases, whereas error-free bypass and extension was observed at the G1-position. The N2-adduct was bypassed and extended by all three polymerases when positioned at the G1-position, and the error-free product was observed. The N2-adduct at the G3-position was more blocking and was bypassed and extended only by Dpo4 to produce an error-free product. These results indicate that the replication of the IQ-adducts of dGuo is strongly influenced by the local sequence and the regioisomer of the adduct. These results also suggest a possible role for pol II and IV in the error-prone bypass of the C8-IQ-adduct leading to frameshift mutations in reiterated sequences, whereas noniterated sequences result in error-free bypass.

'Close-fitting sleeves': DNA damage recognition by the UvrABC nuclease system

DNA damage recognition represents a long-standing problem in the field of protein-DNA interactions. This article reviews our current knowledge of how damage recognition is achieved in bacterial nucleotide excision repair through the concerted action of the UvrA, UvrB, and UvrC proteins.

Site-specific incorporation of N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-AAF) into oligonucleotides using modified 'ultra-mild' DNA synthesis

Aromatic amino and nitro compounds are potent carcinogens found in the environment that exert their toxic effects by reacting with DNA following metabolic activation. One important adduct is N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-AAF), which has been extensively used in studies of the mechanisms of DNA repair and mutagenesis. Despite the importance of dG-AAF adducts in DNA, an efficient method for its incorporation into DNA using solid-phase synthesis is still missing. We report the development of a modified 'ultra-mild' DNA synthesis protocol that allows the incorporation of dG-AAF into oligonucleotides of any length accessible by solid-phase DNA synthesis with high efficiency and independent of sequence context. Key to this endeavor was the development of improved deprotection conditions (10% diisopropylamine in methanol supplemented with 0.25 M of beta-mercaptoethanol) designed to remove protecting groups of commercially available 'ultra-mild' phosphoramidite building blocks without compromising the integrity of the exquisitely base-labile acetyl group at N8 of dG-AAF. We demonstrate the suitability of these oligonucleotides in the nucleotide excision repair reaction. Our synthetic approach should facilitate comprehensive studies of the mechanisms of repair and mutagenesis induced by dG-AAF adducts in DNA and should be of general use for the incorporation of base-labile functionalities into DNA.

Defining the position of the switches between replicative and bypass DNA polymerases

Cells contain specialized DNA polymerases that are able to copy past lesions with an associated risk of generating mutations, the major cause of cancer. Here, we reconstitute translesion synthesis (TLS) using the replicative (Pol III) and major bypass (Pol V) DNA polymerases from Escherichia coli in the presence of accessory factors. When the replicative polymerase disconnects from the template in the vicinity of a lesion, Pol V binds the blocked replication intermediate and forms a stable complex by means of a dual interaction with the tip of the RecA filament and the beta-clamp, the processivity factor donated by the blocked Pol III holoenzyme. Both interactions are required to confer to Pol V the processivity that will allow it synthesize, in a single binding event, a TLS patch long enough to support further extension by Pol III. In the absence of these accessory factors, the patch synthesized by Pol V is too short, being degraded by the Pol III-associated exonuclease activity that senses the distortion induced by the lesion, thus leading to an aborted bypass process.

The biochemical requirements of DNA polymerase V-mediated translesion synthesis revisited

In addition to replicative DNA polymerases, cells contain specialized DNA polymerases involved in processes such as lesion tolerance, mutagenesis and immunoglobulin diversity. In Escherichia coli, DNA polymerase V (Pol V), encoded by the umuDC locus, is involved in translesion synthesis (TLS) and mutagenesis. Genetic studies have established that mutagenesis requires both UmuC and a proteolytic product of UmuD (UmuD'). In addition, RecA protein and the replication processivity factor, the beta-clamp, were genetically found to be essential co-factors for mutagenesis. Here, we have reconstituted Pol V-mediated bypass of three common replication-blocking lesions, namely the two major UV-induced lesions and a guanine adduct formed by a chemical carcinogen (G-AAF) under conditions that fulfil these in vivo requirements. Two co-factors are essential for efficient Pol V-mediated lesion bypass: (i) a DNA substrate onto which the beta-clamp is stably loaded; and (ii) an extended single-stranded RecA/ATP filament assembled downstream from the lesion site. For efficient bypass, Pol V needs to interact simultaneously with the beta-clamp and the 3' tip of the RecA filament. Formation of an extended RecA/ATP filament and stable loading of the beta-clamp are best achieved on long single-stranded circular DNA templates. In contrast to previously published data, the single-stranded DNA-binding protein (SSB) is not absolutely required for Pol V-mediated lesion bypass provided ATP, instead of ATPgammaS, activates the RecA filament. Further discrepancies with the existing literature are explainable by the use of either inadequate DNA substrates or a UmuC fusion protein instead of native Pol V.

