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.

Molecular topology of polycyclic aromatic carcinogens determines DNA adduct conformation: a link to tumorigenic activity.

We report below on the solution structures of stereoisomeric "fjord" region trans-anti-benzo[c]phenanthrene-N2-guanine (designated (BPh)G) adducts positioned opposite cytosine within the (C-(BPh)G-C).(G-C-G) sequence context. We observe intercalation of the phenanthrenyl ring with stereoisomer-dependent directionality, without disruption of the modified (BPh)G.C base-pair. Intercalation occurs to the 5' side of the modified strand for the 1S stereoisomeric adduct and to the 3' side for the 1R stereoisomeric adduct, with the S and R-trans-isomers related to one another by inversion in a mirror plane at all four chiral carbon atoms on the benzylic ring. Intercalation of the fjord region BPh ring into the helix without disruption of the modified base-pair is achieved through buckling of the (BPh)G.C base-pair, displacement of the linkage bond from the plane of the (BPh)G base, adaptation of a chair pucker by the BPh benzylic ring and the propeller-like deviation from planarity of the BPh phenanthrenyl ring. It is noteworthy that intercalation without base-pair disruption occurs from the minor groove side for S and R-trans-anti BPh-N2-guanine adducts opposite C, in contrast to our previous demonstration of intercalation without modified base-pair disruption from the major groove side for S and R-trans-anti BPh-N6-adenine adducts opposite T. Further, these results on fjord region 1S and 1R-trans-anti (BPh)G adducts positioned opposite C are in striking contrast to earlier research with "bay" region benzo[a]pyrene-N2-guanine (designated (BP)G) adducts positioned opposite cytosine, where 10S and 10R-trans-anti stereoisomers were positioned with opposite directionality in the minor groove without modified base-pair disruption. They also are in contrast to the 10S and 10R-cis-anti stereoisomers of (BP)G adducts opposite C, where the pyrenyl ring is intercalated into the helix with directionality, but the modified base and its partner on the opposite strand are displaced out of the helix. These results are especially significant given the known greater tumorigenic potential of fjord region compared to bay region polycyclic aromatic hydrocarbons. The tumorigenic potential has been linked to repair efficiency such that bay region adducts can be readily repaired while their fjord region counterparts are refractory to repair. Our structural results propose a link between DNA adduct conformation and repair-dependent mutagenic activity, which could ultimately translate into structure-dependent differences in tumorigenic activities. We propose that the fjord region minor groove-linked BPh-N2-guanine and major groove-linked BPh-N6-adenine adducts are refractory to repair based on our observations that the phenanthrenyl ring intercalates into the helix without modified base-pair disruption. The helix is therefore minimally perturbed and the phenanthrenyl ring is not available for recognition by the repair machinery. By contrast, the bay region BP-N2-G adducts are susceptible to repair, since the repair machinery can recognize either the pyrenyl ring positioned in the minor groove for the trans-anti groove-aligned stereoisomers, or the disrupted modified base-pair for the cis-anti base-displaced intercalated stereoisomers.

Accomodation of S-cis-tamoxifen-N(2)-guanine adduct within a bent and widened DNA minor groove.

The non-steroidal anti-estrogen tamoxifen [TAM] has been in clinical use over the last two decades as a potent adjunct chemotherapeutic agent for treatment of breast cancer. It has also been given prophylactically to women with a strong family history of breast cancer. However, tamoxifen treatment has also been associated with increased endometrial cancer, possibly resulting from the reaction of metabolically activated tamoxifen derivatives with cellular DNA. Such DNA adducts can be mutagenic and the activities of isomeric adducts may be conformation-dependent. We therefore investigated the high resolution NMR solution conformation of one covalent adduct (cis-isomer, S-epimer of [TAM]G) formed from the reaction of tamoxifen [TAM] to N(2)-of guanine in the d(C-[TAM]G-C).d(G-C-G) sequence context at the 11-mer oligonucleotide duplex level. Our NMR results establish that the S-cis [TAM]G lesion is accomodated within a widened minor groove without disruption of the Watson-Crick [TAM]G. C and flanking Watson-Crick G.C base-pairs. The helix axis of the bound DNA oligomer is bent by about 30 degrees and is directed away from the minor groove adduct site. The presence of such a bulky [TAM]G adduct with components of the TAM residue on both the 5'- and the 3'-side of the modified base could compromise the fidelity of the minor groove polymerase scanning machinery.

Solution structure of the (+)-cis-anti-benzo[a]pyrene-dA ([BP]dA) adduct opposite dT in a DNA duplex.

