Figure 1.

Covalently closed circular and linearized plasmid substrates (2686 bp) containing site-specifically positioned (+)-cis-B[a]P-dG adduct (X) and a radioactive ³²P-internal label. The covalently closed plasmid substrate was generated by an established gapped-vector technology¹⁰ from a pUC19NN plasmid containing two Nt. BbvCI restriction sites (underlined). The linearized plasmid substrate with the lesion X positioned at the 945^th nucleotide counted from the 5′-end was prepared by the selective cleavage of the circular plasmid with a unique ScaI restriction enzyme that generates blunt end cleavage products.