Dynamic conformational heterogeneities of carcinogen-DNA adducts and their mutagenic relevance

Arylamines and polycyclic aromatic hydrocarbons (PAHs), which are known as "bulky" carcinogens, have been studied extensively and upon activation in vivo, react with cellular DNA to form DNA-adducts. The available structure data accumulated thus far has revealed that conformational heterogeneity is a common theme among duplex DNA modified with these carcinogens. Several conformationally diverse structures have been elucidated and found to be in equilibrium in certain cases. The dynamics of the heterogeneity appear to be modulated by the nature of the adduct structure and the base sequences neighboring the lesion site. These can be termed as "adduct- and sequence-induced conformational heterogeneities," respectively. Due to the small energy differences, the population levels of these conformers could readily be altered within the active sites of repair or replicate enzymes. Thus, the complex role of "enzyme-induced conformational heterogeneity" must also be taken into consideration for the establishment of a functional structure-mutation relationship. Ultimately, a major challenge in mutation structural biology is to carry out adduct- and site-specific experiments in a conformationally specific manner within biologically relevant environments. Results from such experiments should provide an accurate account of how a single chemically homogenous adduct gives rise to complex multiple mutations, the earliest step in the induction of cancer.

Nucleotide excision repair and its interplay with transcription

Nucleotide excision repair (NER) is a multistep process capable to remove a variety of DNA distorting lesions from prokaryotic and eukaryotic genomes. In eukaryotic cells, the process requires more than 30 proteins to perform the different steps, i.e. recognition of DNA damage, single strand incisions and excision of the lesion-containing DNA fragment and DNA repair synthesis/ligation. NER can operate via two subpathways: global genome repair (GGR) and a specialized pathway coupled to active transcription (transcription-coupled repair, TCR) and directed to DNA lesions in the transcribed strand of active genes. Both in vivo as well as in cultured cells the fast removal of transcription blocking lesions by TCR is crucial to escape from lethal effects of inhibited transcription inhibition The most delicate step in NER is the recognition of the DNA lesions in their different chromatin context and the mechanism of damage recognition in GGR and TCR is principally different and requires specific proteins. In GGR, the XPC-HR23B is essential for the formation of the incision complex. In TCR the Cockayne syndrome (CS) gene products are key players in the recognition of a stalled RNA polymerase the presumed signaling structure for repair of transcribed strands. In this study, we show that the extent of recovery of UV-inhibited transcription and TCR strictly depends on the amount of CSB protein as well as the amount of DNA damage present in the cell. This indicates that the ratio between DNA damage frequency and CSB protein concentration in the cell is rather critical for acute cellular response, i.e. recovery of inhibited transcription upon DNA damage infliction, and hence cellular survival.

The isomerization of the UvrB-DNA preincision complex couples the UvrB and UvrC activities

In Escherichia coli nucleotide excision repair, the UvrB-DNA preincision complex plays a key role, linking adduct recognition to incision. We previously showed that the efficiency of the incision is inversely related to the stability of the preincision complex. We postulated that an isomerization reaction converts [UvrB-DNA], stable but incompetent for incision, into the [UvrB-DNA]' complex, unstable and competent for incision. Here, we identify two parameters, negative supercoiling and presence of a nick at the fifth phosphodiester bond 3' to the lesion, that accelerate the isomerization leading to an increasing incision efficiency. We also show that the [UvrB-DNA] complex is more resistant to a salt concentration increase than the [UvrB-DNA]' complex. Finally, we report that the [UvrB-DNA]' is recognized by UvrC. These data suggest that the isomerization reaction leads to an exposure of single-stranded DNA around the lesion. This newly exposed single-stranded DNA serves as a binding site and substrate for the UvrC endonuclease. We propose that the isomerization reaction is responsible for coupling UvrB and UvrC activities and that this reaction corresponds to the binding of ATP.

Localization of UvrA and effect of DNA damage on the chromosome of Bacillus subtilis

We found that the nucleotide excision repair protein UvrA, which is involved in DNA damage recognition, localizes to the entire chromosome both before and after damage in living Bacillus subtilis cells. We suggest that the UvrA(2)B damage recognition complex is constantly scanning the genome, searching for lesions in the DNA. We also found that DNA damage induces a dramatic reconfiguration of the chromosome such that it no longer fills the entire cell as it does during normal growth. This reconfiguration is reversible after low doses of damage and is dependent on the damage-induced SOS response. We suggest that this reconfiguration of the chromosome after damage may be either a reflection of ongoing DNA repair or an active mechanism to protect the cell's genome. Similar observations have been made in Escherichia coli, indicating that the alteration of chromosome structure after DNA damage may be a widespread phenomenon.

Solution structure of the 2-amino-1- methyl-6-phenylimidazo[4,5-b]pyridine C8-deoxyguanosine adduct in duplex DNA

The carcinogenic heterocyclic amine (HA) 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is formed during the cooking of various meats. To enable structure/activity studies aimed at understanding how DNA damaged by a member of the HA class of compounds can ultimately lead to cancer, we have determined the first solution structure of an 11-mer duplex containing the C8-dG adduct formed by reaction with N-acetoxy-PhIP. A slow conformational exchange is observed in which the PhIP ligand either intercalates into the DNA helix by denaturing and displacing the modified base pair (main form) or is located outside the helix in a minimally perturbed B-DNA duplex (minor form). In the main base-displaced intercalation structure, the minor groove is widened, and the major groove is compressed at the lesion site because of the location of the bulky PhIP-N-methyl and phenyl ring in the minor groove; this distortion causes significant bending of the helix. The PhIP phenyl ring interacts with the phosphodiester-sugar ring backbone of the complementary strand and its fast rotation with respect to the intercalated imidazopyridine ring causes substantial distortions at this site, such as unwinding and bulging-out of the strand. The glycosidic torsion angle of the [PhIP]dG residue is syn, and the displaced guanine base is directed toward the 3' end of the modified strand. This study contributes, to our knowledge, the first structural information on the biologically relevant HA class to a growing body of knowledge about how conformational similarities and differences for a variety of types of lesions can influence protein interactions and ultimately biological outcome.