Minor adducts, derived from the covalent binding of anti-benzo[a]pyrene-7,8-dihydroxy-9,10-epoxide to cellular DNA, may play an important role in generating mutations and initiating cancer. We have applied a combined NMR-computational approach including intensity based refinement to determine the solution structure of the minor (+)-cis-anti-[BP]dA adduct positioned opposite dT in the d(C1-T2-C3-T4-C5-[BP]A6-C7-T8-T9-C10-C11). (d(G12-G13-A14-A15-G16-T17-G18-A19-G20+ ++-A21-G22) 11-mer duplex. The BP ring system is intercalated toward the 5'-side of the [BP]dA6 lesion site without disrupting the flanking Watson-Crick dC5.dG18 and [BP]dA6.dT17 base pairs. This structure of the (+)-cis-anti-[BP]dA.dT 11-mer duplex, containing a bay region benzo[a]pyrenyl [BP]dA adduct, is compared with the corresponding structure of the (+)-trans-anti-[BPh]dA.dT 11-mer duplex (Cosman et al., Biochemistry 32, 12488-12497, 1993), which contains a fjord region benzo[c]phenanthrenyl [BPh]dA adduct with the same R stereochemistry at the linkage site. The carcinogen intercalates toward the 5'-direction of the modified strand in both duplexes (the adduct is embedded within the same sequence context) with the buckling of the Watson-Crick [BP]dA6.dT17 base pair more pronounced in the (+)-cis-anti-[BP]dA.dT 11-mer duplex compared to its Watson-Crick [BPh]dA.dT17 base pair in the (+)-trans-anti-[BPh]dA.dT 11-mer duplex. The available structural studies of covalent polycyclic aromatic hydrocarbon (PAH) carcinogen-DNA adducts point toward the emergence of a general theme where distinct alignments are adopted by PAH adducts covalently linked to the N(6) of adenine when compared to the N(2) of guanine in DNA duplexes. The [BPh]dA and [BP]dA N(6)-adenine adducts intercalate their polycyclic aromatic rings into the helix without disruption of their modified base pairs. This may reflect the potential flexibility associated with the positioning of the covalent tether and the benzylic ring of the carcinogen in the sterically spacious major groove. By contrast, such an intercalation without modified base pair disruption option appears not to be available to [BP]dG N(2)-guanine adducts where the covalent tether and the benzylic ring are positioned in the more sterically crowded minor groove. In the case of [BP]dG adducts, the benzopyrenyl ring is either positioned in the minor groove without base pair disruption, or if intercalated into the helix, requires disruption of the modified base pair and displacement of the bases out of the helix.

Solution structure of the N-(deoxyguanosin-8-yl)-1-aminopyrene ([AP]dG) adduct opposite dA in a DNA duplex.

Solution structural studies have been undertaken on the aminopyrene-C(8)-dG ([AP]dG) adduct in the d(C5-[AP]G6-C7). d(G16-A17-G18) sequence context in an 11-mer duplex with dA opposite [AP]dG, using proton-proton distance and intensity restraints derived from NMR data in combination with distance-restrained molecular mechanics and intensity-restrained relaxation matrix refinement calculations. The exchangeable and nonexchangeable protons of the aminopyrene and the nucleic acid were assigned following analysis of two-dimensional NMR data sets on the [AP]dG.dA 11-mer duplex in H2O and D2O solution. The broadening of several resonances within the d(G16-A17-G18) segment positioned opposite the [AP]dG6 lesion site resulted in weaker NOEs, involving these protons in the adduct duplex. Both proton and carbon NMR data are consistent with a syn glycosidic torsion angle for the [AP]dG6 residue in the adduct duplex. The aminopyrene ring of [AP]dG6 is intercalated into the DNA helix between intact Watson-Crick dC5.dG18 and dC7.dG16 base pairs and is in contact with dC5, dC7, dG16, dA17, and dG18 residues that form a hydrophobic pocket around it. The intercalated AP ring of [AP]dG6 stacks over the purine ring of dG16 and, to a lesser extent dG18, while the looped out deoxyguanosine ring of [AP]dG6 stacks over dC5 in the solution structure of the adduct duplex. The dA17 base opposite the adduct site is not looped out of the helix but rather participates in an in-plane platform with adjacent dG18 in some of the refined structures of the adduct duplex. The solution structures are quite different for the [AP]dG.dA 11-mer duplex containing the larger aminopyrene ring (reported in this study) relative to the previously published [AF]dG.dA 11-mer duplex containing the smaller aminofluorene ring (Norman et al., Biochemistry 28, 7462-7476, 1989) in the same sequence context. Both the modified syn guanine and the dA positioned opposite it are stacked into the helix with the aminofluorene chromophore displaced into the minor groove in the latter adduct duplex. By contrast, the aminopyrenyl ring participates in an intercalated base-displaced structure in the present study of the [AP]dG.dA 11-mer duplex and in a previously published study of the [AP]dG.dC 11-mer duplex (Mao et al., Biochemistry 35, 12659-12670, 1996). Such intercalated base-displaced structures without hydrogen bonding between the [AP]dG adduct and dC or mismatched dA residues positioned opposite it, if present at a replication fork, may cause polymerase stalling and formation of a slipped intermediate that could produce frameshift mutations, the most dominant mutagenic consequence of the [AP]dG lesion.

Solution structures of aminofluorene [AF]-stacked conformers of the syn [AF]-C8-dG adduct positioned opposite dC or dA at a template-primer junction.

