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 ena ntiomer, and the nontumorigenic mirror-image isomer, (-)-7S,8R,9R,10S, of roxy-t9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) bin d covalently to the exocyclic amino group of deoxyguanosine (N-2-dG) i n native DNA. These adducts can cause structural perturbations such as DNA bends, which in turn may influence the cellular processing of the se lesions. The characteristics of bends in site-specifically modified oligodeoxyribonucleotide duplexes induced by single (+)- and (-)-anti -[BP]-N-2-dG lesions were examined by self-ligation and gel electropho resis techniques. The modified residues (dG) were centrally positione d in the 11-mer oligonucleotide d(CACAXGXACAC) complexed with the nat ural complementary strands, with X = T or C, or in oligonucleotides 16 or 22 base pairs long with the same centrally positioned 11-mer. Amon g the four stereochemically distinct lesions, the 10S (+)-trans-anti-[ BP]-N-2-dG adducts were significantly more bent than any of the other three stereoisomeric adducts and were selected for detailed studies. I n the TGT sequence context (X = T), the retardation factor R-L (appar ent length of multimer/sequence length) is approximately independent o f the phasing (distance, in base pairs, between the lesions) of the ad ducts with respect to the helical repeat (10.5 base pairs/helix turn). In contrast, in the CGC sequence context (X = C), R-L is markedly lo wer in the case of ligated 16-mers than in the case of ligated 11-mer duplexes. The dependence of R-L on the phasing of the bends as a funct ion of the helical repeat, indicate that the bends associated with (+) -trans-anti-[BP]-N-2-dG lesions are relatively rigid in the d(...CGC. ..).d(...GCG...) sequences, and flexible in the d(...TGT...).d(...ACA ...) sequence context. These differences are attributed to the orienta tions of the pyrenyl residues on the 5'-side of the modified deoxyguan osine residues in the minor groove and to the intrinsic roll and tilt characteristics of DNA dinucleotide steps CG, GC, TG, and GT. The infl uence of flanking bases on the extent and character of DNA bending sug gest that base sequence effects may be important in the cellular proce ssing of (+)-trans-anti-[BP]-N-2-dG lesions.