Mechanism of DNA polymerase II-mediated frameshift mutagenesis

Escherichia coli possesses three SOS-inducible DNA polymerases (Pol II, IV, and V) that were recently found to participate in translesion synthesis and mutagenesis. Involvement of these polymerases appears to depend on the nature of the lesion and its local sequence context, as illustrated by the bypass of a single N-2-acetylaminofluorene adduct within the NarI mutation hot spot. Indeed, error-free bypass requires Pol V (umuDC), whereas mutagenic (-2 frameshift) bypass depends on Pol II (polB). In this paper, we show that purified DNA Pol II is able in vitro to generate the -2 frameshift bypass product observed in vivo at the NarI sites. Although the Delta polB strain is completely defective in this mutation pathway, introduction of the polB gene on a low copy number plasmid restores the -2 frameshift pathway. In fact, modification of the relative copy number of polB versus umuDC genes results in a corresponding modification in the use of the frameshift versus error-free translesion pathways, suggesting a direct competition between Pol II and V for the bypass of the same lesion. Whether such a polymerase competition model for translesion synthesis will prove to be generally applicable remains to be confirmed.

Mutations in the p53 tumor suppressor gene in tree shrew hepatocellular carcinoma associated with hepatitis B virus infection and intake of aflatoxin B1

Infection with hepadnaviruses and exposure to aflatoxin B1 (AFB1) are considered to be major risk factors in the development of hepatocellular carcinoma (HCC) in humans. A high rate of p53 mutations at codon 249 has been reported in these tumors. The tree shrew (Tupaia belangeri chinensis) is a useful animal model for the development of HCC after human hepatitis B virus (HBV) infection or AFB1 treatment. Therefore, it was of particular interest to determine whether the p53 gene in tree shrew HCCs associated with HBV infection and/or with exposure to AFB1 is affected in the same manner as in human HCCs. We determined the tree shrew p53 wild-type nucleotide sequences by RT-PCR and automatic DNA-sequencing. Tree shrew wild-type p53 sequence showed 91.7 and 93.4% homologies with human p53 nucleotide and amino acids sequences, respectively, while it showed 77.2 and 73.7% homologies in mice. One HCC and normal liver tissue from AFB1 treated and one HCC from AFB1- and HBV-treated tree shrew showed no change in p53 sequences, while three HCCs from AFB1- and HBV-treated tree shrews showed point mutations in p53 sequences. One HCC showed point mutations at codon 275, which is on the DNA-binding domain of p53 gene, which might be a cause of gain-of-function during the development of HCC. As a result, our finding indicates that tree shrews exposed to AFB1 and/or HBV had neither codon 249 mutations nor significant levels of other mutations in the p53 gene, as is the case with humans.

Sequence-dependent repair of synthetic AP sites in 15-mer and 35-mer oligonucleotides: role of thermodynamic stability imposed by neighbor bases

We previously reported that 15-mer oligonucleotides with a central 1, N(6)-epsilonA were cleaved by alkylpurine-DNA N-glycosylase as a function of T(m), modulated by neighbor bases [Hang, B., Sági, J., and Singer, B. (1998) J. Biol. Chem. 273, 33406-33413]. This type of investigation has now been extended to cleavage by Escherichia coli endonuclease IV of a centrally placed synthetic AP site using both 15-mer and 35-mer duplexes. In 15-mers, the triplet sequences adjunct to the central AP site greatly affected the thermodynamic stability. The repair rate paralleled the thermal stability since endonuclease IV requires a double-stranded substrate. When the AP site-containing duplexes were 35-mers, there was also a general correlation between the thermostability and cleavage efficiency. However, the difference in the cleavage rates between different sequences was much less than with the 15-mers. Since the 35-mers were more than 96% annealed, this difference presumably results from local stability and structure adjacent to the AP site. These results suggest that under enzyme limiting conditions or overproduction of AP sites, sequence-dependent differential repair could occur in vivo.

Sequence context modulation of translesion synthesis at a single N-2-acetylaminofluorene adduct located within a mutation hot spot

Oligonucleotides containing a single N-(deoxyguanosin-8-yl)acetylaminofluorene lesion (dGuo-C8-AAF) at each guanine residue of the sequence (5'-G1G2G3) have been used as templates for in vitro primer extension reactions by several DNA polymerases [Escherichia coli DNA polymerase III holoenzyme, its alpha subunit, DNA polymerase I Klenow fragment proficient (exo+) or deficient (exo-) in its 3' --> 5' exonuclease activity, and Sequenase]. The dGuo-C8-AAF lesion appears to be a strong block for all DNA polymerases: exo+ DNA polymerases stop one nucleotide before encountering the lesion, while partial incorporation opposite the lesion is observed only with enzymes devoid of the exonuclease activity. The efficiency of incorporation across from the adduct depends on both the DNA polymerase and the position of the lesion. When polymerase I Klenow fragment exo- is used, translesion synthesis (TLS) is observed with efficiencies varying according to the position of the adduct (G2 > G1 > G3). Sequencing of the TLS products shows that error-free TLS is observed only when the AAF lesion is bound to G1, while all TLS events occurring at G2- or G3-AAF adducts are mutagenic. The major mutational event is a G deletion (27, 76, and 55% of the events for G1, G2, and G3, respectively), while two-G deletions occur to a lesser extent (17-30%). These results are discussed in view of the slippage model developed for frameshift mutagenesis occurring during translesion synthesis at replication blocking lesions.