A solution structural study has been undertaken on the aminofluorene-C8-dG ([AF]dG) adduct located at a single-strand-double-strand d(A1-A2-C3-[AF]G4-C5-T6-A7-C8-C9-A10-T11-C12-C13). d(G14-G15-A16-T17-G18-G19-T20- A21-G22-N23) 13/10-mer junction (N = C or A) using proton-proton distance restraints derived from NMR data in combination with intensity-based relaxation matrix refinement computations. This single-strand-double-strand junction models one arm of a replication fork composed of a 13-mer template strand which contains the [AF]dG modification site and a 10-mer primer strand which has been elongated up to the modified guanine with either its complementary dC partner or a dA mismatch. The solution structures establish that the duplex segment retains a minimally perturbed B-DNA conformation with Watson-Crick hydrogen-bonding retained up to the dC5.dG22 base pair. The guanine ring of the [AF]dG4 adduct adopts a syn glycosidic torsion angle and is displaced into the major groove when positioned opposite dC or dA residues. This base displacement of the modified guanine is accompanied by stacking of one face of the aminofluorene ring of [AF]dG4 with the dC5.dG22 base pair, while the other face of the aminofluorene ring is stacked with the purine ring of the nonadjacent dA2 residue. By contrast, the dC and dA residues opposite the junctional [AF]dG4 adduct site adopt distinctly different alignments. The dC23 residue positioned opposite the adduct site is looped out into the minor groove by the aminofluorene ring. The syn displaced orientation of the modified dG with stacking of the aminofluorene and the looped out position of the partner dC could be envisioned to cause polymerase stalling associated with subsequent misalignment leading to frameshift mutations in appropriate sequences. The dA23 residue positioned opposite the adduct site is positioned in the major groove with its purine ring aligned face down over the van der Waals surface of the major groove and its amino group directed toward the T6.A21 base pair. The Hoogsteen edge of the modified guanine of [AF]dG4 and the Watson-Crick edge of dA23 positioned opposite it are approximately coplanar and directed toward each other but are separated by twice the hydrogen-bonding distance required for pairing. This structure of [AF]dG opposite dA at a model template-primer junctional site can be compared with a previous structure of [AF]dG opposite dA within a fully paired duplex [Norman, D., Abuaf, P., Hingerty, B. E., Live, D. , Grunberger, D., Broyde, S., and Patel, D. J. (1989) Biochemistry 28, 7462-7476]. The alignment of the Hoogsteen edge of [AF]dG (syn) positioned opposite the Watson-Crick edge of dA (anti) has been observed for both systems with the separation greater in the case of the junctional alignment in the model template-primer system. However, the aminofluorene ring is positioned in the minor groove in the fully paired duplex while it stacks over the junctional base pair in the template-primer system. This suggests that the syn [AF]dG opposite dA junctional alignment can be readily incorporated within a duplex by a translation of this entity toward the minor groove.

A slipped replication intermediate model is stabilized by the syn orientation of N-2-aminofluorene- and N-2-(acetyl)aminofluorene-modified guanine at a mutational hotspot.

The Escherichia coli NarI restriction enzyme recognition site 5'G1G2C3G4C5C63' is a mutational hotspot for -2 deletions in E. coli plasmid pBR322, resulting in the sequence 5'GGCC3' when G4 is modified by the aromatic amine N-2-(acetyl)aminofluorene (AAF) [Burnouf, D., Koehl, P., and Fuchs, R. P. P. (1995) Proc. Natl. Acad. Sci. U.S.A. 86, 4147-4151] even though each G shows similar reactivity [Fuchs, R. P. P. (1984) J. Mol. Biol. 177, 173-180]. Modification at G4 by the related aromatic amine 2-aminofluorene (AF), which lacks the acetyl group of AAF, can also cause -2 deletions, but at a lower frequency [Bichara, M., and Fuchs, R. P. P. (1985) J. Mol. Biol. 183, 341-351]. A specific mechanism has been proposed to explain the double-base frameshifts in the NarI sequence in which the GC deletion results from a slipped mutagenic intermediate formed during replication [Schaaper, B. M., Koffel-Schwartz, N., and Fuchs, R. P. P. (1990) Carcinogenesis 11, 1087-1095]. We address the following key questions in this study. Why does AAF modification dramatically increase the mutagenicity at the NarI G4 position, and why does AAF enhance the mutagenicity more than AF? We studied two intermediates which model replication at one arm of a fork, using a fragment of DNA modified by AF or AAF at G4 in the NarI sequence: Intermediate I can be converted into intermediate II by misalignment. Elongation of intermediate I leads to error-free translesion synthesis, while elongation of intermediate II leads to a -2 frameshift mutation. Minimized potential energy calculations were carried out using the molecular mechanics program DUPLEX to investigate the conformations of the AF and AAF adducts at G4 in these two intermediates. We find that the slipped mutagenic intermediate is quite stable relative to its normally extended counterpart in the presence of AF and AAF in an abnormal syn orientation of the damaged base. An enhanced probability of elongation from a stable slipped structure rather than a properly aligned one would favor increased -2 frameshift mutations. Furthermore, AAF-modified DNA has a greater tendency to adopt the syn orientation than AF because of its greater bulk, which could explain its greater propensity to cause -2 deletions in the NarI sequence.

Effect of ring size on conformations of aromatic amine-DNA adducts: the aniline-C8 guanine adduct resides in the B-DNA major groove.

