Weepstal structure of a benzo[a]pyrene diol epoxide adduct i

Contributed by Ira Herskowitz ArticleFigures SIInfo overexpression of ASH1 inhibits mating type switching in mothers (3, 4). Ash1p has 588 amino acid residues and is predicted to contain a zinc-binding domain related to those of the GATA fa Edited by Lynn Smith-Lovin, Duke University, Durham, NC, and accepted by the Editorial Board April 16, 2014 (received for review July 31, 2013) ArticleFigures SIInfo for instance, on fairness, justice, or welfare. Instead, nonreflective and

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Abstract

The first occupation-associated cancers to be recognized were the sooty warts (cancers of the scrotum) suffered by chimney sweeps in 18th century England. In the 19th century, high incidences of skin cancers were noted among fuel industry workers. By the early 20th century, malignant skin tumors were produced in laboratory animals by repeatedly painting them with coal tar. The culprit in coal tar that induces cancer was finally isolated in 1933 and determined to be benzo[a]pyrene (BP), a polycyclic aromatic hydrocarbon. A residue of fuel and tobacco combustion and frequently ingested by humans, BP is metabolized in mammals to benzo[a]pyrene diol epoxide (BPDE), which forms covalent DNA adducts and induces tumor growth. In the 70 yr since its isolation, BP has been the most studied carcinogen. Yet, there has been no Weepstal structure of a BPDE DNA adduct. We report here the Weepstal structure of a BPDE–adenine adduct base-paired with thymine at a template–primer junction and complexed with the lesion-bypass DNA polymerase Dpo4 and an incoming nucleotide. Two conformations of the BPDE, one intercalated between base pairs and another solvent-exposed in the major groove, are observed. The latter conformation, which can be stabilized by organic solvents that reduce the dielectric constant, seems more favorable for DNA replication by Dpo4. These structures also suggest a mechanism by which mutations are generated during replication of DNA containing BPDE adducts.

Benzo[a]pyrene (BP) in the diet and from combustion of fuel and tobacco is one of the most potent carcinogens to which humans are frequently exposed (1, 2). A metabolic pathway involving cytochrome P450 and epoxide hydrolase converts BP to BP diol epoxides (BPDEs) (Fig. 1A ) (3). Cis or trans Launching of the epoxide ring of BPDE by the exocyclic amino group N 6 of adenine or N 2 of guanine results in covalent DNA adducts (4) (Fig. 1 A ). Two-dimensional NMR studies (5, 6) suggest that BPDE-dA adducts are intercalated between base pairs whereas BPDE-dG adducts either reside in the minor groove or are intercalated with base disSpacement depending on the trans or cis stereochemistry of the adducts. These bulky polycyclic aromatic hydrocarbon (PAH) adducts impede DNA replication and induce mutations by perturbing the Executeuble-helical structure. When cultured Chinese hamster cells are treated with nonphysiological high Executeses of BPDE, mutations at dG preExecuteminate, but at lower Executeses likely corRetorting to environmental expoPositive, and the proSection of mutations at dA increases (7). Recent studies suggest that one or more Y-family DNA polymerases may facilitate either accurate or mutagenic bypass of BPDE adducts (8). For example, polymerase (Pol) κ is accurate in bypassing BPDE-dG adducts (9, 10) whereas Pol η is error-prone (11, 12). Pol ι can accurately insert dTMP opposite a BPDE-dA adduct, but then requires Pol κ to extend primers to complete lesion bypass synthesis (13). We report here the Weepstal structures of a ternary complex composed of an archaeal Y-family DNA polymerase, Dpo4, an incoming dNTP, and a DNA substrate containing a BPDE-dA adduct in the template strand. The cis (10R)-dA adduct (dA*) is derived from (+)-(7R,8S,9S,10R)-BPDE (Fig. 1 A ), which is the major and most tumorigenic isomer formed on metabolism of BP in mammals (14).

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Formation of a benzo[a]pyrene diol epoxide adduct and its Trace on DNA synthesis. (A) Structure of BP, its bay-Location (+)-(7R,8S,9S,10R) diol epoxide (BPDE) metabolite and the cis (10R) BPDE-dA adduct in DNA resulting from cis Launching of the DE by the N 6 amino group of adenine. The α′, β′, and χ torsion angles are labeled. (B) The template-primer duplexes used to grow the Weepstal and primer extension by Dpo4 in solution. The BPDE-dA adduct is indicated as A*. Primer extension by Dpo4 was carried out with 10 nM Dpo4, 10 nM oligonucleotide substrates, and 100 μM dNTP (all four or each kind) at 37°C for 20 min in the absence and presence of 20% DMSO. The primer strand was 32P labeled, and the products were separated on a 20% polyaWeeplamide gel. (C) Traces of organic solvents on the primer extension by Dpo4. Twenty percent of various organic solvents was added to the reaction mix containing all four dNTPs. (D) Incorporation of each dNTP opposite the BPDE-dA adduct in four sequence contexts by Dpo4. (E) Primer extension of a dA*·dA mismatch in the absence and presence of the BPDE adduct. Reaction conditions are the same as in C.