Nucleotide excision repair: from E. coli to man

Nucleotide excision repair is both a 'wide spectrum' DNA repair pathway and the sole system for repairing bulky damages such as UV lesions or benzo[a]pyrene adducts. The mechanisms of nucleotide excision repair are known in considerable detail in Escherichia coli. Similarly, in the past 5 years important advances have been made towards understanding the biochemical mechanisms of excision repair in humans. The overall strategy of the repair is the same in the two species: damage recognition through a multistep mechanism involving a molecular matchmaker and an ATP-dependent unwinding of the damaged duplex; dual incisions at both sides of the lesion by two different nucleases, the 3' incision being followed by the 5'; removal of the damaged oligomer; resynthesis of the repair patch, whose length matches the gap size. Despite these similarities, the two systems are biochemically different and do not even share structural homology. E. coli excinuclease employs three proteins in contrast to 16/17 polypeptides in man; the excised fragment is longer in man: the procaryotic excinuclease is not able by itself to remove the excised oligomer whereas the human enzyme does. Thus, the excinuclease mode of action is well conserved throughout evolution, but not the biochemical tools: this represents a case of evolutionary convergence.

Correlation between sequence-dependent glycosylase repair and the thermal stability of oligonucleotide duplexes containing 1, N6-ethenoadenine

Previous experiments on DNA sequence context reported that base modification, replication, and repair are affected by the nature of neighbor bases. We now report that repair by mammalian alkylpurine-DNA-N-glycosylases (APNG) of 15-mer oligonucleotides with a central 1,N6-ethenoadenine (epsilonA), flanked by 5' and 3' tandem bases, is also highly sequence dependent. Oligonucleotides with the central sequences -GGepsilonAGG- or -CCepsilonACC- are repaired 3-5-fold more efficiently than those containing -AAepsilonAAA- or -TTepsilonATT- when using human or mouse APNG. Melting curves of the same duplexes showed that oligomers with G.C/C. G neighbors were less denatured than those with A.T/T.A neighbors at 37 degreesC. This sequence-dependent difference in denaturation correlates with the relative thermodynamic stability of oligomers with G.C/C.G or A.T/T.A neighbors. The dependence of repair on thermal stability was confirmed by enzyme reactions performed over 0-45 degreesC. Under these conditions, repair of epsilonA flanked by G.C/C.G was dramatically increased at 37 degreesC with continuous increase up to 45 degreesC, in contrast to that with flanking A.T/T. A pairs, which was in agreement with the degree of denaturation of these duplexes. These results indicate that the thermodynamic stability conferred by base pairs flanking epsilonA plays an essential role in maintaining the integrity of the duplex structure which is necessary for repair.

Lack of transcription-coupled repair of acetylaminofluorene DNA adducts in human fibroblasts contrasts their efficient inhibition of transcription

The N-(deoxyguanosine-8-yl)-2-acetylaminofluorene (dG-C8-AAF) lesion is among the most helix distorting DNA lesions. In normal fibroblasts dG-C8-AAF is repaired rapidly in transcriptionally active genes, but without strand specificity, indicating that repair of dG-C8-AAF by global genome repair (GGR) overrules transcription-coupled repair (TCR). Yet, dG-C8-AAF is a very potent inhibitor of transcription. The target size of inhibition (45 kilobases) suggests that transcription inhibition by dG-C8-AAF is caused by blockage of initiation rather than elongation. Cockayne's syndrome (CS) cells appear to be extremely sensitive to the cytotoxic effects of dG-C8-AAF and are unable to recover inhibited RNA synthesis. However, CS cells exhibit no detectable defect in repair of dG-C8-AAF in active genes, indicating that impaired TCR is not the cause of the enhanced sensitivity of CS cells. These and data reported previously suggest that the degree of DNA helix distortion determines the rate of GGR as well as the extent of inhibition of transcription initiation. An interchange of the transcription/repair factor TFIIH from promoter sites to sites of damage might underlie inhibition of transcription initiation. This process is likely to occur more rapidly and efficiently in the case of strongly DNA helix distorting lesions, resulting in a very efficient GGR, a poor contribution of TCR to repair of lesions in active genes, and an efficient inhibition of transcription.

A single N-2-acetylaminofluorene adduct alters the footprint of T7 (exo-) DNA polymerase bound to a model primer-template junction

Bovine pancreatic deoxyribonuclease I (DNaseI) has been used to footprint T7 (exo-) DNA polymerase bound to a model primer-template junction. The polymerase was blocked at a specific position either by the omission of dCTP from the reaction mix or by the presence of a N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dGuo-AAF) adduct. This lesion has been shown to be a severe block for several DNA polymerases, both in in vitro primer elongation experiments, and during the in vivo replication of AAF-monomodified single-stranded vectors. The footprints obtained with unmodified primer-template DNA define two protected domains separated by an inter-region that remains sensitive to DNaseI, and several hypersensitive sites located on both strands. Binding of the polymerase to AAF monomodified duplexes results in the same protection pattern as that obtained with the unmodified duplexes. However, the hypersensitive sites either disappear or are dramatically reduced. The results suggest that the AAF lesion alters the correct positioning of the duplex DNA within the polymerase cleft.