While the one-ring amine aniline (AN) has only slight genetic activity, the polycyclic aromatic amines 2-aminofluorene (AF) and 1-aminopyrene (AP) are significant mutagens and carcinogens. Moreover, the bulkier AP is more mutagenic per adduct than AF in the tetracycline-resistance gene of plasmid pBR322 [Melchior et al. (1994) Carcinogenesis 15, 889]. To elucidate possible conformational origins of the differing mutagenic effects of these three adducts, which may stem from their differing ring sizes, we have examined their conformations in two mutation-susceptible sequences from the above gene: TTGAG*GCCG (sequence I) and GAATG*GTGC (sequence II), where G* = C8-modified guanine. No experimental high-resolution NMR data are yet available for the aniline adduct in a DNA duplex. Minimized potential energy calculations were carried out, using the molecular mechanics program DUPLEX to explore the conformation space of these adducts. In the case of AN, a relatively unperturbed B-DNA helix with the amine in the major groove was strongly favored in both sequences. In the case of AF- and AP-modified DNA, however, several differing conformations were competitive in energy. They included major groove structures, as well as conformations with syn-modified guanine and the polycyclic amine in the minor groove, or the amine rings intercalated into the helix with displacement of the modified guanine, in overall harmony with high-resolution NMR solution structures. Thus, aniline distorts DNA structure to a lesser extent than larger aromatic amine ring systems, since a number of different conformations are energetically feasible and have been observed for the larger systems. This result may be relevant to their enhanced mutagenicity and their repair propensity, in contrast to aniline's low mutagenic effect.

Interactions between DNA polymerase beta and the major covalent adduct of the carcinogen (+)-anti-benzo[a]pyrene diol epoxide with DNA at a primer-template junction.

A molecular dynamics simulation has been carried out with DNA polymerase beta (beta pol) complexed with a DNA primer-template. The templating guanine at the polymerase active site was covalently modified by the carcinogenic metabolite of benzo[a]pyrene, (+)-anti-benzo[a]pyrene diol epoxide, to form the major (+)-trans-anti-benzo[a]pyrene diol epoxide covalent adduct. Thus, the benzo[a]pyrenyl moiety (BP) is situated in the single-stranded template at the junction between double- and single-stranded DNA. The starting structure was based on the X-ray crystal structure of the rat beta pol primer-template and ddCTP complex [Pelletier, H., Sawaya, M. R., Kumar, A., Wilson, S. H., and Kraut, J. (1994) Science 264, 1891-1903]. During the simulation, the BP and its attached templating guanine rearrange to form a structure in which the BP is closer to parallel with the adjacent base pair. In addition, the templating attached guanine is displaced toward the major groove side and access to its Watson-Crick edge is partly obstructed. This structure is stabilized, in part, by new hydrogen bonds between the BP and beta pol Asn279 and Arg283. These residues are within hydrogen bonding distance to the incoming ddCTP and templating guanine, respectively, in the crystal structure of the beta pol ternary complex. Site-directed mutagenesis has confirmed their role in dNTP binding, discrimination, and catalytic efficiency [Beard, W. A., Osheroff, W. P., Prasad, R., Sawaya, M. R., Jaju, M., Wood, T. G., Kraut, J., Kunkel, T. A., and Wilson, S. H. (1996) J. Biol. Chem. 271, 12141-12144]. The predominant biological effect of the BP is DNA polymerase blockage. Consistent with this biological effect, the computed structure suggests the possibility that the BP's main deleterious impact on DNA synthesis might result at least in part from its specific interactions with key polymerase side chains. Moreover, relatively modest movement of BP and its attached guanine, with some concomitant enzyme motion, is necessary to relieve the obstruction and permit the observed rare incorporation of a dATP opposite the guanine lesion.

Solution structure of the aminofluorene [AF]-intercalated conformer of the syn-[AF]-C8-dG adduct opposite dC in a DNA duplex.

We report below on a conformational equilibrium between AF-intercalated and AF-external states in slow exchange for the [AF]dG lesion positioned opposite dC in the d(C-[AF]G-C).d(G-C-G) sequence context. The slow exchange between states is attributed to interconversion between syn glycosidic torsion angle in the AF-intercalated and anti torsion angle in AF-external conformers of the [AF]dG opposite dC containing duplex. The present paper describes an NMR-molecular mechanics study that defines the solution structure of the AF-intercalated conformer for the case of [AF]dG adduct positioned opposite dC in the d(C-[AF]G-C).d(G-C-G) sequence context. The structure is of the base displacement-intercalation type where the aminofluorene ring is intercalated into the helix between intact Watson-Crick dG.dC base pairs, which results in a displacement of the modified guanine ring into the major groove where it stacks with the major groove edge of its 5'-flanking cytosine in the adduct duplex. The conformational equilibrium between AF-intercalated conformer (approximately 70%) with a syn alignment and AF-external conformer (approximately 30%) with an anti alignment for the [AF]dG adduct positioned opposite dC in the d(C-[AF]G-C).d(G-C-G) sequence context can be contrasted with our earlier demonstration that the population is 100% for the AP-intercalated conformer with a synalignment at the N-(deoxyguanosin-8-yl)-2-aminopyrene ([AP]dG) adduct site positioned opposite dC in the same sequence context [Mao, B., Vyas, R. R., Hingerty, B. E., Broyde, S., Basu, A. K., and Patel, D. J. (1996) Biochemistry, 35, 12659-12670]. This shift in population may reflect the much larger size of the pyrenyl ring of the [AP]dG adduct compared to the fluorenyl ring of the [AF]dG adduct which in turn might provide for a greater overlap of the aromatic amine with the flanking base pairs in the intercalated conformer of the former adduct in DNA.