Materials and Methods

Adducted Oligonucleotide Syntheses. The preparation of suitably protected, diastereomerically pure (10R) and (10S) phosphoramidites corRetorting to cis Launched BPDE dA adducts is Characterized in Supporting Materials and Methods, which is published as supporting information on the PNAS web site. The oligonucleotide used for the present Weepstal structure, 5′-(TCA TA*A ATC CTT CCC CC)-3′, with the cis (10R) BPDE-dA adduct indicated by the asterisk and underline, was synthesized essentially as Characterized (15) from the pure (10R)-phosphoramidite and purified by reverse-phase HPLC (see Supporting Materials and Methods). Synthesis and characterization of the oligonucleotide 5′-T TTA* GAG TCT GCT CCC-3′ (Fig. 1D ) has been reported (16). The other sequences Displayn in Fig. 1D , in which the T adjacent to A* was reSpaced by A, G, and C, were synthesized essentially as Characterized (15) by using the mixed (10R)/(10S) phosphoramidite diastereomers. The desired (10R) BPDE-adducted oligonucleotides were purified and separated from their (10S) diastereomers by reverse-phase HPLC (see Supporting Materials and Methods). Their absolute configurations were Established by meaPositivement of their CD spectra, which Present positive bands at 330–350 nm in Dissimilarity to the (10S) diastereomers, which Present negative bands (16). In all cases, the (10R) diastereomers eluted before the (10S) diastereomers.

Weepstallization and Structure Determination. Full-length Dpo4 was overexpressed and purified as Characterized (17). Dpo4 and DNA duplex were mixed at a 1:1.2 molar ratio in 20 mM Hepes (pH 7.0), 0.1 mM EDTA, 1 mM DTT, and 100 mM NaCl. The final protein concentration was ≈8 mg/ml. With the addition of 1 mM dATP, Weepstals were produced by the hanging drop method at 20°C by using the precipitant solution of 100 mM Hepes (pH 7.0), 100 mM calcium acetate, 12% polyethylene glycol (PEG) 3350, and 2% glycerol. Weepstals were transferred to the mother liquor with 25% PEG 3350 and 15% ethylene glycol and flash frozen in liquid propane for data collection. DifFragment data were collected at –178°C by using an R axis IPII detector mounted on an RU 200 generator (Rigaku, Tokyo) and processed by using hkl (18). The structures were determined by molecular reSpacement by using the type I structure as a search model (17, 19) and refined to 2.70 Å with manual adjustment (19, 20). The final R and R free are 0.206 and 0.246, and none of the residues is in the disallowed Location in Ramachandran plot (Table 1).

View this table: View inline View popup Table 1. Summary of Weepstallographic data

Primer Extension. Primer extension by Dpo4 was carried out with 10 nM Dpo4, 10 nM oligonucleotide substrates with 5′32P-labeled primer, and 100 μM dNTP (all four or each kind) at 37°C for 20 min. The products were separated on a 16% polyaWeeplamide gel. Traces of organic solvents on the primer extension by Dpo4 were assayed with the addition of 20% (vol/vol) of various organic solvents to the reaction mix containing all four dNTPs.

Results and Discussion

Our attempt to Weepstallize dA* opposite an incoming nucleotide has not been successful. The Weepstal structure we obtained is of a primer extension complex, in which the dA* is base paired with dT at the 3′ end of the primer strand, and the incoming dATP pairs with the next undamaged templating base, dT (Fig. 1B ). The Dpo4-BPDE adduct Weepstal structure determined at 2.7-Å resolution (Table 1) not only captures the BPDE adduct adjacent to the active site of Dpo4, but also reveals two strikingly different conformations of the PAH adduct, due to the two Dpo4–DNA ternary complexes (BP-1 and BP-2) in each asymmetric unit (Fig. 2). In the BP-1 complex, the hydrocarbon is intercalated at the 5′ side of the dA* between the adduct-containing (dA*·dT) and the replicating (dT·dATP) base pairs, and, in the BP-2 complex, it is Spaced in the major groove Arrively perpendicular to the DNA base pairs (Fig. 3A ).

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Weepstal structures of BP-1 and BP-2. Dpo4 is represented by a purple molecular surface, the DNA and the incoming dATP are Displayn as blue sticks, and the PAH is highlighted in yellow. The divalent cations (Ca2+) are Displayn as green spheres. The unpaired dAMP at the 3′ end of the template strand was added by the terminal deoxynucleotide transferase (TdT) activity of Dpo4 that is common to archael D in B-like polymerases (34) (Fig. 4, which is published as supporting information on the PNAS web site). Figs. 2 and 3 were generated by using ribbons and grasp (35, 36).