What structural features determine repair enzyme specificity and mechanism in chemically modified DNA?

A crucial question in repair is how do enzymes recognize substrates. In surveying the relevant literature, it becomes evident that there are no rules which can be clearly applied. At this time it appears that uracil glycosylase is the only repair enzyme for which all the known substrates can be rationalized on the basis of chemical structure. When surveying the multiplicity of substrates for m3A-DNA glycosylase, it is difficult, on the basis of present knowledge, to explain why 1,N6-etheno-A (epsilon A) is as good a substrate, if not better, than m3A for which the enzyme is named. There is no apparent unifying chemical structure which is required for recognition. It should also be noted that many studies of the mechanism of m3A-DNA glycosylase only utilized-N-3- and N-7-alkylpurines. On this basis, an electron-deficient purine, and later pyrimidine, was considered to be the recognition signal. Since epsilon A and Hx do not fall in this class, this is one illustration of why exploring new substrates becomes important in elucidating enzyme mechanisms. Ubiquitous enzymes, such as 5'-AP endonucleases, are present in both prokaryotes and eukaryotes. The primary function is the same, i.e., repair of an AP site which occurs through natural processes or from the action of DNA glycosylases. There are, however, completely unrelated substrates such as the exocyclic adducts pBQ-dC and pBQ-dG. pBQ-dC is repaired by both the human HAP1 and E. coli Exo III and Endo IV, while pBQ-dG is only repaired by the E. coli enzymes. Yet, when repair of these adducts occurs, it is by the same unusual pathway which differs from the usual base excision repair mechanism. This finding may ultimately not be as unusual as it now seems. The understanding of substrate recognition by repair enzymes, which can have different repair pathways, is complex. For example, three exocyclic derivatives which each have either the same modification (1,N4-epsilon dA and 3,N4-epsilon dC) or the same base with different modifying groups (3,N4-epsilon dC and 3,N4-pBQ-dC) are repaired by three separate enzymes and two mechanism (Figure 9). Investigators have also reported that two separate enzymes and pathways can be found for simple adducts such as m6G and O4T. It is not clear why, for these adducts, both MGMT and excision repair can be utilized. This could be visualized as a "backup" system and may be more common than now known. We cannot think like an enzyme or vice versa. In the absence of enough necessary information, we can only be descriptive. What information is necessary for further understanding? (1) More detailed structural studies of adducts in defined oligonucleotides would be useful. (2) New substrates should be explored. For example, is the mechanism for PBQ-dC (and pBQ-dG) repair unique? This involves guesswork and intuition. (3) For the adducts mentioned in this Perspective and others, understanding enzyme/substrate recognition will be facilitated by cocrystallography and site-directed mutagenesis. (4) Genetic approaches, such as knockouts or targeted mutations in repair genes, should be expanded in order to focus on the physiological role of a specific enzyme. Above all: structure, structure, structure! Enzymologists, organic chemists, physical chemiste, X-ray crystallographers, and others must combine forces if the fundamental problems addressed here are to be understood.

Sequence-dependent modulation of nucleotide excision repair: the efficiency of the incision reaction is inversely correlated with the stability of the pre-incision UvrB-DNA complex

The UvrABC excinuclease is involved in the nucleotide excision repair (NER) pathway. Sequence-dependent differences in repair efficiency have been reported for many different lesions, and it is often suggested that sites with poor repair contribute to the occurrence of mutation hot spots. However, guanine bases modified by N-2-acetylaminofluorence (AAF) within the NarI site (5'-G1G2CG3CC-3') are incised by the UvrABC excinuclease with different efficiencies in a pattern not correlated with the potency of mutation induction. To gain insight into the mechanism of sequence-dependent modulation of NER, we analyzed the formation, the structure and the stability of UvrB-DNA pre-incision complexes formed at all three positions of the AAF-modified NarI site. We show that the efficiency of release of UvrA2 from specific UvrA2B-DNA complexes is sequence-dependent and that the efficiency of incision is inversely related to the stability of the pre-incision complex. We propose that the pre-incision complex, [UvrB-DNA], when formed upon dissociation of UvrA2, undergoes a conformational change (isomerization step) giving rise to an unstable but incision-competent complex that we call [UvrB-DNA]'. The [UvrB-DNA] complex is stable and unable to form an incision-competent complex with UvrC. As the release of UvrA2, this isomerization step is sequence-dependent. Both steps contribute to modulate NER efficiency.

The limited strand-separating activity of the UvrAB protein complex and its role in the recognition of DNA damage

The recognition by Escherichia coli Uvr nucleotide excision repair proteins of a variety of lesions with diverse chemical structures and the presence of helicase activity in the UvrAB complex which can displace short oligonucleotides annealed to single-stranded DNA led to a model in which this activity moves UvrAB along undamaged DNA to damaged sites where the lesion blocks further translocation and the protein-DNA pre-incision complex is formed. To evaluate this mechanism for damage recognition, we constructed substrates with oligonucleotides of different lengths annealed to single-stranded DNA circles and placed a single 2-(acetylamino)fluorene (AAF) lesion either on the oligonucleotide or on the circle. For the substrates with no lesion, the UvrAB complex effectively displaced a 22-mer but not a 27-mer or longer fragments. The presence of AAF on the oligonucleotide significantly increased the release of the 27-mer but oligomers of 30 or longer were not separated. Placing the lesion on the circular strand did not block the release of the fragments. Instead, the releasing activity of UvrAB was stimulated and also depended on the length of the annealed oligonucleotide. These observations do not agree with the predictions of a damage recognition mechanism that depends on helicase-driven translocation. Most likely, the strand-separating activity of UvrAB is a consequence of local changes occurring during the formation of a DNA-protein pre-incision complex at the damaged site and is not due to translocation of the protein along undamaged DNA to locate a lesion.