Solution structure of the aminofluorene [AF]-external conformer of the anti-[AF]-C8-dG adduct opposite dC in a DNA duplex.

The Escherichia coli genome contains a C-G1-G2-C-G3-C-C NarI hot spot sequence for -2 deletion mutations at G3 by aromatic amine carcinogens 2-acetylaminofluorene (AAF) and 2-aminofluorene (AF) that form covalent adducts at the C8-position of the guanine ring. Each of the three guanines are positioned in different sequence contexts (C-G1-G, G-G2-C, and C-G3-C) which provides an opportunity to investigate the potential sequence dependent interconversion between AF-intercalated and AF-external conformers of the [AF]dG adduct positioned opposite dC within the NarI sequence at the duplex level. We have prepared and purified DNA duplexes containing the [AF]dG adduct positioned in C-[AF]G-G, G-[AF]G-C, and C-[AF]G-C NarI sequence contexts and observe the ratio of AF-intercalated to AF-external conformers to be 30:70, 10:90, and 50:50, respectively. We have applied a combined NMR-molecular mechanics approach to define the structure of the AF-external conformer in the G-[AF]G-C NarI sequence context where it is the predominant conformation (90%) in solution. The modified guanine of the [AF]dG adduct aligns through Watson-Crick pairing with its partner cytosine and is stacked into the helix between flanking Watson-Crick dG.dC base pairs. The AF-external conformer with its anti-[AF]dG residue causes minimal perturbations in the DNA duplex at and adjacent to the lesion site with the covalently linked fluorenyl ring readily accommodated in the major groove and tilted toward the 5'-end of the modified strand of the helix. This paper on the structure of the AF-external conformer with an anti-[AF]dG adduct together with the preceding paper in this issue on the structure of the AF-intercalated conformer with a syn-[AF]dG adduct defines for the first time the capacity of the mutagenic [AF]dG lesion to adopt interconverting syn and antialignments with the equilibrium shifting between the conformers depending on nearest neighbor and next-nearest neighbor sequences. Perhaps, recognition of the [AF]dG lesion by the repair machinery would be able to discriminate between the AF-intercalated conformer with its base displacement-fluorenyl ring insertion perturbation of the helix and the AF-external conformer where the DNA helix is essentially unperturbed at the lesion site and the fluorenyl ring is positioned with directionality in the major groove.

Solution conformation of the (-)-trans-anti-[BP]dG adduct opposite a deletion site in a DNA duplex: intercalation of the covalently attached benzo[a]pyrene into the helix with base displacement of the modified deoxyguanosine into the minor groove.

A combined NMR-computational approach was employed to determine the solution structure of the (-)-trans-anti-[BP]dG adduct positioned opposite a -1 deletion site in the d(C1-C2-A3-T4-C5- [BP]G6-C7-T8-A9-C10-C11).d(G12-G13-T14-A15-G1 6-G17-A18-T19-G20-G21) sequence context. The (-)-trans-anti-[BP]dG moiety is derived from the binding of the (-)-anti-benzo[a]pyrene diol epoxide [(-)-anti-BPDE] to N2 of dG6 and has a 10R absolute configuration at the [BP]dG linkage site. The exchangeable and non-exchangeable protons of the benzo[a]pyrenyl moiety and the nucleic acid were assigned following analysis of two-dimensional NMR data sets in H2O and D2O solution. The solution conformation has been determined by incorporating intramolecular and intermolecular proton-proton distances defined by lower and upper bounds deduced from NOESY spectra as restraints in molecular mechanics computations in torsion angle space followed by restrained molecular dynamics calculations based on a NOE distance and intensity refinement protocol. Our structural studies establish that the aromatic BP ring system intercalates into the helix opposite the deletion site, while the modified deoxyguanosine residue is displaced into the minor groove with its face parallel to the helix axis. The intercalation site is wedge-shaped and the BP aromatic ring system stacks over intact flanking Watson-Crick dG.dC base pairs. The modified deoxyguanosine stacks over the minor groove face of the sugar ring of the 5'-flanking dC5 residue. The BP moiety is positioned with the benzylic ring oriented toward the minor groove and the distal pyrenyl aromatic ring directed toward the major groove. This conformation strikingly contrasts with the corresponding structure in the full duplex with the same 10R (-)-trans-anti-[BP]dG lesion positioned opposite a complementary dC residue [de los Santos et al. (1992) Biochemistry 31, 5245-5252); in this case the aromatic BP ring system is located in the minor groove, and there is no disruption of the [BP]dG.dC Watson-Crick base pairing alignment. The intercalation-base displacement features of the 10R (-)-trans-anti-[BP]dG adduct opposite a deletion site have features in common to those of the 10S (+)-trans-anti-[BP]dG adduct opposite a deletion site previously reported by Cosman et al. [(1994)(Biochemistry 33, 11507-11517], except that there is a nearly 180 degrees rotation of the BP residue about the axis of the helix at the base-displaced intercalation site and the modified deoxyguanosine is positioned in the opposite groove. In the 10S adduct, the benzylic ring is in the major groove and the aromatic ring systems point toward the minor groove. This work extends the theme of opposite orientations of adducts derived from chiral pairs of (+)- and (-)-anti-BPDE enantiomers; both 10S and 10R adducts can be positioned with opposite orientations either in the minor groove or at base displaced intercalation sites, depending on the presence or absence of the partner dC base in the complementary strand.

