Sequence dependence and characteristics of bends induced by site- specific polynuclear aromatic carcinogen - Deoxyguanosine lesions in oligonucleotides

The tumorigenic metabolite of benzo[a]pyrene, the (+)-7R,8S,9S, 10R enantiomer, and the nontumorigenic mirror-image isomer, (-)-7S,8R,9R, 10S, of r7,t8-dihydroxy-t9,10-epoxy-7,8,9,10-tetrahy- drobenzo[a]pyrene (anti-BPDE) bind covalently to the exocyclic amino group of deoxyguanosine (N²⁺ dG in native DNA. These adducts can cause structural perturbations such as DNA bends, which in turn may influence the cellular processing of these lesions. The characteristics of bends in site-specifically modified oligodeoxyribonucleotide duplexes induced by single (+)- and (-)-ant/-[BP]- N²-dG lesions were examined by self-ligation and gel electrophoresis techniques. The modified residues (dG*) were centrally positioned in the 11- mer oligonucleotide d(CACAXG*XACAC) complexed with the natural complementary strands, with X = Tor C, or in oligonucleotides 16 or 22 base pairs long with the same centrally positioned 11-mer. Among the four stereochemically distinct lesions, the 10S (+)-trans-anti-[BP]-N²-dG adducts were significantly more bent than any of the other three stereoisomeric adducts and were selected for detailed studies. In the TG*T sequence context (X = T), the retardation factor R(L) (apparent length of multimer/sequence length) is approximately independent of the phasing (distance, in base pairs, between the lesions)of the adducts with respect to the helical repeat (10.5 base pairs/helix turn). In contrast, in the CG*C sequence context (X = c), RL is markedly lower in the case of ligated 16-mers than in the case of ligated 11- met duplexes. The dependence of RL on the phasing of the bends as a function of the helical repeat, indicate that the bends associated with (+)-trans- anti-[BP]-N²-dG lesions are relatively rigid in the d(...CG*C...)·d(...GCG...) sequences, and flexible in the d(...TG*T...)·d(...ACA...) sequence context. These differences are attributed to the orientations of the pyrenyl residues on the 5'-side of the modified deoxyguanosine residues in the minor groove and to the intrinsic roll and tilt characteristics of DNA dinucleotide steps CG, GC, TG, and GT. The influence of flanking bases on the extent and character of DNA bending suggest that base sequence effects may be important in the cellular processing of (+)trans-anti-[BP]-N²-dG lesions.