Binding of the Escherichia coli UvrAB proteins to the DNA mono- and diadducts of cis-[N-2-amino-N-2-methylamino-2,2,1-bicycloheptane]dichloroplatinum(II ) and cisplatin. Analysis of the factors controlling recognition and proof of monoadduct-mediated UvrB-DNA cross-linking

The interactions of the Escherichia coli endonuclease UvrAB proteins with the DNA mono- and diadducts of both the cis-racemic exo-[N-2-amino-N-2-methylamino-2,2,1-bicycloheptane]dichloroplatin um(II) (complex 1) and cisplatin (cis-diamminedichloroplatinum(II) (cis-DDP)), have been studied. Complex 1 reacts faster with DNA than cis-DDP and gives monoadducts with a longer lifetime (8 h 20 min chelation t 1/2 compared with 2 h 40 min for cis-DDP). Using pSP65 plasmid [3H]DNA, the filter binding assay was associated with the analysis of the nucleoprotein complexes to characterize the UvrAB recognition of the platinum adducts and to demonstrate the occurrence of platinum-mediated DNA-protein cross-linking. First, it is shown that the UvrAB proteins recognize the complex 1 mono- and diadducts with a higher affinity than those of cis-DDP. Fifteen times more cis-DDP adducts per plasmid are required than complex 1 adducts, to lead to similar UvrAB binding. However, the UvrAB proteins recognize monoadducts and diadducts of each complex with a similar affinity. Second, it is shown that UvrB is the protein involved in the nucleo-protein complexes formed from mono- and diadducts of complex 1 and cis-DDP. This protein is also partly cross-linked to DNA with a similar efficiency by monoadducts derived from complex 1 and cis-DDP. However, as UvrB has a greater affinity for the DNA adducts of complex 1 than for those of cis-DDP, more UvrB-platinum-DNA cross-links are formed with complex 1 than with cis-DDP. This study, using a bacterial repair system as a model, points to a possible strategy for making new cytotoxic platinum complexes for mammalian cells.

Human and E.coli excinucleases are affected differently by the sequence context of acetylaminofluorene-guanine adduct

Synthetic DNA substrates containing an acetylaminofluorene (AAF) adduct at each of the three guanine in the G1G2CG3CC sequence were constructed and tested as substrates for reconstituted E.coli (A)BC excinuclease and human excinuclease in HeLa cell-free extract (CFE). The (A)BC excinulcease repaired the three substrates with relative efficiencies of G1:G2:G3 of 100:18:66 in agreement with an earlier report [Seeberg, E., and Fuchs, R.P.P. (1990) Proc. Natl Acad. Sci. USA 87, 191-194]. The same lesions were repaired by the human excinuclease with the strikingly different efficiencies of G1:G2:G3 as 38:100:68. These results reveal that the human excinuclease is affected by the sequence context of the lesion in a different manner than its prokaryotic counterpart.

Self-induced correction of the genetic defect in tyrosinemia type I

A mosaic pattern of immunoreactive fumarylacetoacetase (FAH) protein was found in liver tissue in 15 of 18 tyrosinemia type I patients of various ethnic origins. One additional patient had variable levels of FAH enzyme activity in liver tissue. In four patients exhibiting mosaicism of FAH protein, analysis for the tyrosinemia-causing mutations was performed in immunonegative and immunopositive areas of liver tissue by restriction digestion analysis and direct DNA sequencing. In all four patients the immunonegative liver tissue contained the FAH mutations demonstrated in fibroblasts of the patients. In the immunopositive nodules of regenerating liver tissue one of the mutated alleles apparently had reverted to the normal genotype. This genetic correction was observed for three different tyrosinemia-causing mutations. In each case a mutant AT nucleotide pair was reverted to a normal GC pair.

Protein-DNA interactions and alterations in the DNA structure upon UvrB-DNA preincision complex formation during nucleotide excision repair in Escherichia coli

The UvrB-DNA preincision complex is a key intermediate in the repair of damaged DNA by the UvrABC endonuclease from Escherichia coli. DNaseI footprinting of this complex on DNA with a cis-[Pt(NH3)2[d(GpG)-N7(1),N7(2)]] adduct provided global information on the protein binding site on this substrate [Visse, R., et al. (1991) J. Biol. Chem. 266, 7609-7617]. By applying a method developed by Fairall and Rhodes [Fairall, L., & Rhodes, D. (1992) Nucleic Acids Res. 20, 4727-4731], who have used the size and shape of DNasI for the interpretation of a footprint, we were able to define in more detail the region where UvrB-DNA interactions in the preincision complex occur. The potential interactions with phosphate groups could be reduced to less then 14 in the damaged and to 12 in the nondamaged strand. The main UvrB-DNA interactions seem restricted to the major groove on both sides of the lesion. As a consequence UvrB crosses the minor groove just downstream of the damage. Such a binding of UvrB orients the protein away from the damage. The more detailed interpretation of UvrB-DNA interactions was supported by methylation protection experiments. The structure of the DNA in the preincision complex formed on cis-[Pt(NH3)2[GpG-N7(1),N7(2)]] is altered as could be shown diethylpyrocarbonate sensitivity of adenines just downstream of the lesion. However the adenines just downstream of another cisplatin adduct, cis-[Pt(NH3)2[d(GpCpG)-N7(1),N7(3)]], did not become diethylpyrocarbonate sensitive in the preincision complex although this complex is incision proficient.(ABSTRACT TRUNCATED AT 250 WORDS)