Structural alignment of the (+)-trans-anti-benzo[a]pyrene-dG adduct positioned opposite dC at a DNA template-primer junction.

This study reports on the solution conformation of the covalent (+)-trans-anti-[BP]dG adduct (derived from the binding of the highly mutagenic and tumorigenic (+)-anti-benzo[a]pyrene diol epoxide to the N2 of deoxyguanosine) positioned opposite dC at a junctional site in the d(A1-A2-C3-[BP]G4-C5- T6-A7-C8-C9-A10-T11-C12-C13).d(G14-G15-A16-T17-+ ++G18-G19-T20-A21-G22-C23) 13/10-mer DNA sequence. The 13-mer represents the template strand containing the junction [BP]dG4 lesion while the complementary 10-mer models a primer strand which extends upto and is complementary to the modified dG4 residue. The solution conformation has been determined by initially incorporating intramolecular and intermolecular proton-proton distances defined by lower and upper bounds deduced from NOESY spectra as restraints in molecular mechanics computations in torsion angle space and subsequently through restrained molecular dynamics calculations based on a NOE distance and intensity refinement protocol. The duplex segment retains a minimally perturbed B-DNA conformation with all base pairs, including the junctional [BP]dG4.dC23 pair, in Watson-Crick hydrogen-bonded alignments. The pyrenyl ring is not stacked over the adjacent dC5.dG22 base pair but is positioned on the minor groove-side of the [BP]dG moiety and directed toward the 5'-end of the template strand. The pyrenyl ring stacks over the base of the non-adjacent dA2 residue in one direction and the sugar ring of dC23 in the other direction. The solution structure of the (+)-trans-anti-[BP]dG adduct opposite dC in the 13/10-mer in which the modified deoxyguanosine adopts an anti glycosidic torsion angle (this study) is in striking contrast to the structure of the same (+)-trans-anti-[BP]dG moiety in a 13/9-mer of the same sequence but without the dC23 residue positioned opposite the adduct site [Cosman, M., et al. (1995) Biochemistry 34, 15334-15350]. For the latter case, the aromatic portion of the BP residue stacks over the adjacent dC5.dG22 base pair, the modified deoxyguanosine adopts a syn glycosidic torsion angle and is displaced toward the major groove direction. Insights into the factors that affect the sequence and context dependent conformations of stereoisomeric [BP]dG lesions have emerged following comparison of these two structures with the minor groove conformations of the same (+)-trans-anti-[BP]dG lesion in the fully complementary 11-mer duplex [Cosman, M., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1914-1918] and in the base displaced-intercalative conformation of the 11/10-mer deletion duplex containing a -1 deletion site opposite the lesion [Cosman, M., et al. (1994) Biochemistry 33, 11507-11517]. The contributing factors where applicable include Watson-Crick base pairing at the site of the lesion, positioning of the carcinogen within the floor of the minor groove, and the tendency of the bulky hydrophobic aromatic BP residue to assume stacked or intercalative conformations.

Solution structure of the aminofluorene-intercalated conformer of the syn [AF]-C8-dG adduct opposite a--2 deletion site in the NarI hot spot sequence context.

This paper addresses structural issues related to the capacity of aminofluorene [AF] for frameshift mutations of the -2 type on C8 covalent adduct formation at the G3 site in the d(C-G1-G2-C-G3-C-C) NarI hot spot sequence. This problem has been approached from a combined NMR and relaxation matrix analysis computational structural study of the [AF]dG adduct in the d(C-G-G-C-[AF]G-C-C).d(G-G-C-C-G) sequence context at the 12/10-mer adduct level (designated [AF]dG.del(-2) 12/10-mer). The proton spectra of this system are of exceptional quality and are consistent with the formation of an AF-intercalated conformer with the modified guanine in a syn alignment displaced along with the 5'-flanking cytosine residue into the major groove. The solution structure has been determined by initially incorporating intramolecular and intermolecular proton-proton distances defined by lower and upper bound deduced from NOESY spectra as restraints in molecular mechanics computations in torsion angle space and subsequently refined through restrainted molecular dynamics calculations based on a NOE distance and intensity refinement protocol. Strikingly, the [AF]dG.del(-2) 12/10-mer duplex adopts only one of two potential AF-intercalation alignments for the [AF]dG adduct opposite the -2 deletion site in the NarI sequence context with the extrusion of the dC-[AF]dG step favored completely over extrusion of the [AF]dG-dC step at the lesion site. This polarity establishes that the structural perturbation extends 5' rather than 3' to the [AF]dG lesion site in the adduct duplex. This structure of the [AF]dG adduct opposite a -2 deletion site shows distinct differences with conclusions reported on the alignment of the related acetylaminofluorene [AAF]dG adduct opposite a -2 deletion site in the identical NarI sequence context [Milhe, C., Fuchs, R. P. P., and Lefevre, J. F. (1996) Eur. J. Biochem. 235, 120-127]. In that study, qualitative NMR data without computational analysis were employed to conclude that the extrusion at the lesion site occurs at the [AAF]dG-dC step for the AAF-intercalated conformer of the adduct duplex. The structure of the [AF]dG adduct opposite a -2 deletion site determined in our group provides molecular insights into the architecture of extended slipped mutagenic intermediates involving aromatic amine intercalation and base-displaced syn modified guanines in AF and, by analogy, AAF-induced mutagenesis in the NarI hot spot sequence context.