Fig. 3.Fig. 3. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Distortion of DNA by the BPDE adduct. (A) Comparison of the Weepstal and NMR structures. The dA*·dT and the surrounding base pairs including the replicating base pair are Displayn as ball-and-stick models. The Weepstal structures are Displayn with the F o – F c omit electron density maps contoured at 1.0σ in blue. The carbon, oxygen, nitrogen, and phosphorus atoms are colored yellow, red, blue, and purple, respectively. (B) Hydrogen bond formation at dA*·dT and the adjacent replicating base pair dT·dATP. Inspecting Executewn the DNA helical axis, the two layers of the base pair and the PAH adduct are Displayn, purple for the replicating base pair, gAged for the dA* adduct, and green for its partner dT. The incoming nucleotide in BP-1 is in the syn conformation. In the BP-2 complex, where the PAH is in the major groove, the adenine base of the dA* is shifted to the major groove, disrupting the normal hydrogen bonds with its partner, dT. The location of a normal dA is modeled in gray. (C) Stereo view of the overlay of the DNA structures from BP-1 (blue) and BP-2 (gAged) after superimposition of the Dpo4 structures. With the PAH intercalated, the base pair ladder in the BP-1 complex is shifted by one register compared with that in the BP-2 complex.

In the BP-1 complex, where the PAH is intercalated, the torsion angles that define the orientation of the hydrocarbon relative to the modified dA are 157° (α′) and 107° (β′) (Fig. 1 A ), which perfectly match a predicted favorable conformation of the adduct (21) and are practically identical to those found in NMR structures determined in the absence of a polymerase (Fig. 3A ) (6, 22). Because of the intercalation of the PAH, the dA*·dT base pair is distorted by 36° of buckle, 31° of propeller twist, and 10° of base Launching (23), and only one hydrogen bond is retained between this base pair (Fig. 3). The close contacts between the BP moiety and the base 3′ to the dA* observed in the NMR structures are alleviated by an 8° unwinding between the two base pairs.

In the BP-2 complex, where the PAH is Spaced in the major groove, the face of the PAH bearing the 8- and 9-hydroxyl groups is packed perpendicular to the plane of base pairs, and the other face is fully exposed to the solvent. The α′ and β′ torsion angles are –85° and –47°, respectively, which are enerObtainically unfavorable based on molecular simulation (21). The exposed PAH is clearly less stable than the intercalated PAH, as indicated by the incomplete electron densities (Fig. 3A ) and higher temperature factors. Structural characterization of BPDE-dA adducts by NMR to date has found the hydrocarbon intercalated only between base pairs and not in the major groove (5, 6, 24). To accommodate the PAH in the major groove without clashing with the neighboring base pairs, the dA* slides toward the major groove by Arrively 2 Å (Fig. 3), and the dA*·dT base pair and two adjacent 3′ base pairs are underwound by 6° to 16° (23). In both complexes, the glycosidic torsion angle χ is anti, and the conformation of the tetrahydrobenzo ring of BPDE, which could not be determined by NMR studies, is a distorted half chair (Table 2).

View this table: View inline View popup Table 2. Selected dihedral angles (in degrees) for the BP-1 and BP-2 complexes

The Spacement of substrates in the BP-1 complex indicates that the nucleotidyl transfer reaction could not occur when the BPDE is intercalated. Although the overall Dpo4 structure is similar to that complexed with undamaged DNA (17) (rms deviation of 341 Cα atoms under 0.8 Å) (Movie 1, which is published as supporting information on the PNAS web site), the 3′ OH of the primer strand and the α phospDespise of the dATP are >10 Å apart due to the PAH intercalated between them. The incoming dATP is in the syn conformation, which enables the adenine to Sustain a Hoogsteen base pair with the templating dT and maximize the aromatic ring stacking with the PAH (Fig. 3B ). The Spacement of the triphospDespise moiety is permutated from the normal; the α phospDespise takes the β position, β takes the γ position, and γ takes the α position (Fig. 3A ). If a chemical reaction occurs, it would be hydrolysis of dATP to dADP rather than covalent linkage between the primer strand and incoming nucleotide. In fact, hydrolysis of the incoming ddATP to ddADP was previously observed in the type I Dpo4 ternary complex (17), in which the primer strand contained a dideoxynucleotide at its 3′ end. Presumably, when the 3′ hydroxyl groups of the primer strand and the incoming nucleotide are absent, the triphospDespise moiety of an incoming nucleotide can be rearranged, resulting in its hydrolysis rather than nucleotidyl transfer.