The actual incision determines the efficiency of repair of cisplatin-damaged DNA by the Escherichia coli UvrABC endonuclease

The UvrABC endonuclease from Escherichia coli repairs a broad spectrum of DNA lesions with variable efficiencies. The effectiveness of repair is influenced by the nature of the lesion, the local DNA sequence, and/or the topology of the DNA. To get a better understanding of the aspects of this multistep repair reaction that determine the effectiveness of repair, we compared the incision efficiencies of linear DNA fragments containing either a site-specific cis-[Pt(NH3)2(d(GpG)-N7(1),-N7(2)]] or a cis- Pt(NH3)2[d(GpCpG)-N7(1),-N7(3)]] adduct. Overall the DNA with the cis-PtGG adduct was incised about 3.5 times more efficiently than the cis-Pt.GCG-containing DNA. The rate of UvrB-DNA preincision complex formation for both lesions was similar and high in relation to the incision. DNase I footprints, however, showed that the local structure of the two preincision complexes is different. An assay was developed to measure the binding of UvrC to the preincision complexes and it was found that the binding rate of UvrC to the more slowly incised cis-Pt.GCG preincision complex was higher than to the cis-Pt.GG preincision complex. This most likely reflects a qualitative difference in preincision complex structures. For both lesions the binding of UvrC to the preincision complex was fast compared to the kinetics of actual incision. Apparently, direct incision of cisplatin damage requires an additional conformational change after the binding of UvrC.

Nucleotide excision repair

Nucleotide excision repair is the major DNA repair mechanism in all species tested. This repair system is the sole mechanism for removing bulky adducts from DNA, but it repairs essentially all DNA lesions, and thus, in addition to its main function, it plays a back-up role for other repair systems. In both pro- and eukaryotes nucleotide excision is accomplished by a multisubunit ATP-dependent nuclease. The excision nuclease of prokaryotes incises the eighth phosphodiester bond 5' and the fourth or fifth phosphodiester bond 3' to the modified nucleotide and thus excises a 12-13-mer. The excision nuclease of eukaryotes incises the 22nd, 23rd, or 24th phosphodiester bond 5' and the fifth phosphodiester bond 3' to the lesion and thus removes the adduct in a 27-29-mer. A transcription repair coupling factor encoded by the mfd gene in Escherichia coli and the ERCC6 gene in humans directs the excision nuclease to RNA polymerase stalled at a lesion in the transcribed strand and thus ensures preferential repair of this strand compared to the nontranscribed strand.

Repair and replication of plasmids with site-specific 8-oxodG and 8-AAFdG residues in normal and repair-deficient human cells

The in vivo mutagenicity of 7-hydro-8-oxo-2'-deoxyguanosine (8-oxodG) and N-(guanin-8-yl)-N-acetyl-2-aminofluorene (8-AAFdG) in human cells was determined by transfecting various cell lines with plasmids that carried a single adduct at a defined site. 8-OxodG is one of the many DNA modifications formed by oxygen radicals, and was found to be highly miscoding during replication with purified DNA polymerases in vitro. Here we show that the frequency of mutations induced by 8-oxodG during replication in vivo is at most only 2% above background. The most predominant mutation found was a single G----T transversion. The frequency of this transversion was found to be 3 to 5-fold increased in excision repair deficient XP-A cells. Interestingly, also the replication of 8-oxodG containing plasmids was significantly impaired (approximately 4-fold) in the XP-A cells, but not in HeLa cells, normal fibroblasts or XP-A revertant cells. When 8-AAFdG containing plasmids were used, the mutation frequencies did not exceed background levels (less than 2%) with any of the cell lines tested. The presence of 8-AAFdG almost completely inhibited plasmid replication (more than 50-fold) in XP-A cells. Apparently, both 8-AAFdG and 8-oxodG are not or poorly repaired in these cells, causing a block of DNA replication. This suggests that both lesions are substrates for excision repair, although to a varying extent.

(A)BC excinuclease: the Escherichia coli nucleotide excision repair enzyme

Nucleotide excision repair is the major pathway for removing damage from DNA. (A)BC excinuclease is the nuclease activity which initiates nucleotide excision repair in Escherichia coli. In this review, we focus on current understanding of the structure-function of the enzyme and the reaction mechanism of the repair pathway. In addition, recent biochemical studies on preferential repair of actively transcribed genes in E. coli are summarized.