Solution structure of the aminofluorene-stacked conformer of the syn [AF]-C8-dG adduct positioned at a template-primer junction.

A solution structural study has been undertaken on the aminofluorene-C8-dG ([AF]dG) adduct located at a single strand-double strand d(A1-A2-C3-[AF]G4-C5-T6-A7-C8-C9-A10-T11-C12-C13).d (G14-G15-A16-T17-G18-G19-T20-A 21-G22) 13/9-mer junction (designated [AF]dG 13/9-mer) using proton-proton distance and intensity restraints derived from NMR data in combination with a computational protocol, which includes intensity refinement. This single strand-double strand junction models one arm of a replication fork composed of a 13-mer template strand, which contains the [AF]dG modification site, and a 9-mer primer strand, which has been elongated up to, but not including, the modified guanine. The NMR data establish that the duplex segment retains a minimally perturbed B-DNA conformation including Watson-Crick hydrogen-bonding at the junctional dC5.dG22 base pair. The NMR spectra are consistent with the guanine ring of the [AF]dG4 adduct adopting a syn glycosidic torsion angle and being displaced into the major groove with the adjacent dC3 residue displaced into the minor groove. Such a base displacement of the modified guanine is accompanied by stacking of one face of the fluorene ring of [AF]dG4 with the dC5.dG22 base pair, while the other face of the flourene ring is stacked with the purine ring of the nonadjacent dA2 residue in the intensity-refined solution structures of the [AF]dG 13/9-mer. A comparison of structural features of the C8-[AF]dG adduct (this study) with those of the (+)-trans-anti-N2-[BP]dG adduct [Cosman et al. (1995) Biochemistry 34, 15334-15350] in the same 13/9-mer junctional sequence context has identified common features associated with the alignment of the modified guanine adducts at the template-primer junction. Thus, despite differences in the covalent linkage site for the C8-[AF]dG and (+)-trans-anti-N2-[BP]dG adducts, one face of the aromatic ring of the carcinogen stacks over the junctional base pair and in so doing displaces the modified guanine in a syn alignment into the major groove. These results lend credence to earlier proposals that such an adduct alignment may represent a common mutagenic conformer at a template-primer junction associated with a replication fork.

A molecular mechanics and dynamics study of the minor adduct between DNA and the carcinogen 2-(acetylamino)fluorene (dG-N2-AAF).

Experimental studies involving the carcinogenic aromatic amine 2-(acetylamino)fluorene (AAF) have afforded two acetylated DNA adducts, the major one bound to C8 of guanine and a minor adduct bound to N2 of guanine. The minor adduct may be important in carcinogenesis because it persists, while the major adduct is rapidly repaired. Primer extension studies of the minor adduct have indicated that it blocks DNA synthesis, with some bypass and misincorporation of adenine opposite the lesion [Shibutani, S., and Grollman, A.P. (1993) Chem. Res. Toxicol. 6, 819-824]. No experimental structural information is available for this adduct. Extensive minimized potential energy searches involving thousands of trials and molecular dynamics simulations were used to study the conformation of this adduct in three sequences: I, d(C1-G2-C3-[AAF]G4-C5-G6-C7).d(G8-C9-G10-C11-G12-C13-G14+ ++); II, the sequence of Shibutani and Grollman, d(C1-T2-A3-[AAF]G4-T5-C6-A7).d(T8-G9-A10-C11-T12-A13-G14); and III, which is the same as II but with a mismatched adenine in position 11, opposite the lesion. AAF was located in the minor groove in the low-energy structures of all sequences. In the lowest energy form of the C3-[AAF]G4-C5 sequence I, the fluorenyl rings point in the 3' direction along the modified strand and the acetyl in the 5' direction. These orientations are reversed in the second lowest energy structure of this sequence, and the energy of this structure is 1.4 kcal/mol higher. Watson Crick hydrogen bonding is intact in both structures. In the two lowest energy structures of the A3-[AAF]G4-T5 sequence II, the AAF is also located in the minor groove with Watson-Crick hydrogen bonding intact. However, in the lowest energy form, the fluorenyl rings point in the 5' direction and the acetyl in the 3' direction. The energy of the structure with opposite orientation is 5.1 kcal/mol higher. In sequence III with adenine mismatched to the modified guanine, the lowest energy form also had the fluorenyl rings oriented 5' in the minor groove with intact Watson-Crick base pairing. However, the mispaired adenine adopts a syn orientation with Hoogsteen pairing to the modified guanine. These results suggest that the orientation of the AAF in the minor groove may be DNA sequence dependent. Mobile aspects of favored structures derived from molecular dynamics simulations with explicit solvent and salt support the essentially undistorting nature of this lesion, which is in harmony with its persistence in mammalian systems.

Solution conformation of the N-(deoxyguanosin-8-yl)-1-aminopyrene ([AP]dG) adduct opposite dC in a DNA duplex.