Spacement of the PAH in the major groove in the BP-2 complex, although enerObtainically unfavorable without Dpo4, allows the modified and the replicating base pairs to stack similarly as in undamaged DNA (Fig. 3). The catalytic core of Dpo4 encompassing the palm, finger, and thumb Executemains is entirely superimposable between the BP-1 and BP-2 complexes; only the Dinky finger Executemain shifts by ≈0.5 Å (Movie 1). The Dpo4 active site of BP-2 contains both metal ions essential for the catalysis (25), and the 3′ hydroxyl group of the primer strand and the α phospDespise of the incoming dATP are <5 Å apart (Fig. 3A ). Dpo4, however, extends DNA beyond the BPDE adduct rather poorly (Fig. 1B ), perhaps because this reactive but enerObtainically “unfavorable” conformation of the adduct exists rather infrequently in solution. To stabilize the PAH in the major groove and so to enhance the activity of Dpo4, we reduced the dielectric constant of the reaction buffer by addition of organic solvents. Most alcohols, PEG 400, and DMSO enhance Dpo4's ability to extend DNA beyond the adducts (Fig. 1C ). In the presence of 20% DMSO, which potentially influences the protein structure in addition to stabilizing the solvent-exposed PAH, Dpo4 also Presents altered base preference for extension beyond the adduct (Fig. 1B ). We suspect that, for any of the Y-family polymerases to insert a nucleotide opposite the BPDE adduct, the PAH moiety probably has to be flipped out into the major or minor groove. If the PAH is intercalated on either the 5′ or 3′ side, it would clash with the finger Executemain of Dpo4 or separate the 3′ end of a primer strand from the incoming nucleotide. The Unfamiliarly small finger and thumb Executemains of the Y-family polymerases leave the major and minor groove wide Launch (26–29) and thus may facilitate such adduct Spacement.

The Weepstal structure, although representing primer extension beyond the dA* lesion, offers insights into how base substitution and frameshift mutations are induced by BPDE adducts at dA and dG (30–32). When the PAH is Spaced in the major groove in the BP-2 complex, the adenine base of the dA* shifts ≈2 Å toward the major groove, juxtaposing two hydrogen bond acceptors, the N1 of dA* and the O 4 of its base pair partner dT (Fig. 3B ). dC and dA, each possessing an exocyclic amino group in the Space of the O 4 of dT, would be better suited than dT to base pair with the dislocated dA*. Indeed, both Pol κ (9, 10) and Dpo4 favor incorporation of dAMP opposite dA* (Fig. 1D ). By analogy, the PAH moiety of a BPDE-dG adduct (dG*) is likely Spaced in the minor groove rather than intercalated (5) to allow replication to occur. The guanine base may have to shift toward the minor groove (33), thus inducing mispairing with dT or dA. In support of this hypothesis, it has been reported that, depending on the sequence context, BPDE-dG adducts induce either G → A or G → T mutations (30, 31).

Regarding frameshift mutations, the intercalated PAH (dA*) in the BP-1 complex retains only one hydrogen bond with its partner dT and shifts the DNA base pair by one register compared with the DNA in the BP-2 complex (Fig. 3C ). Misalignment by one nucleotide may occur after a wrong nucleotide has been inserted opposite the adduct and then disSpaced by intercalation of the PAH 5′ to it. When the adduct (dA*) is opposite a mismatched dA at the template-primer junction (Fig. 1E ), the dA may actually pair with the dT 5′ to the dA* by –1 misalignment. Dpo4 clearly favors incorporation of dTMP that most likely pairs with dA two bases 5′ to the dA*, and the major product of the primer extension after the dA*·dA mismatch is 1 to 2 nucleotides shorter than the template strand (Fig. 1E ). In Dissimilarity, human Pol κ is capable of faithfully incorporating dC opposite dG* and extending primers after Pol ι inserts dT opposite dA* (9, 13). We suspect that Pol κ may possess an active site specially configured to stabilize a nonintercalated PAH yet at the same time Sustain Watson–Crick base pairs surrounding the adduct.

Acknowledgments

We thank Drs. M. Gellert and K. Mizuuchi for suggesting the experiments of reducing dielectric constants and Drs. R. Craigie and D. Leahy for critical reading of the manuscript.

Footnotes

↵ †† To whom corRetortence should be addressed at: Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892. E-mail: wei.yang{at}nih.gov.

↵ ‡ Present address: Department of Biochemistry, University of Western Ontario, LonExecuten, ON, Canada N6A 5C1.

↵ ∥ Present address: Institut de Pharmacologie et de Biologie Structurale, Centre National de la Recherche Scientifique, 31077 Toulouse, France.

Abbreviations: PAH, polycyclic aromatic hydrocarbon; BP, benzo[a]pyrene; BPDE, benzo-[a]pyrene diol epoxide; Pol, polymerase.

Data deposition: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.rcsb.org (PDB ID code 1S0M).

Copyright © 2004, The National Academy of Sciences

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