Acetylaminofluorene and aminofluorene adducts inhibit in vitro transcription of a Xenopus 5S RNA gene only when located on the coding strand

Unique N-acetyl-2-aminofluorene (AAF) or 2-aminofluorene (AF) adducts were introduced into the Xenopus borealis somatic 5S RNA gene between the intragenic control region and the transcription termination site. The effects of these bulky adducts on transcription were studied in a cell-free extract derived from Xenopus laevis oocytes. AAF and AF adducts inhibit transcription only when they are on the template strand, whereas transcription passes through these adducts when they are placed on the nontemplate strand. In the presence of the AAF or AF adduct on the template strand, transcription usually terminates one nucleotide before the altered guanine residue. Premature termination at these bulky adducts does not block reinitiation of transcription, since several transcripts are produced per gene per hour on these damaged templates.

Polymorphism in N-2-acetylaminofluorene induced DNA structure as revealed by DNase I footprinting

In this paper, we have constructed double stranded helices (60-mers) containing a single N-2-acetylaminofluorene (-AAF) adduct covalently bound to one of the three guanine residues of the Narl site (G1G2CG3CC). This sequence was identified as a strong frameshift mutation hot spot for many carcinogens that bind to the C8 position of guanine. Using DNase I as a probe for DNA conformation we show i) that the average size of the helix deformation extends over 3 to 5 base pairs in both directions from the adduct site, and ii) that there is a strong polymorphism in the adduct induced DNA conformation. The present study supports the idea that adducts induce specific sequence dependent local conformational changes in DNA that are differentially recognized and processed by the enzymatic machineries that lead to repair or mutagenesis.

Strong sequence-dependent polymorphism in adduct-induced DNA structure: analysis of single N-2-acetylaminofluorene residues bound within the NarI mutation hot spot

We have used a set of chemical probes to characterize and to compare the structural deformation of double-stranded oligomers bearing a single N-2-acetylaminofluorene (AAF) adduct covalently bound to each of the three guanine residues located within the frameshift mutation hot spot sequence -G1G2CG3CC-(NarI site). Two classes of chemical probes have been used, probes that sense the geometry of the helix, giving rise to cuts at every nucleotide (for example, 1,10-phenanthroline-copper), and probes that react with specific bases depending on their conformation (e.g., diethyl pyrocarbonate). For all probes that were tested, a distinct pattern of reactivity was observed according to the position of the adduct within the DNA sequence, revealing an important polymorphism in the adduct-induced DNA structure. With 1,10-phenanthroline-copper at least three base pairs 3' of the AAF-modified guanine were reactive on each strand, showing that the deformation of the DNA helix extends over a region of 4-6 bases pairs centered around the adduct and sensed by the probe in both strands. With the base-specific probes, reactivities were limited to the base complementary to the modified guanine and to adjacent bases. Within this sequence context, the three possible AAF adducts have previously been shown to exhibit strong differences in biological responses such as excision repair [Seeberg, E., & Fuchs, R. P. P. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 191-194] and mutagenesis [Burnouf, D., Koehl, P., & Fuchs, R. P. P. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 4147-4151].(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence effect on incision by (A)BC excinuclease of 4NQO adducts and UV photoproducts

Nucleotide excision repair in Escherichia coli is initiated by (A)BC excinuclease, an enzyme which incises DNA on both sides of bulky adducts and removes the damaged nucleotide as a 12-13 base long oligomer. The incision pattern of the enzyme was examined using DNA modified by 4-nitroquinoline 1-oxide (4NQO) and UV light. Similar to the cleavage pattern of UV photoproducts and other bulky adducts, the enzyme incises the 8th phosphodiester bond 5' and 5th phosphodiester bond 3' to the 4NQO-modifed base, primarily guanine. The extent of DNA damage by these agents was determined using techniques which quantitatively cleave the DNA or stop at the site of the adduct. By comparison of the intensity of gel bands created by (A)BC excinuclease and the specific cleavage at the damaged site, the efficiency of (A)BC excinuclease incision at 13 different 4NQO-induced adducts and 13 different photoproducts was determined by densitometric scanning. In general, incisions made at 4NQO-induced adducts are proportional to the extent of damage, though the efficiency of cutting throughout the sequence tested varies from 25 to 75%. Incisions made at pyrimidine dimers are less efficient than at 4NQO-adducts, ranging from 13 to 65% incision relative to modification, though most are around 50%. The two (6-4) photoproducts within the region tested are incised more efficiently than any pyrimidine dimer.

Single d(ApG)/cis-diamminedichloroplatinum(II) adduct-induced mutagenesis in Escherichia coli

The mutation spectrum induced by the widely used antitumor drug cis-diamminedichloroplatinum(II) (cis-DDP) showed that cisDDP[d(ApG)] adducts, although they account for only 25% of the lesions formed, are approximately 5 times more mutagenic than the major GG adduct. We report the construction of vectors bearing a single cisDDP[d(ApG)] lesion and their use in mutagenesis experiments in Escherichia coli. The mutagenic processing of the lesion is found to depend strictly on induction of the SOS system of the bacterial host cells. In SOS-induced cells, mutation frequencies of 1-2% were detected. All these mutations are targeted to the 5' base of the adduct. Single A----T transversions are mainly observed (80%), whereas A----G transitions account for 10% of the total mutations. Tandem base-pair substitutions involving the adenine residue and the thymine residue immediately 5' to the adduct occur at a comparable frequency (10%). No selective loss of the strand bearing the platinum adduct was seen, suggesting that, in vivo, cisDDP[d(ApG)] adducts are not blocking lesions. The high mutation specificity of cisDDP[d(ApG)]-induced mutagenesis is discussed in relation to structural data.