Combined NMR-molecular mechanics computational studies were undertaken on the C8-deoxyguanosine adduct formed by the carcinogen 1-nitropyrene embedded in the d(C5-[AP]G6-C7).d(G16-C17-G18) sequence context in a 11-mer duplex, with dC opposite the modified deoxyguanosine. The exchangeable and nonexchangeable protons of the aminopyrene moiety and the nucleic acid were assigned following analysis of two-dimensional NMR data sets in H2O and D2O solution. There was a general broadening of several proton resonances for the three nucleotide d(G16-C17-G18) segment positioned opposite the [AP]dG6 lesion site resulting in weaker NOEs involving these protons in the adduct duplex. The solution conformation of the [AP]dG.dC 11-mer duplex has been determined by incorporating intramolecular and intermolecular proton-proton distances defined by upper and lower bounds deduced from NOESY spectra as restraints in molecular mechanics computations in torsion angle space. The aminopyrene ring of [AP]dG6 is intercalated into the DNA helix between intact Watson-Crick dC5.dG18 and dC7.dG16 base pairs. The modified deoxyguanosine ring of [AP]dG6 is displaced into the major groove and stacks with the major groove edge of dC5 in the adduct duplex. Both carbon and proton chemical shift data for the sugar resonances of the modified deoxyguanosine residue are consistent with a syn glycosidic torsion angle for the [AP]dG6 residue. The dC17 base on the partner strand is displaced from the center of the helix toward the major groove as a consequence of the aminopyrene ring intercalation into the helix. This base-displaced intercalative structure of the [AP]dG.dC 11-mer duplex exhibits several unusually shifted proton resonances which can be accounted for by the ring current contributions of the deoxyguanosinyl and pyrenyl rings of the [AP]dG6 adduct. In summary, intercalation of the aminopyrene moiety is accompanied by displacement of both [AP]dG6 and the partner dC17 into the major groove in the [AP]dG.dC 11-mer duplex.

Solution conformation of the (-)-cis-anti-benzo[a]pyrenyl-dG adduct opposite dC in a DNA duplex: intercalation of the covalently attached BP ring into the helix with base displacement of the modified deoxyguanosine into the major groove.

This paper reports on the combined NMR-molecular mechanics computational studies of the solution structure of the (-)-cis-anti-[BP]dG adduct positioned opposite dC in the sequence context d(C1- C2-A3-T4-C5-[BP]G6-C7-T8-A9-C10-C11).d(G12-G13-T14- A15-G16-C17-G18-A19-T20- G21-G22) duplex [designated (-)-cis-anti-[BP]dG.dC 11-mer duplex]. This adduct is derived from cis addition at C10 of (-)-anti-7(S),8(R)-dihydroxy-9(R),10(S)-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene [(-)-anti-BPDE] to the N2 position of dG6 in this duplex sequence. The exchangeable and nonexchangeable protons of the benzo[a]pyrenyl moiety and nucleic acid of the major conformation were assigned following analysis of two-dimensional NMR data sets in H2O and D2O solution. There was a general broadening of proton resonances for a three-nucleotide segment centered about the lesion site which resulted in a tentative assignment for the sugar protons of the C7 residue in the spectrum of the adduct duplex. The solution conformation of the major conformation of the (-)-cis-anti-[BP]dG.dC 11-mer duplex has been determined by incorporating DNA-DNA and intermolecular BP-DNA proton-proton distances defined by lower and upper bounds deduced from NOESY data sets as restraints in molecular mechanics computations in torsion angle space. The results establish that the covalently attached benzo[a]pyrenyl ring intercalates between intact Watson-Crick dC5.dG18 and dC7.dG16 base pairs. The modified deoxyguanosine [BP]-dG6 and its partner cytosine dC17 are looped out of the helix into the major groove. The purine ring of the [BP]dG6 residue is directed toward the 5'-end of the modified strand and stacks over the major groove edge of its 5'-side neighbor dC5 residue. The solution structure of the (-)-cis-anti-[BP]dG.dC 11-mer duplex is compared with those of the stereoisomeric (+)-trans-anti-[BP]dG [Cosman, M., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1914-1918], (-)-trans-anti-[BP]dG [de los Santos, C., et al. (1992) Biochemistry 31, 5245-5252], and (+)-cis-anti-[BP]dG [Cosman, M., et al. (1993a) Biochemistry 32, 4146-4155] adducts positioned opposite dC in the same duplex sequence context. A key finding is that the long axes of the intercalated benzo[a]pyrenyl rings in the solution structures of the (+)- and (-)- cis-anti-[BP]dG.dC 11-mer duplexes are oriented in opposite directions with the benzylic ring directed toward the minor groove in the (+)-cis isomer and toward the major groove in the (-)-cis isomer. In addition, a comparison is also made with the solution structure of the (+)-trans-anti-[BP]dG adduct opposite a deletion site [Cosman, M., et al. (1994a) Biochemistry 33, 11507-11517] since this adduct duplex displays several conformational features in common with the structure of the (-)-cis-anti-[BP]dG.dC 11-mer duplex. The structures of both duplex adducts exhibit intercalation of the covalently attached ligand into the helix and displacement of the modified deoxyguanosine into the major groove. Studies of the biological activities of stereochemically defined BP-DNA adducts and the comparison of the solution structure of the (-)-cis-anti-[BP]dG.dC 11-mer duplex with its stereoisomeric counterparts should lead to new insights into the relationships between defined helical distortions and mutagenic specificity and activity.